Computing effective dose in cardiac CT.
We present a method of estimating effective doses in cardiac CT that accounts for selected techniques (kV mAs(-1)), anatomical location of the scan and patient size. A CT dosimetry spreadsheet (ImPACT CT Patient Dosimetry Calculator) was used to estimate effective doses (E) using ICRP 103 weighting factors for a 70 kg patient undergoing cardiac CT examinations. Using dose length product (DLP) for the same scans, we obtained values of E/DLP for three CT scanners used in cardiac imaging from two vendors. E/DLP ratios were obtained as a function of the anatomical location in the chest and for x-ray tube voltages ranging from 80 to 140 kV. We also computed the ratio of the average absorbed dose in a water cylinder modeling a patient weighing W kg to the corresponding average absorbed dose in a water cylinder equivalent to a 70 kg patient. The average E/DLP for a 16 cm cardiac heart CT scan was 26 microSv (mGy cm)(-1), which is about 70% higher than the current E/DLP values used for chest CT scans (i.e. 14-17 microSv (mGy cm)(-1)). Our cardiac E/DLP ratios are higher because the cardiac region is approximately 30% more radiosensitive than the chest, and use of the ICRP 103 tissue weighting factors increases cardiac CT effective doses by approximately 30%. Increasing the x-ray tube voltage from 80 to 140 kV increases the E/DLP conversion factor for cardiac CT by 17%. For the same incident radiation at 120 kV, doses in 45 kg adults were approximately 22% higher than those in 70 kg adults, whereas doses in 120 kg adults were approximately 28% lower. Accurate estimates of the patient effective dose in cardiac CT should use ICRP 103 tissue weighting factors, and account for a choice of scan techniques (kV mAs(-1)), exposed scan region, as well as patient size.
Department of Radiology and Radiological Science, Medical University of South Carolina, 96 Jonathan Lucas Street, MSC 323, Charleston, SC 29425-3230, USA. huda@musc.edu
Walter
W
Huda
Sameer
S
Tipnis
Alexander
A
Sterzik
U Joseph
UJ
Schoepf
D000328:Adult; D001835:Body Weight; D006321:Heart / Q000000981:diagnostic imaging*; D006801:Humans; D008954:Models, Biological; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D013902:Radiography, Thoracic / Q000295:instrumentation / Q000379:methods; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods*; D014867:Water
D014867:Water