Midterm Review Sheet Flashcards
OID on Magnification
Increased OID increases image magnification
SID on magnification
Increased SID decreases magnification
Image sharpness is affected by
Focal spot size
OID - Increases most rapidly with OID
SID
Penumbra
Image unsharpness created because photon are being released from multiple sources and not just one single source
Inverse Square law
I(1) D(2)2
— = —-
I(2) D(1)2
The outcome of the equation is the intensity change of the beam from the original
To know a new technique based on this you would divide the original mAS by the answer to the inverse Square Law. This would give a new mAS that worked for the new distance.
mAs rule
mAs(1) D(1)2
— = —-
mAs(2) D(2)2
Answer to this formula will give the answer used to determine a new technique
Precalculated estimates for changes in SID
40 to 56 = multiply mAs by 2
40 to 72 = multiply mAs by 3
40 to 80 = multiply mAs by 4
Size distortion
Always magnification
Due to divergent beam
Fecreasing SID or increasing OID will be most effective with OID being prime
Shape distortion
A result of unequal magnification of the shape of the structure
Occurs with a different OID of one end or the other and will appear fatter and foreshortened
Occurs with When the x-ray tube is angled and everything else is parallel this results in elongation
Safe Lights inside darkrooms must be
At least 4 feet from work area
Must not exceed 15 watts
Covered by a filter that only allows light of a spectral emission that is not within a damaging range (only allows light from the red end of the visual spectrum)
Films should not be exposed to light for three minutes
Safe light test
Expose an underdeveloped film to the light of the room and every thirty seconds move the film so that more light is exposed
First stripe will be most exposed and give an indication of what time length is safe
The three T’s of processing
Time, Temperature, Titration
Increased TTT will overdevelop film and produce a fog whivh results from the development of unexposed silver halide processing
Protocol for manual processing
68 degrees; development for 5 minutes; fixer for 10 minutes; Washing for 20 minutes; drying 20-40 minutes
estimated overall time is one hour per film
Image exposure
Film is exposed to light
Exposure causes silver bromide crystals to seperate into a silver ion and a bromide ion
Sulfur in the crystal collects electrons and holds them away from the silver ion
Creates latent image
Image development
Developer donates electrons that creates elemental silver (which is black in color)
Latent image becomes the visible image
Image fixing
Fixer removes all unexposed silver halide from the emulsion and permenantly stops any possibility of exposure beyond what is already done
Washing
Final wash throughly clears all chemicals from the film surface and in the emulsion
Prevents fixer chemicals from continuing to cause chemical changes in the emulsion after the finished product is produced
Insufficient washing causes artifacts as teh chemical continue to work
Over time film will degrade (mos. to yrs.)
Developer
Activator - Sodium carbonate softens gelatin protective cover on film (preserves pH at 7
Reducing agents - (key ingredient) Hydroquinone or phenidione (metol or enol) reduces the exposed silver halide to black metallic silver
Preservative - Sodium sulfite
Restrainer - Potassium Bromide, acetic acid
Fixer
(pH 4-4.5)
Clearing agent - Ammomium thiosulfate (key ingredient) Dissolves undeveloped silver halide and removes it from emulsion
Tanning agent - potassium alum
Activator - acetic Acid neutralizes the developer, Stops development and maintains pH
Automatic developer system
Higher temps (92-96)
Stronger acids and bases than hand-tanking solutions
Constant agitation of chemicals
Rapid throughput of chemicals (90-80 secs)
Chemical require replenishment
Under-replenishment causes loss of contrast (grey) and loss of film density
Over-replenishment causes loss of contrast (increased fog) and wastes chemicals
Causes of lowered contrast on a film
Safe light fog Room temperature or humidity too high Chemical fog from processor too hot Chemical fog from chemicals too strong Film is past expiration
Every 1-2 months you must
Complete chemical changes and tank cleaning
Check replenisher rates
Check cycle time
Sensitometer
Exposes film to a step wedge of known densities
H & D Curve
Graphic measure of how a film records density over a range of exposures
Graph plot optical density (y-axis) vs. log relative exposure
Consists of the body, toe, and shoulder of the curve
Toe of the curve
Starts above zero optical density even in the absence of any exposure because there is ALWAYS a small amount of light absorption by the film due to the base and emulsion plus a small amount of “auto exposure” that occurs with any emulsion of film
Body of the curve
Straight line component of the curve
Slope of teh curve determines the latitude of the film
Short latitude film has a very High slope and is higher contrast
Long latitude film has a lower slope and a lower contrast
Shoulder of the curve
represents the D-max or maximum optical density the film will achieve no matter what
Comparison of H and D surves
For two different types of films the line further to the left is faster.
