Intra-operative imaging in modern neurosurgery
By Professor Geirmund Unsgaard MD PhD, of The Norwegian University of Science and Technology - who is also Head of the Neurosurgical Dept. and Manager of the National Centre for 3-D Ultrasound in Surgery, at St Olavs University Hospital, Trondheim, Norway - and research scientists Tormod Selbekk MSc and Frank Lindseth PhD, both research scientists at SINTEF Health Research, and the National Centre for 3-D Ultrasound in Surgery
The brain has few anatomical landmarks. During surgery it is critical that the neurosurgeon knows the exact locality of surgical instruments in relation to important brain structures.
Figure 1 (small): Screen dump from the navigation system, showing corresponding image slice extracted from the MR T2 volume (top) and ultrasound volume (bottom). Note the apparent difference in tip position of the navigated instrument (cross-hair) between the images
Thus neuronavigation systems have become standard tools for planning and guidance. However, conventional navigation systems, based on pre-operative images, are of limited value during surgery due to brain shift caused by the removal of cerebrospinal fluid as well as the surgical procedure. Therefore, there is a strong need for intra-operative imaging either by CT, MRI or ultrasound (US).
Figure 2: Corresponding image slices as displayed on the navigation system during surgery of a glial tumour. Intra-operative US volumes acquired A) prior to resection, B) during resection and C) at the end of resection
Many research groups and large companies have tried to solve the brain-shift problem by using intra-operative MRI, which has proved useful for guiding certain surgical procedures and for control of tumour resection. The industry has invested in developing solutions for the operating theatre.
The concept of operating inside the magnet has not been very successful due to the narrow space and need for special non-magnetic instruments. Recently, firms have made operating suites in which a patient can be transported in and out of the magnetic field. But however well designed, this transportation will interrupt an operation. Another solution is to lift small magnets in and out of the operating field (PoleStar).
Although most intra-operative MRI solutions provide good quality images, the main drawback is cost - too high for most clinics, anywhere. On top of MRI equipment operating rooms need special shielding.
Figure 3: Display from the navigation system during resection of a cavernous haemangioma, showing corresponding tool oriented image slices (perpendicular) of MR volume and US volume. Note the difference in the offset tip position (centre of circle) indicated in the respective image volumes. The real position of the tool is at the edge of the lesion
This solves our brain-shift problem, and also our need to follow the operation’s progress and the final control of the tumour resection. The system is very flexible and can be used for any operation without a time increase - rather, it reduces it, because surgeons feel more confident with the updated information. Compared with intra-operative MR, one disadvantage of 3-D US is that it only shows a region of interest, not the whole brain. This potential problem is minimised by the navigation system, which displays corresponding image slices from both intra-operative US and pre-operative MRI (Fig 1 and 3). Another disadvantage is that neurosurgeons are less familiar with US than MRI. In our experience the learning curve is very steep.
Figure 4: 3-D ultrasound angiography (USA) in aneurysm surgery. Volume rendering of USA data acquired just before the clipping (A) of the aneurysm (arrow) and right after the clipping (B). The aneurysm is seen in the middle of picture A) whereas it has been removed in B). Both A) and B) is rendered with the same view
3-D US is cheap, flexible and very useful equipment for a range of neurosurgical applications. It is the workhorse of the operating theatre. We think that, because neurosurgeons are gradually discovering its usefulness, most operating theatres, worldwide, will be equipped with this type of instrument.
This article was published on 05/01/2006