Card. NH3 (UCLA, 2 Parameters)

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 V_ND representing the distribution volume of ammonia in the C₁ compartment which was assumed to be constant (V_ND=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
V_LV = spill-over fraction of the blood activity in the left ventricle C_LV(t),
V_RV = spill-over fraction of the blood activity in the right ventricle C_RV(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 V_RV 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