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CHAPTER 10
Multislice Computed Tomography (MSCT) and Cardiovascular Magnetic Resonance (CMR) Imaging for Coronary and Structural Heart Disease
Pragya Ranjan, Richard Ro, and Stamatios Lerakis
Multislice Computed Tomography (MSCT) technology has advanced rapidly since its introduction 20 years ago. Currently, 64‐MSCT and dual‐source computed tomography (DSCT) are considered state‐of‐the‐art for cardiac MSCT imaging with 320‐slice systems also emerging in clinical practice. Initially, the main clinical focus of MSCT in cardiac imaging was coronary artery evaluation. This has expanded to several other areas such as evaluation of coronary stents, bypass grafts, and cardiac evaluation prior to procedures such as transcatheter aortic/mitral valve replacement and pulmonary vein isolation for atrial fibrillation ablation.
Coronary MSCT angiography—technique
In MSCT scanners, X‐rays are generated by a tube mounted on a rotating gantry with the patient centered within the bore of the gantry. There are two principal modes of scanning – sequential/axial scanning and spiral/helical scanning. In sequential scanning (prospective ECG triggering), X rays are generated while the table is stationary in a predetermined imaging window during diastole (since cardiac motion is minimized during diastole). In spiral scanning, the table moves continuously at a fixed speed relative to the gantry rotation. After data acquisition, an optimal reconstruction window is chosen within the cardiac cycle to minimize motion artifact (retrospective ECG gating).
Sequential scanning helps reduce the dose of radiation, whereas helical scanning has the advantage of allowing for cine evaluation of ventricular function and having a greater ability to correct for artifact during arrhythmias [1]. Both have similar temporal and spatial resolution [2]. DSCT, consisting of two X ray sources mounted at an angle of 90 degrees, has led to a significant improvement in temporal resolution [3]. The introduction of DSCT has omitted the need to lower patients’ heart rates.
Stenosis detection
Invasive coronary angiography (ICA) is the gold standard for detection of coronary stenosis (Figure 10.1a). Cardiac computed tomography angiography (CCTA) was developed as a non‐invasive, alternative test to ICA for the same purpose. Numerous studies demonstrated high accuracy of 16‐MSCT and 64‐MSCT compared with ICA [1][4–20]. Three large, multicenter prospective studies demonstrated a sensitivity of 95%, 99%, and 85% respectively of CCTA compared with ICA. The first two included patients without known CAD whereas the last study included patients with and without CAD and with calcium scores of less than 600 [1] [21–23]. While sensitivity and negative predictive values (NPV) have consistently been found to be high with CCTA, positive predictive values have been lower because the modality tends to overestimate disease severity. Therefore, CCTA has been considered an appropriate test to exclude disease in low to intermediate risk patients only [24].
Bifurcations and ostial lesions
Bifurcation and ostial lesions are notoriously difficult to evaluate on ICA due to projectional foreshortening and vessel overlap. CCTA has shown promise in the evaluation of side branches and ostial lesions. Van Mieghem et al. [25] found that CCTA had high sensitivity, specificity and NPV for the detection and classification of bifurcation lesions when compared to ICA. CTA‐guided PCI led to the increased use of single stent procedures with less frequent 2‐stent overlap and less stenting of the side branch in another study [26]. CTA was even shown to outperform the current Medina classification and RESOLVE angiographic scoring system in predicting intraprocedural side branch occlusion [27].
Stents
Coronary stents are challenging to evaluate on CTA because of partial voluming artifact that causes apparent enlargement of the stent and obscures the lumen (Figure 10.1c). This is exaggerated with smaller stents and overlapping or bifurcation stents.
To evaluate for in‐stent restenosis (ISR), the in‐stent lumen must be directly visualized (Figure 10.1b). Contrast enhancement distal to the stent could be secondary to retrograde filling by collaterals. Improvement in CTA technology has led to an improvement in evaluation of ISR. A recent meta‐analysis evaluated 35 studies with data from 2656 patients that used CTA to diagnose ISR. Not only did CTA have a high sensitivity and specificity for detection of ISR but also by demonstrating a high positive and low negative Likelihood Ratio, the authors concluded that CTA was an excellent test to rule in and rule out ISR, and serve as a good screening test. CTA was more sensitive for stents with a diameter of >3mm, more accurate for stents with thickness <100microns and less accurate at higher heart rates [28].
CTA has been shown to be reliable in excluding ISR in stents implanted in the left main (LM) and proximal left anterior descending/circumflex (LAD/Cx), as these stents have