YEAR IN CARDIOLOGY SERIES
The Year in Cardiac Imaging
Raymond J. Gibbons, MD*,*,
Philip A. Araoz, MD and
Eric E. Williamson, MD
* Division of Cardiovascular Diseases and Internal Medicine, Department of Medicine, Mayo Clinic and Mayo Foundation, Rochester, Minnesota
Department of Radiology, Mayo Clinic and Mayo Foundation, Rochester, Minnesota
Manuscript received August 30, 2008;
accepted September 25, 2008.
* Reprint requests and correspondence: Dr. Raymond J. Gibbons, Mayo Clinic, Gonda 5, 200 First Street SW, Rochester, Minnesota 55905 (Email: gibbons.raymond{at}mayo.edu).
Key Words: cardiac • imaging • CT
This review is a sequel to our 3 previous reports highlighting the most important recent literature in single-photon emission computed tomography (SPECT), myocardial perfusion imaging, cardiac positron emission tomography (PET), cardiac computed tomography (CT), and cardiac magnetic resonance imaging (MRI). In contrast to our previous reports, which covered 1 year each, this report covers the English-language literature over a 15-month period from April 1, 2007, to June 30, 2008. The increased length of our coverage period has magnified the difficulty of reaching a decision as to which articles merit inclusion. Our previous decisions have pleased some authors and disappointed others. We have again made every effort to avoid our personal biases in reaching these ever more difficult decisions. Our summary is once again organized around topical themes, reflecting the growing interest in integrated, multimodality imaging to solve clinical problems.
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Technical Advances
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Novel imaging approaches.
Amid the technical advances of SPECT, PET, CT, and MRI, work continues on novel imaging approaches. Detter et al. (1) validated fluorescent cardiac imaging using a novel, indocyanine green–based imaging system in a porcine model; impaired myocardial perfusion assessed by this technique correlated well with fluorescent microspheres.
PET.
The potential application of vascular fluorodeoxyglucose (FDG)-PET imaging to coronary arteries is limited by myocardial FDG uptake. Williams and Kolodny (2) examined the effect of a new very high-fat, low-carbohydrate, protein-permitted diet on myocardial FDG uptake. Compared with the usual fasting approach, this new diet reduced the maximum standardized myocardial uptake by >50%, which might permit FDG-PET imaging of coronary atherosclerosis.
CT.
A new 320-detector row CT scanner debuted at the Radiologic Society of North America meeting in November 2007. Rybicki et al. (3) subjectively evaluated image quality of coronary CT angiograms performed using this novel scanner in 40 consecutive patients. They reported excellent image quality in 89% of coronary segments (517 of 583); only a single coronary segment was considered unevaluable (Fig. 1).
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Figure 1 320-Row Coronary CT Angiography
Curved planar reformation of the right coronary artery (RCA) performed using the new 320-row computed tomography (CT) scanner demonstrates predominately calcified atherosclerotic changes in the proximal RCA (white arrow). From Rybicki et al. (3), with permission.
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Dual-energy CT, which is performed with photons produced at 2 different energies, can be used to distinguish individual tissues based on differential X-ray attenuation. Johnson et al. (4) described very early results in 10 patients and demonstrated the ability of this technique to distinguish different tissues (Fig. 2).
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Figure 2 Comprehensive Chest Pain Evaluation
Short-axis computed tomography images in diastole and systole (A, B) demonstrating a low-attenuation defect and corresponding dyskinesia of the anterior left ventricular wall (black arrows), characteristic of myocardial infarction. Curved planar reformation of the left anterior descending coronary artery (C) shows a patent proximal stent (white arrow) and reperfusion of the distal vessel. From Johnson et al. (4), with permission.
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MRI.
Delfino et al. (5) tested an MRI sequence designed to measure tissue velocities (similar to Doppler echocardiography) during free breathing (navigator-echocardiography–gated phase-contrast). After phantom validation, they compared MRI velocity measurements in 17 healthy volunteers with those in 28 patients with heart failure. Peak longitudinal and radial velocities were significantly greater in volunteers than in patients with heart failure (p < 0.05 for all).
