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X-Rays: Electromagnetic Waves

- EW are waves that are created as a result of vibrations between an electric field and a magnetic field

- Oscillations are created as a result of the two fields coming in contact

- Have zero mass and travel at the speed of light (c = 3 x 108m/s= 1.08 x 109km/h (1080 million)

- Can travel through any material or vacuum


Periodic Waves

Amplitude - Maximum excursion of a particle

Wavelength - Horizontal length of one cycle

Period - Time required for one cycle

Frequency - Related to the period (Hz or S-1)


Electromagnetic Spectrum: X-Rays and Gamma Rays

- Short wavelength
- High frequency
- high energy


How Are X-Rays Produced

- Originate from within atoms
- Discovered by Willheim Roentgen in 1895


X-Ray Tube

Generate Electrons - Place electric current through filament


Aims of Diagnostic Imaging

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

- The examination must be justified - valid reason

- Expected to have an impact on the clinical management of the patient


Factors Influencing the Image Quality and Patient Dose

X-Ray Beam Characteristics
- Focal Spot Size
- Filtration of the beam
- Exposure Factors
- Field Size
- Scattered Radiation
- Geometry of image production

The Patient
- Ability to keep still
- Thickness and density of body parts

The Detector and Imaging System
- Computed Radiography and Digital Radiography
- Quantum Detection Efficiency
- The Display System
- Viewing Conditions

Practitioners Skill and Perception


Noise in an Image

Images Contain:
- Useful Information (the signal)
- Background Noise (hides useful information)

Signal-to-Noise Ratio (SNR)


Imaging System Geometry

Focus Receptor Distance (FRD)
Focus Object Distance (FOD)
Object Receptor Distance (ORD)

Focus = X-Ray Source
Object = Patient
Receptor = X-Ray Detector



- Essential to have minimal magnification and unsharpness
- Unsharpness is magnified by increasing the distance between the object (patient) and the receptor (detector)
- Image of object increases in size and distance to receptor increases

Can be reduced by:
- Keeping the receptor (detector) close to the patient
- Minimise patient-detector distance

Standardised FRD in a Department
- Tabletop Work – 100 cm
- Standing Chest Images – 180 cm



Penumbra – All images will have some ‘unsharpness’ or blurring (0.3 mm)

May not be detectable but influenced by several factors
- Movement of the patient (e.g. breathing)
- Geometry of the imaging (e.g. focus, object, detector distance)
- Display monitor (type, resolution, quality)
- Brightness and Contrast of the display monitor (
- Viewing Conditions (e.g. background monitor)
- Perception of the Practitioner (e.g. contrast, resolution, experience


Geometric Unsharpness (Penumbra)

Minimising geometric unsharpness
- Fine focus should be used
- Object close to detector as possible
- FRD as large as possible


Magnification and Sharpness

- Large Magnification = Reduced Sharpness
- Small Magnification = Increased Sharpness



- The ability of a system to distinguish two close objects or a specific part of anatomy
- Can be measured objectively using test object/phantom
- Normally expressed in terms of line pairs/mm

Depends on:
- All elements in the imaging chain
- Focus size
- Source, object, detector geometry
- Monitor Display


X-Ray Detectors

- All primary x-rays exiting patient should be absorbed in the detector
- Scattered radiation should not contribute to the image

- 40-60% of x-rays not detected – pass straight through detector

Two main types of detector systems for x-ray radiography
- Computed Radiography (CR)
- Digital Radiography (DR)

Detective Quantum Efficiency (DQE)
- A measure used to compare different imaging system performance



- Any process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules

- X-Rays are ionising radiations

- Removal of electrons from an atom in an ionised or excited state leads to positively charged atom

- Involves a positively charged atom ‘ion’ and a negatively charged electron


Ionisation and Imaging

Production of X-rays in a Tungsten Target
- Thermionic emission at the filament
- Production of heat in the tungsten x-ray target

Detection of Radiation
- Radiation Measurement
- Dosimetry of Radiation Effects
- Fluoroscopy
- Image Production
- Radiation Protection


Radiation Dose

- Energy absorbed through the ionisation process can have detrimental biological effects on the patient (could potentially break strands of DNA)

- Should be recorded at each imaging session

- Record image acquisition parameters (e.g., kV, mA, exposure time, field size, FOD, FRD


Radiation Dose: Exposure Types

Diagnostic Medical Exposure – representative absorbed doses are required to be determined for patients of a typical size

Therapeutic Exposure – individual absorbed dose values are required to be determined (calculation or direct measurement) representative for the target volume and relevant organs of the patient


Human Perception

- Two practitioners may view images differently

- Differences may be small or large and can be a result of experience and skill of the practitioner