Ex vivo imaging of fatty liver using MPPC-based photon counting CT for evaluation of liver diseases

X-ray computed tomography (CT) is a non-destructive internal examination technology. Photon-counting CT (PC-CT) is its next-generation version, detecting X-rays in photon units to provide CT images with X-ray energy information. This enables quantitative investigation of material identification and density, promising new diagnostic methods and treatment visualization in medicine. We developed PC-CT by combining high-speed scintillators (YGAG:Ce) and multi-pixel photon counters (MPPCs) as photo-semiconductor devices. To evaluate liver diseases, we utilized the developed PC-CT in our experiment. Specifically, we conducted ex vivo CT imaging on healthy and fatty liver rats, injecting them with gadolinium contrast agent (Gd-EOB), which selectively accumulates in healthy hepatocytes. This contrast agent is commonly used in MRI for liver disease assessment. Our study aimed to determine if PC-CT could produce images comparable to or superior to MRI, potentially offering diagnostic options for patients without access to MRI. Our findings revealed that gadolinium contrast agent accumulated throughout the liver in healthy rats ($1-5 \mathrm{mg} / \mathrm{ml}$), but not in those with fatty liver. This demonstrates the potential of PC-CT for evaluating liver diseases using Gd-EOB. Additionally, we estimated the fat fraction in fatty livers by analyzing the energy dependence of soft tissue and adipose attenuation coefficients. This revealed a fat fraction of $\sim 56 \%$, aligning closely with typical values. Such fat fraction estimation with PC-CT aids in assessing liver disease progression and understanding disease mechanisms, contributing to innovative diagnostics.