Cardiac Magnetic Resonance Imaging and Computed Tomography—State of the Art, MK Atalay

Tags: MRI, CMRI, Cardiac Magnetic Resonance Imaging, J Am Coll Cardiol, coronary arteries, spatial resolution, CCT, Computed Tomography, coronary artery disease, magnetic resonance, myocardial infarction, Rhode Island Hospital, magnetic resonance imaging, spiral computed tomography, coronary artery, ECG, Michael K Atalay, temporal resolution, cardiac imaging, contractile function, heart disease, pulse sequence, Cardiac Computed Tomography, Eur Heart J, Congenital heart disease, Cardiovascular Angiography, American College of Radiology, constrictive pericarditis, coronary artery anatomy, Data acquisition, cardiac function, Cardiovascular magnetic resonance, cardiac valve surgery, contrast-enhanced, Magn Reson, assessment, coronary heart disease, myocardial perfusion imaging, tissue characterization, coronary artery Bypass graft, Society of Interventional Radiology, Warren Alpert Medical School of Brown University, MD, PhD Director, Brown Medical School Computed tomography, Johns Hopkins, clinical arena, Princeton University, Cardiothoracic Imaging, cardiac arrhythmia, cardiac anatomy, cardiac MRI, pulse sequences, imaging tools, coronary CT angiography, angiography, cardiac assessment, pericardial disease, coronary angiography, cardiovascular magnetic resonance imaging, heart diseases, CT coronary angiography, ventricular function
Content: Cardiothoracic Imaging
Cardiac magnetic resonance imaging and Computed Tomography-- State of the Art
a report by Michael K Atalay, MD, PhD Director, Cardiac Magnetic Resonance Imaging and Computed Tomography, Rhode Island Hospital, and Assistant Professor, Brown Medical School
Computed tomography (CT) and magnetic resonance imaging (MRI) have been used to evaluate the cardiovascular system for almost three decades. For much of this time, however, clinical cardiac imaging has been dominated by nuclear methods, echocardiography (echo), and catheter-based angiography, with cross-sectional imaging largely relegated to minor roles, usually pertaining to questions of anatomy. Over the past few years the landscape of clinical cardiac imaging has shifted. Recent technological advances have led to remarkable improvements in image quality and diagnostic utility for both MRI and CT.1­8 Unlike echo and nuclear cardiology, MRI and CT are not significantly hampered by patient habitus and can consistently provide high-resolution 3D visualization of myocardial morphology and contractile function. It is even possible, using current methods, to assess myocardial tissue perfusion, to characterize tissue composition, and to quantify flow-related physiology in large vessels. In essence, both MRI and CT are surging ahead (competing in some ways, but complementing each other in others) as they converge toward a comprehensive, noninvasive cardiac evaluation. Because of these developments, cardiac MRI (CMRI) and cardiac CT (CCT) are making rapid and dramatic inroads into the clinical arena. This article briefly reviews the recent technological gains witnessed with MRI and CT, as well as their current roles in evaluating a few of the principal forms of heart disease. General advantages and disadvantages of CMRI and CCT are shown in Table 1. Cardiac Magnetic Resonance Imaging Significant developments in both hardware technology and pulse sequence design have made MRI a versatile and robust modality for imaging cardiac anatomy and function in clinically reasonable timeframes.1­3,5,6,9 MRI has a vast and growing arsenal of pulse sequences that permit accurate characterization of most of the clinically important features of heart disease, including anatomy and morphology, quantitative ventricular chamber sizes, regional and global ventricular Michael K Atalay, MD, PhD, is Director of Cardiac Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) at Rhode Island Hospital and The Miriam Hospital in Providence. He is also an Assistant Professor at The Warren Alpert Medical School of Brown University. After obtaining his undergraduate degree in physics from Princeton University, Dr Atalay simultaneously earned his MD and PhD from John Hopkins medical institutions in Baltimore. His PhD focused on developing techniques in cardiac MRI. He subsequently completed a medical internship and a post-doctoral research fellowship at Harvard hospitals, before returning to Johns Hopkins for a radiology residency and a cross-sectional imaging fellowship. E: [email protected]
