Labelled microspheres can be used to quantify tissue perfusion and blood flow distribution (Prinzen & Bassingthwaighte, 2000). 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 commonly used in humans, because it may cause ischemic organ damage. Microspheres can be used to validate the PET perfusion tracers, usually in animal studies (Kairento et al., 1983). Microspheres can be labelled with radionuclides, or alternatively with fluorescent dyes (Glenny et al., 1993). Albumin macroaggregates (MAA) have the advantage of being metabolized (Zolle et al., 1970). For instance, 131I- and 99mTc-labelled albumin macroaggregates have been commercially available and used for studying myocardial function and pulmonary perfusion and perfusion distribution in dogs (Poe, 1971; Schelbert et al., 1971; Richmond et al., 1973) and humans (Rhodes et al., 1973; Grames et al., 1974; Baker et al., 1984; Del Torso et al., 1985), and concluded to be safe.
For PET imaging, MAA has been labelled with 68Ga and 64Cu for perfusion imaging in dogs (Hnatowich, 1976; Wisenberg et al., 1981), rabbits (Kairento et al., 1983), rats (Richter et al., 2010; Boś et al., 2012; Amor-Coarasa et al., 2014) and baboons (Steinling et al., 1985); and covalently labelled with [11C]CH3-groups (Turton et al., 1984), and used to measure perfusion in humans (Wilson et al., 1984; Brooks et al., 1986; Selwyn et al., 1986). 68Ga-MAA could be used in human studies as surrogate for 99mTc-MAA microspheres (Matthies et al., 2008; Maus et al., 2011).
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) and to assess response after treatment (Alsultan et al., 2021).
CT angiography could be used to locate the best arteries for microsphere injection (Simoncini et al., 2018).
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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. doi: 10.1118/1.3608909.
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Updated at: 2021-11-28
Created at: 2015-12-23
Written by: Vesa Oikonen