Japanese Journal of Radiological Technology

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The x-ray output of 54 inverter-type x-ray apparatuses used at 18 institutions was investigated. The reproducibility and linearity of x-ray output and variations among the x-ray equipment were evaluated using the same fluorescence meter. In addition, the x-ray apparatuses were re-measured using the same non-invasive instrument to check for variations in tube voltage, tube current, and irradiation time. The non-invasive instrument was calibrated by simultaneously obtaining measurements with an invasive instrument, employing the tube voltage and current used for the invasive instrument, and the difference was calculated. Reproducibility of x-ray output was satisfactory for all x-ray apparatuses. The coefficient of variation was 0.04 or less for irradiation times of 5 ms or longer. In 84.3% of all X-ray equipment, variation in the linearity of x-ray output was 15% or less for an irradiation time of 5 ms. However, for all the apparatuses, the figure was 50% when irradiation time was the shortest(1 to 3 ms). Variation in x-ray output increased as irradiation time decreased. Variation in x-ray output ranged between 1.8 and 2.5 compared with the maximum and minimum values, excluding those obtained at the shortest irradiation time. The relative standard deviation ranged from ±15.5% to ±21.0%. The largest variation in x-ray output was confirmed in regions irradiated for the shortest time, with smaller variations observed for longer irradiation times. The major factor responsible for variation in x-ray output in regions irradiated for 10 ms or longer, which is a relatively long irradiation time, was variation in tube current. Variation in tube current was slightly greater than 30% at maximum, with an average value of 7% compared with the preset tube current. Variations in x-ray output in regions irradiated for the shortest time were due to photographic effects related to the rise and fall times of the tube voltage waveform. Accordingly, in order to obtain constant x-ray output, inverter-type x-ray equipment should generate rectangular waveforms with short rise and fall times.

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The method of visual evaluation is frequently used, as is the ranking method, because it is a relatively simple means of evaluation. However, in order to measure a ranking method according to the rule of "attaching ranking", results are given with an ordinal scale. The size relation of the scale is guaranteed, but the interval values. Therefore, the average value does not have any mathematical meaning. In order to compensate for these problems, we attempted to determine the right method of analysis for ordered data and examined the normalized-rank method, which can compute a more advanced distance scale, and its method of analysis. The concept of a scale was introduced in order to carry out analysis suitable for the purpose of visual evaluation. Consequently, the reliability of the ranking method was improved.

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In recent years, radiofrequency(RF)power deposition in humans at the time of magnetic resonance imaging(MRI)studies has increased as a result of the fast spin echo technique. Specific absorption rate(SAR)is used as an index of the biological effects of electromagnetic radiation. The methods used by six MRI manufacturing companies to determine displayed SAR values were investigated. The predicted SAR value for the same phantom under the same conditions was compared for the various MRI systems. The results of the inquiry indicated that the methods of predicting the Q-factor of RF coils differed for each company. It was considered that these differences in predicting the Q-factor are related to differences in SAR values. In addition, the results of the study indicated that the methods used to predict the SAR value on some MRI systems using same phantom should be clearly understood by the technicians who use the systems.

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Flow artifacts, also called ghosts or phase shift artifacts, can be caused by blood flowing fast within the image plane, due to the misregistration effect, or through the plane, because of the pulsatility of the flow. This can result in a repeated ghost signal in the phase encoding direction. Our MR unit allows the scan to be planned while viewing three images in the transverse, sagittal, and coronal directions simultaneously. This characteristic feature results in easy scan planning with complete freedom in the three dimensions. Specifically, it allows us to change the angulation in the phase encoding direction of the scan plane. We report here the clinical usefulness of the technique that controls the direction of the artifacts without having to reposition the patient. This technique consists of slanting the phase encoding direction through the combination of the three magnetic field gradient coils, similar to the double oblique. The technique allows us to have immediate freedom of the scan plan in three directions in all anatomies and sequences. This technique is useful and should be implemented in MR units, because of the easy control of artifacts depending on the phase encoding direction.

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