Nuclear Medicine Bone Scans Part 2

Part 2 review of nuclear medicine bone scans for board preparation. After the final (third) episode on this topic is posted, a free downloadable study guide will be made available here. Prepare to succeed!

Show Notes/Study Guide:

I need to provide a clarification on a question from the first episode regarding heterotopic ossification. I previously said that mature heterotopic ossification has no bone scan uptake, and this was too simplistic of an explanation. Mature heterotopic ossification is best evaluated by a three-phase bone scan serially to assess for a decreasing amount of blood flow and blood pool activity and delayed uptake that may either decline to a steady low level or completely resolve. The points I want to drive home are this: mature heterotopic ossification can have a persistent low-level amount of delayed uptake, or resolved delayed uptake, but importantly has resolution of hyperemia manifest by a decreased or resolved amount of blood flow and blood pool activity compared to baseline.  Additionally, I want to point out that three phase bone scan is often able to detect heterotopic ossification earlier than radiographs, so can also be helpful for initial diagnosis of heterotopic ossification.

 I will expand greatly our discussion of a three-phase bone scan on the next episode which will be the final episode of this three-part series on nuclear medicine bone scans.

What is flair phenomenon on a bone scan?

Flair phenomenon occurs when a patient with osseous metastases starts therapy, the metastasis reduce or resolve, the bone starts healing where the metastasis was, and increased uptake is seen on the first post-therapy bone scan.  While on imaging this can look like the bone metastases are getting worse, or even new areas may be identified wherein bone at tiny, previously undetected metastases heals, but all new/increasing uptake is due to a healing response rather than worsening metastases.

How does one confirm whether increasing uptake on the first post-therapy bone scan is due to flair phenomenon or worsening disease?

The key to this answer is that one must get additional follow-up imaging to confirm flair phenomenon versus worsening metastatic disease.  At follow-up 2, perhaps 2-3 months later, if uptake is now decreasing this confirms a flair phenomenon at post-therapy timepoint one.  If uptake continues to worsen, this is most consistent with disease progression.

What is the classic imaging appearance of a sacral insufficiency fracture on a bone scan?

A sacral insufficiency fracture typically presents with H shaped uptake over the sacrum comprising two more laterally positioned vertical fractures and a horizontal fracture through the center of the sacrum which has been termed the Honda sign. Classic history would be an elderly female with osteoporosis with new sacral region pain.

If only one hot bone lesion is seen on a bone scan, what is the approximate percent likelihood that the solitary site of uptake is due to osseous metastatic disease?

This is a classic question for radiology board exams, and the answer is that a solitary focus of uptake has an approximate 20% likelihood of being secondary to an osseous metastasis. The exception to this rule is solitary sternal uptake in the setting of breast cancer in which the likelihood of malignancy may be greater than 50%. Some reports show that a solitary rib lesion in a person with malignancy may have up to a 40% chance of being malignant, others show a lower rate of about 10%.  Ultimately, I do not like this question as there are many factors at play including any history of recent trauma or poor conditioning that would increase risk of fracture in clinical practice, such as a patient who has frequent falls, but for board exams, I would remember, in general, a 20% likelihood of malignancy with the exceptions I’ve noted.

If you see scattered areas of increased and decreased uptake through the osseous structures as well as a hot or partially hot spleen, what underlying condition should you think of first?

Sickle cell disease with variable bone uptake due to bone infarcts (remember early bone infarcts can have peripheral uptake and chronic bone infarcts have decreased/absent uptake) with additional splenic uptake caused by splenic auto-infarction. Note that sickle cell disease can also be associated with increased renal uptake.

Tc99m elution with aluminum breakthrough can cause what imaging manifestation on a bone scan?

Hepatic uptake.  A potential non-metastatic cause of hepatic uptake on a bone scan is aluminum breakthrough in the Tc-99m solution.  This is a must-know fact for radiology and nuclear medicine board exams.

What are classic potential causes of poor Tc-99m and MDP binding during radiopharmaceutical preparation?

Poor binding of Tc99m to MDP during radiopharmaceutical preparation can result from air in the vial or syringe during preparation causing poor binding or else not enough stannous ion in the preparation to reduce the Tc-99m which is a necessary step for Tc99m and MDP to bind.  This results in imaging contamination from free Tc-99m on the bone scan which results in abnormal salivary gland, thyroid, and gastric uptake.

Beyond metastatic disease, what are 3 classic non-malignant causes of abnormal renal uptake on a bone scan?

