Elsevier

Progress in Cardiovascular Diseases

Volume 46, Issue 2, September–October 2003, Pages 123-134
Progress in Cardiovascular Diseases

Tomographic plaque imaging with CT: technical considerations and capabilities

https://doi.org/10.1016/S0033-0620(03)00082-3Get rights and content

Abstract

X-ray computed tomography (CT) is widely available in the world and has the ability to provide high-definition, thin-section imaging of any body part. In particular, CT over the past decade has been shown in numerous publications to allow for quantitation of coronary calcification, a proven surrogate for coronary artery atheromatous plaque. Electron beam tomography (EBT) and multidetector CT (MDCT) have been studied for these purposes. However, there are methodological differences between types of CT scanners and precision of calcium scoring is a function of their individual technical capabilities and limitations. These technical aspects are detailed here. Although MDCT has shown considerable improvements in recent years, EBT remains the clinical reference standard for noninvasive definition of atherosclerotic plaque.

Section snippets

Coronary calcium and atherosclerotic plaque

Recent studies have confirmed that arterial calcium development is intimately associated with vascular injury and atherosclerotic plaque evolution and is largely controlled by common cellular and subcellular mechanisms.1, 2, 3, 4 Calcium can be seen in all degrees of atherosclerotic involvement and is an active process. Thus, the long held notion of so-called “degenerative” calcification of the coronary arteries with aging is not correct.

The incidence of coronary artery calcium by CT as a

CT methods

This section will discuss methods related to coronary artery calcium identification. Specific methods of contrast-enhanced coronary lumen imaging using EBT and MDCT are discussed elsewhere.

Variability and calibration

When comparing measurement devices, calibration to an external standard is crucial for comparability between both EBT and MDCT over time. Significant variability in the measurement of the 130 HU threshold has been documented within and between EBT scanners even when serviced and maintained at the same site.31 Use of an external calibration phantom was shown to reduce scanner variation with EBT by 25%. Because there may be measurement errors both within the same and between different EBT systems

Imaging speed/temporal resolution

Overall CT image quality is dependent on multiple factors throughout the imaging sequence. These include image noise, blurring, spatial resolution, and other factors related to both the imaging device and patient. In the case of measuring calcified plaques of various sizes, temporal resolution, spatial resolution, and image noise are important to varying degrees. Cardiac CT is dependent on having a high temporal resolution to minimize coronary motion. By coupling rapid image acquisition with

Studies comparing EBT and MDCT for calcium scoring

Becker et al studied 100 patients comparing MDCT with EBT and reported a high correlation between the 2 modalities.43 In this study, however, the percent variability between individuals was 32% for CAC scores. There were relatively few patients with scores <100, and the high correlation may have been driven by those individuals with high scores. Moreover, the level of individual precision was limited and the scores <100 appeared to have the most deviation by MDCT as compared with EBT. Although

Signal versus noise

Early detection of calcified plaque is dependent on distinguishing the plaque from image noise. MDCT systems have reduced image noise compared with EBT systems. Image noise with EBT has been shown to have an association with body mass index,47 which may result in falsely identifying noise as calcified plaque or overestimation of true plaque burden. On the other hand, the image blurring by MDCT may result in false-negative studies or underestimation of true plaque burden.

Prospective gated

Radiation exposure

One drawback of MDCT as compared with EBT is the higher radiation exposure to the patient.32, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58 Radiation exposure from prospectively gated studies is much less than from retrospectively gated studies. The x-ray photon flux expressed by the product of x-ray tube current and exposure time (mA) is generally higher with MDCT. For example, 400 mA with 0.5-second exposure time yields 200 mA in MDCT versus 614 mA (fixed tube current) with 0.1-second exposure time

Reproducibility of calcium scoring

A promise of these technologies is to accurately measure atherosclerosis burden and to track changes over time to assess efficacy of therapy.59 This ability to assess progression is dependent on the reproducibility of the technologies. EBT interscan reproducibility has been shown to be approximately 10%, with interreader variability approximately 3% and intra-reader variability <1%.60, 61 This has been significantly more problematic with MDCT. The interscan variability in several studies is 32%

The issue of slice thickness

The Agatston calcium scoring was designed as an area measurement and is predicated on a 3.0-mm slice thickness. Although this is likely not the ideal tomographic slice thickness for coronary artery imaging, it was chosen historically because the original EBT scanning system in use at the time for research had 3.0 mm as the thinnest slice available. Current EBT systems are now able to perform scanning at 1.5 mm, and the latest MDCT systems can provide slice thicknesses that are ≤1 mm. However,

Summary and conclusions

EBT has undergone rigorous testing for reliability and validity and has proven to be useful in identifying individuals with, or at risk for, CHD. Although MDCT is a promising tool for coronary calcium scoring, more studies are needed comparing EBT and helical CT scans in the same patients, especially with calcium scores <100. MDCT studies evaluating progression, reproducibility, and plaque burden determined by independent methods like intravascular ultrasound or autopsy and outcomes studies are

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