Kilovoltage cone-beam CT: comparative dose and image quality evaluations in partial and full-angle scan protocols.

Purpose: To assess imagingdose of partial and full-angle kilovoltage CBCT scan protocols and to evaluate image quality for each protocol. Methods: The authors obtained the CTdose index (CTDI) of the kilovoltage CBCT protocols in an on-board imager by ion chamber (IC) measurements and Monte Carlo(MC) simulations. A total of six new CBCT scan protocols were evaluated: Standard-dose head (100 kVp, 151 mA s, partial-angle), low-dose head (100 kVp, 75 mA s, partial-angle), high-quality head (100 kVp, 754 mA s, partial-angle), pelvis (125 kVp, 706 mA s, full-angle), pelvis spotlight (125 kVp, 752 mA s, partial-angle), and low-dose thorax (110 kVp, 271 mA s, full-angle). Using the point dose method, various CTDI values were calculated by (1) the conventional weighted CTDI ( CTDI w ) calculation and (2) Bakalyar’s method ( CTDI wb ) . The MC simulations were performed to obtain the CTDI w and CTDI wb , as well as from (3) central slice averaging ( CTDI 2 D ) and (4) volume averaging ( CTDI 3 D ) techniques. The CTDI values of the new protocols were compared to those of the old protocols (full-angle CBCT protocols). Image quality of the new protocols was evaluated following the CBCTimage quality assurance (QA) protocol [S. Yoo et al., “A quality assurance program for the on-board imager®,” Med. Phys.33(11), 4431–4447 (2006)] testing Hounsfield unit (HU) linearity, spatial linearity/resolution, contrast resolution, and HU uniformity. Results: The CTDI w were found as 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the new protocols, respectively. The CTDI w and CTDI wb differed within + 3 % between IC measurements and MC simulations. Method (2) results were within ±12% of method (1). In MC simulations, the CTDI w and CTDI wb were comparable to the CTDI 2 D and CTDI 3 D with the differences ranging from −4.3% to 20.6%. The CTDI 3 D were smallest among all the CTDI values. CTDI w of the new protocols were found as ∼ 14 times lower for standard head scan and 1.8 times lower for standard body scan than the old protocols, respectively. In the image quality QA tests, all the protocols except low-dose head and low-dose thorax protocols were within the tolerance in the HU verification test. The HU value for the two protocols was always higher than the nominal value. All the protocols passed the spatial linearity/resolution and HU uniformity tests. In the contrast resolution test, only high-quality head and pelvis scan protocols were within the tolerance. In addition, crescent effect was found in the partial-angle scan protocols. Conclusions: The authors found that CTDI w of the new CBCT protocols has been significantly reduced compared to the old protocols with acceptable image quality. The CTDI w values in the point dose method were close to the volume averaging method within 9%–21% for all the CBCT scan protocols. The Bakalyar’s method produced more accurate dose estimation within 14%. The HU inaccuracy from low-dose head and low-dose thorax protocols can render incorrect dose results in the treatment planning system. When high soft-tissue contrast data are desired, high-quality head or pelvis scan protocol is recommended depending on the imaging area. The point dose method can be applicable to estimate CBCTdose with reasonable accuracy in the clinical environment.