SKELETAL RADIONUCLIDE IMAGING

VI

Dr. Hussein Rabie Farghaly

Nuclear Medicine Consultant

PSMMC

BONE INFARCTION AND OSTEONECROSIS

Etiologies of Aseptic bone necrosis

BONE SCAN FINDING: Acute interruption of the blood supply ----- newly infracted bone----- cold area-------In the postinfarction or healing phase-------hot area

Legg-Calvé-Perthes Disease• Most commonly affects children between the ages of 5–9 years with predominance in boys (4:1 to 5:1).

• It is a form of osteochondrosis and results in avascular necrosis of the capital femoral epiphysis.

• The best scintigraphic technique for detecting the abnormality in the femoral head is to use some form of magnification and to image in the frogleg lateral projection.

• Classically, early in the course of the disease before healing has occurred, a discrete photon-deficient area can be seen in the upper outer portion of the capital femoral epiphysis with a lentiform configuration . Areas of photon deficiency are well demonstrated by SPECT imaging.

• As healing occurs, increased uptake is first seen at the margin of the photon-deficient area, and gradually the scintigram demonstrates filling in of activity.

• In severe cases the femoral head never reverts to normal. Increased tracer uptake is seen for a prolonged period—many months or more.

• Currently MRI is the imaging modality of choice for the evaluation of Legg-Calvé-Perthes disease, as well as other causes of osteonecrosis.

• Compared with nuclear scintigraphy, MRI has comparable or higher sensitivity and higher specificity.

• MRI also provides a range of additional information, including evaluation of articular cartilage, detection of acetabular labral tears, and visualization of metaphyseal cysts that are indicators of prognosis.

Legg-Calvé-Perthes disease. A, (top row). Scintigrams by standard parallel-hole collimator fail to reveal the abnormality. Pinhole images of the same patient (bottom row) reveal the characteristic lentiform area of decreased uptake on the left. B, Corresponding radiograph done months later reveals deformity of the left femoral epiphysis with flattening, increased density,and increased distance between the epiphysis and the acetabulum.

• Skeletal scintigrams in patients with sickle cell anemia have a number of characteristic features that suggest the diagnosis:

- In the skull, the expanded marrow space results in bilaterally increased calvarial uptake of tracer.

- In the extremities, patients usually have greater relative uptake compared with the axial skeleton than is seen in normal subjects. This increased uptake may be related to the persistence of hematopoietic elements throughout the extremities, including the hands and feet, of patients with sickle cell anemia.

- The overall skeleton-to-background ratio is usually good and is accentuated by the increased appendicular uptake.

- In many patients with sickle cell anemia, the kidneys appear somewhat larger than normal, which may be related to a defect in the ability to concentrate urine.

- spleen uptake, presumably because of prior splenic infarction and calcification .

Sickle Cell Anemia

Sickle Cell Anemia - Infarctions in bone and bone marrow result in both acute and long-standing changes. If the

involvement is primarily in the marrow space, the skeletal scintigram may not reveal the extent of the lesion as it does not involve the cortex where Tc-99m MDP binds.

- The image may be normal acutely. Within a few days as healing begins, the scan typically demonstrates increased uptake.

- MRI can demonstrate marrow infarctions immediately. Bone marrow scans using Tc-99m sulfur colloid are also sensitive and are positive immediately after the infarct.

- Affected areas fail to accumulate tracer and are seen as cold or photon deficient.

- The presence of chronic marrow defects from prior bone marrow infarctions persist. Thus the significance of a photon-deficient area on marrow scanning is somewhat uncertain unless a recent baseline study is available for comparison. Here again, MRI has an advantage in distinguishing acute from chronic changes.

- The correlation between the marrow scan and the bone scan is also important in the differentiation of acute osteomyelitis from infarct. If the marrow shows a defect in the region of increased bone scan activity, it is consistent with infarct. If the marrow shows no change, then any increased activity on the bone scan in an acute situation is most likely osteomyelitis.

Bone marrow scanBone scan

Osteomyelitis• The term osteomyelitis literally means inflammation of bone and its marrow regardless of whether it is

due to pyogenic organisms ,tuberculosis ,syphylis ,virus, fungus,or presence of foreign body .

• Osteomyelitis is an acute or chronic inflammatory process of the bone and its structures secondary to infection with pyogenic organisms.

• Acute hematogenous osteomyelitis typically begins by seeding of the infectious organism in the marrow space

• Osteoymelitis may be Hematogenous , Contiguous-Focus or Peripheral Vascular Disease-associated.

