Labelled microspheres can be used to quantify tissue perfusion in animal studies. Larger microspheres that are trapped in the precapillary arterioles measure total blood flow in the tissue, and smaller microspheres which are only trapped in the capillaries measure the nutritive blood flow. Microsphere method is not applicable in humans, because it may cause ischemic organ damage. In animal studies microspheres can be used to validate the PET perfusion tracers (Kairento et al., 1983). Microspheres can be labelled with radionuclides, or alternatively with fluorescent dyes (Glenny et al., 1993).
In animal studies the microspheres are injected into the left atrium or left ventricle of the heart to avoid initial trapping in the lungs but allowing dispersion via the circulation into organs according to the distribution of the cardiac output (CO). Blood sample is collected from an artery (usually femoral artery) using a flow-rate controlled pump during the time that it takes for the microspheres to be completely distributed into tissues (usually 2-3 minutes). The organ is then removed, weighed, and its radioactivity is measured. Alternatively, radioactivity concentration can be measured with PET, if microspheres are labelled with positron-emitting isotope.
Organ perfusion (blood flow, f) can then be calculated in units mL/min per organ from the equation
, where Fs is the pump flow rate (mL/min), Atis is the radioactivity of the dissected organ, and Ab is the activity of the blood sample.
Alternatively, if radioactivity concentration in tissue (sample) was measured, either by weighing the tissue sample or by PET, the perfusion can be calculated in units mL/(min*g) or mL/(min*mL) from similar equation:
, where Ctis is the radioactivity concentration in the tissue sample (radioactivity per gram or mL).
Also cardiac output can be calculated in units mL/min from equation
, where Atot is the total radioactivity of the injected microspheres.
In selective internal radiation therapy (SIRT), trans-arterial radioembolization (TARE), or radiomicrosphere therapy (RMT), microspheres are loaded with a beta emitter, such as 90Y, and injected to a small artery that supplies the tumour. Size range of the microspheres is such that microspheres are trapped in the pre-capillary arterioles, and cannot be redistributed to other organs. PET can be used to assess in vivo dosimetry after 90Y radioembolization (D’Arienzo et al., 2017).
CT angiography could be used to locate the best arteries for microsphere injection (Simoncini et al., 2018).
D’Arienzo M, Filippi L, Bagni O. Quantitative postradioembolization imaging using PET/CT. In: Handbook of Radioembolization - Physics, Biology, Nuclear Medicine, and Imaging. Pasciak AS, McKinney JM, Bradley YC (eds.), CRC Press, 2017, pp 229-249. ISBN 978-1-4987-4201-6.
Dezarn WA, Cessna JT, DeWerd LA, Feng W, Gates VL, Halama J, et al. Recommendations of the American Association of Physicists in Medicine on dosimetry, imaging, and quality assurance procedures for 90Y microsphere brachytherapy in the treatment of hepatic malignancies. Med Phys. 2011; 38: 4824–4845.
Kairento AL, Brownell GL, Schluederberg J, Elmaleh DR. Regional blood-flow measurement in rabbit soft-tissue tumor with positron imaging using the C15O2 steady-state and labeled microspheres. J Nucl Med. 1983; 24(12): 1135-1142.
Prinzen FW, Bassingthwaighte JB. Blood flow distributions by microsphere deposition methods. Cardiovasc Res. 2000; 45(1): 13-21.
Created at: 2015-12-23
Updated at: 2018-12-02
Written by: Vesa Oikonen