02 Imaging
Grid ratio
Grid ratio r = h/D and Bucky factor for mAs increase.
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Simulation
Grid ratio — 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.
Open this machineFormula
Variables
Results
r
Grid ratio
10
mAs
mAs with grid
20mAs
Explanation
What it means
Grid ratio r = h/D (lead height over interspace) sets how well the grid rejects oblique scatter. Higher ratio → better cleanup, more primary loss, higher Bucky factor and more mAs. 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 machineHow to use it
Typical ratios: 8:1 or 10:1 for 100 cm SID; 12:1–16:1 for longer SID / high kVp. Multiply non-grid mAs by the Bucky factor (≈ 3–5 for 8:1–12:1). Change one input and watch the curve and the simulation follow.
Symbols
- hLead strip height4 mm
- DInterspace width0.4 mm
- BBucky factor4
- mAs₀mAs without grid5 mAs
Worked example
A typical case from the default values: h = 4 mm (Lead strip height); D = 0.4 mm (Interspace width); B = 4 (Bucky factor); mAs₀ = 5 mAs (mAs without grid). Substituting into the relation gives r = 10; mAs = 20 mAs. These are teaching numbers — align them with your machine.
Typical values give
- r = 10
- mAs = 20mAs
Where it comes from
The displayed formula is the working relation. Grid ratio r = h/D and Bucky factor for mAs increase. Usual reference: Bushberg. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Bushberg
Assumptions & limits
Ratio alone is not selectivity; strip frequency (lines/cm) and focus distance matter. Misalignment (off-level, off-centre, upside-down) causes grid cutoff.
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 Do not quote DLP as if it were effective dose without a k-factor. Ratio alone is not selectivity; strip frequency (lines/cm) and focus distance matter. Misalignment (off-level, off-centre, upside-down) causes grid cutoff.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Ratio alone is not selectivity; strip frequency (lines/cm) and focus distance matter. Misalignment (off-level, off-centre, upside-down) causes grid cutoff.
In this specialty