Computed tomography (CT) has been widely used in clinical diagnosis and made a great contribution to diagnosis by providing anatomical information with high-resolution.However,when metal implants exist in patients' body,reconstructed CT images are seriously interfered by metal artifacts.Metal artifacts are usually expressed as many dark and bright radiant streak artifacts which seriously reduce diagnosis reliability and bring errors into the calculation of dose distribution in radiotherapy.Therefore,the study of metal artifact reduction (MAR)is of great importance.This article reviews main methods on MAR developed in recent years,and give deep analysis on some of the methods.

Intensity-modulated radiation therapy is widely used in the treatment of tumor,and the dose distribution highly conforms to tumor target area in three-dimension.However,the factors such as complex beam,data error,algorithm error and machine error may cause large dose deviation in intensity-modulated radiation therapy,which may lead to unnecessary radiation accident.Therefore,standing on the patients' safety point of view,dose verification is usually performed before executing the treatment plan in order to ensure the safe implementation of the treatment plan and to avoid un-planned irradiation dose.Currently,there are many tools and methods of dose verification in clinic,including point dose verification tools like finger-shaped ionization chamber and thermoluminescence dosimeter;two-dimensional dose verification tools like Mapcheck,MatriXX and films;three-dimensional dose verification tools like ArcCHECK,Delta4 and the third-party software.These common clinical dose verification methods are reviewed in this paper.

Objective To study the detective sensitivity for position of multi-leave collimators ( MLC) using Delta-4. Methods First,the small positional deviation of MLC was simulated and measured using the linac (Varian,Trubeam) equipped with EPID.Then,two beam fields 2. 0 cm (x)×6. 0 cm (y), 7. 0 cm (x)×6. 0 cm (y) were designed,the x1 and x2 of MLC were expanded 0. 1 mm,0. 2 mm,0. 3 mm... 0. 9 mm and 1. 0 mm,2. 0...5. 0 mm to external simultaneously,different parameters of 3 mm/3%,2. 5 mm/2. 5%,2 mm/2%, 1. 5 mm/1. 5% and 1 mm/1% were used in Gamma analysis to analyze the difference between dose distribution detected by Delta-4 and original dose distribution with unexpanded MLC position derived from TPS. Results For 2. 0 cm ( x) × 6. 0 cm ( y) beam field, the pass rate of original dose distribution was 100%,and that decreased to 95. 5% when x1 ,x2 of MLC were expanded 0. 3 mm to external, and decreased to 89. 4% when expanded 0. 5 mm at 2. 5 mm/2. 5% statistical standards. For 7. 0 cm ( x) × 6. 0 cm ( y) beam field,the pass rate of original dose distribution was 96. 5%,and that decreased to less than 95% when x2,x2 of MLC were expanded 0. 3 mm to external,and passing rate was above 90% when MLC expanded less than 0. 5 mm at 1. 5 mm/1. 5% statistical standards. Conclusions For MLC' s positional deviation in decimillimeter level,raise standards of Gamma analysis properly may improve the capability of Delta-4 for detecting small positional deviation,but it won' t detect all the positional deviation of MLC in decimillimeter level. For different size of beam field,it is proposed to use different analytical standards for Delta-4.

More and more patients were treated by surgery with metal implants in clinic.Metal artifacts in CT images caused by metal implants brought challenges for postoperative evaluation and diagnosis of tumor.It also led to the possibility of missed diagnosis and misdiagnosis.In recent decades,the improved methods based on filtered-backprojection and iterative reconstruction algorithms have great progresses in reducing effects of metal artifacts.The actuality of techniques for metal artifact reduction were reviewed in this article.

