Principal analysis on radiation dosimetry of CT-SD16 detector and its parameter setting

目的 介绍了一种用半导体电离室测量多排CT辐射剂量的原理和方法,为完善我国现有医疗CT检测规范提供技术信息.方法 通过介绍X射线辐射剂量计Barracuda的测量原理和多排CT辐射剂量测量的参数设置依据,并结合在Philip Brilliance iCT 256机器上的应用实例,阐述了用半导体电离室的探测器测量多排CT辐射剂量的关键技术.结果 基于CT扫描参数合理设置探测器参数后,多排CT辐射剂量空间分布情况得以较好展示.100 mm长杆空气电离室的测量范围远小于多排CT线束覆盖范围,需要对多排CT辐射剂量测量方法和技术进行更新.结论 新型探测器有利于揭示多排CT辐射剂量的特性规律.     

常见探测器的空间分辨率比较

目的:比较指型电离室、针点电离室、平行板电离室、液体电离室、半导体探测器、宝石探测器等多种电离辐射探测器测量剂量时的空间分辨率。方法:以胶片剂量探测器的测量结果为基准,用三维水箱扫描同一高剂量梯度区的剂量,通过测量所选探测器的半影,然后进行分析比较给出不同探测器的空间分辨率。结果:不同探测器的测量结果与胶片相比有显著差异,其差异主要取决于探测器灵敏区域的直径,同时与灵敏体积、几何形状有一定关系。结论:因为半导体探测器具有最小的灵敏区域的直径,它的空间分辨率最高。     

医用直线加速器的X射线剂量的实验研究

本文介绍了用于测量电子直线加速器剂量的电离室和胶片探测器。用这些探测器测量了电子直线加速器在方型和圆形野情况下的几种重要的剂量分布,如百分深度剂量、组织最大比、离轴比等。文中介绍了用这些探测器测量这些参数时的特点。     

立体定向放射治疗小射野探测器研究进展

由于侧向带电粒子平衡的缺失、主光子源的遮挡,以及在小射野测量中可以选择的探测器有限,对现代放射治疗技术进行小野剂量学测定提出诸多挑战,这些挑战极大影响了剂量学的准确性。过量辐射事故表明对小野剂量学测定的适当方法了解不足。本综述介绍了目前适合在小野剂量学测量中应用的各类型探测器,包括微型电离室、半导体探测器、金刚石探测器、塑料闪烁体探测器、放射变色胶片、热释光探测器、放射光致发光玻璃探测器、光受激发光探测器、聚合物凝胶剂量计,为物理师在选择小野探测器时提供参考。     

应用辐射直接显色剂量胶片测量高能光子线的表面和建成区剂量

目的：准确测量高能光子射线剂量建成区的剂量分布，评估三维水箱扫描深度剂量曲线在表浅部位的误差。方法：使用辐射直接显色胶片（EBT胶片）测量加速器6MV光子线由体模表面到最大剂量深度区间的建成剂量分布，并与传统的电离室和半导体探头三维水箱扫描百分深度剂量曲线在该区间的剂量分布进行比较。结果：在接近最大剂量深度的区间（0．6cm-Dmax），EBT胶片与三维水箱扫描测量结果非常接近，差别小于2％；在电离室和半导体探头的有效测量深度至0．6cm深度区间，对不同射野大小，EBT胶片测量值大于两种三维水箱测量值5％～10％；在小于电离室和半导体探头的有效测量深度的区间，EBT胶片的测量值与水箱扫描结果比较差别最大分别达到22.7％（半导体探头）和49.3％（电离室）。结论：EBT胶片可以用于准确测量表面和建成区剂量分布，三维水箱扫描得到的PDD曲线应该进行建成区修正。     

