Katsuyuki Kojima

Computer-based measurement of right and left ventricular volumes in magnetic resonance imaging

An accurate and efficient method for automated measurement of right and left ventricular volumes in magnetic resonance imaging is proposed. The method can be divided into four major stages. They are preprocessing, adaptive thresholding, 2-dimensional boundary extraction, and 3-dimensional display and volume calculation. A simulation study using a generated sphere with known dimension and an anatomic specimen study using two excised pig hearts were performed to validate the accuracy of our algorithm for computerized measurement, followed by a clinical study of 3 healthy-volunteer hearts. The measurement results were compared to that obtained with manual tracing by two human experts. Our preliminary results show that the proposed method provides acceptable accuracy and is clinically applicable.<<ETX>>

Physical characterization of digital radiological images by use of transmitted information metric

This paper presents an information-entropy based metric for combined evaluation of resolution and noise properties of radiological images. The metric is expressed by the amount of transmitted information (TI). It is a measure of how much information that one image contains about an object or an input. Merits of the proposed method are its simplicity of computation and the experimented setup. A computer-simulated step wedge was used for simulation study on the relationship of TI and the degree of blur as well as the noise. Three acrylic step wedges were also manufactured and used as test sample objects for experiments. Two imaging plates for computed radiography were employed as information detectors to record X-ray intensities. We investigated the effects of noise and resolution degradation on the amount of TI by varying exposure levels. Simulation and experimental results show that the TI value varies when the noise level or the degree of blur is changed. To validate the reasoning and usefulness of the proposed metric, we also calculated and compared the modulation transfer functions and noise power spectra for the employed imaging plates. Results show that the TI has close correlation with both image noise and image blurring, and it may offer the potential to become a simple and generally applicable measure for quality evaluation of medical images.

Radiation dose reduction in digital radiography using wavelet-based image processing methods

In this paper, we investigate the effect of the use of wavelet transform for image processing on radiation dose reduction in computed radiography (CR), by measuring various physical characteristics of the wavelet-transformed images. Moreover, we propose a wavelet-based method for offering a possibility to reduce radiation dose while maintaining a clinically acceptable image quality. The proposed method integrates the advantages of a previously proposed technique, i.e., sigmoid-type transfer curve for wavelet coefficient weighting adjustment technique, as well as a wavelet soft-thresholding technique. The former can improve contrast and spatial resolution of CR images, the latter is able to improve the performance of image noise. In the investigation of physical characteristics, modulation transfer function, noise power spectrum, and contrast-to-noise ratio of CR images processed by the proposed method and other different methods were measured and compared. Furthermore, visual evaluation was performed using Scheffes pair comparison method. Experimental results showed that the proposed method could improve overall image quality as compared to other methods. Our visual evaluation showed that an approximately 40% reduction in exposure dose might be achieved in hip joint radiography by using the proposed method.

Efficacy of a Combined Wavelet Shrinkage Method for Low-Dose and High-Quality Digital Radiography

The amount of radiation dose to the patient from digital radiography is of great concern. In particular, it is important to keep the radiation dose exposure to a minimum for patients in their reproductive period, who frequently undergo repeated radiation exposure during the course of diagnostic imaging and treatment follow-up. However, there is a well-recognized trade-off between image quality and radiation dose. The balancing of dose and image quality should be performed explicitly to ensure that patient doses are kept as low as reasonable achievable, while maintaining a clinically acceptable image quality. In response to this issue, many researchers have conducted extensive studies on developing image processing methods. In this paper, we propose an improved wavelet-transform-based method for offering a possibility to reduce the radiation dose while maintaining a clinically acceptable image quality. The proposed method integrates the advantages of our previously proposed wavelet-coefficient weighted method and the existing BayesShrink thresholding method. To verify the effectiveness of the proposed method, we measured and compared the presampled modulation transfer functions and the noise power spectra (NPS) of the processed computed radiography images. Furthermore, variations of contrast and NPS with respect to radiation dose were also examined. Visual evaluations were also performed by five experienced radiological technologists. Experimental results demonstrated that the proposed method could improve the resolution characteristic while keeping the noise level within an acceptable limit. Our visual evaluation showed that an approximately 40% reduction in exposure dose might be achieved with the proposed method in hip joint and lumbar spine radiographs.

A Method for Mammographic Image Denoising Based on Hierarchical Correlations of the Coefficients of Wavelet Transforms

In this work the authors present an effective denoising method to attempt to reduce the noise in mammographic images. The method is based on using hierarchical correlation of the coefficients of discrete stationary wavelet transforms. The features of the proposed technique include iterative use of undecimated multi-directional wavelet transforms at adjacent scales. To validate the proposed method, computer simulations were conducted, followed by its applications to clinical mammograms. Mutual information originating from information theory was used as an evaluation measure in the present study. Moreover, we conducted a perceptual evaluation of the processed images obtained from the proposed method and other conventional methods for confirmation of the effectiveness of the proposed approach. The experimental results show that our proposed method has the potential to effectively reduce noise while maintaining high-frequency information of original images.

Investigation of Noise-Resolution Tradeoff for Digital Radiographic Imaging: A Simulation Study

In digital radiographic systems, a tradeoff exists between image resolution (or blur) and noise characteristics. An imaging system may only be superior in one image quality characteristic while being inferior to another in the other characteristic. In this work, a computer simulation model is presented that is to use mutual-information (MI) metric to examine tradeoff behavior between resolution and noise. MI is used to express the amount of information that an output image contains about an input object. The basic idea is that when the amount of the uncertainty associated with an object before and after imaging is reduced, the difference of the uncertainty is equal to the value of MI. The more the MI value provides, the better the image quality is. The simulation model calculated MI as a function of signal-to-noise ratio and that of resolution for two image contrast levels. Our simulation results demonstrated that MI associated with overall image quality is much more sensitive to noise compared to blur, although tradeoff relationship between noise and blur exists. However, we found that overall image quality is primarily determined by image blur at very low noise levels.

Effects of radiation dose reduction in digital radiography using wavelet-based image processing

In this paper, we investigated the effect of the use of wavelet transform on dose reduction in computed radiography (CR). The physical properties of the processed CR images were measured using the modulation transfer function (MTF), noise power spectrum (NPS), contrast-to-noise ratio, and peak signal-to-noise ratio. Furthermore, visual evaluation was performed by Scheffes pair comparison method. Experimental results showed that sigmoid-type transfer curves for wavelet coefficient weighting adjustment could improve the MTF, and three soft-threshold methods could improve the NPS at all spatial frequency ranges. Moreover, our visual evaluation showed that an approximately 40% reduction in exposure dose might be achieved with the sigmoid-type transfer curve in hip joint radiography.

Density-exposure conversion curve and microblackness characteristic in the radiological domain

The authors obtained two market-available microdensitometers having different characteristics (with negligible flare and with high-level flare) and measured the respective characteristic curves and microblackness characteristic (MBC) curves. They discuss the relationship of the curves. Furthermore, they measured a series of line spread functions (LSF) of a radiographic screen-film system by the use of the characteristic curves and the MBC curves obtained, and then examined the density dependence of the LSF.