Teaching Case

Headaches, and mild brainstem compression.

The patient presented with headaches. Mild signs and symptoms of brainstem compression were noted, and she was referred to MRI. On conventional MRI sequences, a small mass was detected at the left cerebellopontine angle, and diffusion imaging was performed. Two diffusion sensitivity values were used b=0, and b=1000 sec/mm2. Images with b=0 provided a set of T2-weighted images, whereas those with b=1000 sec/mm2 provided three sets of true diffusion images in the x,y, and z directions. The apparent diffusion coefficient (ADC) values were calculated according to the Stejskal-Tanner equation: ADC= -(1/b) ln (S/So), where where So is signal intensity (mean) obtained in the regions of interest (ROI) with the gradient factors b=0, and S is the signal intensity (mean) obtained with the gradient factors b=1000 sec/mm2. ‘ln’ is natural logarithm.

Epidermoid tumor is an ectodermal inclusion cyst. Approximately half of these tumors are located in the cerebellopontine angle cistern. They usually have lobulated appearances, engulfing the surrounding structures. Their density pattern on CT, and intensity pattern on MRI are often identical to that of CSF. Therefore, especially when small, and without lobulated contours, it is almost impossible to differentiate epidermoids from arachnoid cysts by conventional MRI sequences. On diffusion MRI, however, most of epidermoids have high-signal on true diffusion (b=1000 sec/mm2) images, whereas arachnoid cysts have CSF-like (low) signal. The ADC value in the epidermoid was 0.57 x10-3 mm2/sec indicating an apparently lower diffusion rate (restricted molecular motion) similar to cytotoxic edema (ADC for cytotoxic edema = 0.32 x10-3 mm2/sec). This value is apparently lower than that of CSF (ADC for CSF = 3.40 x10-3 mm2/sec) as well as than that of normal cerebral white matter(ADC for white matter = 0.84±0.11 x10-3 mm2/sec). The explanation, restricted molecular motion within an epidermoid, therefore, does not seem realistic. In a recent experiment by Branco (3) it was shown that transition of water from the sol to the gel state, can contribute to the signal on diffusion MRI, and in gel state high signal is observed on b=1000 mm2/sec images. This could be the case for epidermoid tumors.