Nuclear scintigraphy

• Technetium-99m (99mTc) is the unstable (radioactive) form of the element technetium which emits gamma radiation as it decays to 99 Tc (a short half-life of 6 hours).
• 99mTc can be bonded by chelation to methylene diphosphate (MDP). This MDP binds preferentially to hydroxyapatite crystals in bone during osteoblastic activity and therefore binds to the entire skeleton during normal bone turnover. The degree of uptake at a specific location is determined by the rate and extent of osteoblastic activity at that location.

  • Diseased bone is undergoing greater osteoblastic activity as it tries to mend. This means it takes up more MDP and more 99mTc so more gamma radiation is emitted from that location.
• 99mTc-MDP is bought by practices from a radiopharmacy and it is delivered in a lead vial immediately prior to the study. Prior to injection, the horse is exercised, rugged and has its legs and feet wrapped. This causes peripheral vasodilation and protects limbs from urine splash (MDP is primarily excreted in urine so the horse must be isolated for 36 to 48 hours after the procedure). 99m Tc-MDP is then injected IV through a catheter using a lead covered syringe.
• Bone phase image acquisition begins 2 hours after injection where 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 consisting of the whole body / neck and forelimbs / neck, back, pelvis, hind limbs. The horse then enters isolation (for 36 to 48 hours) and the bedding is allowed to decay further before disposal.
• indications are…

  • 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.
• For image interpretation, you are looking for areas emitting more gamma radiation than expected.

  • Compare left to right.
  • Have good knowledge of normal
  • Compare total counts within ‘regions of interest’

  Different colour schemes exist.