[en] Current practices for patient safety control in radiological diagnosis may involve problems. For solving of these problems, we have to bear in mind that the mental and physical capacity of patients tend to be limited during radiological testing. When radiography is performed using a general X-ray device at a source table distance (STD) of 100 cm, the patient's head can touch the X-ray tube housing during position adjustment on the stand (up and down adjustment) or the patient is made to take an unnatural posture during body positioning. With this in mind, we carried out a questionnaire survey about source image receptor distance (SID). This survey disclosed that more than 92% of the institutions have adapted 100 cm STD. We then conducted a three-dimensional analysis of a patient's posture and motion by video taping patients during positioning on a roentgenographic table. This analysis revealed that the adoption of the 120 cm STD resulted in less contact between the patient's head and X-ray tube housing, less of unnatural body position and less time required for positioning adjustment when compared to those at 100 cm STD. These results indicate that the current STD (100 cm) is not suitable for safe and smooth adjustment of the positioning of a patient's body of a roentgenographic table. We examined the optimun STD, taking into consideration the dimensions of patient's movement and posture during an X-ray examination. (author)