F

Figure 3-14. First-pass gadolinium DTPA myocardial perfusion study. From top to bottom, sequential cross-sectional images obtained at the base (left), midlevel (middle), and apex (right) of the heart. The first row of images is acquired before arrival of the contrast agent. The second row demonstrates arrival to the right ventricle. The third row shows arrival to the left ventricular cavity, and the fourth row shows enhancement of the myocardium. The arrows demonstrate an area of subendocardial hypoenhancement in a patient with severe stenosis of a large marginal branch.

Figure 3-14. First-pass gadolinium DTPA myocardial perfusion study. From top to bottom, sequential cross-sectional images obtained at the base (left), midlevel (middle), and apex (right) of the heart. The first row of images is acquired before arrival of the contrast agent. The second row demonstrates arrival to the right ventricle. The third row shows arrival to the left ventricular cavity, and the fourth row shows enhancement of the myocardium. The arrows demonstrate an area of subendocardial hypoenhancement in a patient with severe stenosis of a large marginal branch.

visualization of the coronary arteries. Moreover, the rate of technologic advancement leading to improved coronary angiography with MDCT has rapidly exceeded that of electron-beam computed tomography (EBCT) or MRI. Image quality is undergoing constant refinement, and the number of uninterpretable coronary studies has gradually decreased, from 20% to 40% with the 4-detector system, to 15% to 25% with 16 detectors, and now as low as 3% to 10% with 64-detector systems.

MF 2.02 TT 612.5 TI 260 0 TR 720.0 TE 1.1 TA 00.21-8 BW 1130 0 MÍND/NORM

SP A72.2 SL 8 0 FoV 303*330 141-1925 Corí-Sag(34.2)í-Tra(.29.3)

Figure 3-15. Cross-sectional image at the middle of the left ventricle obtained 20 minutes after injection of gadolinium DTPA demonstrates a large area of subendocardial fibrosis (white rim indicated by arrows) involving 50% of transmural thickness in the septum and anterior walls.

Calcium Scoring

With EBCT scanners, the most widely used measure of calcium burden has been the calcium score, based on a radiographic density-weighted volume of plaques with pixel numbers of a least 130 Hounsfield units (HU). More recently, MDCT has been shown to provide comparable and reproducible results. The prognostic value of coronary calcification has been clearly established. Keelan and colleagues25 demonstrated that a coronary calcium Agatston score greater than 100 was an independent predictor (odds ratio [OR] = 1.88) of cardiovascular outcomes (death and nonfatal myocardial infarction) at 7 years' follow-up. Although very high calcium scores impart an approximate 10-fold increased risk, they do not always imply a tight coronary stenosis. The role of EBCT screening of asymptomatic individuals is controversial, and the incorporation of this type of investigation into a comprehensive risk screening with CRP and cholesterol measurements is ongoing. There is some evidence to support the incorporation of calcium scores into an overall risk stratification of older individuals using clinical algorithms such as the Framingham Risk Score. In the South Bay Heart Watch study,26 a calcium score of more than 300 was associated with a significant increase in coronary heart disease event risk compared with that determined by clinical score alone. These data support the hypothesis that high coronary calcium scores can modify predicted risk, especially among patients in the intermediate-risk category, for whom clinical

All dysfunctional segments

0

1-25

26-50

1 1

51-75

Segments with severe hypokinesia, akinesia, or dyskinesia

Segments with akinesia or dyskinesia

Figure 3-16. Relation between the transmural extent (percentage) of hyperenhancement before revascularization and the likelihood of functional recovery (N = 804 dysfunctional segments in 50 patients). (From Kim RJ, Wu E, Rafael A, et al: The use of contrast-enhanced magnetic resonance imaging to identify reversible myocardial dysfunction. N Engl J Med 2000;343:1445-1453.)

decision making is the most difficult. Those at low risk by clinical score derived no additional benefit from calcium scoring. The use of the calcium score scanning to improve cardiovascular risk prediction in people with no cardiac symptoms who are at low absolute risk is very expensive. Some even suggest that its wide clinical implementation can in aggregate have a detrimental effect on the quality of life of screened populations.27

Computed Tomographic Coronary Angiography

Although the actual acquisition of MDCT studies takes less than 15 seconds, patient preparation and data interpretation require extensive training and extreme attention to detail. Even in expert hands, the average time required for interpretation exceeds by far the time required for interpretation of nuclear perfusion or echocardiographic studies.

