The demonstration of glomus tumours by subtraction MRI.

Thirteen patients with 14 glomus tumours have been examined by subtraction gadolinium-enhanced magnetic resonance imaging (MRI), with T1-weighted MR sequences before and after intravenous gadolinium-DTPA. To eliminate movement between subtraction pairs, the patient remains in the tunnel of the imager during administration of the contrast medium, and the venepuncture is made into the dorsum of the foot. The effect of the subtraction process is to remove the NMR signal from the final image so that the photographic densities recorded are dependent on the vascularity of the tissue concerned, normal or abnormal. A particular advantage is the removal of fat signal: the low vascularity of adipose tissue ensures that it is recorded as of minimal density. The extent of skull base glomus tumours has been shown optimally by this technique. Subtraction can also help differentiate glomus tympanicum from glomus jugulare lesions, which may be of crucial importance when deciding the surgical approach. In addition to diagnosis, the technique is also important post-operatively, when imaging is needed to show residual or recurrent tumour and to monitor the effects of radiotherapy.