Intraoperative Imaging

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 reflecting spheres attached to the instruments and reference frame (Fig. 9.1).

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

Fig. 9.1 Example of the Polaris type localizer (NDI, Northern Digital Inc., Bakersfield, CA, USA) with the Medtronic StealthStation S7^® Navigation System (Medtronic, Minneapolis, MN, USA). Note that patient’s head is fixed rigidly in the Mayfield clamp, and to this is attached the blue reference frame (aka “star”) with retroflective spheres.

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 fixed 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).

• Efforts should be made to ensure that the star housing the reflective spheres is not too far from the patient (usually a fist’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 fiducials (Fig. 9.1), performing surface matching without fiducials, or contact-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).

9.1.4 Tips And Tricks

• Interruptions in tracking are often due to reflective 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 fixed head position.

• Endoscopic ventricular navigation.

Fig. 9.2 Contact-free, laser, surface-match registration using Z-touch^® with the Brain-Lab 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 patient’s head is not more than a fist’s width. The brown adhesive tape fixing the patient’s tracker is seen on the right side of the patient’s forehead.

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).

• Registration is performed with fiducials 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 fixed 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 affecting the registration.