Technical stuff
Inverse square law
*The intensity of the beam is inversely proportional to the square of the distance* => 1/D squared If we halve the distance then we will have 4 times the intensity. If we want the same intensity to the image we need to quarter the original mAs. ¾ SID = 2 x Intensity = ½ exposure ½ SID = 4 x Intensity = ¼ exposure ¼ SID = 8 x Intensity = 1/8 exposure NB: Every time you increase SID by ¼ you double your intensity and vice versa
Deterministic effect
- 'Early' (large dose) - Occurs hours or days after an exposure and is directly related to exposure threshold Eg. radiation burns or radiation sickness
Stochastic effect
- 'late' - Occurs by chance and can happen years after an exposure - NO threshold Eg. cancer or can affect offspring
What are some differences between DR and CR
- DR has a greater sensitivity than CR, meaning lower patient radiation dose - DR is significantly faster (doesnt required MRT to collect cassette and process through CR scanner). Higher patient throughput for DR - CR is more convenient for mobile work - portable DR flat plate detectors weight approx 5kg and require a power supply, cable and computer - CR IR's are smaller and available in different size (this could be the same for DR now...) - CR is cheaper - If DR plate fails that x-ray room will be unusable. If CR plate fails, we are able to grab another one
Macroradiography
- Enlargement distortion is higher than normal - To achieve this with good definition, use a very fine focus of 0.3mm2 - Our tubes have 0.6mm2 which allows for a maximum of 1.5x magnification - 2x = midway between IR and focal spot TECHNIQUE: - No grid required due to air gap - Increase of 20kVp to compensate for large OID DISADVANTAGES: - Increased chance of movement unsharpness - Patient closer to source so increased skin dose Used for fractures of wrist (scaphoid) skull and mamms
High kVp technique
- High kV is used when different tissue types are being imaged (bone, soft tissue, air, etc). - Higher kV produces less variation in attenuation of the x-ray beam resulting in lower contrast. - High kV technique gives fast exposure times which freezes motion (UM) due to either patient movement or involuntary movement. Two main advantages of high kV for CXR are; o Better penetration of the mediastinum o Reduction of the visibility of the ribs relative to the lung structures.
MULTIPLE MYLOMA PROTOCOL
- PA chest (landscape to see shoulders). Low kVp - Lateral skull - Lateral thoracic spine - Lateral lumbar spine - AP pelvis to include upper ends of femurs
Image acquisition (CR reader)
- x-ray taken, exciting electrons inside the IR which then get trapped in the "forbidden zone" (forms a latent image) - Cassette is place in the CR reader - Then swept by a red lazer beam - The red light from the lazer energises the electrons trapped in the forbidden zones, as they are release they loose energy in the form of a cyan light. - Cyan light is picked up by the photomultiplier tube, which then converts this energy into an electrical signal. - The signal is then converted into a digital signal, by giving it an x and y coordinate along with a brightness level. - IR is then erased by a bright white light - Image is then processed via an algorithm
15% kVp rule
15% rule is the equivalent of doubling mAs Therefore increasing kV by 15% allows for mAs to be halved
What is the EI 30% rule?
30% of the cassette must be used for the EI to be accurate. If <30% of the cassette is used EI will be inaccurate.
What are the CR plate layers
4 principle layers • *PROTECTIVE COATING* - The layer closest to the screen, used to reduce damage, caused by handling, to the screen. It provides a surface for routine cleaning without disturbing the active phosphor. It is transparent to light. • *PHOSPHOR* - Active layer of the screen. Emits light (fluoresces) during stimulation by x-rays (photostimuable). -Thickness and size of phosphors in the phosphor layer change characteristics of each cassette (extremity/regular). • *REFLECTIVE LAYER* - Between phosphor and the base. - Made of shiny substance, such as magnesium oxide. -When x-rays interact with the phosphor, light is emitted isotropically (radiation emitted with equal intensity in all directions). The reflective layer intercepts light headed in other directions and redirects it to the screen. • *BASE* - The layer farthest from the screen. - It is made of high grade cardboard or polyester.
kVp
= Quality Is a component that controls the quality of the x-ray beam produced. Also, it is what controls the contrast or greyscale in the produced x-ray film. • The higher the kVp, the lower the radiographic contrast. This is due to less differential absorption of tissues. • Controls beam quality and penetrability. • Higher kVp= higher quality= higher energy, so more likely to penetrate ROI. Increased kVp results in reduced radiographic contrast due to: o Reduced differential absorption. o Increased Compton scatter. o Controlling energy of imaging forming x-rays.
mA
= Quantity • Controls the quantity or the number of x-rays produced. • Primary controller of density. • Change in mA does not change the penetration of the ROI and does not affect the beam quality. • High mA, high patient dose. Double mA, doubles patient dose.
