Visualization of Nerves using Diffusion MRI
Challenges in Conventional MR Neurography include inability to render a 3D or projectional images such as MIP (maximum Intensity projection),
inability to depict the path of peripheral nerves along the length of nerve sheaths,
due to overlapping of structures from adjacent tissues,
such as veins which frequently accompany peripheral nerves ,
can be difficult to distinguish from the neural elements.(e.g. At the level of Brachial Plexus) due to their similar signal characteristics on T1 and T2 weighted images
DWIBS for Diffusion Weighted MR Neurography-Pearls
1.DWIBS sequence yields thin section three dimensional data set of Diffusion weighted images(4-5mm) rather than a series of 2D thick sections(8-10mm).
2.Thin sections and image partitions are possible using this technique because of long efficient scan time afforded by performing the scan during free breathing with excellent signal to noise ratio using a short diffusion encoding time of around 100 sec during which MPGs are applied.(Respiration being a coherent motion causes little or no signal attenuation during short diffusion encoding time of 100 msec (Muro et al 2005,Koh et al 2008)
3.
Thin image sections of peripheral nerves with multiple signal averages are acquired with excellent signal to noise ratio during free breathing,
which is one of the key features of DWIBS
4.
Uses a STIR pre pulse for fat suppression in contrast to most prior DW MRI studies in body imaging where frequency selective or chemical shift selective (CHESS) fat suppression techniques have been used.
STIR pre pulse gives a robust fat suppression over a wide Field of view,
even in areas of body experiencing substantial magnetic inhomogeneity.
5.
STIR Pre pulse when used with DW-MRI produces images with excellent background suppression.
Typical DWIBS based MR Neurography Parameters:
Sequence-SE-EPI –Acquisition plane-Axial FOV_350mm Rectangular FOV -80% TR?TE: 5786/68
EPI factor(ETL)-47,SENSE factor-2 Acquisition Matrix-160x256,Half Fourier scan factor-0.6,Phase encode reduction-80% Slice thickness/gap-4/-1(overlap),Number of slices 60,number of excitations-6
Type of Fat suppression-STIR
Post processing by 3D Maximum Intensity projection
Uni directionally encoded DW MR Neurography-
The technique of diffusion MR Neurography with Motion probing diffusion sensitizing gradients applied orthogonal to long axis of nerve.
This is superior to the techniques using three or six motion gradients and is currently recommended for use in clinical practice for visualization of Brachial and Lumbo sacral plexuses,
nerve roots and peripheral nerves
Technical Aspects: Pearls
Parameter settings
1.Images should be acquired axially ( with Phase encoding in antero posterior direction.) rather than coronally to minimize FOV and reduce image distortion.
2 .Parallel imaging (SENSE or GRAPPA) should be employed to reduce number of echoes(EPI factor or ETL)
3.
STIR Pre pulse for better fat suppression
4.
Use thin 2-4mm but numerous 80-100 image sections across the image volume to enable 3D visualization of the data sets.
Difference between Diffusion MR Neurography and Diffusion Tensor Imaging based Tractography
-In contrast to DTI ,
the DW MR Neurography does not require the application of the Motion probing gradients in multiple directions.
Typically MPGs applied in 3 directions are sufficient ,and peripheral nerve visualization may even be better by applying MPG in only one directions
- DW MR Neurography uses diffusion weighting as contrast mechanism to visualize neural structures and does not attempt to perform neural tracking or tractography.
-In other words DW-MRN is simpler to perform and analyse than DTI and this technique only aims to obtain a high quality neurographic data set for three dimensional evaluation and image display.
-In 2004 Takahara et al introduced a new technique called DWIBS(Diffusion weighted whole body imaging with background body signal suppression)