# Delayed PET scan

In diagnostic imaging, patient is often placed in the PET scanner after a certain waiting time, for instance 60 min, after administration of the radioligand. Bed position can be changed during the late imaging session to cover all or most of the body. Since the uptake kinetics of the radioligand is not recorded, only semi-quantitative measures can be obtained from the data, such as SUV, tumour-to-background ratio, or tumour-to-blood ratio, but those can still offer excellent clinical value with reduced cost.

By default, all PET images are decay corrected to the scan start time. If scan was started after the injection, the PET frame times and decay correction must be corrected to the true injection time (zero time) prior to any other calculation with the dynamic image or sinogram.

If PET scanning was performed in separate session, and you only need to combine these together, then follow this link: Catenation of PET images or sinograms.

## Correction of delayed PET scan to injection time

Carimas has a plug-in for correcting the time and decay correction of DICOM image to zero time.

For data in ECAT 6 or 7 format, use command-line program ecattime to correct late-scan image or sinogram to injection time. You need to feed the program with the actual injection time (hh:mm:ss) or the time delay between the injection and scan start time in minutes.

If another scan (for example another organ) was started at the time of injection, you can get the start time from that time using the same program ecattime.

Not recommended: Instead of correcting the image or sinogram, the corrections can be applied to the time-activity curves (TACs) using program tactime with option -decay. This has to be done to all TAC files, also when additional ROIs are drawn later on, but correction of dynamic image or sinogram needs to be done only once for one study.

Decay correction is not needed for static scans when simple ratio or asymmetry ratio between image regions is to be calculated.

## Do not correct twice

Unless you are sure that the ECAT image/sinogram is not yet corrected to the injection time, you can check this by entering the command ecattime yourfilename If the scan start time matches the injection time, or the start time of the first frame is later than 0, then the image/sinogram has already been corrected.

It is always safe to make the correction by entering the injection time (hh:mm:ss).

You must not correct the regional TACs derived from the image, if the image or sinogram was already corrected to injection time.