Name: quantitative magnetic resonance imaging of skeletal muscle disease
Uploaded: 2016-12-18T13:00:00
Description: Watch this Scientific Journal Video about Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease at JoVE.com

We acknowledge grant support from the National Institutes of Health: NIH/NIAMS R01 AR050101 (BMD), NIH/NIAMS R01 AR057091 (BMD/JHP), NIH/NIBEB K25 EB013659 (RDD), and the Vanderbilt CTSA award RR024975. We also thank the reviewers for the comments and the subject for participating in these studies.

NOTE: The reader is reminded that all research involving human subjects must be approved by the local Institutional Review Board (IRB) for the Use of Human Subjects in Research. Research participants must be informed of the purpose, procedures, risks, and benefits of the proposed research; the availability of alternative treatments or procedures; the availability of remuneration; and of their rights to privacy and to withdraw their consent and discontinue their participation. Prior to the MRI testing session, an investigator must present a p...

Muscle diseases such as the muscular dystrophies and idiopathic inflammatory myopathies constitute of group of diseases that are heterogeneous in etiology and, as individual entities, rare in their incidence. For example, Duchenne muscular dystrophy &#8212; the most common form of muscular dystrophy &#8212; has an incidence of 1 in 3,500 live male births37,38; dermatomyositis, to which this protocol has been applied, has an incidence of 1 in 100,00039. The higher collective incidence of these diseas...

Quantitative magnetic resonance imaging (qMRI) describes the development and use of MRI to quantify physical, chemical, and/or biological properties of living systems. QMRI requires that one adopt a biophysical model for the system, composed of the tissue of interest and an MRI pulse sequence. The pulse sequence is designed to sensitize the images&#39; signal intensities to the parameter of interest in the model. MRI signal properties (signal magnitude, frequency, and/or phase) are measured and analyzed according to the model. The goal is to produce an unbiased, quantitative estimate of a physical or biological parameter having continuously distributed, physical units of measurement. Often the equations describing the system are analyzed and fitted on a pixel-by-pixel basis, producing an image whose pixel values directly reflect the values of the variable. Such an image is referred to as a parametric map.
A common use of qMRI is the development and application of biomarkers. Biomarkers can be used to investigate a disease mechanism, establish a diagnosis, determine a prognosis, and/or assess a therapeutic response. They may take the form of the concentrations or activities of endogenous or exogenous molecules, a histological specimen, a physical quantity, or an internal image. Some general requirements of biomarkers are that they objectively measure a continuously distributed variable using physical units of measurement; have a clear, well understood relationship with the pathology of interest; are sensitive to improvement to and worsening of clinical state; and can be measured with suitable accuracy and precision. Non-invasive or minimally invasive biomarkers are particularly desirable, as they promote patient comfort and minimally disturb the pathology of interest.
A goal for developing image-based biomarkers for muscle disease is to reflect muscle disease in ways that are complementary to, more specific than, more spatially selective than, and/or less invasive than existing approaches. One particular advantage of qMRI in this regard is that it has the potential to integrate multiple types of information and thus potentially characterize many aspects of the disease process. This ability is very important in muscle diseases, which frequently exhibit a spatially variable, complex pathology that includes inflammation, necrosis and/or atrophy with fat replacement, fibrosis, disruption of the myofilament lattice (&#34;Z-disk streaming&#34;), and membrane damage. Another advantage of qMRI methods is that qualitative or semi-quantitative descriptions of contrast-based MR images reflect not just pathology, but also differences in image acquisition parameters, hardware, and human perception. An example of this last issue was demonstrated by Wokke et al., who showed that semi-quantitative assessments of fat infiltration are highly variable and frequently incorrect, when compared with quantitative fat/water MRI (FWMRI)1.
The protocol described here includes pulse sequences for measuring the longitudinal (T1) and transverse (T2) relaxation time constants, quantitative magnetization transfer (qMT) parameters, water diffusion coefficients using diffusion tensor MRI (DT-MRI), and muscle structure using structural images and FWMRI. T1 is measured by using an inversion recovery sequence, in which the net magnetization vector is inverted and its magnitude is sampled as the system returns to equilibrium. T2 is measured by repeatedly refocusing transverse magnetization using a train of refocusing pulses, such as the Carr-Purcell Meiboom-Gill (CPMG) method, and sampling the resulting spin-echoes. T1 and T2 data can be analyzed using non-linear curve-fitting methods that either assume a number of exponential components a priori (typically between one and three) or by using a linear inverse approach which fits the observed data to the sum of a large number of decaying exponentials, resulting in a spectrum of signal amplitudes. This approach requires a non-negative least square (NNLS) solution3, and typically includes additional regularization to produce stable results. T1 and T2 measurements have been widely used to study muscle diseases and injury4-9. T1 values are typically decreased in fat-infiltrated regions of muscle and elevated in inflamed regions4-6; T2 values are elevated in both fat-infiltrated and inflamed regions10.
QMT-MRI characterizes the free water and solid-like macromolecular proton pools in tissues by estimating the ratio of macromolecular to free water protons (the pool size ratio, PSR); the intrinsic relaxation rates of these pools; and the rates of exchange between them. Common qMT approaches include pulsed saturation11 and selective inversion recovery12,13 methods. The protocol below describes use of the pulsed saturation approach, which exploits the broad linewidth of the macromolecular proton signal, relative to the narrow linewidth of the water proton signal. By saturating the macromolecular signal at resonance frequencies sufficiently different from the water signal, the water signal is reduced as a result of magnetization transfer between the solid and free water proton pools. The data are analyzed using a quantitative biophysical model. QMT has been developed and applied in healthy muscles14,15, and a recent abstract appeared describing its implementation in muscle disease16. QMT has been used to study small animal models of muscle inflammation, wherein it has been shown that inflammation decreases the PSR17. Inasmuch as MT reflects both macromolecular and water contents, MT data may also reflect fibrosis18,19.
DT-MRI is used to quantify the anisotropic diffusion behavior of water molecules in tissues with ordered, elongated cells. In DT-MRI, water diffusion is measured in six or more different directions; these signals are then fitted to a tensor model20. The diffusion tensor, D, is diagonalized to obtain three eigenvalues (which are the three principal diffusivities) and three eigenvectors (which indicate the directions corresponding to the three diffusion coefficients). These and other quantitative indices derived from D provide information about tissue structure and orientation at a microscopic level. The diffusion properties of muscle, especially the third eigenvalue of D and the degree of diffusion anisotropy, reflect muscle inflammation17 and muscle damage due to experimental injury21, strain injury22, and disease23,24. Other potential influences on the diffusion properties of muscle include changes in cell diameter25 and membrane permeability changes.
Lastly, muscle atrophy, without or without macroscopic fat infiltration, is a pathological component of many muscle diseases. Muscle atrophy can be evaluated by using structural images to measure muscle cross-sectional area or volume and FW-MRI to assess fatty infiltration. Fat infiltration can be qualitatively described in T1- and T2-weighted images26, but fat and water signals are best measured by forming images that exploit the different resonance frequencies of fat and water protons27-29. Quantitative fat/water imaging methods have been applied in muscle diseases such as muscular dystrophy1,30,31, and can predict the loss of ambulation in these patients31.
The qMRI protocol described here uses all of these measurements to characterize muscle condition in the autoimmune inflammatory myopathies dermatomyositis (DM) and polymyositis (PM). Further details of the protocol, including its reproducibility, have been published previously32. The protocol includes standard pulse sequences as well as radiofrequency (RF) and magnetic field gradient objects specifically programmed on our systems. The authors anticipate that the protocol is also applicable in other neuromuscular disorders characterized by muscle atrophy, inflammation, and fat infiltration (such as the muscular dystrophies).

