Accuracy: Corrections for count losses and randoms

General

Positron emission tomograps are usually able to compensate the dead-time losses and the random losses. So they are able to measure quantitatively.

Purpose

The purpose of this procedure is to measure the accuracy of corrections for dead time losses and random losses.

Method

The phantom used to measure scatter fraction is used for this measurement as well. At low activity the dead-time losses and the randoms don’t influence the true count rate.

1. Radionuclide
For this measure ¹⁸F shall be used. Activity shall be great enough that the dead time losses are 50 %. It’s expected that the vendor says the initial activity value.

2.Source Distribution
A 700±5 mm long part of the line source shall be filled with water-radioactivity mixture. It shall be placed in the hole in the phantom. The line source shall be positioned in the phantom that at both side can’t be hang out more than 5 mm. Phantom shall be positioned at patient table, the line source should be the nearest to the patient table. Phantom shall be in the center of the transversal and axial FOV.

3. Data collection
Data collections shall be more frequent than the half of the half-time. And an acquisition-time (T_acq,j) shall be shorter than the quarter of the half-live. Acquisitions shall be done until the true event losses are less than 1%. Every acquisition shall be have at least 500000 coincidence. It’s expected, that the manufacturer recommends a protocol including starting activity, acquisition time and acquisition durations.

4. Data processing
For tomographs with axial FOV less than 65 cm prompt and random sinograms shall be generated for each acquisition (j) of slice (i). For tomographs with an axial FOV greater than 65 cm, only the central 65 cm of the FOV shall be measured. Randoms and dead-time corrections shall be allpied to the data. Image shall be reconstructed as the whole body examinations. These methods shall be reported.

Analysis

All analysis shall be performed on each reconstructed image (i). The average activity shall be calculated for all acquisition (j) as follows:

The effective activity concentration (a_eff,j)can be calculated by devided with the volume of the phantom (22000 cm3). A circular ROI shall be drawn in each reconstructed image. The center of the ROI is the center of the transverse field FOV, and the diameter of ROI is 180 mm. The number of true events for each slice (i) of each acquisition (j) shall be calculated (C_ROI,i,j). The true event rate can be calculated:

For each slice calculate the extrapolated true event rate (R_Extr,i,j), which would be measured if there wouldn’t be dead-time losses and random losses. Calculate as follows to minimize the effects of statistics:

where: k=1 at the lowest activity
For each slice (i) of each acquisition (j) calculate the relative count rate error (Δr_i,j) in percentage as follows:

Report

For each slice calculate the Δr_i,j and a_eff,j values. Plot Δ_ri,j versus a_eff,j versus.

 

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