Define collimated beam
Collimated light is light whose rays are parallel, and therefore will spread minimally as it propagates. The word is related to “collinear” and implies light that does not disperse with distance (ideally), or that will disperse minimally (in reality).
Technician’s role in imaging
- get imaging info on patients as efficiently as possible
- consistent exposure and processing standards
- protecting all involved from x-ray scatter
- other imaging modalities in addition to radiography (e.g. ultrasonography, endoscopy, clinical photography)
Other aspects to consider with imaging
- keeping images as legal docs
- appreciating digital revolution
- radiology is a patient service and profit center, requiring mgmt
What is an X-ray?
A short wavelength portion of the EM (electromagnetic) spectrum having an ability to penetrate/pass through biological tissue.
What does an X-ray do?
- Hits the target
- Penetrates the target
- Emerges from the target
- Emergent radiation is “captured” by a recording media
- The media is processed to reveal the image
Physical aspects of x-rays
Energy on the EMS
Generating X-rays
Inside the x-ray tube
X-ray tube basics
Outside: Lead housing
Inside: X-ray tube
- heat mgmt of tube is critically important (check tube rotating chart, how much heat can it take?)
- larger machines have rotating anode, smaller machines have fixed anode (heat mgmt strategy)
- anode + is red
- cathode - is green
- voltage comes across wire (like light switch)
- filament creates boiling electrons, generated of off wire, sent to collision w/anode made of tungsten (hard metal), heat & x-ray are produced (instantaneously)
Define focusing cup
Directs electrons to tungsten target creating heat and x-ray
Define effective focal spot size
A reflection from anode’s surface
- smaller than actual
- larger focal spots create more image penumbra (soft image around focal point)
Define anode brake
It slows down motor to a complete stop, once exposure has been made (heat mgmt strategy)
Anode heat
99% of energy is heat
1% of converted energy are x-rays
Define anode
Tungsten target in copper
Define mAs
Milliamperage at filament (amount of current applied accross film) x exposure time
or
Amount of current x length of exposure
***Quantity of x-rays***
- controls electron “boil” from cathode and quantity of x-rays emanating off of the anode
- big knob in center of machine
- timer needs to be set (choice is 100, 200, 300 mAs)
Define Kvp
Peak kilovoltage
Thousands of volts applied to “drive” electrons from cathode to anode
***Quality of x-rays***
- changes EMR wavelength of x-rays to affect the penetrant quality of x-rays
- “65” setting means 65,000 Kvp potential energy controlled by tech to make electrons move from cathode to anode
Film-focal distance
In vet med usually 40”
- set and forget for consistency
- Inverse square law (as we change distance between film focal distance, make exposure compensations)
Technique chart
Each machine needs its own technique chart
- ensure you get a reliable film
- Santees law
Formation of primary beam
- x-radiation is filtered through thin sheet of aluminum upon departure from tube head (removes/absorbs “softer” irradiation to “harden” beam)
- after leaving tube head beam is collimated via movable lead shutters to control vertical & lateral dimensions
Latent image
Potential image on film
Attenuation of x-ray passage creates an image
Attenuation = “slowing down”
Variation of passed vs absorbed x-rays yields an image
- film has emulsion on/in it
- x-rays hit emulsion layer (silver halide crystals) and change/sensitize it; now they’re susecptible to chemical developer
Subject density vs radiographic density
- increasing thickness attenuates x-ray absorption
- the more penetration of x-rays, the greater the exposure
- bone has great stopping power
- lots of sensitized crystals, turn film black
- lots of attenuation, turns film white
- Thickness x what it is = subject density
- All shades of colors = radiographic density
- Density = how black is film?
Image capture …DR
True DR = Digital radiography
- pixel sized sensor arrays detec the intensity and frequency of transmitted x-rays
- froms image directly from sensor output
- digital means 1’s and 0’s (sensor hit = 1; sensor not hit = 0)
- many options for post processing enhancements
Image capture …CR
CR = Computerized radiography
- not a direct process, but results in digitized image
- special cassettes have storage phosphor capturing latent image for later retrieval and processing through scanning
- place CR cassette in reader, latent image gets taken off phosphor and turned into digital image
- laser process erases image so cassette may be reused
Film (analog) image capture
How is an image developed?
Via conversion of silver halide salt into elemental silver
- dense black where there is much, tightly packed silver
- no image (clear film base) where no silver halide was exposed (never sensitized)
- shades of varying grays between black and clear (actual image, various densities of silver)
Film processing
- Develop
- developer solution is a reducer that works best at alkaline pH
- reduces exposed silver halides to “grains” of opaque silver - Stop
- removes developer from emulsion to terminate development process - Fix
- thiosulfate fixer chemical dissolves unexposed silver to remove non-black portion of image to create clear and gray areas - Wash & Dry (w/plain water)
Processing Time/Temperature
To keep film’s image consistent:
- chemical quality, time and temperature of processing are critical (major benefit of automatic processors w/100*F @ 35secs vs. dip tanks w/68*F @ 5 min)
- care in handling film is critical to prevent artifacts
- use special screen cleaners and lint free wipes
**The longer it stays, the warmer the temp, the greater the density of the film!**
Reasons Digital is “nice to have”
More forgiving w/mistakes
No need for developing chemicals
Anode “Heel” Effect
Due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side.
