Sensitivity

General

The sensitivity of a PET is the rate in counts per second that true coincidence events are detected for a given radioactivity. Some material shall be around the source to give to the positron a change for annihilation. But this material attenuates the photons as well. To have an attenuation free measurement, successive measurements are made with the same line source surrounded by known absorbers. The attenuation free sensitivity can be extrapolated from the measurements.

Purpose

The purpose is to measure the sensitivity of the scanner to detect positron.

Method

The phantom for this measurement consists of 5 coaxial cylindrical rings. These are 700 mm long, metal and have the inside-outside diameters as the next table shows:

Radionuclide

For this measurement ¹⁸F shall be used. The activity shall be as low, that count losses are less than 1% and the random event rate is less than 5% of the trues rate. It means around 5 MBq activity, but depends on the scanner. The used activity shall be measured in a dose calibrator and reported

Source distribution

A 700±5 mm portion of a plastic tube shall be filled with water-radioactivity mixture. The phantom must be suspended in the center of the transaxial FOV, any supporting mechanism shall be external to the FOV. In sum there are 5 measurements, with 1, 2, 3, 4, 5 metal tube(s) around the line source.

Data collecting

Data shall be collected for that time, it is sure, that at least 10000 trues per slice are collected. Single slice rebinning shall be used to assign count in oblique LORs to the image slice where LOR crosses the scanner axis. The rate R_j,i, shall be calculated for every slice (i) of every acquisition (j).

Calculation and analysis

1. System sensitivity

Each measure shall be corrected because of the radioactive decay.

wherel* R_corr,j,i: corrected rate

R_j,i: calculated rate from measurement
T_j: the time of acquisition j.
T_cal: time of calibration by dose calibrator
T_1/2: half-value of the radionuclide

Measurements shall be summed for each slice:

A curve shall be fitted on the data:

Where R_corr,0 and µ_M are the unknowns, and X_j represents the thickness of the attenuation material. R_corr,0 represents the attenuation free rate.
The same measure shall be repeated in 10 cm radial distance from the center of system.
The sensitivity of the system shall calculated as follows: