CARDIAC RADIOLOGY: SYMPOSIUM
|Year : 2007 | Volume
| Issue : 2 | Page : 98-108
|MRI in Ischemic heart disease: From coronaries to myocardium
Alessio La Manna, Nilesh Sutaria, Sanjay K Prasad
Cardiovascular Magnetic Resonance Unit, Royal Brompton Hospital, London, United Kingdom
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| Abstract|| |
With an escalating health burden from coronary artery disease (CAD), both as a public health and fiscal issue, evaluation of patients with (CAD) remains an important challenge. With advances in therapy our management options have increased, coinciding with a shift away from treatment alone, to disease prevention and accurate risk stratification. Imaging techniques play an important role in guiding management. Recent advances in cardiovascular magnetic resonance (CMR) enable a detailed non-invasive assessment of patients with a combination of anatomical and functional data to assess the impact of CAD in individual patients. The range of sequences means that myocardial perfusion, viability, coronary angiography and plaque characterization are available in a single study. Outcome data have helped with disease prognostication. In this review, we discuss current and future applications of CMR and consider which patients would benefit from a scan.
Keywords: Coronary viability, MRI
|How to cite this article:|
La Manna A, Sutaria N, Prasad SK. MRI in Ischemic heart disease: From coronaries to myocardium. Indian J Radiol Imaging 2007;17:98-108
|How to cite this URL:|
La Manna A, Sutaria N, Prasad SK. MRI in Ischemic heart disease: From coronaries to myocardium. Indian J Radiol Imaging [serial online] 2007 [cited 2020 Aug 15];17:98-108. Available from: http://www.ijri.org/text.asp?2007/17/2/98/33619
| Introduction|| |
Coronary artery disease (CAD) is the leading cause of mortality in Western Countries. Its' prevalence in India is increasing, especially in the urban areas.  Several non-invasive techniques are available and are used routinely for the assessment of patients with known or suspected CAD. They present a viable alternative to the large number of patients still referred for conventional X-ray coronary angiography and found to have no significant coronary stenosis.  Over the last two decades, cardiovascular magnetic resonance (CMR) imaging has progressively become a very useful tool in the evaluation of patients with CAD. From humble origins as a research curiosity, this technique has evolved to provide valuable and comprehensive data on the anatomic and functional assessment of the heart and vessels. There is growing evidence that CMR can be used in each step of the assessment of patients with CAD, from establishing the diagnosis, to guiding the optimal therapeutic strategy, through to risk stratification. CMR is free of ionizing radiation, has a multiplanar cross-sectional nature and offers a wide range of sequences that are clinically available.
Steady state free precession (SSFP) sequences are primarily used for the assessment of ventricular volumes, function and myocardial mass but their application is expanding to include coronary anatomy and viability assessment. T2W turbo spin-echo (SE) sequences are used to detect increased water tissue content as an indirect measure of tissue inflammation or edema and can be useful in the differentiation of acute from chronic myocardial infarction and for distinguishing between myocarditis and CAD. T1W inversion-recovery (IR) sequences with gadolinium contrast agent (Gd-CA) administration are used for contrast-enhanced myocardial imaging. Single-shot, turbo gradient-echo (GRE) sequences and more recently, echo-planar imaging are now used for myocardial perfusion imaging.
This review will focus on the current and potential clinical applications of CMR in the assessment of patients with ischemic heart disease.
One of the biggest goals in cardiovascular imaging is to make available, a non-invasive technique capable of imaging the coronary arteries without the need for X-ray exposure or the administration of nephrotoxic contrast agents. Over the last decade, CMR has developed as a potential tool capable of replacing conventional X-ray coronary angiography (CA), which is still the gold standard for CAD detection.
Conventional X-ray angiography has the advantage of a high spatial resolution (~0.3mm) but, also has some important limitations, since it is a 2-dimensional technique and does not provide any information about vessel wall structure or plaque morphology.
Coronary arteries are difficult to image by CMR for several reasons. These include the small vessel caliber and tortuous course of the coronaries, the surrounding signal from epicardial fat and myocardium, the near constant motion related to the cardiac and respiratory cycles and time constraints related to the limited data acquisition time available during diastole, when bulk flow occurs through the arteries.
