1

Normal Aortic Dimensions

Because of the ease with which it can be visualized and its clinical relevance, the aortic root is the segment for which the greatest amount of data are available. Several large studies have reported normal aortic root diameters in the parasternal long-axis view by two-dimensional (2D) transthoracic echocardiography (TTE). Measurement of the aortic root diameter should be made perpendicular to the axis of the proximal aorta, recorded from several slightly differently oriented long-axis views. The standard measurement is taken as the largest diameter from the right coronary sinus of Valsalva to the posterior (usually noncoronary) sinus. Most studies report aortic root diameter measurements at end-diastole using the leading edge–to–leading edge technique ( Figure 2 ).

Transthoracic echocardiogram in the parasternal long-axis view (zoomed on aortic root and ascending aorta) illustrating measurement of the aortic root diameter at sinus of Valsalva level at end-diastole using the leading edge–to–leading-edge method. asc Ao , Ascending aorta; LVOT , left ventricular outflow tract.

In adults, aortic dimensions are strongly positively correlated with age and body size. They are larger in men than in women of the same age and body size. Although in several reports, aortic diameters have been normalized to body surface area (BSA), this approach has not been entirely satisfactory because it is systematically lower in smaller than in larger normal adults. Fortunately, among children, the regression line of aortic diameter and height (rather than BSA) has a near-zero intercept, so that normalization to height has proved to be a simple and accurate alternative in growing children. Benchmark values from which the guidelines have been taken come from the work of Roman et al ., who reported normal root dimensions for three age groups ( Figure 3 ).

Aortic root diameter ( vertical axis ) in relation to BSA ( horizontal axis ) in apparently normal individuals aged 1 to 15 ( left panel, blue ), 20 to 39 ( center panel, green ), and ≥40 ( right panel, pink ) years. For example, an individual between the ages of 20 and 39 years ( center panel, green ) who has a BSA of 2.0 m ² ( vertical green line ) has a normal root diameter range (2 SDs) between 2.75 and 3.65 cm, as indicated by the intersections of the two horizontal green lines with the green-shaded parallelogram.

The upper limit of normal aortic diameter has been defined as 2 SDs greater than the mean predicted diameter. The Z score (the number of SDs above or below the predicted mean normal diameter) is a useful way to quantify aortic dilatation. Among normal subjects, 95.4% have Z scores between −2 and 2. Therefore, an aortic diameter can be considered dilated when the Z score is ≥2. Using the Z score allows comparison of a given patient’s aortic size at different time points, accounting for the effects of advancing age and increasing body size, thus distinguishing normal from pathologic growth. The Z score is therefore particularly useful for evaluating growing children.

It should be mentioned that aortic root dimensions may be increased by the hemodynamic effects of both endurance and strength exercise training in competitive athletes. This aortic root enlargement appears to be greater at the sinuses of Valsalva than at the aortic annulus or STJ. However, it should be emphasized that the effects of exercise training on aortic diameters are relatively small and that marked enlargement should suggest a pathologic process.

Recently, making use of a database consisting of a multiethnic population of 1,207 apparently normal adolescents and adults ≥15 years of age, investigators devised equations to predict mean normal aortic root diameter and its upper limit by age, body size (BSA or height), and gender ( Table 1 for men and Table 2 for women). These equations have been used graphically to depict the upper limits of the 95% confidence interval for normal aortic root diameter using surfaces to depict the interacting effects of age and body size (see Figure 4 for men and Figure 5 for women).

Surfaces representing aortic diameters at a 1.96 Z score (95% confidence interval) above the predicted mean for age and BSA in male subjects ≥15 years of age.

(Adapted from Devereux et al . )

Surfaces representing aortic diameters 1.96 Z score (95% confidence interval) above the predicted mean value of aortic diameter for age and BSA in female subjects ≥15 years of age.

(Adapted from Devereux et al . )

A noncontrast gated cardiac computed tomographic study, including 4,039 adult patients, showed age, BSA, gender, and hypertension to be directly associated with thoracic aortic diameters perpendicular to the long axis of the aorta. These associations are concordant with those from echocardiographic studies. In another recent large study using similar methodology, the mean value of the diameters of the ascending aorta was 1.8 ± 0.2 cm/m ² and of the descending thoracic aorta was 1.4 ± 0.2 cm/m ² , with the upper limits of normal being 2.1 and 1.8 cm/m ² , respectively. However, more accurate normal values of thoracic aortic diameters may be obtained by anatomically correct double-oblique short-axis images using electrocardiographically gated multidetector CT or by MRI of axially oriented aortic segments. The upper limits of normal are 3.7 cm for the aortic root at the sinuses, 3.6 cm for the ascending aorta and 2.5 cm for the descending thoracic aorta by CT, and 2.5 cm for the descending thoracic aorta and 2.0 cm for the upper abdominal aorta by MRI. As with echocardiography, aortic root and ascending aortic diameters increase significantly with age and BSA on CT and MRI. Aortic root diameters increase 0.9 mm per decade in men and 0.7 mm per decade in women.

The establishment of normative values and reference ranges, taking into account aging and gender, is of great importance for diagnosis, prognosis, serial monitoring, and determining the optimal timing for surgical intervention. Normal values and proximal aortic diameters have been reported using different imaging techniques, from the pioneer studies based on M-mode and 2D echocardiography to more recent studies obtained using CT and MRI. Despite differences in image acquisition methods, temporal and spatial resolution, and signal-to-noise ratios, CT, MRI, TTE, and transesophageal echocardiography (TEE) have evolved as near equal standards for assessing aortic root size. Each of these modalities has advantages and disadvantages, which have been discussed. It should be emphasized that normal aortic diameters vary systematically by age, gender, and body size, and reference values indexed to those parameters have been provided. Last, it is critically important to emphasize not only methodologic variance but also inter- and intraobserver variability. In several studies, variability of measurement of proximal aortic diameters ranges from 1.6 to 5 mm. Given this degree of variability, apparent small changes in proximal aortic diameters on serial computed tomographic examinations may be within the range of measurement error. Accordingly, for all imaging techniques, we recommend that changes of ≤3 mm by electrocardiographically gated CT and ≤5 mm without electrocardiographic gating be viewed with caution and skepticism.

1

Interface, Definitions, and Timing of Aortic Measurements

The American Society of Echocardiography (ASE) proposed standards for measurement of the aortic root in 1978. The ASE recommended measurement at end-diastole from the leading edge of the anterior root wall to the leading edge of the posterior aortic root wall. This technique was believed to minimize the impact of “blooming” of bright reflectors on this measurement. The ASE-recommended method was followed in many important clinical and epidemiologic studies that have reported normal limits for individuals of differing body size and age, and these normal limits have been incorporated into multiple guidelines for imaging in adults ( Figures 4 and 5 ). As a consequence, much of the available data on normal aortic root size as well as on the prevalence and prognostic significance of aortic dilatation in adults have emerged from echocardiography.

