Problems associated with using in vivo proton (1H) magnetic resonance spectroscopy to quantify liver fat

In-vivo 1H magnetic resonance (MR) spectra of the liver were obtained in 8 patients admitted for liver biopsy. These patients had abnormal liver function and the presumptive diagnosis of fatty liver prior to biopsy. Two patients with NIDDM were also studied but liver biopsies were not performed as liver function was normal. The MR spectra, obtained on a 60 cm clear-bore 1.9 tesla superconducting magnet showed two 1H resonances, one from water and the other from repeating methylene protons - (CH2)n - in triglyceride. The lipid: water signal ratio was used to characterize tissues as subcutaneous fat (high lipid:water ratio), normal liver (low lipid: water ratio) and fatty liver (intermediate lipid: water ratio). The spectra obtained at the greatest depth from the probe surface ~4.5 cm) was used as it was most likely to represent liver tissue. Although all 8 patients were expected to have fatty liver only 2 had evidence of significant fatty changes on microscopy. This was assessed by counting the vacuoles of fat over the area of the biopsy specimen and quantitated as 'fat vacuoles per high power field' (f/hpf). In the 2 patients with NIDDM, unusual stack plots suggested technical difficulties with 1H MR spectroscopy for in-vivo assessment of fatty liver. The first patient, PT had a significant increase in lipid:water ratio on the spectra thought to represent liver (lipid:water ~ 65% cf levels <3% in norma liver and 12.6% + 26.5% in those patients subsequently found to have fat on biopsy). This was later found on MR imaging to represent omental fat lying between the liver and muscle layer. The second patient, OM had a large amount of subcutaneous fat overlying the area assessed. As seen on the stack plot, the probe depth was not great enough to pass through the subcutaneous fat and muscle layer to penetrate liver tissue. There was a significant correlation between the lipid:water signal ratio and visible fat on biopsy in those patients who underwent liver biopsy. Difficulties experienced with probe depth suggests imaging would be necessary prior to spectroscopy to ensure liver tissue is actually assessed.