Non-invasive Coronary Imaging with Multidetector Computed Tomography

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Citation
ICR 2006;2006:1(1):26-27

Rapid advancement of technology has facilitated computerised tomography (CT) coronary angiography (CTCA) to move from being a research tool to being a clinically important diagnostic entity. Whilst the initial four detector technology required relatively long acquisition times and reconstruction of the images took considerable time, the newer generation scanners easily cover the heart in a single breath hold and rapidly offer images and reconstructions. Multidetector CT coronary angiography (MDCTA) can be used as a screening tool, including calcium scoring for risk assessment. It can identify patients with normal coronary arteries with great accuracy. The challenge now lies in establishing the role of MDCTA in clinical practice. Ideally, MDCTA would replace invasive coronary angiography (ICA) in certain target populations.

Current Technology

Following on from the initial 4-slice CT scanners, the 16-slice MDCT had faster gantry rotation improved temporal resolution and reduced breath hold to about 20 seconds. While the temporal resolution can be halved to 93ms with two-segment reconstruction, this impairs the image quality. The relatively poor temporal and z-axis resolution potentiates blooming artefact from coronary stents and coronary calcification.

With 64-slice CT scanners, the data for coronary angiography can be obtained in about five seconds - four times faster than 16-slice MDCT and 16 times faster than the 4-slice MDCT of less than 10 years ago. This has obvious benefits and allows the physician to acquire the images a few seconds after the breath hold begins, when the heart rate often becomes more stable. Acquisition over a fewer number of heart beats ensures a lower risk of ectopic and other heart-rate related artefacts.

The fast acquisition time means that slower table feeds and narrower collimation can be used without substantially increasing the breath hold. This allows tiny isotropic voxels (0.4 x 0.4 x 0.4mm) to be obtained routinely, producing spatial resolution of 15- 30 line pairs per centimetre. This is similar to the magnified field in standard coronary angiography and allows 3D reconstructions in any plane with little z-axis blur. The manufacturers have also improved gantry rotation to 330ms, yielding a temporal resolution of 165ms for standard single segment reconstruction.

The most recent development in CT is a dual source CT (DSCT). Two x-ray sources and two detector panels with a rotation time of 0.33sec acquire images with a temporal resolution of 83ms. Initial experience suggests that this technology has further improved image quality for CTCA by reducing motion artefacts and is less dependent on a low heart rate.1 Furthermore, the radiation dose is significantly reduced.

Calcium Scoring

Initially performed by electron beam CT, this investigation can be undertaken with MDCT without using contrast media. The amount of calcium found in the coronary tree correlates with the risk of a future cardiovascular event. Recently published guidelines on prevention of sudden cardiac death, suggest a role of the calcium score in patients with intermediate risk profiles. In this patient cohort, an increased calcium score adds to the risk profile and can trigger preventive treatment.2 The routine assessment of the coronary calcium score is still under debate.3 Additional information, independent of the conventional risk scores, can be obtained. However, population screening would prove expensive and comes with the additional risk of radiation exposure. It remains to be seen, whether larger population data will further support a more liberal use of calcium scoring.

Measurement of coronary calcification may have a role as a gatekeeper to CTCA. Heuschmid et al.4 showed (in 37 patients using 16 slice CTCA) that if analysis was limited to patients with an Agatson score equivalent of Ôëñ1000, sensitivity for significant stenoses improved 59-93%, specificity, 87-94%, positive predictive value, 61-68% and negative predictive value, 87-99%.

It is suggested to limit contrast CT angiography to patients with a low calcium score in order to avoid studies with low diagnostic accuracy.

Coronary Angiography

Several studies have evaluated the use of 4-, 16- and 64-slice MDCTA for the detection of coronary artery stenoses.5-9 MDCTA tends to overestimate stenoses compared with invasive coronary angiography and overall has positive predictive values of about 80% and negative predictive values about 95%. It therefore has a role in ruling out significant coronary artery disease in those with a low pre-test probability. As many studies included patients with a high pre-test probability of coronary artery disease, the results in low risk groups may, in fact, be even better.

Lim et al.,11 in a prospective study of 30 consecutive patients, showed that a 40-slice MDCT had a sensitivity and specificity of 99% and 98%, respectively for showing the 94 significant stenosis in 480 segments detected on ICA. The negative predictive value was 99%. Ferencik et al.10 produced similar results with a 64-slice MDCT With latest generation scanners, Achenbach et al.1 showed that, in selected patients, 98% of coronary artery segments could be visualised free of motion artefact using a dual source CT.

For bypass grafts, MDCTA offers high diagnostic value. Several studies showed a 100% sensitivity for the detection of graft occlusion and high sensitivities, and specificities for significant graft stenoses.12-15 Pache et al.15 showed that in 36 patients who had both MDCTA and ICA, all grafts and 93 out of 96 of distal anastomoses could be visualised. The three non-visualised anastomoses were due to metal-clip artefact. Patients with atrial fibrillation and heart rates uncontrolled by beta-blockers were included. Three grafts were missed by ICA, but not by CTCA. All graft occlusions on CTCA were confirmed by ICA. Overall sensitivity for detecting significant stenoses for CTCA was 97.8% with a specificity of 89.3%.

For the detection of in-stent restenosis data are generally limited. Cademartiri et al.16 showed that in 74 stents, six had a significant stenoses. Five out of six were detected by 16-slice MDCT. Sixty-seven out of 68 were correctly identified as non-significant restenosis. The two errors were in two mm vessels.

Diagnostic accuracy continues to be limited in certain patient groups and lesion subsets. The major denominators of reduced diagnostic yield are arrhythmia, calcification, intracoronary stents, small arteries and metal clips in graft studies.

MDCTA is a high dose radiation test and is unsuitable for screening or regular follow up. Depending on technique, effective doses are generally in the order of 5-15mSv. In general, doses are greater than expertly performed ICA.17 There are similar risks from iodinated contrast, but clearly no catheter-related morbidity or mortality.

The routine application for MDCT coronary angiography is still limited to selected patients and good image quality depends on pristine protocols and commitment to good post-processing.

In patients with a low probability of coronary disease, presence of sinus rhythm and absence of a tachycardia, MDCTA can reliably exclude coronary artery disease and invasive coronary angiography can be avoided. This is especially true for patients who are scheduled to undergo cardiac, but non-coronary surgery and require a coronary study prior to their operation. Furthermore, patency of bypass grafts can be documented with good accuracy. The use of MDCT coronary angiography for the evaluation and triage of patients with acute chest pain can not yet be recommended Patients with proven ischaemia or a higher pre-test probability of significant coronary artery disease should be referred for ICA with a view to revascularisation.

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