Page 19 - Towards personalized therapy for metastatic prostate cancer: technical validation of [18F]fluoromethylcholine

SUV is the most popular PET semi-quantitative index in the Nuclear Medicine field 1 [28]. It provides a measure of radiotracer metabolism and is defined as the tissue concentration of tracer, as measured by PET, divided by the injected dose, normalized
to patient weight, multiplied by a decay factor. SUV estimation is a semi-automated
procedure, being readily calculated with current software available by the PET scanners. Moreover, SUV is a function of several factors such as: time interval between intravenous administration of the tracer and image acquisition, residual activity in the syringe/ infusion system, decay of the injected dose and partial volume effects [31]. However, standardization of procedures is required and yet only available for [18F] fluorodeoxyglucose ([18F]FDG) [29].
NLR analysis employs the estimation of the net rate of tracer influx (Ki) from dynamic PET data and a standard two or more tissue compartment models, based on arterial input function. It represents a quantitative method with the advantage of being independent of uptake period, by also providing insight into various rate constants [28].
Patlak graphical analysis expresses the regional concentration of a tracer at time t after injection. It is more robust than NLR method, but has the same disadvantage of dynamic scanning, which makes its implementation complex for clinical practice.
Simplified kinetic method requires a single static scan and few venous blood samples during the scan acquisition. The samples are further needed to scale a population – derived average plasma curve. The method has the advantage of estimating tracer metabolism without dynamic scanning and with a reduced blood sampling protocol. However, it implies validation in a large patient population sample [28].
Only a decade after the successful implementation of PET/CT, integration of PET with magnetic resonance imaging (PET/MRI) was introduced as the next hybrid imaging option. Strengths of MRI include its high soft tissue contrast, high spatial resolution, and lack of exposure to ionizing radiation. Since these two modalities (i.e., PET and MRI) are not easily combined in a single scanner, the design has taken years of technical research. Several hybrid PET/MRI systems are currently available clinically: an integrated system housing both components in a single gantry, or a geometrically separated PET and MRI on either side of a rotating table [30]. The former has the obvious advantage of truly simultaneous image acquisition, and the latter does not require any concession for either component, being also equipped with time of flight (TF) capability.
Introduction and outline of the thesis
17