Multidetector-Row CT Imaging of Clinicaland Preclinical Coronary Atherosclerosis 35

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Multidetector-Row CT Imaging of Clinical and Preclinical Coronary Atherosclerosis

C

R. B

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From: Contemporary Cardiology: CT of the Heart:

Principles and Applications

Edited by: U. Joseph Schoepf © Humana Press, Inc., Totowa, NJ

PREREQUISITE FOR THE ASSESSMENT OF CORONARY ATHEROSCLEROSIS

Complete assessment of coronary atherosclerosis with multidetector-row computed tomography (MDCT) requires motion-free, contrast-enhanced images with the highest spatial resolution available. Because of the rather long exposure time with MDCT (approx 200 ms), patient preparation with β-blocker may be necessary. Optimal results will be achieved if the heart rate of the patient is below 60 beats per minute (bpm). The spatial resolution is given by the detector element size. The current reasonably achievable near isotropic spatial resolution is approx 0.4 mm³.

A timely, accurate, and homogenous vascular lumen enhancement is essential for full diagnostic capability of coro- nary MDCT angiography studies and to assess coronary ath- erosclerosis. Higher contrast enhancement is superior to identify small vessels in MDCT. However, coronary athero- sclerosis is commonly associated with calcifications that may interfere with dense contrast material and hinder the assess- ment of the residual lumen. In a randomized control group study, we investigated different contrast agent densities as well as peripheral venous injection rates to determine the optimal contrast protocol for the assessment of coronary atherosclero- sis. We found that a contrast medium flow rate of 1 g/s iodine resulted in an enhancement of approx 250–300 Hounsfield units (HU), which allows for delineation of intermediate (91 ± 21 HU) as well as highly dense plaques (419 ± 194 HU) (1).

The final vessel enhancement will also depend on physi- ological parameters such as cardiac output and central blood volume. The cardiac output and central blood volume (corre- lates with body weight) is inversely related with the final enhancement (2). It needs to be considered that in patients with heart rates above 60 bpm, β-blockers are now frequently applied to achieve good image quality (3,4). The consequent lower cardiac output may lead to a higher final enhancement in these patients. Because MDCT imaging of the coronary arteries is performed during the first pass of the contrast medium, the central blood volume plays a minor role for the final enhancement.

ASSESSMENT OF CORONARY ATHEROSCLEROSIS The morphology of calcifications may already give a first hint for the presence or absence of significant stenoses in the coronary arteries. From electron beam CT (EBCT) studies, Kajinami et al. reported that the positive predictive value for significant stenosis (=75%) was 0.04 and 0.17 in none, 0.18 and 0.59 in spotty (Fig. 1), 0.32 and 0.87 in long, 0.40 and 0.84 in wide, and 0.56 and 0.96 in diffuse (Fig. 2) coronary calcifica- tions, respectively (5).

Besides displaying the coronary artery lumen, MDCT as a cross-sectional modality is able to display the coronary artery wall. Coronary calcifications can easily be assessed even with- out contrast media, and represent an advanced stage of athero- sclerosis. However, as different stages of coronary atherosclerosis may be present simultaneously, calcifications may also be associated with lesions of more early stages of the disease. The entire extent of coronary atherosclerosis will be underestimated by assessing coronary calcifications alone (6). Calcified as well as noncalcified lesions can be completely assessed by contrast- enhanced MDCT of the coronary arteries.

In our initial experience, it seems that different histological stages of atherosclerosis will present with different morpho- logical pattern in MDCT. Because of the limited spatial resolution, it cannot be expected to assess a thinned fibrous cap (65 µm) that is approx 10 times beyond the current resolution of MDCT (750 µm). In addition, contrast uptake as an indicator of inflammatory processes is unlikely to be detected by MDCT. In extremely large atheromas, a lipid core may be visible with negative density values corresponding to fat. More commonly, atheromas may present as well-defined and homogenous humps in the coronary artery wall, without calcifications (Fig. 3). The density of these plaques may vary between 40 and 60 HU, and may reflect the ratio between lipid and fibrous tissue. These kinds of plaques are most often found in asymptomatic patients.

Plaque with densities above 80 HU may be considered to con- tain predominantly fibrous tissue. Fibrous plaques may also frequently be associated with calcifications (Fig. 4), indicating an advanced stage of atherosclerotic disease as well as coronary artery disease, and may therefore be found in patients with chronic and stable angina.

