Introduction to Magnetic Resonance Imaging of the Peripheral Nervous System: General Considerations and Examination Technique
Ever since magnetic resonance imaging (MRI) got introduced in the armamentarium of medical imaging methods, it received highest interest from neuroscience. This method’s superb soft tissue contrast allowed for unprecedented imaging of nervous tissue and developed very quickly into a standard method of brain and spine imaging. Over the past few decades, MR scanners became a routine tool and a mass product within affordable cost and thus widely spread. At the same time, rapid technical advances of the method opened the door for even more detailed imaging and complete new applications: to date, scanners with a field strength of 1.5–3T can be considered standard and allow for even more detailed imaging within acceptable imaging time. Cross-sectional imaging of nervous tissue can today be combined with numerous complementary scanning schemes, such as depiction of the vasculature with various MR angiography methods, insight into the biochemistry of nervous tissue and its pathologies with spectroscopy, and observation of nerve tissue function by exploiting tissue perfusion and blood deoxygenation effects with functional MRI. A more recent development in MRI is the ability to assess the Brownian molecular motion of water molecules termed diffusion-weighted imaging (DWI). This method is routinely used for the early depiction of ischemic stroke where hypoxic injury of the neural tissue leads to a shift of free interstitial water into swollen neurons (cytotoxic edema), thus reducing the free diffusibility of water molecules which results in a drop of the calculated diffusion coefficient. This imaging method is also used to facilitate the differentiation of dense cellular tumor tissue from inflammatory or infectious changes as well as scar tissue after tumor resection. Even more interestingly, DWI is also capable of assessing the foremost direction of free Brownian molecular motion, thus permitting the visualization of the nerve bundles’ course within the spinal cord and cerebral tracts or, in case of injury, their anatomical or functional disruption.