02 Imaging
Scatter-to-primary ratio
SPR = S/P. Grid contrast improvement = (1+SPR)/(1+SPR/K).
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Simulation
Scatter-to-primary ratio — Change the numbers; the scene follows.
Where it works
CT scanner
Gantry
Inside the rotating gantry: tube, bowtie, and detectors that define CTDI and HU.
Open this machineFormula
Variables
Results
SPR
Scatter-to-primary
4
SPR_out
SPR after grid
0.6667
C'/C
Contrast improvement
3
Explanation
What it means
In a thick abdomen most of the photons reaching the detector are Compton scatter, not primary. SPR of 4–7 is typical without a grid; a well-used grid (selectivity Σ = K, often 5–10) brings it down and restores subject contrast. The price is a Bucky factor of 3–6 in dose. This is why grids are mandatory on Bucky tables and usually retracted on thin extremities and paediatrics. This is a working relation in Diagnostic imaging.
Where it is used
Clinically it sits on the CT scanner — Gantry. Inside the rotating gantry: tube, bowtie, and detectors that define CTDI and HU. 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.
CT scanner · Open this machineHow to use it
Enter scatter and primary (or SPR directly via S and P) and the grid selectivity K. Read SPR in, SPR out and contrast improvement. K=1 means no grid. Change one input and watch the curve and the simulation follow.
Symbols
- SScatter4
- PPrimary1
- KGrid selectivity6
Worked example
A typical case from the default values: S = 4 (Scatter); P = 1 (Primary); K = 6 (Grid selectivity). Substituting into the relation gives SPR = 4; SPR_out = 0.6667; C'/C = 3. These are teaching numbers — align them with your machine.
Typical values give
- SPR = 4
- SPR_out = 0.6667
- C'/C = 3
Where it comes from
The displayed formula is the working relation. SPR = S/P. Grid contrast improvement = (1+SPR)/(1+SPR/K). Usual reference: Bushberg / AAPM. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Bushberg / AAPM
Assumptions & limits
Uniform field, no grid cutoff, K independent of energy. Real grids have K that falls at low kV and at wide-angle scatter. Digital scatter correction is a software substitute, not a true K.
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 Do not quote DLP as if it were effective dose without a k-factor. Uniform field, no grid cutoff, K independent of energy. Real grids have K that falls at low kV and at wide-angle scatter. Digital scatter correction is a software substitute, not a true K.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Uniform field, no grid cutoff, K independent of energy. Real grids have K that falls at low kV and at wide-angle scatter. Digital scatter correction is a software substitute, not a true K.
In this specialty