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Editorial |
1 Department of Radiology, Keck School of Medicine, University of Southern California, LAC/USC Imaging Science Center, 1744 Zonal Ave., Los Angeles, CA 90033.
Received April 3, 2006; accepted after revision April 3, 2006.
Address correspondence to P. M. Colletti
(colletti{at}usc.edu).
Keywords: cardiac CT angiography • cardiac imaging • cardiac MRI
Technical developments in MDCT and MRI are driving cardiac imaging applications. These methods and applications are exemplified in this Cardiac Imaging supplement.
Primum Non Nocere (First Do No Harm): The Safe and Efficient Performance of Cardiac Imaging
The application of strong magnetic fields and radiation requires optimal technique applications. Abada and colleagues [1] offer reduced cardiac CT angiography (CTA) patient radiation dose by applying an automatic modulation technique (ECG-pulsed tube current modulation) at a reduced kilovoltage setting (80 kV) for coronary CT. Radiation dose exposure can be reduced by up to 88% for slim patients without impairing image quality. Abada et al. estimate that this ECG-pulsed tube current modulation technique produces a reduced radiation exposure of 2 mSV, which is similar to or less than that of conventional coronary angiography [2].
Cardiac imagers administer a variety of pharmacologic interventional agents before and during imaging. These range from the harmless radiopharmaceuticals and gadolinium chelates to the tricky but relatively safe ß-blockers, dipyridamole, adenosine, and nitroglycerine, and to the more challenging dobutamine. In spite of all of the potential side effects of these agents, the most treacherous cardiac imaging intervention—the iodinated contrast agent—is well known to radiologists. Just as radiologists have developed a relative comfort level with iodinated contrast agents, so must we learn to work with adenosine and ß-blockers [3].
Pannu et al. [4] tell us how radiologists can implement a protocol so that ß-blockers can be safely given, orally or IV, to most patients to lower the heart rate for cardiac CT. Patients can be screened for certain contraindications: heart rate less than 60 bpm, systolic blood pressure less than 100 mm Hg, decompensated cardiac failure, allergy to ß-blockers, asthma or chronic obstructive pulmonary disease (COPD) using a ß2-agonist inhaler, active bronchospasm, and second- or third-degree atrioventricular block.
Perhaps the authors might have added a potential option for patients with COPD, active bronchospasm, or ß-blocker allergy: 60–90 mg of diltiazem orally 1 hour before scanning can be a safe and efficacious alternative to ß-blockers [5]. Pannu et al. go on to add [4]: Are these statements correct? We will see.
Although ß-blockers can help lower the heart rate, they also have a negative inotropic effect and can decrease left ventricular contractility.... This may impact assessment of ventricular function; however, currently ventricular contractility is typically evaluated by echocardiography or nuclear medicine studies and the role of CT is primarily to assess the coronary arteries.
Cardiac Function and Quantitation Required for Every Examination
The interstudy reproducibility coefficient of variability for cardiac MRI compared with echocardiography is statistically significantly superior for end-systolic volume, ejection fraction, and myocardial mass [6]. Considerably lower calculated sample sizes [6] are therefore required by cardiac MRI compared with echocardiography to show clinically relevant changes in left ventricular (LV) dimensions and function. Because of this, cardiac MRI is replacing echocardiography as a major end point determiner in translational and clinical cardiovascular research [7].
Given that cardiac MRI provides accurate and reliable parameters of cardiac function, the Multi-Ethnic Study of Atherosclerosis (MESA) multicenter trial [8] used cardiac MRI to identify normal values of LV function and myocardial mass in 400 men and 400 women in four age strata. Participants had a low cardiovascular risk profile: nonsmoker, systolic blood pressure less than 140 mm Hg, diastolic blood pressure less than 90 mm Hg, fasting glucose less than 110 mg/dL, total cholesterol less than 240 mg/dL, and high-density lipoprotein (HDL) cholesterol less than 40 mg/dL. A variety of scanners and techniques were used at the six participating sites. The convincing results are evidence for the robustness of cardiac MRI volumetry.
