INTRODUCTION
Diagnostic evaluation of cardiac diseases has become very advanced with the advent of cardiac computed tomography (CT). CT is now an indispensable component in the armamentarium of currently available imaging techniques. At present, CT scan can be used as a primary or complementary investigative modality for a variety of cardiac diseases, including ischemic heart disease, congenital heart disorders, aortic and pulmonary artery pathologies, and for pulmonary venous mapping. CT now contributes to screening, diagnosis, treatment optimization, and follow-up of patient with diverse cardiovascular diseases.
BASIC CONCEPTS OF CARDIAC COMPUTED TOMOGRAPHY WITH TECHNICAL DEVELOPMENTS
Technically, a basic CT scanner measures the attenuation of multiple X-ray beams passing through different sequential sections of the body from a multitude of different angles. Using these values, pictures of the internal organs are reconstructed. Classically, CT requires at least 180° of gantry rotation to perform image formation. Initial generation CT scanners required 1–5 seconds (s) to acquire a single slice. Rotation speed of up to 0.3–0.4 s per rotation provided by current generation CT scanners allows for increased temporal resolution.1 Though, adequate, for most organ systems, it is still not optimal for cardiac CT as heart exhibits a fast complex three-dimensional (3D) motion, which varies from patient to patient and also within the cardiac cycle. For images in cardiac CT to be of diagnostic utility, cardiac CT has to provide images of the heart that are motion free and that correspond to a specific phase of the cardiac cycle. This goal is achieved by synchronizing data acquisition with the cardiac cycle. This synchronization is done using the concurrent acquisition of the electrocardiography (ECG) signal and overcomes the problem of cardiac motion making quantitative data calculation, such as ejection fractions and volumetric assessments possible. In the quest for further reducing scan times and improving temporal resolution, a new concept that simultaneously utilizes two X-ray sources—dual source CT (DSCT)—was introduced. These scanners require only a 90° rotation to acquire sufficient data and hence exhibit a high temporal resolution in the order of 50–60 ms.2 Moreover, current CT systems are capable of providing a 3D isotropic acquisition, thereby improving spatial resolution for accurate depiction of smaller cardiac structures, including coronaries. The newer third-generation DSCT scanners provide excellent image quality over a wide range of body sizes and heart rates at lower radiation doses than the previous generation CT scanners. Faster gantry rotation time combined with larger Z-axis volume coverage per rotation combine to enable single-heart beat acquisitions in most patients with heart rates of up to 75–80 beats/min.2 Generally, cardiac CT requires intravenous injection of iodinated contrast agent, with the exception of CT calcium scoring which achieves CT acquisition without contrast. Low contrast protocols described in various studies result in significant reduction in contrast medium requirements, without any difficulties in contrast medium timing or unsatisfactory vessel enhancement.34
Recent advancements in multislice CT technology have made coronary computed tomography angiography (CTA) a very useful modality in imaging the coronaries with submillisievert doses using very low contrast load, which is of paramount importance in patients with reduced renal functions (Flowchart 1).4
Types of Cardiac Computed Tomography (Flowchart 2)
Depending on the area of interest, cardiac CT can be broadly divided into ECG-gated and non-ECG-gated CTs.
Electrocardiography-gated CTs are primarily used for coronary artery assessment. However, they can also be useful in valvular assessment, congenital heart disease (CHD) evaluation, and functional cardiac assessment. Non-ECG-gated CTs are useful in morphological assessment of the pulmonary arteries and veins, pericardial assessment, and anatomical assessment of CHD and valves where functional indices are not required.
Gated Cardiac Computed Tomographies
The least amount of cardiac movement is during the end-systole and end-diastole of a particular cardiac cycle. This phase, however, varies with increasing heart rate which narrows the acquisition window. Therefore, motion-free imaging is usually best acquired in these phases. With higher heart rates, however, this dictum no longer applies, and it is advisable to acquire data in all the phases of the cardiac cycle for better visualization of cardiac and coronary morphology. Two gating techniques are utilized: (1) retrospective gating and (2) prospective gating.
Retrospective gating: This is achieved by gating the acquisition with the cardiac cycle with acquisition throughout all the phases of cardiac cycle. Data from several consecutive cardiac cycles is continuously acquired with simultaneous ECG recording, leading to generation of true 3D data set, consisting of overlapping axial imaging slices. Any cardiac phase can, therefore, be reconstructed, but this occurs at the expense of a relatively high radiation dose.5 Since data from all the cardiac phases are available, ventricular functions (ejection fraction) and volumetric assessment can be performed. This scan is less sensitive to 5the changes of heart rate during the scan. The images can also be retrospectively analyzed and missed or extra beats can be excluded for analysis during ECG correction.
