In-patient to isocenter KERMA ratios in CT

Abstract

Purpose: To estimate in-patient KERMA for specific organs in computed tomography (CT) scanning using ratios to isocenter free-in-air KERMA obtained using a Rando phantom. Method: A CT scan of an anthropomorphic phantom results in an air KERMA K at a selected phantom location and air kerma K CT at the CT scanner isocenter when the scan is repeated in the absence of the phantom. The authors define the KERMA ratio (R K ) as K= K CT , which were experimentally determined in a Male Rando Phantom using lithium fluoride chips (TLD-100). R K values were obtained for a total of 400 individual point locations, as well as for 25 individual organs of interest in CT dosimetry. CT examinations of Rando were performed on a GE LightSpeed Ultra scanner operated at 80 kV, 120 kV, and 140 kV, as well as a Siemens Sensation 16 operated at 120 kV. Results: At 120 kV, median R K values for the GE and Siemens scanners were 0.60 and 0.64, respectively. The 10th percentile R K values ranged from 0.34 at 80 kV to 0.54 at 140 kV, and the 90th percentile R K values ranged from 0.64 at 80 kV to 0.78 at 140 kV. The average R K for the 25 Rando organs at 120 kV was 0.61 6 0.08. Average R K values in the head, chest, and abdomen showed little variation. Relative to R K values in the head, chest, and abdomen obtained at 120 kV, R K values were about 12% lower in the pelvis and about 58% higher in the cervical spine region. Average R K values were about 6% higher on the Siemens Sensation 16 scanner than the GE LightSpeed Ultra. Reducing the x-ray tube voltage from 120 kV to 80 kV resulted in an average reduction in R K value of 34%, whereas increasing the x-ray tube voltage to 140 kV increased the average R K value by 9%. Conclusions: In-patient to isocenter relative KERMA values in Rando phantom can be used to estimate organ doses in similar sized adults undergoing CT examinations from easily measured air KERMA values at the isocenter (free in air). Conversion from in-patient air KERMA values to tissue dose would require the use of energy-appropriate conversion factors.

KERMA ratios in pediatric CT dosimetry

“…Table 2 provides a summary of the radiographic techniques used to perform scans on each of the four pediatric phantoms. For these scanners, it is possible to obtain nominal values of K CT , which is numerically equal to CTDI air divided by the over-beaming factor for that scanner/scan geometry combination [6], and to estimate the radiation dose in a 16-cm diameter acrylic phantom that approximates a pediatric patient [14]. Based on the radiographic techniques, values of K CT used in this work were estimated using the ImPACT calculator to range from~40 mGy at 80 kV to 150 mGy at 140 kV in the GE scanner.…”

“…This method is conceptually simple and based on in-air measurements that can be readily obtained for any clinical CT scanner [6]. KERMA is an acronym for Kinetic Energy Released in Matter, and has the same units as dose (the Gray [Gy]), defined as one joule of energy deposited per kilogram of material.…”

Estimating fetal dose from tube current‐modulated (TCM) and fixed tube current (FTC) abdominal/pelvis CT examinations

“…[1][2][3] Initial attempts to estimate the radiation dose a fetus would receive from a CT exam were based either on phantom measurements, Monte Carlo (MC) simulations of geometric phantoms, or a combination of the two. [4][5][6][7][8][9][10][11][12][13] The approach described by Felmlee et al 4 , for example, uses anthropomorphic phantom measurements and measured Computed Tomography Dose Index (CTDI) values. Some important limitations of these early efforts relate to their use of simplified, geometric models and assumptions of nonvarying maternal anatomy in a single-size patient model.…”

Estimation of primary radiation output for wide‐beam computed tomography scanner

“…However, Huda et al reported conversion factors from _ K air to effective dose or organ dose 17. 2 They then calculated the effects of both primary and scattered photons separately at different positions in the phantom.…”

“…9 One reason for this may be the energy depen- or organ dose. 17 At this point, the _ K 0Àw=oÀA and _ K 0ÀwÀA might be utilized as well, although no definite claim can be made without verification. On the other hand, the unit of K 0 is mGy/s, not mGy/rotation as for CTDI vol .…”