Keiji Inamura

The phantom for quality evaluations of the nonlinear noise reduction filter in multidetector row computed tomography

In this paper we propose the phantom for quality evaluations of the nonlinear noise reduction filter in multidetector row computed tomography (MDCT). A nonlinear filter is a filter whose output is not proportional to its input (CT number or contrast value). Generally, a standard phantom for CT cannot be used for the quality evaluation of such a filter. The maximum feature of our phantom is being able to obtain edge spread functions of a maximum of six edge intensities under the influence of a certain maximum contrast by choosing a slice position...The edge-preserving adaptive filter for selectively eliminating noise in low-dose scanning known as the Quantum Denoising System (QDS) developed by Toshiba Medical Systems was chosen as a nonlinear noise reduction filter for this study. For evaluation of the filter characteristics, three types of evaluation, the noise statistical values (mean and standard deviation of CT numbers), noise characteristics (noise Wiener spectrum, WS) and resolution characteristics (modulation transfer function, MTF) were measured using the phantom. As a result, the statistics value of obtained noise, and WS and MTF were able to demonstrate the performance of the filter clearly. Further, our phantom was able to investigate the capability of edge-preserving adaptive filters such as QDS using MTFs with different edge intensities. As compared with a digital phantom, our actual phantom is considered to be a useful tool in order to know the overall processing results containing the feature of each CT scanner and reconstruction filter of manufacturing companies and the performance of a nonlinear noise reduction filter.

Evaluation of the ability of nonlinear noise reduction filter to the low-contrast object in Multi-Detector Row Computed Tomography using a digital phantom and power spectrum

We propose a quantitative method to evaluate the ability of adaptive image filters for reducing noise to low-contrast objects in Multi-Detector Row Computed Tomography (MDCT). The digital phantom for evaluating the ability of a nonlinear noise reduction filter to a low-contrast object in a MDCT image was produced. The digital phantom consists of a water phantom image of MDCT and a bar pattern with a known contrast and spatial frequency. The edge-preserving adaptive filter for selectively eliminating noise in low-dose scanning known as the “Quantum Denoising System” developed by Toshiba Medical Systems was chosen as a nonlinear noise reduction filter for this paper. In this paper, two non-linear noise reduction filters, Q1 and Q2 in QDS were employed. By analyzing the power spectrum (PS) of a digital phantom, the filter performance as regards the contrast and spatial frequency of an object was investigated. The contrast-to-noise ratio from PS could express the feature of the filter briefly. That is, the validity of a filter can be shown based on the combined information of the contrast and spatial frequency of an object. Probably, this simple method will contribute to evaluating various noise reduction filters efficiently.

SU‐GG‐T‐545: Analysis of Biological Effective Doses for 4D‐SBRT Using a Model Based Simulation

Purpose: Four‐dimensional radiotherapy(4DRT) using respiratory gating system and other techniques to control patient breathing are useful for stereotactic body radiotherapy. However, treatment time in 4DRT also will be elongated by the degree of synchronization. In such a situation, the synchronization, internal margin and the target localization might have effects on the target as to tumorcontrol probability. The purpose of this study was to investigate the relationship of biological effective dose(BED) between respiratory motion patterns, gate parameters and target positioning accuracies using a model based simulation. Method and Materials: A virtual square of dimension 512×512 matrices of interest FOVs were used in this simulation. The pseudo tumor was set on the virtual matrices. The motion of the sphere was modeled by a simple cosine curve, and a sinusoidal breathing curve simulated by mathematical equation. A minimal time interval and various motion parameters such as amplitudes, target volumes, and gate parameters were set in this simulation. Then the probability densities of the target in each parameters ware calculated. Using calculated data sets, BEDs for the target were determined by changing two modeled curves of motion, margins, and gate parameters, respectively. Results: Various patterns such as tumor size, motion function, amplitude, gate parameters and size of margins of BEDs for the target were calculated. The result showed that it depended upon target volume and gate parameter. The approximate equation of optimal internal margins for target volume of 99% BED coverage was also determined. Conclusion: In this study, we have calculated BED based optimal internal margins of the moving target. This study also showed a method of 4DRT taking into account BED. Further research is in progress to evaluate the effect of elongation of patient irradiation and shape of the target by gate parameter settings.

Usefulness of GafChromic EBT film for dosimetry beyond 8.0 Gy

The objective of this study was to extend the usable dose measurement range of Gafchromic EBT film (EBT). EBT has the advantages of high sensitivity and improved film uniformity up to 8.0 Gy over Gafchromic MD-V2-55 film (MD-V2-55). EBT samples were exposed from 0.3 to 80 Gy. Optical densities of the samples were measured using ES-2200 (Seiko Epson Corporation, Nagano, Japan). EBT has an absorption spectrum with a maximum absorption band centered at 630 nm. The wavelength position of a maximum absorption does not shift with increasing dose within usable dose range. High sensitivity measurement of density can be performed using measurement light of 630 nm. We designed a flat-bed document scanner with single emission spectrum centered at 630 nm (IR-4000, iMeasure, Inc.) for EBT. Optical densities of the samples were also measured using IR-4000. The optical density response characteristics obtained by the combinations of EBT/ES-2200 and EBT/IR-4000 were compared with those of MD-V2-55/ES-2200. As a result, optical densities of EBT increased from 8.0 to 80 Gy. From the optical densities the calibration curves of net optical density versus delivered dose for dosimetry could be obtained. The combinations of EBT/ES- 2200 and EBT/IR-4000 had higher sensitivity of densitometry than MD-V2-55/ES-2200. The sensitivity of EBT/IR-4000 was highest. rror of dose measurements up to 80 Gy is within about ±5%, ±8%, and ±4% for MD-V2-55/ES-2200, EBT/ES- 2200, and EBT/IR-4000, respectively. The IR-4000 with single emission spectrum can bring out the capacity of high sensitivity and accuracy of EBT up to 80 Gy.