Societal guidelines for measurement by CT or MRI are not currently available. Consequently, uniformity in measurement methods is lacking. Many research and clinical studies using these modalities have reported aortic measurements made from inner edge to inner edge on electrocardiographically gated or nongated images. The 2010 guidelines for the diagnosis and management of thoracic aortic disease took the opposite approach, recommending measurement of aortic diameter between external surfaces to avoid confounding by intraaortic thrombus or atheroma, as is commonly found in the abdominal but not in the ascending aorta. Furthermore, there is no standardized “trigger time” (end-systole vs end-diastole) for image acquisition. Thus, the use of multiple imaging modalities such as CT, MRI, and 2D and three-dimensional (3D) echocardiography has led to nonuniformity in measurement techniques. Moreover, there is currently no standardized approach for reconciling aortic measurements across imaging modalities (echocardiography, CT, MRI, aortography) by trigger time (end-systole vs end-diastole) or by edge selection (leading edge, inner-inner, outer-outer). This writing committee had hoped to recommend a uniform and consistent measurement technique to minimize differences among these various imaging modalities. However, after much consideration, the group recommends that echocardiographic measurements continue to be made in the standard fashion from leading edge to leading edge, at end-diastole, and perpendicular to the long axis of the aorta. The advantages of end-diastolic measurements include greater reproducibility (because aortic pressure is most stable in late diastole) and the ease of identification of end-diastole by the onset of the QRS complex. Although other techniques use the inner edge–to–inner edge approach, there are currently insufficient data to warrant a change for echocardiography. Available data suggest that the echocardiographic leading edge–to–leading edge approach produces values comparable with those produced by the inner edge–to–inner edge approach on CT and MRI, is reproducible, and links to a large body of historical and prognostic data that have long guided clinical decision making.

For all modalities, it is desirable, whenever possible, to specify the locations of measurements, by referencing them to a given landmark. For example, with TEE, a measurement of the maximal diameter of the ascending aorta may be reported by its distance from the STJ. In the descending thoracic aorta, reference to the location of a measurement or abnormality is usually made by its distance from the incisors. Similar attempts should be made for measurements and findings with CT and MRI.

2

Geometry of Different Aortic Segments: Impact on Measurements

Accurate and reproducible measurement of aortic diameter or cross-sectional area in a given segment requires three measurements of its diameter perpendicular to the long axis. In most cases, the largest correctly oriented measurement is reported.

a

Aortic Annulus

Although the aortic annulus is approximately circular in children and young adults, it may become elliptical in older adults. Thus, 3D imaging by CT or echocardiography or 2D imaging in multiple planes (e.g., long-axis or sagittal and coronal planes) is required to measure a diameter that is accurate enough to be used when selecting patients for transcatheter aortic valve replacement ( Figure 6 ).

Models of the thoracic aorta showing the cut planes of the aortic annulus for each applied imaging modality. (A) Angiography in the 90° left anterior oblique (LAO) projection with an orange arrow indicating the sagittal annulus diameter ( left ) and in the 0° posteroanterior (p.a.) projection with a blue arrow indicating the coronal annulus diameter ( right ). (B) TTE ( left ) and 2D TEE ( right ) left ventricular outflow tract (LVOT) view of the aortic annulus. The cut planes slightly differ because parasternal and midesophageal acoustic are not quite comparable. Both the transthoracic and 2D transesophageal echocardiographic LVOT views resemble a sagittal view ( bright and dark yellow arrows , respectively). The direction of the arrows in the aortic arch model and the echocardiographic images indicate the scanning direction. Individual adjustments in scan plane direction are shown in the model. (C) Three-dimensional transesophageal echocardiographic cropped images of a sagittal ( left ) and coronal ( right ) view with the corresponding diameters ( orange and blue arrows ). The sagittal and coronal cut planes are depicted in the aortic arch model and the anatomic short-axis view ( middle ). (D) Dual-source computed tomographic (DSCT) reconstructed images of a sagittal ( left ) and coronal ( right ) view with the corresponding diameters ( orange and blue arrows ). The sagittal and coronal cut planes are depicted in the aortic arch model and the anatomic short-axis view ( middle ). AO , Ascending aorta; LA , left atrium.

(From Altiok et al . )

b

Sinuses of Valsalva and STJ

Aortic root diameter can be measured perpendicular to its long axis by 2D echocardiography or in analogous nontrue coronal and sagittal plane by MRI or CT. The variability in this measurement resulting from the orientation of the aortic root is overcome by choosing the largest diameter measured from the right coronary sinus of Valsalva to the posterior (usually noncoronary) sinus, parallel to the aortic annulus and perpendicular to the long axis of the proximal aorta in several slightly differently oriented long-axis views. Failure to search for the largest correctly oriented measurement can lead to underestimation of aortic root diameter. Aortic root diameter is commonly measured by CT or MRI between the inner edges from commissure to opposite sinus ( Figure 7 ). Diameters measured using the sinus-to-sinus method are generally a mean of 2 mm larger than those measured by the sinus-to-commissure method ( Figure 8 ). However, using the sinus-to-sinus method has several advantages, including the ease of detecting cusp margins in computed tomographic or MRI transverse planes, close agreement with echocardiographic measurements, and greater feasibility in bicuspid valves. Thus, for aortic measurements by CT and MRI, it is recommended to average the three sinus-to-sinus measurements in end-diastole in the sinus-of-Valsalva plane. When the sinuses are unusually asymmetric, it may be preferable to report the three measurements individually.

Aortic root measurements by CT. The aortic root diameter is commonly measured between the inner edges from one commissure to opposite sinus ( yellow line ) or from one sinus to another sinus ( red line ), as shown in the large image ( left ), which is a zoomed cross-sectional view of the aortic root at the sinus of Valsalva level using a double oblique image for orientation (shown in the right panel ).

Computed tomographic scan image of aortic root illustrates that the mean difference of the aortic root diameter is about 2 mm larger measured by the anteroposterior diameter ( sinus-sinus ) shown by red arrow than by the sinus-commissure diameter ( black arrow ).

c

Ascending Aorta and More Distal Segments

The same basic principles apply to obtaining correct measurements of the other aortic segments. Conventional imaging by all modalities and techniques can be used to measure the diameter of aortic segments that are oriented along the long axis of the body. However, the necessity to avoid oblique imaging that can overestimate the aortic diameter applies to the aortic arch and to portions of the descending thoracic and abdominal aorta that may take a tortuous course ( Figure 9 ).

Diagram illustrating the potential pitfall of obtaining an oblique cut resulting in an “ellipsoid” cross-section that overestimates the true diameter. This is especially a problem when the descending aorta is tortuous.

