Nuclear Medicine

A. Signs of osteoporosis may include the H sign (aka the butterfly sign), where the sacrum and both sacral alae are involved in degenerative changes. Other common areas for degenerative changes are the hands and wrists, where uptake may be seen in proximal and distal interphalangeal joints, as well as at the base of the first metacarpal joint. The shoulder may also be an area of involvement appreciated in bone scans. 

B. Paget’s disease is characterized by a very strong uptake of tracer in the affected region. Characteristic presentations may be the involvement of a hemipelvis or a large section of a long bone, such as the tibia or femur. It is not uncommon for bony structures to show expansion. When the disease involves a long bone, it usually extends to at least one end of the bone. 

C.  Although fibrous dysplasia may have a similar appearance to the images (very high tracer uptake) and may involve long bones, it rarely extends to either end of the bone. Another point to note is the demographic involved. Fibrous dysplasia usually involves a younger patient population. 

D. Sickle cell anemia causes expansion of the bone marrow due to higher production of red blood cells, which incidentally causes thinning of the bone. On a bone scan, the typical signs can include osseous expansion due to the bone marrow expanding. The spleen may also show an increased uptake of the tracer due to calcification. The bone marrow expansion in the skull may appear as bilateral tracer uptake in the calvarial region. Other signs include higher than-normal uptake in the knees and ankles. Long bones may appear as photon deficient due to prior events of bone marrow infarction. 


Answer: B

Discussion:

Paget’s disease is characterized by cycles of high bone resorption followed by bone growth. The newly formed bones are often weak and patients may present with obvious deformities such as bowing of the legs. In order, the most affected areas are the pelvis, spine, skull, femur, scapula, tibia, and humerus. On a bone scan, a specific finding to Paget’s diseases (although rare) is the mickey mouse sign, whereby anterior and posterior sclerotic involvement of a vertebrae causes it to look like a mickey mouse head (see Figure 1). 

​

The radiopharmaceutical used in bone scans is technetium-99m diphosphonate (20 mCi aka 740 mBq), and images are acquired both directly after IV injection and then again 2-4 hours after the injection. Patients are asked to urinate before the injection and frequently after, until the second imaging step. Anterior and posterior views are obtained of the entire skeleton. Technetium-99m diphosphonate is used due to its higher absorption in areas of increased osteogenic activity (which may be due to infection, trauma healing, autoimmune processes, metastases, metabolic or other). The radiotracer is injected intravenously, meaning that blood flow is an important factor to take into consideration. Areas requiring more blood flow (e.g. disease processes) get a higher distribution of the radiotracer, and therefore increased uptake is seen on the bone scan. 

https://radiologypics.com/2014/01/20/child-swallows-mickey-mouse-mickey-mouse-sign/

Figure 1 – Mickey mouse sign in Paget’s disease