Automatic Tube Current Modulation Systems Research Articles

SU‐FF‐I‐39: The Effects of Additional Scatter Media and Tube Current Modulation On MDCT Organ Doses

Purpose: To quantify the effect of scatter media surrounding the scanned volume in CT on organ dose. This information could prove valuable in using patient specific data with Monte Carlo simulations to determine clinical organ doses. The effects of beam width and automatic tube current modulation (ATCM) on organ doses in CT were also explored, as most current Monte Carlo simulations do not account for the ATCM systems incorporated in modern clinical scanners. Method and Materials: Average organ dose measurements were taken in an adult male anthropomorphic phantom using a fiber optic coupled dosimetry system during a routine clinical chest CT exam. The amount of scatter media was increased longitudinally from a normal 33 cm scan length to include the entire 176 cm length of the phantom. Both modulated and fixed tube current scans were performed using multiple beam widths (2.4 mm and 1.2 mm) to determine the effects of these parameters on organ doses. Results: Within experimental error, the presence of additional scatter media did not cause an increase in measured doses. A 2.4 mm beam produced an 11% dose reduction as compared to a 1.2 mm beam. ATCM was found to reduce average organ doses by approximately 24%, with individual organ dose reductions ranging from 36% in the breast to 10% in the lungs compared to fixed current scans. Conclusion: Monte Carlo simulations performed on patient scan data will not significantly underestimate organ doses due to scatter contribution from outside the scan volume for an average‐sized patient. A combination of ATCM and a wider beam was found to reduce average doses by 33% during a chest CT. It was found that a comparison of the ATCM tube current per slice is an accurate predictor of individual and average organ dose savings for ATCM compared to fixed current techniques.

Misoperation of CT Automatic Tube Current Modulation Systems with Inappropriate Patient Centering: Phantom Studies

Inappropriate patient centering on the gantry changes the size of the localizer radiographs used for CT examinations, influencing the operation of CT automatic tube current modulation because tube current is controlled with information from localizer radiographs. The purpose of this study was to examine the influence of inappropriate patient centering on the gantry isocenter on automatic tube current modulation. An elliptical phantom was scanned with four automatic tube current modulation techniques after acquisition of localizer radiographs in the horizontal and vertical directions with the phantom center shifted from the gantry isocenter in the vertical direction. After scanning, the magnification rate of the frontal localizer radiographs, tube current-time product, and image noise were examined. On phantom studies, the magnification rate of localizer radiographs showed a linear relation to the vertical deviation of the phantom from the gantry isocenter. From 50 mm above to 50 mm below the gantry isocenter, tube current-time products ranged from 75% to 141% compared with those at the gantry isocenter. In addition, increases and decreases in the amount of image noise related to changes in tube current-time product were confirmed. Inappropriate patient centering causes misoperation of automatic tube current modulation systems, in which tube current is controlled with information from localizer radiographs, and thus causes increases in tube current or image noise.

Effective dose and image quality evaluations of an automatic CT tube current modulation system with an anthropomorphic phantom

The purpose of this study was to evaluate the consequences of different choices of acquisition parameters on the actual image noise and on the patient dose with an automatic tube current modulation system. The CT investigated was a GE Lightspeed 16-slice and an anthropomorphic phantom was used to simulate the patient. Several acquisitions were made varying noise index (NI), kilovoltage and pitch values. Tube current values were compared for the different acquisitions. Patient dose was evaluated in terms of volumetric computed tomography dose index (CTDI vol) and also as effective dose. The noise actually present in the images was analyzed by a region of interest analysis considering representatively phantom sections in the regions of the shoulders, of the lungs and of the abdomen. The obtained results generally evidenced a good agreement between the noise index and the measured noise for the abdomen sections, whereas for the shoulders and the lungs sections the measured noise was respectively greater and lower of the NI. Varying the kV the automatic current modulation system provided images with a substantially constancy of the actual noise and of the patient dose. An increase of the pitch generally decreased the patient dose, whereas the noise was slightly greater for the lowest pitch and almost constant for the other pitch values. This study outlines some important relationships between an automatic tube current modulation system and other CT acquisition parameters, providing useful informations for the choice requested by radiologists in the task of optimization of the CT acquisition protocols. Unless there are other considerations in place, pixel pitches below 1.375 should be avoided, and kVp settings can be changed with no real impact on dose or image noise.