Respiratory motion-corrected proton magnetic resonance spectroscopy of the liver

Abstract

Purpose: To develop a post-processing, respiratory-motion correction algorithm for magnetic resonance spectroscopy (MRS) of the liver and to determine the incidence and impact of respiratory motion in liver MRS.

Materials and methods: One hundred thirty-two subjects (27 healthy, 31 with nonalcoholic fatty liver disease and 74 HIV-infected with or without hepatitis C) were scanned with free breathing MRS at 1.5 T. Two spectral time series were acquired on an 8-ml single voxel using TR/TE=2500 ms/30 ms and (1) water suppression, 128 acquisitions, and (2) no water suppression, 8 acquisitions. Individual spectra were phased and frequency aligned to correct for intrahepatic motion. Next, water peaks more than 50% different from the median water peak area were identified and removed, and remaining spectra averaged to correct for presumed extrahepatic motion. Total CH(2)+CH(3) lipids to unsuppressed water ratios were compared before and after corrections.

Results: Intrahepatic-motion correction increased the signal to noise ratio (S/N) in all cases (median=11-fold). Presumed extrahepatic motion was present in 41% (54/132) of the subjects. Its correction altered the lipids/water magnitude (magnitude change: median=2.6%, maximum=290%, and was >5% in 25% of these subjects). The incidence and effect of respiratory motion on lipids/water magnitude were similar among the three groups.

Conclusion: Respiratory-motion correction of free breathing liver MRS greatly increased the S/N and, in a significant number of subjects, changed the lipids/water ratios, relevant for monitoring subjects.

Fig. 1

Example MR images and MR spectra. The top row example (A and B) is from a 22-year-old healthy female. The bottom row example (C and D) is from a 51-year-old female with NAFLD and Grade 2 steatosis. (A and C) T₂-weighted axial images showing the location of the MRS single voxel (white box). (B and D) Averaged, motion-corrected spectra showing water and lipid peaks.

Fig. 2

Respiration-induced phase differences reduce the S/N of the total lipids in the averaged spectrum from 238:1 to 17:1, a 14-fold difference. (A) Sample of spectra as acquired. (B) Sample of spectra from (A), frequency- and phase corrected. (C) Full set of 128 acquired spectra averaged without respiratory corrections performed and (D) corrected spectra averaged, shown on the same arbitrary scale as (C).

Fig. 3

Three consecutive spectra from a subject showing large respiration or other artifact, presumed to be due to motion causing spectral acquisition from extrahepatic tissue.

Fig. 4

Box plot of magnitude % difference in lipids/water due to presumed extrahepatic respiratory motion ( $\%{\text{Diff}}_{\text{L}/\text{W}}=\mid \frac{\text{L}/{\text{W}}_{\text{ind},\text{art}-\text{corrected}}-\text{L}/{\text{W}}_{\text{ind}-\text{corrected}}}{\text{L}/{\text{W}}_{\text{ind},\text{art}-\text{corrected}}}\mid$, see the text for more details) for the three populations. The box indicates the first to third quartile values. The horizontal line within the box indicates the median and the whiskers (vertical lines) indicate the outermost data point that falls within the outer quartile+1.5×interquartile range. Individual points are indicated as dots. Note the upper portion has different scaling to plot the very large values.