1. Magnetization transfer ratio

Non-conventional MR techniques such as magnetization transfer (MT) imaging are much more sensitive than conventional MR imaging to changes in brain tissue water content. Additionally, alterations can be quantified through calculation of MT ratios (MTR). Previous studies have shown MTR decreases in the basal ganglia and cerebral white matter of patients with chronic liver failure. Although it has been found that manganese chloride phantoms reduce MTR values, and that MTR of the pallidus decreases with the severity of liver disease, the presence of manganese deposits in the white matter as the origin of MTR reduction is improbable, since there is no concomitant T1 shortening of the white matter (Figure 8).

Axonal loss and demyelination are also unlikely explanations, since these pathological features have not been described in hepatic encephaloptahy, and moreover, the ¹H-MRS findings of normal NAA indexes and low concentrations of choline-containing compounds indicate preservation of axonal density and absence of demyelinating processes. The still unproven mechanism of MTR decrease reflects a diffuse tissue abnormality. Normalization of the MTR decrease after succesful liver transplantation support the hypothesis that it reflects low-grade brain edema produced by astrocyte swelling (Figure 9).

2. T2-weighted imaging (fast-FLAIR)

Signal abnormalities within the brain indicating cytotoxic edema have not been described in conventional MRI studies, obtained from patients with chronic liver failure, probably because of their poor sensitivity for detecting slight, diffuse increases in brain water content. However, our group has recently demonstrated the presence of asymptomatic symmetrical high signal intensity in the hemispheric corticospinal tract on T2W fast-FLAIR MR images in patients with chronic liver failure (Figure 10). This signal abnormality restores to normal after succesful liver transplantation (Figure 11). This time-course normalization and its serial correlation with normalization of MTR (Figure 12) and ¹H-MRS abnormalities suppports the hypothesis that the T2 signal abnormality reflects the presence of low-grade edema. Thus chronic liver failure should be included in the differential diagnosis of diseases producing bilateral T2 increased signal intensity along the corticospinal tract, such as ALS, MS, Leigh disease, and adult Krabbe disease. The pathologic substrate in these diseases is, however, axonal loss, demyelination or Wallerian degeneration, while in chronic liver failure the reversibility after normalization of liver function supports the hypothesis that the MR abnormality may correspond to astrocytic swelling.

3. Ventricular volume

Following liver transplantation we found a significant decrease in ventricular volume that appeared within the first month (Figure 13 and 14). This observation probably reflects a decrease in brain tissue volume that could be explained by resolution of the low-grade brain edema produce by astrocytic swelling.

4. Diffusion-weighted imaging

Preliminary experience with DW MR images in patients with chronic hepatic failure has shown restricted diffusion (low ADC) within the corticospinal tract. After liver transplantation the ADC increased slightly (Figure 15). However a similar increase in ADC values was observed diffusely in the normal appearing white matter. This observation indicates a reversible increase in intracellular water, not exclusively located along the corticospinal tract, and further supports the hypothesis of astrocyte swelling in chronic hepatic failure.