Size-specific Dose Estimates for Chest, Abdominal, and Pelvic CT: Effect of Intrapatient Variability in Water-equivalent Diameter.
OBJECTIVE
To develop software to automatically calculate size-specific dose estimates (SSDEs) and to assess the effect of variations in water-equivalent diameter (Dw) along the z-axis on SSDE for computed tomographic (CT) examinations of the torso.


METHODS
In this institutional review board-approved, HIPAA-compliant, retrospective study, a software program was used to calculate Dw at each image position in 102 consecutive CT examinations of the combined chest, abdomen, and pelvis. SSDE was calculated by multiplying the size-dependent conversion factor and volume CT dose index (CTDIvol) at each image position. The variations in Dw along the z-axis were determined for six hypothetical scanning ranges: chest alone; abdomen alone; pelvis alone; chest and abdomen; abdomen and pelvis; and chest, abdomen, and pelvis. Mean SSDE was calculated in two ways: (a) from the SSDE at each position and (b) from the mean CTDIvol over each scan range and the conversion factor corresponding to Dw at the middle of the scan range. Linear regression analysis was performed to determine the correlation between SSDE values calculated in these two ways.


RESULTS
Across patients, for scan ranges 1-6, the mean of the difference between maximal and minimal Dw within a given patient was 5.2, 4.9, 2.5, 6.0, 5.6, and 6.5 cm, respectively. The mean SSDE values calculated by using the two methods were in close agreement, with root mean square differences of 0.9, 0.5, 0.5, 1.4, 1.0, and 1.1 mGy or 6%, 3%, 2%, 9%, 4%, and 6%, for the scan ranges of chest; abdomen; pelvis; chest and abdomen; abdomen and pelvis; and chest, abdomen, and pelvis, respectively.


CONCLUSIONS
Using the mean CTDIvol from the whole scan range and Dw from the image at the center of the scan range provided an easily obtained estimate of SSDE for the whole scan range that agreed well with values from an image-by-image approach, with a root mean square difference less than 1.4 mGy (9%).
From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Shuai
S
Leng
From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Maria
M
Shiung
From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Xinhui
X
Duan
From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Lifeng
L
Yu
From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Yi
Y
Zhang
From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905.
Cynthia H
CH
McCollough
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D049628:Body Size; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D010388:Pelvis / Q000000981:diagnostic imaging*; D011829:Radiation Dosage; D011860:Radiography, Abdominal / Q000379:methods*; D013902:Radiography, Thoracic / Q000379:methods*; D012189:Retrospective Studies; D012984:Software; D014057:Tomography, X-Ray Computed / Q000379:methods*; D014867:Water; D055815:Young Adult
D014867:Water