[11C]Metomidate ([11C]MTO)
Metomidate
Metomidate is an imidazole-based methyl ester, and a potent inhibitor of 11β-hydroxylase, which is a key enzyme in the biosynthesis of cortisol and aldosterol in the adrenal cortex.
PET imaging with [O-methyl-11C]metomidate can be used in quantification of adrenal masses and to discriminate tumors of adrenal cortical origin from non-cortical lesions (Bergström et al., 2000; Khan et al. 2003; Zettinig et al. 2004). [11C]Metomidate is useful in imaging of adrenal incidentalomas (Minn et al., 2004).
Estimation of [11C]metomidate uptake
Methods for quantification of [11C]metomidate:
- Standardized uptake value (SUV)
- Fractional uptake rate (FUR)
- Multiple-time graphical analysis for irreversible tracers (Gjedde-Patlak plot) (Minn et al., 2004)
A strong relationshipt between SUV and Ki has been seen for all tumor types and normal adrenal glands (Minn et al., 2004).
Plasma data
Usually, manual samples are collected to measure the concentration of total radioactivity in plasma. Arterial or arterialized venous blood samples are processed in the PET blood laboratory to time-activity curves, which can be found in PETO and used as such. For better understanding, see also the documentation on blood data processing.
If radioactivity concentration is measured in blood instead of plasma (for example, image-derived input), the blood time-activity curves must be converted to plasma before any further analysis.
Metabolite correction
There are two major [11C]metomidate metabolites in plasma. The rate of their appearance varies considerably between individual patients (Minn et al., 2004). In order to calculate Gjedde-Patlak plot (or FUR), the plasma radioactivity concentrations must be corrected for metabolites, and metabolite fractions must be individually measured (Minn et al., 2004).
Correction for time delay
For Gjedde-Patlak analysis, correction for time delay is not required.
PET data
ROIs can be drawn and calculated from dynamic and parametric images as usual.