Four-dimensional Computed Tomography Reconstruction And Application In Radiation Therapy

Computed Tomography(CT) image owns some unique advantages in spatiality and density resolution,which has become a landmark in the developing history of medical image technique.Compared with the conventional X-ray fluoroscopy,the ordinary CT scan,as an indirect image technique,cannot synchronously characterize the dynamic features of the motion viscera in thorax and abdomen.Nevertheless, ordinary CT can reflect the three-dimensional(3D) spatial structures by applying 3D-CT reconstruction.In recent years,four-dimensional computed tomography (4D-CT) has been developed in order to characterize the dynamic features of motion targets.Compared with traditional CT scan,4D-CT has greatly reduced the motion artifacts,not only representing the shape of organs in thorax and abdomen such as liver,kidney,lung and diaphragm which will move with the respiration,but also reflecting the extents and features of movement.4D-CT and its application in radiation therapy have brought about a profound effect in precise radiotherapy for the thoracic and abdominal organs:①4D-CT can be applied to analyze the characteristics of volume variation and location displacement of targets and organs at risk during the respiratory motion,which is of great help to design the patient-specific treatment plan that can reduce the irradiated area and increase the target dose and at the same time spare the normal tissues.②In the course of stereotactic radiotherapy of lung and liver,4D-CT can be expediently applied to analyze the tumors and organs of motion at varying levels of abdominal compression,which will guide the application of abdominal compression and implement precise radiotherapy for lung and liver tumor.③4D-CT can be used to investigate the correlation between internal fiducial tumor motion and external marker motion and implement 4D image guided radiation therapy(IGRT),which will promote in the advance of radiotherapy from 3D to 4D.However,there still exist some limitations in the present 4D-CT system. Currently,the main factors that restrict 4D-CT to a wide utilization in precise radiotherapy include the following aspects:①There is not any 4D-CT system developed in native China yet.The imported 4D-CT systems are very expensive, which must depend on external respiration monitor devices while acquiring synchronous respiratory signals and 4D-CT reconstruction can be implemented only by a small number of newer-type CT scanners.②Now there is not perfect commercial 4D treatment planning system(4D-TPS).The 4D-CT data series contain a great number of CT slices,usually 1000～2000 slices or even more.The workload of segmenting targets and organs at risk(OAR) on 4D-CT slices usually needs ten times of that on the conventional CT.As a result,target delineation has become a bottleneck of the application of 4D-CT in radiation therapy.③The dose accumulation and verification of motion targets in 4D treatment planning still are the great problem?? the whole process of 4D radiation therapy.In order to solve the limitations of the present 4D-CT systems,which must depend on the external respiration monitor systems and implement 4D-CT reconstruction on common helical CT scanners,Berlinge et al proposed a simple 4D-CT reconstruction method,which labels the CT images in the respiratory state by a needle that follows the abdominal respiratory motion by means of leverage.This method can implement 4D-CT reconstruction without external breath monitor and has been greatly accepted in clinics due to its simplicity.But,the positions of the needle in Berlinge's 4D-CT reconstruction were binned into only five respiratory states instead of eight to ten bins used by others and it was impossible to distinguish between the intermediate,inhale and exhale directions of the breathing cycle.Another 4D-CT reconstruction method put forward by McClelland et al was based on continuing 4D motion model.And the iterative sorting method proposed by Zeng et al was based on the internal anatomy motion,which can get a high-quality 4D-CT image.However,all the reconstruction methods have limitations.The disadvantage of those methods is that the calculation will cost several hours.So the application value was very limited.Hence,it is of great necessity to improve the 4D-CT reconstruction and its application in radiation therapy.On the basis of summarizing the former achievements and limitations of 4D-CT reconstruction and its application in radiotherapy,two novel 4D-CT reconstruction methods have been proposed in our study.The corresponding software systems have been developed,which will result in simple and fast 4D-CT reconstruction.As a result,the condition of no domestic 4D-CT reconstruction system has changed.Thus, the applications of 4D-CT in radiotherapy have been investigated in the study,the main works of which can be recapitulated below:First,a motion platform that can simulate??an respiration was developed with a step motor,motor driver and low-density foam.Several balls with different sizes made of polyethylene and two potatoes were placed into the low-density foam to simulate tumors with different sizes.The motion phantom was adopted to investigate the respiratory effects