Nuclear Medicine Bone Scans Part 1

Review of nuclear medicine bone scans for board exam preparation. Upon posting all parts of this series on nuclear medicine bone scans a free downloadable study guide will be available at theradiologyreview.com.  Prepare to succeed. 

Show Notes/Study Guide:

What are the two main varieties of bone scans in nuclear medicine?

Tc-99m-MDP (methylene diphosphate) (planar and SPECT or SPECT/CT imaging) and F18 sodium fluoride (PET imaging). Perhaps due to payment coverage of expensive PET scans, F18 sodium fluoride is less commonly used than Tc99m-MDP. A F18 sodium fluoride PET bone scan is overall more sensitive for detection of osseous metastases than a Tc99m-MDP bone scan, but also shows more uptake with reactive processes such as degenerative change with bone remodeling.

What is the critical organ for Tc-99m MDP and F18 sodium fluoride?

Remember the critical organ is the organ that will receive the highest dose from the systemic IV injection of a radiotracer.  For Tc-99m MDP this is the bone itself.  For F18 sodium fluoride it is the urinary bladder.

Does a bone scan show uptake more frequently in blastic or lytic osseous metastases?

Bone scans are more sensitive for, and more frequently show abnormal uptake, with blastic osseous metastases.  While bone scans can also show abnormal uptake with a lytic osseous metastasis, bone scans are less sensitive for lytic metastases compared to blastic metastases.  Remember that FDG-PET/CT is more sensitive for lytic osseous metastases.  Therefore, neither FDG-PET/CT nor a bone scan can entirely exclude presence of osseous metastatic disease, and one exam may be preferred to the other depending on whether osseous metastases are more likely to be blastic (use MDP/NaF) or lytic (use FDG). When considering multiple myeloma, where lytic metastases are expected, both FDG-PET/CT and a radiographic bone survey will be more sensitive for disease detection compared to a bone scan.

If no renal uptake or soft tissue uptake is seen on a Tc-99m MDP bone scan, this is classic on radiology board exams for what process?

A so-called “superscan” denoting diffuse osseous metastatic disease.  The rationale is that one should normally see soft tissue uptake (such as mild uptake in in the soft tissues of the extremities) as well as renal excretion.  If neither soft tissue uptake nor renal excretion/renal uptake of radiotracer is seen, this is abnormal, and the likely mechanism is that there is so much diffuse osseous metastatic disease, and robust bone uptake (superscan named as such because bone visualized super well) that all radiotracer is going to the bone, and none is left over to be excreted by the kidneys or taken up by the soft tissue of the body.  This is a highly tested concept, so it is imperative that you know how to identify a superscan. The tricky thing is that the bones can, in certain cases, look essentially normal on a superscan if the metastatic disease is truly uniform and diffuse, so in certain cases the only clue that the scan is far from normal is the lack of soft tissue and renal uptake, and lack of renal clearance of radiotracer. Other causes of a super scan are metabolic bone diseases that include hyperparathyroidism, renal osteodystrophy, and Paget’s disease. A trick question can be lack of renal uptake due to renal pathology such as renal transplant (renal uptake in pelvis instead of typical bilateral upper abdomen—can confuse this for bladder or metastasis and think it is a superscan), horseshoe kidney with renal uptake about midline and lower than normal, or other renal pathologies with reduced or nearly absent renal uptake.

What is a classic potential cause of diffusely decreased bone uptake on a Tc-99m MDP bone scan?

A primary consideration is poor tagging of Tc-99m to MDP causing poor bone localization of radiotracer.  In this instance, look for signs of free Tc-99m which include salivary and thyroid uptake in the neck as well as increased gastric uptake. 

If skull uptake is above normal on a bone scan, what are key differential considerations?

Renal osteodystrophy (especially if marked uptake of the cranial sutures), Paget’s disease, and other metabolic bone diseases are the main considerations I would consider for a markedly hot skull on a bone scan for board exam purposes. Marked uptake of the frontal bones of an elderly patient, especially with diffuse calvarial thickening on CT, is consistent with hyperostosis frontalis. Diffuse of focal abnormal skull uptake can also be seen with metastases to the skull in the appropriate clinical setting. Uptake from inflammatory bone remodeling can also be seen in the sinuses from sinus disease, or in the maxilla or mandible from dental disease.

Besides abnormal calvarial uptake, what are additional findings of Paget’s disease on a bone scan?

