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