function, tissue characterization, viability assessment, pharmacological stress perfusion, assessment of masses (e.g. thrombus versus tumor), pericardial disease, and valve lesions. Importantly, MRI can also interrogate blood flow physiology using the technique of phase contrast imaging.10 This technique permits quantitative assessment of instantaneous and time-averaged blood flow in major arteries and estimation of peak systolic velocities (and pressure gradients) across stenotic lesions, including valves. Flow data can be used to calculate shunt fractions and to assess the severity of regurgitant valves. With electrocardiogram (ECG) gating, detailed stop-action images of the heart are possible. When multiple images at different cardiac phases are displayed serially, movies--or cines--of myocardial contraction can be generated. Because of the high temporal resolution of CMRI (~40ms), even very rapid and subtle motions can be depicted accurately. Steady-state free precession deserves mention as a particularly useful pulse sequence for CMRI. It is very rapid, affords high image quality, and has high inherent contrast between blood pool and myocardium, and for these reasons has become the foundation of cine imaging in CMRI. In addition to favorable temporal resolution, MRI has high spatial resolution (approximately 40 times that of nuclear cardiology) and excellent tissue contrast. CMRI has the usual relative and absolute contraindications of general MRI and may be limited by severe cardiac arrhythmia or the inability of subjects to perform breath-holds. In general, these limitations can be addressed adequately and diagnostic studies obtained. There are currently several widely accepted applications for CMRI (see Table 2).11 Cardiac Computed Tomography Conventional non-ECG-gated CT has been the workhorse of thoracic imaging for decades. It is generally accurate in evaluating gross cardiac chamber sizes and morphology, pericardium, intra- and extra-cardiac masses, vascular anatomy, and various diseases of the aorta and great vessels. However, for most of its history, detailed imaging of the heart was outside the purview of CT. The increased gantry rotational speeds of newer generations of multidetector CT (MDCT) scanners offer substantially improved temporal resolution. Borrowing from the lexicon of photography, this means that the scanner `shutter speed' is sufficiently fast that images obtained during moments of relative cardiac quiescence, or diastasis, will be clear. By combining this improved temporal resolution with a large (and rapidly growing) number of detector rows that dramatically increase the z-axis volumetric coverage per unit time, these scanners are able to generate remarkably clear stop-action images of the heart in patients with low normal heart rates during short breath-holds.
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© TOUCH BRIEFINGS 2008
Cardiac Magnetic Resonance Imaging and Computed Tomography--State of the Art
Table 1: Advantages and Disadvantages of Cardiac Magnetic Resonance Imaging and Cardiac Computed Tomography
Table 2: Appropriate Indications for Cardiac Magnetic Resonance Imaging
Advantages
Disadvantages
MRI 3D technique
Long total scan times (30­75 minutes)
Safe (no ionizing radiation)
Claustrophobia
High spatial resolution
Not as readily accessible
Very high temporal resolution
Foreign matter causes local artifacts
Excellent soft-tissue contrast
Contrast agents have finite risk for
nephrogenic systemic fibrosis
Can be performed with free breathing Long training time for
technologists/physicians
Arrhythmias are generally not
MRI contraindications (e.g. pacemaker)
a problem
Can image anatomy, function, and
physiology
CT 3D technique
Ionizing radiation
Short total scan times (10­15 minutes) Risks of iodinated contrast agents
Very high spatial resolution
Requires breath-hold and low, regular
heart rate
Moderate temporal resolution
Convenient
Shorter training time for
technologists/physicians
Diagnostic for fat, calcium, air
Can image anatomy and function
CT = computed tomography; MRI = magnetic resonance imaging.
