An Easily Overlooked Cause of High Level of Free Pertechnetate in Lung Perfusion Scintigraphy with ^99mTc-MAA Resulting From Improper Kit Reconstitution

Sir,

There is a diverse range of sources of error that renders ^99mTc-MAA lung perfusion scintigraphy unsuitable for interpretation by nonnegligible extrapulmonary uptake, as a result of poor radiolabeling efficiency. Most of the time, unbound “free” pertechnetate in the reconstituted solution is the main issue. When injected to the patient, though a high level of free pertechnetate in the circulation, accumulation of radioactivity in salivary glands, thyroid and gastric mucosa would be evident. Poor radiolabeling also results from a number of causes including quality of locally available commercial kits, incorrect order of reconstitution steps, excessive amount of eluted technetium added to the vial and even adding a disproportionate volume of normal saline to the vial before labeling.[123] The latter is of serious concern in laboratories with infrequent number of lung perfusion scans on a daily basis. Because in such situations, each vial is mostly used for one or few number of patients. The hot laboratory technician, in order to decrease the amount of activity required to inject into the vial and thus avoid wasting the eluted activity, might considerably reduce the number of particles in the MAA vial by adding normal saline, i.e., particle dilution, and then draw a proportion, say almost one half, then adding the pertechnetate. This violation of the instructions provided by the kit manufacturer may lead to poor labeling as that in our 82-year-old patient referred for the evaluation of pulmonary thromboembolism. The commercially available MAA kit, used regularly in our laboratory, contains 2–4 million particles and the manufacturer recommends adding a dose of 40 mCi. The first scan is performed by the ^99mTc-MAA kit prepared incorrectly by the technician in our laboratory, i.e., by particle dilution to reduce the number of particles, and the second scan is repeated another day by the kit of the same batch number, but with exact following the reconstitution steps recommended in the package insert [Figure 1]. Taken together, the discussion above again emphasizes the adherence to supplied instructions for preparation and use of radiopharmaceuticals, particularly avoiding dilution of particles and using a small volume of reconstitution in vial to obtain an optimal result.[13]

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Figure 1 (a) Anterior and posterior spot view of the thorax and upper abdomen of the first scan shows remarkable uptake in extrapulmonary tissues in thyroid and gastric wall as well as high background. (b) Head and neck view confirms uptake in the salivary glands, indicating high levels of free pertechnetate. (c and d) Anterior and posterior views of the lung in second scan, no activity is discernible outside the lungs, denoting a well-prepared radiopharmaceutical

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