Garcia-Ramirez Jose L, Mutic Sasa, Dempsey James F, Low Daniel A, Purdy James A
Radiation Oncology Center, Mallinckrodt Institute of Radiology, St. Louis, MO 63110, USA.
Int J Radiat Oncol Biol Phys. 2002 Mar 15;52(4):1123-31. doi: 10.1016/s0360-3016(01)02779-1.
The demand for computed tomography (CT) virtual simulation is constantly increasing with the wider adoption of three-dimensional conformal and intensity-modulated radiation therapy. Virtual simulation CT studies are typically acquired on conventional diagnostic scanners equipped with an external patient positioning laser system and specialized planning and visualization software. Virtual simulation technology has matured to a point where conventional simulators may be replaced with CT scanners. However, diagnostic CT scanner gantry bores (typically 65-70 cm) can present an obstacle to the CT simulation process by limiting patient positions, compared to those that can be attained in a conventional simulator. For example, breast cancer patients cannot always be scanned in the treatment position without compromising reproducibility and appropriateness of setup. Extremely large patients or patients requiring special immobilization or large setup devices are often unable to enter the limited-bore gantry. A dedicated 85-cm-bore radiation oncology CT scanner has the potential to eliminate these problems. The scanner should provide diagnostic-quality images at diagnostic-comparable dose levels. The purpose of this study was to independently evaluate the performance of a novel 85-cm-bore CT X-ray scanner designed specifically for radiation oncology and compare it against diagnostic-type, 70-cm-bore scanners that may be used in the same setting.
We performed image quality and dosimetric measurements on an 85-cm-bore CT scanner (AcQSim CT, Marconi Medical Systems, Inc., Cleveland, OH) and a 70-cm-bore, diagnostic-type scanner (UltraZ CT, Marconi Medical Systems, Inc.). Image quality measurements were performed using a manufacturer-supplied phantom (Performance Phantom, Marconi Medical Systems, Inc.), following the manufacturer's suggested testing procedures, and an independent image quality phantom (CATPHAN 500, The Phantom Laboratory, New York, NY). The standard image quality parameters evaluated for the purpose of comparison were as follows: slice thickness accuracy, high-contrast resolution (limiting spatial resolution), low-contrast resolution, uniformity and noise, and CT number accuracy and linearity. Standard head and body protocols were employed throughout most of our measurements and were kept equal between the two scanners. The computed tomography dose index was measured for standard head and body imaging protocols using accepted methods and procedures. For comparison purposes, data for a diagnostic-type, 70-cm-bore scanner (GE HighSpeed CT/i) were extracted from the literature. The results obtained for the 85-cm-bore scanner are compared with the following: (1) manufacturer-provided autoperformance phantom test results (validating its use for routine quality assurance), (2) a similar set of measurements performed on a conventional 70-cm-bore, diagnostic-type CT scanner (UltraZ CT, Marconi Medical Systems, Inc.), and (3) current available data on other diagnostic-type CT scanners (GE HighSpeed CT/i).
Head and body doses seem on average to be slightly (1-2 cGy) higher for the 85-cm-bore unit than for conventional 70-cm units. Measured slice thickness was within acceptable criteria, +/-0.5 mm. There does not seem to be any significant difference in high-contrast resolution. Both units render a limiting value of approximately 7-8 lp/cm for slice thickness 8-10 mm. Both units exhibit comparable CT number uniformity, accuracy, and linearity. Noise levels seem to be slightly increased (by approximately 0.05-0.2%) for the large-bore geometry. Low-contrast resolution for both units was comparable (4.5-5.5 mm @ 0.35%). All image quality parameters are well within diagnostic acceptable levels.
The overall imaging performance and mechanical integrity of the 85-cm-bore scanner are comparable to those of conventional diagnostic scanners that may be employed in a radiation oncology setting.
随着三维适形放疗和调强放疗的广泛应用，对计算机断层扫描（CT）虚拟模拟的需求不断增加。虚拟模拟CT研究通常在配备外部患者定位激光系统以及专门的计划和可视化软件的传统诊断扫描仪上进行。虚拟模拟技术已经成熟到可以用CT扫描仪取代传统模拟机的程度。然而，与传统模拟机相比，诊断CT扫描仪的机架孔径（通常为65 - 70厘米）可能会限制患者的体位，从而给CT模拟过程带来障碍。例如，乳腺癌患者在不影响摆位的可重复性和适宜性的情况下，并不总能在治疗体位进行扫描。体型极大的患者或需要特殊固定装置或大型摆位设备的患者通常无法进入孔径有限的机架。一台专用的85厘米孔径的放射肿瘤学CT扫描仪有潜力消除这些问题。该扫描仪应在与诊断相当的剂量水平下提供诊断质量的图像。本研究的目的是独立评估一款专门为放射肿瘤学设计的新型85厘米孔径CT X射线扫描仪的性能，并将其与可在相同环境中使用的70厘米孔径诊断型扫描仪进行比较。
我们在一台85厘米孔径的CT扫描仪（AcQSim CT，Marconi Medical Systems公司，俄亥俄州克利夫兰市）和一台70厘米孔径的诊断型扫描仪（UltraZ CT，Marconi Medical Systems公司）上进行了图像质量和剂量测量。图像质量测量使用制造商提供的模体（Performance Phantom，Marconi Medical Systems公司），按照制造商建议的测试程序进行，同时还使用了一个独立的图像质量模体（CATPHAN 500，The Phantom Laboratory公司，纽约州纽约市）。为进行比较而评估的标准图像质量参数如下：层厚精度、高对比度分辨率（极限空间分辨率）、低对比度分辨率、均匀性和噪声以及CT值精度和线性度。在我们的大多数测量中采用了标准的头部和体部扫描协议，并且两台扫描仪保持一致。使用公认的方法和程序测量标准头部和体部成像协议的计算机断层扫描剂量指数。为了进行比较，从文献中提取了一台70厘米孔径诊断型扫描仪（GE HighSpeed CT/i）的数据。将85厘米孔径扫描仪获得的结果与以下内容进行比较：（1）制造商提供的自动性能模体测试结果（验证其用于常规质量保证的有效性），（2）在一台传统的70厘米孔径诊断型CT扫描仪（UltraZ CT，Marconi Medical Systems公司）上进行的一组类似测量，以及（3）其他诊断型CT扫描仪（GE HighSpeed CT/i）的现有数据。
85厘米孔径的设备头部和体部剂量平均似乎比传统的70厘米孔径设备略高（高1 - 2厘戈瑞）。测量得到的层厚在可接受标准范围内，±0.5毫米。高对比度分辨率似乎没有显著差异。对于8 - 10毫米的层厚，两台设备的极限值均约为7 - 8线对/厘米。两台设备的CT值均匀性、精度和线性度相当。大孔径设备的噪声水平似乎略有增加（约0.05 - 0.2%）。两台设备的低对比度分辨率相当（在0.35%时为4.5 - 5.5毫米）。所有图像质量参数均在诊断可接受水平内。
85厘米孔径扫描仪的整体成像性能和机械完整性与可用于放射肿瘤学环境的传统诊断扫描仪相当。