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Viability/Infarction
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PET.
Evidence for the impact of viability studies on patient outcomes has generally been limited to observational studies. Beanlands et al. (6) reported the results of the PARR-2 (PET and Recovery Following Revascularization–Phase 2) study, the first large randomized trial using an FDG-PET–guided approach to management of patients with coronary disease and severe left ventricular dysfunction. The primary end point (cardiac death, myocardial infarction [MI], or hospital stay for cardiac cause within 1 year of randomization) was negative; the event rate was 36% with standard care without FDG imaging and 30% with FDG imaging (relative risk: 0.82, p = 0.16) (Fig. 3). The observed event rates were lower than projected in the design of the trial, reducing its statistical power. Although the overall result of this first large randomized trial was inconclusive, and therefore disappointing, PET-guided therapy did demonstrate significant benefit in several patient subgroups.
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Figure 3 Outcomes With PET-Guided Therapy
Survival free of cardiovascular death, myocardial infarction, or hospital stay for cardiac cause within 1 year of randomization in the first large randomized trial of a positron emission tomography (PET)-guided approach to management of patients with coronary disease and severe left ventricular dysfunction. Although the event rate was reduced in the PET arm, this difference was not significant (relative risk: 0.82, p = 0.16). From Beanlands et al. (6), with permission.
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CT.
First-pass myocardial hypoenhancement, which can be seen during routine coronary computed tomography angiography (CTA), may be of value for the diagnosis of MI. Lessick et al. (7) evaluated 72 patients with acute chest pain who underwent CTA, and correlated first-pass myocardial hypoenhancement with the clinical diagnosis of MI. Three patients had to be excluded owing to excessive artifact; in the remaining 69 patients, CT demonstrated moderate accuracy, with 67% sensitivity (33 of 49) and 85% specificity (17 of 20) for the presence of infarction by clinical criteria (Fig. 4).
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Figure 4 Myocardial Infarction Assessment by CT
Short-axis post-contrast computed tomography (CT) images demonstrating a first-pass transmural perfusion defect in the posterolateral left ventricular wall (black arrows), characteristic of an acute myocardial infarction. From Lessick et al. (7), with permission.
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CT-SPECT.
Henneman et al. (8) compared the same technique—hypoenhancement on early (first-pass) CT images—with SPECT tetrofosmin imaging in 69 patients with previous MI and found that CT detected more infarcts than SPECT; semiquantitative infarct scores using the 2 techniques correlated closely (r = 0.93).
A different technique—delayed myocardial hyperenhancement CT—is also promising. Chiou et al. (9) performed delayed enhancement CT, SPECT thallium, and dobutamine echocardiography on 116 patients with recent MI. The CT imaging identified MI in 97 (96%) patients, compared with 88 (87%) patients by SPECT thallium. Using a pre-defined threshold of 50% segmental involvement by CT, there was modest concordance between CT and SPECT thallium (kappa = 0.55) and dobutamine echo (kappa = 0.50). Sato et al. (10) found that the presence and transmural extent of delayed enhancement in 52 patients with acute MI predicted poorer recovery of function and increased LV dilatation after revascularization.
CT-MRI.
Nieman et al. (11) compared both first-pass CT (n = 21) and delayed enhancement CT (n = 15) with MRI in patients with recent MI. First-pass perfusion defects were seen in all 21 patients on both CT and MRI but the CT defect was significantly larger (11 ± 6% of LV myocardium vs. 7 ± 4%). Delayed enhancement was demonstrated on all 15 MRI scans, but on only 11 of 15 CT scans. Three CT scans could not be evaluated. Contrast-to-noise ratio was much better with MRI (p < 0.001), suggesting a need for further research with CT.
MRI.