We emphasize that there is no standardized method for measuring the aorta across imaging modalities (echocardiography, CT, MRI, aortography). Although one of the major goals of this writing committee was to provide a uniform and universally accepted method to minimize differences among these various imaging modalities, no consensus could be reached. After much consideration, it is recommended that echocardiographic measurements continue to be made from leading edge to leading edge. Although other techniques use inner edge–to–inner edge or outer edge–to–outer edge approaches, there are currently insufficient data to warrant a change for echocardiography. Available data suggest that the echocardiographic leading edge–to–leading edge approach gives larger measurements compared with the inner edge–to–inner edge approach on CT (average difference, 2 mm), and the leading edge–to–leading edge method links to a large body of historical and, more important, prognostic data that influence decision making. Out of concern that patient management might be adversely affected (i.e., intervention might be delayed, leading to a catastrophic complication such as rupture or dissection) by switching to a new protocol that would lead to a smaller measurement, it was decided to continue to recommend the leading edge–to–leading edge approach.

1

Local Indices of Aortic Function

Techniques that provide accurate definition of the aortic diameter or volume in systole and diastole can be used to evaluate the elastic properties of the aorta. The most commonly applied indices for clinical purposes are aortic distensibility and the stiffness index, which is less dependent on blood pressure. Aortic distensibility and the stiffness index can be determined from the changes in the aortic diameter from systole to diastole and from changes in the arterial pressure using the following formula:

For these calculations, the pulse pressure should be measured ideally at the same level of the aorta at which the aortic diameter is measured. In clinical practice, however, brachial artery pressure can be used, even though the pulse pressure obtained from the brachial artery may be slightly higher than that obtained from the aorta because of the amplification phenomenon, which is more apparent in young individuals.

2

Regional Indices of Aortic Stiffness: Pulsewave Velocity (PWV)

PWV is defined as the longitudinal speed of the pulsewave in the aorta. PWV is inversely related to aortic elasticity. Hence, a stiffer aorta will conduct the pulsewave faster than a more compliant aorta. Central pressure, at the level of the ascending aorta, is produced as a combination of the antegrade wave from the left ventricle and the retrograde “reflective” waves from the periphery. In young individuals, because the aorta is more elastic, the pulsewave speed is low, so the retrograde flow arrives in the proximal aorta during diastole. As a result of aortic stiffening, the PWV increases, and the retrograde flow arrives in the proximal aorta earlier in systole, leading to a greater LV afterload and decreased diastolic pressure.

Reported normal values for invasively determined PWV measurements in middle-aged humans are 4.4 ± 0.4 m/sec in the aortic root, 5.3 ± 0.2 m/sec in the proximal descending thoracic aorta, 5.7 ± 0.4 m/sec in the distal thoracic descending aorta, 5.7 ± 0.4 m/sec in the suprarenal abdominal aorta, and 9.2 ± 0.5 m/sec in the infrarenal aorta.

Carotid-femoral PWV is considered to be the gold-standard measure of arterial stiffness, especially because it is simple to obtain and because multiple epidemiologic studies have demonstrated its predictive value for cardiovascular events. However, the ability of a given individual’s PWV value to predict aortic events has not been previously evaluated. A recent expert consensus adjusted this threshold value to 10 m/sec by using the direct carotid-to-femoral distance. The main limitation of PWV interpretation is that it is significantly influenced by arterial blood pressure. Multimodality imaging techniques provide a unique opportunity to assess aortic PWV by the formula:

Echocardiography can accurately estimate the transit time between aortic levels by the subtraction of the time between a fixed reference in the QRS complex and the beginning of the flow at the two levels studied. Distance can be grossly estimated externally with a tape.

MRI can measure the PWV using the transit time of the flow curves from a phase-contrast acquisition. Transit time can be calculated by the upslope approach, which has been described previously and correlates more with age and aortic stiffness indices than point-to-point approaches such as foot-to-foot and half-maximum methods. Distance can accurately be measured at the centerline of the aorta between aortic levels studied.

A normal-size aorta may be functionally abnormal. Thus, determination of aortic function may help define the nature of the underlying disease and give prognostic information in some diseases. This was emphasized by Vriz et al ., who stated that aortic stiffness should be taken into account when increases in aortic diameter are detected.

In summary, aortic biophysical properties can be easily and reliably assessed by imaging techniques, particularly Doppler echocardiography and phase-contrast MRI. This evaluation may provide important pathophysiologic and prognostic information that may have clinical implications both in disease states and in the general population.

1

Imaging of the Aorta

As mentioned, the anatomic proximity of the thoracic aorta and the esophagus allows superb visualization of the aorta using TEE. The multiplane transesophageal echocardiographic examination of the aorta is conducted as follows : with the tip of the transesophageal echocardiographic probe in the esophagus, the ascending aorta is best visualized from a 100° to 140° view: the image is analogous to the transthoracic echocardiographic parasternal long-axis view (but “flipped” upside down if the transesophageal echocardiographic probe “bang” is at the top). This view can be optimized by carefully rotating the transducer between 100° and 140°. Short-axis views of the aortic root and ascending aorta can be obtained from the 45° to 60° angle, usually with an anteflexed probe. From the midesophagus at 0°, the probe needs to be rotated posteriorly to obtain short-axis images of the descending thoracic aorta. While keeping the thoracic aorta in view, the probe can be withdrawn to image upper thoracic levels of the descending aorta or advanced to sequentially image the lower thoracic and upper abdominal aorta. With the transducer array at 90°, a longitudinal view of the aorta can be obtained.

By advancing the probe into the stomach, the proximal portion of the abdominal aorta and the celiac trunk can be seen. The mid and distal abdominal aorta are usually not seen because of difficulty maintaining good contact with the mucosa of the stomach. To obtain images of the arch, the transesophageal echocardiographic probe needs to be facing posteriorly and withdrawn from the midesophagus. With the transducer array at 90°, a short-axis view of the transverse arch can be obtained. It is usually possible to visualize the takeoff of the left subclavian artery, but the left common carotid and brachiocephalic arteries can be difficult or impossible to image and usually require careful clockwise rotation of the probe. A portion of the distal ascending aorta and proximal aortic arch may not be visible because of interposition of the trachea. This “blind spot” can be partially resolved with longitudinal views. An additional view, the deep transgastric view, can sometimes image the entire ascending aorta and often the proximal arch ( Figure 12 ).

Transesophageal echocardiographic deep transgastric view, which illustrates the aortic root (Ao R), the entire ascending aorta (AA), and the proximal arch (not labeled). The left coronary artery is also imaged ( black arrow ).

1

Limitations

The normal aorta appears on IVUS as a circular cross-sectional image with an intact wall and a clear lumen. The ultrasound catheter and the guidewire are seen within the lumen. In some instances, it can be difficult to obtain complete cross-sectional images of the aorta within a single frame of the image display at the arch and locations where the aorta is significantly dilated, because of difficulty maintaining the ultrasound catheter in a central and coaxial orientation and because of the limited penetration with high-frequency transducers. This limitation can be partially overcome by periodic reorientation of the ultrasound catheters. There are also concerns with IVUS measurements. Off-center measurements or those taken in tortuous portions of the aorta (tangential measurements on a curve) do not reflect a true centerline diameter, may provide an oblique slice, and are less accurate than centerline computed tomographic measurements. Another major limitation of IVUS is that it lacks Doppler capabilities (color Doppler can detect flow into small arteries, false luminal flow, and endoleaks). Last, the high cost of the disposable transducers and invasive nature of the technique limit IVUS for most clinical applications other than guidance of endovascular procedures.

