02 Imaging

Pixel size and Nyquist frequency

Pixel size from FOV and matrix, plus Nyquist frequency.

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Simulation

Pixel size and Nyquist frequency — Change the numbers; the scene follows.

Where it works

Radiography room

Bucky / detector

In the Bucky / detector: grid, AEC, DQE, and the pixel that samples the image.

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Formula

\Delta x = \frac{\mathrm{FOV}}{N},\quad f_N = \frac{1}{2\Delta x}

Variables

FOV · Field of viewmmN · Matrix size

Results

Δx
Pixel size
0.7813mm
f_N
Nyquist frequency
0.64lp/mm

Explanation

\Delta x = \frac{\mathrm{FOV}}{N},\quad f_N = \frac{1}{2\Delta x}

What it means

Pixel size Δx = FOV / N and the Nyquist frequency 1/(2 Δx) is the highest spatial frequency you can sample without aliasing. A 400 mm FOV on a 512 matrix → 0.78 mm pixels → 0.64 lp/mm Nyquist. This is a working relation in Diagnostic imaging.

Where it is used

Clinically it sits on the Radiography room — Bucky / detector. In the Bucky / detector: grid, AEC, DQE, and the pixel that samples the image. Diagnostic equations live on the tube, the detector, and the patient: magnification, air kerma, CTDI, and why bone lights up at 70 kV. They turn a technique chart into physics you can defend.

Radiography room · Open this machine

How to use it

Choose matrix and FOV together. Zoom (smaller FOV, same matrix) improves pixel-limited resolution until focal-spot or MTF limits take over. Change one input and watch the curve and the simulation follow.

Symbols

FOVField of view400 mm
NMatrix size512

Worked example

A typical case from the default values: FOV = 400 mm (Field of view); N = 512 (Matrix size). Substituting into the relation gives Δx = 0.7813 mm; f_N = 0.64 lp/mm. These are teaching numbers — align them with your machine.

Typical values give

Δx = 0.7813mm
f_N = 0.64lp/mm

Where it comes from

The displayed formula is the working relation. Pixel size from FOV and matrix, plus Nyquist frequency. Usual reference: Bushberg. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Bushberg

Assumptions & limits

Sampling limit only. Focal-spot blur, motion and detector MTF usually bite before Nyquist in large-FOV radiography.

Pitfalls

kVp is not the same as effective energy. CTDI is not patient dose — SSDE and organ dose come after. Do not quote DLP as if it were effective dose without a k-factor. Sampling limit only. Focal-spot blur, motion and detector MTF usually bite before Nyquist in large-FOV radiography.

Keep this

Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Sampling limit only. Focal-spot blur, motion and detector MTF usually bite before Nyquist in large-FOV radiography.

In this specialty

Diagnostic imaging