Diffusion tensor imaging lines up unique view of muscles

Diffusion tensor imaging at 1.5T MRI just was not practical. While MRI at 1.5T could generate diffusion tensor maps, it required lengthy scan times to get resolution decent enough to eliminate noise and reveal fiber bundle tracking. The increased signal-to-noise ratio at 3T makes it possible to produce fiber tracking maps in two to three minutes. As a result, DTI at 3T is becoming a standard part of imaging patients with neurological disease.

The technique capitalizes on the anisotropic movement of water in brain white matter. While water diffuses randomly in gray matter, it moves in only one direction in white matter: parallel to the orientation of the fibers. By determining the degree of anisotropy and by tracking the path of white matter fibers, DTI and tractography help radiologists examine the overall architecture of white matter and assess the integrity of the fibers in a variety of brain pathologies, including multiple sclerosis, AIDS, and Alzheimer's disease, as well as in psychiatric disorders and trauma.

DTI can evaluate any structure whose internal fibers are anisotropic, including muscle. DTI assesses neurological conditions in several ways. The diffusion tensor, which mathematically models diffusion in 3D, elicits measurements of relative and fractional anisotropy (FA) in MS and AIDS, and anisotropy imaging looks at alterations in the aging brain.

In recent clinical investigations, reductions in anisotropy have been seen in areas of brain and spinal cord that have been subjected to traumatic injury and in psychiatric conditions such as schizophrenia and depression. A pair of studies reported at the 2007 annual meeting of the RSNA relate findings on DTI with the site and degree of impairment following traumatic neurological injury.

In one study, from the University of Maryland Medical Center, DTI parameters were significantly decreased at all sites of spinal cord injury, and the most pronounced reductions occurred in patients who had hemorrhagic cord contusion. DTI differed even in patients who exhibited no signs of contusion on conventional MR scans. The research compared imaging results from 50 patients who had neck trauma and 11 normal volunteers.

In the other study, from the Walter Reed Army Institute of Research, in Bethesda, MD, DTI detected cerebral compromise following mild traumatic brain injury that remained hidden on standard anatomical MRI. The study evaluated imaging examinations performed on 11 soldiers who had sustained mild traumatic brain injury while serving in Iraq. At three-month follow-up, FA was lower than baseline values in four soldiers who also had signs of deteriorating attention span, executive function, and memory on standardized neuropsychological tests. However, FA was higher in seven soldiers whose neuropsychological function improved.

A preliminary study from the People's Republic of China suggests that DTI may identify an abnormality in the microstructure of white matter in patients who fail to respond to drug treatment for depression. FA was significantly lower in frontal white matter in nine patients with major depression who did not respond following eight weeks of therapy with a selective serotonin-reuptake inhibitor.

DTI indices corresponded with MR spectroscopy assessment of N-acetylaspartate, a neuronal marker of white matter lesions and areas of axonal loss, in a study of patients with schizophrenia. DTI anisotropy and MRS measurements of NAA were lower in the same anatomic areas, suggesting that patients with schizophrenia have a white matter deficit residing in the medial temporal regions of the brain (BMC Psychiatry 2007;7:25).

The addition of DTI to cortical mapping and blood oxygen level-dependent MR imaging gives neurosurgeons a clearer picture of the extent to which they may resect lesions without compromising cerebral function. In some centers, it is part of the routine workup of any patient slated for neurosurgery.

According to results of an early small study from the University of Tokyo Hospital, tractography may offer the first method for preventing visual field deficits after stereotactic radiosurgery involving lesions near the focus of optic radiations for treatment of arteriovenous malformations (J Neurosurg 2007;107[4]:721-762).

DTI and functional MRI may be the only way to track hand and foot fibers of the corticospinal tract and their relationship to malignancies, according to investigators from Erasmus Medical Center in Rotterdam, the Netherlands, who reached their conclusion following an initial evaluation of the techniques in nine patients and one healthy volunteer (AJNR 2007;28[7]:1354-1356).

Diffusion tensor imaging is well suited to the examination of muscle. Muscle fibers are about 50 or 60 µm in diameter and more than 10 cm long, and their protein structure tends to be aligned in a longitudinal fashion. Water, therefore, does not diffuse to the same degree in all directions; it diffuses more readily along the long axis of the muscle fiber, said Dr. Bruce Damon, a research scientist with the Vanderbilt University Institute of Imaging Science in Nashville.

DTI provides a unique view of muscle tissue. Until the 1990s, the only way to obtain information about muscle architecture was by dissecting cadavers.

"We wind up with the fiber track reconstruction of the entire muscle," Damon said. Researchers believe DTI may be able to relate the underlying architectural scaffolding of muscle to the actual movement of muscle. Damon explained that there are two principal types of muscles. The biceps, hamstrings, and heart have fibers that run in the same direction as the muscle and are built to contract quickly. Calf muscles have fibers that run at an oblique angle to the overall direction of the muscle and are designed for high force production. "What we want to be able to do is relate the underlying architectural scaffolding of muscle to muscle behavior and use that to form biomechanical models," Damon said.

INJURED MUSCLE
Damon and his colleagues have developed a protocol for acquiring the diffusion tensor of muscles such as the hamstring and quadriceps and for ascertaining how the differences in their structure support their functions. They and other researchers are now using diffusion MRI to evaluate injured or diseased muscle. Canadian investigators detected clear differences in FA and the apparent diffusion coefficient as well as the organization of muscle fibers in injured and healthy gastrocnemius and soleus muscles.

The Vanderbilt investigators used DTI and T1-weighted data to construct a biomechanical model of the quadriceps mechanism in healthy volunteers and patients with chronic patellar dislocation, which is caused to some degree by muscular atrophy in the medial portions of the quadriceps.

"The long-term objective is to apply the technique on an individual basis," Damon said.

The next step, at least for the Vanderbilt researchers, is to apply diffusion MRI to the assessment and management of inflammatory myopathies. A study published in January found that MRI provided quantitative data on the movement of molecular fluid in patients with polymyositis or dermatomyositis and the apparent changes in diffusion tendencies after anti-inflammatory therapy (J Magn Reson Imaging 2008;27[11]: 212-217).

Ms. Sandrick is a contributing editor to Diagnostic Imaging.