**3.1 Tc-99m MDP: Pharmacological and physiochemical characteristics**

Tc-99m Methylene-Diphosphonate (MDP) has been extensively used in the work up of osteomyelitis. The mechanism of uptake of diphosphonates has not been completely elucidated but the general presumption is that they are adsorbed to the mineral phase of bone with relatively lesser binding to the organic phase. Some also have postulated chemoadsorption to the hydroxyl-apatite mineral component of the osseous matrix itself. Tc-99m (Technetium 99m) is a generator produced radioisotope with the principle gamma photon of 140 KeV optimal for imaging with low energy high resolution parallel-hole collimator. The physical half-life of Tc-99m is 6 hours making possible delayed imaging with good target to

Ga-67 scintigraphy is primarily characterized by poor spatial resolution and low specificity due to paucity of anatomical and morphological information. (Bar-Shalom et al., 2006) studied patients with multiple infectious conditions including FUO and concluded that SPECT/CT was found to be beneficial in determining the precise anatomical sites of infection in 85% of discordant studies. In particular, substantial benefits were observed in scans of the chest and abdomen. In our own experience the addition of SPECT/CT to Ga-67 scintigraphy has a definite incremental value that helps resolve many difficult clinical

Fig. 4. A 21-year-old female with history of polycystic kidney disease referred for Ga-67 scintigraphy to evaluate suspected infected right renal cyst. Ga-67 scintigraphy shows linear intense tracer accumulation in the infra-hepatic region. SPECT/CT images localize the abnormal uptake to an area in the upper pole of right kidney just above a hypodensity (renal cyst) that shows photopenia on SPECT images. Subsequent guided biopsy drained

Tc-99m Methylene-Diphosphonate (MDP) has been extensively used in the work up of osteomyelitis. The mechanism of uptake of diphosphonates has not been completely elucidated but the general presumption is that they are adsorbed to the mineral phase of bone with relatively lesser binding to the organic phase. Some also have postulated chemoadsorption to the hydroxyl-apatite mineral component of the osseous matrix itself. Tc-99m (Technetium 99m) is a generator produced radioisotope with the principle gamma photon of 140 KeV optimal for imaging with low energy high resolution parallel-hole collimator. The physical half-life of Tc-99m is 6 hours making possible delayed imaging with good target to

**3.1 Tc-99m MDP: Pharmacological and physiochemical characteristics** 

**2.4 Gallium-67: Hybrid SPECT/CT imaging** 

abscess just beneath the renal capsule.

**3. Bone scintigraphy**

scenarios (Figure 4).

background ratio. Approximately 50% of the injected dose is localized to the bone. The tracer uptake is dependent upon the blood flow and the rate of new bone formation.

#### **3.2 Tc-99m MDP: Imaging protocols and pre-requisites**

When bone scintigraphy is performed for the evaluation of osteomyelitis, the study is done in three phases. These include a dynamic sequence termed as the flow or perfusion phase, followed by immediate static image of the area of interest termed as the blood-pool phase or soft tissue phase. The third phase is the static delayed imaging of the area of interest usually acquired 2-4 hours post injection. Some also perform a fourth phase at 24 hours which is usually a static spot view of the region of interest. The usual adult dosage is 740-925 MBq. Patients are instructed to be well hydrated and void frequently after the injection.

#### **3.3 Tc-99m MDP: Clinical utilities and applications**

Three-phase bone scintigraphy is the radionuclide procedure of choice for diagnosing osteomyelitis in non-violated bone i.e. bone that is not affected by underlying conditions. It is highly sensitive for diagnosing osteomyelitis and can detect the process 7-14 days before the manifestation of radiological changes. The reported sensitivity of 90-100% and specificity of 70-95% for identification of osteomyelitis is in non-violated bone. In adults a negative bone scan essentially rules out infection. Focal hyperperfusion, focal hyperemia, and focally increased bony uptake on delayed images are the characteristic scintigraphic findings for osteomyelitis. Many times due to some underlying bone conditions the specificity of the bone scan for osteomyelitis is further compromised. In such situations, the additional images at 24 hours (4th - phase) may improve specificity. This is due to the fact that uptake in woven or immature bone present in osteomyelitis continues for several more hours than normal bone. The accuracy of 4-phase bone scan that is more specific but less sensitive than a 3-phase bone scan is approximately 85%. Further the specificity of bone scan can also be improved by addition of Ga-67 scintigraphy. The overall accuracy of bone scan/Ga-67 scintigraphy is approximately 65-80%.