Prospective gating: This is a sequential CT scan which uses step and shoot technique to overcome the motion of the heart enabling data acquisition in only a certain phase of the cardiac cycle. If only morphological assessment is required, such as only coronary artery imaging, prospectively triggered sequences can be used. Using these sequences, preselection of a fraction of the RR interval width is done, and this allows imaging during a predefined cardiac phase (usually mid-diastole, when there is least cardiac motion—at approximately 75% of the cardiac cycle). With this scan, estimation of the next RR interval is of paramount importance and hence can be performed only in patients with regular heart rates. This type of scan is associated with lower radiation exposure to the patient as compared to retrospectively gated scans but does not allow ventricular functional analysis.6
Acquisition and Interpretation of Cardiac Computed Tomography (Flowchart 2)
For cardiac CT acquisition, injection of contrast is done via an automatic injector which injects the contrast intravenously at a high flow rate (3–5 mL/s in adults and 1–3 mL/s in children/infants). The scan is usually programmed to begin automatically once the contrast reaches its peak value in the vessel of interest (for coronaries and aorta—vessel of interest is the proximal descending thoracic aorta; for pulmonary arteries—vessel of interest is the main pulmonary artery; for CHD evaluation—scan is triggered manually when all the chambers of the heart are optimally opacified). After the data is acquired, the images are reviewed on the CT workstation using a variety of postprocessing techniques (described in Figures 1 to 5) to reach a diagnosis (Table 1).7,8
Uses of Gated Cardiac Computed Tomography (Box 1)
Coronary Artery Calcium Scoring
It involves prospectively gated noncontrast cardiac CT acquisition for the assessment of coronary artery calcium (CAC). It identifies and characterizes the location as well as burden of coronary artery disease (CAD) by detecting calcium within plaques. Calcium being defined as areas with greater than 130 HU, these coronary calcifications are usually automatically detected and color-coded. Semi-quantification of coronary calcium is expressed by the Agatston score, which uses the plaque size, density, and a weighting factor to calculate a value ranging from 0 (absence of detectable calcium) to over 1,000 (indicating extensive coronary calcification).
Fig. 1: Axial section showing thickened calcific pericardium causing constriction of bilateral ventricles. Also noted is mild bilateral pleural effusion (arrows).
Fig. 2: 3D MPR showing calcified thickened tricuspid aortic valve in three different planes. (3D: three-dimensional; MPR: multiplanar reconstruction)
An elevated CAC score has consistently been associated with increased mortality and adverse cardiovascular events, independent of other risk factors. CAC score has been found to be superior to well-established cardiovascular risk factor models for predicting silent myocardial ischemia and short-term outcomes.9 However, the association between CAC score and the extent of coronary atherosclerosis is not linear as severe coronary stenosis can occur at sites with limited calcium deposition and vice versa. Thus, CAC scoring is better used as a risk stratification tool to identify high-risk asymptomatic patients who might benefit from further investigations rather than being a diagnostic test for obstructive CAD.6
Fig. 3: MIP showing anomalous origin of RCA with interarterial course between aorta and pulmonary artery (arrow). (MIP: maximum intensity projection; RCA: right coronary artery)
Fig. 4: Curved MPR showing the entire extent of the coronary arterial tree in one image (arrow). (MPR: multiplanar reconstruction)
Fig. 5: VRT image of a child with truncus arteriosus showing a common arterial trunk arising from the left ventricle. (VRT: volume-rendered reconstruction)
|
Unlike the CAC scan, this scan requires intravenous injection of iodinated contrast. CTCA has a high sensitivity and specificity for diagnosing coronary stenosis. CTCA allows identification of coronary atherosclerotic plaques in the absence of calcium or stenosis, thus permitting visualization of early subclinical atherosclerosis. Studies have demonstrated excellent diagnostic accuracy of CTCA for the detection of coronary stenosis, with a sensitivity of 786–99%, specificity of 92–98%, and negative predictive value of 92–100%.10 The high negative predictive value which approaches 100% is the main advantage of this technique. It can provide detailed information on coronary artery anatomy and assess both coronary artery atherosclerosis as well as luminal stenosis. In view of its high negative predictive value and high sensitivity, CTCA is the most beneficial in symptomatic patients with low to intermediate pretest probability of obstructive CAD.11 Moreover, CTCA can be useful for plaque characterization as studies have shown that plaques with spotty calcification, positive remodeling, or those with a lipid-rich core have a higher (95%) risk of rupture.12 It is also extremely beneficial in identifying the anomalous origin, course, and termination of the coronary arteries (Fig. 4) and is the investigation of choice for the same. CTCA also allows for noninvasive evaluation of hemodynamics in the coronary arterial tree. Using computational fluid dynamics, “three-vessel” fractional flow reserve (CT-FFR) can be calculated from already acquired coronary CTA data with no need for additional imaging or vasodilators. CT-FFR, therefore, can help in assessing the exact area of blockage and its impact on the flow which may guide treatment strategies.13
