Fluctuation of cardiac size on radiographs during a cardiac cycle: re-examination with magnetic resonance imaging

Abstract

Purpose Exposure time within 40 ms for chest radiography is brief, and therefore cardiac timing of exposure may influence cardio-thoracic ratio (CTR). To determine whether or not ECG triggering is required, it is necessary to evaluate the cardiac size during the complete cardiac cycle. Methods A ciné magnetic resonance series was obtained in eight contiguous coronal planes in ten patients. At a single cardiac phase, the maximum horizontal length from the middle to the right cardiac border was measured on each coronal image. The transverse diameter to the right border of the cardiac silhouette projected on the imaginary radiograph was defined as the greatest value of length obtained by magnifying the eight maximum horizontal lengths in inverse projection to the distance from the imaginary X-ray source to each imaging plane. The transverse diameter to the left border was also defined. The two diameters were then summed to describe the cardiac size at the common cardiac phase. To constitute cardiac phase-cardiac size curves this procedure was repeated for eight cardiac phases. Results On cardiac phase-cardiac size curves, the maximum transverse silhouette of the heart (diameter: 155±15 mm) was obtained at a delay time of 0 ms from R wave and the minimum size (diameter: 149±16 mm) was identified at a delay time of 240 ms in all patients. Conclusions Although the fluctuation of the size of the projected cardiac silhouette during a cardiac cycle was statistically significant, the fluctuation magnitude of 2% indicated clinical acceptability of chest radiography without electrocardiographic gating.