The Card. H2O (Tissue fraction) model has was developed by Hermannsen et al. [1] for cardiac PET studies using H215O water bolus injection. The distribution of the freely diffusible inert tracer H215O in myocardium can be described by a 1-tissue compartment model with myocardial blood flow F and the partition coefficient of water p, which is defined as (ml water/ml tissue)/(ml water/ml blood).
Operational Model Curve
The differential equation for the tracer concentration in myocardium is
In practice, the left ventricular time-activity curve is used as the input curve CP(t).
The model curve incorporates two spill-over terms from blood in the left and the right ventriclar cavities which are relatively displaced in time. So the operational equation which is fitted to the measured data is
where TF = tissue fraction, VLV = spill-over fraction from the left ventricle, VRV = spill-over fraction from the right ventricle.
Implementation
As opposed to most models in PKIN a tissue fraction is used for calculating the model curve rather than a strict geometrical correction for compliance with the original model. When using the model from the PCARD module, the data are transferred appropriately. When using it in PKIN the blood data have to be loaded as follows:
▪The left ventricle curve must be loaded as the Plasma activity curve.
▪The right ventricle curve must be loaded as the Whole blood curve.
The following automatic adjustment is performed within the model:
▪The spill-over fraction from the right ventricle VRV is automatically fixed to zero if the string "Sep" is not contained in the name a region. The assumption is that such a TAC is not from septal tissue and should thus be modeled with spill-over from the left ventricle only.
Parameter Fitting
The model encompasses the 5 fitable parameters F, TF, vLV, vRV and p. However, in practice it is impossible to estimate so many parameters from a time-activity curve with reasonable identifiability. The partition coefficient is therefore fixed per default at a value of 0.96 [1], and the spillover fraction from right ventricular blood vRV is only fitted for septal tissue TACs.
Reference
1.Hermansen F, Rosen SD, Fath-Ordoubadi F, Kooner JS, Clark JC, Camici PG, Lammertsma AA: Measurement of myocardial blood flow with oxygen-15 labelled water: comparison of different administration protocols. Eur J Nucl Med 1998, 25(7):751-759. PDF