Effects of scout radiographic imaging conditions on tube current modulation in chest computed tomography

Localiser radiographs in CT: Current practice, radiation dose, image quality and clinical applications

INTRODUCTION

Survey or localiser radiographs are integral to CT imaging. However, the diverse functions and roles of the localiser radiograph are often obscure to radiographers and radiologists. This scoping review reports the full scope of localiser radiograph use and function in contemporary CT imaging.

METHODS

A scoping review was performed. A systematic literature search was conducted using four databases: MEDLINE, CINAHL, Emcare and Scopus from January 2013 to December 2023. Data extraction was conducted by two review authors and validated by a third reviewer. Thirty-six studies were included in this review.

RESULTS

Three major themes emerged: radiation dose management, image quality considerations and clinical protocol applications. Specifically, the number, order of selection and directions of localiser radiographs significantly impact patient dose and image quality; which are additionally impacted by off-centre patient positioning, which can influence the accuracy of body size estimates and CT numbers. Finally, the optimal selection of localiser radiographs, including exposure parameters (kVp, mAs), can be a part of clinical task-based imaging protocol optimisation.

CONCLUSIONS

The utilities of localiser radiographs in CT imaging are varied. It is salient that radiographers and radiologists understand their role and the impacts of poor application to ensure that radiation dose is minimised and image quality maximised through correct use. Radiographers and radiologists should also be aware of the impact of poor patient positioning on ACTM function, dose and image quality. Additionally, localiser radiographs should be used for clinical task-based protocol optimisation.

IMPLICATIONS FOR PRACTICE

The number, order of selection, direction, patient off-centring, and exposure parameters must be considered when utilising localiser radiographs as they impact dose, image quality, and protocol applications. It is essential for radiographers and radiologists to understand these impacts.

Automatic Exposure Control Attains Radiation Dose Modulation Matched with the Head Size in Pediatric Brain CT

We investigated the relationship between the head size and radiation dose in pediatric brain computed tomography (CT) to evaluate the validity of automatic exposure control (AEC). Phantom experiments were performed to assess image noise with and without AEC, and indicated that AEC decreased differences in noise between slices of different section sizes. Retrospective analysis was conducted on 980 pediatric brain CT scans where the tube current was determined using AEC. The water equivalent diameter (WED) was employed as an index of the head size, and mean WED for each image set (WEDmean) and WED for each slice (WEDslice) were used for analysis. For the image-set-based analysis, volume CT dose index (CTDIvol) was compared to WEDmean. For the slice-based analysis, the tube current was compared to WEDslice using 20 of the 980 sets. Additionally, CTDIvol and WEDmean were compared between male and female patients matched for age, weight, or WEDmean. CTDIvol increased with increasing WEDmean, and an exponential curve was closely fitted to the relationship. Tube current changed similarly to the change in WEDslice for each image set, and an exponential curve was well-fitted to the plots of tube current against WEDslice when data from the 20 sets were pooled together. Although CTDIvol and WEDmean were slightly but significantly larger for male than female patients after matching for age or weight, a sex-dependent difference in CTDIvol was not found after matching for WEDmean. This study indicated successful dose modulation using AEC according to the head size for each patient and each slice location. The application of AEC to pediatric brain CT is recommended for radiation dose optimization.

Factors affecting dose-length product of computed tomography component in whole-body positron emission tomography/computed tomography

In whole-body positron emission tomography (PET)/computed tomography (CT), it is important to optimise the CT radiation dose. We have investigated factors affecting the dose-length product (DLP) of the CT component of whole-body PET/CT and derived equations to predict the DLP. In this retrospective study, 1596 whole-body oncology PET/CT examinations with¹⁸F-fluorodeoxyglucose were analysed. Automatic exposure control was used to modulate radiation dose in CT. Considering age, weight, sex, arm position (up, down, one arm up), scan range (up to the mid-thigh or feet), scan mode (spiral or respiratory-triggered nonspiral) and the presence of a metal prosthesis as potential factors, multivariate analysis was performed to identify independent predictors of DLP and to determine equations to predict DLP. DLP values were predicted using the obtained equations, and compared with actual values. Among body size indices, weight best correlated with DLP in examinations performed under the standard imaging conditions (arms: up; scan range: up to the mid-thigh; scan mode: spiral; and no metal prosthesis). Multivariate analysis indicated that weight, arm position, scan range and scan mode were substantial independent predictors; lowering the arms, extending the scan range and using respiratory-triggered imaging, as well as increasing weight, increased DLP. The degree of the DLP increase tended to increase with increasing weight. The DLP values were predicted using equations that considered these parameters were in excellent agreement with the actual values. The DLP for the CT component of whole-body PET/CT is affected by weight, arm position, scan range and scan mode, and can be predicted with excellent accuracy using these factors.

SUBOPTIMAL MODULATION OF RADIATION DOSE IN THE COMPUTED TOMOGRAPHY COMPONENT OF WHOLE-BODY POSITRON EMISSION TOMOGRAPHY/COMPUTED TOMOGRAPHY

Radiation exposure in computed tomography (CT) is automatically modulated by automatic exposure control (AEC) mainly based on scout images. To simulate the whole-body positron emission tomography/CT, CT images of a phantom were obtained using the posteroanterior scout image alone (PA scout) or the posteroanterior and lateral images (PA + Lat scout). Old and new versions of the AEC software were compared. Using the old version of the software and the PA scout, a markedly high dose at the top of the head was observed, which varied depending on the position of the phantom. This issue was resolved in the new version of the software. Radiation dose in the shoulder region was much higher using the PA scout than using the PA + Lat scout, even with the new version of the software. AEC may cause unreasonably high radiation exposure locally, and the appropriateness of the dose modulation pattern should be examined at each facility.