1

Methodology

a

CTA

The combination of wide multidetector arrays with short gantry rotation times in ≥64-detector computed tomographic scanners results in standard acquisition times of 3 to 4 sec for the thoracic aorta and <10 sec for the thoracoabdominal aorta and the iliofemoral arteries.

The minimal technical characteristics of state-of-the-art CTA are a slice thickness of ≤1 mm and homogeneous contrast enhancement in the aortic lumen. Complete examination of the aorta from the supra-aortic vessels to the femoral arteries is needed for evaluation before transthoracic endovascular aortic repair (TEVAR), but as a general rule the scan length (anatomic scan range) on CTA should be individually tailored to avoid unnecessary exposure to ionizing radiation.

i

Noncontrast CT before Aortography

In the acute setting of a suspected AAS, it is important to initiate the protocol by a noncontrast thoracic computed tomographic scan to rule out IMH. This scan identifies concentrated hemoglobin in recently extravasated blood within the aortic wall that shows a characteristically high computed tomographic density (40–70 Hounsfield units), facilitates the characterization of the hematoma, can identify vascular calcifications, and provides a baseline examination for postcontrast evaluation.

ii

Electrocardiographically Gated CTA

Motion artifacts involving the thoracic aorta are evident in most (92%) standard nongated computed tomographic angiograms. Because of the limited temporal resolution of CT, imaging artifacts arising from the peduncular motion of the heart, the circular distension of the pulsewave, aortic distensibility, and the hemodynamic state may appear as a “double aortic wall” on standard nongated CTA. This finding may also lead to a false-positive diagnosis of a dissection flap and impair accurate measurement of the aortic root and ascending aorta.

Prospective or retrospective synchronization of data acquisition with the electrocardiographic tracing eliminates these artifacts, thereby improving the accuracy of diagnosis and reproducibility of aortic size measurements. Low-dose prospective electrocardiographically gated CT protocols have the advantage of decreased radiation exposure compared with the standard technique.

iii

Thoracoabdominal CT after Aortography

A late thoracoabdominal scan (∼50 msec after bolus injection) improves the detection of visceral malperfusion in the acute setting of aortic dissection, detects slow endograft leaks, distinguishes slow flow from thrombus in the false lumen, and allows alternative abdominal diagnoses in the absence of acute aortic pathology.

iv

Exposure to Ionizing Radiation

Radiation minimization protocols include limiting scan range, prospectively electrocardiographically triggered acquisitions, and using low tube voltage (80–100 kV) for low–body weight patients (<85 kg) without risking loss of diagnostic quality. Application of iterative reconstruction algorithms provide the opportunity for even larger reductions in scan acquisition parameters. Despite progress in radiation reduction, the use of alternative methods such as MRI and echocardiography remains a consideration for serial studies.

v

Measurements

In contrast with other aortic imaging techniques, CTA depicts the aortic wall, thereby permitting measurement of both the inner-inner (luminal) and outer-outer (total) aortic diameters. Imaging artifacts from the highly contrasted lumen frequently impair the visualization of a thin and healthy wall in the ascending aorta.

Multiplanar reconstruction of the axial source data can create aortic images in a plane perpendicular to the aortic lumen direction (double-oblique or true short-axis images of the aorta; see Figures 16 and 17 ). This method corrects shape distortions introduced by aortic tortuosity. In cases of noncircular aortic shape, both major and minor diameters should be measured. The manual procedure of double-oblique images is time consuming and may add observer variability. Automated aortic segmentation software is available at many institutions but, like most automated software, has limitations and requires manual adjustment.

Sagittal multiplanar reformatted (A) and double oblique images (B, C) from a computed tomographic aortogram in a patient with AAS. The anatomic locations of the planes of (B) and (C) are marked on (A) . Note the transition from a type B acute IMH involving the descending thoracic aorta to a type B aortic dissection involving the abdominal aorta.

The methodology of double oblique aortic images. Multiplanar reformatted images (A–F) in different planes obtained from a computed tomographic aortogram (C) corresponds to the axial source image and shows an elliptical descending aorta. The sagittal (A) and coronal (B) correlates show the reference planes of (C) as well as the tortuosity of the aortic segment, which results in a distorted shape. The plane is corrected in both the sagittal (D) and coronal (E) images to achieve perpendicularity to the aortic flow, resulting in a corrected true transversal image of the aortic lumen, which is circular in this case.

The measurement technique must be highly reproducible to correctly assess follow-up studies. Accurate assessment of aortic morphologic changes can be achieved by side-by-side comparison of source axial images from two or more serial computed tomographic aortographic examinations with anatomic landmark synchronization and a slice thickness ≤1 mm. Electrocardiographically gated or triggered imaging is an additional refinement that further reduces variability, with a maximum interobserver variability of ±1.2 mm in the ascending aorta. Measures in the axial plane are valid only for aortic segments with a circular shape and craniocaudal axis, like the midascending and the descending aorta. Distortion in the axial image introduced by aortic tortuosity may be minimized by measuring the lesser diameter. Interobserver variability is always higher than intraobserver variability, suggesting that follow-up of aortic disease in a specific patient should be performed by a single experienced observer.

In summary, CTA is one of the most used techniques in the assessment of aortic diseases. Advantages of CTA over other imaging modalities include the short time required for image acquisition and processing, the ability to obtain a complete 3D data set of the entire aorta, and its availability. Moreover, MDCT permits a correct evaluation of the coronary arteries and aortic branch disease. Its main drawbacks are the radiation exposure and need for contrast administration.

1

Black-Blood Sequences

Black-blood MRI sequences, acquired with spin-echo techniques, and often including inversion recovery pulses, are useful for defining morphology across a spectrum of aortic conditions without the need for intravascular contrast medium. With these sequences, the use of multiple radiofrequency pulses nulls the signal from moving blood, causing the dark blood appearance; mobile protons in stable or slowly moving structures (e.g., aortic wall) provide the signal in the image. Aortic wall morphology can be defined and tissue characterized with T1- and T2-weighted sequences and their variants, including T2-weighted dark-blood techniques and T2 turbo spin-echo and short-tau inversion recovery sequences ( Figure 19 ). Each of these imaging protocols has relative strengths and limitations; for example, T2-weighted MRI is sensitive to areas of increased water content, as is often noted in pathologic conditions, but is limited by relatively low signal-to-noise ratio. MRI of the thoracic aorta can be obtained with high spatial resolution, with in-plane resolution typically in the range of 1.5 × 1.5 mm and submillimeter acquisition achievable with more specialized MRI sequences.

