Weight-bearing MRI opens new possibilities for the evaluation of structural, positional, and kinetic changes of the spine

Weight-bearing magnetic resonance imaging (MRI) of the spine has been accomplished with axial compression devices,¹ modified magnets allowing standing or seated positions,^2,3 and dedicated magnets allowing standing, seated, and kinetic positions.⁴

The configuration of the magnet that is open in the front and on the top allows a patient to be imaged in a weight-bearing position, seated or standing, and positions the patient in a physiological position of pain. This may include kinetic positions of flexion, extension, rotation, lateral bending, or any other combination of these positions.

The potential advantages of weight-bearing MRI as compared to conventional recumbent supine MRI include physiological and pathological changes in the relationships of the intervertebral disks, ligamentum flavum, articular facets, central canal, neural foramina, and segmental instabilities.^2,5

Technical Considerations
The only dedicated weight-bearing MRI unit is a 0.6 T electromagnet with a horizontal magnetic field that is transverse to the longitudinal axis of the patient’s body. The configuration of the magnet is open on the top and front with a retractable tilting table, allowing images to be obtained from 180° horizontal to 90° vertical. This configuration permits imaging in seated, standing, kinetic, and recumbent positions.

Results and Review of Literature
Weishaupt et al6 evaluated whether weight-bearing MRI of the lumbar spine can demonstrate nerve root compromise not visible in conventional MR images with the patient in the supine position. Thirty patients with chronic low back or leg pain unresponsive to a trial of conservative treatment for more than 6 weeks were included in their study. All 30 patients underwent weight-bearing MRI in seated neutral, flexed, and extended positions and were compared to conventional supine MR images. The results of this study have shown that the cross-sectional area of the dural sac significantly decreased between the supine neutral and weight-bearing extended position. Another significant finding was a statistically significant relationship between increased pain intensity using the visual analogue scale (VAS) and simultaneous foraminal narrowing supporting the concept of dynamic foraminal stenosis. This contradicts previous assumptions that correlation between foraminal stenosis and symptoms is poor.

A prospective analysis was conducted of cervical and lumbar MRI examinations performed in weight-bearing and kinetic positions, with weight-bearing MRI compared to conventional supine MRI.^4,7,9,10 The results included correction of the sagittal cervical and lumbar lordosis in the weight-bearing positions compared to recumbent MRI, increased severity of focal posterior disk herniation on the extension weight-bearing positions compared to supine and neutral weight-bearing, increased severity of central canal and foraminal stenosis in weight-bearing positions (most severe in extension and least severe in flexion and supine positions), and translational instabilities in weight-bearing positions compared to supine MRI.

It is well-documented that narrowing of the spinal canal is provoked by axial loading, especially in combination with extension of the spine.¹¹ Willen et al¹² evaluated patients with sciatica after conventional MR and compared the findings with axial-loaded supine MR technique. Seventy-seven percent of the patients examined had a significant reduction of the dural sac cross- sectional area at one or more lumbar levels during the axial-loaded MR technique.

Schmid et al,³ Danielson et al,¹ and Danielson et al⁸ demonstrated changes of the spinal anatomy under weight-bearing conditions. The changes become more evident when weight-bearing is combined with additional flexion or extension of the spine. In general, when the spine is moved to the extension position, a reduction takes place in the cross-section diameter of the central canal and neural foramina. Several factors influencing the reduction in size of the central canal and neural foramina include increases in the size of the ligamentum flavum due to buckling, increase in size of disc protrusions, and decreased disc space height.

Myelography
Myelography has been established as an imaging procedure for assessment of the positional component of weight bearing on the dural sac and the neuroforamina. It is documented that narrowing of the spinal canal is produced by axial loading, especially in combination with extension of the spine. However, myelography is an invasive procedure. Wildermuth et al¹¹ performed a study that compared findings of weight-bearing MRI of the lumbar spine and myelography. The sagittal diameter of the lumbar dural sac was shown to have good correlation between the two procedures.

Based on this data, the conclusion can be made that myelography and weight-bearing MRI are comparable for quantitative assessment for sagittal dural sac diameter in the upright neutral, flexed, and extended positions.

