Human (13)N-ammonia PET studies: the importance of measuring (13)N-ammonia metabolites in blood

Abstract

Dynamic (13)N-ammonia PET is used to assess ammonia metabolism in brain, liver and muscle based on kinetic modeling of metabolic pathways, using arterial blood (13)N-ammonia as input function. Rosenspire et al. (1990) introduced a solid phase extraction procedure for fractionation of (13)N-content in blood into (13)N-ammonia, (13)N-urea, (13)N-glutamine and (13)N-glutamate. Due to a radioactive half-life for (13)N of 10 min, the procedure is not suitable for blood samples taken beyond 5-7 min after tracer injection. By modifying Rosenspire's method, we established a method enabling analysis of up to 10 blood samples in the course of 30 min. The modified procedure was validated by HPLC and by 30-min reproducibility studies in humans examined by duplicate (13)N-ammonia injections with a 60-min interval. Blood data from a (13)N-ammonia brain PET study (from Keiding et al. 2006) showed: (1) time courses of (13)N-ammonia fractions could be described adequately by double exponential functions; (2) metabolic conversion of (13)N-ammonia to (13)N-metabolites were in the order: healthy subjects > cirrhotic patients without HE > cirrhotic patients with HE; (3) kinetics of initial tracer distribution in tissue can be assessed by using total (13)N-concentration in blood as input function, whereas assessment of metabolic processes requires (13)N-ammonia measurements.

Fig. 1

Diagram of sequential solid phase fraction extraction procedure for determination of ¹³N-metabolites

Fig. 2

Reproducibility study. Time courses of the ¹³N-ammonia fractions in arterial blood in 5 cirrhotic patients without HE, each studied by two successive ¹³N-ammonia injections with a 60-min interval. Data points from the two injections in a patient are shown by similar symbols, being open and closed, respectively, and points from each injection are connected by lines with the same type of line

Fig. 3

Time course of fractions of ¹³N-ammonia in arterial blood in three groups of subjects as indicated (from Keiding et al. 2006). Curves are double exponential fits (see Table 2)

Fig. 4

Examples of ¹³N-metabolite fractions in arterial blood in a cirrhotic patient with HE (closed symbols) and a healthy subject (open symbols)

Fig. 5

Time-courses of blood concentrations of total ¹³N-radioactivity concentrations (black) and of ¹³N-ammonia (red) and ¹³N-metabolites (blue and black) after intravenous injection of 500 MBq ¹³N-ammonia tracer to a cirrhotic patient with HE (upper graph) and a healthy subject (lower graph). The time-courses of ¹³N-urea and ¹³N-glutamine are coincident in the healthy subject

Fig. 6

Area under the curves (AUC) of blood concentrations of ¹³N-ammonia in the groups of patients indicated (adapted from Sørensen and Keiding 2006). Since the same dose of ¹³N-ammonia tracer was given to all subjects (500 MBq), the AUCs reflect “inverse” whole-body ammonia clearance estimates. Data are given as mean ± standard error of the mean

Fig. 7

Simplified diagram of ammonia metabolism in brain: K₁ is the unidirectional clearance of ¹³N-ammonia from blood into cells (ml blood/min/ml tissue), K_met is the net metabolic clearance of ¹³N-ammonia from blood into intracellular of ¹³N-metabolites (ml blood/min/ml tissue), k₂ the rate constant for back-flux from cell to blood of un-metabolized ¹³N-ammonia (min^-1), and k₃ the rate constant for conversion of ¹³N-ammonia to ¹³N-metabolites (min^-1)

Fig. 8

Plot of the unidirectional clearance K₁ calculated from the model shown in Fig. 7, using either whole-blood ¹³N-concentration (Y-axis) or ¹³N-ammonia concentration (X-axis) as input function using the individual PET-data from the groups of patients indicated (adapted from Sørensen and Keiding 2006)