Images from a 54-year-old woman with an elevated sedimentation rate and dilatation of the descending aorta. Wall thickening is well depicted in dark ( black ) blood images ( left, yellow arrow ). Short tau inversion recovery (STIR) images ( right ) demonstrate bright signal in the aortic wall ( yellow arrow ), a result consistent with edema. Surgical repair was performed in this patient, and histology was consistent with GCA.

2

Cine MRI Sequences

Bright-blood imaging with approaches such as steady-state free precession and gradient-echo techniques is useful for obtaining high–temporal resolution cine images of flow in the aorta. In these images, the blood pool is bright compared with the adjacent aortic wall, which is typically intermediate in signal. Cine imaging can demonstrate flow within aortic lumens (true or false), and areas of low signal caused by intravoxel dephasing can be seen with complex flow patterns associated with valvular stenosis or regurgitation ( Figure 20 ).

Images from a 60-year-old man with moderately severe aortic insufficiency. Three-chamber ( left ) and coronal oblique ( right ) cine steady-state free precession (SSFP) images depict dark signal ( yellow arrows ) caused by intravoxel dephasing associated with the posteriorly directed jet of insufficiency.

3

Flow Mapping

Velocity-encoded phase-contrast imaging can be used to quantify aortic flow. The phase-contrast technique is based on the fact that protons undergo a change in phase that is proportional to velocity when they pass through a magnetic field gradient consisting of equal pulses that are of opposite polarity and slightly offset in time. Blood flow can be quantified by integrating these measured velocities within the aortic lumen throughout the cardiac cycle with values that have shown strong agreement with phantom models and other measurement approaches. Phase-contrast imaging of the aorta can be used to assess forward flow and stenotic and regurgitant valves and can aid in assessment of congenital heart disease. Phase-contrast imaging is typically acquired in a single in-plane or through-plane direction, with some applications allowing flow encoding in multiple directions.

4

Contrast-Enhanced MR Angiography (MRA)

Contrast-enhanced MRA can provide a 3D data set of the aorta and branch vessels, allowing complex anatomy and postoperative changes to be depicted through postprocessing techniques such as maximum-intensity projection and multiplanar reformatting ( Figure 21 ). In patients with contraindications to contrast or in cases of difficult intravenous access, a 3D angiogram of the aorta can still be obtained with unenhanced segmented steady-state free precession angiography. When precise dimensions of the aortic root and proximal ascending aorta are needed, electrocardiographically gated techniques can be used. Improved scanning speed allows time-resolved MRA. Although contrast timing for contrast-enhanced MRA can be a challenge, particularly in the concurrent assessment of the aorta and pulmonary arteries or veins, the use of newer blood-pool contrast agents can circumvent the limitations of traditional interstitial gadolinium contrast agents and in conjunction with electrocardiographic and respiratory gating has been shown to increase vessel sharpness and reduce artifacts.

MIP image obtained from MRA in a 60-year-old man with a dilated ascending aorta ( large yellow arrow ). There was suspicion of coarctation of the descending aorta raised by surface echocardiographic imaging; however, MRA revealed a mild kink in the isthmus without significant stenosis ( large red arrow ) and normal-sized intercostal ( small red arrow ) and internal mammary ( small yellow arrow ) arteries, results consistent with pseudocoarctation.

MRI may also be used as a tool to investigate aortic physiology. Quantification of stiffness, an important predictor of cardiovascular outcome, can be obtained with pulsewave measurements from high–temporal resolution cine imaging. MRI can provide insight into the elastic properties of the aorta, quantify the resultant blood flow, and estimate aortic wall shear stress.

5

Artifacts

Similar to echocardiographic imaging, MRI artifacts occur. Consequently, consistently recognizing artifacts can prevent misinterpretation. The reader is referred to two excellent reviews for a detailed discussion of these.

1

Classification of Aortic Dissection

Accurate classification of aortic dissection is important because significant differences in clinical presentation, prognosis, and management depend on the location and extent of the dissection. Figure 26 illustrates the two commonly used classifications: the DeBakey system (types I, II, and III) and the Stanford system (types A and B). Dissections involving the aortic arch without involving the ascending aorta are classified as type B in the Stanford system. The majority of dissections, whether type A or type B, propagate beyond the diaphragm to the iliac arteries.

Diagram illustrating the two commonly used classification systems for aortic dissection. In the older of the two, the DeBakey system, type I dissection originates in the ascending aorta and propagates distally to includes at least the arch and typically the descending aorta. Type II dissection, not shown (the least common type) originates in and is confined to the ascending aorta. Type III dissection originates in the descending thoracic aorta (usually just distal to the left subclavian artery) and propagates distally, usually to below the diaphragm. The Stanford system, in a simpler scheme, divides dissections into two categories: those that involve the ascending aorta, regardless of the site of origin, are classified as type A, and those beginning beyond the arch vessels are classified as type B. The majority of dissections, whether type A or type B, propagate beyond the diaphragm to the iliac arteries.

The appropriate management of aortic dissections depends not only on the location of the dissection but also on the time that has elapsed between onset of the process and the patient’s presentation. Although the adjectives acute , subacute , and chronic are often applied, there is no standard definition for these time periods. There is a 24-hour “hyperacute” period during which dissections involving the ascending aorta carry a risk for rupture approaching 1% per hour. Studies have shown that 75% of aortic dissection–related deaths occur in the initial 2 weeks. At the opposite extreme are “old dissections” encountered incidentally during aortic imaging or surgery. These are clearly “chronic.” Hirst et al ., Levinson et al ., and DeBakey considered an aortic dissection to be “acute” when the onset of symptoms was <2 weeks in duration at the time of diagnosis. The subsequent 2-month period was designated “subacute,” and beyond the second month, an aortic dissection was termed “chronic”. We endorse this classification, as it has some basis in pathologic observations. The extremely high initial death rate declines after 2 weeks. Moreover, friable aortic tissue extends beyond 2 weeks. By 6 to 8 weeks, the outer aortic wall has largely “healed,” in that it has developed scar, a reasonable marker for the beginning of the chronic stage. It must be acknowledged that any time-based division of “acute” from “subacute” and “subacute” from “chronic” is arbitrary. Nevertheless, such distinctions are necessary for analyzing outcomes. Moreover, some imaging features of acute and chronic dissections are different. In chronic dissection, the dissection flap tends to be thicker, more echodense, and relatively immobile (as distinct from the oscillating flaps seen in acute dissection).

2

Echocardiography (TTE and TEE)

The sensitivity of 2D TTE by fundamental imaging was previously reported to be only 70% to 80% for the detection of type A dissection. However, because of new transducers with improved resolution, harmonic imaging, and contrast enhancement, the sensitivity of TTE has improved to approximately 85% on the basis of recent data from Cecconi et al . and Evangelista et al . Therefore, TTE may be of some use as the initial imaging modality, especially in the emergency room ( Figure 27 ). In addition, TTE provides assessment of left ventricular contractility, pericardial effusion, aortic valve function, right ventricular size and function, and pulmonary artery pressure, which may facilitate the diagnosis of chest pain due to myocardial ischemia and/or infarction, pulmonary embolism, or pericardial disease and may identify dissection complications such as aortic regurgitation (AR) in an early fashion. Moreover, the use of contrast agents may further improve the accuracy, as illustrated in Figure 28 . Nevertheless, because of the potential catastrophic nature of type A aortic dissection, negative results on TTE should not be considered definitive, and further imaging should follow.

