Authors:
R. Vadapalli1, M. A.K2, A. S. Vadapalli3; 1Hyderabad/IN, 2Chennai/IN, 3Pune AFMC/IN
DOI:
10.1594/essr2017/P-0182
Background
Magnetic resonance neurography is an exciting imaging technique
that affords multi-planar anatomic and functional depiction of peripheral
nerves and their related lesions.
Appropriate imaging and accurate interpretation are essential components of successful performance of this ever advancing Technique.
DWIBS MR Neurography:
DW_MRI appears to be very effective method to visualize nerves of Peripheral nervous system.
It is well known that the application of Motion probing gradients(MPGS) using DW MR Imaging results in signal suppression from structures with relatively un impeded diffusion such as blood vessels and cerebrospinal fluid.
In structures with relatively impeded diffusion the MR signal will be little suppressed by the motion probing gradients (MPGs) and will appear as relatively bright as the diffusion sensitizing gradient increases.
Peripheral nerves contain numerous neuronal fibres across which diffusion is relatively impeded orthogonal to the long axis of nerve fibers .Consequently peripheral nerves exhibit high signal intensity at DW-MRI.
(Hajnal et al 1991)
The difference in the water diffusivity being higher in the long axis of the nerves compared to the orthogonal short axes is known as Anisotropy.
The degree of Unequal water diffusion in relation to axes of peripheral nerves can be quantified by calculating the Fractional Anisotropy(FA) and this ranges from 0 to 1 where 0 represents fully isotropic water diffusion (I.e.
no difference in water diffusion in any of the neural axes) and 1 represents fully anisotropic water diffusion.
The FA of peripheral nerves has been reported to be ranging between 0.599 and 0.80 (Kabakci et al 2007,Khalil et L 2008) Suggesting that there is less impedance of water diffusion along the long axis of the nerves due to orientation of nerve fibers.
A variation of DWI MRI called Diffusion Tensor imaging is required to measure FA and this DTI technique can also be used for fibre tracking along the course of peripheral nerve sheaths.
(Kabakci et al 2007)
3.MRN is best performed on 3 Tesla (3T)scanners with dedicated extremity or wrap around flex coils as they can provide higher signal-to-noise ratio (SNR),
which is essential to enhance the smallest structure in the neurovascular bundle,
namely the peripheral nerves
.
4.Uniform fat suppression is
essential to avoid artifactual increase in nerve signal intensity