The role of Computed Tomography (CT) of the coronary arteries in the diagnosis and treatment of coronary artery disease remains is in evolution. Yet in several areas of the world, this technique is already in widespread use, and it is not yet known what effect this has had on other areas of the diagnosis and management of coronary artery disease. This study looked at the impact of the utlization of coronary CT on other diagnostic modalities in the same centre.

1053 consecutive patients underwent coronary CT after the installation of a new 64-slice Siemens Somatom in Columbus, Ohio. The yearly procedural volumes in cardiac catheterization and noninvasive stress testing were reviewed both before and after the introduction of the CT machine. Overall, only 91 patients (8.6%) of the 1053 patients scanned went on to have invasive diagnostic catheterization, and of these, 48% required percutaneous or surgical intervention. This did not represent an increase in procedures over previous years. However, a 30% decrease was noted in patients referred for non-invasive stress testing.

As very few patients who underwent coronary CT went on to cardiac catheterization, the study highlights the high negative predictive value of coronary CT. However, current guidelines recommend noninvasive stress testing to confirm myocardial ischaemia before percutaneous coronary intervention. This small study also reminds cardiologists that even if coronary disease is detected using CT, other non-invasive stress testing must still be used when appropriate.

· Alex J. Auseon, DO, Sunil S. Advani, MD, Charles A. Bush, MD, Subha V. Raman, MD, MSEE. Impact of 64-Slice Multidetector Computed Tomography on Other Diagnostic Studies for Coronary Artery Disease. Am J Med 122(4);388-391.

The potential risks from exposure to ionizing radiation during cardiac computed tomography angiography (CCTA) have led many to question the role of this technique. The Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice I (PROTECTION I) was an observational study of estimated radiation dose in 1965 patients undergoing CCTA over an 11 month period.

The median dose-length product (DLP) of 1965 CCTA examinations performed at 50 study sites was 885mGy per cm, which corresponds to an estimated radiation dose of 12mSv. This equates to 1.2 times the dose of an abdominal CT study, or 600 chest X-rays. Moreover, a large variability in DLP was noted between study sites, with a range of 331-2146 mGy per cm.

The authors conclude that the median radiation doses from CCTA differ significantly between individual centres and CT systems. The dose of radiation received can be difficult to estimate, and the study does not look at any links between the radiation dose received and cancer, but one important finding was that strategies known to reduce radiation dose were not implemented. Results from PROTECTION 2 and 3, due later this year, will determine whether image quality can be maintained with lower doses of radiation.

• Hausleiter J, Meyer T, Hermann F, et al. Estimated radiation dose associated with cardiac CT angiography. JAMA 2009; 301:500-507.
• Einstein AJ. Radiation protection of patients undergoing cardiac computed tomographic angiography. JAMA 2009; 301: 545-547.

Previous studies comparing CT coronary angiography with the current gold standard of coronary angiography have been limited by single-centre design and inherent bias in small studies.  This multi-centre, international study, CORE 64 (Coronary Artery Evaluation Using 64-Row Multidetector Computed Tomography Angiography) used central blinded analysis to evaluate the diagnostic accuracy of MDCT angiography involving 64 slice detectors and a slice thickness of 0.5mm in order to identify symptomatic patients with suspected CAD who should be referred for conventional coronary angiography. Patients with Agatston calcium scores>600 were pre-specified to be excluded from the analysis and entered into a registry. Patients were followed up for the interim occurrence of death, MI, stroke, revascularisation (percutaneous or surgical), hospitalisation for angina or heart failure and other serious adverse events at 7 and 30 days.

291 patients had calcium scores <600 and underwent MDCT and conventional angiography. Stenoses >50% were considered obstructive. 56% of patients had obstructive CAD. The patient-based diagnostic accuracy of quantitative CT for detecting or ruling out stenoses of 50% or more according to conventional coronary angiography revealed an AUC of 0.93 (95%CI 0.90-0.96), with a sensitivity of 85% (95% CI 79-90), specificity of 90% (95%CI 83-94), positive predictive value (PPV) of 91% (95%CI 86-95) and a negative predictive value (NPV) of 83% (95%CI 75-89). CT angiography was commensurate with conventional angiography in its ability to identify patients who subsequently required revascularisation: AUC 0.84 (95%CI 0.79-0.88) for MDCT compared to 0.82 (95%CI 0.77-0.86) for conventional coronary angiography. Assessment on an individual vessel basis of 866 vessels yielded an AUC of 0.91 (95% CI 0.88-0.93).Good correlation of disease severity assessment was seen between the 2 techniques (r=0.81, 95%CI 0.76-0.84).

Although this study suggests that MDCT can accurately diagnose obstructive CAD, the PPV of 91% and NPV of 83% suggest that it cannot replace conventional coronary angiography in symptomatic patients at this time.  The results suggest that MDCT should be considered in the context of other non-invasive stress tests although exposure to radiation is a concern in all methods involving radiography or nuclear isotopes. Further studies will be necessary to determine its precise role in diagnostic algorithms evaluating patients with suspected CAD.

• Miller JM, Rochitte CE, Dewey M et al. Diagnostic performance of Coronary Angiography by 64-Row CT. N Engl J Med 2008;359:2324-6