Brain blood flow using [15O]H2O and PET
The brain receives about 15% of the cardiac output and uses about 20% of oxygen used by the body. The cerebral metabolic rate for oxygen, CMRO2, and cerebral blood flow (CBF, perfusion) are tightly coupled (Raichle et al., 1976a). Young women have higher perfusion than men in the cortical grey matter whole, although men have higher synaptic density than women and CMRO2 is similar in both sexes (Aanerud et al., 2017). Cerebral perfusion decreases with ageing in women but not in men, and at age 65 no differences in perfusion remains between sexes (Aanerud et al., 2017). Caffeine affects cerebral perfusion, and both caffeine consumption and withdrawal are confounding variables in brain perfusion studies (Field et al., 2003).
The analysis method of blood flow (perfusion) in the brain (CBF) is based on the specific one-tissue compartment model for [15O]H2O. [15O]H2O is freely diffusible and metabolically inert. Repeated measurements with [15O]H2O or other tracers can be performed within one scanning session because of the short halflife of 15O. [15O]H2O and [15O]O2 studies are often performed together to quantify CBF, OEF, and CMRO2.
Perfusion reserve (usually 30-40%) can be measured by assessing CBF in baseline and after acetazolamide challenge, either by two bolus PET studies, or during one constant-infusion radiowater PET (Weber et al., 2004).
Blood flow can be estimated from dynamic PET data after a bolus infusion of [15O]H2O or [15O]CO2, either from regional tissue time-activity concentration curves (TTACs), or from dynamic PET image to produce perfusion map, using kinetic model fitting methods, or, also from static imaging using ARG method. These quantitative methods use arterial blood curve (BTAC) as their arterial input function (AIF).
The first quantitative CBF measurements with PET and [15O]H2O were not performed with dynamic scanning, but using steady-state technique. Since modern PET scanners enable collection of dynamic data with good accuracy, and dynamic and ARG methods lead to lower radiation dose, the steady-state method is seldom used.
Alternatively, static or summed dynamic PET image data can be used without blood sampling in brain activation studies (Evans et al., 1992); only an index of regional perfusion changes can then be observed. Population-based input function can be used to improve the quantitation of regional blood flow changes in brain activation studies.
Semi-quantitative perfusion estimates can be calculated using double-integration method (Koopman et al., 2017).
Image-derived input function is difficult to obtain in brain PET studies because the carotid arteries are so thin that spill-in and spill-out effects need to be accounted for. Dose-optimization with TOF-enabled PET/MR enables the measurement of angiograms using both PET and MR, and based on those the AIF can be calculated (Khalighi et al., 2018).
In theory, reference tissue could be used to cancel out the arterial input function from the equations, but in case of radiowater, the perfusion in the reference tissue should be known. Mejia et al (1994) used whole brain as reference, assuming that its perfusion is 50 mL/(dL*min), and assuming that the partition coefficient p is 1 in all brain regions. Lammertsma (1994) noted that fitting both perfusion and p would provide perfusion estimates that would not be biased by variable p and tissue heterogeneity. Watabe et al (1995) validated the method further, using weighted integration method, and generalized it to assess perfusion in two brain regions, serving as reference region for each other; this idea has since then expanded to model-based input function methods (see below). More recently, integrated PET-MRI systems have allowed simultaneous measurement of brain perfusion using radiowater PET and phase-contrast MRI. MRI-based perfusion estimate in the brain can be used as the reference tissue perfusion for the PET method (Ssali et al., 2018).
Methods applying model-based input function have been developed for brain [15O]H2O data analysis (Watabe et al., 1996; Kudomi et al., 2002; Treyer et al., 2003; Kudomi et al., 2016). Fiestra et al. (2018) validated fMRI assessment of cerebrovascular reactivity (CVR) against PET-derived cerebral blood flow reserve, measured using the method by Treyer et al. (2003).
The method of Treyer et al (2003) is based on a population average AIF, which is used to calculate parametric images of K1 and k2, and the K1 image, having better quality, is scaled to correct level, representing CBF, based on cortical average of K1 and k2 values. The method essentially assumed that the partition coefficient p is 1 in all brain regions.
Preprocessing arterial blood data
Arterial blood data, collected using on-line sampling system, must be calibrated and corrected for physical decay, dispersion and time delay. It is recommended that regional tissue TAC from pancreas is used in time delay correction.
- Process the count-rate data, or draw ROI on the whole brain and calculate tissue TAC to be used as “head curve” in time delay correction (below). Make sure that TAC files do not contain ROIs of large blood pools!
- On MS Windows PC in TPC network, do the corrections for blood data using water_input script. Alternatively, these corrections can also be done using a series of low-level commands.
- Verify visually that the corrected blood TAC is fine and that time delay correction has moved
it to start to rise at the same time as the tissue TACs. Previous
water_inputcommand made a graph of these curves. Alternatively you can create the plot by yourself. Sampler blood TAC often contains close-to-zero values in the end, which should be removed with a text editor, or left out in analysis when setting the fit or integration time.
Calculation of CBF
Follow the general instruction for analysis of radiowater data to calculate CBF from either regional TTAC data or to compute perfusion maps using kinetic method, or instructions for ARG method to compute perfusion maps with ARG technique.
Kinetic method can provide also an estimate of the arterial plus capillary blood volume, which may be more reliable hemodynamic parameter than the total blood volume (including venous volume) resulting from [15O]CO PET study (Okazawa et al 2001).
- Radiowater model
- ARG method
- Fick’s principle
- [15O]H2O in the brain
- Analysis instructions by tracer
- Compartmental model
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Updated at: 2018-11-30
Created at: 2014-05-07
Written by: Vesa Oikonen