[Effect of Matrix Size on Lesion Detectability in CT Colonography].

OBJECTIVES: While computed tomography colonography (CTC) has become a non-invasive alternative to traditional colonoscopy, superficial lesions tend to be harder to detect, with reports of lesions <2 mm in height being missed. Therefore, for the evaluation of lesion detectability using CTC, optimizing scan parameters becomes important. In this study, we investigate the effect of the reconstructed matrix size from CTC on the lesion conspicuity. METHODS: A CTC phantom was scanned, and images were reconstructed with 512, 768 and 1024 matrix sizes with the same raw data. The image data in each matrix size were compared in terms of physical assessment of the task-transfer function (TTF) and the noise power spectrum (NPS), and of visual assessment using Scheffé's paired comparison. RESULTS: Comparing 768 and 1024 matrix sizes with that of 512, NPS was higher in high-frequency components. On the other hand, TTF was improved using these larger matrix sizes along with significant statistical differences in the visual assessment. CONCLUSION: Larger matrix sizes (768 and 1024) improve the lesion conspicuity, thereby helping to detect superficial and small lesions (size<2 mm) in CTC.

Effect of Reduced Tube Voltage on Diagnostic Accuracy of CT Colonography.

The normal tube voltage in computed tomography colonography (CTC) is 120 kV. Some reports indicate that the use of a low tube voltage (lower than 120 kV) technique plays a significant role in reduction of radiation dose. However, to determine whether a lower tube voltage can reduce radiation dose without compromising diagnostic accuracy, an evaluation of images that are obtained while maintaining the volume CT dose index (CTDIvol) is required. This study investigated the effect of reduced tube voltage in CTC, without modifying radiation dose (i.e. constant CTDIvol), on image quality. Evaluation of image quality involved the shape of the noise power spectrum, surface profiling with volume rendering (VR), and receiver operating characteristic (ROC) analysis. The shape of the noise power spectrum obtained with a tube voltage of 80 kV and 100 kV was not similar to the one produced with a tube voltage of 120 kV. Moreover, a higher standard deviation was observed on volume-rendered images that were generated using the reduced tube voltages. In addition, ROC analysis revealed a statistically significant drop in diagnostic accuracy with reduced tube voltage, revealing that the modification of tube voltage affects volume-rendered images. The results of this study suggest that reduction of tube voltage in CTC, so as to reduce radiation dose, affects image quality and diagnostic accuracy.

[Computed tomography colonography image assessment using colon phantom-effects of reconstructed slice thickness].

Computed tomography colonography (CTC) is a robust and reliable imaging test of the colon. Recent studies show good sensitivity for the identification of nonpolypoid (flat) lesions as well. The purpose of this study was to determine the accuracy and reproducibility of a volume-rendering (VR) and virtual gross pathology (VGP) technique for detecting a polypoid lesion phantom by varying slice thickness. The scan of a simulated house-made phantom was performed using a 16-slice CT scanner with varying combinations of tube voltage (120 kVp), effective exposure (100 mAs), detector configuration (16×0.75 mm), rotation time (0.75 s), helical pitch (0.688, 0.938, 1.066 and 1.188), reconstruction kernel (A, B and C), and section thickness/reconstruction interval (0.8/0.4, 1.0/0.5 and 1.5/0.75 mm). All image data were transferred to a three-dimensional workstation to assess multi-planar reformation (MPR), VR and VGP. Accuracy of volume measurement using the VR technique for quantitative analysis was compared using a paired t-test. Four radiological technologists also independently evaluated the visual score using the VGP technique for qualitative analysis, and their evaluations were compared using one-way analysis of variance with Fisher's protected least significant difference post-hoc test. There was a statistically significant difference in reproducibility between the three different slice thicknesses as to volume measurement and observer performance test (p<0.01 and p<0.05, respectively). Furthermore, the reproducibility improved when using thinner slices. In conclusion, VR and VGP techniques using a slice thickness of 0.8 mm made it possible to maintain accuracy and reproducibility when using CTC to detect polypoid lesions.

[Overview of the infant body part photography condition using 16-rows multidetector CT].

In infant CT scans, it is important to minimize radiation exposure without lowering the quality of the diagnosis. Therefore, appropriate parameters for infant CT scan should be considered at each institute. In order to determine parameters for infant body CT, we measured the physical characteristics of our current CT machine and evaluated scan parameters as S.D. below 10 on the basis of literature recommendations, which say that we should adopt S.D. of the liver when treated with a radiation exposure dose in an adult abdominal CT scan. As a result, the ideal parameters were 90 kV tube voltage, ultra fast detail resolution, B kernel, and Eff.mAs value calculated from patient body width. Infant body CT scans with the mentioned parameters resulted in an S.D. below 10, and this is thought to be applicable to further examinations.