Research On Pseudo CT Imaging Based On Ultrasound Images In Radiotherapy

The clinical applications of image guided radiation therapy(IGRT)and adaptive radiotherapy(ART)improve the non-uniform or inadequate dose in tumor target area caused by organ movement and positioning error.Cone beam CT(CBCT),as a common image guiding device in clinical radiotherapy,has been widely used in IGRT and ART radiotherapy.However,CBCT has the following disadvantages: in the position verification,the rigid body registration is mostly performed with the patient's bony mark or gray value,and the position and shape of the organ in the patient are ignored.The soft tissue resolution of the CBCT image is not high and the electron density is not accurate,which can not meet the requirements of accurate calculation of treatment plan;CBCT scan has beam sclerotherapy effect,electron scattering and other artifacts,the image quality after reconstruction is poor;CBCT will bring additional radiation dose to patients.Compared with the traditional CBCT guided radiotherapy technology,ultrasound-guided radiotherapy has the advantages of high soft tissue imaging quality,real-time,and no radiation.Under the direct real-time guidance of ultrasound equipment,the irradiation field and the tumor target can be kept relatively fixed,and the dynamic conformity can be achieved,which makes the radiotherapy more accurate.Ultrasound-guided radiotherapy needs to be positioned with CBCT in the treatment stage.Although it improves the accuracy of IGRT,multiple CBCT scans increase the radiation dose of patients.Therefore,the pseudo CT(CTps)guidance method based on ultrasound image acquisition can avoid repeated scanning of CBCT,and is suitable for real-time correction of tumor target in adaptive radiotherapy.This paper mainly studies the pseudo-CT images based on different ultrasonic deformation field methods,and verifies the possibility of using pseudo-CT for clinical adaptiveradiotherapy.Research achievements and contributions are summarized as follows:(1)Image preprocessing: Because the voxel information collected by different imaging devices is inconsistent in clinical treatment,if the ultrasound image(USsim)of the simulated positioning scan and the ultrasound image(USpv)scanned before the position verification are not unified before the registration,the voxel information is not unified.The obtained deformation field applied to the localized CT image does not result in a pseudo CT that meets the treatment requirements.In addition,since the ultrasound image can only reflect the local tissue structure information in the patient's body,it is necessary to intercept the same imaging region in the localized CT image as the positioning ultrasound image before applying the deformation field.Therefore,it is necessary to unify the voxel information such as the size,voxel spacing and initial position of the registration image before the experiment,so that the ultrasound and CT images can be in the same spatial coordinate system to ensure the correct application of the deformation field.(2)Ultrasound image mosaic processing: Since the ultrasound image is narrow-beam imaging compared with CT and CBCT,sometimes the single-shot ultrasound image can not completely include the tumor target area and dangerous organs in the imaging range,so the fusion and registration of CT image is only local,which results in some errors in registration of CBCT and ultrasound images based on the tumor target area,and then affects the accuracy of final positioning verification of patients.In order to overcome the limitation of the existing ultrasound imaging range,an automatic ultrasound image mosaic method based on adaptive threshold selection is proposed.Firstly,the sequential ultrasound images in different directions are acquired by ultrasonic scanning equipment.On this basis,the Harris corner detection algorithm based on adaptive threshold is studied for the ultrasound data acquired in each direction.Through the removal of pseudo-corners and the selection of optimalthreshold,the set of corner pairs between ultrasound pixels is finally determined,and the fusion and stitching of large field of view ultrasound images is completed,which makes the stitched ultrasound images reflect the relative position of patients' tissues and organs more truly and accurately.The similarity measure method is used to compare the stitched ultrasound image with the gold standard ultrasound image,to verify the accuracy of the stitched ultrasound image using the improved algorithm,and to register the stitched image with CBCT,to verify the integrity of the image of abdominal tissue and organs in the stitched ultrasound image.(3)Acquiring pseudo CT based on ultrasound deformation field: Compared with CBCT images,ultrasound images have abundant information,gray-scale section images,clear layers,dynamic real-time display of the active interface,and good display ability for small lesions.Therefore,in this paper,pseudo-CT images based on region of interest(ROI)mask and pseudo-CT images based on step-by-step local registration are proposed to replace CBCT as the calibration image of the patient's posture before each treatment.First,different masks are prepared to prevent skeleton from deforming in the registration process.Then,the deformation field between US images of cervical cancer patients is obtained by free-form deformation registration method,which is used to locate CT images and obtain pseudo-CT images.In the latter,by making masks of multiple ROI regions,different regions of interest are locally registered and several deformation fields are obtained.According to the order of the deformation fields,the superimposed deformation fields are superimposed on the localized CT images,and the pseudo-CT images are obtained.The feasibility of CTps image in clinical radiotherapy was verified by experiments in anatomical structure and dosimetry.The experimental results show that the normalized Mean Square Error(NMSE)and the normalized Mutual Information(NMI)are 0.13 and 0.25 respectively,and0.90 and 0.80 respectively,for the ultrasound image and CBCT stitched by the improved Harris corner detection method and the traditional Harris stitching method.The anatomical structure of tissues and organs in the imaging region of ultrasound image after the improved method splicing is more similar to that of CBCT.In addition,the mean values of NMI measurements of CTps and CBCT based on step-by-step local,single ROI overlapping area mask and overall ultrasound imaging overlapping area are 0.93,0.89 and 0.79,the mean values of correlation coefficient(CC)are 0.96,0.94 and 0.92,and the mean values of NMSE are 0.13,0.27 and 0.38,respectively.The CTps based on step-by-step local generation are more similar to the anatomical structure of CBCT,which can replace CBCT as the guiding image in the placement verification stage to avoid additional radiation for patients due to scanning CBCT.Under the same calculation conditions of Monte Carlo algorithm,the dose distribution of target area and organs at risk of radiotherapy planning based on CTps and localized CT is different.The CTps based on ultrasound image acquisition can modify the radiotherapy plan of fractional treatment,and can reduce the dose of surrounding normal tissues to the greatest extent while giving the dose of radiotherapy target area adequately.