Image guided frameless stereotaxy

Although craniotomy with conventional stereotaxy is feasible, in practice the cumbersome frame, combined with the aiming device, tend to obstruct the operating field. The need for a simultaneous CT or MRI with the head ring and locating rods in place leads to a further inconvenience.

The combination of modern imaging, elaborate computer software and a locating device now permit the surgeon to determine how the tip of a pointer outwith or within the skull, directly relates to a two or even three dimensional CT or MR image.

The accuracy of the technique depends on the quality of the digitised image and on the method used to register the patient's head to the image. The registration of recognisable skin points (e.g. nasion, inner canthus, ears) on the patient to the CT/MR image provides an accuracy of 2-3 mm and this is sufficient for most purposes.

Infra-red beams de tect the position of the probe in space

Alternatively, potentiometers detecting the angles in a multijointed mechanical arm fixed to the patient's head clamp, feed back information on the pointer tip position.

Infra-red beams de tect the position of the probe in space

The position and direction of the pointer tip is displayed on the CT/MR image.

Pointer with infra-red reflectors

The position and direction of the pointer tip is displayed on the CT/MR image.

Pointer with infra-red reflectors

2D or 3D image acquired before surgery

Uses of '1'rameless' stereotaxy

Aids accurate positioning of burrholes and bone flap, and the planning of the safest approach to the lesion.

TUMOURS

- biopsy

- resection: locates, then identifies the tumour margins and the position of important adjacent structures

- brachytherapy (see page 304)

EPILEPSY

- defining the extent of resected tissue e.g. amygdalohippocampectomy

- placement of depth electrodes

ARTERIOVENOUS MALFORMATION

- localisation of lesion and the feeding vessels

ABSCESS

- aspiration

ORBIT

- location of intraorbital lesion

Framed stereotactic methods are still required for functional procedures, most of which require a local anaesthetic 372 and patient cooperation. Head movement prevents accurate registration.

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