In the first international, multicenter trial of delayed enhancement imaging, Kim et al. (12) randomly allocated 282 patients with acute MI and 284 patients with chronic MI to different doses of gadolinium. They found the highest sensitivity (97%) for their highest dose of contrast (0.3 mmol/kg), and the lowest sensitivity (76%) in patients with acute MI in the lowest tertile of creatine kinase-myocardial band values, suggesting a lower limit for the detection of MI by MRI delayed enhancement imaging.
Although the presence of MI can be determined with delayed enhancement, the severity of the MI seems to be best measured with MRI-detected microvascular obstruction. Microvascular obstruction, usually demonstrated as a central dark region inside an area of delayed enhancement, is thought to reflect capillary damage, which restricts contrast access (Fig. 5) (13). Several studies have validated MRI-detected microvascular obstruction against coronary angiography, MRI perfusion, and pathology (Table 1) (14–18).
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Figure 5 Example of Microvascular Obstruction
Delayed-enhancement magnetic resonance imaging (MRI) scan, in a plane comparable to an echocardiographic parasternal long-axis view, shows a rim of increased signal in the inferolateral wall (black arrows) surrounding a dark, nonenhancing core (white arrow). The nonenhancing core represents microvascular obstruction. Modified from Nijeveldt et al. (13), with permission.
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MRI-SPECT.
In contrast, Nagao et al. (19) validated 201-thallium (201-Tl) SPECT and 99m-technetium-hydroxymethylenediphsphonate (99m-Tc-HMDP) SPECT using MRI-detected microvascular obstruction as the external standard in 40 patients with reperfused acute MI. Areas with evidence of infarction by both SPECT radiotracers (namely, both decreased 201-Tl and increased 99m-Tc-HMDP) corresponded to microvascular obstruction on MRI.
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Safety
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SPECT.
The dramatic increase in cardiac imaging during the past decade has led to concern over the increased cumulative population exposure to ionizing radiation. Einstein et al. (20) systematically reviewed the recent literature on this subject and provided a comprehensive review of radiation principles, terminology, and dosimetry, as well as suggestions for strategies to minimize radiation exposure during SPECT, PET, CT, and coronary angiography. Their tabulated data show a 10-fold range of estimated effective radiation dose for different nuclear cardiology procedures (Table 2) (20).
CT.
A second paper by Einstein et al. (21) reported dose estimates for individual organs from CTA using Monte Carlo simulations in mathematical phantoms and determined the lifetime attributable risk of cancer incidence associated with these dose estimates. Lifetime attributable risk primarily reflected the subsequent risk of lung and breast cancer and was highest among young women. Retrospective electrocardiography (ECG)-gating with tube current modulation reduced both the estimated effective dose and lifetime attributable risk by approximately one-third (21). As tube current modulation techniques are refined, further dose reductions are anticipated.
Other studies reported an even greater reduction in radiation dose using prospective ECG-triggering, a nonhelical "step-and-shoot" technique that does not involve the acquisition of redundant data and therefore limits multiphase (cine) reconstructions. Earls et al. (22) reported an 83% decrease in radiation dose (from 18.4 to 2.8 mSv) compared with retrospective ECG-gating without a decrease in image quality. Husmann et al. (23) demonstrated initial clinical feasibility of prospective ECG-triggering in 41 patients, with an average effective dose of 2.1 mSv.
The American College of Radiology published an important white paper on radiation in medicine, with a detailed list of recommendations for referring physicians, radiologists, technologists, and patients (24).
MRI.
Nephrogenic systemic sclerosis is a systemic disease involving fibrosis of multiple organs, particularly the skin, that has been linked to gadolinium exposure in patients with renal insufficiency (25). There have been numerous reports describing the development of nephrogenic systemic sclerosis in small numbers of patients with renal failure exposed to MRI contrast agents. The prevailing theory is that gadolinium dissociates from its carrier molecule and accumulates in the tissue of patients with renal failure because it is not cleared. Singh et al. (26), Swaminathan et al. (27), and Schroeder et al. (28) reported cases in which gadolinium deposits were detected in the skin and other organs in a total of 5 patients with nephrogenic systemic sclerosis. The European Society of Urogenital Radiology released guidelines regarding the use of gadolinium-containing MRI contrast agents (29), suggesting that these agents are contraindicated for patients with a glomerular filtration rate <30 ml/min and recommending caution for patients with a glomerular filtration rate of 30 to 60 ml/min.
