Blood sampling in PET studies
Quantification of PET image data requires measurement of input function. Input function (delivery function) describes the concentration of the unchanged (non-metabolized) tracer in arterial plasma (or blood, depending on the tracer) as a function of time (PTAC or BTAC).
Blood radioactivity concentration can be measured from the PET image, if a large artery or LV heart cavity is visible in the image. Even then some blood samples may be needed for
- scaling image-derived blood data to the correct level, if data are biased because of partial volume effect,
- scaling of population mean or model based blood data to the correct level,
- measuring plasma-to-blood ratio, if concentration in blood needs to be corrected to concentration in plasma,
- measurement of the fraction of unchanged tracer in plasma, if metabolite correction is needed,
- measurement of hematocrit, if it is needed in blood-to-plasma or plasma-to-blood transformation,
- measurement of glucose, FFA, or drug concentration, or oxygen saturation, in case that is needed in the quantitative analysis.
Before the PET study a short catheter has been inserted into artery of the arm, or into antecubital vein of the arm, if hand is heated to gather “arterialized” venous blood. Venous injection catheter can be used for venous sampling (Ponto et al., 2003). Arterial catheterization is safe (Jons et al., 1997) and reliable, but burdensome. Arterialized venous blood sampling can be used in certain, mostly diagnostic, studies when absolute quantification is not necessary; heating the hand leads to vascular dilatation, which increases blood flow but not metabolism, resulting in venous blood becoming almost identical to arterial blood (Brooks et al., 1989; Zello et al., 1990; Jensen and Heiling, 1991; Copeland et al., 1992; Liu et al., 1992; van der Weerdt et al., 2002). Arterialized venous blood sampling is acceptable for collecting PTAC for clinical use in case of certain PET tracers, including FDG, but may still not be valid method for taking samples for metabolite analysis.
Catheter is connected to saline drip via three-way cock. After tracer inhalation or injection via another vein and catheter, blood samples are collected by a syringe. A 2 mL waste sample (containing mainly saline) is taken before collecting the actual blood sample. Blood is transferred from syringe to a heparinized tube, avoiding hemolysis. Heparin and blood is mixed by turning the tube upside-down. Tube is then immediately placed on ice to slow down further metabolism and transport from plasma to blood cells. The exact time of blood sampling is recorded.
Blood sample tubes are centrifuged (at +4 °C) to separate plasma. A certain volume of plasma is then moved to another tube, and its radioactivity is measured using gamma counter. If also concentration in total blood needs to be measured, then the rest of the sample tube is also measured using gamma counter, and the tube is weighed. Concentration in blood can then be calculated from the sum of sample radioactivities and sum of weights, taking into account the physical decay between the measurements. Finally, all radioactivity concentrations are calibrated to kBq/mL and decay corrected to the tracer injection time.
Separate blood samples are collected for metabolite analysis. These tubes may contain inhibitor, preventing further metabolism of the tracer in vitro. Blood tube must still be immediately placed on ice to further slow down the metabolism and transport from plasma to blood cells. Plasma is separated by centrifugation at +4 °C. Plasma proteins are precipitated, usually with acetonitrile followed by centrifugation. Protein free plasma supernatant is then analyzed by chromatographic methods.
The total radioactivity of blood, plasma and urine samples is obtained by two automated gamma counters (1480 Wizard 3”, Wallac, Turku).
Arterial whole blood radioactivity concentration can be measured with an automatic blood sampling system (ABSS, “blood pump” with on-line detector) to record the curve precisely after radiotracer bolus infusion. In long PET studies ABSS is used only during the first 3-5 minutes p.i. and manual blood sampling is continued after that.
ABSS provides more precise results than manual blood sampling when the dynamics of the tracer in the blood is very fast, especially in oxygen-15 studies. However, longer tubing is needed in ABSS than with manual sampling, which may lead to higher dispersion error. ABSS measures only activity concentration in whole blood, and conversion to concentration in plasma may be error prone with some tracers. Arterial catheterization is required with ABSS.
Separate blood samples for metabolite analysis and measurement of plasma-to-blood ratio can be collected while the ABSS is still working. These samples can be taken from the end of the ABSS tubing, so that it does not any disturbance in the on-line data. For this purpose, ABSS can have a carousel system holding the blood tubes (Espagnet et al., 2017).
Internal carotid artery is close to the skin, and therefore positrons from the arterial blood can penetrate into a transcutaneous detector. Positron signal can be separated from γ-ray background, but arterial signal cannot be fully separated from the venous signal; yet, the peaks of arterial and venous curves can be detected, and this kind of system could be used as a complimentary method for determining the circulation time through the brain (Litton & Eriksson, 1990; Watabe et al., 1995). In radiowater studies, an external detector monitoring γ-rays from the superior lobe of the right lung provided BTAC equivalent to the blood sampling (Nelson et al., 1993).
Intra-arterial probe, combined with external detector which monitors the background, could be used to measure AIF (Pain et al., 2004; Lee et al., 2008), but the method may still suffer from background activity from tissues and vein close to the artery, especially when measuring AIF in small animals with small arteries. Radiosensitive probes can also be used to measure tissue radioactivity in small animals without PET scanner (Pain et al., 2002; Weber et al., 2003).
- Input function preprocessing
- Metabolite correction
- Input function
- ABSS data
- Image-derived input function
- Population-based input function
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Created at: 2006-05-23
Updated at: 2018-01-06
Written by: Vesa Oikonen, Pauliina Virsu, Tuula Tolvanen, Anne Roivainen