Radioprotection 60-3 | Page 69

A. Khallouqi et al.: Radioprotection 2025, 60( 3), 268 – 276 269 Table 1. CTPA examination scanning specifications. Specification

Protocol

Traditional methods of estimating radiation dose in CT examinations, such as the volume computed tomography dose index( CTDI vol) and dose length product( DLP), rely on standardized phantoms. These approaches fail to account for the diverse range of patient body habitus encountered in clinical practice, potentially leading to inaccuracies in dose estimation. To address this limitation, the concept of Size- Specific Dose Estimates( SSDE) was introduced by Association of Physicists in Medicine( AAPM)( Hadipour et al., 2022; Hakme et al., 2023; Khallouqi et al., 2024).

The conventional SSDE methodology utilizes the effective diameter( D eff), derived from anteroposterior( d AP) and lateral( d LAT) measurements, to estimate patient-specific radiation dose( El Fahssi et al., 2024; Sekkat et al., 2024b) ⁠. While this approach has proven effective in abdominal and pediatric imaging, it may underestimate the absorbed radiation dose in regions with significant tissue inhomogeneities, such as the chest( Xu et al., 2019) ⁠. To overcome these limitations, particularly in thoracic imaging, the water-equivalent diameter( D W) has been proposed as an alternative metric for SSDE calculation accounting for both patient size and tissue composition( Gabusi et al., 2016a).

The aim of this study is to enhance the accuracy of radiation dose estimation in CTPA by comparing two SSDE methods: one based on effective diameter( SSDE Deff) and the other on water-equivalent diameter( SSDE Dw). Additionally, the study aims to compare the effective diameter( D eff) and water-equivalent diameter( D w) with lateral( d LAT) and anteroposterior( d AP) diameters to assess their correlation and reliability in patient size assessment. The research also evaluates the underestimation of conventional CT dose measurements( CTDIvol) relative to SSDE Dw.

2 Materials and methods

In this retrospective study, a cohort of 200 CT pulmonary angiography scans was curated from an initial pool of 230 examinations. The final sample comprised 96 male and 104 female patients, all of whom underwent imaging in accordance with the standardized CT CTPA helical scanning protocol delineated in Table 1. Exclusion criteria were assessed by removing from consideration any scans that failed to capture the patient’ s entire body within the field of view or those compromised by artifacts resulting from metallic implants. This screening process was essential to maintain the integrity of this study’ s data and the validity of subsequent analyses.

All examinations were performed using a specific CT scanner model Optima CT520 Series from G. E. Healthcare.

Fig. 1. Effective diameter measurement at the mid-slice of the CT lung images.

The utilization of a single scanner model across all examinations minimized potential variability in image acquisition parameters. The contrast injection technique involved tracking a bolus by placing a region of interest( ROI) on the main pulmonary artery. Patients received an injection of iodinated contrast media( 40 – 80 ml / s). The scan was automatically triggered when the density in the ROI reached 100 Hounsfield units, typically 3 – 12 seconds after injection.

For each patient, anonymized CT scans were retrieved from the hospital’ s Picture Archive and Communication System( PACS), ensuring patient confidentiality throughout the analysis.

To manually measure the effective diameter( D eff), each CT pulmonary angiography( CTPA) scan was processed to extract the lateral( d LAT) and anteroposterior( d AP) diameters of the patient. This involved selecting the central axial slice that best represented the midsection of the patient’ s chest knowing that there is no established protocol for measuring these dimensions until nowdays( Sekkat et al., 2024). On this slice, the d LAT and d AP dimensions were measured using a digital caliper tool within the imaging software, ensuring alignment with the outer contours of the patient’ s body( AAPM, 2011)( Fig. 1) ⁠: p D eff ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d Ap d Lat: ð1Þ

The patient D W was determined for each scan location following the methodology outlined in AAPM( 2014) ⁠. This calculation involved the formula( Eq.( 2)): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1 D W ¼ 2

1000 HU AROI

ROI þ 1: ð2Þ p

A ROI is the patient’ s area and HU ROI is the mean Hounsfield Unit value of the patient’ s image.

SSDE was derived both from D eff and D w( respectively SSDE Deff and SSDE Dw. In the first case, SSDE Deff is computed following equation( 3):

� SSDE def f ¼ CTDI vol f D ef f; ð3Þ