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The simplified reference tissue model for SPECT/PET brain receptor studies. Interpretation of its parameters.

AIM
The SRTM (simplified reference tissue model) of brain receptor imaging assumes that the time activity curve in the receptor-rich region of interest can be fitted satisfactorily by the 1-tissue compartment model. This assumption has been formulated by a rather restrictive constraint on the rate constants. Empirically, the SRTM might well describe also tracers which do not fulfil this constraint, such as [(11)C]raclopride, for example. However, this has not been justified rigorously.


METHODS
The requirements for the SRTM to be applicable are analyzed in detail.


RESULTS
The SRTM is applicable under a less restrictive constraint than described previously. The interpretation of the SRTM parameters R(1) and k(2) in physiological terms depends on the constraint, while the interpretation of BP(ND) does not.


CONCLUSION
Correct interpretation of the results of the SRTM is tracer specific. In particular, the parameter R(1), which in case of compliance with the original constraint might be used to detect perfusion and/or extraction effects, might not be appropriate for this purpose in case of raclopride-like tracers.