A study of the relationship between mechanical and ultrasonic properties of dystrophic and normal skeletal muscle

Myeloid cell-mediated targeting of LIF to dystrophic muscle causes transient increases in muscle fiber lesions by disrupting the recruitment and dispersion of macrophages in muscle

Leukemia inhibitory factor (LIF) can influence development by increasing cell proliferation and inhibiting differentiation. Because of its potency for expanding stem cell populations, delivery of exogenous LIF to diseased tissue could have therapeutic value. However, systemic elevations of LIF can have negative, off-target effects. We tested whether inflammatory cells expressing a LIF transgene under control of a leukocyte-specific, CD11b promoter provide a strategy to target LIF to sites of damage in the mdx mouse model of Duchenne muscular dystrophy, leading to increased numbers of muscle stem cells and improved muscle regeneration. However, transgene expression in inflammatory cells did not increase muscle growth or increase numbers of stem cells required for regeneration. Instead, transgene expression disrupted the normal dispersion of macrophages in dystrophic muscles, leading to transient increases in muscle damage in foci where macrophages were highly concentrated during early stages of pathology. The defect in inflammatory cell dispersion reflected impaired chemotaxis of macrophages to C-C motif chemokine ligand-2 and local increases of LIF production that produced large aggregations of cytolytic macrophages. Transgene expression also induced a shift in macrophage phenotype away from a CD206+, M2-biased phenotype that supports regeneration. However, at later stages of the disease when macrophage numbers declined, they dispersed in the muscle, leading to reductions in muscle fiber damage, compared to non-transgenic mdx mice. Together, the findings show that macrophage-mediated delivery of transgenic LIF exerts differential effects on macrophage dispersion and muscle damage depending on the stage of dystrophic pathology.

Magnetic resonance imaging, ultrasound and real-time ultrasound elastography of the thigh muscles in congenital muscle dystrophy

Congenital muscle dystrophy includes a range of genetic disorders characterized by muscle weakness and contractures. We report the magnetic resonance (MR), ultrasound (US) and real-time sonoelastography (RTE) imaging findings of the thigh muscles of a 15-year-old boy with Bethlem myopathy diagnosed with clinical, electromyographic and histopathological criteria. Ultrasound and MR showed hyperechoic appearance and high signal intensity on T1- and T2-weighted sequences respectively at the periphery of the vastus lateralis and the long head of the biceps femoris muscles, and at a central area within the rectus femoris muscles. RTE was employed to examine the elastic properties of the muscle. The elastograms were presented as colour-coded maps superimposed on the B-mode images and revealed that the elastographic pattern correlated with the MR and US pattern of involvement. The abnormal muscle areas were stiffer (blue) than the normal-appearing areas (green), a finding that probably correlates with the presence of dystrophic collagen at the affected areas. This report suggests that RTE could be used as an additional imaging tool to evaluate the pattern of muscle changes in congenital myopathy. Further studies are needed to investigate the specificity and clinical value of RTE in the diagnosis and monitoring of neuromuscular disease.

Transversely isotropic tensile material properties of skeletal muscle tissue

Of the plethora of work performed analyzing skeletal muscle tissue, relatively little has been done in the examination of its passive material properties. Previous studies of the passive properties of skeletal muscle have been primarily performed along the longitudinal material direction. In order to ensure the accuracy of the predictions of computational models of skeletal muscles, a better understanding of the tensile three-dimensional material properties of muscle tissue is necessary. To that end, the purpose of this study was to collect a comprehensive set of tensile stress-strain data from skeletal muscle tissue. Load-deformation data was collected from eighteen extensor digitorum longus muscles, dissected free of aponeuroses, from nine New Zealand White rabbits tested under longitudinal extension (LE), transverse extension (TE), or longitudinal shear (LS). The linear modulus, ultimate stress, and failure strain were calculated from stress-strain results. Results indicate that the linear modulus under LE is significantly higher than the modulus of either TE or LS. Additionally, the ultimate stress of muscle was seen to be significantly higher under LE than TE. Conversely, the failure strain was significantly higher under TE than under LE.

Sensitive ultrasonic delineation of steroid treatment in living dystrophic mice with energy-based and entropy-based radio frequency signal processing

Duchenne muscular dystrophy is a severe wasting disease, involving replacement of necrotic muscle tissue by fibrous material and fatty infiltrates. One primary animal model of this human disease is the X chromosome-linked mdx strain of mice. The goals of the present work were to validate and quantify the capability of both energy and entropy metrics of radio-frequency ultrasonic backscatter to differentiate among normal, dystrophic, and steroid-treated skeletal muscle in the mdx model. Thirteen 12-month-old mice were blocked into three groups: 4 treated mdx-dystrophic that received daily subcutaneous steroid (prednisolone) treatment for 14 days, 4 positive-control mdx-dystrophic that received saline injections for 14 days, and 5 negative-control animals. Biceps muscle of each animal was imaged in vivo using a 40-MHz center frequency transducer in conjunction with a Vevo-660 ultrasound system. Radio-frequency data were acquired (1 GHz, 8 bits) corresponding to a sequence of transverse images, advancing the transducer from "shoulder" to "elbow" in 100-micron steps. Data were processed to generate both "integrated backscatter" (log energy), and "entropy" (information theoretic receiver, H(f)) representations. Analyses of the integrated-backscatter values delineated both treated-and untreated-mdx biceps from normal controls (p < 0.01). Complementary analyses of the entropy images differentiated the steroid-treated and positive-control mdx groups (p < 0.01). To our knowledge, this study represents the first reported use of quantitative ultrasonic characterization of skeletal muscle in mdx mice. Successful differentiation among dystrophic, steroid-treated, and normal tissues suggests the potential for local noninvasive monitoring of disease severity and therapeutic effects.