Algorithm
A computer adapted mathematical calculation appliedto raw data during image reconstruction to ensure the image appears on the screeen as it is expected to look. Each body part has a present characterisc curce and a set density. Scanning the cassette under the correct algorithm will autocorrect the image to align with the curve. This means it is important for each view to have its own assigned algorithm
Exposure creep
A gradual increase in x-ray exposure over time that results in increased radiation dose to patient. • Due to wide exposure latitude of CR and DR. • Radiographers aim to have an exposure just above quantum mottle. CR compensate for under and over exposures
Exposure index
A measurement of the amount of exposure recieved by the IR. Directly proportional to the exposure. To change EI by 300alter mAs by factor of 2. Relates to algorithm and depends on - Body part - Position - Collimation - Exposure factors (especially mAs) - Beam attenuation Indicative of image quality EI RANGES: - 1400-1800 regular (Torso) - 1800-2200 regular (extremity) - 2100-2300 detail (extremity)
Effective dose (E)
A method of converting a non-uniform radiation dose (eg, when a lead gown in worn) to a dose as though the entire body was exposed. Used to work out risk includes tissue weighting factors (Wt) which takes into account the potenital harm to the individual and offspring Eg. Takes into account where the radiation is hitting you
AEC
AUTOMATIC EXPOSURE CONTROL An x-ray exposure termination device AECs are designed to produce radiographs with an optimal density by controlling the amount of radiation reaching the IR. Radiation is transmitted through the patient to the IR and is converted into an electrical signal terminating the exposure. Two types: - PHOTOTIMERS: Detectors are positioned behind the IR. Radiation interacts with these causing them to give off light. This light is converted to an electrical signal. The trimmer is tripped and the radiographic exposure is terminated. - IONIZATION CHAMBERS
Compensatory filtration
Added by the MRT to change the spatial intensity of the beam for specific body parts ie. for AP T-spine a wedge filter can be used.
Density steps
Alters the time at which the AEC terminates the exposure The density steps change the exposure time by 15% which changes the resultant mAs i.e. for a large patient AEC might cut out early therefore an increase in density step is required. For a pelvis with one THJR, go down one density step to decrease the exposure otherwise it will be overexposed due to the metal ware. Will decrease mAs by 15% +3 +2 +1 0 -1 -2 -3
x-ray tube target
Area of the anode struck by the electrons from the cathode. • In stationary anodes, made of tungsten alloy metal, embedded in copper. • In rotating anodes, the entire rotating disc is the target. Made of tungsten alloyed with rhenium to give added mechanical strength and withstand high speed rotation.
Centring devices
Assists the MRT in the direction of the beam to the exact area of interest in order to allow for more precise centring and hence tighter collimation is enabled. Consequently, reducing patient dose. PROJECTED CENTRE POINT - Is a radiolucent perspex sheet with a cross to mark the centre - shows entry point - reasonably accurate to 180cm LASER BEAM - Accurate at any SID - Shows entry point and angle - Can be easily knocked off centre though
Inherent filtration
Attenuates the long wavelengths, the non-image forming x-rays. These filters are caused by the glass, plastic, oil etc.
Why do you use low kVp on an abdomen x-ray?
Because structures within an abdomen are of *similar density*, requiring a high contrast to differentiate between them
Broad focus
Broad focus allows for an exposure higher than 200mA to be used without over heating the anode as the heat is dispersed over a larger area. Shorter exposure times can be used because of the higher mA values. This helps when imaging areas of the patient that has involuntary motion such as the abdo and chest to reduce movement unsharpness in the image. However, with broad focus, detail is lost due to a larger penumbra.
Off level grid
Can cause grid cut-off as the lead strips are not perpendicular to the beam. Can also cause underexposure to the images as some of the primary beam is absorbed. Can occur in both types of grids. EG. ED CXR's have off level grid cut off
X-ray tube glass envelope
Contains the cathode and anode. • To ensure there is a vacuum so electrons can flow. • Pyrex glass and metal used to withstand heat and wear and tear
Phosphorescence
Continuous emission of light after exposure ionizing radiation has stopped Electrons are excited to a higher state during radiation exposure. Electron relaxation is a slow process as the electrons transition between energy states, hence the continuous emission of light. USED IN CR
Importance of collimation
Decreases scatter radiation reaching the IR and increases image contrast, therefore increase image quality.
