PET radioligand dose

The dose of radiopharmaceutical to be administered to subject is analyzed for safety according to pharmacy regulations and radiochemical purity before it can be injected. Radiochemist assesses the total radioactivity and the molar activity in the preparation, which is later decay corrected to the time of administration. Based on the volume, total radioactivity, and molar activity of the preparation, the volume that is drawn into a injection syringe is calculated, so that both the injected dose (MBq) and mass of radiopharmaceutical (µg or nmol) will stay between predefined limits. If the injected mass would exceed the limit, it could affect the distribution of the radiopharmaceutical in the body (“mass effect”) or lead to undesired pharmacological effects. The mass of the radioligand becomes easily the limiting factor for the administered dose in small animals (Jagoda et al., 2004; Kung & Kung, 2005; Wurzer et al., 2018). If the administered dose would be too low, the radioactivity concentrations in the tissues and blood would be too low to allow reliable measurements. Increasing the administered dose enhances the quality of PET image, plasma curve, and plasma metabolite analysis, and enables longer imaging; but it also increases the radiation dose (Gy) of the personnel and study subject, limiting the number of PET studies that can be performed for each subject. Very high administered doses may lead to saturation of detectors just after tracer injection, especially if heart is in the field of view, and with older PET scanners. Detector dead time may be too high also in blood sample analysis, but that can be solved by letting blood samples decay before measurement.

If the body weight of study subjects varies considerably, the administered dose can be given per kg of body weight. This approach provides more similar tissue radioactivity concentrations in individual subjects, and ensures that count level is always optimal for the PET scanner and image reconstruction.

Dose, mass, and molar activity

Administered mass can be calculated from the dose and molar activity:

or from specific activity (SA):

The molar mass (g/mol) of radioligand can be used to convert grams to moles.

Example

The injected dose of [11C]raclopride is 207 MBq, and molar activity is 128 GBq/μmol (specific activity is 370 MBq/μg), then the injected mass can be calculated as:

or


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Updated at: 2020-05-13
Created at: 2020-05-13
Written by: Vesa Oikonen