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Studies with laboratory animals have shown that pertechnetate ion is selectively concentrated in the thyroid gland, salivary glands and stomach, and selectively excluded from the cerebrospinal fluid almost to the same extent as inorganic iodide. Since the radionuclide technetium-99m gives extremely low energy dissipation in tissue, human studies were undertaken to evaluate pertechnetate-99m as a clinical tracer material. After intravenous administration, approximately 30% of the technetium-99m is excreted in the urine over the first 24 hr. The pertechnetate remaining in the body is evidently slowly metabolized, since the urinary excretion decreases markedly while fecal excretion increases progressively to a total of about 20% of the injected dose at 72 hr. In normal subjects, up to 2 % of the radioactivity from intravenously injected pertechnetate99m is accumulated by the ion-concentrating mechanism of the thyroid at 1 hr, which corresponds closely to the accumulation of radioiodide in the gland blocked with methimazole.

The factors governing the skeletal uptake of bone-seeking radionuclides in vivo have not been clearly defined. In this study, the effects of alteration in blood flow (thermal-induced) and alteration in osteogenesis (ricket-induced) on the skeletal uptake of these agents in rats were investigated. In vivo quantitative data were corroborated by the use of autoradiographic and well-counting techniques. Results indicate that the short-term uptake of commonly used bonescanning agents is closely correlated with blood flow and is largely independent of the rate of osteogenesis. S K E LE TAL IMAGING with radionuclides has gained wide clinical acceptance for the detection of metastatic bone disease (8, 13,42,49). Other skeletal disorders, namely primary bone tumors, arthritis, osteomyelitis, aseptic necrosis and metabolic bone disease, have also been studied and detected by means of bone-scanning procedures (4, 24, 39, 41, 43). Early clinical applications (6, 16, 17,21), using ca1cium-47 (t1h = 4.5 days, l' = 1.4MeV) and strontium-So (tl/2 = 65 days, l' = 512 keV), were severely limited due to the high radiation absorbed dose to the patient, which restricted the amount of radioactivity administered and resulted in poor count statistics and low-information images (8,49). The high gamma energies of these radionuclides were additional disadvantages for imaging with conventional equipment (9, 28).The introduction of the short-lived gamma emitters strontium-S'Zm (35) (tl/2 = 2.8 hr., l' = 390 keV) andfluorine-18 (9) (tl h = 1.8 hr., 'Y = 511 ke V) provided a distinct improvement, largely due to the greater amount of radioactivity which could be administered. The lower gamma energy of 87mSr was more efficiently collimated and detected with conventional scanning equipment. A major disadvantage of this agent was the expense of the 87mSr generators. Equally important was the biological property of strontium, namely slow renal and intestinal excretion, which resulted in high blood and soft-tissue levels during the effective halflife of the radionuclide (39, 43, 49). On the other hand, the extremely rapid blood clearance of 18F due to skeletal uptake and renal excretion provided improved target to nontarget ratios (8, 24,49). The short physical half-life of 18F and the necessity for shipping it from the cyclotron or reactor production site on the day of use contributed to its expensiveness and lack of availability for universal clinical application. Additionally, the high-energy photons resulting from positron annihilation were not easily collimated for imaging (28). The applicability of bone scanning in routine clinical practice was greatly expanded with the development of bone-seeking 99mTc_Sn-compounds of polyphosphate (45) and diphosplionate (12) that are easily prepared from commercially available kits. Technetium-Osrn (tl h = 6 hr., l' = 140 keV) is a nearly ideal agent for scanning procedures in nuclear medicine (28), i.e., a gamma emitter with energy suitable for imaging and readily available as an inexpensive generator. Th...

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