Digital or analog
Theoretical and practical effects
Practice = Always point thicker side of anatomy toward cathode (-)
CAVT’s cathode is on left, therefore always have head of animal pointed to left
Very important in canine abdominal films especially in deep chested dogs
X-Ray Scatter
Ricochets and reflections of the primary beam as they interact with tissue on their way through the target
Scatter exposes the media but cannot contribute to image
- creates film fog
- reduces contrast
- degrades sharp edge effects
- lateral scatter and backscatter expose patient and techs
- useless to image formation and harmful to everything else
- the thicker the tissue, the worse the scatter
PPE only protects against scatter, NOT the primary beam!
Grids to control scatter
Grid factors include:
- lines per inch
- grid ratio or spacing ratio
Grid does what:
- reduces scatter
- improves film
- absorbs some of primary radiation (need to compensate for this)
- cut off is 10 cm:
Subject is less than 10cm = table top
Subject is greater than 10cm = shoot w/grid
(in the latter case, the order would be: subject, table, lead grid instrument, cassette)
Focused grids
Progressively angled lead plates
Positioned at angles, allows more of radiation to come through
Factory set to 40” distance for focused grid
If subject is moved out of focus, you will lose some of image
What is Sante’s rule?
Calculation for determining approximate kVp for given exposure, based on patient measurement in cm and grid being used. Used as a starting point for new technique chart.
kVp = Tissue thickness in cm times 2 plus SID in inches plus grid factor
Most veterinary grid ratios are 8 : 1
Stationary vs. Moving grids
Stationary grids may show fine grid lines on the image
Potter-Bucky Grid & Diaphragm (“Bucky”) is a mechanical system to put the under-table grid into motion at exposure, thus blurring the grid lines
Film tray is stationary and is NOT the bucky
Grid movement is via motor or spring
Frequently used in veterinary radiography
Downside of bucky use: the sound/vibration may startle your patient
Summary of grids
Is a grid present?
- if shooting table top = no grid!
- Equine extremeties = no grid!
- shooting under table (larger animal, e.g. beagle), better w/grid
Generally begin using grid at >10cm patient thickness
Increased x-rays are needed to penetrate grids (therefore, different technique charts are needed for table-top vs under-table use)
Grids and bucky are precision instruments (require little care but lots of protection from damage)
Safety w/ionizing radiation
Rapidly dividing cells are most at risk (somatic and genetic cells equally sensitive)
Young cells more vulnerable than older
Animals & humans equally sensitive
Blood or GI cells are more sensitive than skin or fat cells
X-ray energy can damage DNA (disrupt code, create mutation)
Ionizing radiation is mutogenic in low doses
In high doses it can destroy completely
SIEVERT
Dose equivalent of absorbed radiation in biological tissue
Also reported as Millirems
Exposure is cummulative
MPD
Maximum Permissible Dose
ALARA
As Low As Reasonably Achievable
(objective to keep accumulation of radiation as low as possible)
Do CTs and MRIs use radiation?
CT uses radiation
MRI does not use radiation
Key Items to Work Safely
“The Big Three”
Time
Distance
Shielding
- get a diagnostic exposure the first time
- get as much distance as possible between yourself and the primary beam (inverse square law)
- use proper PPE (thyroid shield, apron, gloves = inspect q6 months, radiograph annually to check for holes)
Radiographic Density
Quantity
Degree of blackness in the film emulsion = exposed silver halides
Density is mostly controlled by mAs
Limits to achieving maximum blackness?
- not enough mAs (# of x-rays/quantity)
- not enough kVp (quality of x-rays)
- compromised developer strength & activity
Radiographic Contrast
Quality
The degree of perceptible difference between two adjacent gray “tones”
High contrast = large differences between adjacent densities/grays
Low contrast = small differences between adjacent densities/grays
Major controlling factor is kVp
Difference between shades of gray
Short Scale of Contrast
Considerable differences between densities and a minimal number of total densities
- this is achieved by using a lower kVp
- usually selected for orthopedics and extremities
Long Scale of Contrast
Image having only slight differences between adjacent densities and having a larger total number of densities
- this is achieved by using a higher kVp
- usually selected for abdomen & thorax
Contrast Scale Differences
Fewer gray shades
Called “Higher Contrast”
Called “Shorter Scale”
Use Lower kVp
App = Orthopedics
Vs.
More gray shades
Called “Lower Contrast”
Called “Longer Scale”
Use Higher kVp
App = Soft Tissue
Digital = “Noise”
Film = “Grain”
Noise is graininess mottling or a textured appearance
- in film radiography the amount of grain is mostly set by the film/screen combination
- in digital radiography the amount of noise increases due to underexposure
Exposure changes for digital
To decrease noise, technician may increase mAs to keep DVM happy, however, …
- upward “exposure creep” increases scatter for all involved
- digital systems need a technique chart as much as analog systems
- digital systems should not be considered “magic”
Grids, Grid Lines, Collimation in Digital
Some true DR digital systems are able to process away fog and artifact from scatter and backscatter (grids not needed)
Tight collimation (as typical for film radiography to reduce scatter) is less necessary for digital systems
Computer resources in digital
New systems come onto the market every 5-7 yrs
Image back up required
Acquire high quality, high latitude, gray scale monitor
Technical concerns:
- brightness (lumens)
- resolution
- gray scale
- contrast ratio
- viewing conditions
- scheduled recalibration
DICOM
&
PACS
DICOM = Digital Imaging & Communications in Medicine
- embeds signalment and exposure information
- a universal technical standard for file transmission from computer to computer
PACS = Picture Archival Computing Systems
- electronic film file management
- uses DICOM and displays and “files” film locally
- may help manage the on-site/off-site back-up
