Radiation exposure to patients from coronary CTA and the associated risk of developing cancer is a major deterrent to wide application and acceptance of this technology [2
]. Various studies document that significant dose reduction is achievable with prospective ECG triggering, and radiation doses of coronary CTA are comparable with or even lower than those of conventional coronary angiography [3
]. This fact was supported by a study showing a 78% decrease in ED when a prospective gating technique was used [3
]. Although marked reduction in ED is possible by using this technique, current 64-MDCT and dual-source scanners have limitations (e.g., temporal and spatial resolution, cardiac coverage, heart rate, heart rate variability, BMI, and motion artifacts), resulting in a decrease in the image quality, which prevents the widespread clinical utility of this imaging modality [16
]. Previous literature has shown that heart rate greater than 70 beats/min, heart rate variation greater than 10 beats/min, and BMI greater than 30 are all factors that result in a compromise in image quality [29
]. New-generation MDCT scanners have improved temporal resolution of faster image acquisition in comparison with old MDCT [4
]. New imaging techniques (prospective gating) also result in a marked decrease in radiation doses in comparison with retrospective gating. Baumuller et al. [31
] and Rixe et al. [32
] reported mean EDs of 10.4 ± 1.7 mSv and 8.6 ± 2.8 mSv, respectively, for patients scanned with prospective gating at 64-MDCT in comparison with the previously reported radiation range (11–22 mSv) for retrospective techniques [16
The findings of our study show that prospective MDCT angiography using 320-MDCT is clinically feasible. The ED is lower and image quality is better than with 64-MDCT. However, it is vital to note that the advantage of radiation dose reduction is only noted in patients with a heart rate of 65 beats/min or less (92% of patients in this study). However, this advantage was reversed in a minority of patients (8%) with a heart rate greater than 65 beats/min who had the complete scan in more than one heartbeat. A 320-MDCT scanner acquires the heart rate during breath exercise, and then in patients whose heart rate changes above 65 beats/min at the instant of exposure, the heart is captured in more than one beat for improved temporal resolution [4
]. This important feature is clearly apparent with our results, because there is a substantial increase in radiation dose with heart rate greater than 65 beats/min versus 65 beats/min or less (8.7 vs 5.7 mSv). The role of heart rate in radiation dose exposure and image quality has been well documented [31
]. All possible efforts were made to achieve heart rate below 65 beats/min with beta-blockers. Heart rate control is a vital step in cardiac imaging for low radiation exposure and excellent image quality.
Our study shows that the 320-MDCT results in significantly improved image quality compared with 64-MDCT. Improved temporal resolution with fast gantry rotation not only ensures improved image quality, but also improves edge depiction of coronary vessels when compared with retrospective gated MDCT [34
]. MDCT angiography diagnostic accuracy is dependent on excellent-to-good image quality, and every effort should be made to achieve this goal in addition to reducing radiation exposure [18
To the best of our knowledge, this is the first study comparing the image quality in cardiac MDCT angiography between 64-MDCT and 320-MDCT in a clinical setting. Shuman et al. [18
] compared the image quality for prospective and retrospective ECG gating for 64-MDCT angiography and documented similar results for both groups of patients, although that study showed that the absence of table motion usually has a beneficial effect on image quality. Wang et al. [1
] discussed the image quality comparison for single-source CT versus dual-source CT and found equivalent results for both scanners at lower heart rates, but dual-source CT documented superior image quality at high heart rates (70–79 beats/min). However, our study data showed significantly better image quality among those undergoing prospectively gated 320-MDCT angiography versus 64-MDCT.
The limitations of our study are those inherent in comparing two subject groups and two different scanners. Ours is a retrospective study of patients imaged on two different scanners, rather than scanning the same patient on each scanner. This practice, however, would be prohibitive because of the accumulated radiation dose to a patient undergoing two sequential scans, but accuracy studies by Budoff et al. [7
] for 64-MDCT angiography and Dewey et al. [36
] for 320-MDCT angiography in comparison with the reference standard (i.e., invasive catheterization) showed 99% negative predictive value for both scanners. Another limitation is our use of subjective evaluation of image quality rather than quantitative evaluation by signal-to-noise ratio and contrast-to-noise ratio. However, our image quality scoring scales have been used successfully by other investigators for cardiac image quality [1
]. Further prospective studies are needed to corroborate the diagnostic accuracy between the two scanners.
In this study, we found that prospectively gated 320-MDCT angiography provides robust diagnostic image quality at lower radiation doses than those for 64-MDCT angiography. Regardless of the scanner, good heart rate control (< 65 beats/min) plays a vital role in radiation reduction; however, the ability of the 320-MDCT scanner to image in a single heartbeat will always result in a more beneficial reduction in dose. Therefore, the 320-MDCT scanner has the potential to be used routinely in patients with suspected CAD after a standard heart rate control protocol. This lowered radiation exposure, reduced scan time, and better image quality render 320-MDCT angiography as a viable noninvasive option for cardiac imaging.