不同探测器在多叶准直器质量保证中的定位精度比较

目的:比较辐射自显影胶片、电子射野影像系统、电离室矩阵等不同探测器在多叶准直器质量保证中的定位精度。方法:采用辐射自显影胶片(GAFCHROMIC EBT胶片)、电子射野影像系统、电离室矩阵(IBA公司Matrixx和PTW公司Seven29)测量和比较瓦里安公司Clinac ix加速器的多叶准直器叶片的边缘的边响应函数,比较测量结果,评价不同探测器的定位精度。结果:四种探测器的定位精度均可达到0.1mm,其中电子射野影像系统的灵敏度最高。结论:上述探测器均能满足临床质控需要。     

HW-Plan放射治疗计划系统的实验验证

本文采用辐射胶片结合指形电离室的测量方法,借鉴AAPM 51号报告的电离室测量方法和AAMP 55报告中对放射治疗计划系统验证的推荐标准,对本实验室新研发的HW-Plan放射治疗计划系统进行了实验验证,内容包括点剂量、轴向剂量分布曲线以及等剂量曲线的验证比较.实验采用方形水模和有机玻璃模体,通过CT扫描确定模体的电子密度和模拟靶点(测量位置),采用PTW电离室测量在三野交叉共面、等中心照射条件下等中心点和偏等中心点的照射剂量,采用Kodak EDR2辐射胶片测量该条件下靶区剂量场的相对分布,并与计划系统在相同照射条件下计算的剂量场进行了验证比较,实现了对HW-Plan放射治疗计划系统验证,为计划系统的市场准入和进入临床应用提供了可靠的依据.     

三种不同方法测量术中放疗表面剂量的比较研究

目的：分析比较术中放疗表面剂量三种不同测量方法的结果和精度,总结临床应用经验。方法：比较三种不同方法在模体中测量表面剂量的结果：指形电离室三维水箱、平行板电离室和MOSFET探测器。结果：平行板电离室测量表面剂量的结果与MOSFET剂量仪测量的结果具有较好的一致性,两者测量结果最大相差1．13％。采用指形电离室和三维水箱剂量系统测得表面剂量结果与前两者方法相差比较大,范围从-5．55％到4．55％。结论：采用平行板电离室和MOSFE探测器测量表面剂量的方法能得到比较精确的结果。     

三维分析仪与两维矩阵射野测量的比较

目的:应用不同仪器与方法测量加速器6 MV X线射野的特性,比较各方法的优劣和局限性,探讨快速简便检测射野特性的方法。材料与方法:分别采用电离室和半导体探头配合三维射野分析仪测量加速器6 MV X线不同射野大小的百分深度剂量曲线PDD和离轴比曲线OCR,并以二维电离室矩阵测量相同条件的OCR。(1)比较采用电离室和半导体探头测量PDD的差别。(2)比较两维矩阵与电离室半导体探头测量射野的对称性、平坦度、射野大小和半影等的差别。结果:对小于15 cm×15 cm照射野,半导体探头和电离室测量PDD的结果一致性较好,两者偏差小于1.3%。对于20 cm×20 cm照射野,半导体探头的测量结果大于电离室,最大差别3.5%,偏差为2.6%。用半导体探头与电离室测量射野的大小,两者的最大差别为0.6 mm,两者有较好的一致性,二维电离室矩阵测量与前两者比较,最大差别为2.9 mm,最小差别0.5 mm。三种方法测量的射野平坦度差别在1.2%~2.6%,矩阵的测量数值在半导体和电离室测量范围之内。结论:在检测加速器射野性能时,二维矩阵可以快速检测射野平坦度、对称性,但测量射野大小时可能有较大误差,不宜用作验收加速器和收集...     

螺旋CT的多排多层探测器应用设计

目的设计螺旋CT机的多排多层探测器,提高影像质量。方法将常规多排探测器改进为多排多层探测器,输出端接超高倍光电耦合放大器,可成倍提高探测器与采集系统的信噪比。结果多排多层探测器比同样排数的单层探测器输出的信噪比高、省时、剂量少,且图像质量高。结论多排多层探测器输出的数据量大幅增加,使CT系统的成像质量显著提高。     

