Quantification of fatty acid uptake with[11C]palmitate-PET
Background
Radioactive metabolites
[11]CO2 and [11C]HCO3- are produced by peripheral and local metabolism of [11C]palmitate-PET, and it readily enters all tissues and approaches a transient equilibrium based on the difference between tissue and plasma pH (Brooks et al., 1984). Concentration of [11]CO2 in plasma or blood must be measured or estimated and subtracted from the input curve. Also its contribution to tissue radioactivity must be accounted for.
Also 11C bearing urea, lactate and glucose can be found in tissues and blood.
Myocardium
Heart normally extracts 40-50% of labelled palmitate in a single transit time. The capillary endothelium is a major barrier to the extraction of FFAs.
Myocardial [11C]palmitate PET studies are commonly analyzed by measuring the clearance of [11C] from myocardial regions assuming bi-exponential washout. The faster component correlates with β-oxidation including formation of [11C]-2-acetyl-CoA and production of [11C]CO2 by the tricarboxylic acid (TCA) cycle in the mitochondrial matrix. The slower washout component correlates with incorporation of [11C]palmitate into triglyserides and phospholipids (Schelbert et al., 1986; Grover-McKay et al., 1986).
Bergmann et al. (1996) have developed and validated in dogs a four-compartment model for analysis of myocardial [11C]palmitate PET studies for estimation of myocardial fatty acid utilization (MFAU) and myocardial fatty acid oxidation (FMAO). This method requires blood TAC is measured, [15O]CO2 appearing in blood is measured and subtracted, and that myocardial blood flow is determined from a separate [15O]H2O scan. This method has been used e.g. by Herrero et al. (2005).
Angsten et al (2005) analyzed their animal PET data with a simplified approach: Gjedde-Patlak plot was first used to determine [11C]palmitate net influx rate (KT, or Ki) using arterial curve measured from LV region in the image, and myocardial curve from about half a minute to two minutes after injection. Further, a monoexponential function was fitted to blood curve from about one minute to two minutes after injection and was used to extrapolate the blood curve of parent tracer at later time points. Using this extrapolated blood curve and KT, the total amount of extracted tracer at late times was calculated and compared to the measured tissue concentration at the same time (representing the tracer retained in slow turnover compartment) to calculate the fractional oxidative utilization of palmitate.
Suggested analysis method
Calculate the [11C]palmitate uptake index by dividing the myocardial concentration at 7.5 min (peak value) with the integral of the metabolite corrected plasma curve (Knuuti et al., 2001). Index was then further multiplied with the concentration of non-esterified fatty acids in plasma.
To estimate [11C]palmitic acid β-oxidation rate, a two-exponential function is fitted to the steepest descent of the myocardial curve (11-27.5 min) and the clearance rate (1/min) or half-time (t1/2, min) of the faster component is reported (Knuuti et al., 2001).
Tools for these calculations will be available in a future release of CarimasTurku.
Brain
About 50-60% of [11C]palmitate is oxidized to produce [11C]CO2 and other hydrophilic metabolites. The rest enters rapidly the stable brain lipid pool (Miller et al., 1987; Chang et al., 1994). Brain preferentially incorporates fatty acids into phosphoglyserides instead of neutral lipids.
A three-compartment model for palmitate incorporation into (rat) brain has been presented by Tabata et al. (1988). Simplified method for estimating the rate of incorporation of fatty acids into brain phopholipids and other stable compartments (not β-oxidation rate) was further developed in the study of Robinson et al. (1992). Model was also used to analyze monkey brain PET studies (45 min), now including also cerebral blood volume and the brain [11C]CO2 in the model (Arai et al., 1995): mean net uptake rates were about 0.0027 ml(min g)-1 in cortical regions and 0.0017 ml(min g)-1 in white matter. However, monkeys were anesthetized, which in rats reduced brain uptake rate by 40%. Vb was 3.5 - 4.8% in cortical regions and 2.2% in white matter (Arai et al., 1995). The same model was applied to monkey [11C]arachidonic acid studies (Chang et al., 1997). Time delay correction, a different correction method for the brain [11C]CO2 (and [11C]HCO3-, and for partial volume effects were introduced by Giovacchini et al. (2002 and 2004) for [11C]arachidonic acid human PET studies; these methods would be applicable to [11C]palmitate studies.
