Radiology Flashcards
Radiographs
Images created by Xray which have been projected through and object and interacted with a receptor
Different shades of grey on the image correspond to the different types of tissue and thicknesses of tissue involved
Why are radiographs useful?
Provide ability to see structures within the body, particularly mineralised tissues - many dental related conditions affect the mineral content of tissues
Can show normal anatomy and pathology
Aid diagnosis, treatment planning and monitoring
Common intra and extra oral dental radiographs
Intra
Bitewings
Periapical
Occlusal
Extra
Panoramic
Lateral cephalogram
Electromagnetic radiation
Flow of energy created by simultaneously varying electrical and magnetic fields
No mass
No charge
Travels at speed of light
Can travel in a vacuum
EM spectrum
Shorter wavelength, higher frequency. higher energy
Gamma
Xray
UV
Visible light
Infrared
Microwaves
Radiowaves
Longer wavelength, lower frequency, lower energy
Frequency of EM waves
How many times one full wave cycle is repeated per unit time in Hertz
One Hz = One cycle per second
Speed of EM waves formula
Speed = frequency x wavelength
Energy of EM waves
Measured in electron volts
1 eV = energy (in joules) gained by one electron moving across a potential difference of one volt
Xray photon energies
124eV - 124 keV
Medical imaging uses mostly hard Xrays >5keV
Hard Xrays vs soft Xrays
Have higher energies and are able to penetrate human tissues
Soft are easily absorbed
What is the difference between Xrays and Gamma rays?
Gamma occur naturally
Production of Xrays
Electrons are fired at atoms at very high speed and on collision the kinetic energy of the electrons is converted into EM radiation and heat
Xray photons are aimed at a substance
Xrays cause ionisation - displace electrons from atoms/molecules
Electron shells
Orbits around atom where electrons are found
Electrons fill available spaces in innermost shells first
Shells are called K, L, M, N etc
Max number of electrons in a shell
2n^2 where n is shell number
Binding energy
Energy required to exceed electrostatic force between an electron and it’s nucleus, and remove the electron
Higher atomic number = higher electrostatic force
Amps
Measure of how much charge flows past a point per second
Current
A flow of electrical charge usually by movement of electrons
Why must Xray units modify mains electrical current?
Mains is alternation current, Xray requires direct current - rectification of the curren
Voltage
Difference in electrical potential between two points in an electrical field and is related to how forcefully a charge will be pushed through an electrical field
Measured in volts V
Synonymous with potential difference
Electrical supply to Xray unit
UK mains electricity is AC 13 amps or less, 220-240 volts
Dental Xray requires a direct current with 2 different voltages - one as high as 10s of thousands of volts, one as low as around 10
Transformers alter voltage from one circuit to another
One transformer mains -> Xray tube (cathode - anode)
One transformer mains -> filament
Xray beam
Made up on millions of Xray photons directed in the same general direction
They travel in straight lines but diverge from Xray source
Intensity is the quantity of photon energy passing through a cross sections area of the beam per unit time
Increased number or energy of photons = increased intensity, proportional to the current in the filament and voltage across the Xray tube
Intensity of Xray compared to distance from source
The intensity of Xray beam is inversely proportional to the square distance between the Xray source and the point of measurement
Doubling the distance will quarter the dose
Xray production basic outline
Electrons accelerated towards atoms at very high speed
On collision, the kinetic energy of these electrons is converted to heat and electromagnetic radiation (ideally Xray photons)
The Xray photons are aimed at a subject
Components of Xray unit
Tubehead
Collimator
Positioning arm
Control panel
Circuitry
Xray tube
Made up of a glass envelope with a vacuum inside, containing -ve cathode (filament and focussing cup)and +ve anode (target and heat dissipating block)
What material is the heat dissipating block in Xray tube?
Copper
What material is the focussing cup in Xray tube?
Molybdenum
What material is the filament in Xray tube?
Tungsten, because of its high melting point 3422C and high atomic number 74 so lots of electrons per atom to be released
It is also suitably malleable to form the coiled wire
What occurs at the cathode in the Xray tube?
