Gamma scintigraphy (bone scan)
The horse has 205 bones in its body. All of these 205 bones and related structures have to work in harmony for a horse to perform to its maximum ability in its chosen profession. There will be a number of times in a horses life where, for some reason the fluid relationship between the musculo-skeletal system is broken and the horse becomes lame. This lameness can sometimes be easily identified and localized by regional anaesthesia (nerve blocks). But occasionally due to a multitude of reasons from the horse being lame in multiple legs or the lameness has been identified above the region where vets can routinely nerve block, to the horses temperament, a bone scan or nuclear scintigraphic examination may be indicated.
The procedure involves injecting the horse with a radioactive isotope (technetium 99m). This isotope is bound to a carrier (methylene diphosphonate) which attaches to calcium crystals (hydroxyapetite) in bone and shows areas of inflammation and increased bone turnover i.e. remodeling of bone due to injury or increased biomechanical loads. Areas of increased uptake of the isotope are then detected by a purpose built gamma camera on a hydraulic arm.
Nuclear scintigraphy can be used in three diagnostic forms:
- The Vascular Phase: Images are acquired as the isotope is injected. This technique is highly useful for confirming vascular obstruction e.g. in frostbite but has limited applications in horses, e.g. aortoiliac thrombosis.
- Soft Tissue or Pool Phase: The horse is scanned sequentially for up to 30 minutes after injection of the isotope. This type of scan can be highly useful in identifying ligament damage e.g. to the origin of the suspensory ligament. However, the results are often interpreted in light of the bone phase images as early bone uptake will show up on soft tissue phase images. In addition, because of the very limited time window, this type of scan is usually restricted to one site.
- Bone Phase: This is the most common use for gamma scintigraphy. The horse is scanned two hours after injection and a whole body is scanned. During this examination, the horse is sedated to minimize movement whilst the scan is being acquired which would result in blurring of the image. In addition, this also helps minimize any reaction to the sound and movement of the hydraulic stand and camera. Bone phase images often pick up subtle changes causing subclinical lameness. Common examples would be bilateral increased fore feet uptake suggesting subclinical foot pain, which may manifest itself as poor performance or unwillingness to jump down hills. In addition, it is invaluable for the diagnosis of upper limb problems e.g. pelvic fractures.
The clinic performs about 400 whole body bone scans a year. Preparation of the patient prior to the bone scan is vital in producing a diagnostic scan, as the absorption of the isotope is significantly affected by perfusion (blood flow), particularly in the lower legs. Horses need to come in the day before the appointment to ensure optimal blood flow at time of injection of the isotope. In addition, in accordance with radiation safety legislation, the horse must stay for two days after the scan, to allow the radiation to decline.
Following a bone scan, additional diagnostic test e.g. radiographs, ultrasound or nerve blocks may be recommended to confirm the diagnosis seen on the bone scan. In addition, in cases where more than one hot spot occurs in the same leg, supplemental nerve blocks may also help to determine the importance of these findings.