Principles And Practice Of Imageguided Neurosurgery

difficulties and cost of modifying the usual operating theater set-up to suit the requirements of MR scanning are clear. Several options have been used in various centers:

The use of a vertical open-bore MRI scanner, allowing two surgeons to operate within the scanner. This avoids the need for patient movement but requires the use of non-ferromagnetic instruments as well as magnetic shielding of the operating room [19].

The use of a twin operating theater; surgery is performed in a conventional theater and the patient is transported to an adjacent MRI suite for imaging. The scanner can be rendered more cost effective by allowing its use for diagnostic scans unrelated to theater sessions [17].

The use of a single-shielded operating theater, with the surgery being performed in the fringe field of the magnet. Normal ferromagnetic instruments and microscopes are then allowed [18].

There is a steadily growing volume of literature on intraoperative MRI and it is not possible to discuss this topic in detail here. Its value was demonstrated in a recent study [17] in which 40 patients underwent image-guided resection of gliomas (WHO grades II-IV) using an image-guidance system. An intraoperative MRI scan was obtained after the surgeon felt that the planned extent of tumor resection was achieved. Of these patients, 53% were found on MRI to have had a less than optimal resection. It was noted from this study that patients with a tumor volume higher than 20cm3 are more likely to have incomplete resections, probably because the degree of brain shift is much higher for larger lesions.

The intraoperative MRI system described by Hadani et al. [18] is characterized by a vertical-bore 0.12T open magnet that can be stored below the operating table when not in use. The patient's head occupies a fixed position relative to the magnet. With the magnet in the scanning position, real-time navigation can be employed using an MRI wand. This is independent of the optical tracking system and the magnet is the only reference point. Alternatively, the magnet can be lowered under the operating table once an image update has been obtained; optical tracking using the standard guidance set-up then allows navigation on the updated image data set. Image updating is independent of fiducials and skin markers, because the same relationship between the magnet and the patient is maintained throughout the procedure. Ferromagnetic instruments are used when the magnet is not in the scanning position.

There are several issues relating to intraoperative MRI that are still unclear. The presence of residual tumor on intraoperative images is ascertained on the basis of contrast enhancement around or within the resection cavity [17]. Increased permeability of the blood-brain barrier also occurs as a direct result of surgical manipulation, and indeed, surgically induced nodular as well as diffuse enhancement has been reported. This is probably more likely after contrast agents have been injected several times during the same procedure. The use of contrast agents that bind the tumor for a longer period of time and that are cleared from the circulation before the operation is begun has been investigated; iron oxide microparticles are phagocy-tosed by glioblastoma cells. It would then be possible to avoid administration of contrast agents just before or during the procedure. Bohinski et al. found that biopsy of residual enhancing tissue after partial glioma resection yielded tumor in 81% of cases [17]. Comparison with pre-operative contrasted scans is useful to differentiate tumor-induced enhancement from surgery-induced enhancement.

The edema associated with retraction is easily identifiable on T2-weighted imaging. As enhancement in the dependent part of the resection cavity might also be due to blood clot, it is essential that adequate hemostasis be secured prior to imaging. Blood is also identifiable on T2-weighted images. Oxidized cellulose in the cavity is another factor that interferes with the identification of tumor remnants. The debris of a metal drill produces significant artifact on MRI and it is preferable to use only diamond drills [19]. Even when all of these precautions are taken, however, definition of the borders of low-grade astrocytomas is still a challenge.

Other unanswered questions relate to the frequency of intraoperative scanning. Ideally one should obtain enough information to ensure that the image data set used for navigation is not outdated, and also to control resection of the target, without prolonging the operation or moving the patient unnecessarily. Should

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