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Units of radioactivity in PET data

Plasma and blood time-activity curves (TACs)

Blood and plasma TAC files produced in the PET laboratory are automatically calibrated to kBq/mL.

In other PET centers the unit is often given per mass (gram) because the samples may have been weighted. In Turku PET Centre, the plasma samples are pipetted by known volume, which allows the same unit in plasma and tissue data. Blood samples can not be pipetted because of very high viscosity; sample tubes are weighted and converted to kBq/mL applying the population mean blood density.

PET images

PET-CT

PET images are originally in DICOM format, but can be converted to ECAT 7.x format. Radioactivities are by default in units Bq/cc (cc=cm3=mL). When necessary, the unit can be changed using ecatunit with option -uc.

HR+

PET images are in ECAT 7.x format and the radioactivities are by default in units Bq/cc. When necessary, the unit can be changed using ecatunit with option -uc.

HRRT

PET images are in Interfile or ECAT 7.x format and the radioactivities are by default in units Bq/cc.

GE Advance

PET images are automatically calibrated to units kBq/mL when the images are extracted from the database and saved in ECAT 6.3 format. If necessary, the images can be calibrated using calib4ge.

ECAT 931

PET images are automatically calibrated to units kBq/mL by the reconstruction programs. If necessary, the images scanned with ECAT 931 can be calibrated using program cal4img in UNIX/Solaris workstations.

Regional tissue TACs

If filename extension is *.roi.kbq or *.roi.nci, the unit of radioactivity is kBq/mL or nCi/mL, respectively. In other datafiles (usually *.dft) the unit is written in the datafile, and the unit be seen using program dftunit; if datafile does not contain unit specification, but you know what the unit is, you can set it using dftunit with option -u. Conversion of units of radioactivity concentration can be achieved with option -uc.

History of data units in Turku PET Centre

Before year 1997, most data in were in units nCi/ml. The units should be converted to kBq/mL before any other use using program ecatunit.

The earliest PET images (ECAT 931) are not calibrated at all (which means that units are ECAT counts/sec/pixel). Before these images can be used, they must be calibrated using program cal4img in UNIX/Solaris workstations.

The unit can be seen using program ecatunit; if the images need to be calibrated, please contact Vesa Oikonen or Tuula Tolvanen.

Old tissue and blood/plasma TAC files may have a filename extension *.nci or *.nci.fit. This means that they are calibrated to unit nCi/mL, and need to be changed to unit kBq/ml using program nci2kbq in SUN/Solaris workstations. After that, the PET TAC files (*.roi.kbq), but not the blood/plasma files, may need to be converted to DFT format using program nci2dft (note that despite of the program name, the first step is also required, and the resulting *.roi.kbq are converted further).

Since 1997, the unit of radioactivity concentrations has been kBq/mL in ECAT 6.3 format images (ECAT 931) and in all regional and plasma/blood TACs. In GE Advance images (database and Display program) the unit is MBq/mL (shown as MBq/cc, where cc stands for cubic cm, same volume as mL), but when GE Advance images are transferred to ECAT 6.3 format, the unit is automatically changed to kBq/mL.

The branching ratio has been included in the calibration programs since 1997.

With HR+, the image file format ECAT 7.x was introduced, and in those PET images the unit of radioactivity is Bq/cc, by default, also when the images are scanned withECAT 931 or GE Advance and later converted to ECAT 7.x.

Possible problems with units

Currently most analysis tools do not check what the units are in the PET data. If the units are different in plasma or blood curve and tissue data (image or regional TACs) the results may be invalid, for example they may be wrong by a factor of 1000, if the units kBq/mL and Bq/cc were mixed up. When using old PET data, also units nCi/ml and kBq/ml can be mixed, leading to results that are wrong by factor 0.037 or 27.

If you have to convert between Curies and Becquerels "manually":

Conversion factors:

Gigas and megas:

Name sign factor
giga G 109
mega M 106
kilo k 103
milli m 10-3
micro µ 10-6
nano n 10-9

For example:


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