Coronary Computed Tomography Angiography for Graft and Stents
Computed tomography coronary angiography is the recommended investigation of choice for graft patency in postcoronary artery bypass grafting (CABG) patients with acute chest pain. It is also used for the follow-up of asymptomatic patients in whom CABG has been performed more than 5 years ago. Studies have shown a high sensitivity and specificity each ranging more than 95% with positive predictive value of more than 92–94% and negative predictive value to the tune of almost 99%, in the evaluation of stenosis/occlusion of grafts.14 Patency of coronary stents too can be evaluated with CTCA. However, CTCA is most useful if the diameter of the stent is more than 3 mm. CT has shown high diagnostic accuracy (sensitivity—91%, specificity—91%, positive predictive value—68%, and negative predictive value—98%) for in stent restenosis of stents greater than 3 mm diameter. Thicker stent material and strut thickness may also affect the accuracy of CTCA adversely.15
Assessment of Ventricular Volumes, Wall Thickness, and Mass
Magnetic resonance imaging (MRI) is the investigation of choice for functional assessment. But CT may be an equally accurate investigation with less inter- and intraobserver variability. This information is exclusively available with retrospectively gated acquisitions. Not only is the cardiac and coronary morphology assessed but cardiac functional assessment can also be performed with a single contrast-enhanced retrospective ECG-gated examination. Ejection fraction, end-systolic and end-diastolic volumes, stroke volumes, and cardiac output can be calculated.16 Left ventricular regional function can also be displayed on color-coded maps of wall motion and wall thickness for better assessment of wall-motion abnormalities.
Role of Computed Tomography in Congenital Heart Disease (see Fig. 5)
Though echocardiography (ECHO) remains the investigation of choice in these patients, cardiac CT may be used as a problem-solving tool, especially where only morphological information is required. Moreover, delineation of extracardiac structures, including pulmonary arteries, aortopulmonary collaterals, and aortic arch anatomy and associated anomalies, may be better evaluated with CT.17 Simultaneous assessment of the airway and skeletal abnormalities may influence management options. Techniques such as ECG-gated ultrahigh pitch [fast low-angle shot (FLASH) sequences], ECG padding/pulsing (giving full radiation dose only for a part of the cardiac cycle), using prospective ECG gating, help in radiation dose reduction in this population. Moreover, postoperative evaluation of complications is also better done by CT since it facilitates global and rapid simultaneous assessment of the chest and pleural spaces, airways, lung parenchyma, and vasculature.18
Evaluation of Valves
Earlier used primarily to identify valvular calcifications, CT can now depict subtle valvular anatomy and also evaluate valve function. It plays a role in both preoperative evaluation and postsurgical assessment. Another advantage of CT is that the coronary arteries can also be simultaneously evaluated in the same study.
For Morphological and Functional Valvular Assessment
Computed tomography offers excellent morphological assessment of valve leaflets, apposition zones, chordae commissures, and papillary annulus muscles during all the phases of the cardiac cycle.19 In mitral/aortic stenosis, CT helps in anatomical localization of the stenosis (valvular or subvalvular), presence of calcification (see Fig. 2), and the ancillary information, such as degree of left atrium (LA) dilation, left ventricle (LV) function, and coronary artery status, which can help in deciding management. CT plays a very important role in preoperative assessment for transcatheter aortic valve implantation (TAVI) (percutaneous aortic valve implantation). It can evaluate the aortic root dimensions (for accurate sizing purposes), coronary ostia takeoff as well as LV [specifically left ventricular outflow tract (LVOT)] assessment.8
Fig. 6: Endoluminal view showing prosthetic heart valves with tissue discontinuity at the mitral annulus suggesting dehiscence (arrow).
CT can also be useful in morphological assessment of the tricuspid valve morphology, especially in the cases, such as Ebstein's anomaly where the displacement of the septal leaflet and redundancy of the anterior leaflet can be depicted in detail. Congenital pulmonary valve abnormalities, such as valvular stenosis or agenesis, can be assessed on CT. Excellent visualization of valve prostheses with minimal streak artifacts can be obtained by CT. Even bioprosthetic valves can be evaluated to assess for leaflet degeneration and dysfunction. Postoperative complications, such as pseudoaneurysms, abscess, paravalvular leaks, and valve dehiscence, can be identified and assessed by CT, providing both anatomic and dynamic information important for redo surgery (Fig. 6).
