Monte Carlo simulations to assess the effects of tube current modulation on breast dose for multidetector CT

Abstract

Tube current modulation was designed to reduce radiation dose in CT imaging while maintaining overall image quality. This study aims to develop a method for evaluating the effects of tube current modulation (TCM) on organ dose in CT exams of actual patient anatomy. This method was validated by simulating a TCM and a fixed tube current chest CT exam on 30 voxelized patient models and estimating the radiation dose to each patient’s glandular breast tissue. This new method for estimating organ dose was compared with other conventional estimates of dose reduction. Thirty detailed voxelized models of patient anatomy were created based on image data from female patients who had previously undergone clinically indicated CT scans including the chest area. As an indicator of patient size, the perimeter of the patient was measured on the image containing at least one nipple using a semi-automated technique. The breasts were contoured on each image set by a radiologist and glandular tissue was semi-automatically segmented from this region. Previously validated Monte Carlo models of two multidetector CT scanners were used, taking into account details about the source spectra, filtration, collimation and geometry of the scanner. TCM data were obtained from each patient’s clinical scan and factored into the model to simulate the effects of TCM. For each patient model, two exams were simulated: a fixed tube current chest CT and a tube current modulated chest CT. X-ray photons were transported through the anatomy of the voxelized patient models, and radiation dose was tallied in the glandular breast tissue. The resulting doses from the tube current modulated simulations were compared to the results obtained from simulations performed using a fixed mA value. The average radiation dose to the glandular breast tissue from a fixed tube current scan across all patient models was 19 mGy. The average reduction in breast dose using the tube current modulated scan was 17%. Results were size dependent with smaller patients getting better dose reduction (up to 64% reduction) and larger patients getting a smaller reduction, and in some cases the dose actually increased when using tube current modulation (up to 41% increase). The results indicate that radiation dose to glandular breast tissue generally decreases with the use of tube current modulated CT acquisition, but that patient size (and in some cases patient positioning) may affect dose reduction.

CT Dose Index and Patient Dose: They AreNotthe Same Thing

“…Facilities that adjust their CT technique appropriately for patient size, whether with use of manual technique charts or automatic exposure control ( 35,(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56), will prescribe a wide range of scanner output (CTDI) values. This is a good outcome, refl ecting the facility's conscientiousness in "right-sizing" the dose settings on the basis of specifi c patient body habitus.…”

The feasibility of patient size‐corrected, scanner‐independent organ dose estimates for abdominal CT exams

“…11,12 As a result, the reported CTDI vol -to-organ-dose conversion coefficients were valid only for that specific scan protocol and patient model. Several investigators [13][14][15][16] have demonstrated that patient size has a significant effect on the absorbed dose and, even more specifically, on organ dose for a specific scanner output ͑e.g., CTDI vol ͒. These reports have all shown that for the same exposure conditions ͑i.e., same technical parameter settings͒, organs in smaller patients ͑including pediatric patients͒ receive higher radiation doses than those in larger patients.…”

Thoracic low-dose CT image processing using an artifact suppressed large-scale nonlocal means

“…Thus to limit the radiation dose, low-dose computed tomography (LDCT) could be applied for screening in patients at high risk for developing lung cancer [4]. Low dose CT can be achieved by decreasing the milliamperage and the voltage [5][6], which, however, leads to a degraded signal to noise ratio [7][8][9][10][11]. This is due to a severe increase of the quantum and electronic noise.…”