Tube loading
Depends mainly on the tube heat capacity. The heat capacity is how much heat can be dissipated in a given time from an anode. If it cannot be dissipated fast enough for the set mAs, the tube load will be too much for the heat capacity, leading to heat overloading. This overload of heat at the anode can lead to pitting and cracking. To reduce tube overloading the tube must be warmed up in the morning, and appropriate mAs values set (avoid using high mAs values on cold mornings) in order to increase the life span of the tube.
Collimators
Devices that restrict the beam and minimise the area irradiated. We collimate to reduce patients' effective dose and also to enhance image contrast as scatter is reduced due to a smaller area being irradiated. Collimators are made of two sets of adjustable lead shutters CSP 5 states that misalignment of the visually defined light and the radiation field should not exceed 1% and shall not exceed 1.5% Poor collimation using an AEC increases scatter causing premature termination of the exposure.
Contrast
Differences in shades of greys of adjacent tissues. kVp is responsible for the level of contrast required. - High kVp - low radiographic contrast (wide latitude of greys (tissues) - Low kVp - high radiographic contrast (small latitude of greys)
Somatic effects
Effects appear in the exposed individual
Genetic effects
Effects appear in the exposed individuals offspring. Must be preconception There is no threshold and the incidence is proportional to dose
Absorbed dose (D)
Energy transferred from ionizing radiation per unit mass of irradiated material Eg. Absorbed dose in bone/soft tissue etc - is the number of objects hitting you divided by your size
Fine focus
Fine focus gives *greater detail due to less penumbra* as the area targeted on the anode is smaller. However, the *mA is limited to 100-200* due to chance of tube heating. This mA limitation means there are *long exposure times* which may be a problem when imaging children, patients with Parkinson's and areas that have involuntary motion, i.e. the abdomen, chest
Why does a low kVp give a high contrast image?
Generally x-rays using low kVp results in a radiograph with high contrast. This occurs because *low energy radiation is more easily attenuated*. High contrast = black and white
Ug - GEOMETRIC UNSHARPNESS
Geometric unsharpness refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup. • Caused by the fact that radiation does not come from a single point, but rather over an area (a predetermined area called the focal spot). • Large focal spots cause greater penumbra, so more Ug.
Attenuation
Gradual loss in intensity of an x-ray beam as it passes through an object due to the absorption and scattering of photons The amount of attenuation that occurs depends on the intensity of the original x-ray beam and the physical properties of the object through which the x-ray beam passes
X-ray tube housing
Guards against leakage radiation so dose to the patient and MRT is reduced. Also protects against electric shock. Contains a cooling fan to air cool the oil in which the x-ray tube is immersed.
What is grid ratio, and what do we use at HVDHB?
Height of the lead strip/the distance of the interspace Bucky grids - 12:1 Portable grids - 8:1 (focussed grid = 100cm) Grid cassette - 6:1
HIS
Hospital Information System Contains all patient information over all aspects of their time in the hospital, from admission to discharge.
Why do we use the smallest cassette size possible?
IT will give me the best spatial resolution and optimal image quality. The 18x24 and 24x30 have the same number of phospors, so when imaging a hand, it will cover more phospors therefore giving us more detail.
Why do we require two images at 90 degrees to each other?
Important for: • Localisation of foreign bodies and lesions • Fracture alignment • To see behind anatomy that obscures the area in other images (i.e. behind the heart)
x-ray tube
In an X-Ray Tube 3 functions must happen simultaneously: 1. The cathode (filament) must be emitting electrons 2. There must be a Potential Difference between Anode and Cathode 3. The Anode must be positive in respect of the cathode. PARTS OF X-RAY TUBE: - Housing: - Glass envelope: - Cathode: - Anode: - Target - Induction motor
Magnification
Magnification factor= SID/SOD Macroradiography: • Small focal spot 2x • Large focal spot 1.5x
Exposure (x)
Measures the quantity of radiation (Gamma or x-rays) produced in the air. - eg - is like the number of objects being thrown at you in the air
How do the ionization chambers work in an AEC?