Evaluation of myocardial scarring Location and extent of myonecrosis after acute MI Viability prior to revascularization or medical therapy Viability after `equivocal' or `indeterminate' results on SPECT or dobutamine echo Stress CMRI (e.g. adenosine perfusion) Chest pain syndrome Intermediate PTP of CAD and either ECG uninterpretable or unable to exercise Stenosis of unclear significance on coronary angiography Ventricular and valvular function (LV/RV mass and volumes, MR angiography, quantification of valvular disease, and/or delayed contrast enhancement) Congenital heart disease Evaluation for ARVC in patients with syncope or ventricular arrhythmia NICMs using delayed enhancement Myocarditis or MI with positive cardiac enzymes and obstructive coronary lesions LV function after MI or in heart failure patients when echo is limited LV function when prior tests give discordant data Native and prosthetic valves, with planimetry and quantification, when echo is limited Cardiac masses using contrast to assess vascularity Pericardial disease (e.g. mass, constrictive pericarditis) Suspected coronary anomalies (CT better) Pulmonary vein mapping pre- and post-RF ablation for atrial fibrillation ARVC = arrhythmogenic right ventricular cardiomyopathy; CAD = coronary artery disease; CT = computed tomography; ECG = echocardiography; LV/RV = left/right ventricle; MI = myocardial infarction; NICM = non-ischemic cardiomyopathy; PTP = pre-test probability; SPECT = single-photon emission computed tomography. Modified from Table 19 in Hendel et al.11
With ECG gating and sub-millimeter spatial resolution (~0.06mm3), accurate non-invasive assessment of the coronary artery anatomy is now possible. With CCT, there are two methods of ECG gating: retrospective and prospective. It is instructive to understand a few of their basic principles. (The same is true of MRI, but the implications are less important.) With retrospective gating, the X-ray tube is `on' throughout the examination (usually ramping up to optimal tube current in mid-diastole and returning to some lower baseline value during systole), and the scan trajectory is helical with a very low pitch (~0.2). With prospective gating, the X-ray tube is on only during a short window (usually in mid-diastole), and the acquisition is so-called `step-and-shoot': the gantry spins at one position, the tube turns on for a short burst, data are acquired, the tube turns off, and the table advances to the next scan position (~4cm for a 64-row MDCT), where the sequence is repeated. data acquisition typically occurs every other heartbeat so that the scanner has time to reposition. With both techniques, ECG data are obtained concurrently with the imaging data, and the timing of peak tube current is based on the duration of the R-R interval of the preceding heartbeats. After the study, all imaging data obtained at the same point in the cardiac cycle, across heartbeats, are `stitched' together to create a 3D volume for each desired cardiac phase. Total radiation dose is substantially higher with retrospective gating, but this technique offers three important benefits: first, because data are available from every phase of the cardiac cycle, cine imaging can be performed (as with MRI) and global and regional LV function assessed; second, many more phases can be reconstructed, if necessary, to overcome motion artifacts during coronary artery evaluation; and third, data associated with ectopic beats can be rejected. (A fourth potential benefit
Table 3: Appropriate Indications for Cardiac Computed Tomography Coronary angiography Chest pain syndrome Intermediate PTP of CAD and either ECG uninterpretable or unable to exercise Uninterpretable or equivocal Stress Test Acute chest pain Intermediate PTP of CAD, nECG changes, serial enzymes negative New-onset heart failure to assess etiology Suspected anomalous coronary artery Bypass graft luminal analysis Pre-op evaluation prior to cardiac surgery with low suspicion for CAD Congenital heart disease before or after surgical repair Cardiac masses (limited MRI and/or echo) Pericardial diseases (limited MRI and/or echo) Pulmonary vein mapping pre- and post-RF ablation for atrial fibrillation Coronary vein mapping prior to biventricular pacemaker placement Global and regional ventricular function (CMRI better) Ventricular mass, cavity volumes, and myocardial morphology (CMRI better) ARVC = arrhythmogenic right ventricular cardiomyopathy; CAD = coronary artery disease; CMRI = cardiac magnetic resonance imaging; ECG = echocardiography; MI = myocardial infarction; NICM = non-ischemic cardiomyopathy; PTP = pre-test probability; RF = radiofrequency. Modified from Table 10 in Hendel et al.11 may come from a comparison of systolic versus diastolic subendocardial perfusion for the detection of ischemia.12) Despite these benefits, the radiation dose reduction associated with prospective gating makes this the preferred method in most cases where function is not critical. In addition to radiation exposure (which can be substantial with retrospective gating), CCT requires iodinated contrast agents that can induce or exacerbate renal insufficiency and that have a finite risk for anaphylactoid reaction. For these reasons, appropriate patient selection
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Cardiothoracic Imaging
Figure 1: Magnetic Resonance Imaging Viability Images from a Patient with a History of Myocardial Infarction and Angiographically Confirmed Three-vessel Disease
A
B
occurs when atherosclerotic plaque in epicardial coronary arteries compromises blood flow to downstream tissue, through either progressive luminal narrowing or abrupt plaque rupture with subsequent thrombotic occlusion. A reasonably comprehensive, non-invasive clinical assessment of IHD might include:
· coronary artery lumen analysis;
· plaque composition (`stable' versus `vulnerable');
· effects of stenosis on regional blood flow (tissue perfusion);
Extensive hyperenhancing, subendocardial scar (arrows) is seen throughout the left ventricle (LV) on (A) fourchamber and (B) short-axis views. Normal myocardium is black. Because the scar in each segment is thin relative to the overall segment thickness, this patient's contractile function is expected to improve following bypass surgery. LA = left atrium; RV = right ventricle.
