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Functional magnetic resonance imaging of the cervical spinal cord at 7 Tesla

Periodic Reporting for period 1 - Spinal cord fMRI (Functional magnetic resonance imaging of the cervical spinal cord at 7 Tesla)

Reporting period: 2015-09-01 to 2017-08-31

Magnetic Resonance Imaging (MRI) offers a unique possibility to study anatomy and function of neural tissue in humans without invasive procedures. The spinal cord is a part of the central nervous system and is implicated in a number of debilitating diseases. It is a small structure, measuring only about 1 cm in diameter at the level of the neck, and therefore requires high resolution imaging to study. With normal clinical MR scanners sufficient image resolution is difficult to achieve. MR scanners with higher background field strength (7 Tesla and above) give better signal-to-noise ratio, which can be used to achieve higher image resolution. Such systems are therefore well posed to study the spinal cord. However, they also introduce a number of additional technical challenges, especially related to spinal cord imaging. In this project, we have investigated methods for improving spinal cord imaging on 7 Tesla MR systems. The aim was to facilitate high-resolution anatomical and functional imaging of the spinal cord.
We evaluated the use of a new coil for imaging of the spinal cord at 7 Tesla. This kind of hardware is only available to a handful of research sites world-wide and is therefore still at an early development stage. We evaluated the performance of the coil and developed protocols for anatomical imaging of the spinal cord. We found that high-resolution (0.35x0.35mm in-plane) acquisitions combining images at different echo times yielded the best contrast between gray and white matter inside the cord.

We addressed two types of technical challenges that impair spinal cord imaging at 7 Tesla.

i) MRI requires a homogenous background field. However, the body distorts the magnetic field, because tissue and air have different magnetic susceptibility (i.e. become differently magnetised). The resulting local field distortions give rise to distortions and signal loss in the images. This is particularly a problem in the spinal cord. To minimise the field distortion, it is common to apply compensatory magnetic fields, so called shim fields. The shim fields are usually optimised for the whole volume of interest, in this case the spinal cord. We investigated the possibility of optimising and setting the shim fields separately for each imaging slice, thereby achieving better field homogeneity within the slice.

ii) On top of the local static field distortion, the background field may change over time. One cause for the field to fluctuate is the motion of the lungs and the thorax over the breathing cycle. The field fluctuations cause specific types of artefacts in anatomical and functional images. We measured the magnitude of the breathing-related field fluctuations inside the spinal cord in the neck region, and characterised the variations over time in relation to the breathing cycle. We further investigated if we could use the signal from a respiratory bellows to estimate the state of the field in the spinal cord, and apply corrections to the acquired imaging data..

The obtained results were presented at scientific conferences (ISMRM and ESMRMB) and prepared for publication in scientific journals.
High-resolution anatomical imaging at 7 Tesla yielded high quality images of the spinal cord, with a clear depiction of the gray/white matter distribution and delineation of the nerve roots and dorsal root ganglia. We found that slice-wise shim optimisation yielded higher signal from the spinal cord in slices affected by local field distortions, especially at longer echo times. We also showed that signal correction based on a respiratory trace can greatly reduce the level of artefacts related to field fluctuations caused by breathing in high-resolution anatomical imaging of the spinal cord (see figure).

There are many potential uses of the outcomes of this project. Improved functional imaging of the spinal cord would be of considerable interest for research into the neural mechanisms underlying chronic pain states. The clear depiction of the dorsal root ganglion may also be of relevance to study cervical myelopathy.
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