More common than phototimers. Detectors are positioned infront of the IR. It is made up of a hollow cell containing air which is connected to a timing circuit. When the chamber is exposed, the air becomes ionised creating an electrical charge which travels along a wire to the timer circuit. This trips the exposure and terminates it. The charge is proportional to the exposure received.
NAI PROTOCOL
NOT TO BE DONE BY A STUDENT - AP Skull / C-spine - Lateral skull / C-spine - AP Chest - Lateral chest - Oblique chest for ribs - AP abdomen / pelvis - Lateral full spine - Bilateral AP humerus - Bilateral AP forearm - Bilateral PA hand - Bilateral AP femur - Bilateral AP tib/fib - Bilateral DP feet MUST record all exposure factors and document into RIS
Cathode
Negative side of the x-ray tube. • Made up of filament and focussing cup. • The filament is a coil of wire and emits electrons when heated. • The filament is embedded in a metal cup called the focussing cup. • The focussing cup is negatively charged and restricts electrons to a small area (i.e. changes between fine and broad focus).
What formula do you use to work out a new exposure when the distance is changed? EG. you are doing a bedside and you can't get a distance of 180cm so you drop to 150cm. You were going to use 2mAs, what do you use now?
New exposure New distance^2 ----------------- = ------------------- Old exposure Old distance^2 From 180 to 150cm, it will be 0.694 x old distance. Works out to be almost half the exposure but not quite. EG. 8mAs becomes 5.5mAs, 4mAs becomes 2.8mAs
Grid errors
OFF LEVEL OFF CENTRED OFF FOCUSED UPSIDE DOWN GRID
Fluorescence
Only emits light when it is being exposed by ionizing radiation (FILM ONLY).
Types of grids?
PARALLEL: the lead strips of the grid are parallel. - Both sides of the grid can face the tube and there is no set SID required. - Likely to get grid cut-off at the edges due to divergence of the beam and no change in direction of the lead strips to compensate for this. FOCUSSED: The lead strips are angled to focal point. - Correct side of the grid required and a set SID is needed to not get grid cut-off.
X-ray production
PREP: - Anode starts spinning - Filament is heated EXPOSE: - Electron are emitted. (Number of electrons is proportional to the current applied to the filament.) - A high potential difference is placed across the tube. (To increase PD, kVp needs to be increased) - Electrons emitted from the negative cathode end, slam into the focal spot on the positive anode end. (Anode is made from tungsten due to it's high melting point). - *When the electrons reach the anode, there is rapid deceleration*. This loss of energy is converted. (97% heat and 3% converted into characteristic and bremsstrahlung radiation). - We use a multileaf collimator to shape the primary beam evidence by the light field shown, which is the irradiated area. *The x-rays that are diagnostic and mainly bremsstrahlung*.
Penumbra
Penumbra is affected by SID, OID and focal spot. Penumbra is the blurred edges around a structure on the image. It occurs because the radiation does not originate from a single point but rather over an area. Penumbra is also known as geometric unsharpness. OID: Once the x-ray beam interacts with the subject, the subject then acts as a point source, rather than the initial point source on the anode. If the patient is further away from the IR it gives a greater distance for the beam to diverge, hence increasing penumbra. When the patient is closer to the IR there is less distance for the beam to diverge, so less penumbra. SOD: If SOD is small, the penumbra is greater; this is because once the beam interacts with the subject the subject then becomes the point source, therefore more distance for the beam to diverge and vice versa.
What 3 factors affect photographic unsharpness (Up)?
Photographic unsharpness (Up) is a combination of 3 factors: 1. Us - SCREEN UNSHARPNESS 2. Um - MOVEMENT UNSHARPNESS 3. Ug - GEOMETRIC UNSHARPNESS
PACS
Picture Archiving and Communications System Provides storage and access to images from different hospitals and different modalities
Anode
Positive side of the tube; contains the target. • It conducts electricity and radiates heat. 97% of the energy reaching the anode is lost as heat. • The anode receives electrons emitted from the cathode and conducts them through the tube to the connecting cables, and back to the high voltage generator. • The anode provides mechanical support for the target.
Quantum Mottle
Radiographic noise produced by the random interaction of x-rays with the IR. QM is obvious when mAs is too low, as there are not enough photons to be absorbed by the IR, therefore not enough photons to produce a diagnostic image and give all necessary information. Refers to the 'graininess' on an image.