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Coronary Artery Disease
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Diagnosis.
SPECT
Iskandrian et al. (30) reported a multicenter, double-blinded trial comparing stress with regadenoson, a new selective A2A receptor agonist, with stress with adenosine for 784 patients undergoing SPECT; regadenoson provided comparable diagnostic information with fewer side effects. Ragosta et al. (31) performed fractional flow reserve (FFR) measurements at the time of coronary angiography on 36 selected patients with prior SPECT sestamibi. They reported concordance between coronary angiography, FFR, and SPECT in 61 of 88 (69%) diseased vessels. In the remaining 27 vessels, SPECT underestimated disease burden compared with FFR.
Kane et al. (32) showed that a hypertensive response to exercise does not increase the prevalence of abnormal SPECT scans. Prosnitz et al. (33) extended a prospective study of patients receiving radiation for breast cancer and reported that SPECT perfusion defects were present in a majority of patients 3 to 6 years later.
PET
FDG-PET has increasingly been used to study myocardial glucose utilization in a variety of metabolic conditions. Naoumova et al. (34) reported a randomized trial of 26 patients with familial combined hyperlipidemia, in which pioglitazone improved myocardial blood flow and myocardial glucose utilization compared with placebo.
CT
Chung et al. (35) reported the 95% repeatability limits for CT coronary artery calcification on repeated scans in 3,380 subjects in the Multi-Ethnic Study of Atherosclerosis. The limits varied with body mass index and initial calcium score. The required change in Agatston score that would represent a definite change in the patient (rather than measurement error) was 68 for an initial score of 100 and 169 for an initial score of 400.
There were multiple new studies comparing CTA with catheterization (Tables 3 and 4) (36–56). Although the potential application of CTA for the noninvasive evaluation of noncalcified "soft" plaque is exciting, technical limitations remain. In a phantom study, Horiguchi et al. (57) demonstrated that low heart rates (50 beats/min), coronary enhancement of 250 HU, and plaque area of  50% of the lumen allowed accurate characterization of "fatty" and "fibrous" plaque by 64-row CTA.
Iriart et al. (58) compared 16-row CTA to intravascular ultrasound (IVUS) in 20 patients with acute coronary syndromes. CTA identified 79% of all noncalcified plaques (68 of 86) and 95% of culprit lesions (19 of 20), but could not detect plaque rupture seen by IVUS. Sun et al. (59) compared 64-row CTA to IVUS in 26 patients. CTA correctly identified 97% of coronary segments containing noncalcified plaque (86 of 89) and 90% of coronary segments without any plaque (118 of 131), but the differentiation of individual plaque components was problematic.
Multiple studies showed a clinically significant prevalence of extracardiac findings on cardiac CT (Table 5) (50,60–64). New studies on the diagnosis of chest pain in the emergency department were limited by small numbers of patients (Table 6) (65–68).
CT-SPECT
Van Werkhoven et al. (69) performed CTA in 97 patients with normal SPECT. Eighteen patients (19%) had significant coronary artery disease (CAD), but only 4 patients (4%) had left-main or 3-vessel CAD. Sato et al. (70) compared 64-slice CTA and stress thallium SPECT in 104 patients with stable angina and a low body mass index. The prevalence of ischemia by SPECT was 5% for stenosis severity <60%, 86% for stenosis severity 80%, and 44% for stenosis severity 60% to 80%. Lin et al. (71) compared CTA and SPECT in 163 low-intermediate risk patients; the extent, location, and composition of coronary plaque predicted abnormal SPECT.
