[Development of a converging collimator for thyroid scintigraphy].

Thyroid scintigraphy is generally performed using a pin-hole or a parallel-hole collimator. The former yields a high resolution image, however, the acquisition time prolongs because the collimation system magnifies a thyroid image. The latter has somewhat low resolution, because the collimation system cannot magnify the thyroid image and it is difficult to close the collimator surface to a patient thyroid. The defects of the above two collimators are distortion or blurs at the thyroid edge of the acquired image. To improve the image quality of the thyroid scintigram, we made computer simulations concerning contrast resolution in comparison of a parallel-hole, pin-hole and converging collimator. The simulations results showed that the contrast resolution by the converging collimator is superior to the other collimators. According to the results, we developed a specially-designed converging collimator (thickness: 100 mm, focal length: 195 mm). We cut an edge of the collimator to decrease the distance between the neck and the collimator surface. Under almost the same conditions for spatial resolution and sensitivity, we compared the effectiveness of the converging collimator with that of the other two collimators; the pin-hole type [medium resolution] and the parallel-hole type [high resolution]. As a result of experiments using a phantom, the converging collimator showed less distortion compared with the other collimators at the deep area of the thyroid phantom.(ABSTRACT TRUNCATED AT 250 WORDS)