#### **3.4 Tc-99m MDP: Hybrid SPECT/CT imaging**

Three-phase bone scintigraphy and MRI are considered the modalities of choice for diagnosing osteomyelitis. Moreover, MRI can assess the associated soft tissue complications. MRI has its limitations as well. The replacement of marrow fat with edema and exudate results in a decreased signal on T1 and an increased signal on T2-weighted images. Such findings are not specific for osteomyelitis and can be seen with acute infarction, fracture or even tumour. Therefore the overall sensitivity and specificity of MRI for detection of acute osteomyelitis ranges from 92-100% and 89-100% respectively. CT is more sensitive to detect cortical destruction. On site-based analysis by (Bar-Shalom et al., 2006) scintigraphy (planar & SPECT) and SPECT/CT showed concordant results for diagnosis and localization of 50% of infection sites. SPECT/CT defined the precise anatomical localization of 44% of infectious sites that were erroneous or equivocal on scintigraphy in this study. In another preliminary report, (Horger et al., 2003) found that SPECT/CT improved the diagnostic performance of three-phase bone scan for osteomyelitis avoiding false-positive or equivocal results. In our own experience SPECT/CT has always had an incremental value to routine planar imaging and in many clinical situations in particular further characterizing the abnormalities to reach definitive imaging diagnosis (Figure 5).

Radionuclide Infection Imaging: Conventional to Hybrid 81

electron capture with gamma emissions of 173 and 247 KeV. In-111 labelled leukocytes are normally distributed to the liver, spleen, and bone marrow. Intense pulmonary activity is seen soon after injection, which clears rapidly, and it is probably due to leukocyte activation during labelling, which impedes their movement through the pulmonary vascular bed,

Tc-99m HMPAO is a lipophilic agent that readily crosses the cell membrane of leukocytes. Once inside the cell the compound becomes hydrophilic and remains trapped. It is subsequently bound to intracellular organelles, primarily the nucleus and mitochondria. The bond to granulocytes is more stable than to monocytes and the tag elutes from these cells five times more rapidly. The normal biodistribution of Tc-99m HMPAO labeled leukocytes is more variable. In addition to the reticuloendothelial system, activity is also normally present in the genitourinary tract, large bowel, blood pool, and occasionally the gallbladder. Physiologic bowel excretion limits the usefulness of this agent for imaging

There are a number of methods for labelling leukocytes, however, the basic principles and technique remains uniform. Approximately 40 ml of blood is withdrawn from the patient into a syringe that contains anticoagulant. This syringe containing blood is then kept in an upright position for about 1–2 hours to promote gravity erythrocyte sedimentation, a process that is facilitated by the addition of hydroxyethyl starch. The process can be accelerated by using hypotonic lysis of the red cells instead of gravity sedimentation as well. After the erythrocytes have been separated, the leukocytes must be separated from platelets. The leukocyte-rich plasma is centrifuged, and the leukocyte "pellet" that forms at the bottom of the tube is removed, incubated with the radiolabel, washed, and re-injected into the patient. The usual dose of In-111 labelled leukocytes is 10–18.5 MBq; while the routine dose of Tc-99m HMPAO labelled leukocytes is 185–370 MBq. The majority of leukocytes labelled are neutrophils, and hence the procedure is most useful for identifying neutrophil-mediated inflammatory processes, such as bacterial infections. The procedure is less useful in conditions where the predominant cellular response is other than neutrophilic such as

A total white count of at least 2000/mm3 is needed to obtain satisfactory images. ABO compatible donor WBC's may be used in neutropenic patients (i.e. White cell count less than 2000/mm3). Radiolabelled leukocytes should be administered within 1–2 hours of cell labelling. Labelled cells stored longer than 3 hours have a significant loss of cell viability. Temperatures higher than 70°F tend to increase cell damage and should be avoided. Cell labelling should be performed by trained laboratory personnel in a laminar flow hood using sterile procedures. Care must be taken to ensure correct identification of patients and blood products. All patients and laboratory procedures should have an appropriate quality control

In-111 labelled leukocyte is usually performed 18-24 hours post injection because early imaging at 4 hours usually misses out on two-thirds of the lesions detected on later images. However, it is critical to obtain early (1-4 hour) images when evaluating patients for inflammatory bowel disease or those suspected of possible ischemic bowel disease.

**4.3 Radiolabelled leukocytes: Imaging protocols and pre-requisites** 

prolonging their passage through the lungs.

**4.2 Radiolabelled leukocytes: Labeling technique** 

abdominal infections.

tuberculosis.

program.

Fig. 5. A 19-year old male referred for evaluation of pain left lower limb. Tc-99m MDP images show hyperaemia, increased pool activity and intense increase tracer uptake in the distal end of left tibia. SPECT/CT images show a well-defined low attenuation metaphyseal lesion with central radiolucent area surrounded by peripheral bone sclerosis clearly confined to within the bone itself sparing the joint cavity. Findings consistent with diagnosis of osteomyelitis with features typical of a Brodie's Abscess.