CT-PET
Esteves et al. (72) reported that 34 of 84 (40%) low-intermediate risk patients in a chest pain unit had no detectable calcium. None of these patients had abnormal adenosine PET studies. Di Carli et al. (73) reported simultaneous PET-CT studies on 110 patients with suspected CAD. Almost one-half (47%) of stenoses of >70% by CTA were not associated with ischemia by PET.
MRI
The previous literature on MRI for diagnosis of CAD has been limited by small sample sizes. Nandalur (74) performed a meta-analysis of 37 small studies comparing 754 patients undergoing MRI stress wall-motion imaging and 1,516 patients undergoing MRI stress perfusion imaging with coronary angiography. Stress-induced wall-motion abnormality had a sensitivity of 83% and a specificity of 86%. Perfusion imaging had a sensitivity of 91% and a specificity of 81%. However, these favorable data were based on high-risk populations who underwent cardiac catheterization; data are limited for low-risk patients.
The high spatial resolution of MRI was used to better define the pathophysiology of 2 disease states. Lanza et al. (75) applied dobutamine stress MRI to 18 patients with cardiac syndrome X, who had typical angina, ST-depression associated with angina during a previous stress test, and angiographically normal coronary arteries. They reported perfusion defects in 10 of the 18 (56%) syndrome X patients but in none of 10 age-matched volunteers (p = 0.004), suggesting that patients with syndrome X have impairment of coronary microcirculation (75).
Selvanayagam et al. (76) reported 44 patients who underwent MRI before and after complex PCI. Twenty-five patients (63%) had new delayed enhancement at follow-up, suggesting myocardial injury during PCI. These damaged segments also had decreased perfusion reserve on adenosine stress MRI (Fig. 6).
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Figure 6 Decreased Perfusion Reserve
Decreased perfusion reserve in a patient with myocardial injury after percutaneous coronary intervention (PCI). On the left (A, B), magnetic resonance imaging (MRI) perfusion signal intensity curves in arbitrary units (a.u.) show that in a myocardial segment, perfusion at rest is equivalent before and after PCI (curves, A). After PCI, the stress hyperemic response is reduced (curves, B). On the right, delayed enhancement MRI images before PCI (top) and after PCI (bottom) show a new, small area of delayed enhancement in the inferolateral wall (white arrow), indicating myocardial injury. From Selvanayagam et al. (76), with permission. Solid lines = pre-PCI; dashed lines = post-PCI; circles = pre-PCI; squares = post-PCI. LV = left ventricle.
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Ebeling Barbier et al. (77) performed MRI on 248 volunteers older than age 70 years, randomly chosen from a Swedish municipality, to screen for CAD in the general population. Sixty patients (24%) had MI detected by MRI delayed enhancement imaging; 49 of these patients with MI were clinically unrecognized, and 11 had a clinical history of MI. Patients with a history of MI were significantly different from both the patients with unrecognized MI and those without MI, with respect to carotid intima media thickness, C-reactive protein level, and Framingham risk score (41).
Prognosis.
SPECT
The COURAGE (Clinical Outcomes Utilizing Revascularization and Aggressive Drug Evaluation) trial is the largest randomized trial ever reported in patients with chronic CAD. Shaw et al. (78) reported important results from the nuclear substudy. Of the 2,287 COURAGE patients, 314 underwent stress myocardial perfusion imaging before treatment and 6 to 18 months after randomization. The percent ischemic myocardium was assessed quantitatively by a blinded core laboratory. At follow-up, the mean reduction in ischemic myocardium was greater with percutaneous coronary intervention (PCI) and optimal medical therapy (–2.7%) than with optimal medical therapy alone (–0.5%) (Fig. 7). One-third of those treated with PCI (and optimal medical therapy) had a 5% reduction in ischemia, compared with 18.9% of patients treated with optimal medical therapy. This improvement in the reduction of ischemia with percutaneous intervention did not translate into an improvement in patient outcome in the much larger overall trial.
