基于12和16 bit CT图像的金属伪影校正对放疗剂量分布的影响

目的 利用金属伪影去除技术去除基于12 bit和16 bit CT图像中金属植入物伪影,分析其对图像CT值分布和放疗剂量分布的影响。方法 将金属棒插入模体中,CT扫描得到12和16 bit原始CT图像,运用归一化伪影去除法（NMAR）分别对所得到的原始CT图像进行去伪影处理,得到NMAR修正后图像。临床中选取人工股骨头患者CT图像,对其进行同样处理。比较分析各图像伪影去除前后CT值分布。在放疗计划系统中,基于各图像设计放射治疗计划,计算剂量分布,比较分析各图像的剂量分布差异。结果 12 bit图像中金属CT值为3 071 HU,远小于金属实际CT值11 080 HU;16 bit图像中金属CT值为11 098 HU,与实际值很接近。原始CT图像在金属周围含有大量伪影,CT值与参考图像CT值偏差很大;NMAR校正后图像伪影显著减少,CT值与参考图像较接近。NMAR修正后16 bit图像的剂量分布与参考图像最接近,中心轴上最大剂量偏差为1.8%;12 bit图像与参考图像在金属后方剂量差异很大,最大剂量偏差为81.6%。射线穿过原始图像伪影区域后导致剂量分布与参考图像有明显差异,引起最大剂量偏差达21.6%。结论 含有金属植入物时,基于16 bit图像进行NMAR伪影校正可以得到准确的CT值分布,从而得到准确的剂量分布。     

CT不规则伪影的探讨

CT的图像伪影是CT设备常见的故障，由于数据收集系统(DAS)的故障或失调，阵列处理器(AP)的故障都可能产生伪影，此外其工作环境不良亦可能产生伪影，如电网情况、湿度、温度、周围环境的电磁干扰、接地电阻、电源内阻等因素都可能引起图像伪影的出现。所以，在处理这类故障时应根据伪影的情况做出准确的判断，如果不仔细分析而盲目更换部件，不但故障难以排除，还可能造成经济损失。下面以几例故障情况的分析对此类伪影进行一些探讨。     

CT图像部分重建对消除运动伪影的作用

伪影是CT图像中有碍于诊断准确性的主要因素之一,尤其在动态CT中,伪影常成为既影响诊断,又无法补求的问题。利用CT 扫描机部分重建(partialreconstruction)功能,亦即亚重建功能(subreconstruc-tion),能有效地消除运动伪影对CT 图像的干扰,起到净化图像的作用。     

CT噪声和伪影对成像质量的影响及控制

ＣＴ噪声和伪影对成像质量的影响及控制赵勇李品江ＣＴ噪声和伪影是评价其成像质量的两项重要指标，如何降低影像噪声及减少伪影也是ＣＴ质量控制技术的重要内容，笔者拟结合有关资料及实践的体会对此作一初步的探讨。设备及方法使用的设备为ＳｏｍａｔｏｍＣＲ型全身ＣＴ...     

Toshiba TCT-300 CT伪影排除一则

故障现象 Toshiba TCT-300 CT扫描时图像出现淡环状伪影,校准水模后伪影消失,但2天后伪影重新出现。改变扫描条件发现大场水模图像粗糙,CT值及S．D．值均偏高。     

不同CT值赋值法对脑转移瘤放疗剂量计算影响的研究

目的研究不同CT值赋值法对脑转移瘤放疗计划剂量计算的影响,为基于磁共振(MR)图像进行放疗计划设计提供基础。方法选取35例接受放疗的脑转移瘤患者,每位患者在放疗前同一天分别进行CT和MR模拟定位,基于CT图像制定三维适形放射治疗(3D-CRT)或调强放射治疗(IMRT)计划为原计划Plan1。将CT图像和MR图像刚性配准,在CT和MR图像上勾画主要的组织和器官,计算各组织器官的群体化CT值。基于CT图像,采用3种CT值赋值法生成3组伪CT,分别为:全组织赋予140 HU;空腔、骨骼和软组织分别赋予-700、700和20 HU;不同组织器官分别赋予群体化的CT值。Plan1在3组伪CT上重新计算剂量分布,分别获得Plan2、Plan3、Plan4,然后比较这3组计划和Plan1的剂量学差异。结果骨骼、空腔平均CT值分别为(735.3±68.0)、(-723.9±27.0)HU,软组织的平均CT值基本分布在-70~70 HU。Plan2、Plan3、Plan4相比Plan1的剂量差异依次减小,在剂量指标比较中,眼晶状体最大剂量差异最大,分别可达5.0%以上、1.5%~2.0%、1.0%~1.5%,其余剂量指标差异的95%置信区间上限基本不超过2.0%、1.2%、0.8%。在像素点剂量比较中,局部靶区病例中差异>1%的区域主要分布在靠近射野的皮肤处,而全脑靶区病例中主要分布在骨骼与空腔、软组织交界处,以及靠近射野的皮肤处。此外,CT值赋值法在3D-CRT的剂量学差异大于IMRT,在全脑靶区病例大于局部靶区病例。结论不同CT值赋值法对脑转移瘤放疗计划剂量计算的影响显著,对骨骼、空腔和软组织赋予合适CT值,剂量计算偏差可基本控制于1.2%以内,而对各组织器官赋予群体化的CT值,可进一步将偏差控制于0.8%以内,满足临床要求。     

