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.

Transcript

The overall goal of this procedure is to characterize the spatially heterogeneous pathology, that is characteristic of many neuromuscular diseases, by using noninvasive magnetic resonance imaging methods. This is accomplished by first defining an MRI protocol that is composed of one or more quantitative methods. The second step is to position the patient in the MRI scanner such that she or he feels comfortable and secure, but is not able to perform extraneous movements.

Next, quantitative MRI data is acquired according to an established protocol. The final step is to analyze the data offline using a scientific computing platform. Ultimately, quantitative MRI is used to characterize muscle health.

The implications of this technique extended to therapy for muscle diseases, because it could be is to establish a patient's prognosis or to track his or her responsiveness to therapy. The protocol has been tested specifically for patients with inflammatory myopathies, but it could be used in other patients as well, such as people with muscular dystrophies, or with a muscle strain injury. Prepare for this experiment by first setting up an MRI protocol.

Suggested parameters can be found in the tables accompanying this video. Then obtain written and informed consent from the participant. Also confirm that the subject has abstained from moderate or heavy exercise 48 hours prior to testing and from any substances that may interfere with measurements.

Before entering the MRI room, perform safety screening and ensure that all personnel have removed any magnetic and magnetically sensitive objects. Always be sure to rescreen prior to the entering the room. Prepare the system by placing the receive coil, as well as a mattress with a sheet and pillow on the bed.

Have straps available to place around the thighs and support to place under the knees. Once the room is set up, position the participant on the bed in a supine, feet first position. Place the body part to be imaged as close to the midline of the table as possible.

Provide bolsters or pillows under the knees for back strain relief, and place a pillow under the participants head. To limit motion, gently but effectively, secure the thigh, leg and feet and ensure that the participant is comfortable. Then place the RF receiver coil around the participant's thighs, and connect it to the MRI system.

Be sure to provide the participant with hearing protection as well as a call button. Finally, advance the patient's bed into the MRI scanner, such that the body part to be imaged is in the center of the MRI scanner. Throughout the protocol, communicate regularly with the participant, to ensure comfort and compliance with instructions.

Also be sure to check the instrumental settings and calibrations automatically set by the scanner prior to each scan, such as center frequency and receiver gain calibration. Begin by acquiring a set of localizer images, using parameters described in the tables accompanying this video. Then determine the center slice position for qMRI data acquisitions by identifying areas of damage and or by referencing the slice position relative to reproducible anatomical landmarks.

For this, and all subsequent imaging steps define a region of anatomy in which to optimize the homogeneity of the static magnetic field. This process is known as shimming. Now acquire high resolution, multi slice T1 weighted images using a fast-spin echo sequence with the imaging parameters as seen on screen here.

Next, acquire three dimensional multiple gradient echo data for the calculation of B0 field maps. Examine these field maps to ensure that there are no deviations greater than 60 Hertz across the image. If there are, adopt an alternative approach to shimming such as alternate placement of the volume of interest.

Now acquire 3D data for the calculation of notation angle maps. Examine the field maps to ensure that there are no areas that deviate excessively from the nominal nutation angle. For the RF pulses used in this protocol deviations greater than 30%are considered excessive.

Next, acquire the qMRI data. First, use an inversion recovery sequence to acquire 3D images for calculation of the T1 value. Also acquire single slice images for calculation of the T2 value using a multiple spin echo Carr-Purcell-Meiboom-Gill sequence.

Next, acquire 3D images for calculation of QMT parameters using a pulse saturation sequence with FS as well as multi slice data for calculation of diffusion tensor parameters. Finally, acquire 3D data for calculation of fat and water images using a series of six gradient echo images. After the scan is complete, ensure that all images are of suitable quality by examining them for potentially correctable artifacts and by measuring the signal to noise ratio.

Then for each qMRI dataset define several regions of interest in the image series, and examine the signal as a function of the relevant parameter. For example, for the T1 dependent data, plot the signal as a function of TI, and ensure that the data follow the inversion recovery function. Once the images have been examined, slide the bed out of the scanner, remove all straps and padding, and assist the participant in exiting the MRI scanner.

Transfer the data using methods compliant with health privacy laws to a local workstation for processing. Select a program designed for quantitative image analysis and calculating parametric maps. Examine a histogram of the signal intensities in the image and form a signal treshold based image mask that delineates areas of signal from areas of noise.

Complete the following steps for every pixel in the signal portions of the images. First, define regions of interest on the anatomical images by defining the boundaries of each muscle of interest. Carefully examined each ROI.

If necessary ensure that no pixels are included that contain partial volume artifacts, flow artifacts or non-contractile tissue. Finally, calculate the mean and standard deviation of the qMRI values in all pixels within the selected ROIs. Here we see a delta B0 field map.

Note that the mass is most homogeneous in the shimmed region. In this B1 map, each pixel represents the actual flip angle expressed as a percentage of nominal flip angle. These images display the T1 values calculated for each pixel.

Both T1 and sample T1 FS data are shown. Here we see a masked parametric map of the T2 FS and T2 values. T2-dependent signal decay from a single pixel and best fit of the data to a mono exponential model shows a deviation from mono exponential relaxation.

Non-negative least squares analysis shows a single broad peak that likely includes both fat and water components. In this image, the color scale indicates the pool size ratio a dimensionless quantity reflecting the ratio of macromolecular to free water protons. A parametric map of mean diffusivity and a map of the fractional anisotropy values are seen here.

Note that means diffusivity values are elevated in blood vessels. Lastly, a fat fraction map calculated from the FW MRI data is shown here. After watching this video, you should have a good understanding of how to describe the specially heterogeneous pathology, that is characteristic of many neuromuscular diseases by using noninvasive MRI methods.

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Journal (Medicine)

Journal (Medicine)

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

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