# Metabolic rate of oxygen in the brain using [15O]O2-PET

The cerebral metabolic rate for oxygen, CMRO2, blood flow, CBF, oxygen extraction fraction, OEF (i.e. oxygen extraction ratio, OER), and cerebral blood volume (CBV) are important indices in disease and health, because of the high metabolic rate and demand for oxygen in the brain. All of these can be regionally quantified using PET and 15O tracers (see review by Baron and Jones, 2012), while global OEF can be assessed by blood sampling alone (Hattori et al., 2004). Differences in magnetic susceptibility between oxy- and deoxy-haemoglobin can be utilized in BOLD MRI, and QSM MRI can even provide maps of baseline OEF (Kudo et al., 2016; Uwano et al., 2017). MRI after 17O-enriched O2 inhalation can provide CMRO2 images (Kurzhunov et al., 2017).

CMRO2, as determined using [15O]O2-PET, is similar in both sexes and does not change with age, although there are differences in the perfusion (Aanerud et al., 2017) and in synaptic density (Alonso-Nanclares et al., 2008).

## Model for brain [15O]O2-PET

Figure 1. Cerebral blood flow delivers labelled oxygen into the capillaries of the brain. A fraction of the oxygen-15 that is diffused into the brain tissue is used in the oxidative metabolism of the brain, forming O-15 labelled water; the labelled water and the rest of the remaining fraction of labelled oxygen are flushed from the brain tissue with blood flow. During the PET scan, labeled water is also formed elsewhere in the body, and it is delivered to the brain from arterial blood like labelled oxygen and washed away like the labelled water that is formed in the brain.

The metabolic rate of oxygen in the brain has been measured with steady-state and bolus inhalation techniques (Frackowiak et al., 1980; Depresseux et al., 1981; Herscovitch, 1995). Radiation dose to the patient is smaller in the single bolus inhalation than in the “gold standard” steady-state techniques, because separate [15O]H2O and [15O]CO studies are not necessarily needed. Single inhalation bolus study also require less scanner time. Techniques that require several PET scans may be unreliable if subject can move between the scans (Correia et al. 1985). Steady-state technique is still widely used, and if can be fully non-invasive, if [15O]H2O injection is replaced by [15O]CO2 inhalation. Steady-state technique is even applicable to mice studies, and because of the non-invasiveness, it can be repeated to the same animals in a follow-up study setting (Temma et al., 2017).

Bolus inhalation models are based on the study of Mintun et al (1984), in which perfusion and blood volume were still measured in separate PET studies using [15O]H2O and [15O]CO. In this two-compartmental model it is assumed that cerebral blood flow (CBF, f) delivers a certain amount of labelled oxygen into the blood volume in the tissue. A fraction of this oxygen (OEF, oxygen extraction fraction) is diffused to tissue and metabolized instantly to labeled water ([15O]H2O); the remaining fraction of delivered oxygen (1-OEF) is flushed from the tissue with blood flow. Labelled water is flushed away from the tissue with rate constant f/p, where p is the partition constant of the water. There is no marked back-flux of labelled oxygen from the brain tissue to blood (Iida et al., 2014).

O2 is readily dissolved and diffused in lipid bilayers. The blood-to-brain permeability of O2 is even higher than that of water (Kassissia et al., 1995.

During the PET scan, labelled water is also formed elsewhere in the body, and it is delivered to the brain from arterial blood with perfusion f and washed away like the labelled water that is formed in the brain. If the oxygen concentration in arterial blood, [O2]a, is measured, then the metabolic rate of oxygen (MRO2) can be calculated (Ter-Pogossian et al., 1970) as:

Notice that equation for calculating [O2]a may be different in animals than in humans (Poulsen et al., 1997). In healthy human subjects, OEF is ∼0.4, and relatively uniform in the brain, despite the 4-fold difference in perfusion of the grey and white matter (Raichle et al., 2001). In the brain, perfusion is locally controlled in response to changed neuronal activity, with relatively low partial pressure of O2 at all times.

It is not necessary to measure the blood flow or blood volume separately, because f*OEF and VB can be estimated from a single inhalation [15O]O2 PET study (Holden et al. 1988). However, quantification of OEF requires the measurement of perfusion with [15O]H2O. In bolus and steady-state studies the [15O]CO scan can be omitted without introducing marked error in OEF or CMRO2 (Lammertsma and Jones, 1983; Kudomi et al., 2005; Sasakawa et al., 2011).

Ohta et al (1992) have further simplified the calculation model. This model can be linearized (Blomqvist 1984; Poulsen et al., 1997), which permits pixel-by-pixel computation to produce parametric cerebral MRO2 image. Unlike for the previous models, in this method the separation of [15O]O2 and [15O]H2O in arterial blood is not necessary. When the duration of the study (time that is used in the model fit) is limited to 3 minutes, and the oxygen consumption is in the range 50-300 μmol/(min 100 g), the error caused by ignoring the recirculating radiowater will remain between ±10% (Ohta et al., 1992). This method has been used in TPC (Kaisti et al., 2003; Långsjö et al., 2005) and elsewhere (Aanerud et al., 2017; Blazey et al., 2018).

