山口 弘次郎

In magnetic resonance imaging (MRI) , signal intensity is dependent on the substances in a voxel. The signal is decayed when Air gap is present. In addition to that, the presence of Air gap creates a local inhomogeneity in a magnetic field, causing artifact on MR images. <br>Consequently, it is said that the influence of Air gap is less in fast spin-echo (FSE) sequences than spinecho (SE) sequences. We analyzed the effect of air on slice profiles of both SE sequences and FSE sequences in numerical simulation with Bloch equations. As a result, we confirmed that slice profile is more distorted in FSE sequences compared to SE sequences.

Following the trend of the digitalization of the modalities used for diagnostic imaging, the devices for such imaging have increasingly included monitors. The present study was undertaken to evaluate the usefulness of soft-copy (liquid crystal display; LCD) images of phantoms of small adenocarcinomas using receiver operating characteristic (ROC) analysis of two different display systems: LCD and hard copy (film). A two-tailed paired t-test and the jackknife method (parametric methods) were performed, and no significant differences were found in the area under the ROC curve (AUC) for the pulmonary fields, lungs, ribs, or mediastinum between the film and LCD display systems, and the detectability did not differ between the film and LCD monitors. A Mann-Whitney U test, which is a non-parametric method that applies to the analysis of a small sample, also showed no significant differences in the AUC. The results of this study suggest that LCDs can replace hard-copy film as a display system if the signals.

The recent increase in the incidence of SAR and fever contracted during medical examinations has raised concerns about transmission from the WB coil used during magnetic resonance imaging(MRI)using SENSE. We measured changes in temperature at the body surface during MRI medical examinations using T₂ Fast Spin Echo(T₂FSE)and Hahn T₂ Fast Spin Echo(Hahn-T₂FSE). We initially calibrated the thermometer in an incubator and confirmed that it operated normally in a high magnetic field and that it was not damaged or influenced by heat generated by RF pulses. All experiments were performed using an Excel ART Pianissimo 1.0T(Toshiba Medical Systems)and Thermo clone SL type(KN Laboratories). The temperature at the body surface changed according to the measurement point. The temperature increase was lower when measured with Hahn-T₂FSE than with T₂FSE. One limitation of this study was that temperature was measured only during 1.0 T MRI.

In general, the sinc function is the standard RF pulse employed in MRI studies. In the present study, we experimentally produced &ldquo;the original function&rdquo; by extending the Fourier cosine transform to the sinc-Gaussian function. Theoretic slice profiles obtained using the original function showed a greater improvement in the rectangular shape compared with the sinc function. As to the physical evaluation performed with 1T-MRI clinical equipment, the suppressed decrease in Contrast to Noise Ratio was higher with the original function than the sinc function in the volunteer head image multi-slice evaluation. Thus, all the results from the simulation and the clinical equipment evaluation demonstrate that the original function can improve the quality of MRI images even with shorter Echo Time.

Purpose: q-ball imaging (QBI) reconstructs the orientation distribution function (ODF) that describes the probability for a spin to diffuse in a given direction, and it is capable of identifying intravoxel multiple fiber orientations. The local maxima of ODF are assumed to indicate fiber orientations, but there is a mismatch between the orientation of a fiber crossing and the local maxima. We propose a novel method, multi-shelled QBI (MS-QBI), that gives a new ODF based on the moment of the probability density function of diffusion displacement. We test the accuracy of the fiber orientation indicated by the new ODF and test fiber tracking using the new ODF. Methods: We performed tests using numerical simulation. To test the accuracy of fiber orientation, we assumed that 2 fibers cross and evaluated the deviation of the measured crossing angle from the actual angle. To test the fiber tracking, we used a numerical phantom of the cerebral hemisphere containing the corpus callosum, projection fibers, and superior longitudinal fasciculus. In the tests, we compared the results between MS-QBI and conventional QBI under the condition of approximately equal total numbers of diffusion signal samplings between the 2 methods and chose the interpolation parameter such that the stabilities of the results of the angular deviation for the 2 methods were the same. Results: The absolute value of the mean angular deviation was smaller in MS-QBI than in conventional QBI. Using the moment-based ODF improved the accuracy of fiber pathways in fiber tracking but maintained the stability of the results. Conclusion: MS-QBI can more accurately identify intravoxel multiple fiber orientations than can QBI, without increasing sampling number. The high accuracy of MS-QBI will contribute to the improved tractography.

Interslice sensitivity variation in magnetic resonance imaging (MRI) is said to be attributable to crosstalk. Crosstalk is a phenomenon in which deterioration of the rectangular profile of the RF pulse frequency characteristics leads to poor slice profiles, resulting in interference between slices. In the present study, the ideal characteristics of multislice profiles were analyzed through numerical simulation with the Bloch equation using sinc symmetrical RF pulses. The effects of the slice excitation order on the slice profile were also examined, comparing interleaved acquisition and sequential acquisition. The results of the simulation showed that the order of image acquisition affects the slice profile.

q-space diffusion analysis is a method to obtain the probability density function of the translational displacement of diffusing water molecules. Several quantities can be extracted from the function that indicate a characteristic of the water diffusion in tissue, e.g., the mean displacement of the diffusion, probability for zero displacement, and kurtosis of the function. These quantities are expected to give information about the microstructure of tissues in addition to that obtained from the apparent diffusion coefficient (ADC) however, this method requires high q (i.e., high b) values, which are undesirable in practical applications of the method using clinical magnetic resonance (MR) imaging equipment. We propose a method to obtain certain quantities that indicate a characteristic of the diffusion and that uses low q-value measurements. The quantities we can obtain are the moments of translational displacement, R the n-th order moment is defined as the average of Rn (n: integer). Kurtosis can also be calculated from the second and fourth moments. We tried to map the moments and kurtosis using clinical MR imaging equipment. We also estimated the inherent errors of the moments obtained. Our method requires precision in measuring spin echo signals and setting q values rather than using high q-value measurements. Although our results show that further error reductions are desired, our method is workable using ordinary clinical MR imaging equipment.