二极管半导体探测器(Diode)测量放射治疗患者剂量方法的研究

目的 研究用Diode探测器测量光子线束治疗中患者接受剂量的方法,验证治疗计划系统(TPS)计算剂量,并与Diode探测器测量剂量进行比较.方法 用60Coγ射线、6 MV X射线、水模体和固体模体,开展Diode探测器的重复性、剂量率响应、非线性剂量响应及刻度因子等实验.根据临床治疗需要,选择在不同条件下,研究剂量随机器角度、能量响应、源皮距、照射野、楔形角度、挡块和托盘因子等变化的影响,求出Diode探测器校准因子,用仿真人模体、Diode探测器、6 MV X线束,验证骨盆、头颈等部位剂量.再用Diode探测器测量6 MV X射线照射9例放疗患者的头颈、胸及腹等部位的剂量.结果 仿真人模体骨盆前面,左、右两侧(加楔形和不加楔形角度),以及头颈部左、右两侧(戴面具和不带面具)条件下,Diode测量值与TPS计算值的相对偏差均在±3%以内;放疗患者的头颈部两侧(戴面具)、胸部及腹部,Diode测量值与TPS计算值的相对偏差均在±5%以内.结论 用Diode探测器验证放疗患者剂量方法准确可靠,能快速获得数据.

Abstract：

Objective To explore the measurement method of the treatment dose of the patient with Diode for photon beam in radiotherapy,and to validate the treatment dose by comparing with the treatment planning system (TPS).Methods Experiments of the reproducibility,dose rate dependence,non-linearity dose response,and calibration factor in 60Co γ and 6 MV X beams were carried out with Diode on the surface of solid phantom and in water phantom.According to the needs of clinic treatment,different conditions were chosen to observe the dose changes with the angle of incidence,energy response,distance of source to skin,field size,wedge angle,block and tray using ionization chamber and water phantom.The Diode was placed on the surface of the solid phantom to obtain the correction factors.The doses of the chest,abdomen,and head and neek were verified with the Alderson phantom and Diode.Diode doses of the pelvis,head and neck at 14 points on the patient were measured.Results The Diode was irradiated at the points of the Alderson phantom,such as AP,RL and LL of the pelvis,with and without wedges,RL and LL junction of the neck and chin,with and without mask,the maximum relative deviation of doses was within ± 3% between Diode and TPS.The Diode was placed in different locations on the patient,including chest,abdomen and head and neck.The relative maximum deviation of doses was within ±5% between Diode and TPS.Conclusions The Diode method is reliable for measuring the exposure doses of the patient in radiotherapy.     

Kilovoltage cone-beam CT imaging dose during breast radiotherapy: A dose comparison between a left and right breast setup

The purpose of this study was to investigate the delivered dose from a kilovoltage cone-beam computed tomography (kV-CBCT) acquired in breast treatment position for a left and right breast setup. The dose was measured with thermoluminescent dosimeters positioned within a female anthropomorphic phantom at organ locations. Imaging was performed on an Elekta Synergy XVI system with the phantom setup on a breast board. The image protocol involved 120 kVp, 140 mAs, and a 270° arc rotation clockwise 0° to 270° for the left breast setup and 270° to 180° for the right breast setup (maximum arc rotations possible). The dose delivered to the left breast, right breast, and heart was 5.1 mGy, 3.9 mGy, and 4.0 mGy for the left breast setup kV-CBCT, and 6.4 mGy, 6.0 mGy, and 4.8 mGy for the right breast setup kV-CBCT, respectively. The rotation arc of the kV-CBCT influenced the dose delivered, with the right breast setup kV-CBCT found to deliver a dose of up to 4 mGy or 105% higher to the treated breast′s surface in comparison with the left breast setup. This is attributed to the kV-CBCT source being more proximal to the anterior of the phantom for a right breast setup, whereas the source is more proximal to the posterior of the patient for a left-side scan.     

Fast iterative algorithm for metal artifact reduction in X-ray CT

RATIONALE AND OBJECTIVES: The reduction of metal artifacts in x-ray computed tomography (CT) has important clinical applications. An iterative method adapted from the expectation maximization (EM) formula for emission CT was shown to be effective for metal artifact reduction, but its computational speed is slow. The goal of this project was to accelerate that iterative method for metal artifact reduction. MATERIALS AND METHODS: Using the row-action/ordered-subset (EM) formula for emission CT as a basis, the authors developed a fast iterative algorithm for metal artifact reduction. In each iteration of this algorithm, both reprojection from an intermediate image and backprojection from discrepancy data are performed. RESULTS: The feasibility of the fast iterative algorithm was demonstrated in numerical and phantom experiments. In comparison with the nonaccelerated iterative algorithm, the speed of iterative metal artifact reduction is improved by an order of magnitude given image quality in terms of visual inspection, I-divergence in the projection domain, and the euclidean distance in the image domain. CONCLUSION: The fast iterative algorithm corrects intermediate reconstruction according to subsets of projections and produces satisfactory image quality at a much faster speed than the previously published iterative algorithm. This algorithm has important potential in clinical applications, such as orthopedic, oncologic, and dental imaging.     