Figure 2: Mid-cavity Short-axis Magnetic Resonance Images Obtained During the `First Pass' of Intravenous (IV) Contrast at Rest (A) and During IV Adenosine Infusion (B)
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B
With adenosine, a small subendocardial defect (arrows) is seen in the anterolateral segment that is not present at rest, suggesting the presence of a flow-limiting stenosis in a diagonal branch distribution. This was confirmed at catheterization. LV = left ventricle.
Figure 3: Cardiac Computed Tomography
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B
· effects of stenoses on wall thickness and motion (function); and · the presence and significance of scarring (viability). No technique is currently able to address plaque composition, although CT shows great promise and is currently receiving considerable research attention. The remaining items are clinically feasible with cross-sectional methods and are discussed below. CMRI has emerged as an excellent tool for addressing the final three items on the above list. Among the many significant recent developments in CMRI, one that stands out is the demonstration that MR contrast agents can be used to very accurately delineate areas of myocardial scarring.16­18 In fact, in addition to being widely regarded as the gold standard for the assessment of global and regional myocardial contractile function, many feel that MRI also serves as a new reference standard for myocardial viability, even when compared directly with positron emission tomography, the erstwhile gold standard.19 In the setting of IHD, the likelihood of a dysfunctional myocardial segment to improve function following revascularization is inversely related to the transmural extent of infarct, as defined on CMRI.17,18 In general, if the transmural extent of scarring across a segment of myocardium is less than 50%, that segment stands a reasonable chance of recovering function and hence is considered viable. Figure 1 shows representative viability images.
A: Cardiac computed tomography (CCT) demonstrates a high-grade stenosis (arrow) in the distal left main coronary artery in an elderly man who had an equivocal exercise stress test. The stenosis was subsequently confirmed at conventional angiography (B), and the patient had bypass surgery on the following day. Note that despite the extensive, dense, mural calcification on CT, a confident assessment can in this case be rendered. is paramount. Currently accepted indications for ECG-gated CT are shown in Table 3.11 The recent strides in CCT have been remarkable, and its emerging role in cardiac evaluation is undeniable.8,13,14 Principal Applications of Cardiac Computed Tomography and Cardiac Magnetic Resonance Imaging Ischemic Heart Disease Ischemic heart disease (IHD), which is synonymous with coronary heart disease, continues to be the number one cause of death in the US.15 It
Adenosine myocardial perfusion imaging is gaining recognition as an excellent alternative to other more traditional methods for detecting flowlimiting coronary artery stenoses.20­24 Figure 2 shows an example of perfusion imaging. Despite the growing advocacy, evolving technology, and promising data emerging on this technique, it is technically more involved and has yet to gain significant traction outside of a few major centers. One notable current weakness of MRI in the assessment of IHD remains accurate coronary angiography. Although techniques are available to image the coronary arteries with MRI, spatial resolution (for practical purposes) is inadequate to image atherosclerotic plaque and associated luminal narrowing of native vessels. MRI methods that identify coronary arteries and their proximal courses are available and may be useful, for example in the setting of suspected coronary artery anomalies or for evaluating bypass graft patency.25,26 Nonetheless, CCT, with its superb spatial resolution, is clearly the preferred non-invasive method for evaluating small thoracic vessels, including native coronary arteries and bypass grafts. An example is shown in Figure 3.
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Cardiac Magnetic Resonance Imaging and Computed Tomography--State of the Art
Data indicate that with 64-row MDCT scanners, under optimal conditions, high accuracy can be achieved for the detection of coronary artery stenoses greater than 50%.27­31 Importantly, in these and numerous other studies relating to CT coronary angiography, the negative predictive value for the presence of coronary atherosclerosis is uniformly very high. Because of this, perhaps the most widely advocated application of CCT is for excluding coronary disease in patients who have atypical symptoms and a low to intermediate suspicion for coronary disease. Stent imaging by CCT shows variable success, largely dependent on the size of the stent and vessel in question.32,33 Because bypass grafts tend to be larger and more stationary than native coronary arteries, assessment of bypass graft patency by CCT has shown good success.34,35 Moreover, bypass graft ostia may be challenging to find and engage by conventional angiography, particularly if the vessel is occluded. This also argues for CCT.
