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Fiducials: Achilles heel of image guided neurosurgery
An attempt at indigenization & improvement
Dr Deepak Agrawal
Background
• Image-guided neurosurgery is becoming more popular
• It increases accuracy and …
• Improves the quality of neurosurgical output in a centre
• A step towards consistency in surgical results
• (References to be put)
Limitations
• High initial purchase cost
• Learning curve
• Cost of disposables (fiducials) and MR/CT
• High Initial purchase cost
• Initial systems cost 2-4 crores
• AIIMS was the first centre to have IG system (Stealthstation) in India ?1999
• Now a image guidance system can be purchased for RS 80 lakhs
• Learning curve
• Systems are getting more user friendly
• Initially added 2-3hrs to the operating time
• With experience, only 15-20 minutes are needed to set up the system
• Cost of diposables
• Propriety item.
• In stealthstation 10 disposable fiducials are used
• Literature shows less tha 8-10 fiducials may cause Incr errors
• Set of 10 fiducials costs $100 (approx Rs 4000)
• Disposable fiducials can be reused upto a max of 2-3 times
• Problems may arise in registration due to movement of scalp
Process of image guided surgery
• Fiducials are applied to the patients scalp
• Ideally should be around the site of pathology
• May need to shave the hair
Thin Slice CT/MRI is done with the fiducials in place according to prtocol
1mm slices starting from the hard palate in
Read
• The CT/MRI may be contrast or plain depending on the pathology
• The images are saved on a CD in the standard DICOM format
• CD loaded on the stealthstation wokstation (inside the OT, just prior to surgery)
• Loaded images checked for orientation & windowing
• The fiducials are marked visually
• Following intubation, the patient is positioned & head fixed rigidly on mayfield clamp
• The actual fiducials are then matched with those marked on the MR/CT images
• This gives a 3D localisation of any point in space in the patients brain
• Sources of error
• Wrong marking on CT/MR
• Errors during reg due to movement of scalp & affixed fiducial
• Head fixation not rigid
• Aim of this study:
• To indigenize the fiducials for use in image-guided neurosurgery
• To improve upon the accuracy obtained using conventional fiducials
• Prerequisites for a potential fiducial• Cheap and easily available• Can be used with both CT/MRI• Good contrast ratio with scalp• Easy & good fixation to scalp• Easy to register with stealthstation• Should allow mm accuracy during
registration
• Copper sulphate paste
• Filled into syringe and applied wet to the scalp
• Allowed to dry
• Rubber caps from vials filled with copper sulphate
• Poor contrast ratio with surroundings
• Difficult to stick to scalp
• Vitamin E capsules
• Good contrast ratio (?Fat content)
• Potential problem: Being curved,no identifiable point to register
• Tried making a hole in it-vitamin leaked out
• Vitamin E capsule
• Easier to fix to scalp
• Cleaned the area with Tinc Benzoin
• Put the capsule & taped it with leukoplast/dynapore
• Being compact, whole head shave not required
• Vitamin E (Registration)
• As expected, registration difficult on scalp as no identifiable point present.
• For want of a nail the shoe was lost, For want of a shoe the horse was lost, For want of a horse the rider was lost, For want of a rider the battle was lost, For want of a battle the Kingdom was lost, And all for the want of a horseshoe nail.
• Attributed to Benjamin Franklin
• Vitamin E (Registration)
• Decided to mark the tips of the capsule instead of a point on the scalp
• Registration using this method yielded overall accuracy < 1.5 mm
• Improved accuracy could be attributed to decreased scalp movement
• Only 5 capsules were required
• (each tip on either end serves as a fiducial)
• Total cost comes to Rs 5
• As a backup measure 5 additional capsules may be applied
• Conclusions
• Vitamin E capsules serve as excellent fiducials for image guided neurosurgery
• Save more than 99% costs on the disposables
• Improve accuracy of the procedure as scalp movement is diminshed
• Representative cases (1-2)
• Indigenization of fiducials in Image-guided neurosurgery