Week 4 Principles of Radiation Detection and Image Formation

Explain the fundamental principles of radiation detection and image formation, including the roles of noise, resolution, contrast, and imaging geometry. ● Compare major radiation detector types (film, screen–film, CR, DR, gas-filled detectors) and evaluate their strengths, limitations, and clinical applications ● Interpret key detector performance measures especially DQE, efficiency factors, and temporal response and discuss how they influence image quality and patient dose

LO

Explain the fundamental principles of radiation detection and image formation, including the roles of noise, resolution, contrast, and imaging geometry.

● Compare major radiation detector types (film, screen–film, CR, DR, gas-filled detectors) and evaluate their strengths, limitations, and clinical applications

● Interpret key detector performance measures especially DQE, efficiency factors, and temporal response and discuss how they influence image quality and patient dose

Principles of radiation detection and image formation

Desirable characteristics of radiation detectors

Detective Quantum Efficiency (DQE)

Principles of image formation

• Noise, Resolution, Contrast, Magnification, Speed (sensitivity)

Scatter

Radiographic detection techniques

• Film

• Screen-film radiography

• Computed Radiography (CR)

• Digital Radiography (DR)

Aim of diagnostic imaging

Produce images of optimum quality for diagnosis and management / treatment of disease

Examination is justified

Expected to impact clinical management of patient

• Intervention

• Do nothing

Factors influencing the image quality and patient dose

1. X-ray beam characteristics (CS, Exposure factors, beam filtration)

2. The patient (stillness, thickness)

3. The detector and imaging (CR or DR, Quantum Detection Efficiency (QDE), THe display system)

4. Practitioner skill and perception

Image noise

Images may contain useful information known as the signal

Background noise, which conceives useful information

Signal-to-noise ratio (SNR)

Image noise

is random variation in the recorded signal from pixel to pixel.

Image noise is proportional to the no. of quanta involved in forming the recorded signal

Ability to detect an object depends on the contrast of the object and the noise in the image

Imaging system geometry

Focus Receptor Distance (FRD, AKA SID)

Focus Object Distance (FOD, SOD)

Object Receptor Distance (ORD, OID)

Magnification

Minimal magnification and unsharpness is best

Unsharpness is magnified by increasing OID

Magnification reduced by keeping receptor close to patient

Minimise patient receptor (OID) distance

Typical FRD (SID) is 100cm for tabletop, 180 for standing chest

Magnification factor SID/SOD

Unsharpness

Penumbra - All images has some blurring approx. 0.3mm

Influenced by factors such as:

• Movement of patient

• OID

• Brightness and contrast of display monitor

• Background lighting

Unshapness = Focus x OID / SID - OID

Geometric unsharpness (penumbra)

Minimising geometric unsharpness

• Fine focus should be used

• small OID

• SID large as possible

Magnification and sharpness

Large magnification as a result of large OID means reduced sharpness

Small magnification due to small OID increases sharpness

Resolution / definition

The ability of a system to distinguish two close objects or a specific part of anatomy

Measured objectively using a test object/phantom

Normally expressed in terms of line pairs / mm

Resolution depends on

• all the elements in the imaging chain

• Focus size

• Source, object, detector geometry

• Monitor display

Spatial frequency

The ability to see features in the image that are small or close together, upper limit is usually 3.5lp/mm

Image contrast

Low contrast vs high contrast (more blackening)

Human perceptions

Difference in perceptions can vary due to skill

Desirable characteristics of radiation detectors

Characteristics important for ay kinds of radiation detector

1. Absorption efficiency (% x-ray absorbed)

2. Conversion efficiency (% x-ray converted to electronic signal)

3. Capture efficiency (% of area of the detector that is ‘active’ detector)

4. Dose efficiency (how much incident dose on the detector contributes to image)

Desirable characteristics of radiation detectors

Temporal response

Timing of phosphorescence or afterglow

Wide dynamic range

High reporductibility abd stability

Detective Quantum Efficiency (DQE)

Measures how well incident x-ray is transferred into useful information (signal)

IDEAL DQE is 1 (100%)

A DQE of 0.5 means only 50% of incident x-ray on the detector are used for producing image.

DQE is affected by changes in input signal (mAs, kV) and patient

Evolution of Radiographic Detectors

Film → screen film → Computed Radiography (CR) → Digital Radiography (DR)

Film Radiography

X-rays interact directly with silver halide crystals

Advantage: high spatial resolution

Disadvantage: low efficiency (higher dose required)

Screen Film

Phosphor screen convert x-rays to visible light

advantage (higher efficiency)

Limitation (lower resolution vs film radiography due to light spread)

Intensifiers increase efficiency more (even lower dosage)

CR machine

Laser scanning releases stored energy as light

Advantage: wide exposure latitude

L: Moderate resolution (2.4 - 5lp/mm)

DR machine

Direct electronic conversion to image

A: fast workflow, wide dynamic range

L: costs

Clinical considerations

Higher DQE → better signal to noise ratio → lower dose

Digital systems reduce repeats

Risk of dose creep in digital systems due to wide dynamic range

Gas filled detectors

enclosed volume of detection medium (gas)

Carged electrodes

As radiation passes through, ionisation results from interactions

Number of ion paris produced depends on the LET of the radiation, High LET - more ionising

Ionisation chambers

X-rays interact in the chamber wall surrounding the air cavity

Electrons are generated which transverse the air in the cavity causing ionisations

Xenon gas detectors

Used in older CT scanners

Xenon gas molecules widely spaced in cavity so low absorption efficiency

Fast response