For Assessment of Valvular Endocarditis
Cardiac CT can demonstrate valvular vegetations, with a sensitivity and specificity of 97% and 88%, far superior to ECHO. On CT, vegetations appear as low-attenuation masses attached to the leaflets.20 CT is especially helpful in assessing peri- and paravalvular complications, such as abscess, pseudoaneurysm formation, or remote complications, such as septic emboli to the lungs.
Evaluation of Intracardiac Masses
Cardiac CT is often used concomitantly with ECHO and MRI in the evaluation of cardiac masses.21 Cardiac MRI is the investigation of choice for cardiac masses due to its superior soft-tissue characterization and high temporal resolution. Cardiac CT can be used in patients who cannot undergo an MRI or as an alternative modality. In addition, CT is superior to MRI for the assessment of calcification of the masses. Apart from characterization, CT enables complete evaluation of the chest and lung parenchyma, vascular anatomy, and also allows exclusion of obstructive CAD as a preoperative requisite in older patients.
Pericardial Assessment
Computed tomography enables imaging of the entire pericardium with high spatial and temporal resolution and is an important modality in the diagnostic workup of pericardial diseases (see Fig. 1). Moreover, it is the gold standard to visualize calcified pericardium. Although MRI is best suited for pericardial assessment, CT can help in the initial characterization of pericardial effusion (by using HUs to differentiate between transudative and effusive exudate), identifying inflammatory pericarditis (by visualization of enhancing pericardium), assessing constriction (using functional CT), and in characterization of a pericardial mass (enhancement, presence of calcium/fat).22
Uses of Nongated Cardiac Computed Tomography
Assessment of Pulmonary Veins: Pulmonary Venous Mapping
Preprocedural CT is helpful for morphological assessment of the pulmonary veins and LA, especially to assess the presence of additional pulmonary veins (e.g. middle lobe vein). This information is vital to the electrophysiologist who is attempting to perform a percutaneous ablation of an arrhythmic focus. Volume- and surface-rendered reconstructions allow for a quick overview of the pulmonary anatomy, thus providing the electrophysiologist with a roadmap prior to the intervention. CT is also useful in identifying postprocedural complications commonly associated with percutaneous ablation, such as pulmonary vein stenosis. For this purpose, however, ECG-gated study is performed for better visualization of the stenosis.23
Assessment of Pulmonary Arteries
For evaluation of patients with pathologies of the pulmonary arteries, CT is routinely used.24,25 Noncontrast study is performed when the assessment of lung parenchyma is the objective. Contrast-enhanced CT may be performed (often with a pulmonary angiography protocol) when chronic thromboembolic pulmonary hypertension (CTEPH) or other parenchymal causes of pulmonary hypertension is suspected. Based on the assessment of pulmonary arteries, heart, parenchyma, and bronchial arteries, the proper etiology of pulmonary hypertension may be identified which helps in planning treatment.9
CONCLUSION
Cardiac CT has assumed a wider role in the diagnostic workup of major cardiac pathologies. This has become possible due to continuously evolving technical refinements, such as recent third-generation multislice CT with dual source-dual energy acquisitions, cardiac gating, advanced postprocessing options, and radiation dose reduction strategies. It has now become possible to obtain reproducible functional information in addition to the morphological assessment of cardiac pathologies, thereby influencing the treatment strategies. It is the investigation of choice in emergency settings and in critically ill patients.
REFERENCES
- Schroeder S, Achenbach S, Bengel F, et al. Cardiac computed tomography: indications, applications, limitations, and training requirements: report of a Writing Group deployed by the Working Group Nuclear Cardiology and Cardiac CT of the European Society of Cardiology and the European Council of Nuclear Cardiology. Eur Heart J. 2008;29(4):531–56.
- Scheffel H, Alkadhi H, Plass A, et al. Accuracy of dual-source CT coronary angiography: first experience in a high pre-test probability population without heart rate control. Eur Radiol. 2006;16(12):2739–47.
- Min JK, Lin FY. What makes a coronary CT angiogram nondiagnostic? J Cardiovasc Comput Tomogr. 2008;2(6):351–9.
- Buechel RR, Husmann L, Herzog BA, et al. Low-dose computed tomography coronary angiography with prospective electrocardiogram triggering: feasibility in a large population. J Am Coll Cardiol. 2011;57(3):332–6.
- Einstein AJ, Moser KW, Thompson RC, et al. Radiation dose to patients from cardiac diagnostic imaging. Circulation. 2007;116(11):1290–305.
