Imaging: Echo, CT and MRI



Fig. 6.1a
PLAX zoom view at peak/mid-systole to measure the aortic annulus and LVOT



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Fig. 6.1b
PLAX zoom view at end diastole to measure the SOV and STJ


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Fig. 6.1c
High PLAX view at end diastole to measure the proximal and mid-ascending aorta


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Fig. 6.2a
Simultaneous biplane (X-plane) orthogonal images from the 3D matrix transducer confirming that the measured PLAX aortic annulus dimension is through the correct anteroposterior (AP) plane from the right coronary cusp (RCC) to the fibrous trigone between the left coronary cusp (LCC) and non-coronary cusp (NCC)


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Fig. 6.2b
Incorrect aortic annulus measurement taken from an eccentric oblique plane


However, for the measurements of the aortic root and the rest of the aorta, the recommendation was to use back the leading-edge-to-leading-edge (L-L) convention at end diastole in a plane perpendicular to the long axis of the aorta as the currently available long-standing reference values for the aorta (Table 6.1) were based on the L-L convention [24].


Table 6.1
Dimensions of the aorta in normal adult population











































Aortic regions

Absolute values (cm)

Indexed values (cm/m2)

Men

Women

Men

Women

Aortic annulus

2.6 ± 0.3

2.3 ± 0.2

1.3 ± 0.1

1.3 ± 0.1

Sinus of Valsalva

3.4 ± 0.3

3.0 ± 0.3

1.7 ± 0.2

1.8 ± 0.2

Sinotubular junction

2.9 ± 0.3

2.6 ± 0.3

1.5 ± 0.2

1.5 ± 0.2

Proximal ascending aorta

3.0 ± 0.4

2.7 ± 0.4

1.5 ± 0.2

1.6 ± 0.3


This table is adapted from Roman et al. [2] and Hiratzka et al. [4]

Aortic root dilatation at the level of the Sinus of Valsalva (SOV) is defined as greater than 95 % confidence interval of the distribution in a large reference population. This varies according to different age range (from children to adults above the age of 40) and their BSA; the commonly used reference range is published by Roman, et al. [2].

In addition, direct two-dimensional (2D) measurements of the aorta are now preferable to M-mode measurements because of possible inaccuracies arising from cardiac motion during acquisition of the M-mode images.



Left Parasternal Short-Axis View (PSAX View)


Aortic Annulus

Simultaneous biplane (X-plane) orthogonal images from the 3D matrix transducer using an initial PSAX view can be helpful for confirmation that the measured PLAX aortic annulus dimension is through the correct anteroposterior (AP) plane from the right coronary cusp (RCC) to the fibrous trigone between the left coronary cusp (LCC) and non-coronary cusp (NCC) as depicted in Fig. 6.2a. Errors can arise from incorrect measurements taken from an eccentric oblique plane as shown in Fig. 6.2b. In some patients, the aortic root is more elliptical in structure, and the AP plane dimension is often smaller than the medial–lateral dimension.


Aortic Root

Not uncommonly the dilatation at the level of SOV may be eccentric involving one particular cusp more than others. The SAX view allows the direct measurement of the SOV from one cusp to another. Data from other imaging modalities such as CMR has shown that measuring the diameter of the SOV using the sinus to sinus method (from the middle of one cusp to the middle of another cusp) resulted in a mean 2–3 mm larger diameter than the sinus to commissure method (from the middle of one cusp to the commissure between the two cusps) [5]. It is important to report where the largest aortic SOV dimension was obtained in the serial follow-up of patients with dilated aortic root for meaningful comparisons to previous and for future echo follow-up studies.


Other Echo Views

The aortic root and the proximal ascending aorta may also be visualized in other echo windows such as the apical long axis (apical three chamber) as shown in Fig. 6.3a and apical five chamber (Fig. 6.3b). The mid- to distal part of the ascending aorta and the aortic arch can be visualized in the modified suprasternal view (transducer tilted slightly to the right), and the distal ascending aorta, the aortic arch, and the descending thoracic aorta are best visualized in the suprasternal window as shown in Fig. 6.3c and 6.3d, respectively. The abdominal aorta (Fig. 6.3e) is best seen from the subcostal view to the left of the inferior vena cava in sagittal (superior–inferior) alignment.

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Fig. 6.3
(ae) View of different aortic segments from the various echo windows such as the apical three-chamber view (a), apical five-chamber view (b), modified suprasternal view (c), suprasternal view (d), and subcostal view (e)




6.2.2 Transesophageal Echocardiography (TEE) and Three-Dimensional (3D) Echocardiography in Aortopathy


The most important TEE views are the mid-esophageal TEE long-axis view (between 120 and 150°) and the short-axis views (30–60°) to visualize the aortic valve, aortic root, and proximal ascending aorta as shown in Figs. 6.4a and 6.4b, respectively. With minimal manipulation and rotation of the transducer array, a continuum of transverse and longitudinal image planes of the descending thoracic aorta can be obtained as shown in Fig. 6.4c (simultaneous transverse and longitudinal plane of the descending thoracic aorta using X-plane). The TEE probe can then be advanced or withdrawn to image the lower thoracic/upper abdominal aorta or the upper thoracic aorta, respectively. To image the aortic arches, the TEE probe is rotated posteriorly and withdrawn slowly from the mid-esophagus at 0°. The right brachiocephalic and the left common carotid branches are difficult to see, but the left subclavian artery takeoff can usually be seen.