The one with a lower curve is a more latitude film with lower contrast
Densitometer
Machine with the little arm on it that precisely measures the optical density of each step on a step wedge.
This is reported on a logarithmic scale from 0 to about 4. Zero indicates zero absorption of light
through the film- or the film transmits 100% of the incident light through the film.
Optical density of 4 indicates 100% absorption of light- the film is entirely black and allows no transmission of light through it.
Densitometer can tell us how much light transmits through any one of the steps on a step wedge in very precise ways
Measurements we obtain to track performance of the processor over time:
Speed Index
Contrast Index
Gross fog index
Speed index
Established by finding the step on the sensitometric step wedge strip that has the closest optical density to 1.0 (based on measures using a densitometer)
Contrast index
Difference in optical density between the Speed Index step and the optical density reading (using the densitometer) of the step that is two steps darker on the sensitometric strip
Should be about 1.4-1.7 for films used to obtain skeletal images (lower for films for chest images)
Gross fog index
is film Base plus Fog
It is the inherent optical density of the film when it has not been exposed at all and has just run through the processor blank
Ideally it should be perfectly clear however there is always some stray radiation that causes silver bromide to break into silver and bromine and that then results in some elemental silver
Usual Gross Fog Index
0.15 to 0.20
faster films have a higher gross fog index
Length of the toe of teh H and D curve is related to the film’s sensitivity to fog
The longer the toe the less sensitive to fog
Should not see reading greater than these values on serial (over months) recordings
+/- 0.15 for speed index
+/- 0.15 for contrast index
+/- 0.03 for gross fog index
Spectral sensitivity of film
The bandwidth of light that it is most sensitive to during exposure
Match the spectral emission of the screens that it is paired with
Speed vs. detail film
Function of crystal size
Slow speed with best detail: small crystals in thin layers
Medium speed with medium detail:
Medium sized crystals in a medium thick layer on the film
Large crystals in a thin layer
Small crystals in a thick layer
Fast speed with low detail: large crystals in thick layers
Latitude
Term used to describe the range of densities that can be recorded on the film
Wide latitude film is best when you desire low contrast radiographs (i.e. chest radiograph)
Short latitude film is best when you desire high contrast radiographs
As contrast decreases… latitude increases (“Longest (widest) latitude”)
Increasing film speed causes
More density per unit of exposure less mAs needed to create the film density that you want
Faster films do not produce as much detail
Fog may be caused by: TTT
Secondary radiation (scatter)
Accidental radiation exposure
Excessive or insufficient time, temperatures or chemical titration strength
Exposure to darkroom chemical fumes
Interactions of x-ray with matter
Coherent scattering: same energy of incident photon
photon hits, spins, and is released without breaking apart
Compton Effect (key feature: Compton scatter is the most common mechanism of scatter production in human tissue): photon hits and scattered radiation is released
Photoelectric effect: Hits a positively charged atom, releases characteristic radiation and an excited photon
Things that affect scatter production
The greater the tissue volume the greater the scatter produced (more than 10 cm and a grid is advised)
Increasing the kvp Increases the energy of the beam and increases the number and energy of scattered photons. Can also lead to increased fog due to increased scatter (use high ratio grid to absorb high energy scatter)
Minimize scatter production
Grid - place grid between patient and film to absorb scatter
Air gap technique - Equal effect to an 8:1 grid
Minimize exposed tissue through collimation
May decrease kVP or mAs put this is risky
Grid performance measures
