Progress in biomedical engineering technology has resulted in major advances in noninvasive cardiac imaging, with noninvasive techniques becoming a real alternative to invasive modalities for diagnosing cardiac conditions. Easy availability and wide use of technology has led to reduced costs, thus making these new techniques more accessible and cost effective.
Advances in cardiac imaging
Advances in cardiac imaging can be categorized as either an extension of traditional cardiac imaging modalities or an introduction of new imaging techniques that are still emerging for cardiac conditions.
In the former we include echocardiography, nuclear medicine techniques, magnetic resonance imaging and cardiac computed tomography (CT). The latter includes emerging techniques, such as hybrid PET/CT imaging for combined functional and anatomic information.
Echocardiography
Velocity vector imaging
Velocity vector imaging (VVI) is a recent ultrasound-based development that allows for greater precision in estimating regional myocardial function. VVI permits measurment of myocardial velocity and deformation in the apical and short-axis views of the left ventricle walls. It uses a tracking algorithm to estimate myocardial velocity at a set of points on a contour in a sequence of 2D echocardiographic images. The velocity is displayed over the 2D image in real time. The direction of the vector indicates the direction in which the tissue is moving and the length of the vector indicates the magnitude of the tissue velocity. This technology holds great promise to assess regional dyssynchrony for patients with myocardial dysfunction, who are being assessed for cardiac resynchronization therapy.
Strain rate imaging
Assessment of regional wall motion by 2D echo alone is confounded by translational motion of the myocardium. Strain rate imaging (SRI) is another recent development that can aid in improving assessment of regional myocardial function. This technique was originally used in an MRI setting and can now be performed using cardiac ultrasound. Strain is the myocardial deformation produced when stress is applied and strain rate is the rate at which strain is produced. SRI uses tissue velocity data to measure velocity gradients between two distinct points along the ultrasound beam function. Strain is measured in longitudinal, horizontal and circumferential axes. The disadvantage of SRI is that it is 1D in nature and measurements are angle dependent.
3D echo
Progress in bioengineering technology has now made real-time transthoracic 3D echocardiography feasible for day-to-day use. 3D echo is now becoming established as a better way to assess valve stenosis and cardiac masses. Its use in assessing left ventricular function still requires more data.
Nuclear medicine
PET imaging
There are newer applications of PET imaging emerging in the field of the assessment of myocardial blood flow, myocardial ischemia assessment, and plaque inflammation imaging and quantification.
Traditionally, PET imaging has been restricted to evaluation of cellular viability and myocardial perfusion physiology. Extension of this field using 3D PET, shows promise in the evaluation of microvascular function by assessing myocardial perfusion. In combination with high-resolution CT, noise-free attenuation correction is obtained. The integrated PET/CT platform is technically challenging but offers the promise of combining physiologic and anatomic assessment simultaneously. Furthermore, PET now appears to be superior to single photon emission computed tomography (SPECT) imaging for the assessment of myocardial ischemia secondary to obstructive coronary artery disease in comparative studies. PET imaging appears to be particularly valuable in obese subjects where imaging with SPECT may be challenging. In another area, early reports of PET imaging of inflammation in atherosclerotic plaques offer a new field for future applications of this technology.
Magnetic resonance imaging
Faster acquisition times and advances in processing technology have enabled MRI to expand its frontiers in the field of cardiac imaging. Newer applications include infarct imaging, stress myocardial perfusion imaging and coronary angiography. While MRI has always been at the forefront of the assessment of cardiac anatomy, recent reports suggest that MRI may be the gold standard to assess subendocardial infarction and sizing transmural infarcts. Late enhancement of the myocardium seen with gadolinium clearly delineates the extent of subendocardial infarction and, using algorithms, quantification of infarct size can be done accurately. Using pharmacologic stressors, stress myocardial perfusion imaging is proving to be highly accurate in diagnosing ischemia due to obstructive coronary artery disease. Comparative studies with stress echocardiography and stress nuclear imaging techniques demonstrate that MRI is a viable alternative for diagnosing obstructive coronary artery disease. While the versatility and portability of MRI remains an issue compared with echo and nuclear stress imaging, cost no longer appears to be a major impediment in routine use of MRI.
