[Evaluation of Resolution Characteristics in Three-dimensional MR Imaging Using Single Plate Thin-Ramp Method].

Recent remarkable progress of fast imaging techniques in 3D MRI has emphasized the importance of evaluation of its resolution characteristics. A trial point spread function (PSF) measurement was conducted using the ramp method, a conventional measurement method for the slice profile of 2D imaging, as an approach to evaluate the resolution characteristics of 3D imaging. However, problems peculiar to 3D imaging have arisen, such as artifacts and offsets in the slice selective direction. Therefore, we attempted PSF measurement using a phantom having a single-plate construction with only a simple ramp part (Single Plate Thin-Ramp Method) to respond to these shortcomings. By employing appropriate positioning and simple post processing, we obtained a PSF easily in both phase encoding and slice selective directions without the artifacts and offsets described above. Furthermore, it was possible to evaluate the change of resolution characteristics depending on the scan condition in three-dimensional MR image.

[Slice profile measurement using a crossed thin-ramps method in three-dimensional magnetic resonance imaging].

Recent progress in variable-flip-angle fast spin-echo technology has further extended the utility of three-dimensional (3D) magnetic resonance imaging (MRI) for clinical application. The slice profile in 3D MRI is the point spread function that has a sync form in principle, whereas a slice profile in 2D imaging provides information on characteristics of selective radio frequency excitation. We investigated the optimal condition to measure 3D slice profiles using a crossed thin-ramps phantom. We found that the profile data should cover a large area in order to evaluate both the main lobe and side lobes in the slice profile, and that the appropriate slice thickness was 2 mm. We also found that artifacts in the direction perpendicular to the slice create an offset error in the measured slice profile when 3D imaging. In this paper, we describe the optimal condition and some remarks on the slice profile evaluation for 3D MRI.

[Accuracy of volume measurement of lateral atlanto-axial joint by 3D-CT facet arthrograms].

In conventional 3D-CT image processing, the images are influenced by subjective threshold settings. The purpose of this study was to evaluate an objective threshold setting technique based on the discriminant analysis method. The concentration of contrast medium in a joint of a simulated upper cervical spine phantom was changed, and its threshold was measured from scanned data by using the discriminant analysis method, and mean CT attenuation was measured. On the other hand, an accurate image of the corresponding joint in the phantom was made, and its minimum threshold was measured. Regression analysis between the adjusted minimum threshold and mean CT attenuation of the region of contrast medium was performed. The obtained linear regression formula was applied to the threshold settings in five cases for atlanto-axial 3D-CT facet arthrogram(3D-CTF), and the accuracy of the images was examined. There was a strong correlation between the adjusted threshold and mean CT attenuation, and the obtained linear regression formula was y=0.625x-141(r2=0.991, p<0.01). This equation could be used clinically for correction of the threshold settings. We propose the following method for threshold setting of 3D-CTF: the threshold of the region of contrast medium is measured using the discriminant analysis method, then the adjusted minimum threshold for the threshold settings of 3D-CTF is calculated from mean CT attenuation. The method described herein is an objective, general-purpose methodology that is applicable to various types of 3D-CT.