Current CMR techniques are based on the use of two different approaches to deal with respiratory movement: breath-hold and free-breathing. The first studies comparing coronary magnetic resonance angiography (CMRA) and X-ray CA were published more than 10 years ago using a breath hold 2-D GRE technique, in which data acquisition was performed during a single breath-hold in order to avoid respiratory blurring. ,, With the breath-hold approach, data is acquired in ~15-20 heartbeats in a single scan or as multiple-targeted 3-D volumes with repeated breath-holds to cover the coronary arteries. The breath-hold technique has the advantage of being rapid and effective in co-operative patients. Limitations include the need for patient co-operation, which depends on their clinical condition and the poor reproducibility of breath-holding leading to increased artifacts. Substantial development has occurred from these first generation techniques with the application of navigator echo monitoring which allows real time monitoring of diaphragm motion, avoiding the need for patient breath-holding and allowing data acquisition during free-breathing. ,, Another important step forward has been the development of 3-D acquisition techniques,  which allow better image quality through a higher signal-to-noise ratio and facilitate imaging of the tortuous pathways of the coronary arteries through multiple thin contiguous slices.
Since coronary arteries are embedded in myocardial fat, to improve contrast between coronary blood pool and the surrounding soft tissues, selective pulses are used to suppress signal from the surrounding fat,  particularly around the proximal vessels.
In the international multi-center trial published by Kim et al in 2001,  3-D free-breathing CMRA was performed in 109 patients before elective conventional CA. Overall, 84% of proximal and middle segments were found to be interpretable with sensitivity and specificity of 93% and 42%, respectively for the detection of significant stenosis (>50% reduction in lumen diameter). The diagnostic accuracy was found to be higher in the assessment of left main coronary artery stenosis and three vessel proximal coronary stenoses for which sensitivity and specificity were 100% and 85%, respectively.
Another important improvement in CMRA has been the application of the SSFP  sequence which, by improving image quality, allows visualization of distal coronary segments and better specificity. To help targeted coronary artery acquisition, the most recent and promising development is the so-called whole heart approach,  which has several advantages including visualization of longer segments of all coronary arteries, depiction of all major arteries in one study, minimal pilot scanning and less operator dependence. Single center studies have been published using this technique, which show improved diagnostic accuracy. , Recently, Sakuma et al  published a prospective study of 131 patients who underwent whole heart 3-D CMRA. In this study, the success rate for detecting significant stenosis compared to X-ray angiography was 86% with a per-patient sensitivity of 82% and specificity of 90% with an overall accuracy of 87%.
CMRA can be used for the evaluation of coronary artery bypass grafts (CABG). These are easier to image compared to native coronaries as they are less affected by cardiac and respiratory motion and have a larger caliber and less tortuous course. However, artifacts from clips and motion at anastomotic sites can limit their assessment. CMR allows both anatomic , and functional, , assessment of CABG disease. The diagnostic accuracy of CMR for CABG assessment has improved in the last few years, in particular with the application of flow velocity mapping techniques, but its application in routine clinical practice is still limited.
Coronary stents appear as areas of signal void on CMRA. CMR can be safely performed any time after coronary stent implantation. Hypothetical concerns about dislodging the stent and thus causing thrombosis have not been borne out in clinical practice. The signal void means that accurate visualization of in-stent stenosis is difficult although it can be used to locate the stent position. As with CABG, stent patency has been evaluated using flow velocity mapping techniques with promising results. 
CMR is an accepted gold standard for the assessment of coronary anomalies.  These congenital disorders are rarely encountered in the general population but are a common cause of sudden death in athletes. CMR is useful in defining the origin and proximal course of vessels , and to evaluate inflammatory changes,  as seen in Kawasaki's disease, of the coronary arteries [Figure - 1],[Figure - 2]. The avoidance of radiation exposure allows serial evaluation of these lesions, which is necessary for risk stratification and therapeutic management.
Beyond luminography, the wide range of available multi contrast sequences, makes CMR suitable for characterization of the atherosclerotic plaque.  Black blood techniques , have been used for coronary artery plaque detection and appear to be promising as non-invasive tools for the early assessment of plaque formation and coronary vessel wall remodeling, making these techniques suitable for longitudinal studies of coronary plaque progression/regression  such as in the METEOR trial.