Comparison of calculated dose distributions reported as dose-to-water and dose-to-medium for intensity-modulated radiotherapy of nasopharyngeal cancer patients

Advanced dose calculation algorithms for radiation therapy treatment planning can report external beam photon dose 2-sided, in terms of dose-to-medium (D_m) and dose-to-water (D_w). The purpose of our study was to determinate the effect of D_w and D_m reporting modes built in Elekta Monaco treatment planning system on intensity-modulated radiotherapy dose distributions for patients with nasopharyngeal cancer. For 13 patients involved in this retrospective study, 2 plans were created: 1 using D_w and another according to D_m reporting mode. Treatment plans were normalized such that 100% planning target volume should be covered by 95% of prescribed dose. Dose-volume constraints were assigned according to international standards. The comparison between dose distributions was performed evaluating quantities important for respective volumes of interest. For target volumes, heterogeneity index and conformity index methodology were used along with the maximum dose concept. Also, for the comparisons over particular organ at risk, maximum dose or mean dose as well as dose-volume concepts were used. For all target volumes and majority of organs at risk, the differences between 2 reporting modes are statistically insignificant, but this is not the case for bony structured organs at risks: mandible and cochlea. It was observed that D_w is higher than D_m with mean difference of 9.91% (p?=?0.000009) of the mandible volume covered with 70?Gy. The same trend was observed for left and right cochlea with difference in mean dose of 8.74% (p?=?0.037) and 6.87% (p?=?0.029), respectively. The comparative analysis of dosimetric parameters in this study shows that the selection of reporting modes in Monaco treatment planning system can produce dose differences up to 15% in high-density volumes such as mandible and cochlea, which might have clinical consequences.     

质子治疗计划系统剂量算法的研究进展

质子治疗具有肿瘤区域剂量适形度高,周围危及器官所受辐射少的特点,对于减少患者的放疗辐射损伤具有重要的意义。准确地计算出患者体内的剂量分布是质子精准治疗的必要条件,因此,质子治疗计划系统是质子治疗中不可或缺的关键部分。本文对质子治疗计划系统及其所使用的剂量算法的研究进展进行综述。     

Impact of dose calculation algorithm on radiation therapy

The quality of radiation therapy depends on the ability to maximize the tumor control probability while minimize the normal tissue complication probability. Both of these two quantities are directly related to the accuracy of dose distributions calculated by treatment planning systems. The commonly used dose calculation algorithms in the treatment planning systems are reviewed in this work. The accuracy comparisons among these algorithms are illustrated by summarizing the highly cited research papers on this topic. Further, the correlation between the algorithms and tumor control probability/normal tissue complication probability values are manifested by several recent studies from different groups. All the cases demonstrate that dose calculation algorithms play a vital role in radiation therapy.     

剂量计算中解析组织非均匀性修正方法的比较研究

目的研究和比较目前放射治疗计划系统中常用的几种组织非均匀性修正方法。方法通过真实病人算例对不同修正方法的计算精度和计算速度进行对比测试。结果等效组织空气比修正（ETAR）方法虽然较Batho等修正方法考虑的影响因素多，但ETAR方法在大大增加计算时间的基础上，计算精度并未得到明显的提高。结论可使用新近开发的混合Batho修正方法替代目前计划系统中常用的ETAR方法。     

Optimal electrocardiographically triggered phase for reducing motion artifact at electron-beam CT in the coronary artery

RATIONALE AND OBJECTIVES: The authors performed this study to (a) investigate coronary movement with electron-beam computed tomography (CT) and (b) find the optimal electrocardiographic (ECG) triggering phase for eliminating motion artifact. MATERIALS AND METHODS: One hundred fifty-one patients without arrhythmia were examined with electron-beam CT. First, movie scans were obtained to create displacement and velocity graphs of coronary artery movement. Then, a volume scan with an exposure time of 100 msec was obtained with various ECG trigger settings. RESULTS: Movement patterns of coronary arteries varied with heart rate. Optimal triggering phase was before atrial systole (near 71% of the R-R interval) when heart rate was slower than 68 beats per minute and at ventricular end systole when heart rate was fast. Rate of severe motion artifacts decreased from 43% to 0% when triggering was altered from 80% of the R-R interval to the individual optimal value. Experimental values of the optimal phase at different heart rates were derived, and severe motion artifact was only 3.0% with these values. CONCLUSION: ECG triggering set according to the heart rate enables a great reduction in motion artifacts at electron-beam CT with a 100-msec exposure time. The results may have implications for magnetic resonance imaging of the coronary artery.