Patient selection is critical when performing MDCT coronary angiography. A stable, low HR is required at the time of the procedure, because motion artifacts can occur, given current limitations in temporal resolution of existing scanners. Oral and/or intravenous P-blockers are administered before the study to obtain, ideally, a resting HR lower than 60 beats/min. P-Blockers reduce HR variability during the scan, and for that reason we recommend their administration almost routinely, unless contraindicated.

MDCT provides complex and detailed 3D data sets, which are reconstructed from the raw data file, according to specific phases of the cardiac cycles. In most patients with a heart rate lower than 70 beats/ min, the best phase free of motion is centered on 75% of the R-R interval, corresponding to the diasta-sis phase of diastole. At higher rates, diastasis disappears, so image reconstruction at about 50% of the cardiac cycle is preferred. Nevertheless, there is significant patient-to-patient variability, and often several phases reconstructed at 5% to 10% intervals need to be examined.

Once the best phase for analysis is determined, examination of each vessel is performed from 3D multiplanar reconstructed images. Careful adjustment of image windowing parameters is done to differentiate the iodine-enhanced lumen from calcified and noncalcified plaques. This procedure is repeated for each vessel segment and its branches.

Both EBCT and MDCT are very useful in assessing the origin and course of congenitally anomalous coronary arteries and the 3D relationship of such arteries with the aorta and the pulmonary arterial trunk.28-30 Myocardial bridges and coronary arterial-venous fistulas can also be well visualized by EBCT and MDCT.

Evaluation of Coronary Artery Stenosis by MDCT

Figures 3-17 and 3-18 are MDCT coronary angio-grams obtained, respectively, from a patient with normal coronaries and a patient with severe multi-vessel disease. The corresponding invasive angiogram

Figure 3-17. Multidetector computed tomographic (MDCT) coronary angiography showing normal coronary arteries. A, Volume-rendered maximum-intensity projection of the aortic root and coronary arteries. B, Curved multiplanar reconstruction of the left anterior descending coronary artery (LAD). C, Series of cross-sectional images obtained from the mid-LAD at 1-mm intervals.

Figure 3-17. Multidetector computed tomographic (MDCT) coronary angiography showing normal coronary arteries. A, Volume-rendered maximum-intensity projection of the aortic root and coronary arteries. B, Curved multiplanar reconstruction of the left anterior descending coronary artery (LAD). C, Series of cross-sectional images obtained from the mid-LAD at 1-mm intervals.

from the latter individual is shown in Figure 3-19. Several single-center studies have investigated the accuracy of MDCT coronary angiography for the detection of coronary artery stenosis in patients with known or suspected CAD referred for invasive coronary angiography.31-41 In all of these studies, analysis of the MDCT data was performed by investigators blinded to the results of invasive angiography, and in many, it was limited to coronary segments of more than 1.5 or 2 mm in diameter. In most cases, significant coronary artery stenosis was defined as a reduction in diameter of more than 50%, in order to define sensitivity, specificity and positive and negative predictive values. The prevalence of significant CAD in patients enrolled in these studies was 53% to 83%. A few studies have also reported the performance characteristics of MDCT, using each patient as the unit of analysis. Based on these single studies, the sensitivity of MDCT coronary angiography ranged between 72% and 95% per coronary segment, and between 85% and 100% when using each patient as the denominator unit. The specificity per segment has been reported to be between 86% and 98%, and between 78% and 86% per patient. Positive predictive values have ranged from 72% to 90% per segment and from 81% to 97% per patient; negative predictive values were between 97% and 99% per segment and 82% and 100% per patient. As expected, sensitivity was higher in those studies that excluded segments with a diameter smaller than 1.5 mm. Most experts agree that the ability to detect obstructive coronary disease in smaller-caliber vessels is less important, because myocardial revascularization is often not required or cannot be performed.

We recently completed a multicenter trial that studied the accuracy of MDCT coronary angiography performed with 16-slice scanners.42 We enrolled 238 patients with high or intermediate risk who were clinically referred for diagnostic angiography. Patients first underwent a calcium score scan, followed by MDCT angiography if the Agatston calcium score was less than 600, before invasive angiography. Coronary angiography and MDCT data sets were quantitatively analyzed by blinded independent core laboratories. Among the 187 patients who underwent MDCT, there were 89 segments (5.5%) in 59 patients (32%) with stenosis greater than 50% by conventional angiography. Of 1629 segments larger than 2 mm in diameter, 71% were evaluable on MDCT. All nonevaluable segments were censored "positive," because in clinical practice they would also lead to performance of angiography. Using this

19 Apr 2005 hl 12:03:51.0

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