File-based Data Import

There are slightly different formats for time-activity data for blood and tissue. With blood data it is assumed that instantaneous sampling was performed at well-defined time points. With tissue data it is assumed that the measurements represent the average tracer concentration during a certain observation period, the PET frame duration defined by start-time and end-time.

Blood Data

PET tracers are usually applied by intra-venous injection and then brought to the tissue of interest by circulation. Often, a part of the tracer may be bound to red blood cells. Another part may be processed in organs and end up as labeled metabolites circulating in blood plasma. The remaining fraction of tracer (unchanged tracer, or parent) is available for exchange with tissue and represents the input curve for modeling.

In PKIN, the following three types of blood data are supported to model the contributions of the different forms of tracer in blood to the expected PET signal:

1. The concentration of all forms of tracer in Whole Blood samples: As the average signal from a VOI always contains a fraction of signal from blood vessels and capillaries, the concentration in whole blood is needed for modeling the blood content in the expected PET signal.
2. The concentration of all forms of tracer in Plasma: Although the unchanged tracer in plasma is required as the input curve of the models, it is common practice to measure the tracer activity of the whole plasma sample.
3. The fraction of unchanged ligand in plasma (Parent Fraction): As a result of the metabolite analysis of plasma samples, the relative concentrations of the unchanged tracer and the metabolites are known. This information is represented by the ratio of unchanged tracer to total tracer. This fraction starts with a value of 1 at the time of injection (all tracer is unchanged), and gradually decreases as the metabolites build up. Since plasma analysis is often experimentally complex and therefore error-prone, it is advisable to fit a smooth curve to the measured parent fraction.

Whole Blood and Plasma

The whole blood and plasma concentrations must be prepared in text files, and can be loaded using Load Whole Blood Curve and Load Plasma Curve from Menu. There are two variants of the format:

1. Separate files for whole blood and plasma activity. In this case separate files are prepared with a header line, the sample time in the first column and the sample value in the second column.
   
   and
   
2. A composite file containing whole blood and plasma activity. Note that in this case the keywords sample-time, plasma and whole-blood are required to define the meaning of the columns.
   

Parent Fraction

The parent fraction needs to be prepared in a similar file

and loaded using Load Parent Fraction from Menu. Such text files with the blood data can be prepared for example in MS Excel and then saved as tab-delimited or csv separated text files.

Notes:
1. If no whole blood data is loaded into PKIN, the plasma concentration is used for blood correction.
2. If the activity of unchanged tracer in plasma is loaded instead of the total plasma activity, no correction with the parent fraction is required. No further action is required in this case, because per default it is assumed that the parent fraction equals the constant of 1.
3. There are models which require two input curves, and in principle models with up to 10 input curves can be handled in PKIN. For these models the sub-menu contains appropriately labeled entries.

Tissue Time-Activity Data

Tissue time-activity curves are similarly organized in tab-delimited text files. The frame start and end times are arranged in the first two columns. Next come columns with the measurements from different tissues. Note that the TAC value units are specified after the heading of the second column.

Tissue time-activity curves are loaded using Load Time Activity Curve from Menu.

Data Units

Typically the data will arise in the following units:

• [seconds], [minutes] or [hours] for the time specification, and
• [kBq/cc], [uCi/cc] or [1/1] for the value specification.

The unit string in square brackets should immediately follow the column header. If units are not defined in the column headers or not recognized, it is assumed that the units specified in the configuration apply. They can be defined in the configuration dialog as illustrated below.

Note: During loading, the input unit specification will be used to convert the activity values into the internal representation of kBq/cc. All loaded data will be displayed with the values after conversion.