Objective To reconstruct 16?bit images of metal implants using the extended function of computed tomography ( CT) imaging, and to analyze the effect of the metal CT value on calculation of dose distribution by evaluation of metal CT values in different scanning conditions. Methods A stainless steel rod and a titanium rod were inserted in a phantom. The 12?and 16?bit images and CT value distribution of metal implants were obtained by scanning the phantom using 120 kV tube voltage and 230 mA tube current. The 16?bit images and CT value distribution of metal implants were obtained by scanning the phantom using fixed tube current ( 230 mA) with varied tube voltage ( 100, 120, and 140 kV) or fixed tube voltage ( 120 kV) with varied tube current ( 180, 230, and 280 mA) . In the Varian treatment planning system, a single?field plan and a parallel?opposed field plan were designed based on the CT images. The dose distribution was calculated and compared by the paired t test. Results The CT values of the stainless steel rod and the titanium rod were both 3 071 HU in 12?bit CT images. In 16?bit CT images;however, the CT value of the stainless steel rod was significantly larger than that of the titanium rod. There were no significant differences in CT value of 16?bit image and dose distribution in radiotherapy plan between three scanning conditions with different tube currents. Under three scanning conditions with different tube voltages, the maximum CT values were 13 568, 13 127, and 12 295 HU for the stainless steel rod and 8 420, 7 140, and 6 310 HU for the titanium rod, respectively. Conclusions High?density metal implants cannot be distinguished by 12?bit images, while the distribution of metal CT value can be obtained by 16?bit images. The dose distribution of metal implants based on 12?bit images is different from that based on 16?bit images. Changes in tube voltage cause substantial changes in the CT value for metal implants, leading to changes in dose distribution in radiotherapy. Variation of tube current within a certain range causes slight changes in metal CT value and dose distribution.

Objective To investigate the impact on registration accuracy with the different registration ranges of CBCT images and CT images.Methods CBCT and CT scans were performed on the of 5 patients.The registration ranges of five patients' images of abdomen,head and chest performed CBCT and CT scanning were processed with four modes.Mode 1:the registration range of CT images was larger than the registration range of CBCT images,mode 2:the registration range of CT images and CBCT images were equally,mode 3:taking a 5 cm translation of CT images range from mode 2,mode 4:The CBCT range and CT range reduced 2 cm both sides simultaneously from mode 2.Using the registration program from Insight Segmentation and Registration Toolkit (ITK) to the four modes,the Mean Square Difference (MSD) metric values of four modes after registration were compared and the relationship between mode 2 and another three modes was analyzed by paired t test.Results For the MSD metric values,mode 3 was maximum,mode 2 and 4 were minimum,and mode 1 was centered.The difference between the mode 2 and mode 4 was not statistically significant(P ＞ 0.05).The difference between the mode 2 and mode 1 was statistically significant(t =-4.586,-4.164,-5.618,P ＜ 0.05).The difference between the mode 2 and mode 3 was statistically significant(t =-6.423,-8.109,-19.601,P＜0.05).Conclusion The registration ranges of CBCT and CT images has a certain extent of influence on the accuracy of image registration.The closer the registration range of CBCT and CT is,the higher the registration accuracy is.

Radiomics refers to the comprehensive quantification of information such as tumor biological feature and heterogeneity through assessing a large number of quantitative image features from ROI of CT,MRI and PET images.With the development of data science,radiomics has been paid more and more attention.Feature extraction is an important step of Radiomics.The processes in feature extraction of radiomics were reviewed in this paper.

Objective To apply metal artifact reduction algorithm to reduce metal artifacts based on 12 bit and 16 bit CT images,and aimed to analyze the effects on CT value and dose distribution.Methods The metal implant was inserted into the phantom,and the original 12 bit CT image and original 16 bit CT image were derived from CT scanning.The images were processed using NMAR algorithm,so the corrected 12 bit image and corrected 16 bit image were obtained.A patient's CT was chosen with artificial femur,and used the NMAR algorithm to reduce metal artifacts.Furthermore,the CT values of original images and corrected images were compared and analyzed.In the planning system,dose distribution was calculated based on each image by same radiation treatment plan.The dose distribution difference of each image was compared and analyzed.Results For the 12 bit image,the CT value of metal was 3 071 HU,which was much smaller than the metal's actual CT value ll 080 HU.The metal's CT value for the 16bit image was 11 098 HU,which was very close to the actual value.The original CT images contained a lot of artifacts around the metal,resulting in a large deviation of CT values from the reference image.After NMAR correction,metal artifacts were reduced significantly,and the CT values were close to the reference images.The dose distribution of the corrected 16 bit image was closest to that of the reference image.The maximum dose deviation on the central axis was 1.8％.The difference between the 12 bit image and the reference image downstream the metal was notable,and the maximum dose deviation on the central axis was 81.6％.The X-rays passed through the artifact region in original image,the dose distribution was obviously different from the reference image,and the maximum dose deviation was 21.6％.Conclusions For the patient with metal implant,using the NAMR algorithm on the 16 bit image result in accurate CT value of CT image,so that the accurate dose distribution will be obtained.