A new look at CT dose measurement: beyond CTDI

Equations are derived for generating accumulated dose distributions and the dose line integral in a cylindrical dosimetry phantom for a helical CT scan series from the single slice dose profiles using convolution methods. This exposition will better clarify the nature of the dose distribution in helical CT, as well as providing the medical physicist with a better understanding of the physics involved in dose delivery and the measurement process. Also addressed is the concern that as radiation beam widths for multi-slice scanners get wider, the current methodology based on the measurement of the integral of the single slice profile using a 10 cm long ion chamber (CTDI100) may no longer be adequate. It is shown that this measurement would underestimate the equilibrium dose and dose line integral by about 20% in the center of the body phantom, and by about 10% in the center of the head phantom for a 20 mm nominal beam width in a multi-slice scanner. Rather than making the ion chamber even longer to collect the broad scatter tails of the single slice profile, an alternative to the CTDI method is suggested which involves using a small volume ion chamber, and scanning a length of phantom long enough to establish dose equilibrium at the location of the chamber. With a modern CT scanner, such a scan length can be covered in 15 s or less with a helical or axial series, so this method is not significantly more time-consuming than the long chamber method. The method is demonstrated experimentally herein.     

A preliminary study of the novel application of normoxic polymer gel dosimeters for the measurement of CTDI on diagnostic x-ray CT scanners

Computer tomography dose index (CTDI) is a measurement undertaken during acceptance testing and subsequent quality assurance measurements of diagnostic x-ray CT scanners for the determination of patient dose. Normoxic polymer gel dosimeters have been used for the first time to measure dose and subsequently CTDI during acceptance testing of a CT scanner and compared with the conventional ionization chamber measurement for a range of imaging protocols. The normoxic polymer gel dosimeter was additionally used to simultaneously determine slice-width dose profiles and CTDI in the transaxial plane, the measurements of which are usually determined with thermoluminescent dosimetry or film. The resulting CTDI for all slice widths calculated from the normoxic polymer gel dosimeter were within corresponding ionization chamber CTDI values. Slice-width dose-profiles full-width half-maximum values from the normoxic polymer gel dosimeter were compared to the slice sensitivity profiles and were within the tolerances of the manufacturer. Normoxic polymer gel dosimeters have been shown to be a useful device for determining CTDI and dose distributions for CT equipment, and provide additional information not possible with just the use of an ionization chamber.     

The trouble with CTD100

The computed tomography dose index (CTDI100) is typically measured using a 100 mm long pencil ion chamber with cylindrical polymethyl methacrylate (PMMA) dosimetry phantoms. While this metric was useful in the era of single slice CT scanners with collimated slice thicknesses of 10 mm or less, the efficiency of this metric in multi-slice CT scanners with wide (40 mm) collimated x-ray beams is unknown. Monte Carlo simulations were used to assess the efficiency of the CTDI100 parameter for wider beam collimations. The simulations utilized the geometry of a commercially available CT scanner, with modeled polyenergetic x-ray spectra. Dose spread functions (DSFs) were computed along the length of 12.4 mm diam rods placed at several radii in infinitely long 160 mm diam (head) and 320 mm diam (body) PMMA phantoms. The DSFs were used to compute radiation dose profiles for slice thicknesses from 1 to 400 mm. CTDI00 efficiency was defined as the fraction of the dose along a PMMA rod collected in a 100 mm length centered on the CT slice position, divided by the total dose deposited along an infinitely long PMMA rod. For a 10 mm slice thickness, a 120 kVp x-ray spectrum, and the PMMA head phantom, the efficiency of the CTDI00 was 82% and 90% for the center and peripheral holes, respectively. The corresponding efficiency values for the body phantom were 63% and 88%. These values are reduced by only 1% when a 40 mm slice thickness was studied, so the use of CTDI00 for 40 mm wide x-ray beams is no less valid than its use for 10 mm beam widths. However, these data illustrate that the efficiency of the CTDI100 measurement even with 10 mm beam widths is low and, consequently, dose computations which are derived from this metric may not be as accurate as desirable.     

