Development of an optical measurement method for “sampled” micro-volumes and nano-flow rates

Abstract Radiopharmaceuticals used in nuclear medicine for therapy or diagnosis (molecular imaging, PETscan, scintigraphy) are characterized in terms of volume activity before injection to patients. The current measurement process relies on dose calibrators which have to be calibrated by transfer standards, traceable to primary standards. For very short half-life radionuclides (few minutes), the metrological traceability can only be assured through an on-site calibration with primary standards. However, until now, there is no primary system for the direct measurement of such high activity radioactive solutions. This study presents the sub-system under development for the measurement of a sampled volume of the order of 1 μL with an associated relative target standard uncertainty of 1%. The volume of solution is a key parameter in activity per unit of volume calibration of radiopharmaceuticals solutions. This article focuses on the volume measurement method development and its validation by comparison to the gravimetric method. The paper, in a first part, describes the developed method and the associated hardware and software. The authors have chosen a non-contact optical method implemented by a microscope camera and associated optics in front of a transparent capillary. The second part of the paper describes the measurement process. Several image processing steps are described and the traceability to dimensional units is presented. Finally, the paper presents some validation results by comparison to a gravimetric measurement, including repeatability and accuracy tests. Further development and improvements, necessary for the finalization of the prototype and the measurement of nano-flow rates are discussed.