on 3D reconstruction of motion targets with different periods and initial phases,which were scanned by a GE LightSpeed16 CT scanner with different pitches and slices.The relative deviations of reconstruction volumes between dynamic targets and the stationary ones were calculated.No obvious differences were observed between the 3D reconstruction volumes of stationary targets scanned with different pitches and slices.But great differences were found between the 3D reconstruction volumes of dynamic targets and stationary ones.The relative deviations of reconstruction volumes of targets,which were scanned with different pitches,slices and periods,were variable among the targets,about -39.8%～89.5%for the smaller targets and -18.4%～20.5%for the larger ones,which indicates that the respiration has great effects on the 3D reconstruction of the tumor targets.The CT images adopted in three-dimensional treatment planning system (3D-TPS) must be acquired while the targets are under static condition.Otherwise, severe errors will be introduced into the treatment plan.In order to implement precise radiotherapy,it is of great necessity to reduce or eliminate the motion artifacts and include motion information of targets in the course of the treatment planning and irradiating processes.Based on the comprehensive analysis of 4D-CT reconstruction theory, combining with the intrinsic features of CT images for the same target,a novel 4D-CT reconstruction method based on similarity principle of spatial adjacent images has been proposed in our study.The corresponding 4D-CT reconstruction program was developed by Using VC++ program language and Visual Tools Kits(VTK).The CT data of patients with free breathing were acquired??Cine model at several continuing couch positions.The Cine duration time of every couch position was equal to the respiratory period pulsing one second.The mutual information(MI) of spatial adjacent images was taken as the similarity measure,on which the phases resorting were based.The 4D-CT reconstruction has been implemented with conventional multi-slices CT scanners without any external breath monitor device based on the similarity principle.Advantages of this method include the following aspects:high image quality,lower motion artifacts,characterizing not only reality of the periodic motion target,but also extent and disciplinarian of periodic motion.The most apparent disadvantage of the method is that a single phase 4D-CT reconstruction will take several minutes.For instance,the reconstruction process generally costs 3～6 min to complete a single phase 4D-CT on a lap PC(Dell Latitude 620,1GB memory) for 1000～2000 slices CT,which can not distinguish the respiratory phase directly.In order to improve the method mentioned above and implement the 4D-CT reconstruction more conveniently and effectively,another 4D-CT reconstruction method,based on the volume change of the thoracic and abdominal tissues or lung tissue,has been proposed in our study.The corresponding 4D-CT reconstruction program was carried out by using VC++ program language and VTK software tools, including the following modules:image transmission,readin CT image,threshold segmentation,volume calculation,phase resort,dynamic 4D-CT visualization,result output and print.The representative patients,whose CT data under free breathing conditions were acquired in Cine model at several continuing couch positions,were selected to implement the 4D-CT reconstruction.The phase of each CT image within a breathing cycle was determined according to the pixel volume the thoracic and abdominal contours or lung tissues.The CT images of different phases were resorted into different series according to their phases.Several CT series in diffe?? phases of a motion cycle were acquired,which would result in a 4D-CT data that could reflect the characteristics of the periodical motion targets.Compared with the current imported 4D-CT systems,the advantages of our proposed 4D-CT systems can be summarized as follows:①The 4D-CT reconstruction of periodical motion targets can be implemented with any general multi-slice CT scanners.The process of the reconstruction does not depend on any external respiratory monitor devices and not restricted to the hardware or software of CT scanner.As a consequence,it can be applied universally.After the 4D-CT reconstruction has been completed,arbitrary single-phase 4D-CT,sagittal or coronal or 3D view CTs can be exported,which will facilitate its application in clinical practice and scientific study.②It can not be obstructed by thermoplastic immobilization mask(TIM) during the 4D-CT reconstruction and it is a breakthrough with a great value for the current imported 4D-CT reconstruction system.When patient receiving radiotherapy is immobilized with TIM and his/her breathing amplitude is restricted,it will result in a failure determination of respiration phase for the external respiratory monitor system. Therefore,the imported 4D-CT system can not implement 4D-CT reconstruction under this condition.But,the TIM will not affect the 4D-CT reconstruction system developed by our group,which is based on the volume change of thoracic and abdominal tissues or lung volume.Therefore,it will facilitate the popularization of 4D-CT in precise radiotherapy.