Role of intraoperative imaging in endoscopic skull base surgery

Introduction:
Advances in nasal endoscopic surgery have prompted the surgeon to explore hither to unexplored vistas, like skull base surgery. It has been proved that endoscopic skull base surgery has helped in the reduction of morbidity and mortality of classical craniofacial approaches. Technologically superior endoscopes with wide field of vision, better illumination, camera and monitors have improved the success rate of these surgical procedures. These rigid endoscopes despite their obvious advantages provides only a 2 dimensional view of a complex 3 dimensional space. Image guided surgery was popularized in order to provide a certain degree of 3 dimensional view. Image guided surgery is heavily dependent on preoperative image data collection, resulting in a scenario of non visualization of the anatomic manipulation caused by the surgical procedure. This is where intraop imaging score over conventional image guided surgery because of its real time imaging ability. The operating surgeon will be virtually seeing the imaging of his surgical procedure.

Advantages of intraop imaging:

1.Provides real time images of the surgical field
2.These images can be fed into the image guidance system for better visualization of the surgical field. In other words it gives real time updates for image guided surgery.
3.Intra op imaging may help in performing extensive skull base surgeries using an endoscope.
4.It may be useful in managing complex craniofacial fractures


Equipment specification for intrao op imaging:

1.Equipment for image acquisition
2.Equipment for transferring the acquired image
3.Equipement for viewing and manipulation of acquired images
4.Software necessary to upload these real time images to the navigation system
5.A computer workstation to make all these things possible

Intra op imaging equipment should ideally be:

1.Portable
2.Capable of rapid image acquisition
3.Compatible with already commercially available image guidance systems
4.CT scanner / MRI scanner can be used for intra op imaging. CT scanner is preferrable because of its rapid scanning time, and excellent bone details which it provides.

Role of Cone Beam CT in intra op imaging:

This is slowly replacing conventional in office x-ray machines. It is very useful for rapid diagnosis of sino nasal and otological disorders. This CT machine permits imaging of the structure in question within single rotation. It uses the technique of volumetric tomography. Other major advantages of this imaging modality is reduced time of scanning and reduced exposure of the patient to harmful effects of irradiation. It is also comparatively less expensive than MRI scanners. These devices are highly portable and can be effortlessly wheeled into the operating room.

Image showing portable CT scanner

Three dimensional fluroscopy:

This is the other popular method of intra op imaging. It is highly portable and maneuverable. Patient is placed between the fluroscope and image intensifier. The image acquired is viewed on a portable workstation monitor. Softwares are available for converting the 2 dimensional image data acquired to 3 dimensional ones.

It should always be borne in mind that intra op imaging at the most provides complimentary information to the endoscopic portion of the surgical procedure.

Limitations of intra op imaging:

Are nothing but limitations of Cone Beam CT scanning. Due to technological limitations there is some loss in the quality of soft tissue imaging. Infact fluid / blood within the sinus cavity may be confused with that of residual mass / polyp. Hence intra op imaging should always be interpreted in conjuction with the endoscopic vision.