Additional classic findings of Paget’s disease on a bone scan include a hot, expanded, and sometimes abnormally bowed femur and/or other long bone such as tibia or humerus, often flaring distally toward the knee, abnormal pelvic uptake with an expanded pelvis, which can involve only a hemi-pelvis or be more diffuse, an expanded, hot vertebral body which has been termed a “Mickey Mouse” sign as the spinous process and posterior elements can look similar to the Disney character Mickey Mouse. Finally, severe, and widespread involvement of Paget’s disease can present as a super scan. The “Lincoln sign” has also been described where uptake is seen in an expanded mandible appearing perhaps like Abraham Lincoln’s beard. Note that a burned-out sclerotic form of Paget’s is possible in which minimal to no abnormal bone scan uptake may be seen, but as a rule Paget’s is classically hot on a bone scan.

What is another classic cause of a very abnormal hot mandible on a bone scan?

Fibrous dysplasia.  The most common areas of uptake with fibrous dysplasia are the ribs, femurs, and facial bones to include the mandible. Remember that the polyostotic form of fibrous dysplasia is called McCune Albright syndrome. Look for co-existing radiographs showing an expansile, often ground-glass density lesion with no periosteal reaction to confirm fibrous dysplasia.

What are classic findings of an osteoid osteoma on a bone scan?

Osteoid osteomas often appear as a double-density area of uptake with a central hot nidus within a larger area of abnormal uptake that is slightly less intense. On a three-phase bone scan, an osteoid osteoma is classically hot on all three phases.  Osteoid osteomas are essentially uniformly hot on a bone scan, so a normal bone scan would essentially exclude the presence of an osteoid osteoma. Remember that osteoid osteomas are benign bone tumors most classic in children and teens that on radiography have a characteristic lucent central nidus with surrounding sclerosis.  Classic clinical symptoms on a board exam stem would include pain worse or exclusively at night relieved by NSAIDs.

True or false? Aneurysmal bone cysts are classically hot on a bone scan.

True.

How can bone scans help guide the management of heterotopic ossification?

Bone scans help show whether heterotopic ossification is mature/inactive.  If bone scan uptake is present at a site of heterotopic ossification, it is not mature and you typically would not resect the area of heterotopic ossification if it is a potential surgical case.  If there is no abnormal uptake, it is considered mature and therefore resectable.  The key is that if you resect when heterotopic ossification is still active, this has a higher rate of recurrence/failure of surgery to clear the disease.

If a radiograph has no correlate to account for an area of abnormal or indeterminate uptake on a bone scan, does this increase or decrease the likelihood that uptake is due to malignancy?

Increases. Remember that bone scans can show osseous metastases prior to radiographic changes being present.  The primary purpose of a radiograph in this setting is to evaluate for non-malignant causes of bone scan uptake, such as a subtle fracture or other benign bone lesion.  If no benign radiographic cause for bone scan uptake is seen, this increases the odds that this is a metastasis because benign causes of bone scan uptake typically would show some non-malignant abnormality on an x-ray.

What is expected uptake on a bone scan for early, middle, and late phases of avascular necrosis?

Early=cold

Mid=hot

Late=cold

Early avascular necrosis does not yet have enough bone remodeling present to show abnormal bone scan uptake.  Mid-phase avascular necrosis does have prominent bone remodeling from repair of AVN to result in abnormal uptake on a bone scan. Late phase AVN is essentially as repaired as it will get so there is no residual significant bone remodeling and there is classically no bone scan uptake.

Remember that causes of avascular necrosis that can clue you in to this pathology in a question stem include steroid use, alcohol use, sickle cell, pancreatitis, and trauma.

True or false? Bone infarcts classically have increased uptake on a bone scan.

False. Bone infarction, whether early or late, classically does not show abnormal bone scan uptake, at least in infarcted bone, as the bone is dead, avascular, and remodeling is not present. Note that mild uptake can be seen in the early phase in the periphery around a site of bone infarction due to bone reaction and remodeling around the necrotic bone.

What are classic bone scan findings of hypertrophic osteoarthropathy?

Linear symmetric uptake along the periosteum of the long bones, often the tibias on board exams, causing a “tram track” or “tram line” appearance. On board exams, if you see this finding first consider the possibility of a lung cancer as this is a classic association.  If there is no chest imaging, this may be the next step they will test you on as you need to now exclude presence of lung cancer or other lung pathologies. Remember that hypertrophic osteoarthropathy is associated with causes of chronic hypoxia such as cyanotic heart diseases, cystic fibrosis, mesothelioma, and other cyanotic heart or lung diseases.