Program Information
A Method for Assessment of Scan Length and Size Specific Energy Absorption in CT
X Li*, K Yang , M DeLorenzo , B Liu , Massachusetts General Hospital, Boston, MA
Presentations
SU-K-201-5 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 201
Purpose: CT scanners usually report dose length product (DLP) for the overall energy delivered by individual scan series and complete examinations. But DLP reflects the energy absorbed in a body or head CTDI phantom (material acrylic, diameter 32 or 16 cm), not in the patient. This work was aimed to present a method for assessment of scan length and size specific energy absorption in CT.
Methods: Patient size and attenuation can be assessed with water equivalent diameter in AAPM Report No. 220. The total energy (E) absorbed in a water phantom was the product of scan length (L) and the equilibrium dose (D_eq) in the water phantom. The D_eq (water) to CTDI_vol ratio was adopted from a previous paper. The total energy (E_in) absorbed in the scanned range was assessed from longitudinal dose distribution [D_L(z)] in the water phantom, which was calculated using previously published dose calculation algorithms and CT dose equilibration data for water diameters from 6 to 55 cm. Results were obtained for 50 water diameters (6-55 cm), 5 tube voltages (70, 80, 100, 120, 140 kV), and 7 scan lengths (5, 10, 20, 30, 40, 60, 80 cm).
Results: The ratios of E/DLP=D_eq/CTDI_vol decreased with water diameter, also decreased with tube voltage at small diameters, but increased with tube voltage at large diameters. E_in/E spanned a large range from 0.53 to 0.99. It decreased with diameter, slightly decreased with tube voltage, but asymptotically increased with scan length.
Conclusion: E and E_in enable improved monitoring of energy absorptions in individual patients, and may further facilitate CT protocol optimization with respect to image quality and radiation induced risk.
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