9 Intraoperative Imaging

George Samandouras and Gráinne S. McKenna

9.1 Optical Neuronavigation

9.1.1 Indications

•Frameless stereotactic biopsy.

•Hemispheric gliomas.

•Pituitary and skull base tumors.

•Intraventricular tumors.

•Ventricular catheter/shunt placement.

9.1.2 System Components

•Most optical neuronavigation systems are composed of a compact-footprint, free-standing computer workstation on wheels, a camera array that emits (illuminator) and detects (sensor) the infrared light from light-emitting

diodes (LEDs) sources and refecting spheres attached to the instruments and reference frame (Fig. 9.1).

•The most commonly used localizer is the Polaris localization system, which transmits a fash of infrared light from outer rings on each side of the hardware. The fash is refected by the retrofective spheres in the reference frame and surgical probe, and is detected by the sensors on the transmission unit.

9.1.3 Setup

•Images are transferred to the computer workstation, via ethernet cable, memory stick or CD, where surgical planning is taking place.

•In optical navigation systems, the patient’s head should be fxed in a head clamp (Fig. 9.1). Any subsequent head movement will invalidate the registration accuracy.

•A direct line of sight is required between the LEDs and the camera array (Fig. 9.1).

•Eforts should be made to ensure that the star housing the refective spheres is not too far from the patient (usually a fst’s width away) and will not be blocked by subsequent patient’s draping and surgeon’s or assistant’s positions.

•Patient’s registration and referencing to the pre-operative MRI (Magnetic Resonance Imaging) or CT (Computed Tomography) images, depending on the system, can

be obtained by touching the pointer to fducials (Fig. 9.1), performing surface matching without fducials, or con- tact-free, laser registration based on surface matching (Fig. 9.2).

•For surgeon’s convenience, for right-side lesions the camera and monitors should be placed to the left of the patient and vice versa (Fig. 9.1).

Fig. 9.1 Example of the Polaris type localizer (NDI, Northern Digital Inc., Baker ld, CA, USA) with the Medtronic StealthStation S7®Navigation System (Medtronic, Minneapolis, MN, USA). Note that patient’s he ed rigidly in the Ma ld clamp,

and to this is attached the blue reference frame (aka “star”) with ret tive spheres.

9.1.4 Tips And Tricks

•Interruptions in tracking are often due to refective spheres; cleaning them with a wet swab or replacing them with new ones can restore navigation.

•Although brain shift can reduce navigational accuracy, skull base landmarks remain accurate reference points.

•System generated registration accuracy depends on the size of the lesion. A 2.0 mm registration error might be too large for a biopsy of a 10 mm deep-seated lesion but is acceptable in resecting a 6 cm convexity meningioma.

•The surgeon should not be relying on navigation only and should be prepared to perform the operation based on surgical anatomy and pathology in case of equipment malfunction or hardware failure.

9.2 Electromagnetic

Neuronavigation

9.2.1 Indications

•Ventricular catheter placement in small or compressed ventricles (secondary to trauma or idiopathic intracranial hypertension).

•Awake craniotomies avoiding a fxed head position.

•Endoscopic ventricular navigation.

9.2.2 System Components

•StealthStation® AxiEM™ Electromagnetic Tracking System (Medtronic, Louisville, CO, USA).

•The portable computer workstation is identical to the optical navigation systems.

•Patient tracker attached to the patient’s head.

•Registration probe.

•Emitter of electromagnetic signal (Fig. 9.3).

•The AxiEM™ Portable system box where patient tracker, registration probe and emitter are connected.

9.2.3 Setup

•Similar to optical navigation systems images are transferred to the computer workstation, via ethernet cable, memory stick or CD, where surgical planning is taking place.

•Patient tracker is attached to the patient’s head with a strong adhesive tape (Fig. 9.3).

•The emitter is attached to the articulated vertek arm and placed close to the patient’s head (Fig. 9.3).

Fig. 9.2 Contact-free, laser, surface-match registration using Z-touch® with the BrainLab Cranial navigation system (BrainLab, Feldkirchen, Germany) in the intraoperative MRI for subsequent awake resection of a dominant frontal low-grade glioma. The Z-touch® allows the surgeon to sweep laser beams over the patient’s head and acquire a dataset of surface points for registration. Note the projected red laser beam near patient’s right eye.

Fig. 9.3 Set up of the StealthStation S7® AxiEM Electromagnetic Tracking System (Medtronic, Louisville, CO, USA).

Note that the patient’s head is resting on a horseshoe; the black emitter is attached to the articulated vertek arm on the right side of the patient and the distance to the

tracker is seen on the right side of the patient’s forehead.

•Registration is performed with fducials or surface match similar to optical navigation systems (Fig. 9.3).

9.2.4 Tips And Tricks

•The main advantage of the electromagnetic navigation system is that it

○Allows patient’s head movement.

○Can incorporate a stylet that can be introduced to ventricular catheters for targeting small or slit ventricles, or to guide an endoscope into the ventricle.

•Keeping the patient’s tracker fxed is paramount. Movement of the tracker is equivalent to movement of reference frame in optical navigation systems and will result in impaired or completely invalid registration.

