Relationship between CBCT Image Quality and Image Dose of Accelerator’s Cone Beam CT Equipment with Bowtie-filter_West China Medical Journal

Authors：

PENG Guang , XU Qingfeng , CHEN Gang ,  BAI Sen

Division of Physics Center, West China Hospital, Sichuan University, Chengdu, Sichuan 610041, P.R.China;

Corresponding author：

BAI Sen, Email: mini_rat@163.com

Keywords：

图像质量; 图像剂量; 锥形束CT; Bowtie滤线器; HVL

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【摘要】　目的　研究使用Bowtie滤线器（F1）后，保证加速器CBCT患者扫描图像质量前提下，如何合理设置扫描条件，尽量降低辐射剂量。　方法　使用Piranha辐射测量仪测量CBCT在使用F1与未使用F1时射线的半价层。设定不同的扫描模式，使用直径30 cm的有机玻璃模体测量扫描剂量，并使用XVI附带的Catphan503模体测量客观图像质量。在此基础上，研究扫描剂量以及图像质量与扫描条件的关系，提出了适合临床患者的胸部与腹部不同的扫描条件。　结果　使用F1后射束的半价层增加了0.77～0.92 mmAl，扫描剂量明显减少，中心点减少了22%~29%，边缘点减少了41%~45%，皮肤剂量减少显著。图像质量随着扫描剂量的增大而提高。空间分辨力受FOV影响较大，但一般能识别1～2 mm的物体，完全能够满足分辨细小骨结构与标记点的临床要求。图像伪影在使用L20时的大mAs下明显。胸部低剂量的CBCT图像如100 kV，M20，0.5 mAs的扫描条件亦可满足临床要求。腹部则需要使用较大剂量的扫描模式，CBCT图像才达到进行配准的要求。　结论　 F1的使用在改善图像质量的前提下降低了扫描剂量，使用新的扫描序列能平衡二者的关系。
【Abstract】　Objective　 To explore the influence of bowtie filtration on absorb dose and half-value layer inaluminum (Al) of cone-beam CT, estimate the image dose under different scan protocol, and establish the relationship between the image quality and the scan protocol after using F1.　Methods　 Piranha was used to measure the HVL. Dose measurements were performed with an 0.6 cc Farmer type ionization chamber with a 30 cm-diam cylindrical shaped water phantoms in 100 and 120 kV with a series of mAs and FOV. CNR, noise and uniformity were measured on the Catphan503 images.　Results　 HVL increased 0.77-0.92 mmAl where XVI generally had more penetrating beams at the similar kV settings. Scanning dose significantly reduced, the center point decreased 22%-29%, the edge with a decrease of 41%-45% which meant a very significant reduction in skin dose. Image quality improved with mAs increase. The spatial resolution mainly changed with FOV. But generally can identify 1-2 mm-diam objects, fully meet the clinical requirements of identify small bone structure and marker. Through this clinical investigation, low-dose CBCT images in chest, such as 100 kV, M20, and 0.5 mAs scanning protocol appeared to be an optimal settings. Abdomen image needed a higher dose to reach the requirements of registration.　Conclusion　 Using F1 under the premise of improving the image quality then reducing the scanning dose and using a new scanning sequence can balance the image quality and scanning dose.

Citation： PENG Guang,XU Qingfeng,CHEN Gang,BAI Sen. Relationship between CBCT Image Quality and Image Dose of Accelerator’s Cone Beam CT Equipment with Bowtie-filter. West China Medical Journal, 2010, 25(12): 2151-2155. doi: Copy

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1. 　Islam MK, Purdie TG. Patient dose from kilo voltage cone beam computed tomography imaging in radiation therapy[J]. Med Phys, 2006, 33(6): 1573-1582.
2. 　Song WY, Kamath S, Ozawa S, et al. A dose comparison study between XVI and OBI CBCT systems[J]. Med Phys, 2008, 35(2): 480-486.
3. 　Moore CJ, Amer A, Marchant T, et al. Developments in and experience of kilovoltage X-ray cone beam image-guided radiotherapy[J]. Br J Radiol, 2006, 27(S1): S66-S78.
4. 　Létourneau D, Wong JW, Oldham M, et al. Cone-beam-CT guided radiation therapy: technical implementation[J]. Radiother oncol, 2005, 75(3): 279-286.
5. 　Mail N, Moseley DJ, Siewerdsen JH, et al. The influence of bowtie filtration on cone-beam CT image quality[J]. Med Phys, 2009, 36(1): 22-32.
6. 　尹勇, 袁双虎. 加速器附加锥型束CT图像质量评价[J]. 中华放射肿瘤学杂志, 2007, 16(3): 225-227.
7. 　Amer A, Marchant T, Sykes J. Imaging doses from the Elekta Synergy X-ray cone beam CT system[J]. Br J Radiol, 2007, 80(954): 476-482.
8. 　Bissonnette JP, Moseley DJ, Jaffray DA. A quality assurance program for image quality of cone-beam CT guidance in radiation therapy [J]. Med Phys, 2008, 35(5): 1807-1815.
9. 　McBain CA, Henry AM, Sykes J, et al. X-ray volumetric imaging in image-guided radiotherapy: the new standard in on-treatment imaging[J]. Int J Radiat Oncol Biol Phys, 2006, 64(2): 625-634.
10. 　Smitsmans MH, de Bois J, Sonkes JJ, et al. Automatic prostate localization on cone-beam CT scans for high precision image-guided radiotherapy[J]. Int J Radiat Oncol Boil Phys, 2006, 63(4): 975-984.

West China Medical Journal

Relationship between CBCT Image Quality and Image Dose of Accelerator’s Cone Beam CT Equipment with Bowtie-filter

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