Delay time correction

What is time delay?

Although the shape of radioactivity concentration curve is similar in all arteries (if dispersion is corrected), 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 appearance times in blood (ATB) is insignificant, but in some studies, particularly all bolus studies that are done with radiowater or oxygen, or where a full compartmental model is fitted, the delay time must be considered.

Time delay

After time delay correction the arterial blood curve should start to ascending 1-3 s before the tissue curve.

How to correct for time delay?

What data do we need?

For delay correction, corrected blood curve measured using on-line detector and PET count rate 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 count rate curve can be replaced with regional TACs (see below) or “head curve”, an average TAC from the dynamic image or sinogram made with imghead, if PET time frames are short enough (10-15 s or less) in the beginning of the PET scan.

Count rate 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 count rate 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.


In most cases, the time delay correction is included in the ABSS data pre-processing scripts. 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 tactime.

If it does not work

The most common reasons for failure in automatic delay correction are:

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 file names to the end of the command line.


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Updated at: 2017-06-09
Written by: Vesa Oikonen