Case Study A 28-year-old female patient presented to Mark Hanson, DC, (Action Chiropractic, Dallas) with recurring low-back and right-leg pain 2 years following an L5–S1 discectomy. Conventional supine MRI was unremarkable. Figures 1 to 4 show weight-bearing MR images. In Figures 1 and 2, weight-bearing MR images demonstrate a normal L5 posterior disc contour in the neutral sagittal and axial sequences. In Figures 3 and 4, weight-bearing T2-weighted axial and sagittal extension sequences reveal a focal right paracentral 5–6 mm disk herniation compressing the thecal sac and right S1 nerve root.

Imaging Protocol
Imaging protocol for the cervical spine includes sagittal T1 and T2-weighted 4-mm thickness with the patient in the weight-bearing neutral position, sagittal T2-weighted weight-bearing extension, axial gradient echo 4-mm thickness contiguous from C2 through T1, with the patient in the weight-bearing neutral position.

Thoracic spine imaging is performed with the patient in the weight-bearing neutral position. Sagittal T1 and T2-weighted and axial T2-weighted sequences are performed.

Lumbar spine sequences include sagittal T1 and T2-weighted 4-mm thickness in the weight-bearing neutral position, sagittal T2 weighted in the extension position.

Axial T1 and T2-weighted sequences are performed from L1 through S1 with the patient in a weight-bearing neutral position. Axial T2-weighted parallel with the disc spaces of L1-2 through L5-S1 with the patient in a weight-bearing extension position is also performed.

The potential benefits of weight-bearing MRI include unmasking of occult pathology in the supine recumbent position, including disc protrusion and herniation, central canal and foraminal stenosis, and translational instability.

Weight-bearing MRI opens new possibilities for the evaluation of structural positional and kinetic changes of the spine. Indications for weight-bearing MRI may include inconclusive conventional MRI, suspected positional dependent nerve root compression, clarification of true sagittal spinal lordosis, unmasking of weight-bearing and kinetic dependent degenerative spinal disease, and the potential to scan a patient in a position of clinically relevant pain. CP

Robert J. Longenecker, DC, DACBR, is in private practice in Irving, Tex. He is a postgraduate instructor at Parker College of Chiropractic in Dallas. Contact him at rjldc@yahoo.com. 

References
1. Danielson B, Willen J. Axially Loaded Magnetic Resonance Image of the Lumbar Spine in Asymptomatic Individuals. Spine. 2001;26(23)2601–2606.
2. Weishaupt D, Boxheimer L. Magnetic Resonance Imaging of the Weight-Bearing Spine. Seminars in Musculoskeletal Radiology. 2003;7(4)277–286.
3. Schmid MR, Strucki G, Duewell S, Wildermuth S, Romanowski B, Hodler J. Changes in cross-sectional measurements of the spinal canal and intervertebral foramina as a function body position: in vivo studies on an open-configuration MR system. Am J Roentgenol. 1999;172:1095–1102.
4. Jinkins JR, Dworkin, JS, et al. Upright, Weight-Bearing, Dynamic-Kinetic MRI of the Spine pMRI/kMRI. Rivista di Neuroradiologia 2002:15:333–356.
5. Yochum T, Rowe L. Essentials of Skeletal Radiology, 3rd ed. Hagerstown, MD: Lippincott Williams & Wilkins, 2005.
6. Weishaupt D, et al. Positional MR imaging of the lumbar spine: does it demonstrate nerve root compromise not visible at conventional MR imaging? Radiology 2000;215:247-253.
7. Jinkins JR. Atlas of Neuroradiologic Embryology, Anatomy and Variants. Lippincott Williams & Wilkins, Philadelphia 2000.
8. Danielson B, Willen J: Axial Loading of the spine during CT and MR in Patients with Suspected Lumbar Spinal Stenosis. Acta Radiologica. 1998;39:604–611
9. Jinkins JR, Dworkin, JS, et al: Upright, Weight-Bearing, Dynamic-Kinetic MRI of the Spine pMRI/kMRI. Rivista di Neuroradiologia 2002;15:333–356.
10. Jinkins JR, Green C, Damadian R: Upright, Weight-Bearing, Dynamic-Kinetic MRI of the Spine. Rivista di Neuroradiologia. 2001;14:135.
11. Wildermuth S, Zanetti M, Duewell S, et al. Lumbar spine quantitative and qualitative assessment of positional (upright flexion and extension). MR imaging and myelography. Radiology. 1998;207:391–398.
12. Willen J, Danielson B, et al. Dynamic Effects on the Lumbar Spinal Canal. Spine. 1997;22(24):2968–2976.