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Biomedical subjects

Akio Ogura

Publications and source records attributed to Akio Ogura.

10 recordsLinked to original sources
PubMed2006

[Magnetic resonance image examinations in emergency medical care].

There is a growing consensus in terms of the need for effective use of magnetic resonance imaging (MRI) diagnostic devices in emergency medical care. However, a thorough assessment of risk management in emergency medical care is required because of the high magnetic field in the MRI room. To understand the conditions required for the execution of emergency MRI examinations in individual medical facilities, and to prepare guidelines for emergency MRI examinations, we carried out a questionnaire survey concerning emergency MRI examinations. We obtained responses from 71% of 230 medical facilities and used this information in considering a system of emergency MRI examinations. Moreover, some difficulties were experienced in half of the facilities where emergency MRI examinations had been enacted, the main cause of which was the medics. Based on the results of the questionnaire, guidelines are necessary to maintain an urgent system for MRI examinations. Moreover, we were able to comprehend the current state of emergency MRI examinations in other medical facilities through this investigation, and we are preparing a system for the implementation of emergency MRI examinations.

Emergency Medical Services↗
PubMed2006

[Effect of vibration caused by time-varying magnetic fields on diffusion-weighted MRI].

Diffusion-weighted images (DWIs) with high b-factor in the body are often used to detect and diagnose cancer at MRI. The echo planar imaging (EPI) sequence and high motion probing gradient pulse are used at diffusion weighted imaging, causing high table vibration. The purpose of this study was to assess whether the diffusion signal and apparent diffusion coefficient (ADC) values are influenced by this vibration because of time-varying magnetic fields. Two DWIs were compared. In one, phantoms were fixed on the MRI unit's table transmitting the vibration. In the other, phantoms were supported in air, in the absence of vibration. The phantoms called "solution phantoms" were made from agarose of a particular density. The phantoms called "jelly phantoms" were made from agarose that was heated. The diffusion signal and ADC value of each image were compared. The results showed that the signal of DWI units using the solution phantom was not affected by vibration. However, the signal of DWI and ADC were increased in the low-density jelly phantom as a result of vibration, causing the jelly phantom to vibrate. The DWIs of vibrating regions such as the breast maybe be subject to error. A countermeasure seems to be to support the region adequately.

Diffusion Magnetic Resonance Imaging↗
PubMed2005

[Effects of slice thickness and matrix size on MRI for signal detection].

High-resolution MRI with increased matrix size is becoming widely used. When matrix size is increased, the signal-to-noise ratio (SNR) decreases. To compensate for a poor SNR, a thick slice is often used. The purpose of this study was to assess whether slice thickness affects the signal detectability of MR images. Signal detectability was evaluated for various slice thicknesses using a contrast-detail phantom. The results showed that thinning slices led to increased signal detection but to decreased SNR because of higher contrast in the partial volume effect. In addition, increasing matrix size and slice thickness for high-resolution imaging led to a decrease in signal detection. It is necessary to consider voxel form for two-dimensional MR images and to recognize that cuboid voxels lead to increased signal detection.

Image Enhancement↗
PubMed2004

[Accuracy of contrast-to-noise ratio measurement for magnetic resonance clinical images].

The contrast-to-noise ratio (CNR) is often used to evaluate magnetic resonance images, because it has two components, contrast and SNR, and indicates the detectability of clinical lesions. Two methods (using a phantom and using clinical images) are employed to measure CNR. In addition, there are some methods of measurement that use clinical images. In this report, the accuracy of measurement and correlation for signal detectability were evaluated in four methods of measuring CNR using clinical images. The results indicated that the inter-tissue method using an air signal provided good accuracy and was consistent with signal detectability using observer performance. In addition, a small region of interest (ROI) was better suited as the target for CNR measurement using clinical images.

Artifacts↗
PubMed2003

[Examination of measurement and its method of compensation of the sensitivity distribution using phased array coil for body scan].