RIS
Radiology Information System Database used for patient tracking, includes radiologist reports and appointment dates etc
ROP
Remote Operating Panel Gives MRT the ability to access patient details and assign images to pre-set algorithms
What does the induction motor in the x-ray tube do?
Rotates the anode
What type of crystals are in a CR plate
Screen inside cassette is called a phosporescent screen with photostimulable crystals.
Teratogenic effect
Seen in offspring who were exposed in utero (while pregnant)
Us - SCREEN UNSHARPNESS
Size of the screen phosphors. Detailed screens have more phosphors, meaning they show minor changes in contour. This is because there are more phosphors to image one point in comparison to regular screens. i.e. 1 phosphor may image one point in a regular screen, while 5 may image the same point using detail. However, higher exposures are required.
Distance
Standard clinical distances - 110cm (up to 120cm wherever deemed necessary) - 180cm- Chest Large SID = Less magnification therefore less penumbra and better special resolution (ability of an object to be differentiated from another). - More mAs must be used due to some of the beam interacting and being absorbed by the air.
Contrast resolution
The ability to distinguish between, and image, similar tissues.
Grid factor
The factor at which the exposure need to be increased when using a grid to still obtain an optimal image. • 12:1 - x5 (wall + table buckys) actually x3 • 8:1 - x4 (stationary) actual x2-3 • 6:1 - x3 - (grid cassette) actual x2
Characteristic curve
The graphic *relationship between the optical density (blackness) and the exposure*. - The steeper the curve the more contrast is seen. i.e. the narrower the latitude, the higher the contrasts, the less shades of grey. - Can be adjusted by the 'brightness' setting.
Compton scattering
The interaction between an x-ray and a loosely bound outer shell electron, resulting in ionisation and x-ray scatter
DR and the processes
The main difference between CR and DR is that DR uses flat-panel detectors rather than cassettes. -Indirect and Direct panels INDIRECT PROCESS: X-rays interact with phosphors that emit light with x-rays - light detected by photodetectors - electric charge produced, proportional to light striking - charge stored until ready to produce image and put through analogue to digital converter. DIRECT PROCESS: - Flat-panel detectors in direct digital imaging are called direct flat-panel detectors. - xrays produce a charge when they interact with photoconductor - Charge stored until ready to produce an image. -image sent to computer and image form via algorithm (Uses an amorphous selenium-coated thin-film-transistor (TFT) array to capture and convert X-ray energy directly into digital signals. There are no light-emitting materials as with CR systems. Under a bias voltage (a voltage given to ensure each image turns out the same) applied across the detector structure, incident X-rays directly generate electron-hole pairs in the selenium layer. These charges are collected by individual storage capacitors associated with each detector element (pixels) and converted immediately into an image on the monitor.)
Equivalent dose (H)
The quantity that is used for radiation protection and expresses dose on a common scale for all types of radiation - EG. takes into account the type of objects thrown (knives more harmful than pingpong ball)
Anode heel effect
The x-ray beam has higher intensity at the cathode (negative) side of the tube. This is due to the angle of the anode - X-rays emitted from the steeper end of the anode must pass through a greater amount of the target material, therefore there is more attenuation of the beam at this end; and hence less intensity. - In practice thickest part of body should be at the cathode end (this is marked on the tube as '-'). Can be useful for abdomen, DP feet and AP T-spine x-rays
Fluoroscopy scatter
There is scatter in all fluoroscopic procedures but some areas have higher amounts of scatter than others. Scatter is greatest in the immediate regions of the patient on either side of the Radiologist. Dose to people in the room can be reduced by: o Standing away from the table as much as possible o Wearing lead shielding
Personal monitoring
Thermoluminescent Dosimetry (TLD) badge - At trunk level - The 'chip' is a small radiolucent disc of lithium fluoride - Aluminium filters are on one side only - so must wear the badge right way around (aluminium facing the source) - This disc absorbs radiation, with electrons being stored in traps - When discs are heated they emmit light - The intensity of light is proportional to radiation recieved. - After reading the disc, it is heated to release all electrons and return it to natural state for re-use. ADVANTAGES: - reusable chips - light weight and robust - quick results
Subject Contrast
This is the *contrast determined by the patient*. This includes: *PATIENT THICKNESS*: • The thicker the pt, the more radiation is absorbed so less reaches IR. • Can use compression to reduce volume. *ATOMIC NUMBER OF PART*: • Bone (white, Z=20) lungs with air (black, Z=6). • Pathology can change contrast. *TISSUE DENSITY*: • Different density in tissues, i.e. muscle vs. fat, appears as different shades of greys. *CONTRAST AGENTS* (if used): • Positive or negative. *SCATTER RADIATION*: • Reduces contrast as it is secondary radiation which is non-image forming. • Raising kV increase scatter.
