This model implements a 2-tissue compartment model with only two fitted tissue rate constants. It was developed by Choi et al. for cardiac PET studies using 13NH3 ammonia bolus injection and was called the Modified Two-Parameter Model, [1] eq (5).
Operational Model Curve
The system of compartmental differential equations for this compartment structure is given by
with K1 corresponding to myocardial blood flow F. The k3 rate constant corresponding to glutamine formation was converted into a K1-dependent value by the relation
Similarly, the k2 rate constant of ammonia washout was converted into a K1-dependent value by
with the VND representing the distribution volume of ammonia in the C1 compartment which was assumed to be constant (VND=0.8).
In contrast to Choi's method, the model incorporates not only a spillover from the left ventricle, but for septal regions also from the right ventricle. The operational equation is then given by
where
VLV = spill-over fraction of the blood activity in the left ventricle CLV(t),
VRV = spill-over fraction of the blood activity in the right ventricle CRV(t) .
Because the model does not include a metabolite correction, only the samples within 2 minutes after injection should be used for the fit [1].
Implementation Notes:
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 adjustments are 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.
▪The valid flag for all data samples after 2 minutes is set to false, and they are consequently not considered in the fit.
Parameter Fitting
The model includes the three fitable parameters F, vLV, vRV. Please inspect the %SE standard error to get information about the reliability of their estimates. The input parameters Vnd (0.8), EF scale (1.65) and EF Exp (1.25) are related to the conversion of k2 and k3 as described above and can be adjusted if needed.
Reference
1.Choi Y, Huang SC, Hawkins RA, Kim JY, Kim BT, Hoh CK, Chen K, Phelps ME, Schelbert HR: Quantification of myocardial blood flow using 13N-ammonia and PET: comparison of tracer models. J Nucl Med 1999, 40(6):1045-1055. PDF