PET data units in TPC

Units of PET images

Radioactivity concentrations in PET images are usually in units Bq/cc (cc=cm³=mL). Images from old scanners (GE Advance, ECAT 931) are usually in units kBq/cc, especially when stored in ECAT 6.3 format.

DICOM, ECAT, Interfile, and microPET image formats contain the information on the concentration and time units, and analysis software can usually automatically take care of unit conversions when necessary.

Analyze 7.5 and NIfTI images do not store the concentration units; if these are used in quantitative analysis, then user must convert the radioactivity concentration units when necessary. Frame times are not stored inside Analyze 7.5 and NIfTI image files, but are in seconds in the accompanying SIFs.

Units in image files can be viewed with command-line program imgunit.

Units of regional TACs

Regional TAC files usually contain the concentration and time units. Units can be viewed with command-line program tacunit, or, since TAC files are simple ASCII text files, by opening the file in a text editor.

Old TAC files in Turku PET Centre may have file name extension *.roi.kbq or *.roi.nci; then the unit of radioactivity is kBq/mL or nCi/mL, respectively.

Units of plasma and blood TACs

Plasma and blood TAC files produced in the TPC PET laboratory are automatically calibrated to kBq/mL. Times are normally in minutes, except in ¹⁵O studies times may be in seconds. Units are saved in the TAC files, and can be viewed with command-line program tacunit, or, since TAC files are simple ASCII text files, by opening the file in a text editor.

Unit conversions

Analysis software require that data in all files are in the same calibration and time units, except if the units are stored inside the data files. When working with Analyze 7.5 and NIfTI images, and simple input file formats, which do not contain the units, user must make sure that units are the same in all data files before analysis.

In SUV calculations, the injected dose may need to be converted to suitable units, in order for the radioactivity units to cancel out.

Data in image files in image formats can be converted to another concentration unit using imgunit with option -uc.

Data in TAC files can be converted to other concentration or time units using tacunit with options -yconv and -xconv, respectively. If units are not stored in the TAC file, the units can be added with options -yset and -xset. Program tac2suv can convert TAC data into SUV units.

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 imgunit.

The earliest PET images (the first studies with ECAT 931) are not calibrated at all, which means that data units are ECAT counts/sec/pixel. Before these images can be used, they must be calibrated using program ecalibr, or cal4img in UNIX/Solaris workstations. The data unit can be seen using program imgunit; if the images need to be calibrated, please contact Vesa Oikonen or Tuula Tolvanen.

Old tissue and blood/plasma TAC files may have a file name extension *.nci or *.nci.fit. This means that data are calibrated to unit nCi/mL, and need to be changed to unit kBq/ml using program nci2kbq. After that, the regional TAC files (*.roi.kbq), but not the blood/plasma files, may still need to be converted to more modern file 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 with ECAT 931 or GE Advance and later converted to ECAT 7.x.

Possible problems with units

Currently some analysis tools do not verify the units of the PET data. If the data 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 files, also units nCi/ml and kBq/ml can be mixed, leading to results that are wrong by factor 0.037 or 27.

"Manual" radioactivity unit conversion

If you have to convert between Curies and Becquerels "manually", the conversion factors are:

1 Ci = 3.7 x 10¹⁰ Bq = 37 GBq
1 mCi = 3.7 x 10⁷ Bq = 37 MBq
1 µCi = 3.7 x 10⁴ Bq = 37 kBq
1 nCi = 37 Bq

mCi -> MBq and nCi -> Bq : multiply by 37
nCi -> kBq : multiply by 0.037

MBq to mCi and Bq-> nCi : divide by 37 (or multiply by 0.027027)
kBq -> nCi : divide by 0.037 (or multiply by 27.027)

Giga versus mega

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:

Radiopharmaceutical batch 200 mCi = 7.4 GBq
Injected dose 5 mCi = 185 MBq
Plasma sample radioactivity concentration 800 nCi/mL = 29.6 kBq/mL
Tissue radioactivity concentration 100 nCi/mL = 3.7 kBq/mL