Total liver blood flow (TLBF) is closely regulated by relative contributions from the hepatic artery (HA) and portal vein (PV). Reductions of PV flow are compensated by rises in HA flow in the healthy liver – the ‘hepatic arterial buffer response’ (HABR) – but this response is thought to be impaired in liver disease, resulting in reduced TLBF(1,2). We have previously used phase-contrast (PC) MRI at 9.4T to measure rat baseline PV flow and haemodynamic response to terlipressin (an agent used to reduce PV flow, for instance during variceal bleeding), in the presence of cirrhosis(3). Measuring HA flow directly using PCMRI is challenging in small animals, primarily because of vessel size and tortuosity. In this study, we apply caval subtraction PCMRI, a novel and validated method for measurement of HA and TLBF(4) to study any differences in the response to terlipressin in normal and cirrhotic rats.

Subjects

Healthy Sprague-Dawley rats were underwent bile-duct ligation (BDL) procedure (n=6)(5) or sham laparotomy (n=6). Animals were maintained for 4 weeks until the development of cirrhosis. After induction with isoflurane, a jugular venous line was sited before transfer to a 9.4T Agilent 20 cm horizontal-bore scanner, with a 72 mm birdcage coil (Oxford, UK).

Two-dimensional cine PCMRI

Axial and angled coronal gradient echo images were used to plan PV and caval PCMRI studies. Cardiac and respiratory-triggered 2D cine PCMRI was performed (TR/TE=10/1.2ms, α=10°, 2 mm slice thickness, 192x192 matrix, FOV 40x40mm², 10-15 cardiac cycle phases, V_enc=33 and 66 cm/s for PV, proximal and distal IVC flows). ROIs were positioned manually on each vessel for each frame of the cardiac cycle and flow quantification was performed using in-house developed Matlab code. TLBF was estimated by subtracting proximal IVC flow (above renal but below hepatic venous inlets) from distal IVC flow (above hepatic venous inlets, but below the IVC-right atrial junction). Estimated TLBF measurements were normalised to explanted liver weight.

Terlipressin response

Dosage regimes were based on previous pilot experiments. After baseline caval subtraction PCMRI measurements, an intravenous 100 μg/kg bolus of terlipressin was followed by a 10 μg/kg/min infusion. Post-terlipressin caval subtraction PCMRI measurements were made after 15 minutes.

Expected reductions in sham (mean difference -90.3±11.1 ml/min/100g; p=0.0005) and BDL (mean difference -29.8±6.9 ml/min/100g; p=0.0049) PCMRI PV flow were demonstrated. No significant change in caval subtraction PCMRI TLBF was observed in sham animals (mean difference -2.5±14.0 ml/min/100g; p=0.8630), but a reduction in TLBF was observed in BDL animals (mean difference -65.5 ml/min/100g; p=0.0006) (figure 1a and 1b). These reflect a rise in caval subtraction PCMRI estimated HA flow in sham animals (mean difference +92.8±21.3 ml/min/100g; p=0.0073), but not BDL animals where estimated HA flow declined (mean difference -34.4±7.5 ml/min/100g; p=0.0059) (figure 1c and 1d).We have demonstrated expected reductions in PV flow in response to terlipressin, but also used caval subtraction PCMRI to demonstrate a very differing HA response in cirrhosis. Sham operated rats buffer reductions in PV flow by increases in HA flow, thereby maintaining TLBF. Contrastingly, the compensatory HABR fails in BDL rats, thereby causing reductions in TLBF. Caval subtraction PCMRI therefore represents a promising tool for non-invasive investigation of TLBF and HA flow in small animals, even in the presence of established cirrhosis.This is the first non-invasive demonstration of the HABR and its failure in a model of cirrhosis, made possible through the use of caval subtraction PCMRI.