Delay time correction
What is time delay?
Although the shape of radioactivity concentration curve is similar in all arteries, the measured radioactivity peak seems to arrive at different times to the tissue (PET) and to the blood samples, because of sample tubing and different distance and blood velocity in different arteries. In many PET analyses, the effect of this difference in tracer arrival times is insignificant, but in some studies, particularly all that are done with radiowater or oxygen, or where a full compartmental model is fitted, the delay time must be considered.
How to correct for time delay?
What data do we need?
For delay correction, corrected blood curve measured using on-line detector and PET countrate curve (*.cr) are normally used.
It may be possible to use manually sampled blood or plasma curve instead of on-line detector data, if manual samples have been taken at 10-15 s intervals (at max), and the initial increasing phase of the curve contains several samples.
Also, the countrate curve can be replaced with regional TACs (see below) or "head curve", an average TAC from the dynamic image or sinogram made with ecathead, if PET time frames are short enough (10-15 s or less) in the beginning of the PET scan.
Countrate curve must not be used in certain cases
If the heart, lungs or aorta is inside the PET image volume (or even close to it in 3D studies, in which the spill-over artefacts may be significant), then the use of countrate or head curve is discouraged: In the heart and large arteries the initial radioactivity concentration is relatively high and appears considerably sooner than in the tissue of interest. This may cause a bias of a few seconds in the estimated delay time. For example in perfusion studies of renal cortex and tumours in oral cavity, a region-of-interest TAC should be used in the delay time correction in place of count-rate curve.
Software
In most cases, the time delay correction is included in the blood data pre-processing scripts for oxygen-15 labelled or other radiotracers. Alternatively, certain model analysis software makes automatically or optionally also the time delay correction, for example fit_h2o.
If you need to do the time delay correction by yourself, you can use fitdelay. However, different tracers may require specific settings for this program to work reliably.
In all cases, you must always visually check that time delay correction was successful!
If time delay is known from elsewhere, e.g. by visual inspection, the time delay can be corrected using dfttime.
If it does not work
The most common reasons for failure in automatic delay correction are:
- Outlier (very high value) in the beginning of the on-line detector curve; find and remove the outlier manually in a text editor or by using blozero.
- Steady background before the appearance of actual tracer; remove the background using dftrmbkg, dftslope, or dftcalc.
- Initial bump in the countrate curve, sometimes caused by approaching injection syringe; set the values manually to zero.
- The curves have different time units; check by looking at the first (time) column in the datafiles, and correct using min2sec or sec2min.
- Manual input curve has too few samples during the radioactivity build-up phase.
- Old countrate or blood datafile does not contain sample times; check by looking at the datafile, and add the time column using addtimes.
Note that the failure is not always obvious, but is only seen as biased results, unless you check the correction by plotting fitted input curve and tissue curve together.
Do it only after metabolite correction
The fractions of authentic tracer and metabolites are measured at sample times that are relative to the sample times of the total plasma or blood curve. Therefore, the metabolite correction must be made before the sample times of plasma or blood curve are changed.
If you use fitdelay to make the delay correction, you can correct all plasma and blood curves belonging to the same study by entering the filenames to the end of the command line.