A low voltage, high current electricity is passed through the Xray filament, heating it til incandescent ~2200C
Electrons are then released from atoms in the wire creating a cloud of electrons around the filament
Focussing cup
Metal plate shaped around the filament
Negatively charged to repel electrons towards the anode target
Made of molybdenum due to its high mp 2623C
Between cathode and anode
Electrons are accelerated to a very high speed, and have high kinetic energy on collision with the anode target
Kinetic energy of electrons in Xray tube
60-70keV
Xray anode target
Metal block which gets bombarded with electrons, producing photons (and lots of heat)
Made of Tungsten due to its high mp and the fact that it creates xray photons of useful energies
Orientated at angle from filament as this reduces area from which Xray is emitted while increasing actual SA, for heat dissipation
Focal spot is the precise area on the target at which electrons collide and produce Xrays
Penumbra effect
Blurring of radiographic image due to focal spot not being a single point, minimised by shrinking the focal spot
Glass envelope
Air tight vacuum so that air particles don’t get in the way of electron path
Leaded apart from one small window, so that Xray photons can only escape in the desired direction
Purpose of oil in glass envelope
Heat dissipation
Aluminium filtration in Xray tube
Removes lower energy non diagnostic Xray from the beam as these are fully absorbed, not contributing to image but increased pt dose - reduces photoelectric effect
Thickness required <70kV-1.5mm 70kV+ - 2.5mm
Spacer cone
Helps direct beam
Creates desired focus to skin distance
FSD
<60kV 100mm
60kV+ 200mm
Collimator
Lead diaphragm attached to the end of the spacer cone which reduces pt dose and focuses beam to shape and size of the receptor
When using size 2 receptors, rectangular collimators should (at min) crop the beam area to 50x40mm but preferably 45x35mmo
Why is rectangular collimation recomended?
Can reduce pt dose by approx 50% and improves image contrast by reducing scatter
Xray control panel
On/off switch and light
Electronic timer
Exposure time selector and presets
Warning light and noise
Continuous vs characteristic radiation
Continuous - produces continuous range of photon energies, maximum matches the peak voltage, bombarding electron interacts with the nucleus of target atom
Characteristic - produces specific photon energies, characteristic to the target element, photon energies depend on the binding energies of electron shells, bombarding electrons interact with inner shell electrons of target atom
Typical characteristic spikes of electron energy in Xray unit 70kV
59kV 67kV
3 ways photons can interact with matter
Transmission - passes through unaltered
Absorption - stopped by the matter
Scatter - changes direction
Result of absorption and scatter occurring
Attenuation - reduced intensity of Xray beam
What determines Xray number of photons?
Current in the filament mA
What affects the energy of Xray photons?
Voltage across Xray tube kV
Minimal attenuation appearance
Black
Partial attenuation appearance
Grey
Complete attenuation appearance
White
Photoelectric effect
Complete absorption
Photon in Xray beam interacts with inner shell electron in subject, resulting in absorption of the photon and creation of a photoelectron - gives out light
When does the photoelectric effect occur?
Energy of photon is equal to or just greater than the binding energy of inner shell electrons, therefore photoelectric effect predominates with lower energy photons, since human tissues have relatively low binding energies
Any excess photon energy becomes kinetic energy of photoelectron and this can ionise and potentially damage adjacent tissues
Inner electron shell them filled by cascade which produces light photons and/or heat
Probability of photoelectric effect
~ (p x Z^3)/ E^3
p - density of material
Z atomic number
E photon energy
Compton effect
Partial absorption and scatter
Photon in beam interacts with an outer shell electron in the subject and results in partial absorption and scattering of the photon and creation of a recoil electron
When does the Compton effect occur?
When the energy of incoming photon is much greater than binding energy of electron, therefore it predominates between higher energy photons and outer shell electrons
Effect of recoil electron production by Compton effect
Recoil electrons can ionise and potentially damage adjacent tissues
What happens to a photon after it undergoes the Compton effect?
The photon loses energy and changes direction, it can still undergo the photoelectric or further Compton effect
What is the effect of energy of photon on direction of scatter?
Higher energy are deflected more forward
Lower energy are deflected more backward
What is the effect of photons being scattered slightly obliquely?
