Bone: scintigraphy technique in horses | Vetlexicon Equis from Vetlexicon

Introduction

Bone scintigraphy is a non-invasive imaging modality that is indicated in a wide variety of skeletal problems because of its high sensitivity, and the ease with which the entire skeleton can be imaged.
Images obtained represent physiology in contrast to morphology.
Bony lesions are associated with increased local metabolism of calcium and osteoblastic activity which is demonstrated by increased uptake of a suitable tracer.
The changes visualized represent increased new bone turnover and/or increased bone blood flow.
Factors influencing rate of uptake:
- Quantity of mineralized bone.
- Capillary permeability.
- Local acid base balance.
- Fluid pressure within bones.
- Hormones.
- Dehydration.
Radiopharmaceutical:
- The radiopharmaceutical used are 99mTc-Diphosphonates.
- This is the radionuclide technetium -99m which emits gamma rays of the energy 140 KeV and has a half life of 6 h to which is attached a diphosphonate. The most commonly used are methylene diphosphate (MDP) and hydroxymethylene diphosphate (HDP).
- Diphosphonates are stable in vivo, having their phosphorus bound to carbon rather than oxygen and are therefore rapidly cleared from the soft tissue; because of rapid renal excretion of the diphosphonate they have a high bone to soft tissue ratio by 2-3 h after injection.
- 50% to 60% of the injected dose localizes in bone the remainder being excreted by the kidneys.
The three phases of a bone scan:
- Phase I: an initial dynamic flow study (vascular phase) with rapid images 0-1 min following injection.
- Phase II: the soft tissue image phase at 5-15 min when the tracer is still within the vascular compartment.
- Phase III: static bone phase images at 2-4 h.
The detector system:
- The detector can be a probe based system, or a gamma camera which consists of a sodium iodide crystal, which when exposed to gamma rays produces light flashes which are detected by a photomultiplier tube behind the crystal. This then emits an electrical pulse which is monitored by electronic counting circuitry.

Print off the Owner factsheet on Diagnostic imaging to give to your clients. Clinical tip
Question: Can scintigraphy help my diagnosis?
Answer: Yes; nuclear bone imaging is very sensitive to localizing and quantifying dynamic bone change, before changes are demonstrable by radiography.

Uses

General:
- Acute onset lameness with no localizing signs, if a fracture is suspected.
- Chronic lameness with multi limb involvement.
- Back or pelvic problem Musculoskeletal: back pain .
- Equivocal radiographic findings.
- Equivocal nerve blocks.
- Damaged skeletal muscle, eg rhabdomyolysis Muscle: myopathy - exertional rhabdomyolysis .
- Unable to perform nerve blocks due to horse's dangerous behavior or spontaneous resolution of lameness.
- Measure glomerular filtration.
Vascular phase:
- To evaluate blood flow to an area following degloving injuries.
- Aortoilliac thrombosis.
- Vascular infarcts.
- Detecting inflammatory processes.
Soft tisssue phase:
- Tendon/ligament injuries.
- Synovitis, eg chronic proliferative synovitis MCP / MTP joint: chronic proliferative synovitis , digital sheath tenosynovitis Digital sheath: tenosynovitis , carpal sheath tenosynovitis Carpus: sheath tenosynovitis , thoroughpin Tarsus: tarsal sheath tenosynovitis , bog spavin Tarsus: tarsocrural synovitis (bog spavin) , bone spavin Tarsus: osteoarthritis (bone spavin) .
- Osteomyelitis, eg septic osteitis Bone: osteitis - septic , septic arthritis Joint: septic arthritis - adult .
Bone phase:
- Fractures.
- Degenerative joint disease (DJD) Musculoskeletal: osteoarthritis (joint disease) .
- Periosteal reactions, eg dorsal metacarpal disease MC / MT 3: dorsal metacarpal disease .
- Enthesopathies.
- Myositis (altered muscle metabolism causes retention of the diphosphonate).

Advantages

Nuclear bone imaging is very sensitive to localizing and quantifying dynamic bone change, before changes are demonstrable by radiography.