Multi-parametric MR in Becker muscular dystrophy patients.

Quantitative MRI and MRS of muscle are increasingly being used to measure individual pathophysiological processes in Becker muscular dystrophy (BMD). In particular, muscle fat fraction was shown to be highly associated with functional tests in BMD. However, the muscle strength per unit of contractile cross-sectional area is lower in patients with BMD compared with healthy controls. This suggests that the quality of the non-fat-replaced (NFR) muscle tissue is lower than in healthy controls. Consequently, a measure that reflects changes in muscle tissue itself is needed. Here, we explore the potential of water T2 relaxation times, diffusion parameters and phosphorus metabolic indices as early disease markers in patients with BMD. For this purpose, we examined these measures in fat-replaced (FR) and NFR lower leg muscles in patients with BMD and compared these values with those in healthy controls. Quantitative proton MRI (three-point Dixon, multi-spin-echo and diffusion-weighted spin-echo echo planar imaging) and 2D chemical shift imaging 31 P MRS data were acquired in 24 patients with BMD (age 18.8-66.2 years) and 13 healthy controls (age 21.3-63.6 years). Muscle fat fractions, phosphorus metabolic indices, and averages and standard deviations (SDs) of the water T2 relaxation times and diffusion tensor imaging (DTI) parameters were assessed in six individual leg muscles. Phosphodiester levels were increased in the NFR and FR tibialis anterior, FR peroneus and FR gastrocnemius lateralis muscles. No clear pattern was visible for the other metabolic indices. Increased T2 SD was found in the majority of FR muscles compared with NFR and healthy control muscles. No differences in average water T2 relaxation times or DTI indices were found between groups. Overall, our results indicate that primarily muscles that are further along in the disease process showed increases in T2 heterogeneity and changes in some metabolic indices. No clear differences were found for the DTI indices between groups.

Proton Magnetic Resonance Spectroscopy Indicates Preserved Cerebral Biochemical Composition in Duchenne Muscular Dystrophy Patients.

BACKGROUND: Duchenne muscular dystrophy (DMD) is caused by the absence of dystrophin. DMD is associated with specific learning and behavioural disabilities. In the brain, dystrophin is associated with GABAA receptors and aquaporin-4 in neurons and astrocytes, respectively, but little is known about its function. OBJECTIVE AND METHODS: In this study we aimed to compare the biochemical composition between patients and healthy controls in brain regions that are naturally rich in dystrophin using magnetic resonance spectroscopy. Given previous conflicting results obtained at clinical field strengths, we obtained data using a 7 Tesla system with associated higher signal-to-noise ratio and spectral resolution. RESULTS: Results indicated unchanged biochemical composition in all regions investigated, and increased variance in glutamate in the frontal cortex.

T2 relaxation times are increased in Skeletal muscle of DMD but not BMD patients.

INTRODUCTION: Exon-skipping drugs in Duchenne muscular dystrophy (DMD) aim to restore truncated dystrophin expression, which is present in the milder Becker muscular dystrophy (BMD). MRI skeletal muscle T2 relaxation times as a representation of edema/inflammation could be quantitative outcome parameters for such trials. METHODS: We studied T2 relaxation times, adjusted for muscle fat fraction using Dixon MRI, in lower leg muscles of DMD and BMD patients and healthy controls. RESULTS: T2 relaxation times correlated significantly with fat fractions in patients only (P < 0.001). After adjusting for muscle fat, T2 relaxation times were significantly increased in 6 muscles of DMD patients (P < 0.01), except for the extensor digitorum longus. In BMD, T2 relaxation times were unchanged. CONCLUSIONS: T2 relaxation times could be a useful outcome parameter in exon-skipping trials in DMD but are influenced by fat despite fat suppression. This should be accounted for when using quantitative T2 mapping to investigate edema/inflammation.

Phosphorylated guanidinoacetate partly compensates for the lack of phosphocreatine in skeletal muscle of mice lacking guanidinoacetate methyltransferase.

The effects of creatine (Cr) absence in skeletal muscle caused by a deletion of guanidinoacetate methyltransferase (GAMT) were studied in a knockout mouse model by in vivo (31)P magnetic resonance (MR) spectroscopy. (31)P MR spectra of hindleg muscle of GAMT-deficient (GAMT-/-) mice showed no phosphocreatine (PCr) signal and instead showed the signal for phosphorylated guanidinoacetate (PGua), the immediate precursor of Cr, which is not normally present. Tissue pH did not differ between wild-type (WT) and GAMT-/- mice, while relative inorganic phosphate (P(i)) levels were increased in the latter. During ischaemia, PGua was metabolically active in GAMT-/- mice and decreased at a rate comparable to the decrease of PCr in WT mice. However, the recovery rate of PGua in GAMT-/- mice after ischaemia was reduced compared to PCr in WT mice. Saturation transfer measurements revealed no detectable flux from PGua to gamma-ATP, indicating severely reduced enzyme kinetics. Supplementation of Cr resulted in a rapid increase in PCr signal intensity until only this resonance was visible, along with a reduction in relative P(i) values. However, the PGua recovery rate after ischaemia did not change. Our results show that despite the absence of Cr, GAMT-/- mice can cope with mild ischaemic stress by using PGua for high energy phosphoryl transfer. The reduced affinity of creatine kinase (CK) for (P)Gua only becomes apparent during recovery from ischaemia. It is argued that absence of Cr causes the higher relative P(i) concentration also observed in animals lacking muscle CK, indicating an important role of the CK system in P(i) homeostasis.