- Alkadhi H, Leschka S. Radiation dose of cardiac computed tomography—what has been achieved and what needs to be done. Eur Radiol. 2011;21(3):505–9.
- Lell MM, Anders K, Uder M, et al. New techniques in CT angiography. Radiographics. 2006;26(Suppl 1):S45–62.
- Anders K, Ropers U, Kuettner A, et al. Individually adapted, interactive multiplanar reformations vs. semi-automated coronary segmentation and curved planar reformations for stenosis detection in coronary computed tomography angiography. Eur J Radiol. 2011;80(1):89–95.
- Detrano R, Guerci AD, Carr JJ, et al. Coronary calcium as a predictor of coronary events in four racial or ethnic groups. N Engl J Med. 2008;358(13):1336–45.
- Bax JJ, Schuijf JD. Which role for multislice computed tomography in clinical cardiology? Am Heart J. 2005;149(6): 960–1.
- Paech DC, Weston AR. A systematic review of the clinical effectiveness of 64-slice or higher computed tomography angiography as an alternative to invasive coronary angiography in the investigation of suspected coronary artery disease. BMC Cardiovasc Disord. 2011;11:32.
- Taylor AJ, Cerqueira M, Hodgson JM, et al. ACCF/SCCT/ACR/AHA/ASE/ASNC/NASCI/SCAI/SCMR 2010 appropriate use criteria for cardiac computed tomography. A report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force, the Society of Cardiovascular Computed Tomography, the American College of Radiology, the American Heart Association, the American Society of Echocardiography, the American Society of Nuclear Cardiology, the North American Society for Cardiovascular Imaging, the Society for Cardiovascular Angiography and Interventions, and the Society for Cardiovascular Magnetic Resonance. J Am Coll Cardiol. 2010;56(22):1864–94.
- Norgaard BL, Leipsic J, Gaur S, et al. Diagnostic performance of noninvasive fractional flow reserve derived from coronary computed tomography angiography in suspected coronary artery disease: the NXT trial (Analysis of Coronary Blood Flow Using CT Angiography: Next Steps). J Am Coll Cardiol. 2014;63(12):1145–55.
- Hamon M, Champ-Rigot L, Morello R, et al. Diagnostic accuracy of in-stent coronary restenosis detection with multislice spiral computed tomography: a meta-analysis. Eur Radiol. 2008;18(2):217–25.
- Smith SC, Jr., Feldman TE, Hirshfeld JW, Jr., et al. ACC/AHA/SCAI 2005 guideline update for percutaneous coronary intervention: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (ACC/AHA/SCAI Writing Committee to Update the 2001 Guidelines for Percutaneous Coronary Intervention). J Am Coll Cardiol. 2006;47(1):e1–121.
- Rizvi A, Deano RC, Bachman DP, et al. Analysis of ventricular function by CT. J Cardiovasc Comput Tomogr. 2015;9(1):1–12.
- Dillman JR, Hernandez RJ. Role of CT in the evaluation of congenital cardiovascular disease in children. AJR Am J Roentgenol. 2009;192(5):1219–31.
- Fidler JL, Cheatham JP, Fletcher SE, et al. CT angiography of complications in pediatric patients treated with intravascular stents. AJR Am J Roentgenol. 2000;174(2):355–9.
- Chen JJ, Manning MA, Frazier AA, et al. CT angiography of the cardiac valves: normal, diseased, and postoperative appearances. Radiographics. 2009;29(5):1393–412.
- Koo HJ, Yang DH, Kang JW, et al. Demonstration of infective endocarditis by cardiac CT and transoesophageal echocardiography: comparison with intraoperative findings. Eur Heart J Cardiovasc Imaging. 2018;19(2):199–207.
- Kassop D, Donovan MS, Cheezum MK, et al. Cardiac masses on cardiac CT: a review. Curr Cardiovasc Imaging Rep. 2014;7:9281.
- O'Leary SM, Williams PL, Williams MP, et al. Imaging the pericardium: appearances on ECG-gated 64-detector row cardiac computed tomography. Br J Radiol. 2010;83(987):194–205.
- Manghat NE, Mathias HC, Kakani N, et al. Pulmonary venous evaluation using electrocardiogram-gated 64-detector row cardiac CT. Br J Radiol. 2012;85(1015):965–71.
- Dogan H, de Roos A, Geleijins J, et al. The role of computed tomography in the diagnosis of acute and chronic pulmonary embolism. Diagn Interv Radiol. 2015;21(4):307–16.
- Halpern EJ. Clinical applications of cardiac CT angiography. Insights Imaging. 2010;1(4):205–22.