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Figure 7 Nuclear Substudy of the COURAGE Trial
In a nuclear substudy of the COURAGE (Clinical Outcomes Utilizing Revascularization Aggressive Drug Evaluation) trial, the change in percent ischemic myocardium assessed quantitatively by a blinded core laboratory is shown (left) for patients treated with percutaneous coronary intervention and optimal medical therapy and (right) for patients treated with optimal medical therapy. The mean reduction in ischemic myocardium was 2.7% in the percutaneous coronary intervention and optimal medical therapy groups, significantly >–0.5% seen in the optimal medical therapy group. From Shaw et al. (78), with permission. CI = confidence interval.
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Cardiovascular disease remains a major cause of mortality in patients with end-stage renal disease. Hage et al. (79) reported all-cause mortality in 3,698 patients with end-stage renal disease, 60% of whom underwent stress SPECT. Although patients with abnormal SPECT perfusion had worse outcomes, the best predictor of death was the ejection fraction measured by gated SPECT. Nishimura et al. (80) followed up 375 dialysis patients who underwent dual SPECT imaging using iodine-123 beta-methyl-iodophenyl-pentadecanoic acid (BMIPP) and thallium. Severely abnormal BMIPP images were highly associated with subsequent death (p < 0.0001) with a hazard ratio of 21.9. These provocative results require confirmation.
CT
Two large studies assessed the prognostic value of coronary calcium scoring in different ethnic groups. In 6,722 asymptomatic persons, Detrano et al. (81) found that the coronary calcium score predicted hard cardiac events independent of standard risk factors in 4 major racial and ethnic groups. Nasir et al. (82) showed that coronary calcium scores in 14,812 asymptomatic subjects had incremental prognostic value compared with the presence of conventional risk factors in all ethnic groups, but the relative risk ratio for higher calcium scores was greater for African Americans.
Min et al. (83) evaluated a single-center cohort of 1,127 patients who underwent CTA for the evaluation of chest pain. The presence, distribution, and severity of CAD identified by CTA were independent predictors of all-cause mortality. A normal coronary CTA predicted very low mortality (0.3%) compared with the overall mortality in the entire study population (3.5%).
Matsumoto et al. (84) used CTA to assess the prognostic significance of nonobstructive CAD. Acute coronary syndromes were more frequent in patients with low-density noncalcified coronary plaques (1.82%/year) compared with those without (0.86%).
CT-SPECT
Ramakrishna et al. (85) reported outcomes in 835 patients who underwent both SPECT and CT for coronary calcification. Both the SPECT summed stress score and the CT calcium score were independently associated with mortality, as well as other secondary cardiac end points.
Danciu et al. (86) performed CTA on 421 patients with intermediate-risk stress SPECT studies. Only 78 (18.5%) patients were sent to subsequent invasive coronary angiography. The remaining 343 (81.5%) patients were managed medically with infrequent (<2%) invasive coronary angiography and a single revascularization procedure over the next 15 months.
CT-PET
Schenker et al. (87) reported outcomes in 695 intermediate-risk patients who underwent both CT for coronary calcification and rubidium PET. There was a stepwise increase in deaths and MI with increasing calcium scores in patients with and without ischemia by PET.
MRI
The evidence for the prognostic value of MRI continues to expand. Bodi et al. (88) reported that the induction of wall-motion abnormalities (not perfusion abnormalities) on dipyridamole stress predicted death and nonfatal MI in 420 patients (p = 0.002, hazard ratio: 1.15) (Fig. 8). The authors did not address the potential mechanism of this unexpected finding, as dipyridamole is primarily a vasodilator, which is thought to infrequently cause ischemia or wall-motion abnormalities.
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Figure 8 Stress MRI Predicts Adverse Events
Survival curves adjusted for diabetes mellitus, hypertension, and previous myocardial infarction (MI) show that patients with magnetic resonance imaging dipyridamole stress-induced wall-motion abnormalities (dotted line) had more major adverse cardiac events (MACE), defined as cardiac death or nonfatal MI than patients without them (solid line). From Bodi et al. (88), with permission.