Sensitive ultrasonic detection of dystrophic skeletal muscle in patients with duchenne muscular dystrophy using an entropy-based signal receiver

The dystrophinopathies comprise a group of X-linked genetic diseases that feature dystrophin deficiency. Duchenne and Becker muscular dystrophy are characterized by progressive weakness and wasting of skeletal, smooth, and/or cardiac muscle. Duchenne muscular dystrophy (DMD) is the most severe dystrophinopathy, with an incidence of 1:3500 male births. Despite understanding the structural and genetic basis for DMD, the pathogenesis and clinical basis for more severe involvement in specific skeletal muscle groups and the heart are poorly understood. Current techniques, such as strength testing for monitoring progress of disease and therapy in DMD patients, are imprecise and physically demanding for test subjects. Ultrasound is well-suited to detect changes in structure and organization in muscle tissue in a manner that makes low demands on the patient. Therefore, we investigated the use of ultrasound to quantitatively phenotype the remodeling process in patients with DMD. Beam-formed radio-frequency (RF) data were acquired from the skeletal muscles of nine DMD and five normal subjects imaged with a clinical imaging system (HDI5000 w/7 MHz probe applied above left biceps muscle). From these data, images were reconstructed using B-mode (log of analytic signal magnitude) and information-theoretic receivers (H(f)-receiver). H(f) images obtained from dystrophic muscle contained extensive "mottled" regions (i.e., areas with heterogeneous image contrast) that were not readily apparent from the B-Mode images. The 2-D autocorrelation of DMD H(f) images have broader peaks than those of normal subjects, which is indicative of larger scatterer sizes, consistent with pathologic changes of fibers, edema and fatty infiltration. Comparison of the relative peak widths (full width measured at 60% maximum) of the autocorrelation of the DMD and normal H(f) images shows a quantitative difference between the two groups (p < 0.005, student two-tailed paired t-test). Consequently, these imaging techniques may prove useful for longitudinal monitoring of disease progression and therapy.

Ultrasonic tissue characterization in patients with dilated cardiomyopathy: comparison with findings from right ventricular endomyocardial biopsy

AIM

The clinical usefulness of integrated backscatter (IB) imaging was compared with right ventricular endomyocardial biopsy for assessing myocardial damage in patients with dilated cardiomyopathy (DCM).

METHODS

We examined 15 patients with DCM and 20 healthy controls. In addition to the conventional M-mode echocardiographic parameters, we determined the cyclic variation in IB values (CV-IB) obtained from parasternal short axis views of the left ventricle just under the transducer for both the interventricular septum (IVS) and the left ventricular posterior wall (PW). The per cent fibrosis area (%) and the transverse diameter of myocytes (microm) were measured in right ventricular endomyocardial biopsy specimens by computer image analysis. To analyze the relationship between pathological findings and CV-IB, we divided patients into four subgroups on the basis of the pathological characteristics of endomyocardial biopsy specimens as follows: degeneration dominant group (n = 5), fibrosis dominant group (n = 5), dilated phase hypertrophic cardiomyopathy (n = 2), and mixed type (n = 3).

RESULTS

CV-IB in the IVS and the PW was lower in patients with DCM (8.8 +/- 2.9, 8.3 +/- 2.7 dB, respectively) than in normal subjects (14.4 +/- 2.9, 13.6 +/- 2.6 dB, respectively). Biopsy findings showed a mean per cent fibrosis area of 24.0 +/- 12.3%, and a mean myocyte diameter of 14.3 +/- 2.9 microm in patients with DCM. CV-IB was correlated with both of these findings: per cent fibrosis area (r = -0.56 in IVS, r = -0.56 in PW) and myocyte diameter (r = 0.67 in IVS, r = 0.71 in PW). CV-IB was decreased in all DCM subgroups compared with normal subjects, but there was no significant difference between subgroups.

CONCLUSIONS

CV-IB was correlated with both the extent of fibrosis in myocardial tissue and the myocyte diameter. These findings suggest that ultrasonic tissue characterization is a good indicator of the severity of fibrosis and myocyte atrophy in patients with DCM.

Variation in ultrasonic backscattering from skeletal muscle during passive stretching

The purpose of this study was to further validate the scattering mechanism of ultrasound in the skeletal muscle tissue. It was hypothesized that the endomysial collagen fibers are a major determinant of ultrasonic scattering in the skeletal muscle. Previous studies have found that the ultrasonic backscattering from avian skeletal muscle changed as it was passively stretched from 0 to 40%. In this study, ultrasonic backscattering was measured from chicken breast muscles as they were stretched passively in increments of 10% of original length up to 60%. The integrated backscatter was found to reach a peak as the specimen was stretched to 40% and leveled off as it was further stretched from 40 to 60%. This finding was supported by results from scanning electron microscopy (SEM) of the specimens. SEM images showed that the orientation of the endomysial collagen fibers surrounding muscle fibers became approximately parallel to the axis of the muscle fiber when the muscle was stretched up to 40% of its original length, yielding maximal ultrasonic backscatter and as the muscle was further stretched, no apparent alteration of fiber orientation could be observed.