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Diffusion tensor imaging of cerebral white matter: a pictorial review of physics, fiber tract anatomy, and tumor imaging patterns. Pubmed citation 2. Assaf Y, Pasternak O. Diffusion tensor imaging DTI -based white matter mapping in brain research: a review. Kim et al. Since axonal structure and integrity have been closely linked to MR diffusion measurements 34,37 the above correlations emphasize the utility of DTI to mirror both the structural and functional properties of axons.
While DTI can accurately identify the level of white matter disruption, it can also characterize the orientation of the glial scar as well as the degree of axonal dieback and preservation, thereby providing valuable data on regenerative potential following SCI 19, Additionally, the tADC and AI around the injured site have been shown to correlate with behavioral recovery in rats that were transplanted with fibroblasts following SCI In the future, it is expected that spinal cord DTI will be used to monitor transplants and other therapeutic interventions for SCI.
DTI studies of the spinal cord in healthy subjects have established baseline values, thereby allowing us to study diseased states While the magnitude of FA of the whole cord decreases in the rostral-caudal direction, the MD is relatively constant throughout the cord. DTI measurements are age-dependent, and reflect microstructural changes in the spinal cord associated with ageing In 25 healthy subjects studied at our center, we found that the FA across the cervical spinal cord decreased significantly after 55 years of age accepted for publication Figure 3.
While these results further emphasize the need to compare DTI measurements in patients with age-matched controls, they also indicate that DTI values in the elderly need to be evaluated in the light of normal age-related variations.
Facon et al. The authors suggested that the use of ADC be restricted to chronic spinal cord compression. However, Shanmuganathan et al. Acute SCI is characterized by edema, hemorrhage and inflammation that usually subside in 72 hours Neural injury is characterized by axonal injury, demyelination and the disruption of cellular membranes.
The FA and lADC are decreased by the interruption of longitudinal white fibers, while intracellular and intercellular edema contribute to increased tADC. Choosing a DTI parameter that best characterizes SCI remains a challenge and authors have suggested that the individual eigenvalues are more useful than anisotropy measures in representing microstructural changes Additionally, we have shown that axial FA maps and tractography are sensitive to asymmetric cord damage in acute SCI and can supplement conventional MR imaging in this setting In one study, higher ADC values at the injured site were associated with better postoperative neurosurgical cervical spine scale NCSS scores but not Frankel scale measures The ASIA score is a reliable measure of spinal cord injury and is useful in tracking neurological recovery There is a need to use a standardized functional outcome score in order to define the prognostic value of DTI indices.
Moreover, if diffusion metrics of individual white matter tracts or funiculi within the spinal cord are measured, it becomes essential to use scales that measure both sensory and motor function. Chronic SCI is associated with a number of microstructural neural changes including demyelination 81,82 , remyelination 83,84 , axonal loss 83 and atrophy 85 that affect the diffusion of water molecules. MD, tADC, and lADC have been shown to be significantly greater in injured patients compared to corresponding levels in neurologically intact controls.
The FA value at the site of the lesion is greatly reduced and appears to depend on both the level of injury and the completeness of the injury Chang et al. In chronic SCI, markers of neuronal damage are important to rehabilitation and therapeutic interventions. Both cellular and electrophysiological approaches to stimulate neural regeneration in SCI patients rely on accurate delineation of the lesion. It is possible to use the FA values to locate the epicenter of the lesion, thus enabling the targeted transplantation of stem cells or drugs.
While the rostral extent of the lesion can be obtained readily by clinical examination 88 , the caudal boundary is more difficult to discern. DTI offers additional information on the viability of spinal cord tissue below the clinical lesion level and this is particularly important when considering interventions that target the spinal cord below the level of the lesion Additionally, mapping the lesion using DTI could be useful in newer therapeutic modalities that implant biopolymers as scaffolds for neural regeneration CSM is the most common spinal disorder in patients over the age of 55 years The complex pathophysiology of CSM includes mechanical spinal cord compression due to disc protrusion, osteophytes or ossified posterior longitudinal ligament as well as secondary cord ischemia 92, In an effort to study CSM in animal models, a variety of methods to induce chronic cord compression have been used However, chronic compression in rat models is often induced with a dorsal or dorso-lateral approach, which does not replicate the predominantly ventral compression seen in patients.
Also, CSM in humans is affected by multi-directional neck motion that cannot be adequately reproduced in animal models. Reis et al. Other authors have corroborated these findings suggesting that DTI is more sensitive to identify cord damage than regular T2W images of the cervical spine in patients with CSM 11, Across studies, FA has been shown to be lower at the affected level in patients compared to corresponding levels in controls.
MD values, however, are not uniformly sensitive to white matter changes due to chronic spinal cord compression. Mild neural damage in CSM is characterized by edema, demyelination, gliosis and nerve loss.
Subsequently, necrosis and myelomalacia occur signifying permanent cord damage Recently, authors have shown that symptomatic CSM patients have lower FA values and higher ADC measures at the compressed level, as compared to asymptomatic patients It therefore appears that DTI has a role to play in the preoperative planning of CSM patients, but the use of DTI to decide on surgical intervention or monitor recovery is yet to be investigated in detail.