Significant differences in LV volumes and mass between men and women were identified. LV volumes were inversely associated with age for both sexes except for the LV end-systolic volume index. For men, LV mass was inversely associated with age, but LV mass index was not associated with age. For women, LV mass and LV mass index were not associated with age. LV mass was largest in the African-American group and was smallest in the Asian-American group.
The normal LV differs in volume and mass between sexes and among certain ethnic groups. When indexed by body surface area, LV mass was independent of age for both sexes. Studies that assess cardiovascular risk factors in relation to cardiac function and structure need to account for these normal variations in the population.
Further given that cardiac MRI is the gold standard for cardiac motility and volumetry [6, 7], can cardiac CT produce similar information? If one processes ECG-triggered cardiac CT data throughout the cardiac cycle, not only is it possible to optimize individual coronary artery visualization, but cardiac motility and function may also be performed. Juergens et al. [9] show two important concepts: First, LV volumetry and ejection fraction with reformatted 16-MDCT short-axis views correlate well with LV volumes on cardiac MRI. This is in spite of most patients receiving 80 mg of propranolol orally 45 min before MDCT. Second, LV volumes and ejection fractions may be calculated using direct axial CT data. Measured volumes are apparently increased by 7–11%, and ejection fraction differences are minimal, because of increases in both end-diastolic and end-systolic volumes, relatively negating effects on ejection fraction.
The importance of quantitative cardiac function for risk stratification cannot be over-stated. A patient with cardiomyopathy and an ejection fraction of 25% has a substantially greater likelihood of a significant cardiac event, including sudden death, than a patient with a 45% ejection fraction. According to University of Southern California Keck School of Medicine professor Leslie Saxon, "Everyone should know their ejection fraction, just like they know their cholesterol level, blood pressure and other measures of cardiovascular health" [10]. Indeed, for most Medicare patients with cardiomyopathy and an ejection fraction of 30% or less, the federal government provides expanded coverage of implantable cardioverter defibrillators (ICDs). Cardiac quantitation helps with cardiac intervention decision making.
With this background, it is clear that physicians should seek to determine reliable cardiac quantitation whenever possible. Echocardiography reports generally include substantial quantitation. We do routinely quantitate SPECT and PET (for those of us with rubidium programs) because it is easy and automatic. It remains common to see cardiac MRI and CT reports with little or no quantitation. Why is this? Simply, cardiac quantitation with MRI has been operator-intensive. With respect to automatic region-of-interest detection for cardiac MRI, "...visually checked and manually corrected if necessary," "if necessary" can be replaced by "most of the time." I base this on experience with more than 1 million cardiac regions of interest using the common MASS software (Medis).
If gated MDCT can reliably evaluate LV function, might it also be used to evaluate right ventricular (RV) function? Dogan et al. [11] show in 15 patients that axial 16-MDCT with no ß-blocker, using a 30- to 35-sec administration of contrast agent at 4 mL/sec, can be used for RV quantitation. Comparison of LV and RV stroke volumes showed a mean difference of 3.6 mL.
How can this be useful? The ventricular stroke volume difference may be used in single-lesion situations to estimate valvular regurgitation—a regurgitant index—and cardiac shunts—a Qp/Qs flow ratio. Pulmonary CTA has shown that RV diameter on axial views and percentage of cross-sectional area of pulmonary emboli compared with total pulmonary artery area on a central slice correlate with outcomes [12].
Pulmonary CTA has become the examination of choice for the detection of pulmonary emboli. Direct evidence for acute emboli is seen as intraluminal filling defects. Wittram et al. [13] show the CTA findings in acute and chronic pulmonary emboli. Acute emboli may pack the pulmonary artery, with local expansion and constriction of the downstream vessels, whereas chronic emboli coat the pulmonary arterial walls.
