CT and MRI provide impetus in heart imaging revolution

October 2, 2008

Coronary heart disease is one of the leading causes of morbidity and mortality in developed countries.1 Accurate detection of early cardiac disease is of utmost importance for the delivery of appropriate treatment.

 

Coronary heart disease is one of the leading causes of morbidity and mortality in developed countries.1 Accurate detection of early cardiac disease is of utmost importance for the delivery of appropriate treatment.

The top three modalities used by cardiologists to diagnose cardiac disease are echocardiography, SPECT, and cardiac catheterization.2 The use of cardiac CT and cardiac MRI, however, is expected to increase significantly over the next three years and alter this ranking.2 Both modalities have progressed from being novel research techniques to routine diagnostic tools. They promise to become valuable additions to the radiological armamentarium for the diagnosis of cardiac diseases.

Several publications have demonstrated the high degree of diagnostic accuracy that both CT and MRI can deliver and the possibility of an accurate risk assessment with these modalities.3-6 Cardiac imaging has never had such a prominent, expansive body of evidence indicating the role of radiology in patient management.7 The ability of cardiac CT to evaluate the coronary arteries places it in direct competition with cardiac catheterization. Because cardiac CT is noninvasive, morbidity associated with the procedure is substantially lower than that for catheterization. It is also cheaper and can visualize both the lumen and the arterial wall, improving identification of early plaque formation.

A large number of publications have shown that the diagnostic performance of cardiac CT compares well with that of catheterization.8-12 The optimal use of cardiac CT is still a matter of some debate, however. The American College of Cardiology published a consensus document in 2006 defining the appropriate uses of both cardiac CT and cardiac MRI.13 In the meantime, two major innovations in CT technology-64-slice and dual-source scanning-have been introduced. The rapid pace of innovation is outstripping the rate at which strategies for clinical application can be decided.

The ACC consensus statement lists the following indications as being appropriate for cardiac CT:

  • diagnosis of coronary heart disease in a symptomatic patient with a medium pretest probability of coronary artery disease who has an inconclusive ECG or who is not able to undergo a stress examination;

  • evaluation of coronary arteries with new-onset cardiac insufficiency; and

  • evaluation of coronary artery anomalies (Figure 1).13

In practice, a large number of cardiac CT referrals are for evaluation of coronary artery bypass grafts, exclusion of coronary artery disease in patients with a medium pretest probability of the disease, evaluation of coronary arteries prior to noncoronary cardiac or noncardiac surgery, evaluation of cardiac anatomy prior to minimally invasive cardiac valve or coronary surgery, and resolution of questions raised by other imaging modalities.

The most important indication for cardiac CT is the exclusion of coronary artery stenosis in patients suffering from atypical chest pain who have a low to intermediate pretest probability of the disease and an inconclusive stress test (Figure 2). This indication has also been recommended by the European Society of Cardiology14 and the American Heart Association.15 Calcium deposits within the coronary artery wall that are measured by unenhanced low-dose cardiac CT (i.e., calcium scoring) can serve as an independent and powerful prognostic parameter for the subsequent development of major cardiac events.6

MRI offers better contrast resolution than CT and has better temporal and spatial resolution. It cannot, however, provide a detailed assessment of the coronary artery lumen with resolution equivalent to CT.

Developments in high-field technology and the advent of fast imaging sequences have resulted in cardiac MRI emerging as the new reference modality for assessments of myocardial function, perfusion and viability imaging in ischemic and nonischemic cardiomyopathies, assessment of cardiac tumors (Figure 3), and evaluation of complex congenital heart disease prior to and after surgery.16 Whole-heart acquisitions allow the entire heart to be scanned in a manner similar to cardiac CT, producing 3D data sets that can be reformatted in any desired orientation.16

A NONINVASIVE FUTURE?

Additional technical developments should increase the clinical value of cardiac CT and MRI still further. CT systems with greater detector coverage will permit dynamic volume imaging of the entire heart without table movement. This could enable perfusion imaging and allow practitioners to monitor contrast filling and the direction of flow in coronary vessels. Novel cardiac MRI techniques combining high temporal and spatial resolution may make it possible to produce accurate coronary artery assessments.

Concerns over radiation dose will continue to be addressed. Most initiatives are aimed at reducing dose to a level that is as low as reasonably achievable (the ALARA principle). Implementation of step-and-shoot cardiac CT is associated with an effective radiation dose of just 1 to 3 mSv.17 Another strategy is to lower the tube voltage to 100 kV in patients with a normal body weight or body mass index.18

Cardiac CT and MRI already provide more information about atherosclerotic plaque than does catheter angiography. Dual-source CT examinations carried out with two beams of x-rays that have distinctly different energies can be used to improve material differentiation. It is possible that this technique may replace methods of CT plaque characterization that rely on Hounsfield units. MRI contrast agents being developed to identify vulnerable plaques work by targeting mediators of neovascularization or macrophages in inflamed plaques.19,20

Combining noninvasive imaging modalities for assessments of cardiac morphology and function will give new insights into the mechanisms of cardiovascular disease. This approach promises to have a significant impact on clinical decision making.