Improvements in the field of CMRA include the use of contrast agents, the application of higher field strength (3 Tesla) and parallel imaging. Extracellular contrast agents , have not been shown to improve results because of their rapid half-life which does not allow data acquisition within an adequate time-frame using the currently available sequences. Intravascular contrast agents , have a longer available half-life and are under investigation and seem to be promising but are not yet ready for clinical use. CMRA at 3 Tesla,  in particular with the application of parallel imaging, , has been demonstrated to achieve better image quality by increasing the signal to noise and contrast to noise ratios but there are several technical issues still unresolved, including field inhomogeneities, which render the technique more prone to artifacts.
So, is CMRA ready for prime time? It is an established and validated technique for the assessment of patients with known or suspected anomalous coronary arteries and coronary artery aneurysms. Its application for the detection of coronary artery disease is still evolving. In this context, CMRA has been shown to be useful in the assessment of selected patients with suspected left main/multi-vessel disease. Further technical developments are needed to improve the diagnostic accuracy of CMRA in order to replace routine invasive X-ray CA in the assessment of native coronary artery vessel and bypass graft integrity and to be an effective tool for coronary plaque characterization and vulnerable plaque detection. CT coronary angiography has a better spatial resolution but at the cost of high radiation. The strength of CMRA is really in the additional functional assessment that is available with the technique in a single scan.
CMR provides an excellent field of view and allows the study of cardiac structures in multiple planes without geometrical assumptions. Left ventricular dimensions and global and regional function can be very accurately assessed.  The sequence currently used for this purpose is the SSFP which has superior spatial and temporal resolution and higher signal-to-noise and contrast-to-noise ratios compared with the previously used GRE sequences. This allows improved endocardial border detection.  Accordingly, CMR has been shown to have better inter-study reproducibility compared with 2-D-echocardiography in normal subjects and patients.  LV dimensions and global and regional function are evaluated using sequential short axis slices through the left ventricle from the base to the apex as well as in long axis planes (2-chamber, left ventricular outflow tract - LVOT and 4-chamber). CMR can detect typical changes correlating with myocardial infarction, such as a derangement of the normal LV geometry, regional wall motion abnormalities and wall thinning.  Another exclusive application of CMR for regional wall motion assessment is the use of myocardial tagging. This technique consists of the application of presaturation tag grids on the myocardium by manipulation of the magnetic field and their follow-through during the cardiac cycle to allow the evaluation of myocardial regional wall deformation. Strain patterns in the longitudinal, radial and circumferential directions are derived. Even though myocardial tagging is still used predominantly for research purposes, the application in the clinical setting is growing.  Recently, the application of myocardial tagging has been shown to improve the accuracy of dobutamine CMR, for both, the evaluation of viability  and the detection of significant coronary stenosis. 
Diastolic LV function can be evaluated by CMR both qualitatively by assessing relaxation and long-axis function and quantitatively by measurement of peak filling rates, left atrium size and longitudinal movement.
Infarction and viability
One of the most important applications of CMR is in the detection of viable myocardium that would benefit from revascularisation. Following coronary occlusion, the subsequent myocardial histopathological changes can be schematically distinguished as acute or chronic. The acute phase is predominantly characterized by extensive capillary damage manifesting as microvascular obstruction (MVO), also called "no-reflow" phenomenon; inflammatory changes due to myocardial injury, manifestating as increased water content, usually co-exist. In the chronic phase, these areas of myocardial damage are replaced by collagen. CMR can detect and assess the extent of myocardial infarction in both acute and chronic phases. ,, This technique is based on the application of T1W segmented IR pulses after administration of Gd-CA. This agent is given via a peripheral vein using a dose of 0.1-0.2 mmol/Kg as a single bolus. The area of myocardium where this agent accumulates will appear bright. Because of its molecular characteristics, Gd-CA accumulates in the extracellular spaces and is unable to penetrate the intact myocyte cell membrane. Consequently, in normal myocardium, the wash-out of Gd-CA is quite fast, lasting a few minutes, since the extracellular space is small with a low volume of distribution. In cases of both acute (cell membrane rupture and edema) and chronic myocardial injury (collagen formation), there is an increased volume of distribution. Consequently, because of its higher volume of distribution and slower wash-out kinetics, Gd-CA accumulates in the area affected by myocardial necrosis in both the acute and chronic phase of infarction.
