Late-scan tissue-to-plasma ratio (DVsimplified)

Background

Especially if tracer kinetics are very slow, and a minimum duration of dynamic PET scan would be 1.5 h or more with arterial blood sampling, for clinical studies a simplified method of quantification must be found.

When tracer uptake is reversible, tissue-to-plasma ratios stabilize sooner or later, and the ratio will usually have at least some correlation with the equilibrium volume of distribution (VT) of the tracer in the tissue.

Applications

Lehtiö et al. (2003) showed with hypoxia tracer [18F]FETNIM in tumours that simple tissue-to-plasma ratio correlated well with VT, which would require dynamic imaging and arterial blood sampling. Instead, tumour-to-muscle ratio which is commonly used in oncology did not show any correlation with VT.

Mitkovski et al. (2005) have validated the use of "simplified distribution volume" from a single 90-120 min PET scan for 2[18F]F-A-85380.

Calculation of tissue-to-plasma ratio image

From a single-scan (static) PET study, the ratio image can be calculated using ecatcalc, with the plasma radioactivity concentration during the PET scan as the divider. Before calculation, one must be sure that PET image is corrected for physical decay to the time of tracer administration.

For validation of the method the tissue-to-plasma ratio image may have to be calculated from a dynamic PET study and metabolite corrected plasma curve. This can be accomplished by using imgratio. To simulate single-scan PET study, the "static" image can be made with imginteg with option -avg.

Regional ratios can be calculated from the ratio images after the ROIs are drawn with the usual tools.

Calculation of regional tissue-to-plasma ratios

From the regional radioactivity concentration values (DFT files) of a single-scan (static) PET study, the regional tissue-to-plasma ratios can be calculated using any spreadsheet software or using dftcalc, with the (metabolite corrected) plasma radioactivity concentration during the PET scan as the divider. Before calculation, one must be sure that tissue data is corrected for physical decay to the time of tracer administration. Ratios in DFT format can be viewed for example in web browser or Excel after conversion to HTML tables or converted into result file format.

For validation of the method the regional tissue-to-plasma ratios may have to be calculated from a dynamic PET study and metabolite corrected plasma curve. This can be accomplished by using dftratio.


See also:


Lehtiö K, Oikonen V, Nyman S, Grönroos T, Roivainen A, Eskola O, Minn H. Quantifying tumour hypoxia with fluorine-18 fluoroerythronitroimidazole ([18F]FETNIM) and PET using the tumour to plasma ratio. Eur J Nuck Med. 2003; 30: 101-108.

Mitkovski S, Villemagne VL, Novakovic KE, O'Keefe G, Tochon-Danguy H, Mulligan RS, Dickinson KL, Saunder T, Gregoire MC, Bottlaender M, Dolle F, Rowe CC. Simplified quantification of nicotinic receptors with 2[18F]F-A-85380 PET. Nucl Med Biol. 2005; 32(6): 585-591.



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