Imaging of Intraneural Edema by Using Gadolinium-Enhanced MR Imaging: Experimental Compression Injury

Comparison between enhanced MR imaging and fluorescent micrograph of the median nerve.

A and B, No enhancement of a healthy median nerve was found on a gadolinium- enhanced MR image (T1-weighted SE image, 600/25 [TR/TE]). The median nerve showed moderate signal intensities and the signal intensity was similar to that of muscle in normal conditions (A, arrow). Evans blue albumin was limited inside the blood vessels (B, arrow), and the blood-nerve barrier was maintained as seen under fluorescent microscopy.

C and D, After compression with a 30-g force, no enhancement of median nerve was found on gadolinium-enhanced MR image (T1-weighted SE image, 600/25 [TR/TE]; C, arrow); however, there was seen the limited extravasation of protein tracers around intraneural blood vessels (D, arrows).

E and F, Clear enhancement was seen inside the nerve compressed by a 60-g-force clip as seen on gadolinium-enhanced MR image (T1-weighted SE image, 600/25 [TR/TE]; E, arrow). In the nerve, where enhancement was found on MR imaging, Evans blue albumin emits a bright red fluorescence, which leaked outside the blood vessels, and intraneural edema was seen under a fluorescent microscope (F, asterisk). R, radius; U, ulna.

Electron micrographs of the unaffected median nerve after intravenous injection of horseradish peroxidase.

A, In the normal median nerve without compression, electron-attenuated of horseradish peroxidase was found in capillary lumen, but no leakage into the endoneurial space was observed.

B, There are a very small number of pinocytotic vesicles in the endothelium (arrowheads). The tight junction (arrow) among endothelial cells was closed, and the blood-nerve barrier was maintained (A, ×5,000; B, ×40,000). En, endoneurial space, E, endothelium; L, capillary lumen.

Electron micrographs of the affected median nerve after intravenous injection of horseradish peroxidase.

A, After 1 hour of perfusion, the dark reaction product of horseradish peroxidase leaks out of capillary under compression (asterisk).

B, Horseradish peroxidase leaked into the endoneurial space from capillary lumen owing to expansion of the tight junction (large arrow) and increased transcellular transport by pinocytotic vesicles (small arrow). The dark reaction products of horseradish peroxidase pass through from capillary lumen to endoneurial space. Occurrence of intraneural edema was shown (asterisk). The lumen has been cleared of horseradish peroxidase by perfusion fixation (A, ×3,000; B, ×25,000). En, endoneurial space; Et, endothelium; L, capillary lumen.

Classification of the true capillary.

A, Brain type of continuous capillary. In morphologic features, the blood-brain (nerve) barrier of endothelial blood vessels consists of tight junction (zonulae occudentes) between adjacent endothelial cells and a very small number of pinocytotic vesicles. This capillary is mainly located in the central and peripheral nerve.

B, Muscle type of continuous capillary. In morphologic features, their endothelial cells are linked with tight junction; however, pinocytotic vesicles related to transport of nutrients and waste matter are much more abundant in endothelial cells of a capillary on muscle than capillary of brain type.

C, Fenestrated capillary. This capillary has a fenestration with a diaphragm in an endothelial cell and is mainly located in stomach, colon, and kidney.

D, Discontinuous capillary (sinusoid). Between the endothelial cells are large gaps, which permit a direct continuity of access of blood plasma from the lumen to the extracellular space. This capillary is mainly located in liver, spleen, hypophysis cerebre, and bone marrow. Et, vascular endothelium.