Stroboscopic effect
This occurs when the exposure time is shorter than the speed of the grid movement. The grid appears to be 'frozen' and grid lines are pronounced on the image.
Air gap technique - Advantages and disadvantages
Used instead of a grid as the scatter radiation is attenuated in the air between the patient and the IR before it reaches the IR. The air acts as a grid - Requires an increased OID - Due to large OID, SID must be increased from 110cm to 180cm. ADVANTAGES: - Greater the gap, the more effective - Increases contrast DISADVANTAGES: - Magnification is increased (decreasing image detail) - mAs is increased eg. C-spine lateral, skyline patella
Filtration
Used to be needed with film, CR/DR does not require compensatory filters except in extreme cases as the algorithms compensate for this. There are 3 types: - INHERENT - ADDITIONAL - COMPENSATORY
Additional filtration
Usually a non-removable disk mounted into the tube to add extra filtration to reach the ICRP. CSP 5 recommended standards of 2.5mm aluminium equivalence
Scatter
When radiation passes through any material some is absorbed and some is scattered. • Scattered radiation occurs when the radiation emitted changes direction and intensity, mainly when it interacts with body part. • Scatter can degrade the image by decreasing image contrast. Scatter is dependent on: Intensity of the beam Amount of tissue being radiated Type/ thickness of tissue This means close collimation is important in reducing scatter, addition of grid or lead also reduces scatter. Scatter is caused by Compton effects (picture)
What are the maximum and minimum exposures that can be used on the mobile machines
kVp: - Max: 125 - Min: 50 mAs: - Max: 200 - Min: 0.4
Off centred grid
mainly affects focussed grids. The CR needs to be centred to the cassette in order for the diverging beam to pass through the interspace and not be absorbed by the lead. Can cause grid cut-off over the entire image and underexposure
Spatial resolution
number of pixels used to construct a digital image Images having higher spatial resolution are composed with a greater number of pixels than those of lower spatial resolution Factors effecting spatial resolution are focal spot size, geometry and phosphor size.
ROP controls - Contrast - Latitude - Brightness
o CONTRAST: the difference between brightness of light and dark areas of an image. If the contrast is decreased the image will be greyer. Increasing gives a black and white image. o LATITUDE: allows for acceptable images to be obtained over a range of exposures. Wide latitude enables for images of body parts which vary in thickness to be imaged in one exposure. Increasing latitude increases detail seen through thicker areas keeping exposure dose low. o BRIGHTNESS: the intensity of light that represents each individual pixel in the image (density). Increasing brightness means each pixel will be lighter.
Factors that can affect distortion?
o SID o OID o Object-IR alignment o CR alignment
Off focused grid
the focussed grid needs to be used at a set distance, the further the distance used from the focal point the greater grid cut-off. This is because the lead strips have been angled in such a way to compensate for the divergence of the beam at this set focal point. Grid cut-off will be seen at the edges of the image.
Upside down grid
the lead strips are angled in a certain direction to allow the beam to pass through, if upside down the angle of the strips is opposite to that of the beam divergence. This results in severe grid cut-off. On either side of the CR.
Grid frequency
the number of lines per cm
Pixels
• A digital image is recorded as a combination of rows and columns, known as a matrix. • The smallest components of the matrix is the pixel (picture element), which is recorded as a single numerical value. • The locations of the pixel within the image matrix correspond to an area within the patient, or volume of tissue, called the voxel. • The larger the FOV, the larger the matrix and the greater the number of pixels.
What is noise
• Any information on the screen that does not contribute to the diagnostic image. • Appears as speckled background and occurs more often when fast-screen and high kVp techniques are used. • Noise degrades the image. Can be quantum noise (mottle) due to low photon energy, scatter or electronic or system noise. • Electronic/system noises are random effects which degrade the image quality. • The noise causes decreased visibility and detail on the image and reduces image contrast. EG. An image with quantum mottle is underexposed, need to increase exposure.
