This paper describes a method for estimating spectral distributions from X-ray attenuation data obtained by inserting metal filters in the beam. First, the principle of the estimation method is briefly outlined. The spectral distribution is approximated by a histogram with an appropriate number of cells whose heights are estimated by recursive algorithm. The algorithm is based on a gradient method that seeks the minimum of a least-square criterion function. Next, the following two problems to which special attention should be paid in practical applications are discussed: 1) appearance of discontinuity in estimated distribution; and 2) termination criterion of iterative computation. The problem 1) is investigated theoretically and the improved algorithm is devised. Computer simulation is performed for the problem 2), and a proper termination criterion based on the decreasing curve of the least-square criterion function is given. The validity of estimation results is illustrated about the spectral distribution of 140 kV constant potential X-ray source.
Mercury contents in head hair of 58 dentists, working at the hospital of Nihon University School of Dentistry, were determined using neutron activation analysis. The values ranged from 2.47 ppm to 13.7 ppm. The frequency distribution of the mercury contents increased steeply at about 2 ppm to the maximum of 4-5 ppm, and decreased to about 11 ppm, its distribution pattern followed a log-normal one with a median of 5.46 ppm. The dentists were divided into two groups and the mercury contents in head hair of them were compared with each other. The arithmetic and geometric means of the values for dentists who handled amalgam in dental practice were 5.66 ppm (SD±1.80) and 5.40 ppm (SD×÷1.36), while those for the dentists who did not handle amalgam in dental practice were 5.96 ppm (SD±2.82) and 5.38 ppm (SD×÷1.59) . The values of them agreed well with each other. The arithmetic mean for all dentists was 5.79 ppm (SD±2.28) and the geometric mean of them was 5.40 ppm (SD×÷1.46) . They were much lower than the values reported by Suzuki, Miyazawa or Sai-renji, et al, in the past year, and they agreed well to those for normal Japanese men with the same age. Therefore it is concluded that proper mercury handling might be established in the dental office.
Gamma dose to the gastric wall from a capsule containing 1.85 GBq (50 mCi) of131I was estimated in 6 patients who had received total thyroidectomy for thyroid carcinoma some years before. The tests were done with a 37 MBq (1mCi) capsule each in 5 patients and with a 185 MBq (5 mCi) capsule in one patient. All the patients were requested to fast in the morning. The capsule was given with a glass of water (200ml) . Then, the patient kept supine position under the scintillation camera for a period of one hour except one patient on whom the test was suspended at 30 minutes because of early clearance of the radioactivity from the stomach. In one of 5 patients who were tested for a period of one hour, serial scinticamera images showed almost no movement and minimum dissolution of the capsule. The remaining 4 patients show-ed slight to moderate movements of the capsules with a variety of dissolution speeds. Data processings were done by Scintipac-1200. The estimated doses at the distance of 0.5 cm from the source were 3.820, 2.074, 1.445, 1.154 and 1.462 grays (382.0, 207.4, 144.5, 115.4 and 146.2 rads) per initial one hour and 375 mGy (37.5 rad) per initial 30 minutes, respectively. From these data, it is thought to be wise to advise the patient to rotate or shake the body on bed occasionally after swallowing the capsules containing a large dose of131I for the treatment of thyroid cancer. It is also desirable to recommend the patient to walk around even though the controlled patient's room is small. Additional water may be also meaningful to avoid unnecessary irradiation to the gastric wall.