A study of the midpoint dose to CTDIvolratio: Implications for CT dose evaluation.
OBJECTIVE
In multidetector CT, the volume CT dose index (CTDIvol) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scanner-independent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length.


METHODS
A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDIL(water)/CTDIw(PMMA), where L was dose integration length. The ratio was equal to the midpoint dose DL(0) (water) to CTDIvolratio in a CT scan series with a scan length equal to L.


RESULTS
For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of DL(0)/CTDIvolfrom that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDIvolto SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, DL(0)/CTDIvoldecreased considerably as L decreased. DL(0)/CTDIvolwas shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, DL(0)/CTDIvoldecreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm.


CONCLUSIONS
The midpoint dose to CTDIvolratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT.
Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts 02114.
Xinhua
X
Li
Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts 02114.
Kai
K
Yang
Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts 02114.
Bob
B
Liu
D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011874:Radiometry; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods*