**Saia Unit 4 Flashcards
Energy deposited into the image receptor
Density
The controlling factors of density are
mA
Time
mAs
mAs
The number of X-rays in polyenergetic primary beam
mAs has the primary control of
Image density
Density is placed in one of three categories
Acceptable
Underexposed (too light)
Overexposed (too dark)
30% rule
The human eye requires a change in density of at least 30% before it can be visualized
Fuji exposure indicator
S number
Range 180-220
Indirect relationship
Kodak exposure number
EI number
Range 1800-2200
Direct relationship
Agfa exposure indicator
LGM number
Range - 1.9-2.5
Direct relationship
A perfect exposure indicator value is
1mR of exposure to imaging plate/panel
Increasing the KVP will increase the quality or energy energy of the beam giving the beam more
Penetration ability
As KVP is increased, density…
Will increase
Energy deposited into the image receptor is
Density
15% rule
Used to alter KVP settings to change density and contrast
A 15% increase in KVP = a 50% decrease in mAs
A 15% decrease in KVP = a doubling in mAs
As KVP goes up, image contrast
Goes down
As KVP goes down, image contrast
Goes up
If SID is increased with no other change, density is
Decreased
If SID is decreased with no other change density is
Increased
SID and density have an ? relationship
Inverse
Density
Overall blackening of a film/image
Inverse square law
The intensity of the beam is inversely proportional to the square of the distance.
I1 (D2)2/I2 (D1)2
Density maintenance law
Formula that will compensate for changes in SID to maintain density
mAs1 (D1)2/mAs2 (D2)2
If you increase the OID less scatter reaches the film resulting in a decrease in
Density
Less OID will result in a greater density due to
More radiation reaching the film
OID and density have a ? relationship
Inverse
Imaging system speed
As speed increases the efficiency of placing energy into the IR increases, and density increases also.
Grids are used to
Absorb scatter to prevent it from reaching the film.
Formula to maintain density with grids
Mas1 GCF1 / mAs 2 GCF2
If collimation is increased (smaller light field) then
Less radiation leaves the tube
Less scatter produced
Less energy reaches IR
LESS DENSITY IS THE RESULT
Radiolucent tissue
Tissue that let’s xray energy through
Radiopaque
Tissue that will absorb ray energy
Disease
Physical and chemical changes in tissue
Anode heel effect
Variation in the beams intensity across its longitudinal axis,
More X-rays on cathode side than anode side
As filtration is increased,
More photons are absorbed resulting in fewer X-rays and less xray energy in the primary beam
Contrast
The difference in adjacent shades across the radiographic image
Contrasts primary function
To make recorded detail visible
Contrast is primarily controlled by
KVP
Energy deposited into the IR is
Density
The more energy values = more shades of density
High contrast
Few shades of gray
Increased contrast
Lower KVP
“Short scale” contrast
Low contrast
Many shades of gray
Decreased contrast
High KVP
“Long scale” contrast
Differential absorption
Differing materials absorb xray at differing degrees
As KVP decreases patient dose ?
Increases (weaker xray, more absorbed in patient)
As KVP increases patient dose ?
Decreases
Stronger xray beam, more passes through patient
Grid ratio
Height of lead strips to distance between
Grid frequency
Number of lead strips per unit distance
Increasing beam restriction - smaller field
Less tissue irradiated
Less scatter produced
- “short scale”, higher contrast
An increase in OID results in
Less scatter reaching the film - higher contrast
As beam filtration increases
Shorter scale
Higher contrast
Positive contrast media
(Iodine/barium) - temporarily make anatomy more dense than normal
Negative contrast media
Oxygen, carbon dioxide, room air
Allow more X-rays to penetrate due to the low atomic number resulting in an increase in density
Barium atomic number
56
Iodine atomic number
53
High contrast
Big difference in shades, short scale, low KVP, very black and white
Think of the top of shade triangle (small peak)
Low contrast
Little difference between shades, long scale, high KVP, many grays
Think of the bottom of shade triangle (large base)
Photographic factors
Density
Contrast
Density
Overall image blackness
Contrast
Visibility of adjacent structures
Geometric factors
Detail
Distortion
Detail
Visibility of fine anatomical structures
Distortion
Misrepresentation of an objects size/shape
Recorded detail aka
Resolution, sharpness, definition, detail
Measured in lp/mm
Line pair
A line and it’s adjacent space, as recorded in an image of the tool (resolution tool)
Human visual acuity is
5 lp/mm
Unsharpness
Degree of loss of detail of the anatomical structure by the imaging system
Penumbra
Geometric unsharpness around the periphery of the structure of the image
As focal spot size increases
Penumbra increases
Effective focal spot
focal spot projected toward the patient
Usually .5-2.0mm
Line focus principal
The effective focal spot is smaller than the true/actual focal spot
True/actual focal spot
Area where electron stream strikes the anode target
Small focal spot
Maximum image detail
Umbra
Objects true size
Why not always use a small focal spot
Small focal spot uses small filament which are mA limited, large focal sometimes needed
Increased SID will result in
Less penumbra
Increased OID will result in
More penumbra
Macro radiography
Magnification radiography
Intentionally magnifying to exaggerate small structures
Most common in mammography
IR speed
Faster speed is used to save patient dose, however resolution not good
Quantum mottle
Not enough X-rays in the primary beam to adequately form image
Distortion
Misrepresentation of an objects true size and shape
Size distortion is always
Magnification
Magnification
The misrepresentation of the actual size of an object
As SID increases, magnification?
Decreases
As OID increases, magnification ?
Increases
Magnification factor
SID/SOD (SID divided by SOD)
SID/SOD
Image width/object width = SID/SOD
Shape distortion
Misrepresentation of an anatomical part due to misalignment of source, IR, and/or part
Results in elongation or foreshortening
When the xray beam or IR is misaligned =
Elongation
When the body part is misaligned=
Foreshortening
Three points to perfect alignment
CR perpendicular to body part (most important)
CR perpendicular to IR
Body part parallel to IR
Magnification formula
Mag=SID/SOD
Percent magnification =
Image size - object size / object size X 100 = % magnification
Intensifying screens
Used to convert xray energy into visible light energy.
- 90-99% of film exposure is due to visible light
- 1-10% of film exposure is due to direct xray
Xray film is much more sensitive to
Visible light energy than it is to xray energy
Spectral emission
The color of visible light emitted by a phosphor crystal when stimulated
Intensifying screen construction:
Base layer
Reflective layer
Phosphor layer (phosphor crystals)
Protective layer
Function of radiographic film
To record and store a permanent anatomical image
Radiographic film construction
Protective layer Emulsion layer (silver halide crystals) Adhesive layer Base layer Adhesive layer Emulsion layer (silver halide crystals) Protective layer
Density
Energy deposited into an image receptor
Emulsion layer
Silver halide crystals - gelatin whose function is to absorb and then release the processing chemistry and water
Colloid
A material that can absorb and release water without itself being dissolved in the process.
Like a sponge.
Function of the silver halide crystals
Respond when stimulated by xray or light energy
Latent image
Formed in the films emulsion as a shadow representation of anatomy. Due to chemical changes in AgBr crystals. Invisible to human eye.
Manifest
Visible
Latent
Hidden
Manifest image
Formed in films emulsion due to chemical changes in AgBr crystals, made visible to the human eye by processor.
Gurney Mott theory
Photon breaks ionic bond releasing shared electrons which drift to and are captured by speck. Electrons attaching to the development center. Silver ion attracted to the development center. Sensitivity speck is now neutral and able to attract an electron.
More energy absorbed = more debris = bigger sensitivity speck
Grid conversion
No Grid = 1
5: 1 = 2
6: 1 = 3
8: 1 = 4
12: 1 = 5
16: 1 = 6
Filtration
Minimum of 2.5mm Al/Eq
Purpose of filtration
Remove weaker X-rays from primary beam
Half value layer
Amount of filtration required to lower xray intensity to half of its original volume
Collimator illuminance
Brightness of collimator bulb and field
- at least 15 foot candles (160 lux) at 40”
Xray to light field
Collimator to xray beam alignment
- +/-2% semiannual
Nine penny test
Test done for xray to light field (diameter of penny is .8 inches)
Positive beam limitation
Auto collimation of light field to IR size, can be smaller - never larger!
