Bone scintigraphy
Vascular phase:
• Perfusion defects result in little to no
radiopharmaceutical uptake at the level of the affected area; the diagnosis is usually straightforward.
Soft tissue (blood pool) phase:
• IRU in the soft tissue phase is passive and may be due to increased blood flow or to expansion of the
extracellular fluid spaces.
• Increased soft tissue uptake is almost always significant; however, a normal blood pool distribution does not rule out the presence of soft tissue pathology.
• Soft tissue phase images are best used to detect active (acute) inflammation and, in combination with vascular and bone phase images, to differentiate between soft tissue and bone infection.
Bone phase:
• There are five main differential diagnoses for an area of marked IRU: neoplasia, fracture, infection,
enthesopathy and subchondral erosion.
• Localization of the area(s) of IRU within the skeleton, in conjunction with patient signalment, history and the results of any ancillary tests, will aid in the ranking of the differential diagnoses:
• Primary bone tumours most commonly result in areas of marked IRU at the level of the metaphyses of the distal radius, proximal humerus, distal femur and proximal tibia, but can occur in other locations (Figure 5.4)
• Bone metastases are areas of IRU that can be located anywhere in the body, but are often
associated with the diaphyses of long bones, ribs or vertebrae (Figure 5.5)
• It should be noted that neoplasms that are primarily or exclusively lytic are not readily identified using bone scintigraphy, because they lack osteoblastic activity; multiple myeloma is a typical example
• The sensitivity of bone phase scintigraphy to detect acute fractures is high: in humans, 80% of patients with fractures have a positive bone scan by 24 hours, and 95% have a positive bone scan by 72 hours.
However, the intensity and pattern of uptake is similar for acute traumatic fractures, pathological fracture, healing fractures and malignant processes, highlighting the lack of specificity of bone scintigraphy
• Three-phase bone scintigraphy can aid in
differentiating between soft tissue and bone infection.
With cellulitis, uptake will be mildly increased in the vascular phase, moderately to markedly increased in the soft tissue phase, and normal in the bone phase.
With osteomyelitis, uptake will be moderately
increased in both the vascular and soft tissue phases, and will be markedly increased in the bone phase
• Further evaluation of inflammation/infection can be carried out using radiolabelled leucocytes, 111InCl3 or
67Ga citrate. These techniques have been rarely used in veterinary medicine and have several limitations, including poor labelling efficiency (leucocyte imaging), and lack of specificity in differentiating between inflammatory and infectious lesions.
• Degenerative joint disease results in mild to moderate IRU in the subchondral bone on both sides of the affected joint. Soft tissue phase images may reveal mildly increased soft tissue uptake around the joint if synovitis (especially acute synovitis) is also present.
• Scintigraphy has been used for the diagnosis of medial coronoid disease; it can be especially helpful in older dogs where clinical and radiographic changes may be ambiguous. Abnormal medial coronoid uptake is characterized by a focal area of IRU at the proximal ulna, superimposed over the proximal radial physis on the lateral view (Figure 5.6).
Bone phase lateral image of the right femur in an 11-year-old female spayed Husky obtained using 99mTc-MDP. There is marked IRU along the diaphysis of the femur, more intense along the cortical margin; the final diagnosis was femoral osteosarcoma. Notice normal activity in the urinary bladder, the normal route of excretion for bone-seeking radiopharmaceuticals.
5.4
Bone phase lateral images of the (a) right and (b) left scapulae, humeri and elbows of the same dog as in Figure 5.4.
There are focal areas of marked IRU at the level of the proximal diaphysis of both humeri: radiographs only revealed mildly increased medullary opacity at these levels. Both sites were confirmed to be
osteosarcoma metastases.
5.5
(a) (b)
Bone phase images of the elbows of a 12-month-old male neutered Labrador Retriever with waxing and waning multiple limb lameness. (a) Lateral and (b) caudal views of both elbows. There is IRU in the proximal left radius/ulna (right side of (a)). It has a semilunar shape which differs from the pattern of uptake at the level of the normal right proximal radial physis. On the caudal view, the left limb IRU (left side of (b)) is more intense medially. The final diagnosis was disease of the medial coronoid process of the left elbow.