Gird ratio: Height of lead strips: distance between lead strips
10:1 or 12:1 is best
Grid frequency: Number of lead strips per square inch
Bucky factor - Ratio of x-rays arriving at the grid to those transmitted through the grid
Contrast improvement factor: the ratio of image contrast with a grid vs. image contrast without a grid
Focused grids
Lead strips are aligned to the direction of the diverging primary beam
Grid radius are the specific SID the grids are attuned to
Grid focal range is the range of SIDs that will function with grid
Grid cutoff: when the grid absorbs excessive amounts of remnant radiation resulting in visible and undesirable degradation of the image
Cross hatch grid
lamination of 2 grids with lead strips at 90 degree angles to each other
Stationary girds should
have at least 100 lines per inch
Grid cutoff
Undesireable attenuation of the primary beam by grid
Oblique grids cause loss of image density on the side closer to the tube
Sir gap technique
Increases OID 10 80-12 inches in order to reduce the volume of scatter that hits the film
Must increase the SID to compensate for the magnification caused
This in turn causes more mAs to accommodate for the increased distance
Air gap will cause the scatter to miss the film so grid is not necessary
Density
Level of overall darkness in the film
25-30% change is required for a visible effect on the film
Film too dark: Over penetrated, too dense, overexposed, “midnight under a skillet”
Film too light: Under penetrated, too light, Under exposed
mAs and number of electrons
Relates to the number of electrons required required to make the beam
QED. the number of photons in the beam
mAs varies directly with the image density
Double = double the optical density of the image Half = cut the density in half
When manipulating image density we try to use only ___
mAs
It affects density in a linear fashion and is easily changed on control panel
Few unintended consequences
Occasionally, manipulating milliamperage (mA) separate from time (s) may be desirable because
Lengthening or shortening the exposure time to allow breathing during exposure or to stop motion in a tremor
Can select a lower mA to get a smaller focal spot for greater detail and use a longer exposure time
Using a lower mA setting and a long exposure to reduce the heat load build-up during large exposures (allows better heat dissipation during the exposure, thus lowering peak tube load)
kVp effect on image density
As the energy of the beam increases a higher percentage of the photons in the beam will get through the target and hit the film
Relationship is NOT linear
Increasing the kVp 15% will double the density and lower the contrast
Decreasing the kVp 15% will half the density and raise the contrast
SID and image density
Increasing SID reduces beam intensity
Decreasing SID increases beam intensity
Effect is not linear - inverse square law
Decreasign SID increases penumbra because the more divergent rays of the beam are being used
Grids and image density
Grids selectively filter scatter and allows beam to pass through
Scatter adds darkness in the form on non-image producing fog and this is always bad
Grids also absorb some remnant radiation (the primary beam that would have made an image, this is BAD)
The image will be underexposed unless one compensates with increased mAs
higher contrast, less fog greater detail, higher radiation to patient
Effects of a smaller focal spot
No magnification change
Less penumbra
Increased detail/sharpness
Effects of a smaller OID
Less magnification
Decreased penumbra
Increased detail/sharpness
Effects of Larger SID
Less magnification
Decreased penumbra
Increased detail
Best sharpness with least magnification is achieved with
Smallest OID and Largest SID
OID has a greater effect than SID
Grid cutoff with grid characteristics
Off-level grid will cause visible cut-off on the side closest to the tube
an inverted grid will acuse immediate and severe cut off beyond the central ray region
CT
Modern scanners have multiple detector arrays that are stationary around the entire 360 degrees of the gantry rotations around the patient.