May still reach the receptor but will interact with the wrong area, causing darkening of the image in the wrong place
Results in fogging of the image and reduces image contrast and quality
Types of ionising radiation
- Byproducts of radioactive decay (alpha large 2n 2p particle, travels few inches, beta small particle travels few feet, gamma high energy travels long distance)
- Artificially produced radiation - Xray
Xray and gamma rays are identical apart from Xrays being produced artifically
Most significant effect of ionising radiation
DNA in cells nuclei can be damaged - faulty repair of chromosome breaks resulting in the development of abnormal cell populations and therefore cancer
Can damage DNA directly (radiation interacts with DNA molecule or another important part of the cell) or indirectly (radiation reacts with water within the cell, producing highly reactive free radicals which can join in pairs to form hydroxyl radicals, which can cause damage to the cell)
Ionising radiation
Ionising radiation has enough energy to turn atoms into ions by knocking away electrons in the atoms orbit, this results in a positive ion and a free electron
Each ionisation process will deposit approx 35eV locally, greater than the energy involved in atomic bonds (approx 4eV)
Damage to DNA double stranded helix
One broken strand - usually repairable
Double strand break - more difficult to repair, usually due to alpha radiation, if repair is faulty, can lead to mutations affecting cell function
Biological effect will depend on type of radiation, amount of radiation, time over which dose is received, tissue/cell type irradiated
Organ cancer risks and radiation
There are only increased risks of cancer after irradiation of certain tissues
Most medical exposures do not irriadiate the body uniformly
Risk will vary depending on the organ getting the highest dose
What determines radiosensitivity of tissues?
Function of the cells that make up the tissue
If the cells are actively dividing
Highly radiosensitive tissues
Stem cells
Bone marrow
Lymphoid tissue
GI
Gonads
Embryonic tissue
Moderately radiosensitive tissues
Skin
Vascular endothelium
Lung
Lens of the eye
Least radiosensitive tissues
CNS
Bone
Cartilage
Connective tissue
3 possible outcomes after radiation causes DNA mutation of a cell
Mutation repaired
Cell death
Mutated cell survives, can cause cancer
What does LNT Linear No Threshold model estimate?
The long term biological damage from radiation
Absorbed dose
Energy deposited by radiation
Units Gy Gray
Equivalent dose
Absorbed dose multiplied by weighting factor depending on the radiation type
Beta, gamma, xray - weighting 1
Alpha weighting 20
Measured in Sieverts Sv
Digital receptors
Phosphor plates
Solid state sensors
All multiple use
What is the difference between digital and film radiography?
Differ in how the Xray beam is dealt with after it has interacted with the patient - difference in the film and how the film is processed
Digital has mostly taken fil radiography’s place due to multiple benefits but film is still sometimes used
Film receptors
Direct action or indirect action
All single use
Sizes of receptors
Variety, exact measurements may differ between companies
Size 0 - ant PA
Size 2 - BWs, post PA
Size 4 - Occlusal
Xray shadow
When beam passes through an object some photons are attenuated - this cross section is an Xray shadow
The shadow is basically the image information held by the Xray photons after a beam has passed through an object
Image receptor detects this Xray shadow and uses it to create an image
How do digital receptors create an image?
The receptor measure the Xray intensity in each defined area
Each area is given a value relating to the intensity (typically 0-225, most-least)
Each value corresponds with a shade of grey
Lower number darker shade
Displayed as a grid of squares called pixels
More pixels - better detail
What is the effect of increasing the number of pixels on a digital Xray image?