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Fig. 6.4a
High TEE long-axis view at 120° showing the aortic valve, the aortic root, and the ascending aorta


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Fig. 6.4b
High TEE short-axis view at 35° showing the en face view of the aortic valve


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Fig. 6.4c
Deep transgastric view at 99° showing the aortic valve, aortic root, and proximal Asc Ao in a patient with post bioprosthetic aortic valve replacement for severe aortic stenosis

It is important to be wary of occasional reverberation artifacts in the ascending thoracic aorta presenting as linear horizontal lines due to reverberation from the motion of the posterior wall of the ascending aorta as shown in Fig. 6.4d. Extra precaution should also be taken when measuring the descending thoracic aortic diameter (transverse plane) in patients with tortuous aortas as the transverse cut is often in an oblique plane with resultant overestimation of the aortic diameter.

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Fig. 6.4d
Transverse and longitudinal plane of the descending thoracic aorta

TEE has added advantage over TTE in imaging the aorta because of its superior image quality (especially in adult patients with poor windows) and the ability to image the aortic valve, aortic root, and nearly all the ascending and descending thoracic aorta except for a blind area (area between the distal ascending aorta and the proximal aortic arch due to interposition of the trachea and right bronchus).

A summary of the different echo modalities and windows for optimal visualizations of the various aortic segments are as shown in Table 6.2.


Table 6.2
Different echo modalities and windows for optimal visualizations of the various aortic segments






























































































Echo modes and views

Segments of the aorta adequately visualized in adults

Aortic valve

Aortic root

Ascending aorta

Aortic arch

Desc thoracic aorta

Abdominal aorta

(A) TTE
           

 (i) PLAX and SAX

X

X

X
     

 (ii) A3C and A5C

X

X

X
     

 (iii) Subcostal

X

X

X
   
X

 (iv) Suprasternal
   
X

X

X
 

(B) TEE
           

 (i) Upper esophagus
   
X (mid)

X

X
 

 (ii) Mid-esophagus

X

X

X (prox)
 
X
 

 (iii) Deep transgastric

X

X

X

X (prox)
 
X


Key: Desc descending, prox proximal, TTE transthoracic echocardiogram, PLAX parasternal long axis, SAX short axis, A3C apical three chamber, A5C apical five chamber

Real-time three-dimensional imaging of the aortic root with cropping along the long axis may help in understanding the true shape, size, and dilatation of the LVOT, aortic annulus, SOV, and Sino-tubular junction (STJ).


6.2.3 Pulse Wave, Continuous Wave, and Tissue Doppler Echocardiography in Aortopathy


Pulse wave (PW) Doppler allows measurement of blood velocity at a single point and may be used (in the absence of aliasing) to confirm and differentiate multiple areas of discrete stenosis such as subaortic, valvular, or supra-aortic stenosis. It is also helpful in differentiating the severity of aortic regurgitation by looking for the presence of pan-diastolic flow reversal in the thoracic or abdominal aorta. Continuous wave (CW) Doppler is used in the assessment of peak pressure gradient across aortic coarctation.

TDI of the upper ascending aortic wall in early diastole for evaluation of aortic elastic properties is possible but to date remains more in the research arena and not widely used clinically [6].


6.2.4 Computed Tomography (CT)


Multi-detector row CT (MDCT) is one of the most used techniques in the assessment of aortopathies. Its major advantages are excellent spatial and temporal resolution, widespread availability, and ability to image the entire aorta within seconds. Furthermore, it elegantly shows the aortic lumen and wall, resulting in precise and reproducible measurements. Current scanners with higher detector rows allow acquisition of isotropic volumetric datasets, which can be reconstructed in any plane for optimal display and measurements of a structure.


6.2.4.1 CT Angiography


CT Angiography (CTA) acquisition uses iodinated contrast medium (ICM) delivered at rate of 3–5 mL/s by a power injector and usually followed by a saline bolus. The injection site (right vs left, arm vs leg vein) needs consideration, particularly when assessing arch pathologies or Fontan pathways. Optimal contrast enhancement in area of interest is ascertained by “bolus tracking” or “test injection” methods.


6.2.4.2 ECG Gating


On conventional non-ECG-gated CT, outline of the ascending aorta is indistinct due to cardiac motion, resulting in inaccurate measurements and appearance of “pseudo-dissection.” ECG “gating” eliminates these artifacts and also allows proximal coronary arterial assessment in the same setting. ECG gating can be “retrospective” or “prospective,” of which the latter results in significantly lower radiation doses [7].