Neither CT nor MRI will ever replace any of the existing cardiac imaging modalities. Echocardiography can be performed as a bedside examination, which is a considerable advantage for acutely ill patients. Cardiac catheterization offers the option of performing coronary interventions on the basis of imaging results. Cardiac CT and MRI are both purely diagnostic techniques and offer no such therapeutic advantage. But cardiac CT and MRI have become an integral part of the diagnostic workup. They will be used more in the future as an alternative to traditional modalities, eventually becoming the methods of choice for a wider range of indications.

 

References

1. Kalender WA. Considerations for multi-slice spiral CT. In: Kalender WA, Computed tomography: fundamentals, system technology, image quality, applications. Munich: Publicis MCD Verlag, 2000:76-78.
2. International Commission on Radiological Protection. 1990 Recommendations of the ICRP: ICRP Publication 60. Ann ICRP 1991;21(1-3).
3. Morin RL, Gerber TC, McCollough CH. Radiation dose in computed tomography of the heart. Circulation 2003;107(6):917-922.
4. Schoepf UJ, Becker CR, Ohnesorge BM, Yucel EK, et al. CT of coronary artery disease. Radiology 2004;232(1):18-37. 5. Ohnesorge B, Flohr T, Becker C, et al. Cardiac imaging by means of electrocardiographically gated multisection spiral CT: initial experience. Radiology 2000;217(2):564-571.
6. Wintersperger BJ, Nikolaou K. Basics of cardiac MDCT: techniques and contrast application. Europ Radiol 2005;15(Suppl 2):B2-B9.
7. Flohr TG, Stierstorfef K, Ulzheimer S, et al. Image reconstruction and image quality evaluation for a 64-slice CT scanner with z-flying focal spot. Med Phys 2005;32(8):2536-2547.
8. Mollet NR, Cademartiri F, van Mieghem CA, et al. High-resolution spiral computed tomography coronary angiography in patients referred for diagnostic conventional coronary angiography. Circulation 2005;112(15):2318-2323.
9. Raff GL, Gallagher MJ, O'Neill WW, et al. Diagnostic accuracy of noninvasive coronary angiography using 64-slice spiral computed tomography. J Am College Cardiol 2005;46(3):552-557.
10. Hunold P, Vogt FM, Schmermund A, et al. Radiation exposure during cardiac CT: effective doses at multi-detector row CT and electron-beam CT. Radiology 2003;226(1):145-152.
11. Bae KT, Hong C, Whiting BR. Radiation dose in multidetector row computer tomography cardiac imaging. J Mag Reson Imag 2004;19(6):859-863.
12. Coles DR, Smail MA, Negus IS et al. Comparison of radiation doses from multislice computed tomography coronary angiography and conventional diagnostic angiography. J Am College Cardiol 2006;47(9):1840-1845.
13. Jakobs TF, Wintersperger BJ, Herzog P, et al. Ultra-low-dose coronary artery calcium screening using multislice CT with retrospective ECG gating. Europ Radiol 2003;13(8):1923-1930.
14. Hohl C, Mühlenbruch G, Wildberger JE, et al. Estimation of radiation exposure in low-dose multislice computed tomography of the heart and comparison with a calculation program. Europ Radiol 2006;16(8):1841-1846.
15. Kalra MK, Maher MM, Toth TL, et al. Techniques and applications of automatic tube current modulation for CT Radiology 2005;233(3):649-657.
16. Francone M, Napoli A, Carbone I, et al. Noninvasive imaging of the coronary arteries using a 64-row multidetector CT scanner: initial clinical experience and radiation dose concerns. Radiol Med (Torino) 2007;112(1):31-46.
17. Francone M, Castro E Di, Napoli A, et al. Dose reduction and image quality assessment in 64-detector row computed tomography of the coronary arteries using an automatic exposure control system. J Comput Assist Tomogr. In press. 18. Jakobs TF, Becker CR, Ohnesorge B, et al. Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation. Europ Radiol 2002;12(5):1081-1086. 19. Flohr TG, McCollough CH, Bruder H, et al. First performance evaluation of a dual-source CT (DSCT) system. Europ Radiol 2006;16(2):256-268.
20. Johnson TR, Nikolaou K, Wintersperger BJ, et al. Dual-source CT cardiac imaging: initial experience. Europ Radiol 2006; 16(7):1409-1415.
21. McCollough CH, Primak AN, Saba O, et al. Dose performance of a 64-channel dual-source CT scanner. Radiology 2007; 243(3):775-784.
22. Stolzmann P, Scheffel H, Schertler T, et al. Radiation dose estimates in dual-source computed tomography coronary angiography. Europ Radiol 2008;18(3)592-599.
23. Sablayrolles JL, Treutenaere JM, Feignoux J, et al. Cardiac CT exam at 5 mSv average without compromising the image quality with the SnapShot Pulse mode on LightSpeed VCT XT. Presented at annual meeting of European Society of Cardiac Radiology, Rome; October 2007:2714.

 

Dr. Alkadhi is an associate professor and section head of body CT, Dr. Leschka is a research fellow, and Dr. Marincek is director and head of medical radiology, all at the Institute of Diagnostic Radiology, University Hospital Zurich in Switzerland.