•The top of the forehead is a good position to place the patient’s tracker. The practice of the senior author is to place strong adhesive tape over the tracker and further secure with four staples.

•All other components including the emitter and the patient’s head can be moved without afecting the registration.

•All metals close to the operative feld can afect neuronavigation. Although can be used with a Mayfeld clamp, (Integra,

Plainsboro, NJ, USA) navigation may be impaired or lost due to magnetic interference.

•During catheter placements removing self-retaining retractors from the operative feld can restore impaired neuronavigation.

9.3 Intraoperative Ultrasound

9.3.1 Indications

•Real time neuronavigation to determine extent of resection during tumor surgery (glioma or metastasis).

•Localization of cystic lesions/ventricles in the brain.

•Localization of intradural spinal lesions before opening the dura.

9.3.2 System Components

•There are two main types of ultrasound based neuronavigation

○Ultrasound systems that can be integrated with optical neuronavigation systems, including Medtronic (Medtronic, Louisville, CO, USA) and BrainLab (BrainLab, Feldkirchen, Germany) (Figs. 9.4–9.6).

○All-in-one systems combining preoperative CT/MRI with intraoperative 3D (three dimensional) ultrasound

images, 3D Power Doppler and classic neuronavigation, as for example in SonoWandTM (Sonowand AS, Trondheim, Norway) (Fig. 9.7).

Fig. 9.4 A portable ultrasound machine integrated with the Medronic StealthSation S7® using the SonoNav™ ultrasound navigation interface to aid orientation

of the ultrasound probe.

9.3.3 Setup

•Portable ultrasound systems are less expensive and can be integrated to optical neuronavigation systems.

The ultrasound probe and cable are inserted into a sterile plastic sleeve (Fig. 9.5). The probe houses refective spheres allowing real-time navigation. The software allows to swap the image between ultrasound and optical navigation

(Fig. 9.6).

•SonowandTM (Sonowand AS, Trondheim, Norway) is a single-rack system with an ultrasound scanner and a built in navigational computer and an optical tracking system (Fig. 9.7).

9.3.4 Tips And Tricks

•Knowledge of normal ultrasound anatomy is essential to diferentiate normal anatomy from pathology.

•Rotating the head of the ultrasound probe produces images in diferent planes. Knowledge of 3D-anatomy in all planes is required to avoid confusion.

•Ultrasound systems that integrate neuronavigation can be more useful, especially when surgeons have limited experience with normal ultrasound anatomy.

•As the learning curve is steep, gaining experience with ventricular surgery or cysts are good starting points before moving to gliomas and particularly to difuse low grade gliomas that can be more challenging to diferentiate from normal structures.

9.4  Immunofuorescence-Guided

Surgery

9.4.1 Indications

•Anaplastic gliomas

•Glioblastomas

9.4.2 System Components

•5-aminolevulinic acid (5-ALA) is a photosensitive drug, which is converted to Protoporphyrin IX (PpIX) in malignant gliomas via an oral-intake of exogenous 5-ALA.

•Illumination of PpIX at an appropriate ultraviolet (UV) wave length (440 nm, nanometer) induces a visible (635 nm) red fuorescence.

9.4.3 Set Up

•5-ALA dose 20 mg/kg (Medac GmbH, Wedel, Germany) is administered orally, 5 to 6 hours prior to the estimated time of dural opening.

•Patients should avoid direct sunlight/bright external light exposure after taking 5-ALA and immediately postoperatively to avoid the rare complication of superfcial burns.

•The operating microscope should have been manufactured or adapted to emit ultraviolet light of wavelength 440 nm.

•During surgery, and under the UV light the tumor appears bright pink/red while the adjacent normal brain does not fuoresce (Figs. 9.8A, 9.8B).

9.4.4 Tips and Tricks

•The presence of even a thin layer of blood can obscure the fuorescence. A completely dry surgical feld is required for optimal visualization.

•5-ALA is more useful at the edges of malignant gliomas rather than in the necrotic center.

•Only parts of the tumor cavity with direct exposure to UV light will fuoresce. Extra care should be taken to inspect and illuminate all corners of the tumor cavity.

•The fact that a part of the tumor is fuorescent does not mean it cannot be functional. Knowledge of surgical and functional anatomy or cortical/subcortical mapping during awake procedures may be required in gliomas afecting eloquent areas.

•With experience, time spent under the UV will gradually increase as surgeons will be more comfortable operating in darker surgical feld.

•Vessels will not fuoresce under the blue light. When operating in this less well illuminated environment, extra care should be taken to avoid vascular injury.

9.5 Intraoperative MRI (iMRI)

9.5.1 Indications

•Glioma surgery (High-grade or low-grade gliomas).

•Pituitary surgery.

•Skull base surgery.

•Brain biopsies.

•Deep brain stimulation.

•Cyst drainage/ventricular surgery.

9.5.2 System Components

•Low feld magnets [<0.3T (Tesla)] are housed below the table and are raised during scanning, with minimal interruption to the surgical workfow. Surgical instruments and materials must be MRI compatible.

