Examination of these figures reveals varying levels of consistency for normalized dose (CTDIw/mAs) reported by sites with the same model of CT scanner. This value would be expected to very consistent. Two independent factors could influence the disparate values. First, some scanners could be behaving differently from others of the same model. Second, there could have been some error or inconsistency in the measurement process or reporting procedures. Thus, the variation among scanners of the same model could be real, or could be due to measurement variability, or (more likely), due to a combination of these two factors. (The large standard deviations observed for Philips and Toshiba scanner models may be due to a combination of a small number of scanners and the relative unfamiliarity of physicists testing those scanners, and cannot be interpreted with great confidence.)
The values reported in this manuscript are reasonably consistent with those available from the ImpACT CT Patient Dosimetry Calculator (www.impactscan.org
) which reports CTDI values for many different CT scanners. The mean values reported in agree with the values reported by ImPACT to within 10% for 7 of the 13 models listed; with 4 additional scanners having differences between 10 and 15% and only one scanner (Philips MX8000 IDT) reporting larger differences; this last scanner also had one of the larger standard deviations in our study. In addition, data from one of the more recently introduced scanners, the Philips Brilliance 64 (of which there was only 1 in the NLST trial), was not available from ImPACT. The data presented here does represent measured values from a number of different sites and from several different scanners of the same make and model and thus may represent the range of measurement variation due to both measurement variation and scanner variation.
From & we can readily appreciate that the evolving technology with more data channels did not result in noticeably higher dose per mAs. In fact, it appears that the more advanced technology was associated with a consistently lower normalized dose measurement, indicative of greater dose efficiency. This could be due to fewer overlap regions between successive dose profiles that results from wider total x-ray beam collimations available in some newer scanner models, as well as to improvements in the scanner software.
It is important to appreciate the incorporation of more advanced technology as the NLST progressed ( & , ). From trial initiation through completion, the increased speed of the scanners required shorter and shorter breath-holds. The increased complexity of the scanner acquisition parameters required additional physics oversight as the trial progressed.
Normalized dose varied among all scanners by almost a factor of two (minimum of 0.070 mGy/mAs, maximum of 0.127 mGy/mAs). Please note however that the minimum value of 0.07 mGy/mAs was recorded from a single measurement session on a single scanner.
CTDIvol (which is defined as CTDIw divided by pitch to account for a pitch greater or less than unity) was not analyzed for this manuscript. The authors elected to examine CTDIw instead of CTDIvol in order to present data describing machine specific dose, independent of pitch, which is a user selected parameter. (For future estimations of population dose, the effect of pitch will have to be explicitly included.)
It should be noted that at the time of this study, there was no DICOM standard widely available for several key values to determine radiation dose performance such as: pitch, total beam collimation, rotation time, and table feed. These have since been incorporated into the DICOM standard (Enhanced CT DICOM object module) and are starting to be implemented by the CT manufacturers. The widespread implementation of these fields will make estimation of radiation dose performance easier and more accurate in the future.
This study of scanner performance represents exclusively dose values that were measured and reported. Although the LSS and ACRIN investigators performed independent visual image quality assessments, such image quality metrics were not factored into these data. It is unknown whether the scanners that delivered relatively higher dose using the technique charts developed for this trial obtained relatively better image quality.
What can be said from these data is that mAs alone cannot be used as a universal indicator of image quality such as noise, or even machine output, since radiation output per mAs varies between scanner manufacturers and models. As radiation dose becomes an increasingly important factor in radiology, the need to collapse many technical factors down to one or two to gain a clearer understanding of the long term effects also increases. Unfortunately, this study indicates that simplifying image quality or noise based on an mAs type parameter is not likely to ultimately be useful due to the output variability among CT scanners.
A large collection of dose measurements was obtained on current vintage multi-detector row CT scanners during a multi-site lung screening research trial. These dose measurements (CTDIw) were normalized and reported on per mAs basis, by CT scanner model. This study demonstrated a statistically significant difference in normalized CT dose index among CT scanner manufacturers, likely due to design differences such as filtration, bow-tie design and geometry. Our findings also indicated a statistically significant difference in normalized CT dose index among CT scanner models within GE, Siemens, and Philips. We also demonstrated a statistically significant difference in normalized CT dose index among all models and all manufacturers. And, we demonstrated a statistically significant difference in normalized CT dose index from CT scanners among manufacturers when grouped by 4 or 8 data channels vs 16, 32, or 64 channels, suggesting improved dose efficiency in more complex scanners. Average normalized CT dose index values varied by almost a factor of two across all scanners from all manufacturers. This study was focused on machine specific normalized CT dose index; this is one of many factors that influence image quality and patient dose.