What is the difference between paralyzable and non-paralyzable dead time?

The difference between paralyzable and non-paralyzable dead time lies in how a detector behaves when a second photon arrives during the dead time following a previous event.

In a non-paralyzable system, any event arriving during the dead time is simply ignored and does not extend the dead time period. The detector resumes normal operation once the original dead time interval has passed.

In non-paralyzable systems, missed events do not extend dead time; in paralyzable systems, missed events prolong dead time and can cause severe count loss at high rates.

In a paralyzable system, any event arriving during the dead time resets or extends the dead time interval. At very high count rates, this can cause the detector to become effectively “paralyzed,” leading to severe count losses.

These models describe idealised detector behaviour and help explain how measured count rate deviates from true count rate at high activities.

Understanding the physics

When a photon interacts with a radiation detector, the system requires a short time to process the signal. During this processing interval, the detector cannot accurately record additional events.

In a non-paralyzable model, if a second photon arrives during the dead time:

It is not counted.
It does not alter the duration of the dead time.
The system recovers at the original scheduled time.

As count rate increases, the measured count rate gradually plateaus but does not collapse.

In a paralyzable model, if a second photon arrives during the dead time:

It is not counted.
It extends the dead time interval.
If photons continue arriving rapidly, the detector may remain continuously in dead time.

In this case, the measured count rate initially increases with activity, then reaches a maximum, and may eventually decrease as activity continues to rise.

In reality, detector systems may behave somewhere between these two ideal models, but these frameworks help explain count losses at high activity levels.

Where this matters clinically

Understanding dead time behaviour is important in high-count-rate imaging such as PET, dynamic studies, and imaging shortly after radiopharmaceutical injection. Excessively high activity can lead to underestimation of true activity and degraded image quality.