Week 1 - Introduction/Definitions Flashcards
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
Magnification
- 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
Unsharpness
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
Resolution/Definition
- 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
Ideally
- All primary x-rays exiting patient should be absorbed in the detector
- Scattered radiation should not contribute to the image
Reality
- 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
