Tomokazu Ishida

Evaluation of dose efficiency index compared to receiver operating characteristics for assessing CT low-contrast performance

The dose efficiency index (DEI) is a dose independent measure that quantifies the low-contrast performance of CT scanners. The purpose of this paper is to compare the results of DEI analysis with those of ROC analysis. A custom-made phantom consisting of diluted contrast targets of various sizes and densities was scanned at 80 & 120 kV on a multislice CT scanner (Hispeed Advantage RP, GE). Eight radiographers reviewed the images and identified discernable targets. The likelihood of detection was measured as a function of the target size and scan conditions. The results of the DEI & ROC analyses showed that the low-contrast resolution was higher at 80kV than at 120kV. The p-value obtained by the paired-t test was 0.000 for both analyses, indicating that the difference was significant. Under the conditions used in this study, the DEI analysis was found to be an effective alternative to ROC analysis for characterizing the low-contrast performance of CT scanners. The evaluation time was about 1/6 compared with ROC analysis. It is much simpler to calculate than ROC and is useful in comparing scanners from different manufacturers or as part of ongoing quality assurance.

Improvement of Low-dose MDCT Images by Applying a Novel Adaptive Median Filter with Local Averaging

In order to reduce the quantum noise level without the expense of feature preservation of small tumors, a novel 3D adaptive median filter with local averaging is developed according to the size of tumors and the distribution of voxel value in a local region. In this paper, two new methods based on entropy of correlation and mutual information of correlation are also proposed to evaluate the validity and efficiency of the proposed filter quantitatively. The proposed filter is applied to processing low-dose abdominal datasets, and experimental results show that the filtering technique is able to reduce the level of the quantum noise and preserve the features of the edge region and the shape of the tumors for low-dose abdominal 3D MDCT images.

Analysis of 3D Linear and Non-linear Filtering Effects Based on 3D MDCT Medical Images

The risk of X-ray exposure will increase when applying the 3D-CT methods such as MDCT. To decrease this risk it is better to lower the dose-level of X-ray, but it reduces the resolution of obtained image and causes negative effect on imaging diagnosis. We think that 3D-filtering methods will recover the reduction. In order to obtain a better filtered result, 3D linear and non-linear filters are designed and applied to 80%, 60%, 40% and 20%-dose simulated low dose clinical MDCT images of abdomen with a tumor. Direct observation and 3D voxel value profile is extracted to evaluate the filtered results. Experimental results show both the filtering and the evaluating method perform well.