Increases the resolution which will provide a more diagnostic image up to a limit
Greyscale bit depth
Number of different shades of grey available
Typically at least 8 bits
Shades of grey = 2^8
DICOM
Format for digital images
International standard format for handling digital medical images
PACS
Picture archiving communication system
Storage and access to images
Not connected to dental practices
Main components - inputs, secure network, workstations, archive
Environment for viewing digital radiographs
Subdued lighting
Avoid glare
Monitor - clean, adequate resolution, high enough brightness and suitable contrast level
SMPTE test pattern
Society of Motion Picture and Television Engineers
Available online
Can be used to assess the resolution, contrast and brightness of your monitor
Phosphor plates/photostimulable phosphor plate/storage phosphor plate
Not connected to computer
Thinner than solid state
After receptor is exposed to Xrays, it must be put in a scanner and read to create final image
Image creation using phosphor plates in patient’s mouth
Receptor exposed to Xray beam
Phosphor crystals in receptor excited by the Xray energy, resulting in the creation of a latent image
Image creation with phosphor plates in the scanner
Receptor scanned by a laser
Laser energy causes the excited phosphor crystals to emit visible light
This light is detected and creates the digital image
Types of solid state sensors
CCD charge coupled device
CMOS complimentary metal oxide semiconductor
Solid state sensors image creation
Usually wired but can be wireless
Latent image created and immediately read within the sensor itself
Final image created virtually instantly
Solid state sensor components
Back housing and cable
Electronic substrate
CMOS imaging chip
Fibre optic face plate
Scintillator screen
Front housing
Identification dot
Located in corner of receptor to aid orientation of image
Only useful if receptor was positioned correctly during exposure
Cross infection control when taking digital radiographs
Intra oral receptors have purpose made covers to prevent saliva contamination - single use
Receptors still disinfected between uses
Why is careful handing of digital receptors important?
Certain types of damage will impact every subsequent image obtained from that receptor
Reduces their diagnostic value and may render receptor unusable
Hold by their edge not their flat surfaces
What causes white lines on digital receptors??
Scratches/tears
Fingerprints
Bending/creases
Phosphor plates vs solid state sensors
PP - thinner, lighter, usually flexible, wireless (more comfortable). Variable room-light sensitivity, risk of impaired image, latent image needs to be scanned separately, handling similar to film, delicate
SS - bulkier, more rigid, usually wired, smaller active area, no issues with room-light control, replace less often, more expensive
Components of film packet
Radiographic film
Protective black paper to protect film from light exposure, damage from fingers, saliva
Lead foil backing to absorb some excess Xray photons
Outer wrapper usually plastic which prevents saliva ingress, and indicated which side of the packet is the front
Radiographic film
Material in which the actual image is formed
Sensitive to Xray and visible light photons
Photons interact with emulsion on film to produce latent image which only becomes visible after chemical processing
Radiographic film structure
Transparent plastic base to support the emulsion
Adhesive to attach emulsion
Emulsion layered on both sides
Protective coat of clear gelatin to shield the emulsion from mechanical damage
Radiographic emulsion
Silver halide crystals embedded in gelatine binder
Microscopic crystals effectively become the pixels of the final image
Film generally higher resolution than digital
Silver halide crystals in film radiography
Usually silver bromide
Become sensitised on interaction with Xray (and v. light) photons
During processing sensitised crystals are converted to particles of black metallic silver -> dark parts of final image and non sensitised crystals are removed -> light parts of final image
Lead foil in film radiography
In packet, behind the film
Absorbs some excess photons - those in the beam that continue past the film AND those scattered by pt tissues and returning back to the film
Embossed textured pattern to make it obvious if receptor was placed the wrong way around
Film speed
Relates to the amount of Xray exposure required to produce an adequate image
Increase speed leads to reduced radiation required to receive an image
Affected by number and size of silver halide crystals in the emulsion
Larger crystals -> faster film but poorer resolution, because crystals act as pixels
Film speeds
E 2x faster than D, half the exposure time, half the dose
F 20% faster than E, 20% reduction in time and dose
If changing to different film speed, you must either convert settings on Xray unit or install a filter to absorb part of the primary Xray beam
Intensifying screens
Used alongside indirect action film for extra oral radiographs (too bulky for intraoral use)