Acute aortic dissection evaluated by TTE. ( Top left ) Parasternal long-axis view showing a flap in the proximal ascending aorta (Ao) ( arrow ) and a flap in the thoracic descending aorta ( arrow ). ( Top right ) Parasternal short-axis view of the proximal ascending aorta showing the presence of a typical true lumen (TL) and false lumen (FL) divided by a flap. ( Bottom left ) Five-chamber apical view showing the presence of a flap in the proximal ascending aorta ( arrow ). ( Bottom right ) Subcostal view of the descending abdominal aorta with a clear flap inside the aortic lumen. LV , Left ventricle; RV , right ventricle.

Proximal descending thoracic aorta visualized from supraclavicular view. Use of contrast echo illustrates entry tear ( arrow ) by showing contrast emanating from true lumen (TL) to false lumen (FL).

Moreover, TTE is less sensitive for the diagnosis of type B dissection, because the descending thoracic aorta (located farther from the transducer) is imaged less easily and accurately. Therefore, although TTE may be diagnostic in many instances, its role is predominantly that of a screening procedure. TEE, on the other hand, is highly accurate for establishing the diagnosis of both type A and type B acute aortic dissection. Since the landmark multicenter European Cooperative Study, several additional studies have demonstrated the high accuracy of TEE, with sensitivity approaching 100%.

a

Echocardiographic Findings

The diagnostic hallmark of aortic dissection is a mobile dissection flap that separates the true and false lumens ( Figure 29 ). Important features of the dissection flap include oscillation or motion that is independent of the aorta itself, visualization in more than one view, and clear distinction from reverberations from other structures, such as a calcified aortic wall, catheter in the right ventricular outflow tract, pacemaker wire, or pericardial fluid in the transverse oblique sinus.

Transesophageal echocardiographic longitudinal view of the aortic root and ascending aorta (ASC’G AO) illustrating a folded, convoluted dissection flap ( arrow ) that had marked oscillation in real-time. LA , Left atrium.

The true and false lumens can almost always be differentiated. In the descending thoracic aorta, the false lumen is usually larger than the true lumen. The dissection flap typically moves toward the false lumen in systole (systolic expansion of the true lumen) and toward the true lumen in diastole (diastolic expansion of the false lumen), sometimes causing compression of the true lumen. Moreover, the characteristics of blood flow vary in the true and false lumens. In the true lumen, antegrade systolic flow is rapid enough to create brighter shades of red or blue on color Doppler. In contrast, flow in the false lumen is generally slower, producing duller colors. In fact, flow in the false lumen may be absent or in the opposite direction (retrograde) to that of the true lumen. The sluggish flow in the false lumen may result in the presence of spontaneous echo contrast, sometimes referred to as “smoke.” The false lumen may also contain variable degrees of thrombus. Additional findings in patients with aortic dissection include dilatation of the aorta, compression of the left atrium, AR, pericardial and/or pleural effusion, and involvement of the coronary arteries. Table 7 summarizes the main echocardiographic findings in aortic dissection. Three-dimensional TEE may provide information beyond what can be obtained with 2D TEE. For example, the size of the entry tear size and its relationship to surrounding structures may be shown in greater detail, allowing better morphologic and dynamic evaluation of aortic dissection ( Figure 30 ). Such information may be particularly helpful when the flap spirals around the long axis of the aorta. Moreover, 3D TEE demonstrates the dissection flap not as a linear structure but as a sheet of tissue of variable thickness in the long, short, or oblique axis. This may make it possible to distinguish a true dissection flap from an artifact when it is relatively immobile. In addition, multiplane 3D TEE provides a more rapid and accurate evaluation of the aortic arch than 2D TEE.

Table 7

Role of echocardiography in detecting evidence of aortic dissection and echocardiographic definitions of main findings

Diagnostic goals	Definition by echocardiography
Identify presence of a dissection flap	Flap dividing two lumens
Define extension of aortic dissection	Extension of the flap and true/false lumens in the aortic root(ascending/arch/descending abdominal aorta)
Identify true lumen	Systolic expansion, diastolic collapse, systolic jet directed away from the lumen, absence of spontaneous contrast, forward systolic flow)
Identify false lumen	Diastolic diameter increase, spontaneous contrast and or thrombus formation, reverse/delayed or absent flow
Identify presence of false luminal thrombosis	Mass separated from the intimal flap and aortic wall inside the false lumen
Localize entry tear	Disruption of the flap continuity with fluttering or ruptured intimal borders; color Doppler shows flow through the tear
Assess presence, severity and mechanisms of AR	Anatomic definition of the valve (bicuspid, degenerated, normal with/without prolapse of one cusp); dilation of different segments of the aorta; flap invagination into the valve; severity by classic echocardiographic criteria
Assess coronary artery involvement	Flap invaginated into the coronary ostium; flap obstructing the ostium; absence of coronary flow; new regional wall motion abnormalities
Assess side-branch involvement	Flap invaginated into the aortic branches
Detect pericardial and/or pleural effusion	Echo-free space in the pericardium/pleura
Detect signs of cardiac tamponade	Classic echocardiographic and Doppler signs of tamponade

Three-dimensional TEE showing the entry tear of a type B aortic dissection located in the proximal descending aorta. ( Left ) Live 3D image showing a large entry tear ( asterisk ). ( Right ) Maximum orthogonal diameters (D2 and D1) are 17 and 11 mm, and area measured by full volume is 1.5 cm ² .

b

Detection of Complications

AR occurs in approximately 50% of patients with type A aortic dissection. The presence, severity, and mechanism(s) of AR may influence surgical decision making and aid the surgeon in deciding whether to spare, repair, or replace the aortic valve. The mechanisms of AR are listed in Table 8 , and several of these are illustrated in Figure 31 . These mechanisms will be discussed in greater detail in section III.B.6, “Use of TEE to Guide Surgery for Type A Aortic Dissection.”

Table 8

Mechanisms of AR in type A aortic dissection

1. Dilatation of the aortic root leading to incomplete aortic leaflet coaptation 2. Cusp prolapse (asymmetric dissection depressing cusp[s] below annulus) 3. Disruption of aortic annular support resulting in flail leaflet 4. Invagination/prolapse of dissection flap through the aortic valve in diastole 5. Preexisting aortic valve disease (e.g., bicuspid valve)

Mechanisms of AR in the setting of aortic dissection. (A) Transesophageal echocardiogram demonstrating absence of coaptation of aortic leaflets due to dilatation of the aortic root (the most common mechanism of aortic insufficiency associated with type A dissection). Arrow designates the dissection flap. (B) Transesophageal echocardiogram of the aortic root illustrating prolapse of the aortic valve ( small arrow ) due to extension of the dissection to the annulus causing AR (not shown). FL , False lumen; LA , left atrium; TL , true lumen. (C) Transesophageal echocardiogram of the aortic root and ascending aorta (Ao) illustrating a dissection flap ( arrow ) prolapsing through the aortic valve into the left ventricular outflow tract (LVOT), resulting in AR in this patient.

