Radio Principles Theory 7 & 8 Flashcards
Troublehooting of Image (quality/ artefacts).
What factors would you consider as a cause of image density that is suboptimal
− Did you use correct factors/ did you set them correctly on the console?
− Did you measure pt accurately for view taken
− Is the SID set correctly for view taken?
− Is the film/screen speed correct for body region?
− Is pt body type frail or muscular, or is it child?
− Do you have problems with power surges
Factors to consider if image density is suboptimal consistently:
− Processing chemicals- are they fresh and at proper levels?
− Are you using the correct film/screen combination
− Are films expired? Is stored
− Is developer temp correct?
− Is X-ray equipment functioning properly
If image contrast is suboptimal?
− Contrast is mainly controlled by kVP
− Most views use pre-determined kVp values (so itsn not usually cause of contrast issues)
− To increase contrast (decrease grey scale) → Decrease kVP (+ decrease density)
− To decrease contrast (increase grey scale) → increase kVp- it will aslo increase your film Density
Factors to consider as cause of image contrast that is suboptimal consistently
− Are processing chemicals fresh?
− Are you using correct film/screen combination?
− Are your films expired, or are you storing them in too warm environment?
− Is your developer temp correct?
− Is your safe-light too bright or the wrong color?
− Is your X-ray equipment functioning properly
− Motion blur
o Exposure time too long
o Pt needs to be compressed
o Pt not responding correctly to directions
o Unnecessary use of grid (leading to increased exposure time)
− Blur (not from motion)
o Poor film/ screen contact
Magnification and geometric unsharpness
o Wrong SID
o Worng IOD
o Wong focal spot size
What are causes of film fogging
(too much scatter)
− Processing and darkroom issues
− Cassettes left in room while taking other exposures
− Film/screen speed too fast
Artefacts due to darkroom errors
− Light struck film
− Light turned on or darkroom door opened
− Film storage bin door open when lights on (COSTLY error)
− Film left neae safelight too long (fogging)
− Film left near an inappropriate light source in the darkroom.
− Overlapping films in processor (feeding films too close together)
− Kissing artefact (films contact each other during development
− Processor marks/ scratches on films from uncleaned rollers
− Dropping films on floor→ scratching
− Impropper washing or failure to turn on water (film discolouration and fingerprint artefacts)
Artefacts from film storage and cassette loading errors
− Film bending artefacts
− Empty cassettes (failure to reload after processing)
− Failure to close cassette properly
− Loading copy film in the cassette
− Putting the cassette in place back to front
− Static electricity
− Dirty screens
Artefacts due to equipment failure, improper exposure and patient positioning
− Double exposures o Same view o Different views − Pt/ cassette decentring − Improper placement of patient shielding or side marker − Hands or other body parts overlying area of interest − Movong grid fails to move − Warped cassettes
Lec 5a
Optimization
Simple principle is
ALARA
As low as reasonable achievable.
Within social and economic constraints.
The process of optimization should bot detract from the diagnostic quality of the image.
What is the unit of measurment of absorbed dose in the air?
Roentgen- measurment of absorbed dose in air
What is the unit of measurement of radiation?
Gray
Equilivent to one joule per kilogram
Rad: Old term (seperseeded by gray)
The gray is 100 times larger (1Gy= 100 Rads)
Sievert (sv) is what measurment?
SI unit, measure of total biologically effective dose
Calculated by multiplying the number of Grays of radiation by a quality factor or Q factor specified for the type of radiation and its energy, after which these amounts are added together
The Q factor for x- and gamma rays is 1; therefore, 1 Sv = 1 Gy. The factor for the neutrons in atomic-bomb radiation is 10; therefore, 1 Sv = 0.1 Gy
Example of other types of radiation:
Neutrons: Q of about 5
Alpha particles: Q of about 20
What is the effective dose?
Equivalent dose of an exposure multiplied by the weighting factor for the type of tissues involved (essentially sensitivity)
Determines probability of certain biological effects
Also measured in Sieverts
examples:
-gonads (highest tissue weighting factor) (0.20)
Occupational Exposures & Dose LImits/ constraints
Dose limits for occupational exposure are as follows (as units of effective dose):
- 20 mSv per year averaged over 5 consecutive years
- No single annual exposure can exceed 50mSv
Separate equivalent dose levels for:
- Lens of the eye (150 mSv annually)
- Skin, hands, feet (each allowed 500 mSv annually)
Exposures can also be assessed with greater frequency, using pro-rated doses based on the time frame considered [eg, 1.67 mSv monthly (20 mSv/12)]
Medical Exposures and Dose Limits/ constraints
Medical exposures occur in:
Patients
As part of diagnostic workup or treatment
Volunteers during medical research
Persons willingly located in proximity to a radiation source (e.g. parent helping to immobilise a child)
Dose limits are not appropriate for persons undergoing medical care
Justification and optimisation are in affect
Recommended dose guidance levels for some exposures can assist in optimisation.
Public Exposures and Dose Limits/ Constrai nts
Dose limits for public exposure are as follows (as units of effective dose): 1 mSv per year Separate equivalent dose levels for: Lens of the eye (15 mSv annually) Skin (50 mSv annually)
Public exposure covers all exposures not related to either of the above
Workers in a medical practice who have no direct involvement in radiation exposures viewed as “public” for dose limit and control considerations
For pregnant employees, the embryo/foetus should be considered as a member of the public for dose limitation purposes
Persons under 16 years of age should not be exposed to radiation occupationally
Should be considered “public” for radiation protection purposes
Dose reduction Techniques:
Patient related
Patient-related:
Previously discussed- fast film/screen systems error and repeat view reductions patient compression / thinning longer SID non-grid techniques proper patient selection, etc. - and - Upcoming sections Shielding Collimation Filtration
I cant be bothered with the rest of this lecture- read through it
s
What is the purpose of filtration?
