The Study Of The Effect Factors Of Radiation Field And Image Quality In Wide-detector CT

PART Ⅰ THE STUDY OF THE EFFECT FACTORS OF RADIATION FIELD IN WIDE-DETECTOR CTObjective To explore the distribution characteristics of radiation field on Z-axis and X-axis on wide-detector CT with different scan modes,pitch and detector widths to provide the basis for the radiation protection of the chaperone and the operator.Materials and Methods Thermoluminescence dosimeters(TLDs)were placed at the central axis(Z-axis)of the scan hole and the X-axis on the frame.The CT head standard-dose phantom was scanned by GE Revolution CT under the sequential scan mode(with detector widths of 40 mm,80 mm and 160 mm)and the spiral scan mode(with detector widths of 40 mm and 80 mm).The scan parameters were as follows:tube voltage 120 k V,effective m As 200,pitch(for spiral scan):40 mm/0.516:1,40 mm/0.984:1,80 mm/0.508:1,80 mm/0.992:1.All scans were repeated for 4 times,and all TLDs were measured after exposure for the further analysis.Results No matter what detector width was selected,the scattered radiation in the Z-axis was attenuated by exponential function and the scattered radiation in the X-axis showed a bell-shaped with sequential scan mode and spiral scan mode.In the Z-axis,the scattered radiation on the head side of body were all higher than the foot side of body(Z=-2.366,-2.197,-2.366,-2.371,-2.028,-2.236,-2.028,P<0.05).There was no statistically significant difference in scattered radiation in the Z-axis and X-axis with the detector width was 40 mm in sequential scan mode and with the detector pitch of 40 mm/ 0.984:1 in spiral scan mode(Z=-0.345,-0.863,P>0.05).The remaining groups scattered radiation of sequential scan mode were smaller than that of spiral scan mode with same detector width.Under the sequential scan mode,the differences in the distribution of Z-axis and X-axis scattered radiation under different detector widths were statistically significant(X~2= 28.000、18.500,P<0.05),The maximum increase for detector width of 40 mm and 160 mm was 67.5μGy.Under the spiral scan,the largest distribution of scattered radiation was found to be the largest at the detector width of 80 mm and the smallest at the detector width of 40 mm.The maximum increase for detector width of 80 mm and 40 mm was 97.67μGy(Z=-2.981,-3.233,-2.353,-2.982,P<0.05).Furthermore,when the detector width and effective m As were same,the scattered radiation at a pitch of 0.516:1 and 0.508:1was greater than that at a pitch of 0.984:1 and 0.992:1,the maximum increase was 49.95μGy(Z=-2.510,-3.296,-2.903,-3.296,P<0.05).Conclusions In a wide-detector CT,the selection of different detector widths can significantly influence the distribution and radiation value of radiation field,thus a suitable detector width and relevant parameters shall be chosen according to the specific clinical requirements to reduce the radiation dose for the on-site operating medical staffs and accompanies.Significance The dose and distribution characteristics of radiation field under different scan modes and detector widths were analyzed.It has important guiding significance to reduce the radiation dose of patients and personnel in the CT room during clinical scanning.PART Ⅱ THE STUDY OF THE EFFECT OF SCAN MODES AND DETECTOR COMBINATIONS ON IMAGE QUALITY IN WIDE-DETECTOR CTObjective To evaluate the image quality of wide-detector CT with different scan modes,pitch,detector widths and detector positions and discuss the effect factors of image quality.Materials and Methods The body standard dose phantom and the Catphan500 phantom with high and low contrast resolution module were scanned by GE Revolution CT.The scan was performed under the sequential scan mode with the detector coverage of 40 mm,80 mm and 160 mm and under the spiral scan mode with the detector coverage of 40 mm and 80 mm with tube voltage 120 k V and tube current 200 m As.The resolution modules were placed at the central and peripheral of the selected detector coverage,and the overlapped positions between two axial scans.The diameter of the body standard dose phantom,CT number and SD were measured.Then,the high and low contrast resolutions of the acquired images were evaluated.Results When the body standard dose phantom was scanned,regardless of scan modes,the diameter of the body standard dose phantom were 320 mm.The CT number of 80 mm was the minimum(123.400±1.914HU)on sequential scan mode using different detector widths.And the difference between it and CT number(40 mm)or CT number(160 mm)was statistically significant(Z=-2.905,-2.825,P<0.05).The difference between CT number(40 mm)and CT number(160 mm)was not statistically significant(Z=-0.392,P>0.05).SD(40 mm)>SD(80 mm)(Z=-2.432,P<0.05),The other groups(Z=-0.903,-1.824,P>0.05).CT number(overlap)>CT number(central)(Z=-2.983,P<0.05)on sequential scan mode using different detector positions.SD(peripheral)>SD(overlap)>SD(central)(Z=-2.118,-2.590,-3.059,P<0.05).CT number(80 mm)>CT number(40 mm)(Z=-2.095,P<0.05)on spiral scan mode using different detector wide.SD(P0.5)<SD(P1)(Z=-2.664,P<0.05)on spiral scan mode using different pitch.There were no significant difference of CT number and SD on spiral scan mode using different detector positions(Z=-1.397,P>0.05),(Z=-0.103,P>0.05).The high contrast resolution of Catphan500 phantom was 7LP/ cm.When a larger detector width was selected,the low contrast resolution was worse,and it was higher in the central area than in the peripheral area of the detector using the sequential scan.The low contrast resolution was worse with the wider detector width and the larger the pitch using the spiral scan.Conclusions The selection of different scan modes,detector widths and detector positions will affect the image quality using wide-detector CT.Therefore,appropriate combination of detectors should be selected according to actual diagnosis.Significance This experiment analyzes the effect of scanning mode,detector width and detector position on image quality,which plays a guiding role in parameter selection during scanning.