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Two articles, 1 by Lund et al. (13) and 1 by Nijveldt et al. (89), which employed MRI to predict remodeling after a first, acute ST-segment elevation MI, came to different conclusions about the relative importance of MRI-measured infarct size and microvascular obstruction.
In 55 patients, Lund et al. (89) reported that MRI-measured infarct size was the strongest predictor of remodeling (p < 0.001, odds ratio: 1.18), defined as a 20% or greater increase in left ventricular end-diastolic volume index after 8 months (Fig. 9). They also used receiver-operating characteristic analysis to compare infarct size and microvascular obstruction and found that infarct size was more predictive.
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Figure 9 Infarct Size Correlates With Remodeling
Infarct size by delayed enhancement magnetic resonance imaging plotted against change in left ventricular end-diastolic volume index (LVEDVI) 8 months after myocardial infarction shows a correlation (r = 0.56, p < 0.001). Vertical line shows a cut off of 24% infarct size, which was found to be the best cut off for predicting a 20% increase in LVEDVI. From Lund et al. (89), with permission.
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In contrast, in 53 patients, Nijveldt et al. (13) reported that the presence of microvascular obstruction was more predictive than infarct size of improvement in left ventricular ejection fraction and left ventricular end-systolic volume after 4 months (Fig. 10).
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Figure 10 MVO and Remodeling
Patients without microvascular obstruction (MVO) have an improvement (decrease) in end-systolic volume at follow-up (FU) compared with baseline (Base). Patients with MVO show no improvement. From Nijveldt et al. (13), with permission.
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Structure and Function
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SPECT.
The identification of patients who are most likely to benefit from cardiac resynchronization therapy remains a challenge. Trimble et al. (90) described a novel technique to develop normalized phase histograms from gated SPECT images and showed that multiple histogram indexes differed between normal controls and abnormal subjects. Henneman et al. (91) compared multiple parameters from phase analysis by gated SPECT, with LV dyssynchrony assessed by echocardiographic tissue Doppler imaging in 75 patients with heart failure who were candidates for cardiac resynchronization therapy. Histogram bandwidth and phase standard deviation both correlated well with echocardiographic left ventricular dyssynchrony (r = 0.89, p < 0.001; and r = 0.80, p < 0.001, respectively). A second study by Henneman et al. (92) reported that these 2 parameters predicted the response to cardiac resynchronization therapy at 6-month follow-up of 42 patients with severe heart failure.
CT.
The use of ECG-gated CT for the assessment of cardiac function remains a popular topic of investigation. This year marked the first pilot studies describing the use of dual-source CT for ventricular function. In a small series of 15 patients, Busch et al. (93) reported that CT modestly overestimated ejection fraction (3.8%) compared with MR with wide confidence limits (–15% to +22%). Brodoefel et al. (94) compared dual-source CT and MRI in 20 patients and reported no systematic difference in ejection fraction, with much narrower confidence limits (approximately –5% to +5%).
Several studies have described the use of contrast-enhanced CT for the detection of left atrial thrombus before pulmonary vein ablation for atrial fibrillation. Kim et al. (95) reported that CT had a sensitivity of 93% and a specificity of 85% for the detection of left atrial appendage thrombus or severe spontaneous echo contrast by transesophageal echocardiography in 223 patients; CT detected all cases of thrombus. Patel et al. (96) reported that CT had a sensitivity of 100% and a specificity of 72% compared with echocardiography in 72 patients. Shapiro et al. (97) reported that CT had a sensitivity of 80% and a specificity of 73%. Because CT is frequently done in these patients to define pulmonary venous anatomy, the detection of thrombus is a potential added benefit without additional cost.
MRI-CT.
Multiple studies evaluated valves with cardiac MRI and CT (Table 7) (98–103).
Tissue characterization by MRI could provide a definitive diagnosis of cardiac amyloidosis. Vogelsberg et al. (104) studied 33 patients, 15 of whom had cardiac amyloidosis confirmed by myocardial biopsy. A characteristic pattern of diffuse, endomyocardial delayed enhancement (Fig. 11) was 80% sensitive and 94% specific.
