Model Configuration and Assumptions

The **Simplified Reference Tissue Model** (SRTM) of Lammertsma and Hume [1] is used for the analysis of studies with reversibly binding neuroreceptor tracers. A reference tissue devoid of receptors is required which can be modeled by a single-tissue compartment model.

The assumptions of the model are:

- The distribution volume is the same for the tissue of interest and the reference tissue: K
_{1}/k_{2}=K_{1}'/k_{2}'. - The kinetics in the receptor-rich tissue of interest is such that it is difficult to distinguish between the specific and the non-displaceable compartment; ie. the tissue TAC can be fitted by a 1-tissue compartment model with an uptake rate constant k
_{2a}= k_{2}/(1+BP_{ND}). Note that this assumption may not be valid for all tracers, and in this case SRTM calculates biased BP_{ND}estimates.

The model is a further development of the **4 Parameter Reference Tissue** method. It has been shown to be less dependent on the initial parameter values, more stable in the presence of noise, and provide more consistent estimates of relative tracer delivery R_{1}.

Operational Model Curve

Defining the ratio of tracer delivery R_{1} as K_{1}/K_{1}' and the binding potential BP_{ND} as k_{3}/k_{4}, the following operational equation can be derived for the measured tissue TAC in a receptor-rich region:

For convolution with the exponentials, the reference tissue TAC C'(t) is resampled on a regular grid, which can be specified by the **Resampling** parameter.

Parameter Fitting

The operational equation includes four unknowns: **R1**, **k2**, and **BPnd **, which can be fitted using nonlinear fitting techniques. **k2'**, the transfer of tracer from the reference tissue back to the plasma, is provided as a derived parameter, as well as **k2a**.

**Note:** The reference methods MRTM2 and SRTM2 require k_{2}'_{ }as an input parameter. The k_{2}' resulting from the SRTM method is a suitable estimate. Therefore, when switching in PKIN from the SRTM model to the SRTM2 or MRTM2, k_{2}'_{ }is automatically copied from SRTM, in case the **Model conversion** option in the **Extras **panel is enabled.

Reference

1. Lammertsma AA, Hume SP: Simplified reference tissue model for PET receptor studies. Neuroimage 1996, 4(3 Pt 1):153-158. DOI