Our study is the first to report on the effect of tube voltage and IR on PCAT attenuation in a phantom study using ex vivo porcine hearts. The main findings of our study are as follows: (1) The contrast-enriched agar injections into coronary arteries of porcine hearts provide a usable CCTA phantom for PCATMA quantification. (2) Increasing tube voltages for CCTA acquisition were associated with higher PCATMA values. (3) The implementation of IR resulted in higher PCATMA values, as compared to conventional FBP reconstruction.

PCAT attenuation has recently received increased attention as a novel imaging biomarker due to its predictive value for cardiovascular outcomes [3, 7, 15]. Nevertheless, differences in acquisition protocols and reconstruction parameters may introduce bias in the analysis of PCAT attenuation by affecting CT numbers. For example, the tube voltage is often adapted in the clinical routine according to the patient’s characteristics and clinical questions. In addition, the use of low-kilovoltage CCTA has rapidly increased in recent years thanks to the technical advancements of CT scanners [16, 17]. Indeed, the mean photon energy of polychromatic x-ray beams generated by low tube voltage protocols is closer to the k-absorption edge (33 keV) of iodine. Thus, the photoelectric effect is enhanced, resulting in improved vessel-to-tissue contrast while reducing the radiation dose and the amount of contrast agent [18]. The significant increase of PCATMA observed with increasing kVp in our study is the direct result of the energy dependence of the underlying linear attenuation indices. These findings have been also confirmed by a recent phantom study, which investigated the spectral behavior of fat attenuation by using a photon-counting detector CT. In particular, the authors reported increasing PCAT attenuation values with increasing energy levels on virtual monoenergetic image reconstructions [19].

Owing to their ability to mitigate noise and provide diagnostic-quality CT images at lower radiation dose, IR algorithms have become the standard reconstruction technique, replacing the traditional FBP [20,21,22]. Therefore, we reported conversion factors limited to IR datasets. However, the attenuation differences between FBP and IR detected for both CT manufacturers highlight the importance of taking into consideration the reconstruction algorithm in the initial study design of studies investigating PCAT attenuation. While the results related to the tube voltage are more general, the results for IR are specific to the algorithms evaluated and cannot be generalized to other approaches or manufacturers. Similarly, since it has been previously demonstrated that reconstruction kernels influence CT numbers [8, 10], the results reported in this study cannot be transferred to scans reconstructed by using different filters.

Our study design allowed for the indirect evaluation of the effect of phantom positioning in the CT scanners on CT attenuation. Like the findings of a previous report [23], different positions of the hearts from the scanner isocenter resulted in variations of the CT numbers and, therefore, of the conversion factors at each tube voltage applied. Although these values were not calculated from the same heart imaged at different positions, this highlights the importance of accurate and consistent patients’ centering in CT at baseline and follow-up scans to avoid misdiagnosis.

Overall, the specific conversion factors for PCAT attenuation derived from our experiments are in agreement with existing literature values on pericardial adipose tissue [7] and PCAT attenuation [11]. However, in our study, PCAT values were relatively higher in comparison to those reported by Ma et al [11], thus leading to an increase in the relevant conversion factors, especially at low kVp values. While we report k80 = 1.267, the PCAT values reported by Ma et al would result in \( _^=1.137 \). On the other hand, Oikonomou et al used \( _^=1.11485 \) and \( _^=0.89095 \) based on attenuation values of pericardial adipose tissue [9], which are in line with k100 = 1.111 and k140 = 0.970, respectively, derived from our observations. The postmortem nature of our study, in combination with the injection of contrast-enriched agar and the lack of the surrounding anatomical structures, could potentially account for the observed shift in PCAT values at lower tube voltages.

Our study has some limitations. (1) Since the porcine hearts were obtained from the slaughterhouse, no detailed health information was available. Nevertheless, the hearts were generally considered healthy. (2) The phantoms were motionless, lacking motion artifacts that may occur during real-life CCTA scans. (3) Due to the ex vivo nature of the study, attenuation and beam hardening from chest structures expected in patients could not have been evaluated. Despite this, given the paucity of in vivo data on the effect of different tube energies on adipose tissue, we believe that our approach simulates better in vivo conditions than previous experiments limited to the use of tubes filled with oil. Since the aim of our study was to show the magnitude of the effect of tube voltage on CT numbers, rather than proposing conversion factors for the immediate implementation in clinical routine, it was deemed acceptable not to simulate additional absorptive and scattering effects from thoracic structures. (4) Although a minimal delay in the scanning of the hearts after preparation was ensured, diffusive effects might have propagated the contrast agent into the perivascular tissue and caused an apparent dilation of the vessel, potentially leading to the exclusion of perivascular adipose tissue during segmentation. Since the co-registration of the images ensured direct comparability of PCATMA at different tube voltages, the effect on the results should be negligible. (5) Despite preliminary data from human studies showing that body mass index and cardiovascular risk factors might have an impact on PCATMA [24], our study design did not allow the assessment of these clinical factors.

In conclusion, our study showed that PCATMA values vary considerably by tube voltage and reconstruction type. Therefore, standardized acquisition and reconstruction protocols are advisable to assure accurate, reproducible, and comparable PCAT attenuation values in multicenter and longitudinal studies.