Dosymmetric estimates of 99mTc (MAA) and 133Xe in newborn lungs using Cristy-Eckerman / Segars representations

Using the Cristy-Eckerman (C-E) / Segars anatomical representations and the MIRD formalism, the Absorbed doses in lungs of newborn patients scanned with radiopharmaceuticals 133Xe (ventilation) and 99mTc (MAA) (perfusion) are estimated. These representations are phantoms used in Monte Carlo calculations to determine specific absorbed fractions, which, associated with the pharmaceutical residence time, determine the absorbed dose. Concerns about the dosimetric impact of using these ventilation / perfusion agents, as well as the use of different phantoms, were explored in newborn patients. When the lungs were scanned with 99mTc (MAA), the relative difference in total dose between the C-E / Segars anatomical representations was 1.0%. When the lungs were scanned with 133Xe, the relative difference in total dose between the anthropomorphic representations of C-E / Segars was 0.5%. Regardless of the radiopharmaceutical used for the pulmonary studies of a newborn patient, the substitution of the C-E representation for that of Segars does not reflect very significant changes in the calculation of the absorbed dose in the lungs, where the greatest dosimetric contribution is its self-dose, which is supplied mainly by the electrons produced during the 99mTc and 133Xe decay.

Dosimetry of radiopharmaceuticals used in adult patients with suspected pulmonary embolism

The absorbed dose of radiopharmaceuticals is estimated in adults with suspected pulmonary embolism explored by ventilation/perfusion studies. For pulmonary ventilation studies 81mKr, 133Xe, 99mTc (Technegas)-aerosol and 99mTc (DTPA)-aerosol are used. For perfusion agents, 99mTc(MAA), 99mTc (MSA) (macroaggregates and albumin microspheres) are used. For the dose calculation, the MIRD methodology and the anthropomorphic representation of the biokinetic organs of Cristy-Eckerman are used. In ventilation/perfusion studies, the lowest dose absorbed by the lungs with suspected embolism is due to 81mKr/ 99mTc (MSA), and the highest dose is due to 99mTc (Technegas)/99mTc (MAA) calculated for activities of 150 MBq for perfusion agents and 40 MBq for ventilation agents.

Cardiac PET-CT: A Brief Review of Indications and Feasibility in the Indian Scenario

Cardiac positron emission tomography (PET) is a functional imaging modality with the predominant indications being an assessment of myocardial perfusion and viability using short-lived positron emitters. PET procedures are used for perfusion assessment only when single-photon emission computed tomography is equivocal. Perfusion agents are ultra short-lived and have limited availability in our country, that is, only at centers with onsite cyclotron. Viability assessment with fluorodeoxyglucose (FDG) is widely performed in many centers across the country due to the availability of FDG. The other important indication is in the detection of inflammatory myocarditis, the most common being cardiac sarcoidosis done with special dietary instructions and FDG as a tracer. This indication is widely used for early detection of myocardial inflammation which would be reversible if treated early. It is also used in the assessment of treatment response. Newer tracer 68 Ga Dotapeptide is also used in this application.