Based on Gd-CA kinetics, two phases of myocardial distribution can be distinguished after bolus administration. In the early phase, the contrast agent penetrates from the blood pool into the myocardium, spreading through the extracellular space in normally perfused myocardium, but not in regions subtended by tight coronary stenosis or where there is microvascular obstruction. In other words, it reflects myocardial rather than coronary perfusion. Microvascular obstruction will appear as a dark rim of hypo enhancement [Figure - 3]. In the second more delayed stage (10-20 minutes after Gd-CA administration), a T1W IR pulse will show hyper enhancement in areas of necrosis and inflammation in the acute phase of myocardial infarction; in the chronic phase, once the inflammation has resolved, the area of high signal will be smaller, confined to the area of myocardial scarring [Figure - 4].
The importance of CMR in the evaluation of myocardial viability has been demonstrated by Kim et al .  In their study, 50 patients with ventricular dysfunction underwent contrast-enhanced CMR imaging before and after revascularization interventions. Of these patients, 80% had some regions of hyperenhancement. Of the 2093 myocardial segments analyzed before revascularization, 38% had abnormal contractility. The likelihood of wall motion improvement was strongly correlated to the transmural extent of hyperenhancement. In fact, in patients without hyperenhancement, 78% of segments showed improved contractility after revascularization while only 1 of 58 segments recovered function if there was more than 75% transmural hyperenhancement. In patients with myocardial hyperenhancement involving 51% to 75% of the wall, only 10% of revascularized segments showed improved contractility. Another important finding was that the likelihood of functional improvement after revascularization appeared independent of the grade of contractile compromise since even areas of akinesis or dyskinesis without evidence of hyperenhancement recovered function in all cases. The results of this study have had a great impact on the therapeutic management of patients with LV dysfunction and hibernating myocardium since it is possible to target the revascularization to the vessel supplying blood to a myocardial territory where functional recovery is predictable with high accuracy. The ability of CE-CMR to discriminate between viable and non-viable myocardium in the same region has led to a paradigm shift in the understanding of myocardial thinning. While previous studies have indicated that in patients with CAD and LV dysfunction, myocardial thinning (<5mm wall thickness) represents scar tissue that cannot recover contractile function after revascularization,  late enhancement imaging studies have shown that thinned myocardium, which does not show late enhancement can still recover after revascularization, sometimes with dramatic improvement in wall thickness and wall motion. 
The ability of CE- CMR to detect and characterize myocardial scar has been compared to single photon emission tomography (SPECT) and positron emission tomography (PET). In the study of Wagner et al ,  91 patients with suspected or known CAD underwent both CE-CMR and SPECT and the results were validated with an animal model. While both techniques were equally effective in detecting transmural infarction (>75% LV wall), of the 181 segments with subendocardial infarction (<50% transmural extent) detected by CE-CMR, 47% were missed by SPECT. On a per patient basis, 13% of patients with subendocardial infarction detected by CE-CMR were not diagnosed by SPECT. Klein et al  compared CE-CMR with PET in 31 patients with ischemic heart failure. While both techniques were equally effective in assessing the infarct mass, CE-CMR was superior to PET for the detection of subendocardial infarction. The results of both studies are not surprising, since CMR has a higher spatial resolution (~1mm) compared with both SPECT and PET (~5-8mm), allowing visualization of even microinfarcts with a threshold of 0.16g, which cannot be achieved by other techniques. Accordingly, CMR is now emerging as the gold standard for myocardial viability assessment. Further clinical outcome data is however required before such a transition is made.
The ability of CMR to detect the presence and extent of MVO has important effects on prognosis. MVO is usually localized to the subendocardial rim. The occurrence of MVO has been shown to reduce the likelihood of recovery after revascularization and to be a strong adverse prognostic marker even after control of infarct size. 
