Nuclear Medicine Bone Scans Part 3
Part 3 review of nuclear medicine bone scans, with focus on triple-phase bone scans, for radiology and nuclear medicine board review. Download the free study guide for this website by clicking here.
Study Guide/Show Notes:
What are classic imaging findings that can be seen on a bone scan for patients who are on bisphosphonate therapy?
The first thing I would remember is a proximal femur fracture, sometimes termed a “bisphosphonate fracture” or even more specifically a “bisphosphonate-related proximal femoral fracture”. On a bone scan, this would manifest as an area of focal uptake in the proximal femur that involves, often exclusively, the lateral femoral cortex of the proximal femoral diaphysis, located distal to the lesser trochanter. This can be unilateral or bilateral. Note that this is an insufficiency-type fracture and is associated with delayed fracture healing and has a risk for progression to complete fracture. When this is seen, correlation with radiographs of both femurs is often helpful, and would be predicted to show a predominantly simple horizontal fracture, classically non-comminuted. No associated mass on imaging or history of major trauma is expected in a bisphosphonate-related proximal femur fracture. Classic history would be an older woman with osteoporosis. Treatment often involves withholding further bisphosphonate therapy and surgical management to prevent completion of the fracture and promote healing.
Second, I would remember bisphosphonate-related osteonecrosis of the jaw. This is development of exposed bone with osteonecrosis in the maxillofacial region in patients on typically longer-term bisphosphonate therapy. Note that biopsy of the bone for diagnosis can worsen the process, and diagnosis is often clinical and imaging only. On a bone scan, expect abnormal, focal, often intense uptake in the jaw region. Note that bone scan is more sensitive than radiography in the early diagnosis of bisphosphonate-related osteonecrosis of the jaw.
What are key differences of physiologic uptake on a bone scan of a child or teenager compared to a physically mature adult?
Children and young adults show intense activity in the physes secondary to bone growth with associated osteoblastic, bone-building activity that is not seen in a physically mature adult. Normal physiologic uptake can also be more pronounced in children and young adults in the sternum, sacroiliac joints, articular surfaces, and nasopharynx.
What are the three phases of a three-phase bone scan?
First is evaluation of blood flow, second is evaluation of blood pool, and third is delayed bone imaging.
In the first phase, imaging immediately following injection of the radiotracer is performed to evaluate for increased blood flow. This is a dynamic phase that usually requires positioning the patient in front of the camera first, and then immediately starting imaging at time of injection, taking images every few seconds over about 60 seconds. The second phase is blood pool, in which imaging is performed after the blood flow phase, over several minutes duration to evaluate for abnormal radiotracer accumulation in the soft tissues. Finally, about 2-4 hours after injection, delayed phase images are obtained to evaluate for abnormal radiotracer accumulation in the bone itself.
Think about the first two phases as telling you what is going on in the soft tissues around the bone in terms of infection or inflammation and the delayed phase telling you if bone pathology is present.
Often, a three-phase bone scan is performed to evaluate a clinical or imaging finding of concern, and imaging is focused over a certain region of the body only. However, it may not be unusual in cancer patients particularly to obtain the initial imaging in all three phases over a specific area of concern, and then to also get whole body delayed imaging to evaluate the entire skeleton for metastases or other abnormalities.
Finally, SPECT/CT fused images may be obtained at the area(s) of concern on delayed imaging, as clinically directed.
What are general findings of infection on a three-phase bone scan?
First, look for asymmetry. Infection or inflammation will show asymmetric uptake due to increased asymmetric blood flow and blood pool due to inflammation in the affected tissues. Infection in the bone will also show abnormal delayed radiotracer uptake on the delayed phase of imaging.
How can one differentiate cellulitis from osteomyelitis on a three-phase bone scan?
