Patient doses and projection angle in cone beam CT.
OBJECTIVE
To investigate how x-ray tube projection angle affects organ and effective doses to patients undergoing a CT examination on a cone beam scanner.


METHODS
The authors investigated two cone beam CT systems that use a flat panel detector to capture the x-ray pattern transmitted through patients. One system had the flat panel detector and x-ray tube mounted on a conventional CT gantry (gantry CT), whereas the other CT scanner had the x-ray tube and flat panel detector mounted on a C-arm apparatus (C-arm CT). PCXMC software package (version 2.0.1) was used to compute absorbed doses at a constant x-ray beam output as a function of the x-ray tube projection angle. This software uses a mathematical hermaphroditic phantom with a weight of 73.2 kg and a height of 178.6 cm. Average absorbed doses were generated and recorded for five radiosensitive organs (i.e., breast, colon, lung, red bone marrow, and stomach), as well as the corresponding effective dose. Doses for both CT gantries were obtained every 15 degrees of the x-ray tube projection angle, at each of six locations in 10 cm increments along the patient long axis. The authors also investigated the effect on patient dose of filtrations ranging from 2.5 to 9.5 mm Al, with x-ray tube voltages ranging from 80 to 140 kV.


RESULTS
There were substantial differences in organ doses as a function of the projection angle, with higher organ doses for anteroposterior projection and lower doses for lateral projections. The maximum to minimum ratios of organ dose as a function of the x-ray tube angle were approximately 2.2 for the lungs, approximately 3.7 for the colon, approximately 5.9 for the red bone marrow, approximately 19.8 for the breast, and approximately 36 for the stomach. At the same x-ray tube voltage (kV) and intensity (mA s), the ratio of dose maxima for the two cone beam CT geometries for the five organs investigated was 1.85 +/- 0.10, which is mainly attributed to differences in source to isocenter distances. Effective doses varied with x-ray tube angle by a factor of approximately 2.7 for chest CT examinations and by a factor of approximately 4.0 for abdomen/pelvis examinations. Changes in the x-ray tube voltage and beam filtration showed the expected changes in absolute organ doses, but had little effect on relative doses.


CONCLUSIONS
There are major differences in organ and effective dose as the x-ray tube rotates around the patient. The results suggest that the use of x-ray tube current modulation could produce substantial reductions in organ and effective dose for body imaging with cone beam CT.
He
Wenjun
W
Department of Bioengineering, Clemson-MUSC Bioengineering Program, Clemson University, Charleston, South Carolina 29425, USA. whe@clemson.edu
Huda
Walter
W
Magill
Dennise
D
Tavrides
Emily
E
Yao
Hai
H
D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D011829:Radiation Dosage; D011860:Radiography, Abdominal; D013902:Radiography, Thoracic