Um - MOVEMENT UNSHARPNESS
• Caused by movement of a patient during exposure. • The further the movement, or the greater the exposure time, the greater the Um. • Um is more obvious if movement is left or right of the tube as compared to towards or away from the tube. • Large OID's increase magnification, making movement more obvious.
mAs
• Controls radiographic density (too dark (overexposed) too light (underexposed). • Double mAs to increase EI by 300 • Halve mAs to decrease EI by 300
Bremsstrahlung
• Electron emitted from the cathode are influenced by the attractive forces of the nucleus of an anode atom. This changes the path of the electron. • This braking slow the electron and the loss of energy is emitted as electromagnetic radiation (mostly x-rays) • Energy level of x-rays emitted can be anywhere from min to max (hence a curve)
Characteristic radiation
• Electrons from the cathode collides with an inner electron of an anode atom • Electron is ejected from inner shell •Vacancy is filled by an outer shell electron • Excess energy emitted as x-rays
Thermo-luminescent Dosimeter (TLD)
• Film badges replaced in 2013 by TLD badges. • Badges are assigned to an individual with her name , date of issue and the area being monitored (i.e. whole body) • Worn for 3 months, whole unit is returned to NRL to be read, cleared and reissued. • Aluminium filters are only on one side therefore badges must be worn with name and barcode at the front. How it works... • Contains 2 small round discs of thermo-luminescent material (lithium fluoride). • When exposed to ionizing radiation it absorbs and stores the electrons. • When the discs are heated (250-300°C), they emit light. • The intensity of the light is proportional to the radiation received. • After reading disc is heated to release all electrons and is ready for reuse.
mAs rules
• Increase or decrease by 30% to see a visible difference (slight change). • To increase EI by 300, double the mAs. • Doubling mAs, doubles radiation dose to patient.
Disadvantages of a rotating anode
• Less robust. • Bearing wear out. • Mechanical stress on glass (esp. at certain speeds). • Costly.
What are some challenges when using AEC?
• Pre-set exposures for average patient habitus so MRT must alter for each patient dependant on body type. • Patient positioning must be accurate; therefore anatomy of interest must be adequate placed over selected chambers in order to gain appropriate exposure. • Correct chambers must be selected, so pre-set chambers may need to be altered dependant on patient condition; i.e. pleural effusion. • Poor collimation increases scatter which can lead to premature termination
4 types of buckys and how they work
• SINGLE STROKE: makes one movement and is propelled by springs, couldn't cope with short exposure times so is now obsolete. • DECELERATING IMPULSE: movement by tightly coiled spring, movement during the exposure, but is now obsolete. • RECIPROCATING: movements with constant back and forth motions. Moves once the MRT has prepped. Advantages of this bucky are that exposures of any length can be used and they are suitable for autotimers. However it can cause a stroboscopic effect. • OSCILLATING: grid is held by four springs, one in each corner, solenoid tugs the grid and the grid oscillates or shimmies. Degree of movement gradually diminishes to avoid the stroboscopic effect, and is ideal for use with auto-timers. The bucky movement continues for long than most exposure times.
Time (in relation to mA)
• Shortest time practicable to minimise pt dose, but mainly to minimise movement unsharpness. • mA x s
mAs critique points
• There is adequate density over the entire image to visualise all anatomy and pathology, and to see the fine trabecular patterns. • There is no sign of quantum mottle. • EI is within range.
kVp critique points
• There is adequate penetration to visualise the entire ROI. • There is optimal radiographic contrast to visualise the whole range of tissues. • Bony cortical outlines can be visualised.
What is a back up timer?
• To terminate the exposure at the maximum length of time. This may be required for extremely large patients, malfunction of AEC or incorrect bucky selection. • The backup timer is not attached to the primary circuit in case of primary circuit malfunction. • The backup timer protects the patient from unnecessary exposure and the tube from exceeding its heat loading capacity.
kVp rules
• Use as high as practicable to reduce pt dose (ALARA). • Increase by 15% to half mAs - this changes radiographic contrast. • Under peneration: too light, high rad con • Over penetration: too dark, low rad con
Buckys
A bucky is a moving grid that moves during an exposure so the grid lines do not show. Our buckys use a grid ratio of 12:1 (x5)
What material coats the CR plates
Barium-fluoro-bromide
Advantages of a rotating anode
• Greater thermal loads (allowing for higher mA, kV and longer exposure time). • Thermal load is more evenly distributed. • Repetition rate can be higher.