5.6 (a)
(b)
• Extraskeletal uptake during bone phase imaging can be seen in cases of calcinosis cutis, dystrophic mineraliza-tion, pulmonary mineralizamineraliza-tion, rhabdomyolysis and osteosarcoma metastases. Uptake in regional lymph nodes can be seen after perivascular radiopharmaceuti-cal extravasation. Thyroid and salivary uptake is visible when free pertechnetate is present, secondary to oxidation of 99mTc-HDP/MDP, and liver uptake can be seen with the presence of colloidal 99mTc, due to the presence of moisture in the radiopharmaceutical vial.
Extremity lymphoscintigraphy
• Lymphoedema results in abnormal drainage and slow absorption of 99mTc-SC from the injection site.
• Lymphoedema can be primary, due to abnormal development of lymphatic vessels and/or lymph nodes, or secondary, usually due to neoplasia, trauma, surgery, infection or radiation therapy affecting the lymphatic channels and lymph nodes. Differentiation between primary and secondary lymphoedema is not always possible because there is some overlap of imaging abnormalities. However, with primary
lymphoedema there is usually diminished visualization of lymphatic vessels and lymph nodes; secondary lymphoedema is characterized by dilated and tortuous lymphatic vessels, the presence of collateral vessels and interstitial retention of the radiopharmaceutical (Figure 5.7).
(a)
Lymphoscintigraphy using
99mTc-SC in a 4-year-old Irish Wol ound with a 2-month history of left pelvic limb swelling.
Lateral images of (a) the right and (b) the left pelvic limbs. Radioactive 57Co markers (‘string of pearls’
appearance) placed near the body were used to identify key anatomical locations (tarsus, ischium, popliteal lymph node (PLN)). There is normal uptake of radiopharmaceutical in (a) the right pelvic limb, with normal visualization of the PLN in the last two (i.e. 25- and 30-minute) images.
(continues) 5.7
(continued) In the left pelvic limb (b) absorption is abnormal, with lack of visualization of normal lymphatic channels to the level of the PLN, and abundant interstitial activity. The popliteal and medial iliac lymph nodes are visible on both sides. The final diagnosis was secondary lymphoedema of unknown cause.
5.7
(b)
References and further reading
Berry CR, Ackerman N and Monce K (1994) Pulmonary mineralizations in four dogs with Cushing’s syndrome. Veterinary Radiology 35, 10–16
Daniel GB, Avenell JS, Young BS et al. (1996) Scintigraphic detection of subcutaneous metastasis in a dog with appendicular osteosarcoma. Veterinary Radiology and Ultrasound 37, 146–149
Lamb CR (1990) Non-skeletal distribution of bone-seeking radiopharma-ceuticals. Veterinary Radiology 31, 246–253
Poteet BA (2006) Small animal skeletal scintigraphy. In: Textbook of Veterinary Nuclear Medicine, 2nd edn, ed. GB Daniel and CR Berry, pp.143–164. American College of Veterinary Radiology, Harrisburg
Steyn PF and Uhrig J (2005) The role of protective clothing in reducing radiation exposure rates to personnel during equine bone scintigraphy. Veterinary Radiology and Ultrasound 46, 529–532
Tucker RL and Broome M 00 Scintigraphic imaging of inflammation and infection. In: Textbook of Veterinary Nuclear Medicine, 2nd edn, ed. GB Daniel and CR Berry, pp. 363–376. American College of Veterinary Radiology, Harrisburg
Van Bruggen LWL, Hazewinkel HAW, Wolschrijn CF et al. (2010) Bone scintigraphy for the diagnosis of an abnormal medial coronoid process in dogs.
Veterinary Radiology and Ultrasound 51, 344–348
Wisner ER, Daniel GB and Hornof WJ (2006) Lymphoscintigraphy. In: Textbook of Veterinary Nuclear Medicine, 2nd edn, ed. GB Daniel and CR Berry, pp. 353–
361. American College of Veterinary Radiology, Harrisburg
oung AM 013 Dose rates in nuclear medicine and the effectiveness of lead aprons: updating the department’s knowledge on old and new procedures.
Nuclear Medicine Communications 34, 254–264