CT

How Much Does Number of Slices Matter for Cardiac CT?

Will a patient be willing to pay three times as much to reduce breath-hold from five-to-six seconds to 1-2 seconds?

"While it is certain that the new technologies will not replace 64 Slice Cardiac CT in the near future imaging for coronary arteries is bound to improve in years to come" - Rahil Shah CEO NM Medical Diagnostic Chain Mumbai

Technical developments in Computed Tomography (CT) resulted in the introduction of Multi-Detector Computed Tomography (MDCT) in 1999. Since then the increased number of detectors has enabled us to scan large volumes with thin slices within a single breath-hold.

The greatest benefit of this technological development has been witnessed in the realm of cardiac imaging with the emergence of Cardiac CT as a reliable and in many cases the first choice of screening test for coronary artery disease. Since then, various equipment vendors have aggressively marketed new generation scanners, with each new scanner boasting of a higher number of slices than the previous ones. However, beyond a certain level, the incremental value of increasing the number of slices vis-à-vis the additional cost burden on the purchaser and ultimately on the patient is debatable. Presently, there is worldwide consensus that 64 Slice Cardiac CT allows imaging of the coronary arteries with adequate spatial and temporal resolution and acceptable patient comfort with significant improvements over four slice and 16 slice Cardiac CT. The verdict is still out though on the incremental value of newer generation 128, 256, dual source, and even 320 slice Cardiac CT scanners.

Cardiac CT was initially performed on four slice multi-detector CT scanners.

These examinations were confined to evaluation of the proximal coronary arteries because the studies were limited in spatial and temporal resolution and were plagued by long acquisition times that required intolerably long breath-holds.

16-slice scanners produced tremendous improvements in CCTA technique: for the first time, complete coronary circulation exams with breath-holds in the 15 to 20-second range could be routinely obtained. 16 slice Cardiac CT achieved very favourable results in detecting a more than 50 per cent stenosis compared with catheter angiography, far exceeding any sensitivity achieved with routine stress tests or thallium tests. More importantly, 16 slice Cardiac CT demonstrated negative predictive values more than 97 per cent, effectively ruling out coronary artery disease.

However, the long breath-hold associated with 16 Slice Cardiac CT often meant that studies would be marred with breathing artifacts leading to step like reconstructions of the coronary arteries resulting in poor diagnostic quality. Furthermore, obese patients or patients with high heart rates were very difficult to scan on 16 slice scanners. The poor temporal resolution of images also meant that patients with heavy calcification in the coronary arteries could not be evaluated further.

After 16 Slice Cardiac CT, 40 Slice Cardiac CT was introduced with some but not enough improvements to overcome the limitations of 16 Slice Cardiac CT. It was the really the advent of 64 Slice Cardiac CT that revolutionised the application of CT for evaluation of coronary arteries.

64 slice CT allowed imaging centres to obtain 64*0.625 mm scans with true four-cm coverage. Acquisition times and hence breath hold was reduced to five to six seconds only, improving the image quality of patients with high heart rate, and allowing for stabilization of heart rate during the scan, volume reduction of contrast media, and wider coverage of ECG-gated scan compared with 16 slice CT. These advantages reduce the motion artifact, increase the percentage of assessable segments, and widen its clinical application.

64 Slice Cardiac CT also proved to be more effective for the evaluation of in-stent stenosis and also to evaluate the patency of bypass grafts due to the greatly improved spatial and temporal resolution. With 16 Slice Cardiac CT the volume of tissue coverage in a bypass graft case required a breath-hold of greater than 25 seconds at a 0.625 mm slice thickness. For this reason 16 Slice Cardiac CT scanned bypass grafts at a 1.25 mm slice thickness which reduced acquisition time by 50 per cent but sacrificed spatial resolution as a result. 64 Slice CT has also allowed for the introduction of triple rule out studies: evaluation of the pulmonary arteries, coronary arteries, and aorta in a single study. Finally, 64 Slice CT has reduced the radiation dose for Cardiac CT patients as compared to 16 Slice CT. The lower acquisition time produces a radiation dose between 6 and 14 mSv, which is well within the historically acceptable range of radiation exposure for other invasive and non-invasive cardiac imaging techniques. Today, most imaging centres worldwide use 64 Slice Cardiac CT for evaluation of coronary artery disease.

In recent years, however, some equipment vendors such as Siemens and Toshiba have introduced scanners with greater number of slices. Siemens has launched a 128 slice scanner as well as their single beat scanner Flash, whereas Toshiba has launched a much-hyped 320 Slice scanner. In contrast, GE, which has the largest installation base for 64 Slice Cardiac CT in the US, continues to build on its 64 Slice platform with improvements in gantry rotation time.

The immediate benefits of increased number of slices are lower scanning times, resulting lower radiation dose, and enhanced anatomical coverage for evaluation of more vessels. The Siemens Flash Cardiac CT promises coronary CT angiograms with one beat thus further improving patient comfort. 128 Slice and 320 Slice CT allow for wider vascular coverage extending the coronary scan to renal arteries in the same contrast injection.

However, all of these benefits come at a heavy cost. The Siemens Flash and Toshiba 320 slice scanner are almost three times as expensive as a 64 slice CT Scanner. Ultimately, these high purchase costs will have to be passed on to the patient. In a country like India where healthcare affordability is already a burning issue it remains to be seen whether this is a viable proposition. Will a patient be willing to pay three times as much to reduce breath-hold from 5-6 seconds to 1-2 seconds? Will there be such vast improvements in image quality to justify these costs? Will reduction in radiation dose from 5-6 mSv to 2-3 mSv motivate a patient to pay more?

It is too early still to answer these questions as these technologies have just recently been introduced in the market. And while it is certain that these technologies will not replace 64 Slice Cardiac CT in the near future imaging for coronary arteries is bound to improve in years to come.

rahil.shah@nmmedical.com