How will new cardiac guidelines affect CT cardiac angiography services?
INTRODUCTION Cardiac computed tomography (CT) has evolved at extraordinary speed from little more than an interesting concept a decade ago to a mature diagnostic tool today. Advances in CT technology and robust evidence from a multitude of research studies have led to the development of guidelines and recommendations for its appropriate use by organisations such as the American College of Cardiology1,2 for Health and Clinical Excellence3
and the National Institute .
Now it is a mainstream investigation for patients with known or suspected cardiac pathologies. Over the next few years it will become an everyday procedure in most United Kingdom hospitals, part of the routine workload of CT radiographers. It is the most complex of all of the CT investigations we currently perform, making it a challenging, fascinating, and rewarding endeavour.
TECHNOLOGY The earliest contribution of CT to cardiac imaging dates back to the early 1980s in the shape of electron beam CT (EBCT) scanners, which could effectively freeze cardiac motion because of the great rapidity with which they acquired an image – their very high ‘temporal resolution’. They found a niche in the evaluation of coronary artery calcification scoring in the 1990s4
Figure 1. 3-D reconstruction of a CT coronary angiogram/cardiac CT scan.
but their spatial
resolution was inadequate for coronary angiography. Their high cost, compared to conventional CT scanners, has always limited the number of EBCT scanners.
The era of CT coronary angiography (CTCA) really began with the advent of 4-slice
multislice CT scanners in 1998, with research papers on its use for cardiac imaging appearing soon after5
. Since the turn of the century, and in keeping with Moore’s Law,
multislice scanners have ‘morphed’ from 4-slice to 16-slice to 32-slice to 64-slice machines and beyond, and image quality has improved apace. These scanners have the necessary speed to image the contrast-enhanced coronary arteries in a single breath hold (figure 1). Currently, 128-slice scanners are commonplace, while we also have dual- source scanners that increase temporal resolution by a factor of two, and even a 320-slice scanner, which can image the whole heart in end-diastole of a single heartbeat.
CTCA is, however, still inferior to conventional catheter angiography in both its spatial and its temporal resolution. The spatial resolution of current CT scanners is such that each pixel represents a voxel measuring approximately 0.35mm x 0.35mm x 0.35mm, while conventional angiography pixels measure approximately 0.16mm x 0.16mm.
Given the small size of the coronary arteries - the proximal coronaries usually measure 2mm to 5mm in diameter6
- this is a significant disadvantage, particularly for the smaller distal and branch arteries.
The temporal resolution of conventional catheter angiography is in the region of 30ms, while most CT scanners have a temporal resolution of approximately 150ms. (This can be reduced to less than 100ms by using a low-pitch spiral acquisition and multisegment reconstruction7
it is Complex and demanding
, and dual source CT scanners have a temporal resolution
of approximately 75ms.) There is a greater potential for motion artefact with CT which can result in blurred, non-diagnostic images. Another significant problem for CT is that it does not depict accurately calcified plaques within the arterial wall. As a consequence
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IMAGING & ONCOLOGY
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