Another way to evaluate myocardial viability is by assessing contractile reserve. Similar to echocardiography, CMR can be performed using low dose dobutamine stress testing but with more accurate assessment of wall motion and wall thickening because of better endocardial border visualization. This technique has been recently validated with PET. Baer et al  compared the two techniques in 35 patients with mild LV dysfunction and for the detection of viable myocardium, CMR demonstrated a sensitivity and specificity of 88% and 87%, respectively. However, these results were not confirmed by Gunning et al  in a study that involved 30 patients with more severe LV dysfunction; the study showed higher specificity (81%) but lower sensitivity (50%). A comparison between dobutamine CMR and CE-CMR has been recently published by Wellnhofer et al .  In this study, 29 patients with CAD and LV dysfunction underwent both tests. Low dose dobutamine CMR was superior to CE-CMR in terms of sensitivity and specificity when a cut-off value of 25% transmural extent was used; however when myocardial segments with >50% transmural scar extent were analyzed, CE-CMR showed equivalent or superior accuracy compared to low dose dobutamine CMR.
In view of these conflicting results, the optimum CMR technique for assessment of viability has not been defined yet. CE-CMR seems to be a more accurate test in predicting the likelihood of recovery after revascularization in patients with more severe LV dysfunction and, compared to dobutamine CMR, carries less procedural risk and lower inter-observer variability. We anticipate that a combined approach may provide the highest accuracy - particularly in assessment of segments with 25-50% transmural hyperenhancement.
CMR imaging offers several techniques that can be used to non-invasively detect the presence and functional significance of CAD. Dobutamine stress CMR (DSMR) is already a well-established technique for the diagnosis of CAD, but perfusion CMR is controversial. Coronary MRA is a well-recognised method for the assessment of coronary anomalies but not clinically ready for screening purposes. Several studies have shown the high diagnostic accuracy of DSMR. Nagel et al  directly compared dobutamine stress echocardiography (DSE) with harmonic imaging and DSMR in 172 patients prior to conventional coronary angiography. Both tests were performed at rest and during stress according to the standard dobutamine-atropine protocol. DSMR was more accurate than DSE in detecting stress induced regional wall motion abnormalities with a sensitivity and specificity that increased from 74.3% to 86.2% and 69.8% to 85.7%, respectively. Hundley et al  performed DSMR in 153 patients with inadequate echo windows on second harmonic DSE. Both sensitivity and specificity were 83% for the detection of coronary stenosis >50% luminal diameter. Kuijpers et al  compared DSMR with and without tagging in 211 patients before coronary angiography. DSMR with tagging was able to detect significantly more patients with new wall motion abnormalities compared with standard DSMR. Despite the positive results of these studies, DSMR has not yet gained wide acceptance in the clinical arena because of concerns relating to patient safety and monitoring inside the MR scanner.
Perfusion CMR is a less established clinical technique compared with DSMR. Potentially, perfusion CMR could be superior to radionuclide imaging because of its superior spatial resolution, allowing the identification of even small subendocardial defects, without the need for radiation exposure. From a pathophysiologic point of view, perfusion CMR should be more sensitive than DSMR since decreased perfusion is the first step of the ischemic cascade. Gd-CA is the most widely used contrast agent for perfusion CMR. T1W sequences are used to visualise gadolinium during its first passage through the heart. The dosage of Gd-CA varies from 0.025 to 0.15 mmol/Kg. A higher dose of gadolinium was found to be superior to lower doses.  Adenosine is the vasodilator drug most widely used for pharmacologic stress. The dose commonly used is 140 µg/Kg/min for three to four minutes before injection of Gd-CA. Areas of myocardial hypoperfusion will appear as areas of decreased signal intensity. Perfusion CMR analysis is based on time-intensity curves and can be performed in a quantitative, semi-quantitative or qualitative fashion.
Several studies have been recently published comparing perfusion CMR with other imaging techniques including coronary angiography. Schwitter et al  studied 48 patients and 18 healthy subjects with dipyridamole CMR and 13N-ammonia PET before coronary angiography. Receiver-operator characteristic analysis of subendocardial upslope data revealed a sensitivity and specificity of 91% and 94%, respectively, for the detection of coronary artery disease as defined by PET and a sensitivity and specificity of 87% and 85%, respectively, in comparison with quantitative coronary angiography (diameter stenosis >50%). Paetsch et al  compared the accuracy of DSMR and both adenosine stress and perfusion CMR in 79 patients with suspected or known coronary disease and no history of prior myocardial infarction, scheduled for cardiac catheterization. Inducible wall motion abnormalities (IWMA) and segmental perfusion were assessed. Sensitivity and specificity for detection of coronary artery stenoses >50% by dobutamine and adenosine stress and adenosine perfusion were 89% and 80%, 40% and 96% and 91% and 62%, respectively. Thus, DSMR proved superior to adenosine for detection of reversible ischemia.