A pericardial effusion in an ascending aortic dissection is an indicator of poor prognosis and suggests rupture of the false lumen in the pericardium. Echocardiography is the best diagnostic technique for estimating the presence and severity of tamponade. Periaortic hematoma and pleural effusion are best diagnosed by CT. The presence of periaortic hematoma has also been related to increased mortality.

TEE is capable of imaging the ostia and proximal segments of the coronary arteries in nearly all patients and may demonstrate coronary involvement due to dissection (flap invagination into the coronary ostium and origin of coronary ostium from the false lumen). Color Doppler is useful for verifying normal or abnormal or absent flow into the proximal coronary arteries. Detection of segmental wall motion abnormalities of the left ventricle by TTE or TEE may also help identify this complication. Color Doppler also reveals reentry sites (often multiple, as in Figure 32 ), which explain why the false lumen often remains patent over time.

Longitudinal view of a transesophageal echocardiogram with color Doppler illustrates multiple reentry sites ( arrows ) demonstrating flow from true lumen (TL) to false lumen (FL). Reentry sites are the major reason the false lumen remains patent over time.

3

CT

Data from the IRAD published in 2000 showed that among 464 patients with acute aortic dissections (62% with type A), nearly two-thirds underwent CTA as the initial diagnostic imaging. The computed tomographic data in this study were acquired on older generation scanners, which may explain the fact that most patients underwent several imaging tests (average, 1.8 tests).

A more recent IRAD publication, now including 894 patients, showed that the “quickest diagnostic times” were achieved when the initial test was CT, whereas the initial use of MRI or catheter-based aortography resulted in significantly longer diagnostic times.

Today, newer generation modern multidetector computed tomographic scanners are ubiquitous even in remote-area hospitals throughout the United States and Europe and are usually staffed and readily available 24 hours a day. In 2007, according to 2011 health data from the Organisation for Economic Co-operation and Development, there existed 34.3 computed tomographic scanners per million population in the United States, and 185 computed tomographic examinations were performed per 1,000 patients in US hospitals.

Computed tomographic angiographic protocols are robust and relatively operator independent. Computed tomographic angiographic protocols that are designed to exclude dissections typically begin with low-dose noncontrast CT to exclude the possibility of IMH, followed by contrast-enhanced computed tomographic angiography. The coverage includes the entire thorax, abdomen, and pelvis to allow delineation of the extent of a flap and its extension into branch vessels and to evaluate for end-organ ischemia (e.g., bowel or kidneys), and possible extravasation. Examples of computed tomographic angiography are illustrated in Figures 33 and 34 .

Axial source images from the computed tomographic aortogram ( left ) and the late-phase computed tomographic study ( right ) performed in a patient with AAS. The additional late acquisition rules out false lumen thrombosis, showing late enhancement and retention of contrast-enhanced blood in the false lumen.

Evolutive changes in a type B chronic aortic dissection. The comparison is performed by synchronizing thin (0.75-mm) axial images of the baseline and follow-up computed tomographic aortograms. The images show an expansion of the false lumen ( asterisk ) with compression of the true lumen, with an overall mild external expansion of the dissected descending thoracic aorta. Note the similarity of mediastinal and posterior thoracic wall anatomic markers.

Diagnostic accuracy is extremely high for the exclusion of aortic dissection (98%–100%). However, false positives for the detection of type A dissection near the aortic arch may infrequently occur with older generation computed tomographic scanners, which may lead to unnecessary operations. Single-slice spiral computed tomographic scanners and early-generation multidetector computed tomographic scanners frequently demonstrate pulsation artifact in the ascending aorta, which occasionally may mimic type A dissection (pseudoflaps). However, aortic pulsation artifact and pseudoflaps can be completely eliminated with the use of electrocardiographically gated computed tomographic angiographic acquisitions. Therefore, it is advisable to use electrocardiographic gating or triggering if ascending aortic pathology is suspected. False-positive results on CT leading to unnecessary surgery for aortic dissection have not been reported to date with the use of newer generation electrocardiographically gated multidetector computed tomographic angiographic scans.

Surgery or transcatheter intervention in type B dissection may be indicated if there is occlusion of major aortic branches leading to end-organ ischemia or expansion of the aortic diameter or interval extension of the dissection flap. MDCT allows imaging of the entire aorta and iliac system within seconds and allows delineating the intimal flap extension into aortic arch vessels and the abdominal aorta and its branches as well as the iliac system, which may determine the feasibility of stent-graft repair. Entry and reentry sites, aortic diameters, and the relationships between true and false lumen can be defined using multiplanar multidetector computed tomographic reformations. MDCT also allows the determination of end-organ perfusion, such as asymmetric or absent enhancement of kidneys in case of renal artery occlusion.

Given the multiplanar reformation capabilities that, unlike MRI, can be applied post hoc, and 3D imaging capabilities, CT has extremely high retest reliability for measurement of aortic diameters on follow-up scans. The multiplanar reconstruction capabilities facilitate endovascular treatment planning and may allow the determination of proximal fenestrations that may be amenable to endovascular repair. Because determination of these features is important, reporting of the extension of dissection and aneurysms into branch vessels and secondary end-organ hypoperfusion are considered “essential elements” of aortic imaging reports. Gated MDCT may determine proximal extent of the flap into coronary artery ostia, or the aortic valve, as well as presence of pericardial effusion or hemopericardium.

Gated MDCT may simultaneously exclude the presence of obstructive coronary artery disease in acute dissections, as well as coronary artery dissection and aortic valve tears. In addition, combination of a gated or triggered thoracic computed tomographic angiographic acquisition with a nongated abdominal and pelvic acquisition is feasible at low radiation doses.

Further dose reduction using axial prospective electrocardiographic triggering (compared with spiral retrospective gating) computed tomographic angiography at a tube potential of 100 kV allows the further reduction of radiation doses without impairment of image quality of the aorta or coronary arteries.

The “triple rule-out” protocol for assessing acute chest pain in the emergency room is rarely needed and is neither technically suitable nor medically necessary on a routine basis. Optimal protocols for coronary CT angiography, for pulmonary embolism, and for aortic dissection differ, and “triple rule-out” CT is not optimal for all three. Given the increased radiation and contrast exposure and the lack of accurate diagnostic data for aortic dissection, there are no grounds to recommend triple rule-out CT for this condition. If there is a reasonable clinical suspicion for aortic dissection, then the highest quality study for this specific indication should be performed.