Objective To predict the tolerance range of set-up errors in clinical practice by predicting the virtual set-up errors of postoperative radiotherapy patients for breast cancer. Methods A retrospective analysis was made of the patients who underwent radiotherapy after breast cancer surgery in recent 3 years. According to different treatment method, 10 cases of breast cancer after modified radical mastectomy and 10 cases after breast conserving radical mastectomy were selected. The target area was delineated, the volume modulated radiation therapy plan was made, the virtual moving error of the center point of the plan was moved, and the maximum moving error was 5 mm by 1 mm step. After recalculating the dose, the corresponding data including the clinical target areas ( CT ) , CTV ( V50 ) , average heart volume, V20 of the affected lung and CTV volume were recorded. SPSS 19. 0 software was used for statistical analysis, and repeated measurement of variance analysis was used to analyze the changes of the actual dose in the target area after moving the center point. Linear regression analysis method was used to analyze the correlation between the CTV volume and the dose change after the CTV moving midline. Results The virtual set-up error had little influence on the axis direction of the approximate mammary tangent direction, while the vertical direction of the approximate mammary tangent direction had greater influence. After moving more than 3 mm on the vertical axis, the CTV ( V50 ) decreased to below 90％, which was lower than the general requirement for the target area of CTV. Statistical analysis of set-up errors in all directions showed that there was significant difference in dosimetric changes ( F=34. 182, 12. 877, 16. 443, 9. 846, 46. 829, 10. 122, 57. 931, P <0. 05) in all directions except the B direction of left breast set-up errors (P>0. 05). Between breast conserving surgery and modified radical mastectomy, there was little effect on target movement. Through correlation analysis, it was found that the volume of CTV in the target area of breast cancer patients was linearly related to the errors caused by the movement of B, C and B directions of left breast and right breast(F=5. 733, 18. 496, 6. 630, P<0. 05). Conclusions In postoperative radiotherapy for breast cancer, the errors perpendicular to the section of breast should be paid special attention to whether left or right breast. When the error of this direction exceeds 3 mm, CTV is obviously less than V50 below 83. 85％. The effect of set-up error on the dose of CTV was not related to the operation mode of breast cancer and there was no obvious correlation with the volume of the target area.

Objective:To develop the real-time radiotherapy monitoring system of three-dimensional (3D) point cloud by using depth camera and verify its feasibility.Methods:Taking the depth camera coordinate system as the world coordinate system, the conversion relationship between the simulation CT coordinate system and the world coordinate system was obtained from the calibration module. The patient's simulation CT point cloud was transformed into the world coordinate system through the above relationship, and registered with the patient's surface point cloud obtained in real-time manner by the depth camera to calculate the six-dimensional (6D) error, and complete the positioning verification and fractional internal position error monitoring in radiotherapy. Mean and standard deviation of 6D calculation error, Hausdorff distance of point cloud after registration and the running time of each part of the program were calculated to verify the feasibility of the system. Fifteen real patients were selected to calculate the 6D error between the system and cone beam CT (CBCT).Results:In the phantom experiment, the errors of the system in the x, y and z axes were (1.292±0.880)mm, (1.963±1.115)mm, (1.496±1.045)mm, respectively, and the errors in the rotation, pitch and roll directions were 0.201°±0.181°, 0.286°±0.326°, 0.181°±0.192°, respectively. For real patients, the translational error of the system was within 2.6 mm, the rotational error was approximately 1°, and the program run at 1-2 frames/s. The precision and speed met the radiotherapy requirement. Conclusion:The 3D point cloud radiotherapy real-time monitoring system based on depth camera can automatically complete the positioning verification before radiotherapy, real-time monitoring of body position during radiotherapy, and provide error visual feedback, which has potential clinical application value.