Computed tomography simulator conversion curve dependence on scan parameters and phantom dimension

Abstract

Introduction: Using computed tomography (CT) and treatment planning systems (TPS) in radiotherapy, due to the difference in photon beam energy on CT and linear accelerator, it is necessary to convert Hounsfield units (HU) to relative electron density (RED) values. The aim of this dosimetric study was to determine whether there is a significant effect of potential in the CT tube, field of view size (FOV), and phantom dimensions on the CT conversion curve CT-RED. The second aim is whether there are significant differences between the CT-RED obtained by the Computerized Imaging Reference Systems (CIRS) Thorax 002LFC phantom and the “reference” curve in the TPS, obtained by the CIRS 062M pelvis phantom, at the same CT conditions.Methods: Heterogeneous CIRS 062M and CIRS Thorax 002LFC phantoms were used, which anatomically and dimensionally represent the human pelvis, head, and thorax, with a set of known RED inserts. They were scanned on a CT LightSpeed GE simulator and obtained CT-RED.Results: The high voltage in the CT tube had a significant effect on the HU (t = 10.72, p < 0.001) for RED values >1.1, while FOV as a parameter did not show statistical significance for the 062M pelvis phantom. Comparing the slopes (062M pelvis and head) of the CT-RED for RED ≥ 1.1, the obtained value is t = 1.404 (p = 0.163). In the case of a 062M pelvis and a 002LFC phantom, we have seen a difference in RED values (for the same HU value) of 5 % in the RED region ≥ 1.1 (bone).Conclusion: Patients should be imaged on a CT simulator only at the potential of the CT tube on which the conversion curve was recorded. The influence of the FOV and scanned phantom dimensions is not statistically significant on the appearance of the calibration curve (RED ≥ 1.1).

The Role of a DirectDensity® CT Reconstruction in a Radiotherapy Workflow: A Phantom Study

“…As expected, the curves related to the standard reconstruction algorithm Qr40 had a strong dependence on the kVp value set during the image acquisition, both at low and high densities, as shown in the graphs to the left of Figure 6. In contrast, the mean calibration curves associated with the reconstruction algorithm DD exhibited this dependence only for densities above 1.82 g/cm 3 , which corresponds to the density of the cortical bone, as shown in the middle graphs in Figure 6. The same behavior could be observed for the calibration curves associated with the combined reconstruction DD + iMAR, as shown in the graphs to the right in Figure 6.…”

“…The calibration curves for each kVp and reconstruction technique were built using the nominal density of each insert and the corresponding HU mean value. The acquisition and reconstruction processes were repeated with a titanium (mass density of 4.51 g/cm 3 ) and a stainless-steel (mass density of 8 g/cm 3 ) insert to study the dependency of the calibration curves on kVp at densities exceeding the range of human tissues (Table 1). In these cases, the images were also reconstructed with a combination between the DD algorithm and the iMAR algorithm (indicated as DD + iMAR), giving an additional set of calibration curves.…”

“…Table 2. The mean HU in the selected cylindrical ROIs in the Cheese phantom with inserts of densities ranging from 0.30 g/cm 3 to 8 g/cm 3 . Data were divided on the basis of the kVp used during the acquisition of the image, on the basis of the reconstruction algorithm (Qr40, DD or the combined algorithm DD + iMAR), and on the basis of the metal insert placed in the phantom.…”

“…To derive this density map from the CT scan, it is necessary to implement in the TPS a conversion curve between Hounsfield units (HU) and relative electron density (RED) or mass density (MD) in the form of a calibration curve [1,2]. However, this curve depends on the kVp set during acquisition [3,4]. This is due to the beam energy dependence of the linear attenuation coefficients defining the HUs.…”

Influence of Computed Tomography Parameters on the Radiotherapy Plan Calculation

“…The calibration curve is a function of HU versus RED for a range of materials with different densities represented as a table . If there is a discrepancy between the HU values on the CT image for specific tissue types and the HU values in the TPS calibration curve, errors would be introduced in the TPS calculation. Thus, any inaccuracy in the CT number will have an impact on the dose calculated by TPS (Cozzi et al, 1998;Kolarevic et al, 2020). The tube voltage (kV) is the most important parameter affecting the calibration curve.…”