In summary, CT angiography is readily available throughout the United States and Europe; is most often the first imaging test when acute aortic dissection is suspected; has extremely high diagnostic accuracy; allows the evaluation of the entire aorta and its branches, the coronary arteries, the aortic valve, and the pericardium; and results in the shortest time to diagnosis compared with other imaging modalities, therefore allowing rapid initiation of therapy. Disadvantages of CT include the need for iodinated contrast material and ionizing radiation, although substantial dose reductions have recently been achieved with newer hardware technology and imaging protocols, and this issue may be of less concern in the setting of AAS.

4

MRI of Aortic Dissection

Early identification of aortic dissection and precise characterization of anatomic details are critical for clinical and surgical management of this condition. Imaging of suspected dissection should address not only the presence of a dissection flap and its extent but also the entry and reentry points, presence and severity of aortic insufficiency, and flow into arch and visceral branch vessels. MRI, which can address all of these issues noninvasively, provides high spatial and contrast resolution and functional assessment with an imaging time of 20 to 30 min. Specifically, MRI has very high sensitivity (97%–100%) and specificity (94%–100%) for diagnosing dissection. MRI also provides imaging without the burden of ionizing radiation, an important consideration for patients who undergo serial assessments of a known aortic dissection.

MRI does have potential limitations in this patient population. Although the scan times for MRI are relatively short, they are significantly longer than the scan times for CT angiography. Additionally, physiologic waveforms are challenging to obtain within the MRI scanner environment. Although cardiac rhythm, blood pressure, and oximetry can be monitored with MRI-appropriate equipment, caring for patients within an MRI scanning area can be difficult in emergent or unstable clinical scenarios that may be associated with aortic dissection.

A combination of dark-blood and bright-blood images in axial and oblique planes oriented to the aorta allows the detection and characterization of intimal flaps. True and false lumens can be differentiated by patterns of flow and by anatomic features ( Figures 35 and 36 ). The false lumen can often be identified on spin-echo images by a higher intraluminal signal intensity attributable to slower flow and may be characterized by web-like remnants of dissected media. Cine bright-blood imaging can also be used to directly visualize flow patterns within true and false lumens. Associated anatomic findings outside of the aorta on MRI may also be of interest, such as high signal intensity within pericardial effusion on dark-blood imaging, indicating the possibility of the ascending aorta rupturing into the pericardial space. Phase-contrast imaging can provide flow quantification of aortic insufficiency associated with dissection and can also allow definition of entry and reentry sites and differentiation of slow flow and thrombus in the false lumen. Newer 3D phase-contrast approaches have shown promise in further defining the flow characteristics and associated parameters of aortic dissection, such as wall stress.

MR image extracted from a dynamic cine steady-state free precession (SSFP) sequence in a patient with type B aortic dissection arising just after the origin of the left subclavian artery. The arrow shows the entry tear. FL , false lumen; TL , true lumen.

Images from a 55-year-old woman with chronic type B aortic dissection. The true lumen ( yellow arrow ) is characterized by lack of signal in the dark blood image ( left ), bright signal in the single-shot steady-state free precession (SSFP) image ( middle ), and bright signal (caused by contrast filling) in the MR angiographic image ( right ). False lumen ( red arrow ) is notable for intermediate signal on dark blood and single-shot SSFP sequences, and lack of signal is noted in the thrombosed false lumen on MRA.

Contrast-enhanced 3D MRA provides 3D data, results that allow postprocessing and detailed assessment of aortic and large-branch vessel anatomy in cases of dissection. The dynamics of aortic flow can also be evaluated with time-resolved MRA. Imaging with blood-pool contrast agents allows steady-state phase scanning, which can improve spatial resolution and better demonstrate the amount of thrombus within the false lumen.

5

Imaging Algorithm

Aortic dissection is a life-threatening condition that is associated with high early mortality and therefore requires prompt and accurate diagnosis. Numerous publications have sought to establish the relative merits of CT, TEE, and MRI as first-line imaging modalities. In truth, each diagnostic method has its strengths and weakness, as previously discussed. The optimal choice of imaging modality at a given institution should depend not only on the proven accuracy (all three are highly accurate) but also on the availability of the techniques and on the experience and confidence of the physician performing and interpreting the technique. CT has become the most commonly used first-time imaging modality partly because it is more readily available on a 24-hour basis. TEE may be the preferred imaging modality in the emergency room, if an experienced cardiologist is available, because it provides immediate and sufficient information to determine if emergency surgery will be required. Although CT may be less accurate for determining the degree and mechanism of AR, this can be evaluated by TTE and/or intraoperative TEE. The relative advantages and disadvantages of the various imaging modalities are summarized in Table 9 .

Table 9

Recommendation for choice of imaging modality for aortic dissection

Modality	Recommendation	Advantages	Disadvantages
CT	First-line	• Initial test in >70% of patients ∗ • Widely available, quickest diagnostic times • Very high diagnostic accuracy • Relatively operator independent • Allows evaluation of entire aorta, including arch vessels, mesenteric vessels and renal arteries	• Ionizing radiation exposure • Requires iodinated contrast material • Pulsation artifact in ascending aorta (can be improved with ECG gating)
TEE	First- and second-line	• Very high diagnostic accuracy in thoracic aorta • Widely available, portable, convenient, fast • Excellent for pericardial effusion, and presence, degree and mechanism(s) of AR and LV function • Can detect involvement of coronary arteries • Safely performed on critically ill patients, even those on ventilators • Optimal procedure for guidance in OR	• Operator dependent (depends on skill of operator) • “Blind spot” upper ascending aorta, proximal arch • Not reliable for cerebral vessels, celiac trunk, SMA, etc. • Reverberation artifacts can potentially mimic dissection flap (can be differentiated from flaps in vast majority) • Semi-invasive
TTE	Second-line	• Often initial imaging modality in ER • Provides assessment of LV contractility, pericardial effusion, RV size and function, PA pressure • Presence and severity of AR	• Sensitivity not sufficient distal to aortic root • Descending thoracic aorta imaged less easily and accurately • Misses IMH and PAU
MRI	Third-line	• 3D multiplanar, and high resolution • Very high diagnostic accuracy • Does not require ionizing radiation or iodinated contrast • Appropriate for serial imaging over many years	• Less widely available • Difficult monitoring critically ill patients • Not feasible in emergent or unstable clinical situations • Longer examination time • Caution with use of gadolinium in renal failure
Angiography	Fourth-line	• Rarely necessary	• Often misses IMH (up to 10%–20% of ADs) • Long diagnostic time • Requires ICM • Morbidity • Less sensitivity than CT, TEE, and MRI

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Definitions for a Common Standard for 2D Speckle Tracking Echocardiography: Consensus Document of the EACVI/ASE/Industry Task Force to Standardize Deformation Imaging
2. Echocardiography in Hypertrophic Cardiomyopathy: In with Strain, Out with Straining?
3. Continuing Education and Meeting Calendar
4. Heart Month and the ASE Foundation

Stay updated, free articles. Join our Telegram channel

Join