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quantitative magnetic resonance imaging of skeletal muscle disease

Quantitative magnetic resonance imaging (qMRI) describes the development and use of MRI to quantify physical, chemical, and/or biological properties of living systems. Neuromuscular diseases often exhibit a temporally varying, spatially heterogeneous, and multi-faceted pathology. The goal of this protocol is to characterize this pathology using qMRI methods. The MRI acquisition protocol begins with localizer images (used to locate the position of the body and tissue of interest within the MRI system), quality control measurements of relevant magnetic field distributions, and structural imaging for general anatomical characterization. The qMRI portion of the protocol includes measurements of the longitudinal and transverse relaxation time constants (T1 and T2, respectively). Also acquired are diffusion-tensor MRI data, in which water diffusivity is measured and used to infer pathological processes such as edema. Quantitative magnetization transfer imaging is used to characterize the relative tissue content of macromolecular and free water protons. Lastly, fat-water MRI methods are used to characterize fibro-adipose tissue replacement of muscle. In addition to describing the data acquisition and analysis procedures, this paper also discusses the potential problems associated with these methods, the analysis and interpretation of the data, MRI safety, and strategies for artifact reduction and protocol optimization.

Figure 1 shows representative axial anatomical images acquired at the mid-thigh of a patient with polymyositis. Also shown is the location of the in-plane projection of the shim volume. Representative parameter maps for each qMRI method, all obtained from this same patient, are provided from Figures 2 - 7.
Figures 2A and 2B show the &#916;B0 ...

Watch this Scientific Journal Video about Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease at JoVE.com

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

Neuromuscular diseases often exhibit a temporally varying, spatially heterogeneous, and multi-faceted pathology. The goal of this protocol is to characterize this pathology using non-invasive magnetic resonance imaging methods.

Quantitative magnetic resonance imaging (qMRI) describes the development and use of MRI to quantify physical, chemical, and/or biological properties of ...

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