Magnetic Resonance Imaging and Computed Tomography: Diagnostic Imaging, Image Management, and Assessment of Physiology and Function

Spin Echo ( Fig. 21.1 )

Also known as black-blood images, spin echo can be used to assess vascular anatomy; however, because these are 2D images that often require long breath holds to acquire, they are less frequently used in day-to-day practice. They are useful for diagnosing subtle rings, flaps, and ledges within vessels or outflow tracts and for assessing changes within the vascular walls. The blood pool appears black, while the heart and the vessel wall appear bright. The acquisition is electrocardiogram-triggered and limited to a single phase of the cardiac cycle and therefore the resulting images are static. At normal heart rate the acquisition is usually set in mid-diastole, whereas at higher heart rate can be set in end-systole to minimize cardiac motion artifacts and improve image quality. The use of breath holds or respiratory navigation minimizes respiratory motion. These are among the first sequences used in CMR for CHD. Compared with other sequences, they are useful in the presence of blood flow acceleration (e.g., valve stenosis or vessel obstruction) and in the presence of metallic devices such as stents or sternal wires because they are less affected by metal artifacts.

Gradient Echo and Balanced Steady State Free Precession

In gradient echo (GRE) imaging, the blood pool appears brighter than the myocardium. The balanced steady-state free precession (bSSFP), a type of GRE, is the most commonly used sequence in CMR and can be used for cine functional and 3D anatomic imaging.

Cine images are acquired as multiple slices ideally during multiple breath holds at end expiration to minimize variations in the diaphragm position between the different slices. Retrospective ECG gating is used to allow image acquisition throughout the duration of the cardiac cycle, and the resulting images are therefore in motion. Common planes of acquisition are described in Fig. 21.2 and include vertical long axis, horizontal long axis, four-chamber, short axis, and left ventricular and right ventricular outflow tracts, although cine images can potentially be acquired in any plane. Slice thickness can be adjusted according to the structure being investigated and the patient's size. In small children the recommended slice thickness for cine bSSFP images is 4 to 6 mm, whereas for larger children and adults the slice thickness could be set up to 8 mm. The presence of incessant arrhythmias could reduce the image quality.

Cine images provide a good contrast between the blood pool and the endocardium, allowing accurate segmentation of the cardiac chambers, and therefore are used for volumes and EF quantification ( Video 21.1 ). Moreover, the high spatial and temporal resolution allows evaluation of myocardial thickening in systole and regional wall motion abnormalities. In patients with CHD, when quantification of RV volumes and function is a crucial aspect of decision making and clinical management, cine images represent a very powerful tool compared with echocardiography, which is not ideal for the quantification of the right cardiac chambers. The need for multiple breath holds may not be tolerated in children. Alternatively, it is possible to use real-time cine images that are performed during free breathing (see section on “ Single Shot and Real-Time Acquisitions ”). Thanks to the rapid acquisition of the data during real-time cine, the images are less affected by irregular heart rate and cardiorespiratory motion. Compared with standard breath-hold cine images, real-time cine has a lower spatial and temporal resolution, which may lead to inaccuracy in the volume measurement. To overcome this issue, new sequences have been developed with the intent to combine a rapid image acquisition with a high spatial and temporal resolution as the k-space and time sensitivity encoding imaging, which represents a useful alternative in patients for whom rapid image acquisition is desirable.

The bSSFP sequence can also be used to acquire a 3D image of the heart and great vessel that is cardiac (ECG-gated) and respiratory gated (navigator, diaphragm gated), without the need of contrast administration ( Videos 21.2 and 21.3 ). It is usually acquired at end-diastole, to minimize cardiac motion, but this could be changed from patient to patient to minimize artifacts; for instance, in the presence of high heart rates, acquisition in end-systole may provide better motion compensation. It is particularly useful in patients with complex CHD because it allows analysis and reconstruction of the images in any plane, similarly to CT. The most common application of the technique is the study of the coronary artery anomalies ( Fig. 21.3 ). The main disadvantage of this sequence is the long acquisition time that can be a problem in noncompliant children.

