The previous imaging techniques discussed create contrast by differential reaction of tissues to the stimulus of the modality being used, whether it is radiation, sound waves, or magnetic spin. Bone scans act in a similar fashion: Technetium 99-m-labeled diphosphonates (a radiotracer "contrast agent") are injected and differentially taken up by the tissues (Love et al. 2003). This compound initially disperses throughout all tissue and is ultimately taken up by bone, particularly in areas of rapid bone formation and good blood flow (Love et al. 2003). Patients are initially injected and then told to drink fluids and come back in 2-6 h for detection. By waiting and by encouraging fluid intake, the labeled diphosphonates are washed out of non-bony tissue and allowed to concentrate in bone resulting in greater distinction of uptake of the radiolabeled compounds and a more detailed picture. This picture is obtained by asking patients to lie in front of a gamma camera for about an hour (Love et al. 2003). Front and back images are typically obtained as well as any specific locations required. Thus dimension is limited to flat views much like in plain radiographs except that the entire body (front and back) may be imaged at once. The images do not carry the resolution detail that an MRI may have; they are actually somewhat "fuzzy" by comparison (see Fig. 6.4) but do offer good sensitivity (Love et al. 2003). Specificity is obtained by examining the pattern and distribution of the radiotracer uptake. For example, radiotracer accumulation in both the vertebral body and pedicles is suggestive of metastatic disease while sparing of the pedicles suggests benign disease. Patterns are not always constant and thus specificity is not superb, but the good sensitivity of this test and its ability to image the entire body make it a good screening test, particularly in its principle use: identification of distant metastases in cancer. It is more sensitive than plain radiograph and more efficient when imaging of the entire body is needed (Love et al. 2003).
Figure 6.4 Bone scan images do not carry the resolution detail that an MRI may have; they are actually somewhat "fuzzy" by comparison but do offer good sensitivity.
In identification of metastases, the radiotracer concentrates in areas of bone formation and thus osteoblastic metastases may preferentially be identified compared to osteoclastic metastases. Furthermore, following hormone therapy or other chemotherapies, bone lesions may "flare" as part of the treatment response for about 3 months and worsening of the bone scan may simply reflect this "flare" and not worsening disease. However, a worsening bone scan beyond 6 months should raise concern for metastases (Love et al. 2003).
Bone scans are also useful for diagnosing acute stress fractures. This is only natural because fracture repair involves both bone formation and increased blood flow factors which concentrate the radiolabeled diphosphonates (Love et al. 2003). Furthermore, when plain radiographs fail to identify occult fractures (such as in the hip), a bone scan may be useful because of its greater sensitivity in determining areas of bone formation (Mettler 2005).
Shin splints, "or medial tibial stress syndrome, can be described as a clinical entity characterized by diffuse tenderness over the posteromedial aspect of the distal third of the tibia" caused by inflammation of the tibialis and soleus muscle insertions at the tibia (Love et al. 2003, Wilder and Sethi 2004). In a similar fashion, excessive walking or standing can cause inflammation of the plantar fascia on the bottom of the foot, referred to as plantar fasciitis. Sometimes referred to as a heel spur, it is aggravated by excessive use but is also worse in the morning as the plantar fascia may contract overnight and increase pain. Bone scan is useful in these diagnoses as radiotracers localize at the site of tendinous insertion of the muscles and fascia onto the bone.
Triple-phase bone scan is an imaging modality classically used to identify pain from complex regional pain syndrome but is more useful for osteomyelitis (Love et al. 2003). A triple-phase bone scan, as its name applies, has three phases: a dynamic phase (performed immediately after radiotracer injection), a blood pool phase (performed 3-5 min after injection), and a delayed bone phase (performed 2-6 h after injection) (Nagoya et al. 2008). In this scan, both blood flow and bone turnover are also evaluated as opposed to evaluation of only bone turnover in a plain bone scan. In the dynamic phase, the general amount of blood flow to an area is determined; in the blood pool phase, the amount of extravasation of tracer into the surrounding tissue is detected; while in the delayed phase, bone uptake is measured (Love et al. 2003). Because infections lead to increased blood flow in the area of infection as well as leaky tissue (osteomyel-"i'fc"), the two initial phases of at three-phase bone scan are useful in their diagnosis. The final phase, the delayed bone scan phase, localizes this infection to the bone ("osfeo"-myelitis) by demonstrating increased bone turnover. Both fractures and metastases, as well as infections, may cause hyperperfusion and hyperemia resulting in positive three-phase bone scans. When diagnostic doubt exists and greater specificity is needed, a subsequent scan using indium-111 tagged leukocytes will be positive for infection but not the other conditions. In this scan, leukocytes are withdrawn from a patient, labeled with indium-111 and re-injected. Detection is performed 24 h later with the belief that these labeled leukocytes will concentrate at an area of infection.
Some have advocated use of bone scans to assess problems with prosthetic joints such as loosening and infection. Radiographs can provide some initial information on loosening but cannot elucidate presence or absence of infection (Nagoya et al. 2008). While CT scan and MRI can provide image on infection with the use of contrast, the presence of joint hardware can obscure the images by creating artifacts. Studies have indeed demonstrated good sensitivity and specificity for both prosthetic joint infection and loosening when all three phases
Table 6.5 Conditions for which bone scan may be used as initial or follow-up test.
Bone scan as primary modality
□ Stress fracture
□ Tendonitis and fasciitis
Bone scan following plain radiograph
□ Occult hip fracture
□ Prosthetic joint, infection, or loosening of a bone scan are conducted, but follow-up tissue diagnosis is recommended (Nagoya et al. 2008). What may be of greatest benefit is a negative triple-phase bone scan as that strongly argues against any pathology and obviates the need for tissue specimens (Love et al. 2003) (Table 6.5).
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