A Comparison Of Image Quality Between Spectral And Routine CT Scanning Mode For Bone Subtraction Cerebral CT Angiography

Object:To investigate the keV level with optimal contrast noise ratio (CNR) between cerebral artery and cerebral tissue on contrast-enhanced spectral CT images using fast kVp switch technology. To prospectively and comparatively evaluate image quality between spectral and routine CT scanning mode for bone subtraction cerebral CT angiography (CTA).Materials and methods:Seventy-four consecutive patients who underwent cerebral CTA were distributed into spectral mode group and routine mode group randomly on DISCOVERY CT750HD.The optimal CNR analysis for cerebral artery against cerebral tissue was applied on the contrast-enhanced spectral CT images before subtraction, and the energy levels for optimal CNR were recorded as keV1and keV2according to the two peaks of the curve. The subtracted images were obtained both from spectral CT images (140kVp, keV1, keV2) and from120kVp images (routine).Cerebral CTA images were reformatted with the subtracted images. CT values of the cerebral artery, contrast between cerebral artery and cerebral tissue, background noise, CNR of the artery against cerebral tissue, signal noise ratio(SNR) of the artery and subjective evaluation on bone removal and image quality(both ranks from1to5mean bad to excellent) were compared between different image sets.Results:There were two peaks on the optimal CNR curve for the cerebral artery against cerebral tissue with contrast-enhanced spectral CT images before subtraction. The mean value of keV1and keV2were (55.19±2.05) keV and (64.92±1.16) keV, respectively. There were no difference among the four groups on subjective evaluation of bone removal (χ2=3.85, p=0.28>0.05).Consistency of the two doctors is good (Kappa=0.65, u=12.53, p=0.00(<0.05)).There were differences for the subjective evaluation of image quality of the reformatted CTA images among the four groups.(R1140kVp=0.33,RlkeV1=0.7,R1keV2=0.55,R1routine=0.42,χ2=35.52,P=0.00<0.05;R2140kVP=0.31,R2keV1=0.68,R2keV2=0.57,R2roUtine=0.44,x2=33.52,P=0.00<0.05).The image quality of CTA of the four groups was ranked from excellent to bad by the order of keV1group,keV2group, routine group,140kVp group. The image quality of CTA on keV1was the best. The differences between140kVp,keV1,keV2,and routine groups with bone subtraction for CT values of the cerebral artery, contrast between cerebral artery and cerebral tissue, background noise, CNR of the artery against cerebral tissue, SNR of the artery were significant(F=38.849,38.620,7.995,11.342,10.982(p=0.00<0.05),respectively).The mean CT value of cerebral artery and contrast between cerebral artery and cerebral tissue of the four groups decreased by the order from keV1group,keV2group, routine group, to140kVp group.KeV2group has the lowest background noise. CNR of the artery against cerebral tissue and SNR of the artery of keV1group were similar to those of keV2group. CNR and SNR of140kVp group were similar to those of routine group. CNR and SNR of keV1group and keV2group were higher than those of routine group and140kVp group.Conclusion:1. There were two peaks on the optimal CNR curve for the cerebral artery against cerebral tissue with contrast-enhanced spectral CT images before subtraction. The mean value of keV1and keV2were (55.19±2.05) keV and (64.92±1.16) keV, respectively.2. There was no difference between the four groups with two scan mode on subjective evaluation of bone removal using subtraction and VR, segment/autoselect software synthetic method, which was better than the contemporary method.3. Compared to routine group, keV1group and keV2group had better image quality of cerebral CTA. The140kVp images obtained from spectral imaging by fast kVp switch technology could only be considered as the reference images for quality control considering the image quality of140kVp CTA image were not good enough when compared to routine images.4. The image quality of CTA for keV1group and keV2group was better than that of routine group and the image quality of CTA for keVl group was better than keV2group. Thus,55keV which is the mean of virtual monochromatic energy level in keV1group is recommended as the default monochromatic image for post-processing of cerebral CTA, which can improve the image quality of CTA and optimize the workflow of spectral CT imaging.