The influence on the quality of images by measurement of a sensitivity distribution and the use of a sensitivity compensation filter was considered using an opposite-type phased array coil and volume-type phased array coil. With the opposite-type phased array coil, the relation between coil interval and filter was investigated for the IIC filter, SCIC filter (GE), and the Normalize filter (SIEMENS). The SCIC filter and Normalize filter showed distance dependability over the coil interval of SNR and uniformity was observed, and the existence of an optimal coil interval was suggested. Moreover, with the IIC filter, distance dependability over a coil interval was small, and the decrease in contrast with use was remarkable. On the other hand, with the volume-type phased array coil, the overlap of an array element was investigated to determine the influence it had on sensitivity distribution. Although the value stabilized in the arrangement center of an array element, and its near, was shown when volume-type phased array coil was used, SNR and uniformity decreased as it separated from the center. This showed that overlap of an array element does not have an adverse influence on image quality when the already optimized arrangement is used.

Image Enhancement↗
PubMed2003

[Accuracy of signal-to-noise ratio measurement method for magnetic resonance images].

The signal-to-noise ratio (SNR) of a magnetic resonance image is a common measure of imager performance. However, evaluations for the calculation of the SNR use various methods. A problem with measuring SNR is caused by the distortion of noise statistics in commonly used magnitude images. In this study, measurement accuracy was compared among four methods of evaluating SNR according to the size and position of regions of interest (ROIs). The results indicated that the method that used the difference between two images showed the best agreement with the theoretical value. In the method that used a single image, the SNR calculated by using a small size of ROI showed better agreement with the theoretical value because of noise bias and image artifacts. However, in the method that used the difference between two images, a large size of ROI was better in reducing statistical errors. In the same way, the methods that used air noise and air signal were better when applied to a large ROI. In addition, the image subtraction process used to calculate pixel-by-pixel differences in images may reach zero on a minus pixel value when using an image processor with the MRI system and apparatuses associated with it. A revised equation is presented for this case. It is important to understand the characteristics of each method and to choose a suitable method carefully according to the purpose of the study.

Magnetic Resonance Imaging↗
PubMed2003

[Signal detectability of mammography depends on film-screen system and luminance of view box].

High-density film and the high-luminance view-box system are being recommended for mammograms owing to the improved detection of masses. However, this system causes an increase in radiation. Therefore, the purpose of this study was to assess whether the detection of masses would improve using the normal-luminance view box and normal-density film with different types of contrast systems. Low-contrast detection using ROC analysis and high-contrast detection using an ACR phantom were evaluated for the following systems: high-density film and high-luminance view box, normal-density film and normal-luminance view box, and normal-density film with wide latitude and normal-luminance view box. The results showed no significant variation in the detectability of the system with high-density film and high-luminance view box and the normal-density film with wide latitude and normal-luminance view box. However, in terms of low-contrast visibility, the system using normal-density film and normal-luminance view box was significantly reduced in comparison with the others. Therefore, the system with normal-density film with wide latitude and the normal-luminance view box is recommended because of reduced radiation dose.

Breast Neoplasms↗
PubMed2002

[Comparison of contrast resolution between dynamic MRI and dynamic CT in liver scanning].

Dynamic studies employing contrast agents have been used in the detection of hepatocellular carcinoma (HCC). Some studies have indicated that the detectability of small nodules in liver is significantly better with MRI dynamic study than with CT dynamic study. However, the reasons for this are unclear. In an attempt to clarify this difference, we compared contrast resolution using two contrast phantoms made for MRI and CT. Both phantoms were constructed using the same ratios of dilution of the contrast materials Gd-DTPA and iodine. The signal intensity of different dilution ratios of contrast medium were measured in order to compare the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and contrast. The results showed that the contrast of phantoms in MRI and CT dynamic studies was almost the same except for certain MRI scan techniques. In terms of SNR and CNR, MRI dynamic study was superior to CT dynamic study. In addition, T(1) relaxation of the contrast agent should be considered for suitable scanning, because the contrast of MRI was affected by scan parameters.

Contrast Media↗