• In children, Staphylococcus aureus is the most common organism and is probably responsible for 50% and the common site is the end of long bones.

• Osteomyelitis in adults may occur by hematogenous spread or by direct extension in an area of cellulitis. The foot is a common site of direct extension in diabetic patients,whereas the spine is commonly involved by the hematogenous route. In nondiabetic adults, the axial skeleton is more commonly involved than the appendicular skeleton . Osteomyelitis may involve any skeletal structure.

Bone Scan in Osteomyelitis

Osteomyelitis of the calcaneous

osteomtelitis of the proximal phalynx of the right second toe.

Blood pool Delayed image

Dynamic

Prosthesis Evaluation

• The distinction between component loosening and infection is critical in guiding management.

• The findings on skeletal scintigraphy are not specific enough for a reliable distinction between loosening of a prosthesis and infection .

• Reactive bone around a loose prosthesis may be indistinguishable from increased tracer uptake resulting from osteomyelitis.

• In cases of a loose prosthesis, uptake is usually increased in the region of the greater and lesser trochanters and at the tip of the prosthesis. Loosening may also be seen as diffusely increased uptake around the acetabular component.

• This increased activity is presumably due to remodeling of bone in response to movement of the prosthesis.

• Some increased uptake is expected as a normal healing response for 1 year after placement of a cemented prosthesis and for 2–3 years after placement of a non cemented prosthesis. However, increased activity may persist indefinitely.

• Although seldom performed, a baseline study 6 months to 1 year following surgery is very useful for future comparison.

• In osteomyelitis, activity is increased in the bone surrounding the prosthesis. However, a negative bone scan is useful because it helps rule out both osteomyelitis and prosthesis loosening.

• The differential diagnosis between loosened prosthesis and infection is better made with tracers such as radiolabeled white blood cells.

• Detection of an infected prosthesis with In-111 or Tc-99m HMPAO-labeled white blood cells offers the highest sensitivity and specificity. This tracer localizes in areas of infection and not in areas of remodeling or reactive bone.

• Use of labeled white blood cells has three pitfalls.

• First, false-negative studies may occur in low-grade chronic osteomyelitis.

• Second, cellulitis can be difficult to distinguish from septic arthritis.

• Third, false-positive studies can result from normal radiolabeled white cell

• uptake in bone marrow around a prosthesis.

• The combination of white blood cell and sulfur colloid marrow scanning is combined to avoid this pitfall. Infection is diagnosed only in areas of radiolabeled white blood cell uptake that are negative for marrow activity.

• In osteomyelitis, activity is increased in the bone surrounding the prosthesis. However, a negative bone scan is useful because it helps rule out both osteomyelitis and prosthesis loosening.

• The differential diagnosis between loosened prosthesis and infection is better made with tracers such as radiolabeled white blood cells.

• Detection of an infected prosthesis with In-111 or Tc-99m HMPAO-labeled white blood cells offers the highest sensitivity and specificity. This tracer localizes in areas of infection and not in areas of remodeling or reactive bone.

• Use of labeled white blood cells has three pitfalls.

First, false-negative studies may occur in low-grade chronic osteomyelitis.

Second, cellulitis can be difficult to distinguish from septic arthritis.

Third, false-positive studies can result from normal radiolabeled white cell uptake in bone marrow around a prosthesis.

• The combination of white blood cell and sulfur colloid marrow scanning is combined to avoid this pitfall. Infection is diagnosed only in areas of radiolabeled white blood cell uptake that are negative for marrow activity.

Prosthesis Evaluation cont.

Normal bone tracer uptake in joint prosthesis

Loosening Loosening infection

BONE MARROW SCINTIGRAPHY• Bone marrow scintigraphy is not important in current practice but does have a small number of

indications.

• The procedure is most commonly performed with Tc-99m sulfur colloid, which localizes in the reticuloendothelial elements of the red marrow.

• In patients with sickle cell anemia,bone marrow imaging demonstrates the extent of marrow expansion into the extremities.

• In most normal subjects, the marrow is confined to the proximal thirds of the femurs and humeri.

• In patients with hemoglobinopathies, the marrow uptake is typically seen throughout the appendages.

• Marrow imaging is highly sensitive for detection of bone marrow infarction and can also define the extent of involvement.

• The major limitation is not being able to distinguish new from chronic infarctions.

• Moreover, areas involved by osteomyelitis demonstrate defects on marrow imaging, so the technique is not useful in distinguishing infarction from osteomyelitis. In current practice, marrow imaging is often used in conjunction with labeled white blood cells to diagnose osteomyelitis.