To improve the accuracy of perfusion CMR for the detection of CAD, Klem et al  recently proposed a visual interpretation algorithmic approach combining perfusion CMR with CE-CMR. In this study 92 patients with suspected CAD scheduled for X-ray coronary angiography were prospectively enrolled. The combination of perfusion and CE-CMR had a sensitivity, specificity and accuracy of 89%, 87% and 88%, respectively for CAD diagnosis (≥70% stenosis), compared with 84%, 58% and 68%, respectively, for perfusion-CMR alone. The high specificity of CE-CMR contributed to the improved accuracy of this combined approach.
The prognostic role of perfusion CMR has been recently evaluated by Ingkanisorn et al  who performed adenosine CMR on 135 patients presenting to the emergency department with troponin negative chest pain. Adenosine perfusion abnormalities had 100% sensitivity and 93% specificity as the single most accurate component of the CMR examination. An abnormal CMR was a stronger predictor of future cardiac events than clinical risk factors. In receiver operator curve analysis, adenosine CMR was a more accurate predictor than cardiac risk factors ( p < 0.002). A normal adenosine stress CMR scan predicted excellent prognosis at one-year follow-up.
CE-CMR can also be used in the differential diagnosis of myocardial fibrosis [Figure - 5]. McCrohon et al  showed that myocardial scarring related to ischemia invariably involves the subendocardium whereas the hyperenhancement seen in myocarditis or post-myocarditis fibrosis is usually located in the mid myocardium, often with a patchy distribution. These findings have a great impact on the patient's management and prognosis since it is possibile from the evaluation of the cause of fibrosis and its transmural extent to select the optimal therapeutic treatment.
The combination of T2 weighting and CE-CMR imaging has proven useful in different clinical scenarios. Abdel-Aty et al  have shown that an approach combining T2 weighting and CE-CMR may accurately differentiate between acute and chronic myocardial infarction. The same approach can be used in the assessment of patients with suspected acute myocarditis  and in those with chronic myocarditis presenting with heart failure or recurrent arrhythmias.  Furthermore, from the coexisting pattern of distribution of late enhancement (subendocardial and/or midwall) and T2W imaging, it is possible to differentiate between ischemic and inflammatory causes of acute chest pain and myocardial necrosis, in the absence of obstructive atherosclerosis on coronary angiography. 
T2W imaging has been recently proposed by Aletras et al  to assess the area at risk for reperfused acute myocardial infarction in a dog model. In their study, they found that the area at risk, as measured with microspheres, was comparable to the size of the hyperintense zone on T2W images two days after 90-minutes coronary artery occlusion while the infarcted zone was significantly smaller. Two months later, a repeat scan performed in eight animals showed that the edema resolved and the regional radial systolic strain partially improved. This technique could be used to assess the efficacy of infarct reduction and reperfusion therapies.
The ability of CE-CMR to detect even small myocardial scars has important implications for the diagnosis, risk stratification and therapeutic management of patients with suspected, known or occult CAD.
In the study by Kwong et al ,  195 patients with a clinical suspicion of CAD but without a history of prior myocardial infarction (MI), underwent cine- and CE-CMR. The findings of this study are impressive: the presence of late enhancement was a stronger multivariable predictor of major adverse cardiac events (MACE) (p<0.0001) and cardiac mortality (p<0.0001) compared with conventional variables; a primary threshold effect was noted, wherein even a very small myocardial scar, detected by CE-CMR (<2% of average LV mass) was associated with a >7-fold increase in MACE hazards. Since the prognosis of patients with an unrecognized MI is comparable to or worse than that of patients with a recognized MI, CE-CMR appears to be a potential useful tool for the risk assessment of patients without a prior known MI or with possible CAD. These findings seem particularly meaningful when it is considered that myocardial scars detected by CE-CMR can be more frequent than expected.  One possible reason for worse outcomes in those patients with myocardial scarring is the association with life-threatening arrhythmia. Bello et al  recently reported in 48 patients with known CAD, that the infarct surface area and mass, as measured by CE-CMR, have a stronger correlation with inducible monomorphic ventricular tachycardia compared with LV ejection fraction.