Esterified re-circulating palmitate seems not to make any measurable contribution to (rat) brain uptake (Purdon et al., 1997).
Suggested analysis method
Time-delay correction
Time-delay between plasma and tissue TACs must be corrected before analysis using fitdelay. If blood TAC has been measured, it can be corrected for time-delay at the same time.
Preparing the blood curve
Because the fatty acid uptake into the brain is very low, the impact of vascular radioactivity is very high and must be considered in analysis. If blood curve has not been measured, it can be calculated from the total plasma curve (not corrected for metabolites) using p2blood.
Correcting tissue data for [11C]CO2
Before this correction, weights for fitting should be added to the regional tissue data files. This is necessary because the early time frames are usually much shorter (and numerous) than the later frames.
The fractions of [11C]CO2 of the total blood radioactivity has been determined using evaporation method from eight subjects in our laboratory, and the population average of fraction curves and individual total plasma TACs can be used to calculate a good estimate of plasma [11C]CO2 concentration during the PET study using metabcor. This curve is then used as input in calculation of the tissue concentration of [11C]CO2, using one-tissue compartment model and assuming that K1=0.3 mL/(min*mL) and K1/k2=0.43, as suggested by Giovacchini et al. (2002) based on the study of Brooks et al. (1984). This tissue [11C]CO2 curve is then subtracted from the measured regional tissue curves.
Correction for radioactive metabolites in plasma
In the previous phase, as the plasma [11C]CO2 concentration was calculated using metabcor, also the plasma curve corrected for [11C]CO2 was calculated as well.
However, the evaporation method measured only the fraction of [11C]CO2, not the fraction of other labelled metabolites that appear in plasma at later times. In the relatively long brain [11C]palmitate studies also these metabolites have to be subtracted from the plasma.
One method to correct for these is to fit a single exponential function to
the descending part of plasma curve, corrected for [11C]CO2,
before the appearance of other labelled metabolites (usually about 15 minutes)
and extrapolate the plasma curve after this time using the fitted exponential
function;
extrapol
can be used for this purpose with option -e=15.
Estimation of K
One-tissue compartment model with two parameters, blood volume fraction,
VB and a single rate constant, K, or K1, are estimated
from the metabolite correct plasma curves, blood curve, and
[11C]CO2 corrected and weighted tissue curves using
fitk2,
version 1.0.1 or later. Specify the following parameter initial values and
constraints in a file with option -i=filename :
K1_initial := 0.002 K1_lower := 0 K1_upper := 0.01 K1k2_initial := 0 K1k2_lower := 0 K1k2_upper := 0 Vb_initial := 0.045 Vb_lower := 0 Vb_upper := 0.75
Liver
Insulin has a strong effect on liver metabolism of [11C]palmitate (Guiducci et al., 2006), suggesting that PET studies with [11C]palmitate and palmitate analogs will provide very interesting insight in regulation of lipid metabolism in liver.
There is no published analysis method for analyzing hepatic [11C]palmitate data. However, the results of Yamamura et al. (1998) suggest that the first component of two-exponential function fit could provide useful information on β-oxidation. DeGrado et al. (2000) calculated volume of distribution (VT) from liver PET data, not a [11C]palmitate study, but another tracer with clearly reversible kinetics, and showed that VT (nonlinearly) correlated with [3H]palmitate oxidation rate. Therefore, multiple-time graphical analysis for reversible uptake (Logan plot) with metabolite corrected plasma input might be a usable analysis method for quantitation of [11C]palmitate PET data in liver.
Skeletal muscle
At rest, up to 90% of the energy requirements of muscle are obtained from fatty acid oxidation. During fasting condition, the extraction of FFAs is about 40%.
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