Will Multislice CT Angiography Replace Cardiac Catheterization as a Diagnostic Tool?
July 28, 2006 -- A study from the Cleveland Clinic was published earlier this week in JAMA comparing multidetector computed tomography ((a.k.a. Multislice CT) to the more invasive catheter-based angiography, currently considered the "gold standard" for measuring the blockage in coronary arteries. The Coronary Assessment by Computed Tomographic Scanning and Catheter Angiography (CATSCAN) study was funded by Philips Medical Systems, a manufacturer of CT equipment. News media reports on the study were mixed:
So one might wonder exactly what the study means for patients: are CT coronary scans good or not? Some of the above news reports imply that the medical profession has been overzealous in its acceptance of Multislice CT. Even an article in the professional journal theheart.org leads off by stating that the results of this study "should curb enthusiasm for this fast-evolving technology". How, for example, can one reconcile the media commentary with this observation from imaging expert Dr. Daniel Berman of Cedars-Sinai Medical Center in Los Angeles:
or by the fact that physicians in the cardiology and radiology fields have seen fit to found a new professional organization, the , devoted to establishing standards for and advocating the use of multislice CT (in fact, Dr. Mario Garcia, principal investigator for the CATSCAN study, is also on the Board of Directors for the Society). Is the Cleveland Clinic study flawed?
As with any scientific study, one needs to read carefully what this study measured, how it was done, what the criteria and protocols were and how the conclusions relate to current clinical practice. 187 patients were studied at 11 centers worldwide -- these were non-emergency patients who had been referred for standard coronary angiography. All patients were studied first by non-invasive multidetector CT and then in the cath lab with standard invasive angiography.
What the study clearly showed was that multidetector CT had a very high negative predictive value of 99%. Out of the 187 patients, 69 (37%) could have avoided the invasive angiography -- the CT scan showed no disease, and that finding was confirmed by the catheterization. Only 1 patient was missed by the CT, and that patient had a single blockage in a smaller distant part of an artery. In short, if the CT exam showed no disease, you could trust it. This is a critically important finding, because standard catheterization is almost 10 times as expensive as a CT scan, requires much more time, prep and involves a team of technicians and nurses, in addition to the cardiologist. There is also a 1 in 1,000 risk of death and a risk of complications (vascular or other) of 2 or 3 in a 100 for cardiac catheterization.
The part of the study that showed CT to perform less well than expected was that there were a high number (29%) of "nonevalueable" segments -- parts of the coronary artery that could not be judged as positive (diseased) or negative using the CT scan. It turned out that 95% of these nonevalueable segments were negative under catheterization. And of the segments that CT showed as diseased, 64% turned out not to be. Again the take-away conclusion from the study was if CT showed no disease, you could be sure it was accurate -- otherwise, you probably would need further testing, either via an invasive angiogram or nuclear stress test.
One of the most important aspects of this study, however, was that it was conducted using 16-slice CT scans -- a first generation tool that has since been overshadowed by 32 and 64-slice units. The newer generation is more accurate for a number of reasons, two of which, higher resolution, faster image acquisition (shutter speed), go a long way in realizing sharper images. The Cleveland Clinic study concludes that
Such a study is in fact being conducted right now: the CorE 64 (Coronary Evaluation on 64) study is an international multi-center comparison of 300 patients. The CorE 64 study is funded by Toshiba Medical Systems with noted imaging expert Dr. Joao A.C. Lima of Johns Hopkins as principal investigator.
Besides the use of first generation equipment, the high rate of nonevaluable segments and false positives may be due to other reasons. There were certain protocols used in the Cleveland Clinic study that differ from the norm in standard clinical practice -- mostly these were used to "level the playing field" when comparing CT with catheterization. The authors of the study acknowledge that these parameters may be why the CT technology performed less well than has been seen in most other studies.
There were four technical departures. For example, sublingual nitroglycerine was not administered, in order to match the standard practice for catheterization, although it is often used in multislice CT angiography to improve blood flow and thus image contrast. Lower than usual amounts of beta blockers were used for similar reasons -- this drug is used to slow the heart rate for CT scanning -- the equivalent of the photographer saying, "Hold still for the camera!" Beta blockers are not needed for catheterizations. Somewhat lower doses of radiation were used in the CT scan for all patients (usually lower doses can be successfully used only with patients who have normal heart beat rhythms). And finally, the segments were measured using quantitative rather than qualitative analysis. In other words, looking at a segment and saying "it's blocked" wasn't enough. One had to precisely measure the blockage. If the image wasn't clear enough to measure precisely, it was declared "nonevaluable" -- thus the unusually high number of nonevaluable segments.
The study itself, however, is very important in that it clarifies one fact -- that not everyone is a candidate for multislice CT scanning as the first and primary diagnostic screening tool. This is agreed upon by most imaging experts. Patients who are at very low risk and have no symptoms would do better to have a CT Calcium score done -- much lower radiation, but high correlation to see if one is in fact at risk (see our article on the SHAPE guidelines). Likewise, there is little reason for patients who are at very high risk for coronary artery disease to get this diagnostic test -- they should probably go straight to the cath lab, since they will most likely be needing some type of intervention (balloon and/or stent) which can be done during in the same procedure.
But for the patient with intermediate risk of coronary obstructive disease (an unclear result on a stress test, intermittent symptoms, etc.) the multislice CT angiogram is right now a fast, safe, noninvasive tool that, with the most current equipment and operator training, can save time, money and possible complications. As Dr. Peter Fitzgerald of Stanford noted in the introduction to his lecture on Cardiac CT at this year's American College of Cardiology meeting: "Nowadays it takes longer time to order a caffe latte at the nearby Starbucks than to get a cardiac CT."
Will Cardiac CT replace catheterization as a primary diagnostic tool? Dr. Berman thinks it already has:
Interview with Daniel S. Berman, MD -- Angioplasty.Org
Multislice CT Angiogram and Cardiac CT -- Angioplasty.Org
Safety Risk of Multislice CT Angiogram Compared to Cardiac Catheterization -- Angioplasty.Org
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