DTT has been used to describe the orientation and location of white matter fibers around brain tumors Recent studies have employed tractography in spinal cord astrocytomas and ependymomas , The use of fiber tracking to delineate displaced white matter tracts seems to be particularly useful in solid tumors. In cystic tumors and tumors with considerable vasogenic edema, the increased diffusion of water molecular leads to erroneous fiber tracking.
As such, a recent study found DTT to have a sensitivity of The authors of this study also classified tumors into 3 types based on the number of fibers coursing through the tumor. Type 1 tumors, which had no fibers within the tumor, were deemed resectable while type 3 tumors that had fibers completely encased by the tumor were considered unresectable. Type 2 tumors had variable number of fibers within the tumor substance and resectability was based on the proportion of fibers within the tumor volume.
However, this study did not correlate functional outcomes with the type of tumor and this relationship needs to be explored. Overall, the use of tractography shows much promise in the surgical planning of spinal cord tumors, as it has in brain tumor resection. DTI has been used in a variety of other spinal cord disorders including multiple sclerosis , syringomyelia , , and myelitis Although, many of these studies are able to characterize DTI parameters in diseased states, the routine use of DTI in the clinical setting is yet to be realized.
Spinal cord DTI in humans still has a number of limitations. Adequate spatial resolution remains a problem and it is difficult to visualize the individual funiculi on diffusion-weighted images, particularly in the lower thoracic cord DTI of these segments is affected more by artifacts arising from cardiac and respiratory motion as well as CSF pulsation The use of faster imaging techniques such as parallel imaging, single shot echo-planar imaging as well as the use of cardiac pulse-gating have helped to reduce these artifacts.
However, scan acquisition time is still a limitation for patients with acute SCI since these patients cannot withstand even 30 minutes of additional scanning time in the MRI suite. The signal to noise ratio in human SCI is sub-optimal in most studies and can lead to overestimation of anisotropy measures, particularly in low-anisotropic tissues such as the central gray matter The use of DTI postoperatively is hampered significantly by the use of spinal instrumentation, which creates numerous artifacts and this issue is currently unresolved.
Additionally, standardized software to process tensor images is essential to make this a feasible option for routine clinical use. DTI has given us a unique insight into the pathophysiology and microstructural alterations associated with spinal cord disorders.
While initial studies in rat models have primed this modality for human research, more data is required on the accuracy and reliability of DTI indices in defining cord pathology. Once these challenges are overcome, we can expect the use of DTI in mainstream clinical practice, both to prognosticate as well as monitor patients with spinal cord disease. Stejskal E, Tanner J. Spin diffusion measurements: spin echoes in the presence of a time-dependent field gradient.
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Extent of microstructural white matter injury in postconcussive syndrome correlates with impaired cognitive reaction time: a 3T diffusion tensor imaging study of mild traumatic brain injury. Diffusion tensor imaging during recovery from severe traumatic brain injury and relation to clinical outcome: a longitudinal study.
Clinical utility of diffusion-weighted magnetic resonance imaging in the assessment of ischemic stroke. Ann Neurol. The role of diffusion tensor imaging in establishing the proximity of tumor borders to functional brain systems: implications for preoperative risk assessments and postoperative outcomes. Technol Cancer Res Treat.
Diffusion tensor imaging in spinal cord: methods and applications - a review. NMR in Biomedicine. Diffusion-weighted MR imaging with apparent diffusion coefficient and apparent diffusion tensor maps in cervical spondylotic myelopathy.
Applications of diffusion-weighted MRI in thoracic spinal cord injury without radiographic abnormality. Int Orthop. Basser PJ, Pierpaoli C. Microstructural and physiological features of tissues elucidated by quantitative-diffusion-tensor MRI.
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Beaulieu C. Diffusion-weighted MRI and the evaluation of spinal cord axonal integrity following injury and treatment. Exp Neurol. Ex vivo evaluation of ADC values within spinal cord white matter tracts. Spinal cord diffusion tensor imaging and fiber tracking can identify white matter tract disruption and glial scar orientation following lateral funiculotomy.
J Neurotrauma. Apparent diffusion coefficients in spinal cord transplants and surrounding white matter correlate with degree of axonal dieback after injury in rats. First, an anatomic scan is performed. You must lie still. This is the sound of the pictures being taken. Next, you will be asked to perform a number of tasks.
You will see a monitor through a system of mirrors while in the magnet. Instructions and tasks will be displayed on this screen. It is important that you devote all of your effort to the tasks in order for the images of your brain to be meaningful.
The most common tasks performed include:. Finger tapping: You will be instructed to tap your thumb and fingers in one or both hands.
Word generation: When you see a letter, think of a word that starts with that letter. Do not say the word out loud. Verb generation: When you see a noun, think of a verb that is associated with that noun. Sentence completion: You will see a sentence with a blank.
Think of the word that completes the sentence. The exam usually takes 1 to 2 hours. It is important that you relax and lie as still as possible. Any movement during this time will blur the picture. You may be given an injection of contrast dye into your arm or through an IV to enhance the images. After the test is complete, you are free to go.
You may be told to drink lots of fluids to help your kidneys remove the contrast dye from your body. There are no known health risks associated with the magnetic field or the radio waves used by the machine.
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