Velocity-Encoded Phase-Contrast Images ( Video 21.4 )

Another form of GRE imaging, velocity-encoded phase-contrast MR (PC-MR) imaging, allows for noninvasive assessment of flow in the vessels. It is the best in vivo method to quantify flow and is essential to acquire physiologic information, such as SVs, cardiac output (CO), regurgitant volumes, and shunt quantification with a good accuracy compared with invasive assessment. Flow can be measured in arteries, systemic and pulmonary veins, the aortic and pulmonary valves, and atrioventricular valves. Several technical aspects need accurate planning to avoid errors and unreliable results. The slice needs to be acquired perpendicular to the flow direction, and careful planning of the slice position is crucial to obtain reliable data. It is also important to accurately plan the maximal velocity acquired (Venc), to avoid aliasing.

Four-dimensional flow is a new 3D sequence where the three directions of flow are measured over time ( Video 21.5 ). It enables advance postprocessing; in particular, it allows the evaluation of flows in every direction needed, without need of accurate location planning before the acquisition. It is not routinely used in clinical practice due to the long acquisition time and the need for complex postprocessing. All the flow in a selected volume of the body is acquired, and a plane can be position in any direction to visualize and quantify the flow through it. Postprocessing analysis also allows for visualization of vectors, which are a visual representation of the velocity and direction of the flow, and path lines and streamlines, which are imaginary lines that connect all the vectors through the vessel, to visually assess the flow pattern, the vortices, and the displacement of the flow ( Fig. 21.4 ). Moreover, it is used to calculate the wall shear stress, which represents the force of the flow viscosity on the endothelium of the vessel. Current research applications of 4D flow mostly study bicuspid aortic valve, aortic disease, and aortic valve interventions, but more recently the technique has been used to assess intracardiac flow in acquired and CHDs.

Contrast-Enhanced Magnetic Resonance Angiography ( Fig. 21.5 )

Contrast-enhanced MR angiography is acquired after gadolinium-based contrast administration, which increases the contrast between the blood pool and the extravascular structures. This sequence is useful to study the thoracic vasculature. It is crucial to decide carefully the timing of acquisition of the images after injection because arteries or veins will appear with greater contrast at different times. In clinical practice, a series of data sets are acquired consecutively to depict the entire thoracic vasculature in the venous and arterial phases. When reviewing and analyzing the images, it is important to remember that the reconstruction represents a mean shape throughout the cardiac cycle because the acquisition is not ECG gated. Therefore it is not possible to measure the vessel in systole or diastole. Moreover, being non-ECG gated, the images will be affected by cardiac motion and therefore intracardiac anatomy cannot be adequately investigated.

Stress Perfusion Imaging ( Video 21.6 )

Stress perfusion MRI has limited application in children. However, it can be used to assess inducible myocardial ischemia in conditions involving the coronary arteries, such as Kawasaki disease, or when the coronary arteries have been manipulated (e.g., post arterial switch operation) and ischemia is suspected. It can be performed using either a vasodilator agent (adenosine/regadenoson) or, more rarely, an inotropic agent (dobutamine). Perfusion images are acquired at peak stress during infusion of gadolinium-based contrast media. The perfusion defects are visualized as hypointense dark regions of the myocardium that correspond to the ischemic territories. Compared with nuclear imaging techniques, adenosine stress CMR has the advantage of being radiation free and much quicker, allowing the acquisition of both stress and resting perfusion in a single examination. Careful evaluation of the patients, including medical history, medications, and ECG, is necessary prior to administration of adenosine, to screen for contraindications (asthma, allergy, conduction defects).

Tissue Characterization Sequences

Tissue characterization sequences are increasingly used in pediatric CMR. It is important to know that CMR can offer not only anatomic and functional information but also reveal insights on the myocardial tissue characteristics noninvasively. By choosing the appropriate sequences, myocardial edema, scarring, fibrosis, iron, and lipid overload can be investigated. These sequences are particularly useful also in the assessment of cardiac masses and cardiomyopathies.