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Figure 11 Delayed Enhancement in Cardiac Amyloid
Short-axis delayed enhancement magnetic resonance imaging shows diffuse, subendocardial delayed enhancement, a pattern the authors found to be 80% sensitive and 94% specific for biopsy-proven cardiac amyloidosis. Modified from Voglesberg et al. (104), with permission.
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Petersen et al. (105) reported a comprehensive study of hypertrophy, fibrosis, and hyperemic myocardial blood flow in 35 patients with hypertrophic cardiomyopathy. Hyperemic blood flow was decreased, particularly in the endocardium, with increasing hypertrophy.
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Stem Cell Therapy
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The enormous literature in this emerging field is beyond the scope of this article. The fate and function of stem cells is an unresolved issue. Amsalem et al. (106) and Terrovitis et al. (107) showed that the residual MRI signal several weeks after injection of iron-labeled stem cells is in macrophages, not in viable stem cells. Several other investigators labeled and imaged stem cells, targeted to the cardiovascular system, using MRI and nuclear methods (Table 8) (106–109).
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Appropriateness/Utilization
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Both public and scientific interest in the appropriateness of cardiac imaging continues to increase. Gibbons et al. (110) reported the first application of the American College of Cardiology Foundation/American Society of Nuclear Cardiology appropriateness criteria for SPECT to 284 patients who underwent stress SPECT and 298 patients who underwent stress echocardiography in an academic medical center. Fourteen percent of stress SPECT studies and 18% of stress echo studies were inappropriate (Fig. 12). Askew et al. (111) examined the value of screening for CAD with SPECT in 374 asymptomatic patients with atrial fibrillation. SPECT had a low yield, similar to that in matched patients without atrial fibrillation, suggesting that the appropriateness criteria for this indication should be revised.
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Figure 12 Application of SPECT Appropriateness Criteria
The overall classification of 284 stress single-positron emission computed tomography (SPECT) studies and 298 stress echo studies at Mayo Clinic Rochester is shown, according to the American College of Cardiology Foundation/American Society of Nuclear Cardiology appropriateness criteria for stress SPECT myocardial perfusion imaging. The results for stress SPECT and stress echo were very similar. From Gibbons et al. (110), with permission.
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Gazelle et al. (112) examined the frequency of SPECT, PET, and gated blood pool imaging during >1 million episodes of outpatient care for cardiac disease in a large national health plan. Physicians referring patients for imaging to physicians of the same specialty were 3 times more likely to perform imaging than were physicians referring patients to radiologists, after adjustment for patient age and comorbidity score.
Two distinguished senior cardiologists published commentaries on the potential value of SPECT for screening asymptomatic diabetic patients. Diamond et al. (113) argued that statin treatment without SPECT imaging would save the nation 3.2 billion dollars annually. Beller (114) suggested that a sequential screening strategy utilizing CT coronary calcium scoring followed by SPECT merits further evaluation.
Merhige et al. (115) compared the frequency of subsequent coronary angiography, revascularization, and costs in 2,159 patients who underwent PET, and compared them with 2 separate control groups studied with SPECT. This provocative study found that PET produced considerable cost savings.
On a separate, but equally important topic, Cerqueira et al. (116) and Budoff et al. (117) published new American College of Cardiology Foundation standards for training in SPECT/PET and cardiac CT, respectively.
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Conclusions
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The authors continue to hope that this review will encourage you, the reader, to examine at least some of these articles in more detail, and to utilize an integrated, multimodality approach in the application of imaging to clinical problems.
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Footnotes
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Dr. Gibbons has received a research grant from King Pharmaceuticals, which is developing an adenosine agonist for pharmacologic stress testing, and is a consultant for Molecular Insight Pharmaceuticals. Dr. Williamson is a consultant for GE Medical and for Siemens, and he has received a research grant from and is a consultant for Bayer Medical.
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References
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