A theoretical re-examination of Spencer-Attix cavity theory

The aim of radiation dosimetry is to evaluate, under specific conditions, absorbed dose in a medium of interest using a detection device. In comparison to what is meant to be evaluated, the distinctive composition of the detector causes particle fluence perturbation and shifted absorbed-dose response, both effects depending on beam quality. For electron and megavoltage photon beams, Spencer-Attix cavity theory further adapted by Nahum remains the accepted standard method used to convert absorbed dose in a wall-less detector to absorbed dose in the medium of interest. For several decades, the approach has been widely used in protocols to generate data for ionization chamber dosimetry. As a considerable effort was made towards accurate Monte Carlo methods, computation techniques are nowadays available to determine absorbed dose accurately in complex geometries, including radiation detectors. In the development of nonstandard beam protocols, direct Monte Carlo dose calculations using realistic models are being suggested and used to generate data for ionization chamber dosimetry. This indicates that for a general dosimetric context, including nonstandard beams, a more general cavity theory in agreement with what is currently being done could be adopted. Not only this could be of interest in the dosimetry standards community, but also for educational purposes. This paper re-examines Spencer-Attix theory from first principles, using a new general cavity theory rigorously derived from radiation transport equations. The approach is based on the same schematization as for Spencer-Attix's (i.e. groups of slow and fast electrons) and yields a general expression of absorbed dose for suitably implemented Monte Carlo methods. The Spencer-Attix-Nahum formulation is shown to be a special case of the presented model, outlining specific issues of the standard method. By providing an expression of absorbed dose which reflects the gold standard calculation method (i.e. Monte Carlo), the proposed theory could be adopted by the radiation dosimetry community.     

Dosimetric characterization of a large area pixel-segmented ionization chamber

A pixel-segmented ionization chamber has been designed and built by Torino University and INFN. The detector features a 24 x 24 cm2 active area divided in 1024 independent cylindrical ionization chambers and can be read out in 500 micros without introducing dead time; the digital charge quantum can be adjusted between 100 fC and 800 fC. The sensitive volume of each single ionization chamber is 0.07 cm3. The purpose of the detector is to ease the two-dimensional (2D) verifications of fields with complex shapes and large gradients. The detector was characterized in a PMMA phantom using 60Co and 6 MV x-ray photon beams. It has shown good signal linearity with respect to dose and dose rate to water. The average sensitivity of a single ionization chamber was 2.1 nC/Gy, constant within 0.5% over one month of daily measurements. Charge collection efficiency was 0.985 at the operating polarization voltage of 400 V and 3.5 Gy/min dose rate. Tissue maximum ratio and output factor have been compared with a Farmer ionization chamber and were found in good agreement. The dose profiles have been compared with the ones obtained with an ionization chamber in water phantom for the field sizes supplied by a 3D-Line dynamic multileaf collimator. These results show that this detector can be used for 2D dosimetry of x-ray photon beams, supplying a good spatial resolution and sensibly reducing the time spent in dosimetric verification of complex radiation fields.     

Measurements of output factors with different detector types and Monte Carlo calculations of stopping-power ratios for degraded electron beams

The aim of the present study was to investigate three different detector types (a parallel-plate ionization chamber, a p-type silicon diode and a diamond detector) with regard to output factor measurements in degraded electron beams, such as those encountered in small-electron-field radiotherapy and intraoperative radiation therapy (IORT). The Monte Carlo method was used to calculate mass collision stopping-power ratios between water and the different detector materials for these complex electron beams (nominal energies of 6, 12 and 20 MeV). The diamond detector was shown to exhibit excellent properties for output factor measurements in degraded beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of output factors, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth (especially for lower electron energies). Application of ionization-chamber-based dosimetry, according to international dosimetry protocols, will introduce uncertainties smaller than 0.3% into the output factor determination for conventional IORT beams if the variation of the water-to-air stopping-power ratio is not taken into account. The IORT system at our department includes a 0.3 cm thin plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. This was verified by the measurements. In small-electron-beam dosimetry, the water-to-air stopping-power ratio variation with field size could mostly be ignored. For fields with flat lateral dose profiles (>3 x 3 cm2), output factors determined with the ionization chamber were found to be in close agreement with the results of the diamond detector. For smaller field sizes the lateral extension of the ionization chamber hampers its use. We therefore recommend that the readily available silicon diode detector should be used for output factor measurements in complex electron fields.     

