MAPD—an objective way to select mAs for paediatric brain CT

Abstract

CT is an advanced imaging modality, but the imaging parameters are normally selected subjectively. For standard head examinations, most of the parameters used are consistent amongst different centres, with the exception of large variations in the selection of the tube current-exposure time product (mAs). As a result, CT images may contain unacceptable levels of noise, or the patient may receive excessive radiation. In this study, the maximum anteroposterior diameter (MAPD) was shown to be a good criterion for mAs selection, and could be measured in a pilot view. 200 paediatric brain CT studies were randomly selected to determine the MAPD at the mid brain level. With knowledge of MAPD distribution, a phantom study was performed to determine the relationship between MAPD and the mAs required for consistent and acceptable image noise. It was found that the required mAs increased linearly with MAPD. Assuming the manufacturer's recommended value is "appropriate" for the average MAPD, the appropriate mAs value could be estimated. Using this method, appropriate mAs values were calculated retrospectively for a group of 240 randomly selected paediatric brain CT studies and compared with the actual mAs subjectively determined by the radiographer. Although their average values were similar, the difference between the calculated and actual values deviated markedly in some cases. When the actual mAs was smaller than the calculated value, higher image noise was observed. However, reduction of image noise was barely observed when the applied mAs was larger than the calculated value. Thus, this method is more objective and appropriate for determination of the mAs value for paediatric brain CT than the traditional subjective method.

Assessment of image quality of a standard and three dose-reducing protocols in adult cranial CT

“…Recently, interest is growing in decreasing the radiation dose in various CT applications [4][5][6][7]. However, studies evaluating the feasibility of reducing the radiation dose in head CT applications are limited [3,8,9]. As neuro-radiologists prefer relatively noise-free CT images, scanning is usually performed with high mAs values, such as 350-560 mAs, and scan doses are between 6 and 11 cGy [10].…”

“…Using the lateral scout views, we measured the MAPD. The MAPD is defined as the largest anteroposterior distance drawn parallel to the orbital meatal (OM) line [3].…”

“…The purpose of this study was to analyze the effect of different tube current settings (mAs) and optimize the image quality and dose for the adult cranial CT protocol to assure a dose as low as possible, but still consistent with sufficient image quality. Wong et al showed a correlation between head size and CT attenuation values in the pediatric age group [3]. However, it is not known whether there is such a correlation between changing head size and CT attenuation in the adult age group.…”

Radiation exposure from CT in early childhood: a French large-scale multicentre study

“…Few paediatric values of organ doses are available [6,[15][16][17]35] and concern mostly optimisation efforts [6,[18][19][20][21]36]. The CT-Expo software allowed us to calculate doses for the paediatric population.…”

“…An almost twofold difference in dose values has been observed among MDCT models of scanners after normalising technical parameters [24]. The discrepancy between technical protocols appeared to be linked both to the radioprotection awareness of the radiologist, as shown by several studies on dose reduction by optimisation [18][19][20][21], and to the referral diagnosis. The wide range of doses linked to the lack of protocol optimisation should no longer be observed.…”