- +/-2% semiannual
Focal spot size is tested by
Pinhole camera, star pattern or slit camera
- +/-50% annual
KVP accuracy is measured with
Voltmeter and radiation meter
- +/-10% annual
Exposure timer accuracy
+/-5% greater than 10ms
+/- 20% less than 10ms
Exposure linearity
Using the same mAs but different combinations of mA and S
i.e. 100mA @1/2S or 200mA @1/4S
AEC backup timer is set to self terminate if
If 600 mAs is reached
If AEC backup timer is manually set, it should be set to
150% of expected mAs
AEC optical density versus change
+/-.3 OD
Film illuminator standard is
15W daylight bulb
Fluoro exposure rate ESE shall not exceed
10R/min
Interventional exposure rate ESE shall not exceed
20R/min
Repeat analysis goal
To minimize patient exposure
Wire mesh test is used to evaluate
Contact between intensifying screen and film
- annually
Speed uniformity is tested to assure that all like speed intensifying screens
Respond to the same xray stimulation by releasing the same amount of visible light
- +/-10% annual
Darkroom fog density can be no greater than
.08 (xray) or .05 (mammo)
- semiannual
Sensitometry
Measuring the response of film to exposure and processing
- daily
Developer is sensitive to changes in
Temperature, oxidation, concentration, contamination
Sensitometer
Device that emits varying intensities of light in a star pattern on a film
Pemetrometer
Aluminum step wedge that is exposed on to radiographic film and developed
21 step sensitometer is
Most common (41% difference between steps) Also a 11 step (100% difference between steps)
Densitometer
Reads/measures image density, measures how much light is emitted to film and how much is transmitted
Optical density
Incident light striking the film to the intensity of light transmitters thru film
The higher the optical density, the less light transmitted
Therefore the greater the density
Density difference or contrast indicator
Found by subtracting Dmin from Dmax
+/- .15OD
Hyporetention (fixer = hypo)
Must be less that .05Gr/m2 - emulsion turns brown in storage due to inadequate washing of film in processor
- quarterly
+/-2%
Collimator dial accuracy (2% of SID)
SID accuracy (2% of SID)
Xray to light field
Positive beam limitation
+/-5%
Exposure reproducibility
AEC reproducibility
+/-10%
KVP accuracy mR/mAs (from installation) Exposure linearity AEC density steps Film illuminators Speed uniformity
+/-50%
Focal spot size
Daily
Sensitometry
Quarterly
Hyporetention
Semiannual
Collimator dial accuracy
Xray to light field
Positive beam limitation
Darkroom fog
Annual
Filtration Half value layer Collimator illuminance SID accuracy Focal spot size KVP accuracy mR/mAs Exposure timer accuracy Exposure linearity Exposure reproducibility Protective apparel Film illuminators Film screen contact Speed uniformity
developer solution
electron soup - lots of electrons, when silver added = black metallic silver
the automatic processors four sections
developer
fixer
washer
dryer
function of developer
convert exposed silver halide crystals into black metallic silver.
latent to manifest.
95 degrees
function of fixer
remove unreduced silver halide crystals.
permanently “fix” image into emulsion
function of washer
to wash residual fixer from emulsion to improve archival quality
hyporetention
retention of fixer by films emulsion resulting in degrading of the stored image over time
dryer section
like a hair dryer, heated air blows across film to evaporate water from emulsion; emulsion is hardened and ready for handling.
130-135 degrees.
the basic theme to ALL digital imaging systems
to convert X-ray energy into electrons
analog
a device or system that represents info as continuously variable physical quantities, like a mechanical watch. X-ray film is analog.
digital
a device or system that represents info as continuously variable numeric values, like a digital watch. CR/DR are digital.
CR - Computerized Radiography
Projection Radiography which uses a PSP (photostimulable phosphor plate - IR/IP). No film.
A PSP is composed of
Europium Barium Fluorohalide
Phosphor plate function
latent image is formed by remnant radiation striking the plate. Xray is absorbed within the PSP crystal.
fading
the loss of latent image information over time
CR scanner
IR casette is read by a CR scanner which is placed in the scanner and opened up internally to remove the phosphor plate.
readout
CR scanner scans the IR plate with a Helium Neon light and reads the latent image fro the phosphor
raster pattern
side to side pattern by the laser to obtain image
f centers
electrons elevated out of their normal orbital rings and into higher orbitals, containing potential energy, latent image is held in this chemical code of rearranged electrons
“F” in F Center is for
Fabre - the German word for color
Latent to Manifest Image
As laser beam travels over the plate, the latent image is released as visible light, the electrons elevated to a higher orbital are “slapped” back to their normal orbit by laser. The transfer of electron energy results in the release of visible light energy.
