Navigation in ENT (Image-Guided Surgery)

MS ENT University Exam - 20 Marks

Definition

• Surgical planning
• Surgical simulation
• Intra-operative image guidance
• Robotics and prosthetics design

"Image-guided surgery systems routinely used today share the same principle: to provide the surgeon with a real-time link between the preoperatively obtained image dataset and the surgical field." - Cummings Otolaryngology

Historical Background

• Use of X-ray images to guide surgeons dates to the mid-20th century
• Pioneering use of Cartesian coordinates to locate target points in 3D space predated computers
• Over 40 stereotactic apparatus developed by the 1950s
• Development of computers + digital imaging opened the field to include diagnosis, pre-operative planning, surgical simulation, intra-operative guidance, robotics, and telemedicine
• Modern IGS has been continuously developed since the early 1990s

Principle

• Accurately represents patient anatomy
• Can display skin, bone, muscles, tumour, vessels, and nerves
• Can be made transparent, semi-transparent, or colour-coded
• Multiple datasets (MRI fused to CT) can be superimposed

1. The preoperative image dataset (virtual world)
2. The actual surgical field (real world)

1. Registration - relates the patient in the operating theatre to pre-operatively acquired image data
2. Tracking - follows the position of the patient or an instrument within the operative field

Why ENT / Skull Base is Ideal for IGS

• Structures near the skull base are embedded in or closely adherent to bone
• CT has great spatial accuracy for bony anatomy
• Normal anatomy is frequently distorted by disease, trauma, or previous surgery
• Minimally invasive techniques (FESS) reduce direct visualization
• Critical adjacent structures (brain, orbit, optic nerve, carotid) must be avoided

Registration

Types of Registration:

Key Points about Registration:

• Registration error = accuracy of correlation between selected points in the virtual dataset and the fiducial/landmark on the patient
• Target error = error at any random point within the surgical field; influenced by registration error
• Points used for registration should be within or adjacent to the surgical field; targets outside the described volume have proportionately greater error
• Fiducials should be placed away from areas of skin movement (e.g., not on mobile skin) for accuracy

Tracking

Requirements of a Tracking System:

• Very precise and consistently accurate
• Fast: >2.5 readings per second
• Insensitive to changes in air temperature
• Unaffected by metal objects
• Able to track at least two objects simultaneously

Types of Tracking Systems:

• Fast and accurate
• Cumbersome, restricted range of movement, hindered tracked object movement
• Now obsolete

• Based on magnetic field distribution
• Relatively cheap and flexible
• No line-of-sight issues - ideal for ENT applications
• Disadvantage: tracking accuracy affected by metal objects (improving with newer technology)
• Latest EM devices are flexible - ideal for difficult areas like the frontal sinus

• Two subtypes:
  • Active devices: sense infrared light from LEDs attached to the patient/probe
  • Passive devices: detect infrared light reflected from metallic balls on patient/probe (infrared source on sensing device)
• Head reference attached via Mayfield clamp, halo, or arch with LEDs/reflective spheres
• Disadvantage: requires line of sight - easily interrupted
• Accurate if light path is not impeded

Indications for Navigation in ENT

1. Revision sinus surgery - distorted anatomy from scarring and previous operations
2. Extensive sinonasal polyposis - poor landmark visibility
3. Frontal sinus surgery - high-risk area near skull base and orbit
4. Skull base surgery - pituitary surgery, CSF leak repair, sella/clivus tumors
5. Complicated orbital/intracranial FESS - lamina papyracea involvement
6. Anatomical variants - dehiscent lamina papyracea, low-lying skull base, Onodi cells
7. Extensive/malignant sinonasal tumors
8. Iatrogenic complications - intraoperative orbital or intracranial injury identification

Note: Navigation is NOT required for routine uncomplicated FESS with normal anatomy.

• Revision sinus surgery
• Distorted sinus anatomy
• Extensive sinonasal polyposis
• Pathology involving frontal, posterior ethmoid, or sphenoid sinuses
• Disease abutting skull base, orbit, optic nerve, or carotid artery
• CSF rhinorrhea or skull base defects

Components of an IGS System

1. Preoperative imaging (CT/MRI scan acquired in special navigation protocol)
2. Workstation/computer with navigation software
3. Patient reference frame (head tracker - halo, headband, or bone-fixed)
4. Instrument tracker (tracked suction, probe, or instrument)
5. Tracking device (optical camera or electromagnetic field generator)
6. Display monitor showing three orthogonal views (axial, coronal, sagittal) or 3D reconstruction

Accuracy and Limitations

Accuracy:

• Clinical accuracy of modern systems: 1-2 mm
• Acceptable error threshold: generally <2 mm for sinus surgery

Limitations:

1. Brain shift / tissue shift - anatomy changes during surgery (blood, CSF egress, tissue removal) are not updated on the preoperative image = "anatomic drift"
2. Registration error - inaccurate placement of fiducials or landmark identification
3. Line-of-sight issues (optical systems)
4. Metal artifact interference (EM systems)
5. Cost - adds to procedure cost and setup time
6. Not a substitute for sound anatomical knowledge and surgical judgment
7. May give false security - experienced surgeon with good anatomy knowledge is still required

Applications in ENT Surgery

Augmented Reality and Future Directions

• Augmented reality systems superimpose 3D image data directly onto the surgical field (e.g., through the operating microscope)
• Future systems will likely replace traditional IGS with more immersive augmented reality
• Intraoperative CT/MRI will address the limitation of brain shift by updating image data in real-time
• Robotic systems will integrate navigation for more precise drilling/dissection
• IGS systems have capacity to store operative data - useful for auditing surgical procedures

Key Points Summary

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 1 - Chapter 48: Image-Guided Surgery, pp. 607-614
• Cummings Otolaryngology Head and Neck Surgery - Image-Guided Surgery section