Page 16 - Delaware Medical Journal - December 2015
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SCIENTIFIC ARTICLE
of the body.” The elbow had 2 positions: 1) side of body (9) and 2) above head (5). CTDlvol (mGy) was 3.31 and 9.00 for positions 1 and 2. CTDLP (mGy-cm) was 47.16 and 136.66 for positions
1 and 2.
All lower extermity scans were in the supine position. Frequency of consistent joint scout start and end varied among all joints: ankle (distal tibia–foot, 13/25, 52 percent); elbow (no consistent scout); femur (iliac crest–proximal tibia, 2/8, 25 percent); foot 4/5, 80 percent); hip (iliac crests–mid femur, 4/32, 12.5 percent and lower abdomen–distal femur, 4/32, 12.5 percent); knee (distal femur–proximal tibia, 11/14, 78 percent); shoulder (nose–mid and lowest radiation values by joint were the following: CTDlvol (mGy), hip (7.11) and ankle (1.4); CTDLP (mGy-cm), femur (191.1) and ankle (27.1).
DISCUSSION
As the frequency of CT scans continues to increase in the pediatric patient population, it becomes increasingly important
to accurately quantify the radiation exposure to each child and
of extremity CT scans were performed for lower extremity evaluation, and these studies were also associated with the highest radiation doses.
There are a variety of ways to report radiation exposure, and
at this time there does not appear to be a consensus.13 CT dose index (CTDI) is the historical metric for reporting radiation in CT scan volume. The CTDIvol is an adjusted derivation that accounts for gaps and overlays between beams that occur in modern CT devices and can be thought of as a weighted sum representing the radiation dose of a single slice. Dose length product (DLP) is the CTDIvol multipled by the scan length and is independent of what is being scanned. Both of these values represent output from the machine itself, and, while they vary appropriately with different scanning parameters, they do not account for variations in patient characteristics that impact the effective dose for each patient. Patient size, for example, alters the overall dose so that CTDIvol and DLP overestimate doses for larger patients and underestimate doses for smaller patients.14
Accurate dose estimates require incorporation of machine output with with adjustments for patient size and the specific organs exposed.
many techniques have arisen to improve assessment of effective dose. Accurate dose estimates require incorporation of machine organs exposed. Traditional CTDIvol output and effective
dose data are based on radiation calculations in reference to
an idealized phantom that weights the different organs by radiosensitivity. In 2011, the American Association of Physicists in Medicine published conversion tables that allow a more accurate calculation of radiation dosing in patients of different sizes. The group used two distinct methods (empirical derivation and computerized simulations) to develop the conversion factors, and both arrived at similar values for the conversion factors.15 The data have since been implemented in clinical practice and have been shown to produce a better estimate of absorbed dose.
The degree of radiation exposure for most extremity CT scans
in body CT imaging. For example, Sharp et al retrospectively reviewed patients undergoing abdominal CT scans at non-pediatric facilities and at a dedicated children’s hospital. They reported average DLP values of 586.25 at non-pediatric facilities and 143.54 at the pediatric facilities.16 The same group reported DLP averages of 786.28 for head CT scans. Both series demonstrate higher DLP values than for the majority of the scans in our study.
The goal of this study was to report radiation doses for pediatric patients undergoing lower extremity CT scans. One of the anatomic imaging studies often varied because of differing limitation of this study may be generalizability. As a pediatric specialty institution, our facility has strived to incorporate ALARA and Image Gently protocols for all scans. Consequently, the radiation dose calculations in this study may be lower than what can be expected in institutions with more traditional
imaging protocols.
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Del Med J | December 2015 | Vol. 87 | No. 12