Non-malignant causes of renal uptake on a bone scan include renal insult from nephropathy which can result from chemotherapy (can be seen on the post-treatment scans as new diffuse renal uptake) or other renal insults, as well as hemochromatosis, and urinary obstruction in which case hydronephrosis and hydroureter would also be seen.

True or false? A primary breast cancer can show abnormal breast uptake on a bone scan?

True.  Soft tissue uptake in the breast on a bone scan can be a manifestation of breast cancer.

True or false? Tc99m-MDP uptake is a function of blood flow and osteoblastic activity, with binding of MDP to hydroxyapatite on the mineralizing bone surface.

True.

How long after radiotracer injection is imaging for a single-phase bone scan typically performed?

2-4 hours.  For purposes of board exams, I would remember 4 hours. If a patient has renal failure, one could image after 4 hours to allow improved renal excretion which clears tracer from soft tissues, improving signal to noise.

What is the classic imaging pattern of rib fractures on a bone scan?

Rib fractures on a bone scan classically appear as focal areas of uptake in multiple contiguous ribs in a linear pattern reflecting contiguous rib fractures at and surrounding site of impact to the ribs, often with similar uptake at all sites as all are typically the same age if from a single traumatic event.

What is the classic imaging pattern of rib metastases on a bone scan?

Scattered areas of rib uptake, often with varying intensities of uptake due to different ages of metastases, that are more diffuse than the pattern noted with rib fractures, especially if uptake is seen spreading through the marrow space of a rib rather than a focal single point. If you see uptake that follows a portion of the length or curvature of the rib, this is highly suspicious for metastatic rib involvement. If you scattered areas of rib uptake combined with suspicious bone uptake elsewhere, suspicion for rib metastases is high.  Also, remember that pathologic fractures, wherein metastatic bone that is weakened undergoes a so-called pathologic fracture from minor trauma or load bearing, cannot be differentiated on a bone scan, but may be suspected, particularly if a coexisting lytic or soft tissue lesion is seen on CT or MRI imaging at the site of fracture.

What is the typical pattern for osteoporotic compression fractures of the spine on a bone scan?

Multilevel linear abnormal areas of uptake of different intensities are classic for osteoporotic compression fractures on a bone scan.  Note that a single vertebral body with linear-type uptake is also consistent with a compression fracture, but whether this is pathologic or due to osteoporosis would be uncertain based off a bone scan alone.  In such setting, correlating with CT or MRI can be helpful to evaluate for any underlying lesion to suggest a pathologic compression fracture.  I would remember for board exams that multilevel linear areas of uptake of varying intensity are the typical pattern for multilevel osteoporotic compression fractures. These would be expected to show decreased uptake on follow-up bone scans due to healing and reduced osteoblastic activity.

If you are called by the technologist to check a bone scan prior to the patient leaving, and uptake over the pelvis is seen that is indeterminate for metastases versus urine contamination, what is the next best step?

Remove potentially contaminated clothing, clean the skin, place a clean gown, and reimage the pelvis.  If the indeterminate activity goes away this confirms urine contamination. Note also that a tail-on-detector (TOD) view can also help differentiate uptake external to the pelvis from true bone metastasis.

If you see focal abnormal myocardial uptake on a bone scan, what are the primary diagnostic considerations for board exams?

I would first consider myocardial infarction with myocardial necrosis or ventricular aneurysm as possible considerations.  This is also reported with unstable angina.  Appropriate cardiac workup would be indicated.

If you see diffuse abnormal myocardial region uptake on a bone scan, what are the primary diagnostic considerations for board exams?

Think diffuse processes to include cardiac amyloidosis, pericarditis, chemotherapy-induced cardiotoxicity, and cardiomyopathies such as alcoholic cardiomyopathy.

If you see diffuse increased lower-level activity throughout the abdomen, what should you first consider for board exams?

Ascites.  The same principle holds true for the thorax wherein low-level diffuse symmetric or asymmetric uptake projecting over portions of the lungs, such as part of a hemithorax, can be a manifestation of pleural effusion. One should evaluate for causes of malignant ascites or pleural effusion, if not already known.

Note that soft tissue extraosseous uptake can result from abnormal blood flow, capillary permeability, or extensive calcifications in the tissues to include calcified femoral or other major arteries. Hypercalcemia and other calcium-depositing diseases can cause abnormal uptake on a bone scan. Other considerations include various malignancies whenever uptake is focal and abnormal, such as in the breast with potential breast cancer.