Analysis of colorectal liver metastases in photon-counting detector CT : optimizing imaging through spectral reconstruction

Abstract

Purpose The aim of this study was to evaluate the visibility of colorectal liver metastases (CRLM) using photon-counting detector computed tomography (PCD-CT) and to determine the optimal virtual monoenergetic image (VMI) and iodine map reconstructions for improved contrast detection between metastases and surrounding liver parenchyma.

Materials and methods A total of 117 patients with 227 CRLM (up to three measurements per patient) who underwent abdominal PCD-CT for staging between 09/2022 and 08/2024 were retrospectively included. VMI were reconstructed at energy levels between 40 and 90 keV (in 10 keV increments), and scanner-generated iodine maps were additionally analysed. To quantify contrast between CRLM and liver parenchyma, the parenchyma-to-lesion ratio (PLR) was calculated for each VMI and iodine map. The contrast-to-noise ratio (CNR) was determined based on attenuation values of the metastases and the bilateral musculus erector spinae, as well as its standard deviation. For the iodine map, lesion and parenchyma iodine concentrations were used analogously. Subjective assessment of metastases visibility on the three best VMIs in PLR and CNR (40–60 keV) and iodine maps were independently performed by three radiologists.

Results Lesion and liver attenuation decreased steadily with higher keV levels. Iodine maps showed markedly higher iodine concentration in liver parenchyma than in metastases. The PLR was highest on the iodine map (3.29 ± 2.01), followed by 40 keV (2.19 ± 0.73). Regarding CNR, the 40 keV VMI showed the highest value (1.49 ± 1.70), followed by the iodine map (1.09 ± 0.99). CNR values decreased further at higher energies and significantly reduced at 70–90 keV. Paired superiority testing confirmed 40 keV as the best-performing VMI, showing significantly higher CNR than the iodine map, whereas PLR remained superior on the iodine map. Subjective ratings indicated that the 50 keV VMI provided the best visibility of CRLM. The iodine map consistently received lower subjective ratings across all criteria.

Conclusion Both iodine maps and low-keV VMIs, particularly at 40 keV, demonstrated high PLR and CNR values, contributing to improved depiction of CRLM in PCD-CT. The complementary use of these reconstructions may enhance lesion detection and overall diagnostic confidence.