Figure 4: Cardiac Computed Tomography and Conventional Angiography in a 33-year-old Man with Chest Pain
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D
First-pass perfusion imaging and delayed viability assessment continue to be works in progress for CCT, but are also showing promise.8,12,36­38 For the moment, these methods are not ready for routine clinical use. As stated earlier, with retrospective gating CCT can provide useful functional information, as shown in Figure 4. Although vendors have developed creative solutions for fast imaging, the temporal resolution of CT is, in general, substantially poorer than that of MRI, so subtle or highfrequency motions are less well characterized. Nevertheless, volumetric quantification appears to compare favorably with CMRI.13,39
The cardiac computed tomography (CCT) coronary artery reconstructions (not shown) were completely normal. However, on a two-chamber re-formatted image (A), a hypodense perfusion defect (arrowhead) can be identified in the distal inferior wall. A single cine frame from mid-systole (B) shows a focal bulge (solid arrow) in the same region. Subsequent coronary angiography (C) demonstrates a distal left anterior descending coronary artery cut-off (dashed arrow), and the ventriculogram (D) confirms the wall motion abnormality (solid arrow). Figure 5: Magnetic Resonance Imaging Delayed Enhancement Images Demonstrate Extensive, Predominantly Subepicardial Scarring (arrows) in (A) Four-chamber and (B) Short-axis Views
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Non-ischemic Cardiomyopathies CMRI may be particularly useful for evaluating non-ischemic cardiomyopathies (NICMs). Myocardial scarring is not unique to infarction and may be seen in a variety of NICMs, such as hypertrophic CM, idiopathic dilated CM, cardiac sarcoid, myocarditis, and others.40­43 While scarring from myocardial infarction propagates outward from the subendocardium (respecting vascular territories), scarring in other CMs may be subendocardial, subepicardial, or mesomural (mid-wall), often in a patchy distribution (see Figure 5). The presence and pattern of scarring may therefore be useful in diagnosis and prognosis, making MRI an invaluable tool for characterizing heart disease. MRI also has well described benefits in evaluating a host of NICMs in which scarring is not a prominent imaging manifestation, including cardiac amyloid, hemochromatosis, and arrhythmogenic right ventricular cardiomyopathy (ARVC).44­46 Importantly, the primary application of CCT for the diagnosis and evaluation of NICMs is to exclude IHD as an unsuspected cause. CCT, like MRI, may also assist in the assessment of morphology and function.
This patient had moderate biventricular wall thickening, severe global hypokinesis, and an ejection fraction of 13%. Endocardial biopsy confirmed the diagnosis of hypertrophic cardiomyopathy. LA = left atrium; LV/RV = left/right ventricle. Figure 6: Magnetic Resonance Images from an Adult Woman Being Evaluated for an Enlarged Right Heart
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Congenital Heart Disease Both CMRI and CCT are well suited to imaging anatomical abnormalities associated with congenital heart disease (CHD). When congenital lesions present early in life, the management decisions may be predicated on clinical presentation, echo, plain films, and catheterization findings. Later in adulthood, however, because of normal developmental changes of the thorax, echo may be inadequate for comprehensive evaluation of congenital lesions, whether found as a new diagnosis or followed from infancy. Here, the volumetric, 3D nature of both MRI and CT permit more detailed visualization of the structural abnormalities and post-surgical sequelae.47 MRI further distinguishes itself in its ability to also offer quantitative bi-ventricular functional information as well as quantitative
A: Short-axis diastolic frame from a cine loop clearly showing marked right ventricular (RV) enlargement. B: A single frame from another cine loop acquired in a transaxial plane passing though both left and right atria (LA/RA). This movie loop was created by superimposing colorized blood flow data (from phase-contrast imaging) onto grayscale magnitude images. Left-to-right blood flow across a secundum atrial septal defect appears as a blue jet originating from the interatrial septum. C: The defect (arrow) shown en face using the same technique. This was a new diagnosis of atrial septal defect. LV: left ventricle. blood flow data. In the setting of intra- and extracardiac shunts, such as an atrial septal defect or an anomalous pulmonary vein, MRI allows quantification of the shunt severity (i.e. Qp/Qs). Qualitative flow information rendered by MRI may also be useful (see Figure 6).
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Figure 7: Axial Cardiac Computed Tomography Images from a Patient with Symptomatic Atrial Fibrillation
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make this determination. Sometimes, the clinical problem is a suspected `pseudomass.' Both MRI and CCT can frequently make the diagnosis (see Figure 7). Pericardial Disease Many conditions of the pericardium, such as simple and complex effusions, hematoma, masses, and diffuse thickening, are generally well imaged by both MRI and CT.50,53­55 With its ability to show motion, MRI also demonstrates how the pericardial disease affects chamber sizes and myocardial function.
