Study Of The Application Of Low-dose CT In The Reconstruction Of Three-dimensional Models Of The Maxillofacial Hard And Soft Tissues

Maxillofacial defects not only cause patient’s physiological function, but also result inserious psychological disorders, owing to appearance damaged. Parts of maxillofacialdefects can be repaired by Maxillofacial Surgery and Plastic Surgery, but serious defectswhich can be unable to surgical reconstruction or can not obtain satisfactory results,require prosthesis to rehabilitate. Traditional methods for producting prosthesis havetedious steps and long production cycle, and they require high skills for the physician.they are the reasons that hampered the development of maxillofacial prosthetic technology.In recent years, CAD/CAM technology was applied to maxillofacial prosthetics,whichoffers a new way for rehabilitate maxillofacial defects fast and simulated. Acquisition ofmaxillofacial three-dimensional data as a basis for CAD/CAM technology, has a directimpact on the final repairing effect. CT scan as one of the main methods of dataacquisition, has the advantage in the design for ear prosthesis and the skull implant. but itsbiggest drawback is causing potential radiological hazards for the patient. Under thepremise of ensuring the imaging diagnosis, how to reduce the CT radiation dose becomeimportant safety issues for obtaining3D data of the maxillofacial. The LDCT as a new method was proposed to resolve the problem of CT radiation safety. Scholars have carriedout studies of the application of low-dose CT in the maxillofacial, but has yet to seereports about the application of low-dose CT in the acquisition of three-dimensional dataof the maxillofacial hard and soft tissues.In this study, we first analyzed differences between3D models of the maxillofacialhard and soft tissues, which reconstructed by different scanning parameters, and whichwere reformed by de-noising algorithm. So the purpose of the first part was to study thefeasibility of LDCT in the reconstruction of three-dimensional models of the maxillofacialhard and soft tissues, and select appropriate scan parameters and de-noising algorithmeventually. The second part was to detect the actual radiation dose in this scan parametersby scanning the standard dose phantom. The third part was to evaluate the actual clinicalapplication, in order to explore optimal parameters and de-noising algorithm for CT scanto reconstruct3D models of the maxillofacial hard and soft tissues, and preliminarilyestablish a low-dose CT scan specification in the reconstruction of3D models of themaxillofacial hard and soft tissues. On this basis, the fourth part was to scan standardperformance phantom with conventional-dose and low-dose parameters, and measured thecorresponding image performance indexs, then analysised the influence of low-dose scanparameters to every indicator, and preliminarily established the detection indexs forlow-dose CT in the reconstruction of three-dimensional models of the maxillofacial hardand soft tissues.Followings are the results:1. Lightspeed16-slice spiral CT scanner were used to scan adult cadaver specimenswith different tube current parameters, and the differences between3D models of themaxillofacial hard and soft tissues, which reconstructed by different scanning parametersand de-noising algorithm were analyzed. The results showed: With the reduction of thetube current, the models’ surface become rough gradually. Compared to the280mA scanresults, the models’ surface of35mA was still fairing. but when fell to less than25mA, themodels’ surface become so rough to distinguish its exact shape. After de-noising, themodels’ surface of25mA become fairing, but the exact shape of15mA and5mA were still unable to distinguish, although the surface roughness was reduced. It came to the sameresults of the test results of models’ surface after registration. Based on the results, the lowdose scanning parameters of Lightspeed16-slice spiral CT scanner could be set as follows:tube current was35mA, tube voltage was120kV, slice thickness was1.25mm, pitch was1.0, and scan field was head mode. Combined with two de-noising methods, WaveletCoefficient Local Adaptive or Stationary Wavelet Adaptive Threshold, tube current couldbe further reduced to25mA. And the effects of the two denoising methods were obviousand Consistent.2. On the basis of having selected the low-dose scan parameters, the dose phantomwas used to measure actual radiation dose in this scan parameters accurately. The resultsshowed: Low dose scanning compared to conventional dose scanning,three radiation doseindicators(CTDIvol、DLP、ED) were significantly reduced. The35mA scanning CTDIvolvalue, DLP values and ED values were10.43mGy、156.45mGy·cm、0.36mSv,onlyapproximately20.10%relatived to280mA scanning. The25mA scanning CTDIvol value,DLP values and ED values were8.06mGy、120.90mGy·cm、0.28mSv, onlyapproximately16.23%relatived to280mA scanning. And there were no significantdifference between the calculated values of scanning dose phantom and the self-test valueof CT workstations. It is suggested that the radiation dose suffered by the patient werereduced greatly, compared the low dose CT scanning to conventional scanning in theacquisition of three-dimensional models of the maxillofacial hard and soft tissue.Therefore, It was safer to use low-dose CT acquire data than conventional CT in facialprosthetic CAD/CAM technology, and the smaller possibility of causing the patient’ sradiological hazards.3. The low-dose scan parameters above were used to scan Patients with maxillofacialdefects, and the practical application effect were evaluated. The results showed: There wasstatistical significance(P<0．05)in CTDIvol and DLP value, rainbow diagram generated by3D comparison of3D model of the hard and soft tissues, showed the differences of thesurface morphology was not greater than0.5mm, There was no statistical significance(P>0.05)in coordinate values(x, y, z value) of each corresponding marked points of the hard and soft tissues. In addition, the3D models of the maxillofacial soft tissue by35mAscanning were used to design and product prosthesis, and achieved good repair effect. Itshowed that the low-dose parameters scanning could be used to obtain satisfactory3Dmodels of the maxillofacial hard and soft tissues, and could effectively reduce radiationdose.4. The76-410-4130CT performance phantom were scanned with conventional-dose（280mA）and low-dose parameter（s35mA、25mA）, and the influence of low-dose scanparameters to every performance index were analyzed. The results showed: When the tubecurrent was35mA, Spatial resolution was0.75mm, density resolution was4.5mm, CTvalue uniformity was5.6HU and noise was10.916%; When the tube current was25mAand combined with two de-noising methods, Wavelet Coefficient Local Adaptive orStationary Wavelet Adaptive Threshold, Spatial resolution was1.00mm, density resolutionwas5.5mm, CT value uniformity was7.8HU and noise was19.315%. It showed that theaccuracy requirements in the reconstruction of three-dimensional models of themaxillofacial hard and soft tissues were meeted, when the low-dose scan test items ofLightspeed16-slice spiral CT scanner achieved targets above. And the indicators can beused as a reference for other brands or models of CT scanner, to determine the LDCTscanning parameters in the acquisition of three-dimensional models of the maxillofacialhard and soft tissues.