Who should be scanned?
CMR may be useful in several clinical scenarios of CAD. Clinical indications from a Consensus Panel endorsed by the Society for Cardiovascular Magnetic Resonance and the European Society of Cardiology have been proposed by Pennell et al .  According to this report, CMR may be used as a first line imaging technique in order to assess global left and right ventricular function and mass, detect coronary anomalies and assess myocardial viability (class I indication).
More recently, appropriateness criteria for cardiac computed tomography and CMR have been reported under the auspices of the American College of Cardiology Foundation.  Accordingly, CMR is considered appropriate in the following conditions involving CAD patients:
- DSMR and perfusion CMR for both the detection of CAD in symptomatic patients with intermediate pre-test probability of CAD and either uninterpretable ECG or inability to exercise and for risk assessment in patients with stenosis of unclear significance on coronary angiography
- Coronary MRA for the evaluation of suspected coronary anomalies
- LV function assessment in patients following myocardial infarction or heart failure with poor echocardiographic windows or in patients with discordant information from prior tests
- Evaluation of myocarditis or myocardial infarction with normal coronary arteries
- CE-CMR to evaluate myocardial necrosis, including no-reflow regions, in post-acute myocardial infarction; to establish the likelihood of recovery of function with revascularization or medical therapy and to evaluate viability when SPECT or DSE has provided equivocal or indeterminate
The cost of imaging tests for cardiovascular disease represents a large economic burden placed upon society. CMR imaging is expensive compared with other imaging modalities but it may offer advantages in terms of diagnostic accuracy that could result in a dominant strategy of improved life years saved and cost savings, in particular for high-risk patients. 
Comparison with other techniques
Currently SPECT plays a major role in the assessment of perfusion in clinical practice. A pooled analysis of 79 studies from Underwood et al  showed a weighted mean sensitivity and specificity of 86% and 74%, respectively. The application of ECG-gated SPECT allows the assessment of LV function  and when coupled to perfusion evaluation, leads to improved specificity.  Nonetheless, limitations of the technique include limited spatial resolution with consequent inability to detect subendocardial defects and a potentially risky radiation burden.
Echocardiography may be used for the detection of ischemia with either stress or perfusion modalities. Exercise stress echocardiography showed a 84% sensitivity and 82% specificity, whereas dobutamine stress testing showed a sensitivity and specificity of 80% and 84%, respectively.  Recently, myocardial contrast echocardiography has been shown to be comparable to SPECT for the detection of CAD;  the possibility of combining perfusion and stress echocardiography could improve accuracy.  However, a large proportion of patients still have suboptimal acoustic windows and inter-observer variability could be a limiting factor.
Computed tomography angiography (CTA) with 64-slice scanners allows us to perform full heart scans in a few seconds with submillimetrer resolution. Recent studies in selected cohorts of patients have reported sensitivities ranging from 83% to 99% and specificities between 93% and 98% for the detection of coronary stenoses in comparison to invasive coronary angiography. This suggests that coronary CTA could be useful to rule-out the presence of coronary stenoses in patients with low pre-test likelihood of CAD.  However, there are some concerns regarding this technique. Firstly, calcification often limits the interpretation of some coronary segments making the test unreliable for detection and quantification of atherosclerotic lesions. Secondly, the need for iodinated nephrotoxic contrast agents is an additional risk factor for contrast-induced renal dysfunction in patients who often require further invasive assessment. Thirdly, the required radiation dose is twice that of invasive diagnostic angiography  and radiation burden may increase in those cases in whom further functional nuclear assessment and, if necessary, invasive catheterization and interventions are required.
| Conclusion|| |
CMR is a valuable tool in the evaluation of patients with CAD. A combination of functional, viability and perfusion assessment provides a comprehensive and detailed assessment of disease presence and burden. Challenges remain to improve coronary imaging, increase access and maintain a balance between the information provided and costs.
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Sanjay K Prasad
Cardiovascular Magnetic Resonance Unit, Royal Brompton Hospital, Sydney Street, London SW3 6NP
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5]
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