Multiple echocardiograms (not shown) revealed a mass in the left atrial appendage (LAA), felt to either represent thrombus or artifact from stationary blood (`spontaneous echo-contrast'). Cardiac computed tomography demonstrates an LAA defect (arrowhead) on arterial phase imaging (A) that fills in after a delay of one minute (B), confirming no thrombus. Where valve lesions are present, hemodynamic severity can be quantified. Moreover, the changes in both anatomy and function can be monitored in a quantitative manner as the patients with CHD are followed. Although the decision to intervene surgically or percutaneously will still be based largely on clinical grounds, the quantitative data available by MRI can contribute substantially.
The clinical and hemodynamic findings of constrictive pericarditis often overlap substantially with those of restrictive cardiomyopathy. treatment options for these two conditions are quite disparate, with medical management preferred for restrictive disease and surgical pericardiectomy of potential benefit for pericardial constriction. Accurate diagnosis is Computed tomography and magnetic resonance imaging have emerged as
Of course, in cases where detailed anatomical assessment is paramount (suspected anomalous coronary artery, for example), CCT is the clear choice.48 In addition, CCT is useful to exclude coronary disease in young patients prior to their undergoing cardiac surgery for CHD or valvular disease.49 Although few investigators would encourage coronary screening by CCT, this is one application that could reasonably gain traction, particularly since invasive angiography is currently used for this purpose. Mass Masses in and around the heart are often detected incidentally by routine echo or chest CT. If a clear diagnosis can be made (such as a laminated intracavitary thrombus or a pericardial cyst), no further evaluation is necessary. Frequently, however, this is not the case, and additional characterization is desired. Using a variety of pulse sequences, MRI can often narrow the differential possibilities significantly. MRI can readily characterize the size, mobility, site of attachment, and functional In cases where detailed anatomical assessment is paramount (suspected anomalous coronary artery, for example), cardiac computed tomography is the clear choice. significance of a mass.50­52 Regarding the evaluation of an intracavitary mass, perhaps the most important first step is to determine whether the mass is a tumor or a thrombus. This distinction has significant therapeutic implications. The presence of internal vascularity essentially excludes the diagnosis of thrombus; conversely, the absence of vascularity strongly suggests thrombus. With the aid of intravenous contrast and serial imaging, both CCT and CMRI are generally able to
excellent imaging tools for diagnosing and characterizing a nearly exhaustive spectrum of heart diseases. therefore paramount. Pericardial thickening (>4mm) and calcification are suggestive (but not diagnostic) of constrictive pericarditis and are both readily appreciated on CT imaging. Although MRI is unable to `see' calcifications, its ability to show anatomy, pericardial thickening, and cardiac function make it very sensitive and specific for the diagnosis of constrictive pericardial disease. Valvular Disease Direct visualization of cardiac valves is best achieved by echo, with its high spatial resolution and very high temporal resolution. However, MRI does have a useful adjuvant role in its ability to confirm and quantify the severity of various valvular lesions (i.e. pressure gradients, planimetry, and percentage regurgitation) and to accurately measure various dimensions pertinent to valve replacement, such as valve annulus diameter and aortic root or main pulmonary artery diameter. The rapid motion of valve leaflets limits the utility of CT for assessment of function. However, CCT can identify important valvular disorders,56,57 and the extent of calcification of the aortic valve has been linked with the severity of stenosis.58 Summary Over the last few years, the landscape of cardiac imaging has changed dramatically. CT and MRI have emerged as excellent imaging tools for diagnosing and characterizing a nearly exhaustive spectrum of heart diseases. These new methods provide clinicians with substantial additional information, including tissue characterization, accurate measurements of ventricular volumes and function, and--in the case of MRI--quantitative data on blood flow physiology. Importantly, MRI (as the slightly more mature cardiac imaging modality) has recently made
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Cardiac Magnetic Resonance Imaging and Computed Tomography--State of the Art
dramatic strides in the evaluation of IHD. CT has also seen remarkable advances and has become a viable alternative to invasive angiography in the anatomical assessment of obstructive coronary artery disease. Moreover, the rapid evolution of CCT suggests that it may soon provide many of the advanced tools of cardiac assessment that CMRI offers (e.g. perfusion and viability), with much greater resolution and in much
shorter examination times. The future of both of these modalities in the non-invasive evaluation of heart disease is bright. For the moment, it seems reasonable to consider CMRI when ventricular function, tissue characterization, or blood flow physiology are the principal clinical concerns. If detailed anatomical evaluation is the major clinical focus, CCT should be used.
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