Diffusion Weighted Imaging and Diffusion Tensor Imaging in Spine and Spinal Cord Diseases
Majda M. Thurnher
- • Diffusion weighted imaging (DWI) is the method of choice for the detection of spinal cord infarction.
- • Diffusion tensor imaging (DTI) can be helpful for differentiating ependymoma from astrocytoma in adults.
- • DTI is a promising technique for evaluating patients with spinal cord injury (detecting the injury epicenter, differentiating injured from normal spinal cord in the absence of T2-signal changes).
- • DWI is a useful tool for differentiating benign from malignant vertebral body fractures (when used with conventional sequences).
- • DWI can be a helpful tool for differentiating Modic I changes from acute spondylodiskitis.
The role of diffusion weighted magnetic resonance imaging (DWI) in the evaluation of brain diseases was established shortly after its introduction. DWI became an irreplaceable part of the routine brain imaging protocol. Further developments with diffusion tensor magnetic resonance imaging (DTI) and fiber tractography (FT) have catapulted brain imaging to an even higher level.
Since the first published report on the usefulness of DWI in the differentiation between osteoporotic and malignant vertebral body fractures in
1998.1 physicists and clinicians continue to grapple with constructing clinically applicable and reproducible DWI and DTI sequences for the spine and spinal cord. Nevertheless, obtaining spinal cord DWI and DTI remains a challenge due to numerous technical difficulties. The size of the spinal cord, the macroscopic motion related to the surrounding cerebrospinal fluid (CSF) pulsations, breathing and swallowing movement artifacts, and local field inhomogeneities are the major technical issues. Technical solutions are needed to optimize both image acquisition and image processing. Studies published on DWI and DTI of the spine are limited and have mainly been restricted to the cervical spine.
Recently, fairly good technical solutions have been developed, such as multishot echo-planar imaging (EPI), diffusion weighted periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER), spin-echo navigator spiral DTI, and parallel imaging. The acquisition times have been reduced, allowing implementation of DWI/DTI in routine clinical protocols. Through spatially selective excitation, zoomed DWI allows for acquisition of high-resolution images, with a reduced scan time, due to a reduced field-of-view (rFOV) in the phase-encoding direc- tion.2 rFOV sequences34 have shown promise in producing high-quality, in vivo human spinal cord DTI. In one larger study of 223 patients, performed on a 1.5 T clinical scanner, an rFOV DWI sequence added clinical utility in 33% of cases where pathology was done.4 The rFOV DWI sequence was found to be helpful in the evaluation of acute infarction, demyelination, infection, neoplasm, and intradural and epidural collections.4 In another study, the diagnostic value of DWI and the measurement of the apparent diffusion coefficient (ADC) were evaluated in 33 patients who presented with a spinal cord syndrome due to a noncompressive myelopathy.5 ADC values measured in that study were significantly lower in spinal cord infarct (mean ADC 0.81 x10-3 mm2/s) when compared with inflammatory spinal cord lesions (mean ADC 1.37 x10-3 mm2/s) and with those measured in healthy control spinal cord (mean ADC 0.93 x 10-3 mm2/s).5
The usefulness of DWI and DTI of the spine has also been reported in children. In one study, five controls and five children with cervical spinal cord injury (SCI) were imaged by using a single-shot, echo-planar, diffusion weighted sequence. Children with SCI showed reduced fractional anisotropy (FA) and increased diffusion (D) values compared with controls. The study has shown that the differences in diffusion metrics between noninjured and injured spinal cords can be demonstrated in the pediatric population.6