If CT volumetry is possible, can regional wall motion and thickening be evaluated qualitatively? It is instructive to view the AJR Online 4D cardiac CT videos by Lawler et al. [14]. Clearly, cardiac motion can be shown on ECG-gated CT. Boll et al. [15] compare quantitative regional wall motion on 16-MDCT with steady-state free precession (SSFP) cine cardiac MRI in 20 patients. Again, this study was performed without ß-blockers or other means for heart rate control. This study showed that cardiac motility assessment based on ECG-gated 16-MDCT and MRI data sets offers comparable regional ventricular function results for normofrequent patients. Boll et al. clarify that the high spatial resolution of cardiac MDCT cannot compensate for limited temporal resolution in patients with tachycardia. It is therefore important to report ventricular motility analysis results in combination with heart rate to allow consideration of this possible cause for measurement variation.
It is reasonable to call for consistency in reporting cardiac imaging. There is an agreed on "standardized myocardial segmentation and nomenclature for tomographic imaging of the heart: a statement for healthcare professionals" from the Cardiac Imaging Committee of the Council on Clinical Cardiology of the American Heart Association (AHA) [16]. This is a standardized format for radionuclide imaging, echocardiography, MRI, and CT of the heart based on a 17-segment model. Boll et al. [15] refer to this model and then go on to present data using 18 segments rotated 30° anticlockwise from the AHA model.
Quantitation may also be useful for risk stratification in aortic disease. Sueyoshi et al. [17] used MDCT to monitor 54 patients with type B intramural hematoma for 5–136 months. The affected aortas tended to increase in size after 1 year from onset. An initial diameter of 40 mm or greater and the presence of blood flow in the false lumen were important risk factors for enlargement during the follow-up period.
Coronary Artery Imaging: The Ultimate Challenge
Raptopoulos et al. [18] compared four ECG-gated and nongated MDCT angiography protocols based on monitoring vascular attenuation in four groups of 20 patients with acute chest pain caused by vascular, nonvascular, and cardiac abnormalities. The chest pain protocol (200 H at the pulmonary artery) can be used to assess both the pulmonary arteries and the thoracic aorta, whereas the ECG-gating protocol (150 H at the pulmonary artery) appears to be a promising adjunct for a comprehensive single chest pain protocol.
Vernhet-Kovacsik et al. [19] used 16-MDCT (without ß-blockers) to evaluate 29 bypass grafts in 19 patients 5–14 days after off-pump surgery. Two patients had a greater than 50% distal anastomotic stenosis on both MDCT and conventional coronary angiography; and in one patient with an LIMA–LAD (left internal mammary artery–left anterior descending artery) bypass graft, the LAD had a dissection at the distal anastomosis, with stenosis greater than 50% detected on coronary angiography but missed on MDCT.
Kantarci et al. [20] discuss 22 cases of myocardial bridging in 626 patients who were examined with 16-MDCT (nitroglycerin, 5 mg sublingual, 1 min before the examination; and metoprolol tartrate, 5 mg/mL IV bolus). Fifteen cases of myocardial bridging were located at the middle third of the LAD, five at the distal third of the LAD, and two at the proximal third of the LAD. In these patients, the length of tunneled artery was 6–22 mm, and the depth of tunneled artery was 1.2–3.3 mm.
One may be able to identify lipid-rich plaque on unenhanced cardiac CT. Dey et al. [21] apply automated software to identify lipid-rich regions of coronary artery plaque on unenhanced cardiac CT. The mean lipid density was –19.6 ± 3.0 (SD) H in a low-risk group, –25.3 ± 8.2 H in the high-risk group, and –34.3 ± 13.0 H in the calcium group. In five patients, the locations of lipid-rich plaque correlated well with available intravascular sonography findings.
Finally, with all the interest in coronary CTA, it is not surprising that MRI coronary techniques continue to develop. Katoh et al. [22] performed radial k-space sampling and SSFP coronary artery imaging using three high-resolution navigator-gated and cardiac-triggered 3D black blood sequences (cartesian gradient-echo [GRE], radial GRE, and radial SSFP) with identical spatial resolution (0.9 x 0.9 x 2.4 m3) in 11 healthy participants. Radial k-space sampling resulted in fewer motion artifacts and improved signal-to-noise and contrast-to-noise ratios. Although SSFP imaging did not help, consistently good image quality and improved coronary vessel wall visualization were seen with the radial GRE sequence. Such techniques may compete with coronary CTA for investigational and clinical coronary vessel wall analysis.
References
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