Imaging of Peripheral Nervous System Flashcards
What is the PNS?
Everything outside the brain and the spinal cord
All the caudate quania, rootlets and the plexus as well as the peripheral nerves
What is the nerves?
- Numerous parallel axons
- Axons surrounded by Schwann cells
- Wrapped in connective tissue
- Endoneurium
- Perineurium
- Epineurium - Nerve fascicles - groups of axons bound into bundles
What are some limitations to nerve conduction studies?
- Can be time consuming
- May be uncomfortable for patients
- May not be feasible (anti-coagulant/skin disorder)
- May not reveal precise site of lesion - proximally in the legs
- May not characterise structural lesions - axonal or demyelination
Magnetic Resonance Imaging
- Non-invasive
- Operator dependent
- High spatial and contrast resolution
- Precise anatomic lesion localisation
- Can characterise underlying structural cause
- May detect denervation change earlier than EMG
What is ultrasound used for?
- Difficult to assess proximal nerves
- Useful Ix in the appropriate clinical setting
- Superficial peripheral nerves
- Operator dependent denervation changes more difficult to assess
What are the indication of when to image?
• Lesion localisation when clinically difficult
(spinal root vs plexus vs peripheral nerves)
• Lesion characterisation
• Single vs multiple nerve involvement
• Extent of abnormality/disease burden
• Establish continuity of nerve (trauma and compression)
What are the potential issues?
- Small structures
- Similar contrast to muscles & vessels on conventional sequences
- Adjacent to vessels
- Anatomic complexity
What are the common requests?
- Lower limb: sciatic nerve
- Upper limb: median, ulnar, radial nerve
- Pelvis: fermoral, pudendal nerve
Why should imaging be targeted as possible?
- Shorter examination
- Improved patient compliance
- Smaller field of view (FOV)
- Improved diagnostic quality
What are the sequences?
- T1w (anatomic detail)
- T2w with fat suppression (tissue contrast)
- Post-gadolinium T1 with fat suppression (BNB)
- More advanced:
- 3D T2w (e.g. SPACE)
- Hybrid T2w & DWI (e.g. 3D DW-PSIF)
- DWI/DTI & tractography
What are the technical aspects?
- 3 Tesla (higher signal:noise ratio)
- Phased-array multi-channel coils
- Dedicated joint/extremity coils
- Flexible surface coils wrapped around limbs - Small FOV
- High in-plane spatial resolution
- Thin slices - axial and longitudinal planes
What is found in the lower limbs?
Sciatic nerve
Where is sciatic nerve formed from?
L4-L5 to S3 nerve roots
Where do sciatic nerve pass in front of?
poiriformis muscles and then down the back of the leg through the foramen and they lie just behind the hamstring muscles
What are the 2 nerves that the sciatic nerve is split into?
- Tibial nerve - carry into the posterior calf
2. Common peroneal nerve wounds around to the lateral side of the lower leg
Where do the sciatic nerve go through?
The sciatic foramen and in between gluteus maximus and the hamstring muscles
How not to do it
- They have not put flexible coils over the front of the body – lost signal
- Large FOV – it is very hard to pick out the sciatic nerve
- Large FOV for the size of the patient
What are the three main nerves of the upper limb?
- Median nerve [to look for brachial artery and it is medial to it]
- Between brachial artery and percilic vein
Where do median nerve descend down to?
the arm, initially lateral to the brachial artery. Halfway down the arm, the nerve crosses over the brachial artery, and becomes situated medially. The median nerve enters the anterior compartment of the forearm via the cubital fossa.
Where do ulnar nerve sit behind?
Percilic nerve on top of the triceps
Radial nerve sits lateral
What do radial nerve innervate?
The muscles located in the posterior arm and posterior forearm
Where do ulnar nerve sit on top?
Of the triceps and behind the basophilic vein
Where do the median nerve sit between?
Artery or lateral to the artery
What to look for in the nerve (direct evidence)?
- Increased STIR
- Nerve calibre (focal vs diffuse)
- Contrast enhancement
What to look for muscle (indirect evidence - denervation)?
- Increased STIR signal (water deposition)
- Increased T1 signal (fatty replacement)
- Pattern of involvement
What to look for in a normal nerve?
- Size = similar to adjacent arteries
- Signal intensity = minimally hyperintense on STIR
- Fasicular pattern = preserved
- Enhancement = Absent (except DRG)
- Perineural fat = Preserved
What to look for in an abnormal nerve?
- Size = Focal or diffuse enlargement
- Signal intensity = Hyperintense on STIR
- Fascicular pattern = Enlarged or disrupted
- Enhancement = Present if BNB disrupted
- Perineural fat = Effaced
What is the importance of muscle denervation?
- Neuronal integrity necessary for muscle function
- Occurs distal to level of nerve lesion
- Pattern can help distinguish site of lesion
- L5 nerve root compression vs common peroneal
What is the prognostic significance of muscle denervation?
- Water deposition (acute) vs fatty replacement (chronic)
- Acute denervation change is reversible
- Early treatment –> potential functional recovery