Performance of the model can be studied with simulations, for an example see the study by Duval et al (2002). Valabrègue et al., (2003) have developed a model where the assumption of insignificant [O2] in the brain tissue is relaxed; the model may not be applicable to analysis of PET data, but can still be useful in simulations.

## Analysis of data from single-inhalation study

In single-inhalation model the parameter K1 in the one-tissue compartment model is assumed to represent f*OEF:

, thus, estimation of MRO2 requires simply estimating K1 from the PET data, and multiplying it with [O2]a.

### Pre-processing of arterial blood curve

Arterial blood data from the on-line sampler needs to be processed before it can be used as input function in the calculation. Because blood TAC does not need to be corrected for [15O]H2O in the single-inhalation model, the blood data can be pre-processed using same procedure as the input function in [15O]H2O PET studies.

If you are working in TPC using computer with Windows XP, use the script water_input to process the on-line detector (blood pump) data prior to the analysis. It requires the countrate curve or similar data for the time delay correction; time delay correction is necessary in order to obtain non-biased oxygen consumption estimates (Poulsen et al., 1997).

Notice that in [15O]O2 PET studies the count rate curve has often been unusable, probably because of high random counts from dose collection system and/or exhaling of 15O gases; then we would advice to create “head curves” from dynamic PET images using imghead for all studies and use these instead of countrate curves.

The corrected blood TAC should always be plotted and controlled visually. Water_input script creates a plot of corrected input curve and count rate curve. Corrected blood curve often contains close-to-zero values in the end, which should be removed with a text editor, or left out when determining the fit time.

### Compute K1

Make sure that PET data (dynamic image or regional TAC file) are in the same calibration units.

If dynamic PET image is very noisy, filtering dynamic PET image may be needed before proceeding. Modern image reconstruction methods are recommended over FBP to reduce the noise level (Ibaraki et al., 2009). Even after filtering, the noise-level in dynamic image may be too high to allow fitting vascular volume fraction (VB) as one of model parameters. If necessary, correct the PET data for the contribution of vascular radioactivity using a measured or population average based VB.

#### Calculation of K1 image

Compute the K1 image using imgflow with the following command-line arguments:

1. option -Va=none to prevent an additional VB correction, if vascular volume was previously corrected
2. corrected arterial blood datafile (times in seconds)
3. Dynamic [15O]O2 image file, preferably corrected for VB
4. fit time in seconds (max 180 in this model)
5. file name for the K1 image

The units in the [15O]O2 image are (mL blood)/(min * mL tissue) by default.

#### Calculation of K1 from regional TACs

Regional K1 can be estimated using fit_h2o with the following command-line arguments:

1. Preferably, option -Va=4 to use pre-determined VB value (4% in the example) instead of estimating it as one model parameter
2. We recommend using option -svg=filename to save fitted TACs in SVG format for verifying the goodness-of-fit
3. Option -ml to save results in units (ml blood)/(min * ml tissue)
4. Corrected arterial blood datafile
5. Regional TAC file
6. fit time in minutes (max 3.0 in this model)
7. file name for the K1 results

### Conversion of K1 values to MRO2

K1 is multiplied by the concentration of O2 in arterial blood. Program imgcalc can be used to process K1 image. Regional K1 result file can be read into a spreadsheet program and processed further there.

Turku PET Centre receives the arterial oxygen concentrations from the hospital laboratory in units ml O2/l blood, which have to be converted to ml O2/100 ml. The concentrations are normally about 20 ml O2/100 ml blood. If MRO2 is required in molar units, then [O2]a must be divided by the molar volume of an ideal gas, 22.4 ml/mmol; thereafter [O2]a values are about 0.9 mmol O2/100 ml blood.

After the multiplication, the unit of the MRO2 is either ml O2 / (min * 100 ml tissue) or mmol O2 / (min * 100 ml tissue), depending on the unit of [O2]a. If MRO2 is required per tissue mass instead of volume, the values can be divided by tissue density (specific gravity) of the brain, 1.04 g/ml (Reference Man).

Cerebral MRO2 in normal subjects are in the range of 2.2 to 3.5 ml O2 / (min * 100 g tissue) in gray matter (Perlmutter et al. 1987; Leenders et al. 1990). Division by 22.4 ml/mmol gives range 0.10 - 0.16 mmol O2 / (min * 100 g tissue).

#### Example of MRO2 image

Below are two MRO2 images calculated from the same PET study, where dynamic image was reconstructed with FBP and normal parameter settings. In the latter case, dynamic image was further filtered before calculation of K1 image using imgdysmo with options -m=5 -s=4. Images are not in the same colour scale.

The level of results from parametric images should always be verified against results from regional curves. Noise in dynamic image may lead into biased results with distorted variance. Filtering of dynamic images may be needed to achieve the same quantitative results as in the regional analysis. To prevent artefacts and excessive loss of image resolution, the strength of filtering must not exceed the level that is required to achieve comparable results.

## Potential problems

In brain PET studies with gaseous radiotracers the high radioactivity in face mask may cause severe image artifacts when conventional scatter correction methods are used, but appropriate selection of scatter correction method can solve the problem (Magota et al., 2017).

Caffeine may reduce CMRO2, via reduced cerebral perfusion that is only partially negated by increased OEF (Merola et al., 2017).

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Updated at: 2019-06-07
Created at: 2007-09-18
Written by: Vesa Oikonen