Application Of Iterative Reconstruction And Spectral Techniques In CT Colonography:a Phantom Study

Part One Adaptive statistical iterative reconstruction versus model-based iterative reconstruction on image qualities and diagnostic efficiencies of CT colonographyObjective: To evaluate the effacacies of adaptive statistical iterative reconstruction (ASIR) and model-based iterative reconstruction (MBIR) to improve image quality and polyp detection rate in CT colonography (CTC) comparing with filtered back projection (FBP).Subjects and Methods:A series of thirty phantom polyps sized from1mm to10mm (ten≤3mm, ten>3mm and≤5mm, ten>5mm and≤10mm) were established by ligation on an isolated segment of porcine colon. Volume data were acquired on High Definition CT scanner at different doses (10,20,30,40,50,60,70,80,90,100,120,140,160,180,200,240and320mA at120kVp) using four different reconstruction algorithms (FBP、50%ASIR、100%ASIR and MBIR). CT virtual endoscopy (CTVE), virtual dissection (VD) and multiplanar reconstruction (MPR) were reformatted. The detection rate of phantom polyps, the image noise levels, the contrast to noise ratios (CNR), the noise reduction rate, and the CNR elevation rate were assessed by two radiologists blindly and compared statistically among of them.Results:All of50%ASIR,100%ASIR and MBIR yielded significantly reduced image noise compared with FBP (F=21.896, P<0.05), the image noise of MBIR were lower than FBP、50%and100%ASIR under10-70mA tube currents (P＜0.05). Compared with FBP, both MBIR and100%ASIR had higher CNR at all tube currents (F=4.120, P<0.05). MBIR had the highest CNR than FBP.50%and100%ASIR when tube current ranged at10～80mA (P<0.05). All of the CT colonography image quality scores were over3points. Under all tube currents, all of the30polyps were detected on CTVE and VD by all four reconstruction algorithm, however, those polyps smaller than3mm could not be distinguished from the mucosal folds on MPR images, and all of the polyps over3mm were detected on MPR.Conclusion:Compared with FBP, both ASIR and MBIR could significantly reduce image noise and improve image quality on CT colonography, MBIR is superior to ASIR in improving image quality at low dose, rendering the possibility of low dose CTC screening exam for colonic polyps. Part Two The efficacy of spectral CT colonography in detecting phantom polyps which masked by bowel fecesObjective:To evaluate the efficacy of monochromatic energy images and quality control (QC) images of Gemstone Spectral Imaging (GSI) in detecting small phantom colonic polyps masked by bowel feces. and to choose the optimal monochromatic energy image.Subjects and Methods:A series of twenty phantom polyps sized from1mm to10mm (ten≤5mm, ten>5mm and≤10mm) were established by ligation on an isolated segment of porcine colon. To simulate the tagged feces in CTC, simulated iodine-tagged fecal material which was a mixture of100g of corn flour,100ml water and2ml iohexol (Omnipaque,300mgl/ml) was filled into the phantom. Then the phantom was inflated by air until adequate distension. Volume data were acquired on a High Definition CT scanner, monochromatic energy images at40,50,60,70,75,80,85,90,100,120,140keV and QC images were reconstructed. Multiplanar reconstruction (MPR), iodine-based images and water-based images were reconstructed. The image noise levels, the contrast to noise ratios (CNR) and the detection rate of phantom polyps were assessed by two radiologists blindly and compared statistically among of them.Results:The image noise levels reduced from40keV (13.00±0.45) to140keV (4.01±0.22) monochromatic energy images, the image noise of70keV (5.00±0.21) achieved relatively low point which significantly lower than40,50,60,75,80and85keV monochromatic energy images (F=443.946, P＜0.05), the image noise levels of over70keV images increased slowly then decreased again along with the energy arising. The CNR of monochromatic energy images showed fluctuation. The70keV (47.97±2.74) monochromatic energy images had the highest CNR, and the CNR of70keV significantly higher than other monochromatic energy images except60keV (F=15.138, P<0.05). The image noise of70keV monochromatic energy images were significantly lower than QC images (t=-2.807, P＜0.05), and the CNR of70keV monochromatic energy images were significantly higher than QC images (t=2.619, P<0.05). The detection rates of colonic polyps were75%(15/20) for70,75,80,85, 90keV images and QC images, which higher than other monochromatic energy images. The MPR images of60keV and70keV were superior to other monochromatic energy images and QC images for the detection of polyps and mucosal folds. Application of iodine-based images and water-based images could not eliminate the simulated fecal materials completely.Conclusion:GSI has the ability to detect small colonic polyps which masked by bowel feces in phantom study. The optimal monochromatic energy images of GSI-CT is70keV in detecting small phantom colonic polyps which masked by bowel feces.