Dosimetry of low-energy protons and light ions

For the vertical beam facility at the 14 MV Munich tandem accelerator, various techniques for dosimetry were tested for radiation fields of low-energy protons and light ions (4He, 12C and 16O). A reference dose was determined from the fluence of particles by counting individual particles. A parallel-plate Markus chamber with a small sensitive air volume was used for beam dosimetry applying the ICRU protocol. The doses measured with the ionization chamber were compared with doses evaluated from the fluence measurements. Alternative dose measurements were performed using MTS-N LiF:Mg, Ti thermoluminescence detectors (TLDs) and a photometrically evaluated Fricke chemical dosimeter. An uncertainty of 8% was found in the determination of the dose relative to the reference method. Effects of an inhomogeneous energy loss and a finite track length of the projectiles in the sensitive detector volume of the dosimeters had to be taken into account.     

Performance evaluation of a multi-slice CT system

Our purpose in this study was to characterize the performance of a recently introduced multi-slice CT scanner (LightSpeed QX/i, Version 1.0, General Electric Medical Systems) in comparison to a single-slice scanner from the same manufacturer (HiSpeed CT/i, Version 4.0). To facilitate this comparison, a refined definition of pitch is introduced which accommodates multi-slice CT systems, yet maintains the existing relationships between pitch, patient dose, and image quality. The following performance parameters were assessed: radiation and slice sensitivity profiles, low-contrast and limiting spatial resolution, image uniformity and noise, CT number and geometric accuracy, and dose. The multi-slice system was tested in axial (1, 2, or 4 images per gantry rotation) and HQ (Pitch = 0.75) and HS (Pitch = 1.5) helical modes. Axial and helical acquisition speed and limiting spatial resolution (0.8-s exposure) were improved on the multi-slice system. Slice sensitivity profiles, image noise, CT number accuracy and uniformity, and low-contrast resolution were similar. In some HS-helical modes, helical artifacts and geometric distortion were more pronounced with a different appearance. Radiation slice profiles and doses were larger on the multi-slice system at all scan widths. For a typical abdomen and pelvis exam, the central and surface body doses for 5-mm helical scans were higher on the multi-slice system by approximately 50%. The increase in surface CTDI values (with respect to the single-slice system) was greatest for the 4 x 1.25-mm detector configuration (190% for head, 240% for body) and least for the 4 x 5-mm configuration (53% for head, 76% for body). Preliminary testing of version 1.1 software demonstrated reduced doses on the multi-slice scanner, where the increase in body surface CTDI values (with respect to the single-slice system) was 105% for the 4 x 1.25-mm detector configuration and 10% for the 4 x 5-mm configuration. In summary, the axial and HQ-helical modes of the multi-slice system provided excellent image quality and a substantial reduction in exam time and tube loading, although at varying degrees of increased dose relative to the single-slice scanner.     

Radiotherapy dosimetry using a commercial OSL system

A commercial optically stimulated luminescence (OSL) system developed for radiation protection dosimetry by Landauer, Inc., the InLight microStar reader, was tested for dosimetry procedures in radiotherapy. The system uses carbon-doped aluminum oxide, Al2O3:C, as a radiation detector material. Using this OSL system, a percent depth dose curve for 60Co gamma radiation was measured in solid water. Field size and SSD dependences of the detector response were also evaluated. The dose response relationship was investigated between 25 and 400 cGy. The decay of the response with time following irradiation and the energy dependence of the Al2O3:C OSL detectors were also measured. The results obtained using OSL dosimeters show good agreement with ionization chamber and diode measurements carried out under the same conditions. Reproducibility studies show that the response of the OSL system to repeated exposures is 2.5% (1sd), indicating a real possibility of applying the Landauer OSL commercial system for radiotherapy dosimetric procedures.