Lakhasly

99mTc-Phosphonate Compounds The rationale for using phosphonate compounds for bone scanning lies in the composition of the bone matrix containing calcium phosphate that can be exchanged with phosphonate compounds.It does not appear to bind to plasma proteins and therefore the plasma clearance is very rapid with two components having half-times of 0.4 and 2.6 h. Its urinary excretion is of the order of 20% in 2 h after injection in normal subjects.About 10% of the injected dosage remains in the blood at 1 h post-injection and less than 1% at 24 h. Urinary excretion is 50% and the remaining 50% is retained by the skeleton in 24 h. Approximately 10-20 mCi (370-740 MBq) 99mTc-MDP or 99mTc-HDP is injected intravenously and scanning is performed with the patient supine 2-3 h after injection.In the hydroxyapatite uptake theory, it has been suggested that hydroxyapatite crystal removes the phosphonate component successfully from 99mTc-phosphate complexes, thus setting the reduced technetium free to bind independently to hydroxyapatite at another binding site.To distinguish between cellulitis and osteomyelitis in the distal extremities, three-phase bone images (flow, blood pool, and bone uptake) are obtained by giving a bolus injection of 30 mCi (1.11 GBq) 99mTc-MDP.18F-Sodium Fluoride 18F-Sodium Fluoride (18F-NaF) was approved for bone imaging in 1972, but with the introduction of 99mTc-phosphonate compounds, it was discontinued in mid-1970s because of its high-energy photons unsuitable for scintigraphic imaging.In the collagen uptake theory, it has been suggested that 99mTc-phosphonate complexes localize in both inorganic and organic matrices of bone, the latter uptake depending on the amount of immature collagen present.Following IV administration of 18F-NaF, 18F rapidly equilibrates in the extracellular space and then is cleared by bone deposition and urinary excre- tion.Static spot images are obtained with a single-head camera, whereas both anterior and posterior scans are obtained simultaneously using a dual-head whole-body camera.Due to hyperemia in cellulitis, the tracer localizes in both the flow and blood pool phases, but disappears in the delayed bone uptake phase.

99mTc-Phosphonate Compounds
The rationale for using phosphonate compounds for bone scanning lies in the composition of the bone matrix containing calcium phosphate that can be exchanged with phosphonate compounds. Two compounds, 99mTc-MDP and 99mTc-HDP, are commercially available for bone imaging, of which 99mTc-MDP is most commonly used.
Following IV administration, 99mTc-MDP is cleared from the plasma with a half-time of 3–4 min. About 10% of the injected dosage remains in the blood at 1 h post-injection and less than 1% at 24 h. Urinary excretion is 50% and the remaining 50% is retained by the skeleton in 24 h.
Approximately 10–20 mCi (370–740 MBq) 99mTc-MDP or 99mTc-HDP is injected intravenously and scanning is performed with the patient supine 2–3 h after injection. The 2- to 3-h waiting period is needed to reduce the background against the bone, and the patient is asked to void before imaging so that the bladder activity does not blur the pelvic region on the image. Whole-body scanning is performed by moving the detector from head to toe of the patient using either a single-head or a dual-head camera equipped with
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298 13. Diagnostic Uses of Radiopharmaceuticals in Nuclear Medicine
a low-energy, all-purpose parallel hole collimator. Static spot images are obtained with a single-head camera, whereas both anterior and posterior scans are obtained simultaneously using a dual-head whole-body camera.
To distinguish between cellulitis and osteomyelitis in the distal extremities, three-phase bone images (flow, blood pool, and bone uptake) are obtained by giving a bolus injection of 30 mCi (1.11 GBq) 99mTc-MDP. In the flow phase, images are obtained every 2 s for 60 s, followed by blood pool imaging immedi- ately and bone uptake imaging at 3–5 h after injection. Due to hyperemia in cellulitis, the tracer localizes in both the flow and blood pool phases, but disappears in the delayed bone uptake phase. On the other hand, in osteomyeli- tis, wherein some associated hyperemia exists, the tracer uptake is seen in the flow and blood pool phases, with a further increase in localization in the bone uptake phase. In some cases, the background clearance is not optimum because of vascular insufficiency; a fourth phase bone image may be required, which is usually performed the next day to delineate bone uptake better.
Regional bone blood flow rate, bone formation rate, and extraction efficiency are the major factors that influence the bone uptake of phosphonate complexes. In general, the higher the rates of blood flow and bone formation, the greater the bone uptake of radiotracer. There are two hypotheses on the bone uptake mechanism of phosphonate compounds: hydroxyapatite uptake and collagen uptake. In the hydroxyapatite uptake theory, it has been suggested that hydroxyapatite crystal removes the phosphonate component successfully from 99mTc-phosphate complexes, thus setting the reduced technetium free to bind independently to hydroxyapatite at another binding site. In the collagen uptake theory, it has been suggested that 99mTc-phosphonate complexes localize in both inorganic and organic matrices of bone, the latter uptake depending on the amount of immature collagen present. It has also been found that 99mTc- phosphonate complexes localize in soft tissues and tumors to a variable degree.
18F-Sodium Fluoride
18F-Sodium Fluoride (18F-NaF) was approved for bone imaging in 1972, but with the introduction of 99mTc-phosphonate compounds, it was discontinued in mid-1970s because of its high-energy photons unsuitable for scintigraphic imaging. With the advent of PET imaging, interest has again grown to use 18F- NaF for bone imaging. Studies have shown 18F-NaF to be superior to 99mTc- MDP imaging in bone imaging, but its use is somewhat limited by the higher cost of its cyclotron production.
Following IV administration of 18F-NaF, 18F rapidly equilibrates in the extracellular space and then is cleared by bone deposition and urinary excre- tion. It does not appear to bind to plasma proteins and therefore the plasma clearance is very rapid with two components having half-times of 0.4 and 2.6 h. Its urinary excretion is of the order of 20% in 2 h after injection in normal subjects. The bone uptake of fluoride ions is a function of blood flow to the bone and bone extraction efficiency of the 18F ion.