The radiation field shape for cardiac catheterization has changed from being circular to being rectangular with the spread of flat-panel detector systems. A rectangular radiation field provides a wide fluoroscopy field to the four corners of a subject area; however, in cardiac catheterization, there is not much usable information around the four corners at several angles, and this tends to be regarded as a meaningless radiation exposure. Hence, overlap of radiation fields has been of concern recently. The authors changed field sizes/fluoroscopy angles and examined entrance dose rates to study radiation field shapes and configurations of radiation exposure to patients, and they discussed a radiation exposure reduction method. In measurements of phantom entrance dose rates, we considered right anterior oblique (RAO) directions, cranial (CRA) directions and RAO-CRA directions and found that entrance dose rates rose considerably, particularly at the RAO-CRA. In the study of radiation field overlap, we discuss radiation field shapes (rectangular/circular) as well as angles. In the RAO-CRA directions, differences occurred in angles of non-overlapping radiation field by differences in radiation field shapes. For RAO-CRA, compared with the RAO and CRA directions, entrance dose rates increased with an increase in angle. When convenience in clinics is considered, the utilization frequency of the four corners of a rectangular field is low. When one considers the increases in radiation exposure caused by radiation field overlap, it is more effective to use circular radiation fields.