PXMOD Implementation

The PXMOD implementation differs from that described in [35] in the following points

An unweighted fitting is employed.
Instead of performing a pixel-wise SRTM calculation and averaging k₂' in non-reference pixels, k₂' is calculated by applying SRTM to an averaged TAC from a VOI covering non-reference pixels.

Acquisition and Data Requirements

Image Data	A dynamic PET data set with an neuroreceptor tracer which behaves kinetically similar to a 1-tissue compartment model.
TAC 1	TAC from a receptor-rich region (such as basal ganglia for D2 receptors).
TAC 2	TAC from a receptor-devoid region (such as cerebellum or frontal cortex for D2 receptors).

Model Preprocessing

Two regional TACs (TAC1 and TAC2) are needed for Model Preprocessing.

PXMOD SRTM2 Preprocessing

k2a min	Minimal value of k_2a (slowest decay of exponential).
k2a max	Maximal value of k_2a (fastest decay of exponential).
# Basis	Number of basis functions between k2a min and k2a max. Note that increments are taken at logarithmic steps. This number is directly proportional to processing time.
Resampling	Specifies the interval of curve resampling which is required for performing the operation of exponential convolution. Resampling should be equal or smaller than the shortest frame duration.
Threshold	Discrimination threshold for background masking.
k2'	k₂ of the reference tissue. It can be fixed at a specified value, or fitted. If it is checked, k2' is fitted using the SRTM method implemented in PKIN. This value will be used in pixel-wise SRTM2.
BPnd	Estimated binding potential (= k₃/k₄ according to the underlying model).
R1	Ratio of tracer delivery in each pixel relative to the reference tissue (R1=K₁/K₁').
k2	Estimated rate constant k₂.
k2a	k_2a value which provides the best least squares fit in each voxel.

The result of the fit during Model Preprocessing is shown in the Result panel for inspection.

PXMOD Gunn Pre-Processing Result

Map Parameters

PXMOD SRTM2 Parameter

BPnd	Estimated binding potential (BPnd= k₃/k₄ according to the underlying model).
k2	Estimated efflux rate constant k₂ .
R1	Ratio of tracer delivery in each pixel relative to the reference tissue (R₁=K₁/K₁'). Therefore the map often has a similar appearance to a perfusion image.
k2a	k_2a value which provides the best least squares fit.

Note: The k2a parametric map should be checked in the initial setup of a processing protocol. The estimated k2a values should not be truncated by too narrow k2a min and k2a max values.