Reduce radiation dose but also reduce detail
Becoming less common as digital receptors become more commonplace
Indirect action film placed inside a cassette with an intensifying screen either side
Screens release v light upon exposure to xrays, this visible light creates the latent image on the film
Film processing
Sequence of steps which converts the invisible latent image into a visible permanent image
MUST be carried out under controlled standardised conditions to ensure consistent image quality
Different methods - manual, automated, self developing (less common)
Common steps of all film processing methods
Developing - converts sensitised crystals to black metallic silver particles
Washing - removes residual developer solution
Fixing - Removes non sensitised crystals and hardens emulsion (which contains the black metallic silver)
Washing - removes the residual fixer solution
Drying - Removes water so that film is ready to be handled/stored
Manual (or wet) cycle
Person dips film into different tanks of chemicals at precise concentrations and temps for specific durations, then washes film after each tank
Must be carried out in a dark room with absolute light-tightness and adequate ventilation
Fairly laborious process
Automated cycle
More common way of film processing
All necessary steps are carried out within a machine
Exposed film goes in one end, processed film comes out other
Faster and more controlled than manual processing and avoids the need for a dark room
BUT more expensive
Opening a film packet for automated processing
Disinfect the surface of the packet
Hold the packet under the hood of the processor unit
Peel back flap of outer wrapper
Fold back lead foil
Pull back paper flap
Hold film by edges and slide out
Insert film into processor slot/shelf
Self developing films
Not recommended
Advantages - no darkroom or processing facilities required, faster
Disadvantages - poorer image quality, image deteriorated more rapidly over time, no lead foil, easily bent, difficult to use in positioning holders, relatively expensive
Processing issues with developing
Developing (silver halide -> black metallic silver) involves a chemical reaction affected by time, temp and concentration, developer solution oxidises in air and becomes less effective in time - needs to be replaced regularly
Potential causes of a pale image in film radiography
Exposure issue - radiation exposure factors too low
Developing issue - film removed from solution too early, solution too cold, dilute or old
Potential issues in fixing stage of film developing
Fixing involves a chemical reaction which removes non sensitised crystals and hardens the remaining emulsion
Inadequate fixing means non sensitised crystals are left behind meaning
Image greenish-yellow or milky
Image becomes brown over time
Potential issues in fixing stage of film developing
Fixing involves a chemical reaction which removes non sensitised crystals and hardens the remaining emulsion
Inadequate fixing means non sensitised crystals are left behind meaning
Image greenish-yellow or milky
Image becomes brown over time
Potential issues with film developing washing stage
Developer and fixer solution will continue to act if not washed off
Advantages of digital radiograhy
Computer filing - easy storage and archive
Easy back up of images
Images can be integrated into pt notes
Easy transfer of images
Images can be manipulated
No chemical processing required
Disadvantages of digital radiography
Worse resolution - risk of pixelation
Requires diagnostic level computer monitors for optimal viewing
Risk of data corruption/loss
Hard copy print outs generally have reduced quality
Image enhancement can create misleading images
Extra oral radiography
Xray source and receptor outside patient
Allows visualisation or teeth, jaws, facial bones etc
Extra oral radiography purposes
Imaging larger sections of the dentition
Alternative when pt unable to tolerate intra oral
Imagine non dento-alveolar regions
Common extra oral radiograph types
Panoramic
Cephalometric - lateral, postero-anterior
Oblique lateral
Skull radiographs - occipitomental, poster-anterior skull/mandible, Reverse Towne’s, true lateral
True vs oblique
True - Xray beam perpendicular to head
Oblique - not perpendicular, off at an angle
Mid sagittal plane
Line down middle of the face
Interpupillary line
Connects both pupils
Frankfort plane
Connects infraorbital margin and superior border of external auditory meatus
Orbitomeatal line
Connects outer canthus and centre or EAM
Cephalostat
On all units that take cephalograms
Ensure standardised positioning of equipment and patients head - avoids discrepancies between radiographs and reduces magnification/distortion
Includes ear rods and forehead support
Lateral cephalograms receptor to focal spot distance
1.5-1.8m from Xray focal spot to receptor, to minimise magnification
Problem and solutions for visualising soft tissues on lateral cephalograms
Soft tissues show up poorly when exposure settings are optimised for hard tissues
Solutions
