Imaging
Figure 3: A 92% Dose Reduction Was Realised with Use of Prospective Triggering Compared with Retrospective Gating in a 53-year-old Male with Chest Pain
2.8mSv – 83% less than with the retrospective helical technique (mean 18.4mSv).13
Despite the reduced dose, the image quality score
was significantly greater for images obtained with a prospectively triggered technique. Many other studies have since evaluated the use of prospective gating for CCTA. In the PROTECTION I study, prospective ECG triggering led to a 71% reduction in effective radiation dose but was used in only 6% of studies.9
The 6% figure
reflects the fact that the PROTECTION I data were accumulated prior to widespread release of prospective triggering protocols.
Diagnostic accuracy of prospectively triggered CCTA compares well with conventional coronary angiography (CA). Scheffel et al. reported overall patient-based sensitivity, specificity and positive (PPV) and negative predictive value (NPV) for the diagnosis of significant stenoses of 100, 93, 94 and 100%, respectively.43
Stolzman et al.
reported overall sensitivity, specificity, PPV and NPV of 98, 99, 95 and 100%, respectively.50
Dewey et al. recently evaluated 320-row MDCT
before same-day CCA and reported per-patient sensitivity and specificity of 100 and 94%, respectively.46
The two images on the left, depicting a left anterior descending (LAD) stenosis (arrows), were acquired using retrospective gating and had an effective dose of 19.2mSv. On the right is an exam performed two years later after he had the LAD stenosis stented; this was prospectively gated and had an effective dose of 1.6mSv.
Figure 4: Portions of the Left Anterior Descending Depicted After Reconstruction with Increasing Amounts of Adaptive Statistical Iterative Reconstruction
There are limitations to prospective ECG triggering. There is no capability to evaluate cardiac function because data are acquired only during a limited portion of the cardiac cycle. Most studies performed with prospective triggering have used upper heart rate criteria of between 65 and 70bpm because image quality degrades at higher heart rates. Husmann et al. reported that 98.9% of the coronary segments were clinically assessable for heart rates less than 63bpm; however, only 85% were assessable at higher heart rates.41
Other
studies have reported a similar inverse correlation between image quality or segment assessability and heart rate.15,51,52
Heart rate
variability has also been independently correlated with overall image quality for prospectively triggered CCTA.15,51,52
Newer techniques and
faster CT systems may be helpful in raising the upper heart rate limitation. Using prospectively triggered DSCT, Xu et al. evaluated patients with a heart rate of 70–110bpm and found coronary evaluability of 99.7%, similar to 98.7% with retrospective gating.53
However, with careful heart rate control, prospective triggering was eventually used for 92.1% of coronary CTAs and 83.2% of CTAs following coronary artery bypass graft (CABG).
Prospective triggering at most centres relies on the use of beta- blockers to lower the heart rate. In our clinical practice we use prospective triggering for all cases as long as the heart rate is less than 70bpm, heart rate variability is less than 10bpm and cardiac function is not required. In an analysis of 2,124 consecutive clinical exams, approximately 50% of patients initially failed one or more of the listed criteria.42
0% top left, increasing in 20% increments to 100% adaptive statistical iterative reconstruction (ASIR) (bottom left). A noticeable decrease in image noise is seen as the percentage of ASIR is increased.
scanners,44–47 CT (DSCT)48,49
two to three heartbeats with 128-detector dual-source or five to eight heartbeats on a 64-row system.13
Prospective ECG triggering enables acquisition of CCTA at doses generally in the 1–4mSv range (see Figure 3).13–15,17,40–43 reported a mean effective dose for prospectively triggered exams of
We first
18
High-pitch Spiral Prospectively Triggered Dual-source Computed Tomography
With the recent introduction of second-generation DSCT systems, a new fast-table-speed/high-pitch-scan mode has been developed.18–22 Achenbach et al. recently evaluated this scan mode in 37 consecutive patients with bodyweight <100kg and low heart rates (<60bpm).19 They reported an estimated effective radiation dose of 0.94±0.06mSv without degradation of image quality. This technique utilises a very high pitch factor of 3.2 and two large (64-slice) detectors with scan initiation at 60% of the R–R interval. This high helical pitch of 3.2 was previously not possible with single-source scanners. With single- source techniques, the spiral or helical pitch (table movement per rotation divided by the collimated beam width) is limited to a
EUROPEAN CARDIOLOGY
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