6.2.4.3 Challenges and Comparison with Other Modalities


Main drawbacks of CT are the need for ICM administration and ionizing radiation exposure. ICM may cause allergic reactions and need cautious use in patients with renal impairment. Ionizing radiation exposure in CT may limit its use in younger people, especially when serial follow-up is needed. Various newer dose reduction techniques are used in modern CT scanners to significantly reduce effective dose, some of which include prospective ECG triggering, ECG-based tube current modulation, lower peak kilovoltage (kVp), and iterative reconstruction algorithms [8, 9]. Compared to other modalities such as TTE and MRI, CT lacks flow assessment capabilities which are useful in assessment of aortic insufficiency and shunts.


6.2.4.4 Measurements


There is still some inconsistency in methods used in different modalities in measuring the aorta, like leading edge to leading edge used in TEE [1] vs external diameters in CT/MRI [4] and sinus to sinus vs sinus to commissure at SOV level. It is thus important to note the method used and follow it on subsequent studies for comparison. Standardized levels for measurements of aortic diameters are shown in Fig. 6.5, adapted from Hiratzka et al. 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 [1, 10, 11].

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Fig. 6.5
Anatomic landmarks for standardized reporting of diameters of the aorta (Adapted from Hiratzka et al.)


6.2.5 Magnetic Resonance Imaging (MRI)


MRI is well suited for the diagnosis of aortic diseases, given its ability to delineate the intrinsic contrast between blood flow and vessel wall. Key indicators for decision-making can be reliably obtained through MRI. Spin-echo black-blood sequences outline the shape and diameter and show intimal flaps nicely [12]. Gradient echo sequences show changes in the diameters during the cardiac cycle and blood flow turbulences. Contrast-enhanced MRI can show the aorta and its branches rapidly, without the need for ECG gating. Gadolinium-enhanced sequences are used to differentiate slow flow from thrombus in the false lumen.


6.2.5.1 Quantitative Analysis


Multilevel measurements of aortic diameters through MRI are obtained on double oblique multi-planar images perpendicular to blood flow at standardized levels [4]. See Fig. 6.5, adapted from Hiratzka, et al. These measurements should be obtained at diastole if possible. The diameters of sinuses or sinotubular junction may not be measured on un-gated images since motion artifacts can lead to blurring and may result in the under- or overestimation of the diameters. These require ECG-gated acquisitions, either from contiguous stack of cines aligned to transect the axis of the aortic root, or three-dimensional balanced steady state-free precession (b-SSFP) images acquired in late diastole. Three-dimensional b-SSFP gives sharp-edge profiling and is easy to acquire and post process. It also shows good interobserver correlation [13]. Volume-rendered techniques may be used for demonstration purposes, but not for detailed analysis.


6.2.5.2 Analysis of the Aortic Wall


Assessment of the aortic wall thickness and irregularities is best achieved by reviewing the turbo spin-echo images.


6.2.5.3 Advantage over Computed Tomography


Cardiac MRI avoids the use of radiation in young patients, who may require repeated scans over their lifetime. Its main advantage is its ability to allow tissue characterization and the analysis of flow and dynamic movement of cardiovascular structures.


6.2.5.4 Challenges


The usual technical difficulties faced when performing cardiac MRI examinations are further amplified in young adults with congenital heart disease. Optimal image quality may be compromised because of the smaller size of structures and the reduced time for image acquisition due to inability or difficulty when breath-holding for patients who have intellectual disability.

Due to the variety of morphology presented to us, it is advised that a trained cardiologist or radiologist be present during the scan so that the appropriate planning of the sequences and imaging planes can be done during the visit.

Potential of gadolinium nephrotoxicity appears to be lower than for CT contrast agents and should be avoided with a glomerular filtration rate of less than 30 ml/min/1.73 m2.


6.2.5.5 Spatial Resolution


Smaller fields of views (FOVs) and the use of thinner slice are required to image small anatomical structures. The resultant reduction in signal-to-noise ratio can be compensated by increasing the number of acquisitions, removing parallel imaging features, or using a coarser matrix.


6.2.5.6 Strategies to Reduce Motion Artifact


This can be overcome by using the following techniques: manual shimming techniques, increasing the number of acquisitions, respiratory compensation methods, or acquiring data using real-time imaging sequences.


6.2.5.7 Consideration of Prior Implants


The presence of implants and devices has been evaluated extensively in scanners with static magnetic fields of 1.5 tesla or less. In addition to the challenges posed due to distortion of the image around the implant, deleterious effects such as heating, dislodgement, and acoustic damage must also be considered. These effects can be further amplified if scanning is done in a 3-tesla environment. Further information specific to the safety of implants can be obtained through “The LIST” found at www.​MRIsafety.​com.



6.3 Imaging of the Aorta in Specific Congenital Heart Disease



6.3.1 Marfan Syndrome



6.3.1.1 Echo


TTE is the main imaging modality for the diagnosis and follow-up of patients with Marfan syndrome where the dilatation of the aorta occurs mainly in the aortic root (Fig. 6.6a, b) at the level of the SOV with relative sparing of the STJ and proximal ascending aorta. An aortic root z-score (at the level of the SOV) greater than 2 is one of the major Ghent criteria needed for diagnosis [14].
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Aug 30, 2017 | Posted by in CARDIOLOGY | Comments Off on Imaging: Echo, CT and MRI
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