Evaluation of Superparamagnetic Iron Oxide for MR Imaging of Liver Injury: Proton Relaxation Mechanisms and Optimal MR Imaging Parameters

Abstract

Purpose: To investigate the proton relaxation mechanisms and the optimal MR imaging parameters in superparamagnetic iron oxide (SPIO)-enhanced MR imaging of liver injury.
Methods: A liver injury model was created in the rat using carbon tetrachloride. The T₁ and T₂ relaxation effects of SPIO in normal and injured liver were estimated by ex vivo relaxometry. In vivo laser confocal microscopy of the liver was performed to simulate the distribution and clustering of SPIO particles in the hepatic macrophages. SPIO-enhanced MR imaging (1.5T) of normal and diseased rats was performed with variable parameters. The liver specimens were prepared for histopathological examination.
Results: Histopathological and laser confocal microscopic findings showed diffuse macrophage distribution but decreased intracellular clustering of SPIO in injured liver. Ex vivo relaxometry showed sustained T₁ and T₂ relaxation effects of SPIO in liver injury. On MR images obtained with moderate echo time (spin echo [SE] 2000/40 and gradient echo [GRE] 130/9.0/60°), injured liver showed significantly lower decrease in signal-to-noise ratio (SNR) than the normal liver, whereas little difference in SNR was found between the normal and injured liver on heavily T₂-(SE 2000/80) and T₁-weighted (SE 300/11 and GRE 130/2.0/90°) MR images.
Conclusion: Pulse sequences with a moderately long echo time (TE) may be more appropriate than heavily T₁- or T₂-weighted images for distinguishing normal and injured liver in SPIO-enhanced MR imaging because of the maintained T₁ and T₂ relaxation effect but decreased T₂* relaxation effect of SPIO in injured liver.