[Usefulness of high helical pitch acquisition for reduction of patient radiation dose in cardiac multidetector computed tomography].

Helical pitch (HP) usually has been decided automatically by the software (Heart Navi) included in the MDCT machine (Aquilion 64) depending on gantry rotation speed (r) and heart rate (HR). To reduce radiation dose, 255 consecutive patients with low HR (< or =60 bpm) and without arrhythmia underwent cardiac MDCT using high HP. We had already reported that the relationship among r, HP, and the maximum data acquisition time interval (Tmax) does not create the data deficit in arrhythmia. It was represented as Tmax= (69.88/HP-0.64) r; (equation 1). From equation 1, HP=69.88 r/ (Tmax+0.64 r); (equation 2) was derived. We measured the maximum R-R interval (R-Rmax) on ECG before MDCT acquisition, and R-Rmaxx1.1 was calculated as Tmax in consideration of R-Rmax prolongation during MDCT acquisition. The HP of high HP acquisition was calculated from equation 2. In HR< or =50 bpm, Heart Navi determined r: 0.35 sec/rot and HP: 9.8, and in 51 bpm< or =HR< or =66 bpm, r: 0.35 sec/rot and HP: 11.2. HP of the high HP (16.4+/-1.2) was significantly (p<0.0001) higher than that of Heart Navi HP (10.9+/-0.6). The scanning time (6.5+/-0.6 sec) of high HP was significantly (p<0.0001) shorter than that of Heart Navi (9.0+/-0.8 sec), and the dose length product of high HP (675+/-185 mGy x cm) was significantly (p<0.0001) lower than that of Heart Navi (923+/-252 mGy x cm). The high HP could produce fine images in 251/255 patients. In conclusion, the high HP acquisition is useful for reduction of radiation dose and scanning time.