02 Imaging

Geometric magnification

Image magnification from SID over SOD.

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Simulation

Geometric magnification — Change the numbers; the scene follows.

Where it works

Radiography room

Table / SID

On the table geometry — SID and SOD between tube, patient, and the Bucky.

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Formula

M = \frac{\mathrm{SID}}{\mathrm{SOD}} = \frac{\mathrm{SID}}{\mathrm{SID}-\mathrm{OID}}

Variables

SID · Source-to-image distancecmOID · Object-to-image distancecm

Results

M
Magnification
1.25
SOD
Source-to-object distance
80cm

Explanation

M = \frac{\mathrm{SID}}{\mathrm{SOD}} = \frac{\mathrm{SID}}{\mathrm{SID}-\mathrm{OID}}

What it means

Geometric magnification M = SID/SOD = SID/(SID−OID). The image is larger than the object because the beam continues to diverge beyond the object. Magnification also enlarges focal-spot blur. This is a working relation in Diagnostic imaging.

Where it is used

Clinically it sits on the Radiography room — Table / SID. On the table geometry — SID and SOD between tube, patient, and the Bucky. 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

Keep OID small and SID large to minimise magnification and blur. In mammography a 1.1–1.5 mag view is deliberate (air-gap, small focus). Change one input and watch the curve and the simulation follow.

Symbols

SIDSource-to-image distance100 cm
OIDObject-to-image distance20 cm

Worked example

A typical case from the default values: SID = 100 cm (Source-to-image distance); OID = 20 cm (Object-to-image distance). Substituting into the relation gives M = 1.25; SOD = 80 cm. These are teaching numbers — align them with your machine.

Typical values give

M = 1.25
SOD = 80cm

Where it comes from

The displayed formula is the working relation. Image magnification from SID over SOD. Usual reference: Bushberg, The Essential Physics of Medical Imaging. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Bushberg, The Essential Physics of Medical Imaging

Assumptions & limits

Point-projection geometry. Patient thickness means different planes magnify differently. Distortion from angled CR is not included.

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. Point-projection geometry. Patient thickness means different planes magnify differently. Distortion from angled CR is not included.

Keep this

Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Point-projection geometry. Patient thickness means different planes magnify differently. Distortion from angled CR is not included.

In this specialty

Diagnostic imaging