The direct determination of rare earth elements, Th and U in fresh water was still difficult because of the lack of sensitivity, although such high sensitive methods as inductively coupled plasma mass spectrometry (ICP MS-quadrupole type) and activation analysis have been introduced. Recently, double focusing and high resolution ICP MS has been developed to allow the separation of overlapped ions (polyatomic ions and analyte ions at the same nominal mass) . This instrument also has a greater sensitivity and lower background as compared to those of the conventional quadrupole type ICP MS. In this study, a double-focusing and high resolution ICP MS with an ultrasonic nebulizer which gives increased sensitivity was used. This combination enabled the direct determination of the rare earth elements, Th and U in fresh water without pre-concentration. In the analysis of ultra trace level of elements, interferences from overlapped ions become a serious problem. In fresh water, however, it is found that high resolution measurement is not necessary to analyze the above elements, except Sc. Excellent detection limits of the order of 0.5-11.6 pg/l were achieved for these elements, with the exception of Sc (92 pg/l) . Although the concentrations of these elements in fresh water were as low as ng/l-pg/l, it was possible to determine the elements by this direct method. The concentrations of the rare earth elements, Th and U in 86 samples of fresh water collected from various places in Japan were measured by this method. The concentrations were in the range of 0.6 ng/l (Tm) -38.2 ng/l (Y) on average, and span a wide range (e.g., 0.9-199 ng/l U) . The mean concetrations of these elements in river waters were slightly higher than those of lake waters.
Image quality in seintigrams is affected by various factors, such as count density, image size, and image density. Therefore, these factors were evaluated in affection for image quality. First, Rollo phantom images were taken by changing these three factors. Image quality was shown by calculating the score of visual evaluation that was done by comparing a pair of images. Next, changes of image quality were analysed using analysis of variance. Image quality became superior by taking a scintigram under the condition with increased count density, smaller image size, and darker image density on the CRT intensity. But, even if count density was increased, the image quality became inferior by taking a scintigram under the condition with too small image size and too dark image density on the CRT intensity, Furthermore, the proportions affecting image quality became the following results by analysis of variance, count density (A) : 60, image size (B) : 21, image density (C) : 10, AB: 81, AC: 72 and BC: 33 (%) .
Myocardial images of SPECT system, especially those of the inferior wall are degraded due to attenuation of gamma ray and the increased distance of gamma camera from the body because of the presence of the patient table. To avoid this difficulty of conventional patient table a newly developed patient table was used with 45 degree right semidecubital position. In 16 cases data acquisition was performed in the both supine and semidecubitul positions. With the use of semidecubitus position imaging of the inferior wall was improved. Transposition of the heart in the semidecubitus position was observed, but its effect on the quality of the myocardial images was minimum. The semidecubitus position technique demonstrated the inferior wall lesions to better advantage when the images in the supine position were not sufficient for the diagnosis.
We performed myocardial scan using201TlCl and 99mTc-methoxy isobutyl isonitrile (99mTc-MIBI) in 16 patients of myocardial infarction and compared their diagnostic value. In view of image quality, 99mTc-MIBI scan was superior to201Tl scan in 11 cases, and201T1scan was superior in no case. There was no statistically significant difference between99mTc-MIBI scan and201T1 scan in sensitivity (88.9% and 83.3%, respectively) and specificity (83.9% and 80.6%, respectively) . It is concluded that99mTc-MIBI enables us to perform high quality myocardial scan and is clinically useful.
In the measurement of environmental radioactivities by Ge detector, the calibration curve of the full-energy peak efficiencies needs to be obtained from standard γ-ray sources. When high energy γ-ray emitters viz., 60Co or88Y are used as standard sources, the correction of coincidental summing effects is necessary for estimating true peak efficiencies. It, however, requires complicated calculations and a simpler way to obtain the ralibration curve has been desired very much. It is well known that the more the distance between a counting sample and a detector endcap is, the less becomes the summing effect, though the efficiency of the measurements may increase. This interrelation, however, seemed still indistinct and the present paper intended to find the optimum source distance to minimize the measurement error and the summing effects without the complicated calculation for the correction. The results indicates that the error of the measurements can be practically low enough without the correction of coincidental summing effects for our detecting system, when the source distance from the detector endcap is about 3 cm.
We bored four samples (5cm diam. by 50cm length) from concrete side walls in the operating radiation facilities. And the gamma rays of radioisotopes induced in samples were measured with a pure Ge detector. So we identified eight radioactive nuclides of46Sc, 59Fe, 60Co, 65Zn, 134Cs, 152Eu, 54Mn and 22Na. From the measurement it was confirmed that the radionuclides induced by thermal neutrons through the (n, γ) reaction are dominant, and that the induced activity by thermal neutron is higher at 5 to 10 cm depth rather than at the surface of concrete and decreases rapidly according to the exponential function beyond about 20 cm depth.