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D eff consistently exceeding D W by an average of 7.73 %. Nearly parallel trend lines for D W and D eff suggest a consistent relationship across the AP range.
4 Discussion
This study presents findings that aim to contribute to the understanding of dose estimation complexities in CT, particularly within the context of computed tomography pulmonary angiography examinations. The results highlight the limitations of conventional dose estimation methodologies, especially the reliance on the standard 32-cm-diameter waterfilled phantom. This traditional approach has been shown to substantially underestimate the radiation dose received by patients, particularly when considering the diverse range of patient morphologies encountered in clinical practice. The analysis revealed that the CTDIvol consistently underestimates patient dose when compared to more tailored metrics, such as the size-specific dose estimate based on water-equivalent diameter( Ponnusamy et al., 2019; Sekkat et al., 2024a). This discrepancy, which varies from 8 % to 26 % depending on patient size, with an average difference of approximately 18 %, underscores the urgent need for more accurate dose estimation techniques that reflect individual patient characteristics. This is a direct consequence of using Automatic Tube Current Modulation( ATCM). This technology adjusts tube current based on tissue attenuation, resulting in a CTDIvol increase of approximately 60 % for patients of larger build compared to those of smaller stature as presented in the previous study( Sekkat et al., 2024c).
The SSDE calculation based on effective diameter( D eff) marked a significant advance in CT dosimetry. This methodology enables a more patient-specific assessment of radiation exposure while retaining clinical practicality. SSDE Deff is distinguished by its simplicity and efficiency, requiring only linear measurements from a single CT image to calculate the effective diameter. This approach strikes an optimal balance between improved accuracy and ease of implementation, enhancing the precision of dose estimates without compromising workflow efficiency. However, it is essential to recognize that this method is not without its limitations. The study indicates that in anatomical regions characterized by substantial tissue inhomogeneities, the D EFF- based approach may lead to either under- or overestimations of patient dose. This variability is contingent upon the average tissue attenuation within the scanned volume, emphasizing the need for ongoing refinement in dose estimation methodologies.
The findings reveal a strong positive correlation between d LAT and both D eff and D w, with determination coefficients of R 2 = 0.9175 and R 2 = 0.7578, respectively. These correlations suggest that both D eff and D w provide comparable estimates of patient size for SSDE calculations, aligning with recent studies in the field.
However, the analysis also uncovered systematic differences between these metrics. SSDE values derived from D eff were consistently higher than those based on D w for the same patient, with an average difference of 7.73 %. This systematic overestimation of D eff relative to D w is particularly notable in the thoracic region.
The observed discrepancies can be attributed to the distinct calculation methods employed for each metric. Both D eff and D w are derived from lateral and anteroposterior diameters, but they utilize different formulae( Pace et al., 2022) . D eff is calculated as the square root of the product of lateral and anteroposterior diameters, while D w incorporates tissue attenuation information, effectively replacing heterogeneous tissues( such as lungs) with an equivalent thickness of water.
Radiation attenuation at a given anatomical location, such as the thoracic region, is influenced by tissue composition and density, which can vary significantly between patients. While radiation attenuation is accounted for in the calculation of SSDE DW, it does not impact the SSDE Deff value. Even when D eff values are the same across patients, the attenuation of radiation can differ due to tissue heterogeneities. This variability in tissue composition results in differences in radiation attenuation, which may affect dose calculations.
The relationship between the D eff d W ratio( denoted as F D) and the fraction of low-attenuating regions in the body( represented by F LA) was examined. A stronger correlation between F D and F LA was found in male patients compared to female patients. This difference can likely be attributed to the greater variability in anatomical proportions among women, including differences in fat distribution and the presence of breast tissue, which impact the fraction of low-attenuating tissues in the thoracic region.
More specifically, these gender-related anatomical differences could help explain the biphasic curve observed for D w in Figure 5 which is not observed for D eff. The presence of lowattenuating tissues, such as fat or breast tissue in women, may result in the observed bimodal distribution of D w, whereas D eff remains more consistent due to its relative insensitivity to tissue heterogeneity.
As the fraction of low-attenuating tissues decreased( F LA approached 0), both D eff and D w values tended to converge, with F D approaching 1. This convergence is a result of increasing tissue homogeneity, where radiation attenuation becomes more uniform across the body, leading to more consistent dose estimates.
Clinically, these results underscore the importance of clearly specifying the size metric used( D eff or D w) when reporting SSDE, as the choice of one or the other can influence the reported numerical value by 5 – 10 %. This is particularly critical when comparing doses across different hospitals or against diagnostic reference levels based on SSDE( Satharasinghe et al., 2022) . Radiologists and medical physicists must be aware of these differences when interpreting and communicating doses to clinicians and patients.
This study’ s strength lies in its comprehensive inclusion of diverse patient morphologies, encompassing diameters from 22 to 38 cm. This broad range facilitated an in-depth examination of the size-SSDE relationship across a spectrum of clinically relevant dimensions.
A notable limitation, however, is the absence of analysis regarding the influence of acquisition parameters such as tube voltage, current-time product, and pitch on the size-dose relationship. Further investigation is warranted to elucidate these intricate interactions and their implications for dose estimation accuracy.