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 ^{13}NH_{3} 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 K_{1} corresponding to myocardial blood flow F. The k_{3} rate constant corresponding to glutamine formation was converted into a K_{1}-dependent value by the relation

Similarly, the k_{2} rate constant of ammonia washout was converted into a K_{1}-dependent value by

with the V_{ND} representing the distribution volume of ammonia in the C_{1} 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 k_{2} and k_{3} 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