Discussion
The use of a high-pitch versus standard-pitch protocol significantly reduces radiation and iodine-based contrast medium load, while achieving comparable diagnostic image quality both with respect to the aorta and the whole coronary tree in patients being assessed for TAVI. However, a high-pitch acquisition is not sufficient if evaluation of the proximal coronaries is required.
Patients being considered for TAVI often undergo numerous investigations to assess their suitability for the procedure.3 7 These focus on the measurement of aortic root dimensions to facilitate device selection and assessment of potential access routes for the procedure.6 Although other modalities such as cardiovascular magnetic resonance (CMR) can provide comparable information,3 a significant number of patients have implanted devices that contraindicate this, or experience difficulty with lying flat in the scanner for significant lengths of time.
Cardiovascular CT offers the versatility of being able to assess access routes and aortic root dimensions in a single study.6 In contrast to CMR, it also has the temporal and spatial resolution to be potentially able to simultaneously assess the coronary arteries. This represents a significant advantage over other modalities as the height of the coronary ostia from the aortic valve annulus can be readily determined by CT and may significantly impact on the safety of device deployment.4 It is therefore an important consideration when determining the appropriateness of TAVI. Cardiovascular CT can also potentially allow the concomitant detection of significant coronary disease, which may increase the risks of ischaemia during the procedure, particularly during the rapid ventricular pacing required immediately prior to device deployment. Given the high burden of coronary disease in this population, this has traditionally been determined by invasive coronary angiography. The latter imposes an additional contrast load, and for those who ultimately prove unsuitable for TAVI, exposure to the risks of the invasive procedure with minimal gain. It would therefore be desirable to obviate the risks of this additional invasive assessment using CT-coronary angiography. Such an approach has been successfully used in other patient groups undergoing non-coronary cardiothoracic surgery15 and in other settings where it is desirable to obviate invasive assessment.16
We found that gating the acquisition of the CT-aortogram undertaken as part of TAVI assessment regardless of which protocol was used produced comparable results in terms of aortic assessment. None of the patients in either the high-pitch or standard-pitch groups experienced any artefact that significantly affected image quality when assessing the aorta. We also found that with the use of a test bolus to optimise the timing of contrast delivery and image acquisition, similar degrees of contrast enhancement were obtained in the aortic root irrespective of which protocol was employed. However, the mean CT number for the distal aorta was significantly higher with the high-pitch protocol, reflecting the combined effects of the lower kV generally employed with this approach, and the greater X-ray attenuation by the denser abdominal viscera versus the less attenuating air-filled lungs in the thorax. The CNR was statistically significantly higher with the standard pitch protocols, again reflecting the generally higher tube potentials employed. These small although statistically significant differences in objective image quality however did not translate into meaningful clinical differences. The mean aortic attenuations and CNR achieved in the high-pitch protocol were higher or comparable to those described by Wuest et al,17 who used a high-pitch only approach, with a similar effective radiation dose. However, the latter study did not make any direct comparisons with standard-pitch approaches or formally evaluate the coronary tree. Nevertheless, the work by Wuest et al demonstrates that further reductions in contrast dose are possible with high-pitch approaches.17
In the present study, there was no significant difference in the subjective diagnostic quality of aortograms obtained by either protocol. Furthermore, motion artefact significantly affected the quality of the CT-coronary angiograms obtained, regardless of protocol, such that over two-thirds were non-diagnostic across the whole cohort. The choice of protocol had no impact on the ability to obtain diagnostic image quality with respect to motion artefact when the entire coronary tree was considered. However, the standard-pitch protocol performed significantly better than the high-pitch approach when considering only the proximal segments of the three main epicardial coronary arteries. Disease in the proximal segments may be of greater potential prognostic importance than distal disease, particularly if there is concomitant left ventricle (LV) dysfunction.18 Information about these vessel segments alone may therefore retain utility in some clinical settings.
Coronary imaging in this patient cohort is particularly challenging as beta-blockers are contraindicated in severe aortic stenosis, making adequate heart rate control difficult to achieve. The median heart rate for patients in both groups was ~70 beats/min, significantly higher than the regular 60 beats/min or less normally desirable for coronary CT-angiography.10 This reflects the fact that with a limited aortic valve area, and therefore stroke volume, such patients maintain their cardiac output by augmenting their resting heart rate. It is likely that this had a greater impact on the frequency of diagnostic scans than the choice of protocol.
Although this was not formally assessed in this study, we have previously found that patients being considered for TAVI have significant coronary calcification in association with their aortic valve disease, which can preclude adequate coronary evaluation by CT-coronary angiography, regardless of image quality.19 This is unsurprising given that the majority of these patients are being considered for TAVI rather than conventional surgery for the very reason that they are elderly or have significant comorbidities such as chronic kidney disease, diabetes and coronary disease. Based on these considerations and the low diagnostic yield obtained in the present study, irrespective of the protocol employed, we suggest that CT-coronary angiography can still as yet not routinely be used to evaluate the coronary arteries in patients being considered for TAVI.
However, if information is required about the coronary arteries, particularly the proximal vessel segments, a standard CT-coronary angiogram should be considered, although even with this approach we found that the diagnostic yield was at most 59%. The high-pitch approach however by definition only provides one single phase for coronary evaluation offering less versatility for coronary evaluation, particularly at more rapid heart rates. More recently, we and others have found that end-systolic imaging (20%–40%) with our dual-source scanner is adequate for coronary assessment at higher heart rates,20 and so the highest dose retrospective acquisitions are no longer performed in our institution.
Although with respect to both the aorta and the coronary arteries we found no significant differences in the rates of diagnostic imaging using the high-pitch or the standard-pitch approaches, the total radiation dose and contrast load with the high-pitch protocols were significantly lower than with the standard-pitch approach. Given their underlying severe aortic stenosis, age and other comorbidities, radiation dose may seem of less relevance to patients being considered for TAVI6; however, iodine-based contrast medium load is important as many patients being considered for TAVI have associated chronic renal impairment. Our data suggest that high-pitch protocols should be considered as part of efforts to adhere to the ‘As Low As Reasonably Achievable’ principle and to minimise the contrast load used in these patients. While attempts to reduce radiation dose should not be at the expense of image quality, our data suggest that the adoption of high-pitch protocols has the potential to significantly minimise radiation exposure and more importantly contrast load without compromising diagnostic value. With improvements in TAVI techniques and devices, this technique may in future be used for lower risk patients in whom open surgery would have been previously considered.21 We speculate that lowering radiation exposure among this cohort may therefore assume greater importance on a population basis as TAVI begins to be offered for younger patients with fewer comorbidities and longer anticipated life expectancy.
Limitations
Our study has several potential limitations. First, it was retrospective in nature risking selection bias. Second, patients were not randomly assigned to one protocol or another, but the impact of a systematic change in protocol was assessed as part of an ongoing programme of service improvement and protocol development. However, consecutive patients were studied over the period during which the protocol change took place and both groups were comparable with respect to heart rate and demographic factors that may have influenced image quality. Finally, all patients were scanned at a single tertiary centre on the same platform. While this facilitated the use of a standardised approach, it is possible that different results may be obtained on different platforms or with different hardware. Careful evaluation of any new strategy is therefore essential as part of any service improvement or systematic change in scan protocols.