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.

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.

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.

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.

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].

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.

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].

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.

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

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.

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.

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.

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.

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.