Place an aluminium wedge filter in the unit to attenuate the specific area of the beam exposing the soft tissues
or
Used software to enhance the soft tissues post exposure
Thyroid collar
Almost always used when taking lateral cephalograms
Thyroid gland is relatively radio sensitive
May obscure hyoid bone and cervical vertebrae, irrelevant to most cases, but sometimes used to assess maturity of skeleton
Thyroid collar
Almost always used when taking lateral cephalograms
Thyroid gland is relatively radio sensitive
May obscure hyoid bone and cervical vertebrae, irrelevant to most cases, but sometimes used to assess maturity of skeleton
Benefits of CBCT
No superimposition or magnification of anatomy
Images can be viewed at any angle
(not indicated currently due to increased radiation dose)
Indicated for orthognathic surgery for pre op assessment
Benefits of CBCT
No superimposition or magnification of anatomy
Images can be viewed at any angle
(not indicated currently due to increased radiation dose)
Indicated for orthognathic surgery for pre op assessment
Benefits of CBCT
No superimposition or magnification of anatomy
Images can be viewed at any angle
(not indicated currently due to increased radiation dose)
Indicated for orthognathic surgery for pre op assessment
Benefits of CBCT
No superimposition or magnification of anatomy
Images can be viewed at any angle
(not indicated currently due to increased radiation dose)
Indicated for orthognathic surgery for pre op assessment
Oblique lateral radiography
Provides view of posterior jaws without superimposition of contralateral side
Uncommon nowadays - difficult to master technique
Superseded by panoramic in most situations
Oblique lateral radiography
Provides view of posterior jaws without superimposition of contralateral side
Uncommon nowadays - difficult to master technique
Superseded by panoramic in most situations
Indications for oblique lateral radiography
(Similar to panoramic)
Assessment of dental pathology
Assessment of presence/position of unerupted teeth
Detection of mandibular fractures
Evaluation lesions/conditions affecting jaws
As an alternative to panoramic when pre-cooperative, learning difficulties, tremors, unconscious
Cone beam CT
Sectional images - thin slices 0.4mm or thinner
Why do we use radiographic localisation?
To determine location of a structure or pathological lesion in relation to other structures
Clinical situations where radiographic localisation may be used
Positions of unerupted teeth - most common
- normal but ectopic or impacted, supernumerary, proximity to important structures
Location of roots/canals for endo or surgery
Relationship of pathological lesions to normal features
Trauma
Soft tissue swellings
Parallax
An apparent change in position of an object caused by a real change in the position of the observer
Sequence of events for parallax
Identify direction of tube head shift
Identify structure we want to know the location of
Choose a reference point which can be seen in both images
Observe the movement of the desired structure in relation to the reference point
Options for parallax with horizontal tube shift
Equivalent views such at 2 PAs, 2 BWs, 2 oblique occlusals
Options of radiographs for parallax with vertical tube shift
Different views such as panoramic and oblique occlusal, panoramic and bisecting angle lower periapical
Purpose of quality assurance in dental radiology
To ensure consistently adequate diagnostic information, whilst radiation doses to patients (and others) are kept ALARP taking into account the relevant requirements from IMRER17 and IRR17
Quality assurance programme components
Procedures - risk assessment, local rules, contingency plans
Staff training
Xray equipment
Patient dose
Display equipment
Image quality
Regular checks for digital image receptors
Check receptor itself for visible damage
Check image uniformity by exposing the receptor and checking if image is uniform - constant shade of grey
Check image quality - radiograph of a test object
How does delamination of digital receptors appear?
White areas around edge
How does cracking of digital receptors appear?
Network of white lines
How do scratches on digital receptors appear?
White lines
What could black marks on digital radiographs be caused by?
Sensitisation of radiographic emulsion
Quality assessment of image quality 3 parts
Image quality rating - grading
Image quality analysis - reviewing images to calculate success rate and identify any trends for suboptimal images, carried out periodically e.g. every 4 month, last 150 images reviewed
Reject analysis - recording and analysing each unacceptable image
Diagnostically acceptable positioning factors for BWs
Show entire crowns of upper and lower teeth
Include distal aspect of the canine and mesial aspect of last standing tooth (may require more than one)
Every approximal surface shown at least once without overlap (where possible, may be impossible if crowding)
Diagnostically acceptable positioning factors for PAs
Shows entire root
Shows periapical bone
Shows crown