•High feld magnets (1.5T - 3.0T) lie within dedicated operating room (OR) suites. Surgery is performed using normal surgical instruments and anesthetic equipment in an area outside a 5G (Gauss) line (usually 4.2-4.5 m from the magnet) to avoid radiofrequency interference with the MRI.

•Neuronavigation is integrated in the iMRI software allowing integration of functional data (fMRI and DTI), if required.

9.5.3  Set Up Low-Field IMRI

•In low-feld iMRI the magnet is stored under the OR table and raised for imaging only, resulting in a fast, minor remodeling of the OR. Example is the PoleStarTM N-20 (Medtronic Navigation, Louisville, CO, USA) with a vertical gap of 27 cm.

•Major advantages of the low-feld iMRI systems is they do not require patient movement in and out of the MRI; there is no need for undraping and re-draping of patients; ofers closer to real time imaging and freedom of surgical access to the patient. In addition, the magnetic feld’s fall-of is rapid.

•In low-feld (<0.3T) iMRI systems, MRI compatible instruments and materials must be used.

9.5.4  Set Up High-Field IMRI

•High-feld iMRI systems (Fig. 9.9) are practically fully diagnostic MRI scanners providing diagnostic image quality and allowing acquisition of all MRI sequences with decreased susceptibility to radiofrequency interference (RFI).

•When not used during surgery, high-feld scanners can be used for diagnostic imaging of inpatients or outpatients.

•High-feld iMRI systems have demanding space requirements, requiring major remodeling of the OR with signifcant cost implications.

•High feld iMRI systems are, at present, either 1.5T or 3.0T and have a design featuring either a fxed MRI scanner with a moving operating room table (Fig. 9.9) or a fxed operating room table with a ceiling-mounted, mobile magnet that moves on tracts from one room used for diagnostic imaging to the operating room used for intraoperative imaging. Systems of the former design include the BrainLab intraoperative MRI (BrainLab AG, Feldkirchen, Germany) incorporating image-guided surgery with high-feld diagnostic MR imaging from diferent manufacturers and the Philips

Gyroscan (Philips Medical Systems, Best, The Netherlands) while systems of the later design include the mobile VISIUS

Surgical Theatre (IMRIS, Deerfeld, New York, USA).

•In high feld (1.5–3.0T) iMRI systems ferrous materials and instruments can be used provided that they remain outside the 5 Gauss (G) line (Fig. 9.10). The 5G line specifes a perimeter line the fux density of the static magnetic feld is strong enough, so that ferrous materials are at risk of developing kinetic properties and becoming a ballistic hazard. On highfeld MRIs, and according to the magnetic feld, the 5G-line is typically 4.2 to 4.5 meters from the gantry aperture.

•High-feld iMRI systems allow combination of neuronavigation systems with integration of functional data including fMRI, DTI, and trans-cranial magnetic stimulation data (TMS) (Fig. 9.11).

9.5.5Tips And Tricks

•iMRI cases are lengthy due to intraoperative preparation and scanning time. Patient selection is crucial to maximize benefts without underutilizing operative time.

•On average 1-2 intraoperative MRIs are required for tumor surgery with an approximate preparation and scanning time of 40 minutes per scan.

•Certain iMRI models have head fxation devices with minimal freedom of movement. Parietal and occipital pathologies cannot always be reached.

•Recent models allow more degree of freedom in head positioning. Certain models allow MR-compatible head-clamp.

Fig. 9.9 Setup of the iMRI suite at the National Hospital for Neurology and Neurosurgery, Queen Square, London, United Kingdom during an awake craniotomy for dominant temporal low grade glioma. The system features a wide-bore, high ld, 1.5T MRI and the Vector Vision version of BrainLab.

•Extra care should be taken during un-draping/re-draping cycles to minimize the risk of infection.

•Awake craniotomies on iMRI are possible (Figs. 9.9, 9.12) but require strict patient selection as in addition to being placed within the scanner, the awake patients have a coil placed close to their faces (Fig. 9.12).

•Expert anesthetic involvement is mandatory in awake resections on iMRI (Fig. 9.12).

•Care should be taken not to cross the 5G-line (Fig. 9.10). where surgical instruments and anesthetic equipment can acquire ballistic properties, and become hazard to patients, personnel, and the MRI magnet itself.

•iMRI is not a replacement for surgical skills, knowledge of anatomy, and operative technique. This statement applies for all technologies of intraoperative imaging.

Fig. 9.12 The patient is transferred from the operating table to the MRI table with the coil placed over his head and connected to MRI-compatible anesthetic equipment.

References

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2014;1(1):CD009685

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3.Hall WA, Truwit CL. Intraoperative MR-guided neurosurgery. J Magn Reson Imaging 2008;27(2):368–375

4.Wu JS, Gong X, Song YY, et al. 3.0-T intraoperative magnetic resonance imaging-guided resection in cerebral glioma surgery: interim analysis of a prospective, randomized, triple-blind, parallel-controlled trial. Neurosurgery 2014; 61(Suppl 1):145–154

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Cranial Approachesxxxx