Parametric exploration of the liver by magnetic resonance methods

Abstract

MRI, as a completely noninvasive technique, can provide quantitative assessment of perfusion, diffusion, viscoelasticity and metabolism, yielding diverse information about liver function. Furthermore, pathological accumulations of iron and lipids can be quantified. Perfusion MRI with various contrast agents is commonly used for the detection and characterization of focal liver disease and the quantification of blood flow parameters. An extended new application is the evaluation of the therapeutic effect of antiangiogenic drugs on liver tumours. Novel, but already widespread, is a histologically validated relaxometry method using five gradient echo sequences for quantifying liver iron content elevation, a measure of inflammation, liver disease and cancer. Because of the high perfusion fraction in the liver, the apparent diffusion coefficients strongly depend on the gradient factors used in diffusion-weighted MRI. While complicating analysis, this offers the opportunity to study perfusion without contrast injection. Another novel method, MR elastography, has already been established as the only technique able to stage fibrosis or diagnose mild disease. Liver fat content is accurately determined with multivoxel MR spectroscopy (MRS) or by faster MRI methods that are, despite their widespread use, prone to systematic error. Focal liver disease characterisation will be of great benefit once multivoxel methods with fat suppression are implemented in proton MRS, in particular on high-field MR systems providing gains in signal-to-noise ratio and spectral resolution.

Fig. 1

Signal intensity vs. time curves of the liver, aorta, portal vein and nodule a in a man with hepatocellular carcinoma (the total blood flow is higher in the hypervascular tumour than in the liver parenchyma) and b in a woman with colorectal metastatic cancer (the total blood flow is lower in the hypovascular metastasis than in normal liver) (reproduced with permission from Abdullah et al. [4])

Fig. 2

Relationship between liver T2* values and serum ferritin concentrations in the entire study population. Dashed lines represent the reference range for liver T2* values and an arbitrary limit of 2,000 μg/L for serum ferritin (n = 64; r = 0.465; P < 0.011) (reproduced with permission from Perifanis et al. [19])

Fig. 3

DW-SS-EPI images for b = 50 s/mm² (a) and b = 600 s/mm² (b). c ADC map. The hepatocellular carcinoma in segment IV (arrowhead) shows a merely moderate signal loss from the b = 50 s/mm² to the b = 600 s/mm² DW-SS-EPI image. The cyst in segment II (arrow) displays markedly high signal intensity on the b = 50 s/mm²DW-SS-EPI image and becomes isointense on the b = 600 s/mm² DW-SS-EPI image. On the corresponding ADC map, the tumour shows a low ADC value (1.18 × 10⁻³ mm²/s), whereas the cyst has a high ADC value (3.09 × 10⁻³ mm²/s) (reproduced with permission from Bruegel et al. [33])

Fig. 4

Patient with hepatic adenoma. a T2-weighted MR image shows hyperintense 8-cm adenoma (arrow) in the right lobe of the liver. b Gadolinium-enhanced 3D spoiled gradient-recalled echo MR image shows intense arterial phase enhancement (arrow). Washout was evident in the portal venous phase (not shown). c Axial MR elastographic wave image shows good illumination of the tumour (circle). Waves in the tumour have a slightly longer length than those in the surrounding normal liver parenchyma. d Elastogram with a region of interest corresponding to the tumour shows a shear stiffness value of the tumour of 3.1 kPa and of the surrounding liver of 2.4 kPa (reproduced with permission from Venkatesh et al. [54])

Fig. 5

The volume of interest (a), spectral map (b) and a curve fitted spectrum (c) of a healthy (obese) volunteer. The water and fat peaks in the 32 quantified voxels yield a liver fat content, i.e. the ratio of fat/(water + fat), of 21.3 ± 5.5%. Unlike single-voxel MRS giving a single value, multivoxel MRS provides a mean with a standard deviation, the latter being a measure of tissue heterogeneity (unpublished material from a study by Irwan et al. [68])

Fig. 6

a {¹H}-³¹P MRSI spectrum and corresponding hepatic histology of a woman with alcoholic liver fibrosis. The biopsy shows extensive periportal and parasinusoidal fibrosis in addition to typical ballooning of hepatocytes. b {¹H}-³¹P MRSI spectrum and corresponding hepatic histology of a man with cirrhosis of the liver. Biopsy shows micronodular alcoholic cirrhosis with typical pseudolobules. Compared with spectra of healthy controls and the patient with noncirrhotic liver disease increased PE/PC and, compared with hepatocellular carcinoma, increased GPE/GPC ratios can be observed. The position of the voxel observed is indicated in orthogonal slices (reproduced with permission from Schlemmer et al. [88])