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.
