We applied an extended-cycle process, in which the processing time is longer than the manufacturer's recommendation, to a low-speed, high-resolution system in mammography and investigated to what extent sharpness, contrast, and speed would be maintained or increased and whether this process could be applied clinically. In this study, a single-screen/single-emulsion film, Konica M-100/CM-H, was used in the lowspeed, high-resolution system, and M-200/CM-H was employed for comparison. Developing temperatures of 30, 32, and 34 degrees centigrade and a processing time of 210 seconds were selected as conditions of the extended-cycle process. M-100/CM-H with the extended-cycle process(M-100/E system)and M-200/CM-H with the standard-cycle process were compared at 34 and 90 seconds(M-200 system)in terms of contrast, speed, modulation transfer function(MTF), and Wiener spectrum(WS). In addition, the visibility of mammograms obtained using an RMI 165 phantom was evaluated by the method of paired comparisons. In the comparison of the M-200 system with the M-100/E system at 34 and 210 seconds, the speed was almost equal and the average gradient and MTF value at 2.5 cycles/mm increased about 20% and 8%, respectively. Ws was higher due to the increase in quantum mottle with increases in the average gradient and MTF. In the scale of the method of paired comparisons, M-100/E systems showed better results. Moreover, entrance skin exposure in air of the M-100/E system at 34 and 210 seconds was almost equal to that of the M-200 system. These results suggested that it was possible to improve visibility by the extended-cycle process in a low-speed, high-resolution system.
Certain types of ionization chambers exhibit a response that depends on the length of time elapsed since the previous irradiation. This effect is called hysteresis. The effect of hysteresis on cylindrical and parallelplate ion chambers used in radiation therapy was measured in this study. The hysteresis of these chambers was examined for Co-60 γrays, x-rays(4-and 10MV), and electron beams(4, 6, 9, 12MeV). We found that this phenomenon was observed in all chambers and beams studied. Compared with the stable reading, the first reading was up to 0.5% lower in a cylindrical chamber and 1.0% lower in a parallel-plate chamber. In a cylindrical chamber, the dose rate dependence of hysteresis was also observed for the 4 MV x-ray beam. In contrast with photon beams, the first reading was lower for electron beams regardless of the type of ion chamber. The Markus chamber, a type of parallel-plate chamber, had a lower reading and a more gradual response curve in reaching a stable reading than the NACP parallel-plate chamber. This hysteresis phenomenon was confirmed not only when radiation was not used for several days but also when it was halted for a few minutes. As this phenomenon may increase errors on estimates of the effects of polarity, it is necessary to be aware of it in correcting these effects.
It has been reported that the rate of the crossover effect is affected by the screen light spectrum, but the crossover effect on each wavelength has not been clarified. We obtained the spectra of the crossover effect on ortho and regular x-ray film by exposing the film with monochromatic light from a monochrometer. The spectra had peaks at 500 nm and 450 mm for the regular and ortho films, respectively, and the crossover effect became less at the shorter and the longer(550nm)wavelengths than at peak wavelengths. Obtaining spectra of the crossover effect appears to be useful for the development of a screen that has an appropriate fluorescence spectrum for sensitivity and sharpness.
We examined the possibility of selective contrast enhanced renal MR angiography through the trial of a test injection technique with a small amount of contrast medium(test injection). The subjects were two volunteers and four patients suspected of having renal disease. With test injection, we instituted a transaxial section above the renal artery and used the 2D-FAST SPGR method at pulse sequence. The SAT technique was carried out in the superior and inferior directions, to suppress the influence of pulsation. With the images we obtained, we set an ROI on the abdominal aortic artery and inferior vena cava, and plotted the time intensity curve. Out of consideration for the difference in peak time between the artery and vein(A-V window), we determined the delay time on 3D contrast enhanced MR angiography. We were able to acquire only selective renal artery images. It was found that the renal vein will be drawn or no renal artery will be drawn on studies performed without determining adequate injection timing. This method was found to be useful in improving the above-mentioned disadvantages.
The authors have performed many MR examinations for patients with magnetic keepers fixed in the mouth as part of a dental magnetic attachment system, who had been refused MR examinations because of the risk of heating the keeper as a result of exposure to RF radiation. The purpose of this study was to evaluate the risk of RF heat associated with MR imaging procedures. In this study, the RF heat of the keeper in a simulated phantom head was measured with a fiber optic temperature measurement system and 1.5-Tesla MR equipment. Measurement revealed that the RF heat of the keeper in the phantom was a maximum of 0.3℃. The authors concluded from this result that the RF heat of keepers fixed in the mouth does not exceed 1℃ during clinical MR imaging procedures of the brain. In this study, however, the risk of burn in the mouth caused by RF heat was not directly estimated. Therefore, it is important to exercise caution in performing MR examinations on patients with magnetic keepers.