Neurography

Section 1: Case Presentation

Teenage patient presenting with atraumatic acute onset severe ulnar motor neuropathy.   The primary team requested imaging to assess the ulnar nerve and to evaluate for any underlying cause to explain the patient’s symptoms. Although ultrasound can visualize nerves, it is less sensitive than magnetic resonance imaging (MRI) to assess for pathology.  Thus, an MRI was ordered to assess the ulnar nerve from the elbow to the hand.

As seen in Movie 1, the patient has limited motion of the 4th and 5th digits. 

Movie 1

 

 

Section 2: Introduction to MRI Neurography

At Lucile Packard Children’s Hospital at Stanford University, a unique and specialized MRI protocol to assess the nerves has been developed. MRI neurography utilizes multiple sequences to visualize nerves, increasing sensitivity. The primary sequences acquired are:

  1. A dual-echo volumetric gradient echo sequence
  2. Small field-of-view T2 weighted fat-suppressed imaging
  3. Volumetric high resolution post contrast T1 weighted fat-suppressed imaging
  4. Volumetric high resolution T2 weighted fat suppressed sequence

Section 3:  T2 Weighted Fat-Suppressed Imaging

An T2 weighted fat-suppressed sequence of the target anatomy perpendicular to the course of the nerves is acquired using a small field-of-view and an echo time (TE) of at least 80ms. Higher TE increases conspicuity of nerves. When edematous (swollen), the nerve becomes particularly bright.

Figure 1 shows increased T2 signal of the ulnar nerve at the level of the elbow (red arrow), signifying edema.

Figure 1

Movie 2 demonstrates diffuse ulnar nerve edema from the distal upper arm and extending distally to the carpal tunnel (Movie 3).

Movie 2 Movie 3

The normal median nerve (Figure 2, white arrow) has similar brightness to adjacent muscle, compared to the bright abnormal ulnar nerve (red arrow).

Figure 2

Section 4:  Axial Dual-Echo Volumetric Gradient Echo Sequence

For additional sensitivity to nerve abnormalities, volumetric imaging with two sets of images (echoes) is obtained.  The second set of images has been tuned to assess diffusion of water molecules in the nerve.

Figure 3 shows the ulnar nerve at the level of the elbow. Both the target nerve and the adjacent vascular structures will have relative increased signal compared to the surrounding structures (Figure 3, left image, white arrow).  The diffusion-sensitized images show near complete suppression of vascular signal and highlights inflammation of the nerve (Figure 3, right image, red arrow). Specifically, the ulnar nerve appears brighter diffusion-sensitized images.

Figure 3

To emphasize the difference between normal and abnormal nerves, Figure 4 shows the median and ulnar nerve at the level of the distal forearm. The normal median nerve (white arrow) has similar signal intensity on both the non-sensitized image (left) and diffusion-sensitized image (right). The abnormal ulnar nerve (red arrow) becomes evident with diffusion sensitization (right image). Note that the vasculature adjacent to the ulnar nerve has been suppressed on the diffusion sensitized image (right, dashed white arrow).

Figure 4

Movie 4 and Movie 4 hows long segments of the nerves at the elbow and forearm respectively.

Movie 4 Movie 5

Section 5: Axial High Resolution Volumetric T1 Weighted Post Contrast Fat Suppressed Sequence

Using a custom two-point Dixon sequence, an axial T1 weighted volumetric post contrast sequence is acquired. This sequence allows high resolution imaging high enough to depict small individual nerve fascicles. Enhancement of the nerve may be indicative of active inflammation.

Figure 5 shows abnormal enhancement of the ulnar nerve fascicles at the level of the elbow (red arrow).

Figure 5

Figure 6 at the level of the distal forearm, shows normal degree of enhancement of the median nerve (white arrow) and abnormal enhancement of the ulnar nerve (red arrow).

Figure 6

Movie 6 shows the entire sequence at the level of the elbow

Movie 6

Section 6: Volumetric Image Reconstruction

Volumetric sequences with custom technique can yield high spatial resolution isotropic voxels, enabling multiplanar reconstructions (i.e., sagittal, coronal, curved). Thus, visualization of the entire target nerve is possible by utilizing specialized 3D post-processing software.

Figures 7-9 show curved reconstructed images of the ulnar nerve from the mid humerus to the mid forearm, for volumetric T2, Dual-echo, and T1 post-contrast sequences respectively. The ulnar nerve can be seen to have abnormal signal (red arrows), extending from the upper arm above the elbow and distally out of the field-of-view.

Figures 7-9

Section 7: Case Wrap-up

Based on physical examination and the MR neurogram, the patient was diagnosed with acute ulnar neuritis from the distal upper arm just above the elbow to the wrist, secondary to entrapment at the cubital tunnel. A combination of surgical intervention (cubital tunnel release), physical therapy, and anti-inflammatory medications resulted in near complete return of hand strength.