Why does increasing acquisition time improve image quality?

Increasing acquisition time improves image quality in radionuclide imaging because it increases the number of detected photon counts. Since radioactive decay follows Poisson statistics, image noise is proportional to the square root of the number of counts ( $N$ ), while signal is proportional to the total number of counts ().

As acquisition time increases, more photons are detected, and the signal-to-noise ratio (SNR) improves according to:

Increasing acquisition time improves image quality by increasing detected counts, thereby improving signal-to-noise ratio according to a square-root relationship.

Because total counts increase linearly with acquisition time, SNR improves with the square root of acquisition time. This reduces statistical noise and produces smoother, clearer images.

Understanding the physics

In nuclear medicine, each detected photon is an independent event governed by probability. Over a short acquisition time, relatively few photons are detected, and statistical fluctuations are large compared to the signal. This results in noisy images.

If acquisition time is increased, more decay events are recorded:

N = (count rate) x (acquisition time)

Since noise is √

This means that while absolute noise increases slightly, the proportion of noise relative to signal decreases. The image appears less grainy and lesion contrast becomes more apparent.

However, the improvement follows a square-root relationship. Doubling acquisition time increases SNR by only a factor of $2$ . To double SNR, acquisition time must increase fourfold. This diminishing return explains why simply prolonging scanning time cannot indefinitely improve image quality.

In addition, increasing acquisition time does not improve intrinsic spatial resolution. It reduces noise, but it does not change detector physics, collimator geometry, or positron range effects.

Also to note:

This assumes:

Constant count rate
No motion artefact
No detector saturation
Pure Poisson noise dominance

In practice:

Longer acquisition improves SNR
But increases risk of motion
And gives diminishing returns

Where this matters clinically

Longer acquisition times improve lesion detectability in low-count studies. However, extended scanning increases the risk of patient motion and reduces workflow efficiency. Optimising acquisition time requires balancing image quality against motion artefact and practical constraints.