A number of paediatric and adult neurosurgical units are investing significant resources in the development of intra-operative magnetic resonance imaging (ioMRI). In Alder Hey Hospital we have been fortunate enough to have this service available since October 2009 and so we have accumulated significant experience in this field. In our institution it is used routinely as an adjunct in the resection of paediatric brain tumours, in lesional epilepsy surgery and in other challenging cases. Investment in this expensive technology has been driven by the improvement in prognosis gained by more complete primary resection. In paediatrics, low-grade glioma is the most common tumour and total resection can be curative without the need for adjuvant therapy but improved degree of resection has also been shown to be a major prognostic factor in high-grade tumours, such as medulloblastoma, high-grade glioma and ependymoma.1–3
Components of Intra-operative Magnetic Resonance Imaging
MRI has been widely available for many years in neurosurgical units, but there are significant considerations when utilising this technology for intra-operative imaging. The choice of technology is the main consideration at the planning stage in terms of structural set-up of the theatre that will be used and budgeting for a costly component of the service. At Alder Hey, we have a three Tesla (3T) (high-field) MRI scanner utilising a ‘two-room solution’ (see Figure 1). This enables the use of the scanner for diagnostic imaging while the theatre is in use, but not requiring ioMRI. This ability to use the two components independently makes the facility economically viable. Not all theatre complexes would be able to house a 3T scanner due to the weight, the installation and the screening required. In such institutions options would include opting for a low-field imaging solution or accepting the inherent problems of using a theatre separated from the normal theatre complex. Current high-resolution ultrasound equipment is safe and gives accurate real-time information aboutresection margins but it will not be able to replace the need for MRI.4,5 It may, however, be a reasonable alternative to low-field systems that have a low signal-to-noise ratio (SNR).
If a two-room solution is adopted the major components are the scanner, an operating table with head clamp and a transfer system. Considerations with each of these components are outlined below.
Magnetic Resonance Scanner
Our department is equipped with a Philips Achieva® 3-T scanner (Philips Healthcare, Best, Netherlands) with a length of 157 cm and an inner bore of 65 cm. The size of the inner bore is important as the patient needs to fit in with a head coil on. This does normally not pose any problems in the neutral supine position but becomes more challenging in prone surgery with the patient’s neck flexed and retracted for posterior fossa surgery. The use of a 3T system allows advanced imaging and has a high SNR, which is especially important in small, immature brains of infants. However, the higher field strength does increase the magnetic susceptibility and chemical shift artefacts are increased. This will be discussed further in the next section.
Head Clamp and Coils
Cranial surgery using optical navigation requires a head clamp in order to maintain the position of the patient’s head in relation to a reference array. With an ioMRI all components leaving the theatre to enter the MRI suite must be MRI compatible. Coils around the patients head are required to enable scanning, but must maintain the sterility of the operative field. It is possible to use a carbon fibre head clamp and parallel flexible coil placed over the drapes, but better results are obtained using a dedicated intra-operative head coil (NORAS MRI Products, Hochberg, Germany).
The NORAS 8-channel inter-operative head coil has an integral fivepin head clamp and 14 fiducial markers, which enables automatic registration of the navigation system to the new ioMR images following a scan (see Figure 2).
The patient is positioned for their procedure on a theatre table that enables the normal changes in position required by a surgeon during surgery, but is compatible with a transfer trolley that can move the patient in the head clamp to the MR scanner. Alder Hey uses a Maquet system (Maquet, Rastatt, Germany), which docks securely with theoperating table and the MR table allowing the patient to slide from one table to the next while remaining in the head clamp.
Considerations in Use
Prior to performing surgery, when ioMRI is intended, the radiographers, theatre staff, anaesthetist and radiologist should be informed. This is to make certain that the appropriate planning is put in place to ensure sufficient experience and availability of staff in order to avoid delays with the patient under anaesthesia.
The anaesthetist has several considerations to bear in mind due to the fact that access to the patients head is difficult after starting surgery. For this reason, the endotracheal tube should be fixed securely prior to positioning. All lines and monitoring equipment should either be MRI compatible or be able to be removed prior to ioMRI. We would advise the use of MRI compatible drip stand and drug infusion pumps from the start of the case so that there is no need to interrupt drug delivery by changing systems for the scan. Conventional electrocardiograms (ECG)electrodes, diathermy plate and temperature probes must be removed and so should be placed with this in mind at the start of the anaesthetic. Vigilance to avoid leaving equipment, such as laryngoscope blades, on the operating table is equally important.