③Another advantage of the 4D-CT reconstruction system based on the volume change induced by respiration is very convenient and effective.For example,a lap PC with IGB memory(Dell Latitude 620) will take only 40s to input 1200 slices CT (512×512),less than 10s to resort phases,and less than 2s to reconstruct arbitrary single phase 4D-CT,sagittal or coronal view images.If a powerful PC with 2GB memory is used to input 2000 slices CT,which covers the whole thorax?? abdomen, the 4D-CT reconstruction process will be less than 10s.So it is much more efficient than the hours-consuming conventional iterative computation methodology.④Real 4D view:The present literatures about 4D-CT reconstructions have only reported sagittal and coronal 4D-CT images.Not only two-dimensional sagittal and coronal 4D-CT images have been reconstructed,but also 3D sagittal and coronal views changing with time have been achieved.Additionally,the living viscera motion induced by respiration can be displayed periodically.After the 4D-CT system has been developed,the variety characteristics of dose distribution of motion targets receiving three-dimensional conformal radiation therapy(3D-CRT) and intensity modulated radiation therapy(IMRT) have been investigated,based on 4D-CT,averaged intensity projection(AIP) and maximal intensity projection(MIP) CT:①The difference of the target volumes among 4D, AIP and MIP CT of the same patient were analyzed for the tumors located separately at upper lung,middle lung and abdomen.②Based on the intrinsic features of AIP and MIP CT,a software system has been developed to create AIP and MIP CT directly through Cine scan CT data without 4D-CT reconstruction and any external breathing monitoring devices.It can create AIP and MIP CT with any multi-slice CT scanners.③The various characteristics of phantom in AIP and MIP CT images have been investigated with different window widths and levels,which will become the application foundation of AIP and MIP CT image in precise radiotherapy.④The dose distribution on different phases 4D-CT,AIP and MIP CT series were achieved by utilizing the fuse module of conventional 3D-TPS,which can characterize intuitively the dose distributions of motion targets.For the moving target,the dose distributions based on an extreme phase 4D-CT can not cover the whole tumor volume on the other extreme phase of 4D-CT.If the dose calculation is based on the tumor contours on AIP or MIP CT images,it??l cover the entire tumors on two extreme respiratory phase 4D-CT images and the irradiation area will not be enlarged egregiously.Hence,the irradiation to the motion target is reasonable.Furthermore, when the AIP and MIP CT series are used to create the planning for periodic motion targets,it will reduce a lot of work for target delineation on all of 4D-CT images.It is a very effective method to utilize the motion information contained in the 4D-CT data. ⑤According to the features of the clip motion targets,a simplified iterative model of dose calculation matrix was developed in our study,and the corresponding simulation program has been accomplished by using Matlab 7.0 software tools,which can predict the dose distributions of clip motion targets.On the basis of treatment planning on 4D-CT and AIP/MIP CT images,a 2D air vented ionization chamber array MatriXX system and a platform which can mimic the clip motion of lung tumor were adopted to investigate the influences of respiratory motion on target dose distribution.The edge dose blurring effect was found in the motion phantom experiment and the random effect of the IMRT segment beam based on multileaf collimator(MLC) has been analysed carefully.Because of the random dose deviation for different field sizes,irradiation locations and doses,the accumulated dose deviation can be greatly reduced.Compared with the static phantom in 3D-CRT,the penumbra of dose distribution of the periodic moving phantom along the moving directions has increased by 6～9 mm.The high dose area has shrinked by about 5mm and the low dose area extended by 5mm.But the area of 50%isodose and the dose center area changed little.When a single segment beam of IMRT irradiation was measured and the maximum dose of measuring plane was normalized to 100%,the averaged difference of dose distribution between the static and periodic phantoms was±27%(from -56.4%～56.1%).When all of the segment beams of IMRT were delivered and the integrated dose distribution was measured,the differences were less than±3%?? maximum difference of dose distribution was about±15%,mainly appearing at the field margin,which was similar to 3D-CRT.The results indicate that the dose distributions of most center areas of the periodic moving targets irradiated by multi-fraction 3DCRT/IMRT beams are similar to those of the static targets,while the high dose area of the former has shrinked and the low area extended.The calculated dose distributions match with the measuring results and the iterative model of dose calculation matrix can be served as a new predictor of the dose distributions of the periodic motion targets.At the end of the study,the achieved results and deficiencies were summarized, and part of the 4D-CT program frames and codes were listed for further study.