Antiscatter grids have been used widely in diagnostic examination for reducing scattered radiation. In mammography, however, the grid lines become an important problem when trying to detect microcalcifications in the breast. In this study, we examined the effect of grid lines, the reduction of scattered radiation, the increase in radiographic contrast and the patient dose, when using an ultra-high-strip-density grid (80 lines/cm) for mammography. It was found that the grid lines did not interfere with the detection of details on the clinical image. For thicker breast, the image contrast obtained with a grid at 33 kV was higher than that obtained without a grid at 25 kV and also with regard to patient dose the former was smaller than the latter. The grid was very useful in mommography involving ineffectual compressing or young breast.
In this paper, the basic concepts of nuclear magnetic resonance (NMR) are surveyed and the principle of the recent conventional NMR-CT is described briefly. The topics of this survey include : concepts of NMR ; pulsed NMR systems ; spinechos ; method for measuring T_1, T_2 ; principles of projection reconstruction and 2-D FT NMR imaging techniques ; and pulse sequences for these NMR-CTs.
In the NMR-CT system, images are obtained from NMR signals. NMR signals are collected by use of three kinds of magnetic fields : static main magnetic field, gradient magnetic field, and RF magnetic field. To generate the main magnetic field, three types of magnets can be used ; a superconducting magnet, normal conducting magnet, and permanent magnet. Presently. superconducting magnet gives the best image quality. Principle of superconductivity and cryogenic cooling are also discribed.
Our hospital has two types of MRI machines : one is a resistive-type installed in March 1982 and the other is a superconducting-type installed in January 1984. The clinical tests of the resistive system carried out for the Japanese Drug Administration were finished in September 1982 and this type of machine (0.15T) was first approved in May 1983. The clinical tests of the superconducting system (0.35T & 0.5T) were performed in February 1985, but approval is still pending. In accordance with the experience I have obtain over the last three years concerning MRI, I am going to discuss about the role of MRI regarding radiological technologists, In my discussion. I will expand on such things as the controls of magnetic field circumstance safety, operations, maintenance, and so on.
Three years have already passed since the clinical used of NMR-CT began in Japan. The techniques for the NMR-CT scanner were developed and improved over these last three years. The images which are produced with the resistive type NMR-CT scanner progressed very much and they were able to supply useful clinical information. The surface coil is able to produce a high S/N ratio or local Image in which we have great interest. We tried to achieve more efficient calculated T_1 images and relaxation rates or BWF (bound water fraction) images which were calculated using T_1 images. Direct sagittal images were able to be obtained easily with NMR-CT, and these images are very useful to check some organs, for example mediastinum, vertebra and pelvis, etc. The relaxation rate image is also useful for the functional analysis, using a contrast medium also, because the relaxation rate depends on the density of the contrast medium in the tissue. BWF image is useful to get the theoretical parameter of the bound water fruction, which depends on the theory of Fullerton et al. In the case of a uterine disease, using the TI image we were able to perform a differential diagnosis between cervical cancer and myoma.
Clinical applications of the superconducting MRI system were started in April of 1985 in our hospital. We had performed examinations of 575 cases for seven months at a field strength of 0.35T. In April of 1985 clinical studies using the 1.5T MRI system were started. Spatial resolution as much as X-ray CT was possible by the superconducting MRI system. It is thus very effective for the treatment of central nervous system, pelvis and extremity, disorders since it is hardly in fluenced by motion artifacts. In thoracic and abdominal regions spatial resolution by MRI was inferior to that by X-ray CT, but its high contrast resolution and ability to provide optional plane images show it highly useful for clinical diagnosis and therapeutic plan therapy. By 1.5T MRI system the spectroscopy of P-31 is possible as well as high resolutional MR imaging for high signal to noise ratio. Many problems, remain to be solved, such as those of motion artifacts and chemical shift artifacts.