MR spectroscopy of the liver: principles and clinical applications

Abstract

Magnetic resonance (MR) spectroscopy allows the demonstration of relative tissue metabolite concentrations along a two- or three-dimensional spectrum based on the chemical shift phenomenon. An MR spectrum is a plot of the signal intensity and frequency of a chemical or metabolite within a given voxel. At proton MR spectroscopy, the frequency at which a chemical or compound occurs depends on the configuration of the protons within the structure of that chemical. At in vivo proton MR spectroscopy, the frequency location of water is used as the standard of reference to identify a chemical. The frequency shift or location of chemicals relative to that of water allows generation of qualitative and quantitative information about the chemicals that occur within tissues, forming the basis of tissue characterization by MR spectroscopy. MR spectroscopy also may be used to quantify liver fat by measuring lipid peaks and to diagnose malignancy, usually by measuring the choline peak. Interpretation of MR spectroscopic data requires specialized postprocessing software and is subject to technical limitations including low signal-to-noise ratio, masking of metabolite peaks by dominant water and lipid peaks, partial-volume averaging from other tissue within the voxel, and phase and frequency shifts from motion. MR spectroscopy of the liver is an evolving technology with potential for improving the diagnostic accuracy of tissue characterization when spectra are interpreted in conjunction with MR images.

Figure 1

Diagram shows metabolite frequency relative to water frequency. The peaks in ¹H MR spectra correspond to different metabolites and are identified primarily by their frequencies. Metabolite frequency is expressed as a frequency shift relative to a reference, usually the water frequency, in parts per million. The dominant peaks in liver are those of water and lipids.

Figure 2

MR spectrum obtained in healthy liver in a 36-year-old woman shows the frequency locations of water and lipid peaks. By convention, the x-axis (frequency scale in parts per million) is plotted as a downward shift relative to water frequency.

Figure 3

Diagram shows the T1 recovery curves of fat and water. Fat has shorter T1 relaxation time and recovers longitudinal magnetization faster than water. Recovery initially is rapid, then slows toward equilibrium, when protons are aligned along the static magnetic field (M₀ ). The effects attributed to T1 properties of tissue components are reduced by using a long TR when the different tissue components, such as fat and water, have reached equilibrium. M_Z = longitudinal magnetization.

Figure 4a

Uncorrected MR spectra obtained in liver. (a) Multiple spectra acquired with free-breathing technique in a patient with grade 3 steatosis, without phase and frequency correction. The lipid and water peaks appear below as well as above the x-axis and are shifted toward the left because of respiratory motion. (b) Averaged spectrum obtained from uncorrected spectra in a shows a marked reduction in SNR, mischaracterization of the actual relative peak size, and inaccurate quantification of compounds. The larger peak on the right is lipid, and the water peak is on the left. Both peaks are barely detectable because of respiration-induced phase differences.

Figure 4b

Uncorrected MR spectra obtained in liver. (a) Multiple spectra acquired with free-breathing technique in a patient with grade 3 steatosis, without phase and frequency correction. The lipid and water peaks appear below as well as above the x-axis and are shifted toward the left because of respiratory motion. (b) Averaged spectrum obtained from uncorrected spectra in a shows a marked reduction in SNR, mischaracterization of the actual relative peak size, and inaccurate quantification of compounds. The larger peak on the right is lipid, and the water peak is on the left. Both peaks are barely detectable because of respiration-induced phase differences.

Figure 5a

Frequency- and phase-corrected MR spectra obtained in liver (same patient as in Fig 4). Averaged spectrum (b) derived from multiple spectral acquisitions corrected for phase and frequency (a) shows improved SNR. The large peak on the right is lipid, and the larger of the small peaks on the left is water.

Figure 5b

Frequency- and phase-corrected MR spectra obtained in liver (same patient as in Fig 4). Averaged spectrum (b) derived from multiple spectral acquisitions corrected for phase and frequency (a) shows improved SNR. The large peak on the right is lipid, and the larger of the small peaks on the left is water.

Figure 6

Phase-corrected spectra from a patient with grade 3 steatosis. Even after correction of the 180° phase shifts attributed to respiratory motion, a large fluctuation is seen in the central spectrum, which shows a very different lipid-to-water ratio than the spectra acquired immediately before and after, a find-ing thought to result from displacement of the region of interest into adjacent tissues.

Figure 7a

(a) Axial T2-weighted MR image shows the correct placement of the MR spectroscopy voxel (□) at the periphery of the liver, at least 1 cm from the edge, to avoid major vessels. (b) Liver spectrum from a healthy volunteer shows a smaller lipid peak relative to that of water. (c) Liver spectrum from a 25-year-old woman with nonalcoholic fatty liver disease shows a greater lipid peak relative to that of water.

Figure 7b

(a) Axial T2-weighted MR image shows the correct placement of the MR spectroscopy voxel (□) at the periphery of the liver, at least 1 cm from the edge, to avoid major vessels. (b) Liver spectrum from a healthy volunteer shows a smaller lipid peak relative to that of water. (c) Liver spectrum from a 25-year-old woman with nonalcoholic fatty liver disease shows a greater lipid peak relative to that of water.

Figure 7c

(a) Axial T2-weighted MR image shows the correct placement of the MR spectroscopy voxel (□) at the periphery of the liver, at least 1 cm from the edge, to avoid major vessels. (b) Liver spectrum from a healthy volunteer shows a smaller lipid peak relative to that of water. (c) Liver spectrum from a 25-year-old woman with nonalcoholic fatty liver disease shows a greater lipid peak relative to that of water.

Figure 8

Diagram shows the dominant lipid peaks in liver MR spectra, which are produced by -CH₃ and -CH₂ resonances at 0.9–1.1 ppm and 1.3–1.6 ppm, respectively. Other liver metabolites generally are difficult to identify because of the small size of their peaks relative to those of water and lipids.

Figure 9a

MR spectra show increasing size of lipid peaks relative to the water peak with increasing steatosis grade, from grade 0 (a) to grade 3 (d). Cho = choline, PME = phosphomonoesters.

Figure 9b

MR spectra show increasing size of lipid peaks relative to the water peak with increasing steatosis grade, from grade 0 (a) to grade 3 (d). Cho = choline, PME = phosphomonoesters.

Figure 9c

MR spectra show increasing size of lipid peaks relative to the water peak with increasing steatosis grade, from grade 0 (a) to grade 3 (d). Cho = choline, PME = phosphomonoesters.

Figure 9d

MR spectra show increasing size of lipid peaks relative to the water peak with increasing steatosis grade, from grade 0 (a) to grade 3 (d). Cho = choline, PME = phosphomonoesters.