02 Imaging

Subject contrast

Fractional difference in transmitted intensity between two regions.

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Simulation

Subject contrast — Change the numbers; the scene follows.

Where it works

Radiography room

Patient

At the patient entrance — skin dose, subject contrast, photoelectric absorption in tissue.

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Formula

C = \frac{I_2-I_1}{I_1} \approx e^{-(\mu_2-\mu_1)x}-1

Variables

I₁ · Intensity region 1a.u.I₂ · Intensity region 2a.u.

Results

C
Contrast
-0.15
C
Contrast
-15%
C_mod
Michelson contrast
-0.1622

Explanation

C = \frac{I_2-I_1}{I_1} \approx e^{-(\mu_2-\mu_1)x}-1

What it means

Subject contrast is the relative difference in transmitted fluence between two regions, before the detector. It is driven by photoelectric Δμ at low kVp and by density differences at high kVp. This is a working relation in Diagnostic imaging.

Where it is used

Clinically it sits on the Radiography room — Patient. At the patient entrance — skin dose, subject contrast, photoelectric absorption in tissue. 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

Enter two intensities (or compute them from μ and thickness). Michelson contrast (I2−I1)/((I1+I2)/2) is bounded ±1 and is common in imaging science. Change one input and watch the curve and the simulation follow.

Symbols

I₁Intensity region 1100 a.u.
I₂Intensity region 285 a.u.

Worked example

A typical case from the default values: I₁ = 100 a.u. (Intensity region 1); I₂ = 85 a.u. (Intensity region 2). Substituting into the relation gives C = -0.15; C = -15 %; C_mod = -0.1622. These are teaching numbers — align them with your machine.

Typical values give

C = -0.15
C = -15%
C_mod = -0.1622

Where it comes from

The displayed formula is the working relation. Fractional difference in transmitted intensity between two regions. Usual reference: Bushberg. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Bushberg

Assumptions & limits

This is subject contrast, not displayed (window/level) contrast or detector contrast. Scatter reaching the detector reduces it by 1/(1+SPR).

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 Do not quote DLP as if it were effective dose without a k-factor. This is subject contrast, not displayed (window/level) contrast or detector contrast. Scatter reaching the detector reduces it by 1/(1+SPR).

Keep this

Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. This is subject contrast, not displayed (window/level) contrast or detector contrast. Scatter reaching the detector reduces it by 1/(1+SPR).

In this specialty

Diagnostic imaging