Dosimetry of small radiation fields for 6 MV and 18 MV x rays have been evaluated with the use of silver halide film such as the XV2 Ready-pack (Kodak) and TVS-EP (CEA) films, and with the radiochromic film MD-55 (ISP Technologies). We compared the characteristics of the films with a 0.006 cm³ ionization chamber (MC-110, Applied Engineering).
The sensitivity of the radiochromic film is less than one hundred ths of that of silver halid films. It was also that there were found linear relationship between a optical density of the radiochromic film and an irradiation dose only in a very narrow range. For this reason, the use of radiochromic film is unsuitable in the dosimetry for the accelerator.
Compared to XV2 Ready-pack, there is linea rity between optical density of the TVS-EP film and a much larger range of the irradiation dose. The sensitivty of the TVS-EP film is not dependent on the direction of the incident x-rays and is independent on the energy of the x-rays. The optical density of the TVS-EP film agreed with the ionization chamber dosimetry when the field size were varied. But, there is a limitation in using silver halide films in dosimetry. This limitation is due to various difficulties associated with intrinsic characteristics of the film, such as in developing processes and also in the measurement of the optical density. Therefore, dosimetry with the improved small size ionization chamber seems to be the most suitable on small field dosimetry.

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A method to convert absolute depth-dose curves of electron beams obtained in plastic phantoms to distributions in another phantom material is proposed. The procedure has been verified by using depth-dose distributions for plane-parallel beams calculated by the Monte Carlo method. Results better than other scaling methods have been obtained along the complete depth-dose curve for incident energies between 5 and 20 MeV.

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A synchrotron radiation scanning slit radiography sy stem was designed and developed for the purposes of reducing scattering effect, enabling high speed image acquisition and obtaining high quality image. In this system, one dimensional synchrotron radiation beam and one dimensional active area on two dimensional detector should be scanned electronically.
In this paper, performance parameters for scatter rejectio n and results of synchrotron radiation scanning slit radiograph obtained from EGS4 simulation are discussed, and compared with experiments in which radiograph images were taken for bone phantom with one dimensional synchrotron radiation beam in which a monochromator was rotated stepwise. The window function was applied on the images in place of electronic scanning of the active area on the detector. The effect of the window function on the elimination of scattered radiation was studied by changing width of the function. In order to study the relation of performance parameters to slit width, two dimensional dose distributions on the detector surface were calculated using the EGS4 Monte Carlo simulation method.
Scatter re jection and the contrast in this system were found to be improved by applying a suitable window function.

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Quantitative regional myocardial blood flow can be assessed noninvasively using 0-15 labeled water and positron emission tomography (PET). Since PET device provides regional radioactivity distribution in vivo, mathematical modeling based on a compartment model provides estimation of physiological myocardial perfusion. Validity of this technque has been demonstrated by several investigators independently, and this technique has already been applied to clinical research, demonstrating unique findings in a number of research areas. PET is thus a unique technique that is able to provide bio physiological fundamental functions in human.

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Coronary angiography has been used as fundamental diagnostic means of coronary disease since it enables us to recognize the configuration of coronary artery and stenotic portion. However, the coronary angiography is not suited to examine the disease type and evaluate quantitatively results of angioplasty by PTCA and so on. Although intravascular scope is useful for examining directly the plaque on the lumen, it can not provide the information on intrastructure and tissue characterization of vessel wall. On the other hand, intravascular ultrasound (IVUS) has been recently developed and can examine not only structural deformation but also tissue characteristic change of the vessel wall by using ultrasonic transducer attached on the head of catheter. From now on, IVUS is expected to be more important technique for the assessment of theraputic results by angioplasty and arterectomy.

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We have been developing a two-dimensional imaging system using an 11-TV system for intravenous coronary angiography, a high-resolution CT system, a fluorescent CT system, a highresolution TV system using a HARP TV tube for angiography and mammography, and other medical imaging systems using synchrotron radiation monochromatic x-ray at the accumulation ring, KEK. Recent results on the two-dimensional imaging system for intravenous coronary angiography will be described here.

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Magnetic resonance (MR) imaging presents several approaches to the assessment of coronary artery disease. The narrowing of coronary arteries can be directly assessed with MR coronary angiography. Though its spatial resolution is inferior to conventional X-ray angiography, MR coronary angiography becomes one of the screening methods for the clinical evaluation of coronary artery stenosis. MR imaging can also estimate coronary blood supply as an essential function of coronary artery. Blood flow velocity in coronary artery is measured by breath hold velocitye ncoded cine MR imaging. Myocardial perfusion is evaluated with high-speed MR imaging after administration of contrast enhancement agent. Finally, the abnormality of wall motion resulting from ischemia is another important object for clinical management of coronary artery disease. Myocardial tagging technique demonstrates the absolute changes of myocardial location. This technique adds a unique aspect of wall motion analysis to conventional cine MR imaging. Present status of MR imaging technique in the diagnosis of coronary artery disease is reviewed and summarized.

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