[Examination of reducing misregistration for lower tube voltage of the mask image in CT angiography using subtraction method].

Computed tomographic angiography (CTA) has been used recently for the evaluation of intracerebral aneurysms, but it is difficult to use this technique to visualize aneurysms near the base of the skull because of the presence of bone. So, subtracted CTA has been used to separate vessels from bony structures. However, we see some misregistration when using subtraction method because of the patient moving, the disaccord of the X-ray tube orbit between the mask image and the live image, the expanding focus, and the bed bending. So, attentioning the difference of bone CT number in any tube voltages, we examined to make the image containing less misregistration by changing the tube voltage of mask image. Making a sham blood vessel, we examined the bone misregistration, the blood vessel volume, and the smoothness when changing the tube voltages of mask images. Comparing with 120 kV, as the tube voltage of the mask image was 80 kV, the bone misregistration decreased significantly, however the blood vessel volume decreased. As for the tube voltage of 100 kV, the bone misregistration decreased significantly, and the blood vessel volume and the smoothness were not significantly different so we could get coordinative image of 120 kV. When the tube voltage of the mask image becomes lower than that of the live image and the effective energy becomes different, the effect of misregistration is less. This method deals with changing the tube voltage only. So, it may be easy to make volume rendering (VR) image and this method may be used in every facility.

[Effectiveness of deep breath-hold SPECT in torso area: examination concerning improvement of resolution].

Because SPECT images are acquired under normal respiration, the respiratory motion induces artifacts and decreases resolution. In this study we developed a novel method of acquiring SPECT data during deep inhalation breath-hold (BrST) and assessed its efficacy in reducing motion artifacts and improving resolution. Reproducibility studies found that variations in SPECT image homogeneity were reduced using the BrST method to within a clinically non-problematic range. An experiment using a custom-built respiration phantom showed almost complete elimination of motion artifacts and significant improvement in resolution using the BrST method. Clinical assessment confirmed a significant reduction in motion artifacts along with the improvement in resolution. The BrST method enabled visualization of lesions that previously had been impossible to detect by standard acquisition under normal respiration. The BrST method is expected to both significantly reduce motion artifacts and improve resolution.

[Evaluation of scatter rejection system using a metal filter].

Grids are used for the purpose of reducing scattered radiation. In portable radiography, however, accurate positioning of the grid is difficult. Errors in alignment may cut off the primary beam, which can result in misdiagnosis. We devised a metal filter with scatter rejection properties and evaluated its performance. The filter, which has no intrinsic alignment mechanism, is placed in front of the IP. We evaluated aluminum (Al), copper (Cu), and tantalum (Ta) filters by comparing them with low-ratio grids of 3:1 and 5:1. In the total evaluation, Ta (0.03mm) demonstrated high visualization of light and minute vascular shadow and visceral pleura, and the Bucky factor was lower than that of the 3:1 grid, while the clinical target was very clear. Because of its high atomic number, Ta can absorb a low energy component effectively even if it is very thin. Because of the K absorption edge, Ta also decreases the high-energy components that cause photographic contrast to decline. Therefore, Ta proved to be a highly suitable material for this research. An air gap within 4cm was not effective for the purpose of supporting the reduction in scattered radiation of the metal filter. The method of placing a metal filter in contact with the patient is more practical and makes the system very thin. This system is thought to be effective for portable radiography because it is light, easy to use, and flexible in structure and does not cause misalignment.

[Evaluation of the inhibiting effect due to flow artifact in radial scan].

Radial scan method collects blades rotated about the k-space origin. The one blade is the group of the echo acquired each repetition time (TR) in concentric strips. Because each of the blades has the phase-encode direction, it can be expected to be dispersing flow artifact which appear in the phase-encode direction. We examined how to produce flow artifact in radial scan method without motion correction. Flow artifact does not appear like a mass in radial scan method. Background noise in radial scan method tended to decrease when width of the blade is thinner and a number of blades is larger under a number of echo is keeping. In conclusion, unlike past countermeasures against flow artifact in Cartesian method, flow artifact becomes inconspicuous to be dispersed by 360 degrees in radial scan method.