Hepatobiliary contrast agents (HBAs) are gadolinium-based contrast agent characterized by hepatocellular uptake and hepatobiliary excretion. The hepatobiliary phase has become an essential tool of modern magnetic resonance imaging (MRI) of the liver and refines the diagnostic capabilities of MRI. The specific characteristic of HBAs allows the combination of MRI dynamic vascular imaging with unique lesion characterization, biliary anatomy and liver functional assessment. The practical use of HBAs in the non-invasive diagnosis of benign hepatocellular lesions must be known, as imaging patterns are specific for focal nodular hyperplasia and hepatic adenomas, and may vary according to adenoma subtypes. HBAs for the non-invasive diagnosis of hepatocellular carcinomas are integrated into Western and Eastern guidelines and on specific criteria for use and interpretation. Metastatic liver disease also represents a major application. Emerging applications include advanced biliary imaging, functional assessment of the liver, and potential role in surveillance strategies. This review summarizes the properties of HBAs, including their pharmacokinetics, safety, and technical considerations for image acquisition. It also discusses their role in characterizing focal liver lesions, highlighting their strengths and limitations.

The purpose of this study was to assess the image quality of color K-edge imaging obtained with a spectral photon-counting CT (SPCCT) scanner using a spectral phantom with a mixture of iodine-based and gadolinium-based contrast agents. A clinical SPCCT scanner prototype was used to scan a spectral phantom. Three dedicated cavities were filled with three contrast agents including iodine alone, gadolinium alone and a mixture of both. Two concentrations of 0.5 and 2 mg/mL were evaluated using nine helical PCCT scans at 120 kVp and 150 mAs. Conventional, color iodine and color K-edge gadolinium images were obtained through a material decomposition algorithm using three basis materials (water, iodine, gadolinium). Attenuation (in Hounsfield unit [HU]), iodine and gadolinium concentrations and task-based transfer function (TTF) were measured on each cavity and image. The noise power spectrum (NPS) was calculated on the phantom's background. Color K-edge imaging differentiated iodine and gadolinium but underestimated their concentrations. Gadolinium concentrations were underestimated by 9.4 ± 2.2 (standard deviation [SD]) % and 9.2 ± 1.0 (SD) % for gadolinium alone, 14.9 ± 2.3 (SD) % and 11.4 ± 1.2 (SD) % for the mixture, at 0.5 and 2 mg/mL, respectively. Similar TTF values at 50 % were found for color iodine (0.43 ± 0.01 [SD] mm-1) and color K-edge gadolinium (0.45 ± 0.03 (SD) mm-1) images for respective cavities at 2 mg/mL but the lowest values were found for color K-edge gadolinium images (0.43 ± 0.01 [SD] mm-1vs. 0.29 ± 0.01 [SD] mm-1) at 0.5 mg/mL. The value of noise magnitude was 24.75 HU, 0.06 mg/mL and 0.03 mg/mL for conventional, color iodine and color K-edge gadolinium images, respectively. Color K-edge imaging helps distinguish between contrast agents while being associated with lownoise magnitude, high-frequency spatial noise and high spatial resolution.