Remove unwanted low energy x-ray photons from the beam
Otherwise absorbed by the patient
Does not include compensatory filtration, which is used to even out film density when body thickness changed substantially in an area.
Filtration is measured in mm of Aluminium Equivalent
Filtration and half value layer.
One way to measure the “hardness” of the x-ray beam is by the half-value layer (HVL).
Simply put, a baseline measurement of beam energy is taken, and then increasing thicknesses of Al. filtration are placed in the beam and the average beam energy is again measured.
The thickness of Al. filtration needed to reduce the average beam energy in half is the HVL.
Thickness of an absorber required to reduce the intensity of the original beam by half
(I hope you appreciate that I’m skipping a whole bunch of hideous mathematics here)
Monochromatic radiation
Changes quantity only, not quality
Each layer absorbs the same percentage of photons
Gives EQUAL half value layers
Polychromatic radiation Think of it as Darwinism for the beam More energetic photons go further Quantity and overall intensity reduced but QUALITY of remnant beam is INCREASED (higher percentage of more energetic photons) Gives INCREASING half value layers
The greater the amount of filtration needed to reduce the beam energy in half, the harder the beam, and the better it is for reducing patient absorbed dose.
A “soft” beam is unacceptable, and more added filtration must be put in place to harden the beam.
Alternatively, using a higher kVp will also increase the HVL, and thus the hardness of the beam.
Legal requirments of Filtration
The minimum value of added filtration is established by regulation. X-ray tubes in Australia with a kVp potential greater than 100 kVp require a minimum of 2.5mm Al. equiv. of added filtration.
Up to double that amount can be used without significantly affecting image quality.
DIGITAL IMAGING
What are the advantages of digital imaging procedures?
reduced long-term operating costs
reduced image storage requirements
high contrast resolution imaging
exceptional imaging latitude. This directly relates to patient dose reduction, as the reduction in retakes due to poor factoring is virtually eliminated.
What are features common to both basic imaging and digital imaging?
With digital imaging systems, the x-rays that reach the receptors form an electronic image which are manipulated by the computer and displayed as a matrix of intensities which have a dynamic range of values.
The image matrix refers to what?
Refers to a layout of cells in rows and columns, with each cell corresponding to a specific location in the image.
A number value is assigned to each cell, which represents its brightness.
Each cell in the matrix is called a “pixel”.
The number of pixels in the matrix helps determine the size of the matrix.
For a given field of view (ie, image plate size), the larger the matrix the better the spatial resolution of the image (ie, the sharper the image).
With our system, using a 35 x 43 cm cassette, we can process an image with a spatial resolution of 6 pixels/mm (2320 x 2826 matrix) or 10 pixels/mm (3480 x 4240 matrix). The latter matrix would provide better spatial resolution, or image detail, with the trade-off being larger image file size.
What is the dynamix range of the image?
Refers to the number of shades of grey that can be represented by the image.
A dynamic range of 21 would only have two shades (black and white).
The dynamic range of the human eye is ~ 25 (i.e., 32 shades).
The dynamic range of the x-ray beam leaving the patient exceeds 210. This range can only be fully appreciated by the computer.
The greater the dynamic range, the more gradual the change in shades of gray will be, and the better the contrast resolution.
Most digital imaging systems use a dynamic range of 28, 210, 212.
Practically, each shade of grey is assigned a numeric value within the range allowed by the system (0 - 255 for 8-bit to 0 - 4095 for 12-bit).
This value is “recorded” in each pixel, which in turn creates the various densities on the image.
A large dynamic range allows for windowing of the image to optimize contrast and density overall, as well as at specific regions of interest
e.g. when the AC joint is “burned out” on a shoulder view, you can manipulate the density of the image to enhance that particular region.
Major advantage of digital over analogue imaging
Computed Radiography is:
Transitional system
Cost-effective
Allows for transforming existing analogue-based radiographic equipment into a digital format
Film-screen portion replaced with imaging plate in a cassette that looks like traditional cassette
Chemical-based film processing system replaced with dry laser-based digital system
The functional component of the imaging plate is the phosphor layer, which contains photostimulable phosphors – barium fluorohalide compounds (BaSrFBrl:Eu for our system). The phosphor layer will only maintain the information for a finite period of time, so it is recommended that they be processed within an hour of imaging.
The latent image is stored as trapped valence electrons, which are released by exposure to a small, high intensity laser beam.
The released electrons return to their valence band, emitting blue light in the process.
This light is viewed by a photomultiplier tube, which creates an electronic signal to be digitized and stored for display.
Digital Radiography (DR)
Another film-less system of radiography.
Unlike CR, fixed detector arrays are used to produce the image rather than imaging plates. As such, no processor is required – the images are directly sent to the computer. This is the same method used for CT imaging.
Detector array technology is highly complex, and will not be discussed in this class.
There are two ways the image is acquired that will be touched on here: Fan beam vs. area beam collimation.
Fan Beam Collimation
The x-ray beam is wide, but thin (~2-10mm). The advantage is greater radiographic contrast as scatter can be dramatically reduced.
The disadvantage is that only a narrow band of the region can be imaged, so the patient must:
a) be translated through the beam (this is the standard for CT scout views) or
b) the tube is moved across the patient, who remains stationary (as with most current DR x-ray systems).
This takes several seconds to perform, and patient motion can actually eliminate any advantage gained by scatter reduction
Area Beam Collimation
This is the same type of beam used in normal radiographic equipment.
The complexity of this method does not lie in the collimation, but in the type of imaging receptor array needed to collect the large amount of information in the area being imaged.
The advantages and disadvantages are reversed from fan beam collimation.
