The Experimental Study And Clinical Application Of Mulati-slice CT With Automatic Tube Current Modulation

PART 1 THE INFLUENCES OF IMAGING PARAMETERS ON MULTI-SLICE CT WITH AUTOMATIC TUBE CURRENT MODULATION:A PHANTOM STUDYObjective: To explore the influences of imaging parameters on image quality and radiation dose in multi-slice CT with automatic tube current modulation (ATCM).Materials and methods: A irradiation equivalence anthropomorphic phantom was scanned using a 16-slice multi-slice spiral CT with ATCM. ATCM was studied with variable parameters for projection radiograph: tube angle(0°, 90°), tube current(10 mA, 50 mA, 100 mA), tube potential(80 kV, 100 kV, 120 kV, 140 kV); and spiral acquisition parameters were changed for different axial images, which were noise index(9, 12, 15, 20), pitch(0.562, 0.938, 1.375, 1.75), detector collimation configuration(8×2.5㎜, 16×1.25㎜, 8×1.25㎜, 16×0.625㎜), and tube voltage (80 kV, 100 kV, 120 kV, 140 kV). Changes in mA , CTDIvol and image noise resulting from the modulation were calculated and compared. Results: The value of CTDIvol from the projection radiograph of 90°by 28.45% over the projection radiograph of 0°. The value of CTDIvol had a certain decrease when the tube current and the tube voltage increased for the projection radiograph. The image noise increased alongside the growth of the NI, while the value of CTDIvol decreased. The image noise could keep constant in spite of the changes of pitch, detector collimation or tube voltage. The value of CTDIvol came to top at the pitch of 0.938. The value of CTDIvol by 6.44% when the pitch changed from 0.562 to 0.938, and decreased respectively 11.67% and 23.05% when the pitch changed from 0.938 to 1.375 and 1.75. The X-ray dose utilization ratio (78.8% for 16×0.625㎜, 78.41% for 8×1.25㎜, 97.4% for 16×1.25㎜, and 96.99% for 8×2.5㎜) and radiation dose were different when detector collimation configuration changed. Compared with the detector collimation width of 10mm, the value of CTDIvol at the width of 20mm increased by 51%(pitch=0.875)and 63.39%(pitch=0.938). For the same detector collimation width but different slice thickness, the value of CTDIvol at 16×0.625㎜ increased by 31.63% when compared with that at 8×1.25㎜, and at 16×1.25㎜ increased by 21.64% when compared with that at 8×2.5㎜. The value of CTDIvol decreased when the tube voltage went up.Conclusions: The projection radiograph acquisition parameters must be standardized to ensure ATCM technique to exert effects normally. When spiral acquisition parameters must be selected rationally, ATCM technique could control the image noise well and reduce the radiation dose.PART2 THE INFLUENCE OF PATIENT OFF-CENTERING ON IMAGE QUALITY AND RADIATION DOSE WITH AUTOMATIC TUBE CURRENT MODULATIONObjective: To evaluate the influence of patient off-centering in the gantry iso-center on CT image quality and radiation dose with automatic tube current modulation(ATCM) technique.Materials and methods: An quality assurance water phantom was scanned in iso-center and various off-center positions, using a 16-slice spiral CT with fixed-tube current and with ATCM technique. Other acquisition parameters remained the same. The size and CT value of the frontal localizer radiographs, mA and noise from the images were recorded after scanning.Results: The size of the frontal localizer radiographs became smaller when the distance of the phantom increasing from the tube but there were no significant change of CT value. In fixed-tube current scanning, the image noise increased alongside the growth of the distance apart from the gantry iso-center. With ATCM technique, from 100㎜ above to 70㎜below the gantry iso-center, the tube current products ranged from 168% to 78% of that at the gantry iso-center , and image noise ranged from 91% to 122% of that at the gantry iso-center.Conclusions: With ATCM technique, patient off-centering may bring to a notable increase the image noise or radiation dose. In clinical practice, the technicians must make correct location using the infrared light to ensure the center of scan target coincide with the gantry iso-center.PART3 AUTOMATIC TUBE CURRENT MODULATION TECHNIQUE IN NECK SCAN WITH 64-SLICE SPIRAL CTObjective: To assess the value of automatic tube current modulation (ATCM) technique in control of image quality and radiation dose of CT scan on neck.Materials and methods: A total of 90 patients underwent neck CT scan with 64-slice spiral CT, and were randomized into 3 groups: fixed-tube current, ATCM 1 with a noise index(NI) of 8, and ATCM 2 (NI=10). Objective noise, CTDIvol and mA of each group were recored.Results: The image qualities of all the patients were diagnostically acceptable, though the objective noise of upper neck in the ATCM groups was higher than that in the group of fixed-tube current(p<0.05). The radiation dose in the ATCM groups[ATCM: (9.39±2.30) mGy, (8.71±2.59) mGy] were significantly inferior to that in the group of fixed-tube current(13.4 mGy)(p<0.05).Conclusions: Compared with fixed-tube current technique, ATCM technique can significantly reduce the radiation dose in neck CT scan with diagnostic acceptability of the image quality.