Nuclear scintigraphy

Question 1

What molecule is used in nuclear scintigraphy?
What is the half life of this molecule?
What compound is this molecule bound to?

Answer 1

• Technetium-99m (99mTc) is the unstable (radioactive) form of the element technetium.
• 99mTc emits gamma radiation as it decays to 99Tc (half life 6 hours).
• 99mTc can be bonded by chelation to methylene diphosphate (MDP).

Question 2

What is the principle behind the mechanism of action and production of images by nuclear scintigraphy?

Answer 2

• MDP binds to hydroxyapatite crystals in bone during osteoblastic activity.
• MDP therefore binds to entire skeleton during normal bone turnover.
• Degree of uptake at a specific location determined by rate and extent of osteoblastic activity at that location.
• Diseased bone is undergoing greater osteoblastic activity as it tries to mend.
  → takes up more MDP (an necessarily more 99mTc)
  → more gamma radiation is emitted from that location
  → can be used to locate areas of the skeleton that are diseased

Question 3

What is the process of perfroming nuclear scintigraphy

Answer 3

• 99mTc-MDP is bought by practices from a radiopharmacy.
  → delivered in a lead vial immediately prior to the study
• Prior to injection the horse is exercised, rugged and has legs & feet wrapped.
  → causes peripheral vasodilation
  → protects limbs from urine splash (MDP is primarily excreted in urine)
• 99mTc-MDP is injected IV through a catheter using a lead covered syringe.
• Careful radiation safety protocols must be followed.

Question 4

How/when are images aquired for nuclear scintigarphy?

Answer 4

• Bone phase image acquisition begins 2 hours after injection.
• A gamma camera is used to detect emitted gamma radiation.
• A thick lead collimator is covers the surface of the camera.
  → many small straight holes
  → only gamma rays perpendicular to the camera are detected.
• An anatomic image is created in the imaging computer.
• Whole body or neck & forelimbs or neck, back, pelvis, hind limbs (takes several hours)

Question 5

What happens to a horse after a nuclear scintigraphy study?

Answer 5

• MDP is primarily excreted in urine
• Isolation for 36-48 hrs
• Bedding allowed to decay further before disposal

Question 6

What are the indication for nuclear scintigraphy?

Answer 6

Not a replacement for a good clinical examination and perineural anaesthesia

• When regional anaesthesia has failed to locate the source of pain.
• Multi-limb lameness.
• Suspected back and pelvis problems when radiography is not possible.
• To confirm / reject a suspected diagnosis and look for concurrent pathology.
• Where a fracture is suspected.
• When lameness is intermittent / horse just underperforming.

Question 7

How do you interpret nuclear scintigraphy images?

Answer 7

• Looking for areas emitting more gamma radiation than expected.
• Compare left v right. (might be differnt between horses but should be symetrical on the horse)
• Knowledge of ‘normal’.
• Different colour schemes exist.
• Compare total counts within ‘regions of interest’

Question 8

These are nucelar scintigraphy images from a horses hind limbs? describe what you can see and suggest a suitable follow up treatment

Answer 8

There has been more uptake of Technetium in the tarsal metatarsal joint of the left leg than the right, suggessting this is where the issue is.
Next step would be to radiograph both of the tarsus to compare the two and then to ultrasound the soft tissue structures in this area to look at the origin of the susspensory ligament.