T₂ mapping provides multiple approaches for the characterization of muscle involvement in neuromuscular diseases: a cross-sectional study of lower leg muscles in 5-15-year-old boys with Duchenne muscular dystrophy

Abstract

Skeletal muscles of children with Duchenne muscular dystrophy (DMD) show enhanced susceptibility to damage and progressive lipid infiltration, which contribute to an increase in the MR proton transverse relaxation time (T₂). Therefore, the examination of T₂ changes in individual muscles may be useful for the monitoring of disease progression in DMD. In this study, we used the mean T₂, percentage of elevated pixels and T₂ heterogeneity to assess changes in the composition of dystrophic muscles. In addition, we used fat saturation to distinguish T₂ changes caused by edema and inflammation from fat infiltration in muscles. Thirty subjects with DMD and 15 age-matched controls underwent T₂ -weighted imaging of their lower leg using a 3-T MR system. T₂ maps were developed and four lower leg muscles were manually traced (soleus, medial gastrocnemius, peroneal and tibialis anterior). The mean T₂ of the traced regions of interest, width of the T₂ histograms and percentage of elevated pixels were calculated. We found that, even in young children with DMD, lower leg muscles showed elevated mean T₂, were more heterogeneous and had a greater percentage of elevated pixels than in controls. T₂ measures decreased with fat saturation, but were still higher (P < 0.05) in dystrophic muscles than in controls. Further, T₂ measures showed positive correlations with timed functional tests (r = 0.23-0.79). The elevated T₂ measures with and without fat saturation at all ages of DMD examined (5-15 years) compared with unaffected controls indicate that the dystrophic muscles have increased regions of damage, edema and fat infiltration. This study shows that T₂ mapping provides multiple approaches that can be used effectively to characterize muscle tissue in children with DMD, even in the early stages of the disease. Therefore, T₂ mapping may prove to be clinically useful in the monitoring of muscle changes caused by the disease process or by therapeutic interventions in DMD.

Copyright © 2012 John Wiley & Sons, Ltd.

Figures

Figure 1

Top panel: a) Demonstration of manually traced muscles of lower leg on an MR image; b) T2 map of the lower leg of a 10-year old control boy and; c) 11 year old boy with DMD. Note the higher T2 values in the muscles of the boy with DMD in comparison to healthy control. Lower panel: Normalized T2 histogram of SOL muscle from a control subject (d, e, black) and from a DMD subject (e, red). Widths of T2 histogram are measured as the full width at half maximum (FWHM) and full width at quarter maximum (FWQM); a threshold T2 value (e) of 48 (ms) is represented by the green line. Note the rightward shift of the DMD histogram (red); and a “shoulder” on the histogram representing broader distribution at base. The number of pixels above the threshold value is used to determine the percent-elevated pixels in the DMD SOL muscle.

Figure 2

Comparison of percent elevated pixels between control and DMD subjects in non-fat sat data (a) and fatsat (b) data. Boxes represent 25 and 75 percentiles and arrow bars indicate 5 and 95 percentile with the median presented with a horizontal line within the DMD box plots. ** Significant difference between groups (p≤0.001).

Figure 3

Example plots showing correlation of percent elevated T2 pixels in the TA muscle with time to walk 30ft (a) and time to stand from supine (b). Subjects not able to complete the task are not included in the figure.

Figure 4

Top panel: Comparison of Mean T2 in PER (a) and TA (b) muscles between controls and children with DMD in different age groups. Lower panel: Changes in percent elevated pixels across different age groups in PER (c) and TA (d) in children with DMD in comparison to controls. Boxes represent 25 and 75 percentiles and arrow bars indicate 5 and 95 percentile with the median presented with a horizontal line within the DMD box plots. **Significantly different between groups (p≤0.001); *Significantly different between groups (p≤0.01); $Significantly different between groups (p≤0.05