Theranostics – present and future

Theragnostics in nuclear medicine constitute an essential element of precision medicine. This notion integrates radionuclide diagnostics procedures and radionuclide therapies using appropriate radiopharmaceutics and treatment targeting specific biological pathways or receptors. The term theragnostics should also include another aspect of treatment: not only whether a given radioisotopic drug can be used, but also in what dose it ought to be used. Theragnostic procedures also allow predicting the effects of treatment based on the assessment of specific receptor density or the metabolic profile of neoplastic cells. The future of theragnostics depends not only on the use of new radiopharmaceuticals, but also on new gamma cameras. Modern theragnostics already require unambiguous pharmacokinetic and pharmacodynamic measurements based on absolute values. Only dynamic studies provide such a possibility. The introduction of the dynamic total-body PET-CT will enable this type of measurements characterizing metabolic processes and receptor expression on the basis of Patlak plot.

Relative Patlak Plot for Dynamic PET Parametric Imaging Without the Need for Early-time Input Function

The Patlak graphical method is widely used in parametric imaging for modeling irreversible radiotracer kinetics in dynamic PET. The net influx rate of radiotracer can be determined from the slope of the Patlak plot. The implementation of the standard Patlak method requires the knowledge of full-time input function from the injection time until the scan end time, which presents a challenge for use in the clinic. This paper proposes a new relative Patlak plot method that does not require early-time input function and therefore can be more efficient for parametric imaging. Theoretical analysis proves that the effect of early-time input function is a constant scaling factor on the Patlak slope estimation. Thus, the parametric image of the slope of the relative Patlak plot is related to the parametric image of standard Patlak slope by a global scaling factor. This theoretical finding has been further demonstrated by computer simulation and real patient data. The study indicates that parametric imaging of the relative Patlak slope can be used as a substitute of parametric imaging of standard Patlak slope for certain clinical tasks such as lesion detection and tumor volume segmentation.

Validation of Microcirculatory Parameters Derived from the Standard Two-Compartment Model with Murine Xenografts Model

The purpose of this study was to validate DCE-MRI parameters such as blood flow (F), permeability surface area product (PS), fractional intravascular space (v1), and fractional extracellular extravascular space (v2), obtained using a standard two-compartment model against other established analysis methods and histological indices. DCE-MRI datasets of 28 mice implanted with various human cancer xenografts were acquired and analyzed. Statistically significant correlations were found between the parameters derived from the standard two-compartment model (v1, v2, F, and PS) with the histological markers of intravascular and interstitial space and with the corresponding flow and permeability estimates obtained by the initial slope method and Patlak plot, respectively.