This auxiliary model is aimed at receptor tracers for which a reference tissue without specific binding is available. It allows getting a quick estimate of the binding potential BPND using two different methods as described by Ito et al. [1].
At the time, when specific binding C2 peaks, its derivative equals zero (transient equilibrium) and BPND = k3/k4 = C2/C1 (non-specific/specific binding).
In practice, however, neither the C1 part nor the C2 part of a measured tissue TAC are known. As a pragmatic solution it is assumed that the non-displaceable compartment is equal among tissues, so the CT'(t) of reference tissue without specific binding is regarded as C1, and C2 is calculated by CT(t)-CT'(t).
Operational Model Curve
Two different ratios are calculated by the Tissue Ratio Methods model:
This ratio is calculated for all times, but it is only valid at the time of transient equilibrium. There is no automatic routine for detecting this peak. Rather, the user has to find this time by looking at the difference curve (enable check box).
The second ratio calculated is called the interval method in [1].
Here, the two curves are integrated in a time interval [tb, te] which must include the time of transient equilibrium (tb=9min. te=45 min for raclopride, where the time of transient equilibrium is in the range 20 to 24 min [1]).
Implementation
1.After switching to Tissue Ratio Methods a suitable reference tissue without specific binding must be selected.
2.The integration start time of the interval method must be specified by the user as the input parameter Integral start time.
3.The ratios are calculated for all times (for the integral method only after the Integral start time) and presented as curves. These curves can easily be exported as values using the save button.
Reference
1.Ito H, Hietala J, Blomqvist G, Halldin C, Farde L: Comparison of the transient equilibrium and continuous infusion method for quantitative PET analysis of [11C]raclopride binding. J Cereb Blood Flow Metab 1998, 18(9):941-950. DOI