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The Card. Rb-82 (2 Compartments) model has been implemented according to the method described and evaluated by Herrero et al [1]. The model is based on the following compartment structure to describe the kinetics of rubidium in the myocardium:
where C1(t) represents the fast exchangeable compartment (vascular and interstitial spaces), and C2(t) the slow exchangeable compartment (intracellular space), myocardium flow F, and rate constants k1 and k2.
Operational Model Curve
The differential equations for the activity concentrations in the different compartments are given by
The model uses the activity in the left ventricular cavity CLV(t) is used as the input curve (CP(t)=CLV(t)). The operational equation which is fitted to the measured data is
where FMM denotes the tissue recovery coefficient and FBM the blood to myocardium spillover fraction.
Implementation
When using the model from the PCARD module, the data are transferred appropriately. When using it in PKIN the left ventricle curve must be loaded as the Plasma activity curve. The right ventricle curve is not used.
Parameter Fitting
The model encompasses 5 fitable parameters. However, in practice it is impossible to estimate so many parameters from a time-activity curve with reasonable identifiability. Therefore, at least the the recovery coefficient FMM is usually fixed. The default from Herrero et al [1] is 0.65, but it depends on the image resolution and should be determined experimentally.
Reference
1. Herrero P, Markham J, Shelton ME, Bergmann SR: Implementation and evaluation of a two-compartment model for quantification of myocardial perfusion with rubidium-82 and positron emission tomography. Circulation research 1992, 70(3):496-507. DOI