The tube rotates around the patient in the axial plane
Helical CT scanners acquire data continuously as the patient is moved within the gantry, creating a “spiral” acquisition of data
Multi-slice scanners create the ability to acquire data in very thin slices creating what is referred to as volumetric acquisition. When the data is reformatted the data in total is as good as the axial data
Reformatting (MPR= Multi Planar Reformatting)
Views of data from multiple orientations
Helical CTs can produce an isotropic voxel (pixel with volume) which has the same depth width and height meaning that it can be viewed from any plane without losing resolution
Hounsfield units
Used to describe the shade of gray that a tissue presents on a scan (-1000 to 3072)
water = 0 HU Air = -1000 HU Cancellous bone = 400 HU cortical bone = 700-2000 HU Metal = 2500 to 3000 HU
Windowing
The process of emphasizing certain ranges in the HU scale
Soft tissue windows allow fo rbetter viewing of the low level ends of the spectrum
Bone values emphasize the higher end of the HU spectrum
Creating the MRI image
Strong magnetic field (.3 to 3 teslas) is used to align the vector of spin of protons in the body
A radiofrequency will be introduced to cause resonance of protons causing them to gain energy and spin away from their axis and wobble away from the dead center orientation at a higher energy
when the radiofrequency is turned off the excess energy is expelled from each tissue as the precession returns to the aligned state of the magnet. The strength and character of the emitted energy is unique to the proton concentration of in that specific tissue and can be differentiated from other tissues
Radiofrequency detectors pick up the emitted signals and are converted to a visible image
T1 weighted image (MRI)
Fat is brightest
Cord higher signal than CSF
Medullary bone and spinal cord are higher signal
water and cortical bone are lower signal
T2 weighted image
Water and edema have brightest signal CSF higher than spinal cord Less precise detail Bright nucleolus pulposus fat loses signal Cortical bone is still dark
STIR (Short Tau Inversion Recovery) images (AKA fat saturation sequences)
Suppresses fat signal (makes it appear black) so that edematous tissue and fluid can be better seen.
Signal intensity and description
High signal = bright or light or white on the image Low signal = dark or black on the image Intermediate = anywhere in between
Contraindications to MRI
Anything with batteries or ferrous metals (those including iron)
Any patient who can’t hold still due to seizures, etc. Patients who are clinically unstable and require electrical monitoring equipment in the MRI suite to ensure their medical stability
Anything that has a battery or relies on radiofrequency signal to operate
Insulin or infusion pump o Implanted drug infusion device
Bone growth/fusion stimulator
Cochlear, otologic, or ear implant
CT vs. MRI
CT uses X-rays (ionizing radiation), MRI uses radiofrequency and magnets
CT refers to densities in tissue; MRI refers to signal intensity
CT provides exemplary detail; MRI provides exemplary tissue differentiation
CT axial data can be reformatted in multiple planes data; MRI directly acquires multiplanar images- no reformatting o CT is very fast; MRI is not so fast
CT can be done on patients with metal or battery implants; MRI can only be used if there are no batteries or ferrous metal in the patient
CT can be done on clinically unstable patients; MRI is harder to use with clinically unstable patients
Scintigraphy
Uses radioisotopes chelated to substances that are inert, but processed by various tissues in the body
Technetium 99m is the most common
Hot spot is an area of increased physiological activity
Cold spot is an area lacking vascular supply
Scintigraphy identifies ___
Physiological activity in organs and bones
Characterizes areas of abnormally high or low metabolic activity
High quality screening examination: Highly sensitive for areas of increased bone metabolism but poorly specific
Types of sctintigraphy
2D
SPECT - Single Proton Emission Computed Tomography
Hybrid SPECT - Combining CT or MRI with scintigraphy to gain anatomic or hysioogical comparison
Scintigraphy scan comprehensiveness
Whole body - entire skeletal system
Limited - for specific region and usually for fractures
Three phase - 3 phases - Flow, Blood pool, Delayed images.Used for infection, especially to determine osteomyelitis from cellulitis (extremely important to detect early infection in diabetics)
Ultrasonography
Use of sound waves to create images
Changes in tissue density will cause sound waves to “bounce back” to the ultrasound transducer at differing rates, creating the images we see on the scan.
The transducer is a device that sends sound waves and has receiver microphones that detect echoes that come back… so it sends and hears sound waves. The typical diagnostic ultrasound wave frequencies range from 2-20 MHz (that’s 2-20 million Hz). Air will ruin image capture
Because it is dynamic, it is highly user dependent
Does not employ radiation
Strengths of an ultrasound
Obstetric, gynecological and genitourinary evaluation
Solid internal organ evaluation o Cardiac, abdominal and pelvic evaluation
Large and medium blood vessel evaluation (arteries or veins)
Evaluation of large and medium size joints- ligaments, menisci, tendons, labra, cartilage surfaces, fluid accumulations, intraarticular plica, etc.
Muscles, tendons, ligaments and fascial bands, syndesmoses, compartment boundaries, etc. in the MSK system
Spinal evaluation is limited to identifying perispinal structures and measuring muscle volume
Intraoperative and interventional applications Biopsies
Needle or device placement guidance
Localizing tissues at surgical depth