Another promising area for MRI in terms of coronary disease appears to be direct visualization of the three major epicardial coronary arteries. Single 3D acquisition of images for MR coronary angiography appears to be feasible for native vessels as well as coronary grafts. However, current limitations include long acquisition times and lack of complete visualization of anatomy due to tortuosity of vessels in many cases. Currently, it does not have enough accuracy to contend for replacing percutaneous coronary angiography. The high-spatial resolution by state-of-the-art 9 Tesla MRI scanners show great promise for plaque imaging – especially in carotid arteries where plaque accessibility is easier owing to relatively simple vessel anatomy.
Cardiac CT
Cardiac CT has been used for the assessment of congenital anomalies, cardiac masses and aortic disorders for a number of years. More recently, use of CT has been extended to the assessment of coronary artery disease. This has been done indirectly by using CT to diagnose the presence of coronary artery calcification (which requires minimal radiation dose) and more recently, using contrast dye, CT angiography is being performed to visualize the coronary artery anatomy. Coronary calcification appears to have good negative predictive value for the presence of coronary disease and, therefore, may be particularly appealing for use in the low-risk population where prevalence of disease is low. Negative coronary calcium or low coronary calcium score is also associated with good long-term prognosis.
With newer multislice and higher resolutions scanners, CT angiography yields high-quality anatomic definition of native coronary arteries and bypass grafts. The recent introduction of a 64-slice CT scanner offers high-spatial and temporal resolution that allow for slices less than 1 mm thick and temporal resolution of 165 ms. Image acquisition is achieved during one breath hold or less than 12 s, thus greatly eliminating motion artifacts. Stenosis location and estimation can be done after 3D reconstruction of acquired images, with reasonable accuracy compared to percutaneous coronary angiography. It again appears to have most impact in patient populations where there is a low prevalence of coronary disease. The advantage of CT angiography over conventional methods is obviously the lack of risks associated with invasive procedures. However, the risk of renal damage and dye allergy is not eliminated due to use of contrast.
Myocardial enhancement using contrast with CT imaging appears to be similar to hyperenhancement on MRI for detecting infarcted muscle. Studies in animals and humans confirm the feasibility of CT for infarct imaging. Other potential applications of CT are simultaneous vessel wall imaging and plaque imaging, both of which are currently research applications of this technique.
PET/CT hybrid imaging
Assessment of coronary disease frequently requires demonstration of inducible ischemia to precisely quantify stenosis significance in addition to anatomic delineation. Standard techniques for this are stress imaging studies using echocardiography or thallium/technetium-based myocardial perfusion imaging. Recently, preliminary data show that stress imaging with PET may have higher accuracy than echocardiography for detecting ischemia. This may be partly owing to the higher spatial resolution offered by PET scans. Recent reports show the feasibility of combining PET perfusion imaging with CT angiography in a single platform to deliver functional and anatomic information in one setting. Early reports suggest that the accuracy of this hybrid platform is 87% (with CT used for attenuation correction). It may be particularly useful to use this technique in situations where echo or SPECT imaging may be problematic – such as in obese patients or women (owing to breast artifacts and post-thoracotomy subjects). Further data on accuracy and cost effectiveness will further define the role of this emerging technology.
Conclusion
The field of noninvasive cardiac imaging is evolving exponentially and applications of existing technologies is breaking new ground. While this has opened great new opportunities, it has also created new challenges. The challenges are in the field of controlling costs, demonstrating effectiveness of new techniques, regulating appropriateness of use, defining quality standards and last but not the least establishing adequate training standards to ensure that trainees ‘certified’ to perform them actually have the skills.
One should note that the zeal to do a new test should be tempered by the lack of multiple, independent, confirmatory studies with these and long-tem data that only exist for established techniques. What we know and what we think we know can be very different.
Financial & competing interests disclosure
The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.