Other types of plaques (Table 1) present with low densities (20 HU) and inhomogeneous structure with irregular defined

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borders (Fig. 5). We have frequently observed these kinds of lesions in patients with acute and unstable angina. We currently believe that these plaques may correspond to intra-coronary thrombi. In some cases, the entire coronary artery may be filled with low-density material with a bright rim surrounding it, in- dicating a complete thrombus occlusion. Acute thrombus oc- clusion may lead to enlargement of the coronary artery vessel,

whereas in chronic vessel occlusion, organizing fibrous tissue may lead to shrinkage and increased density of the vessel.

As mentioned above, spotty, calcified lesions may com- monly be associated with minor wall changes in conventional coronary angiography (5). However, it is known that such cal- cified nodules may also be the source of unheralded plaque rupture and consequent thrombosis, and may lead to sudden Fig. 1. The positive predictive value of spotty coronary calcifications

(calcified nodules) for predicting coronary artery stenosis is between 18% and 59%.

Fig. 2. The positive predictive value of diffuse coronary calcifications is between 56% and 96%.

Fig. 3. Homogenous and well-defined lesion with approx 40-HU density in asymptomatic patients most likely corresponds to atheromas.

Fig. 4. Homogenous plaques with approx 100-HU density and calci- fications correspond most likely to fibro-calcified plaques and may be found in patients with chronic stable angina.

CHAPTER 35 / MDCT OF CLINICAL/PRECLINICAL CORONARY ATHEROSCLEROSIS 379

coronary death in very rare cases (7). Since atherosclerosis is a continuously ongoing process, different stages of the disease may be found simultaneously in the coronary artery wall. In such cases, coronary calcium may be found together with thrombus formation (Fig. 6).

QUANTIFICATION OF CORONARY ATHEROSCLEROSIS

As already shown in many coronary calcium studies, quantification may have an impact on the risk assessment and follow-up of patients with coronary atherosclerosis. Coronary calcifications can easily be detected and quantified even in the presence of contrast material if the density of calcium (>350 HU) is above the density of the contrast medium in the coronary artery lumen (250–300 HU). The mass of coronary calcifica- tions can best be determined by measuring the volume and density of the plaques. Using correction factors, the mass of calcium can be similarly derived from MDCT angiography

Table 1

Coronary Artery Plaque Entities and Morphological Appearance in Multidetector-Row CT

Plaque entity AHA type Calcification Density Shape Remodelling Symptoms

Atheroma IV No approx 40 HU Smooth Positive No

Fibroatheroma Va No approx 60 HU Smooth Positive/negative No

Fibrotic lesion Vc No approx 100 HU Smooth Positive/negative No

Fibrocalcified plaque Vb Yes approx 100 HU Smooth Negative Chronic stable angina

or absent

Thrombus VI No approx 20 HU Irregular High-grade stenosis Acute unstable angina

or occlusion

studies, as well as from coronary calcium screening studies obtained with no contrast and thicker slices (8).

However, because of partial-volume effects and the close relationship to the myocardium, it is much more difficult to quantify noncalcified plaques and vessel wall changes in the coronary arteries. In addition, with image reconstruction algo- rithms that are used to visualize the soft tissue in the coronary artery wall, dense material such as calcifications will become exaggerated. In patients with extensive coronary calcifications, noncalcified plaques are rarely found, most likely because the so-called blooming artifact of calcium prevents its assessment.

Therefore, the optimal quantification algorithm for determina- tion of the complete amount of noncalcified and calcified ath- erosclerosis by MDCT is still under development.

CONCLUSION

The newest generation of MDCT scanners now allows for consistently good image quality in patients with regular sinus Fig. 5. Irregular, nonhomogenous, and low-density plaques in the left

anterior descending coronary artery in a patient with acute coronary syndrome. The lesion most likely corresponds to a thrombus in the coronary artery.

Fig. 6. In a patient with acute coronary syndromes, a calcified nodule may have led to a consecutive thrombus formation (arrow).

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rhythm and heart rate in the range between 40 and 60 bpm. With targeted administration of contrast medium, an enhancement can be achieved that allows for simultaneous assessment of calcified as well as noncalcified atherosclerotic lesions in the coronary artery wall. Morphologic criteria allow for distinguish- ing between basic types of atherosclerosis such as thrombus, atheroma, or fibro-calcified plaques. In addition, calibrated HU allows for estimation of the predominant component of lesions.

These new features of coronary MDCT angiography may offer the chance to detect, quantify, and follow up vulnerable plaques with a high fraction of lipids. However, partial-volume effects with the myocardium and coronary calcifications may interfere with the detection of these plaques. Therefore, the optimal quan- tification algorithm is still under development. With these tools, large prospective cohort studies in an asymptomatic population will be necessary to determine the predictive value for future cardiac events and the change over time under therapy of athero- sclerosis detected by MDCT.

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