Cellulitis demonstrates increased uptake on a three-phase bone on the flow and blood pool phase but shows no abnormal uptake on the delayed phase. To explain further, cellulitis is a soft tissue infection associated with increased blood flow and inflammation in the soft tissues, but no bone involvement. Therefore, increased uptake is expected on blood flow and blood pool imaging but, importantly, not delayed imaging.
On the other hand, osteomyelitis is infection of the bone, and has inflammation of the surrounding soft tissues, and therefore is expected in classic cases to show uptake on all three phases. Remember that osteomyelitis can be detected on bone scan several days to weeks before changes will be seen on a radiograph and the sensitivity of a bone scan for osteomyelitis is something like 95%. Note that blood flow and blood pool phases should resolve within months of treatment of osteomyelitis, but abnormal bone uptake may persist for something like 2 years.
Remember that osteomyelitis is three-phase hot on a bone scan. Cellulitis is only hot on the first two phases (i.e., blood flow and blood pool).
What are classic imaging findings of complex regional pain syndrome/reflex sympathetic dystrophy on a three-phase bone scan?
Increased blood flow and blood pool activity and delayed activity in a juxta-articular distribution, which is most classically shown in a hand on a bone scan on board exams. This is essentially an Aunt Minnie finding so make sure and look up the imaging appearance of complex regional pain syndrome on a bone scan if you don’t know what this looks like. Remember that clinical symptoms of complex regional pain syndrome include pain, swelling and vasomotor instability that is often post-traumatic in etiology.
How can one differentiate orthopedic hardware infection versus loosening on a bone scan?
Infection of hardware would typically show a more generalized increase in activity around the prosthesis whereas loosening without infection would show a more localized, focal area of uptake, particularly on blood flow and blood pool imaging. This rule is not perfect but in general, expect more intense and diffuse abnormality around the prosthesis with infection, and a more localized, focal area of uptake, often around the more distal stem of a prosthesis, with aseptic loosening.
Name several abnormalities that are three-phase positive on a bone scan?
Acute fracture, osteomyelitis, complex regional pain syndrome, inflammatory arthritides, orthopedic hardware infection or loosening, aggressive and vascularized tumors, and the immature phase of myositis ossificans, and osteoid osteoma.
What are two classic entities to keep in mind on board exams for something that is only two-phase (i.e., blood flow and blood pool) positive on a bone scan?
First, consider cellulitis of the soft tissues. Second, consider a soft tissue malignancy without bone involvement.
What are classic entities to keep in mind for board exams for things that are only hot on the delayed phase of a three-phase bone scan?
When blood flow and blood pool are normal but delayed images are positive on a three-phase bone scan consider osteoblastic metastases, metabolic bone diseases, non-acute fractures, and medial tibial stress syndrome/shin splints.
What are classic findings on a bone scan for medial tibial stress syndrome/shin splints?
Longitudinal uptake along the posterior and medial aspect of the tibia, often best depicted on lateral views, with classically normal uptake on blood flow and blood pool images. Remember that if you also see early phase uptake this suggests possible stress fracture which may be acute which is a known complication of medial tibial stress syndrome if stress from running is excessive.
If radiotracer fails to adequately clear from the soft tissues on the delayed phase of a bone scan, what classic entity should you consider first? What should you do about this?
Renal failure. Poor renal function limits the clearance of radiotracer from the soft tissues of the body. If this happens, obtain delayed imaging up to 24-hours after injection to allow more time for radiotracer to clear from the soft tissues, to improve signal to noise. Given lower counts that will be present in the body due to radiotracer decay, a longer imaging acquisition time can be helpful when extended delays in imaging are necessary.
Some refer to a delayed 24-hour imaging timepoint as the “4th phase” of a bone scan. Another scenario where a delayed 24-hour imaging timepoint may be helpful is in a neuropathic foot where peripheral blood flow may be reduced, and one needs to evaluate for cellulitis versus osteomyelitis for which radiotracer clearance from the soft tissues is helpful to improve signal-to-noise of the bone itself.