Helium Neon laser
red in color
photomultiplier TUbe
a device that converts light energy to electrical energy
digitizing
the process of converting colors, shades, and shapes held as analog information into numbers
erasing the PSP plate for re-use
exposure to high intensity fluorescent light
3 inherent factors responsible for image resolution capability
image plate size
laser beam size
monitor matrix size
current resolution for CR digital imaging is
2-5 line pairs per millimeter, conventional radiography can demonstrate 6 lp/mm
spatial resolution
ability to see small objects
measured as lp/mm
Image resolution is synonymous with
image detail or image quality
Contrast resolution is
the number one advantage of digital imaging versus film/screen imaging
Quantum Detection Efficiency (QDE)
measures the efficiency of a CR system to convert remnant radiation into useful image signals
High QDE systems
reduce patient dose due to requiring less radiation to produce an acceptable digital signal
Fuji Systems
"s" number Inverse relationship (
Kodak (carestream)
"EI" value Direct relationship (>2050=overexposed) Expected range - 1950-2050 (2000 ideal)
Agfa
"LGM" number Direct Relationship (>2.5=overexposed) Expected range - 1.9-2.5 (2.2 ideal)
Direct Radiography
the use of detectors that convert X-ray energy into electrical energy that is then delivered to a computer where the anatomical image is digitally processed and displayed
DR is a direct readout image acquisition technology.
the detector is in the flat panel and directly sends the image to a computer upon exposure. No need for processor, ADC, digitizer.
Steps in indirect digital image formation with Amorphous Silicon
Remnant xray beam enters Amorphous Silicon
X-ray energy converted to visible light
Photoconductive material receives light (converts light to electron energy)
Electron energy captured by TFT device (information converted to digital signal)
Steps in Direct DR Formation with Amorphous Selenium
Remnant xray beam enters Amorphous Selenium and ionizes - electrons released
Electrons collected by storage capacitor and transferred to TFT
(information converted to digital signal)
Direct DR
Amorphous Selenium
xray to electron to digital
Indirect DR
Amorphous Silicon
xray to light to electron to digital
No visible light in the image formation process with ?
Direct DR
Thin Film Transistors
Used with both Direct and Indirect DR
Collect electric charges, positioned in a matrix, detect charges on a pixel by pixel basis.
Capable of very high spatial resolution (greater than 20p/mm)
DELS
Detector elements of a thin film transistor
Receiving the DR signal
Each Pixel (DEL) from the detector array (TFT) collects electrons in a capacitor, the number collected is directly proportional to the strength of the X-ray received.
Algorithms
A finite sequence of instructions often used for calculation and data processing. The computers mathematical recipe for processing the digital signal from the pixels.
Window Width
image contrast
Window level
image brightness/density
Histograms
Anatomy specific.
a graphical representation of the distribution of numerical data.
with a histogram the raw data is collected, then…
then interpreted and presented to the human eye
Look up table
a table of predetermined luminance values, a mapping function in which every pixel is changed to a new shade of gray simultaneously resulting in an image with appropriate brightness and contrast.
The look up table controls
automatic rescaling
automatic rescaling
computers ability to display an image with a diagnostically correct gray scale (within limits)
Overexposed pixels are “turned down”
Underexposed pixels are “turned up”
digital image quality factors
pixel/matrix size spatial resolution contrast resolution automatic rescaling histogram windowing phosphor material exposure indicator technical factors
contrast manipulation
A post processing tool used to manipulate the contrast of an image to achieve a desired image.
dynamic range
The range if grays that can be assigned to a pixel.
The higher the range, the more grays available to display the image.
image stitching
process of combining multiple images with overlapping fields of view to produce a high-resolution image
sampling
Process used to digitize the spatial information in an image, typically achieved by dividing an image into a square or rectangular array of sampling points.
shutter
The process of removing border from image as a post processing tool. Not to be used to compensate for poor collimation.
modulation transfer function
the ability of a system to record spatial frequencies (lp/mm) and maintain adequate contrast between adjacent line pairs
Nyquist Theorem
when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal so that image reconstruction will be nearly perfect.
Aliasing
When an image contains frequencies above its lp, they are under-sampled upon digitisation and appear as lower frequency information in the digital image
aliasing in DR occurs when
spatial frequency is greater than the Nyquist frequency
sampling occurs less than twice per cycle
aliasing in CR occurs when
grid errors, grid frequency must be at least 178 lpi
edge enhancement
computer looks for areas of common pixel signal strength across the matrix, which are then averaged and amplified to the area of interest and other tissues suppressed resulting in decreased contrast, potential for loss of detail/quantum mottle
smoothing
computer looks for areas of common pixel signal strength across the matrix, which are then averaged resulting in increased contrast and reduction of unwanted noise.
edge enhancement aka
high pass filtering
useful for viewing large anatomical structures such as organs
smoothing aka
low pass filtering
useful for viewing smaller anatomical structures such as trabecula
Signal to noise ratio (SNR)
wanted (useful) anatomical information (signal) versus the unwanted electrical interference (noise)
noise
detected information that adds nothing to the anatomy - quantum mottle is an example
spatial resolution
the ability to see small objects (lp/mm)
controlled by crystal size in CR, detector element size in DR and pixel size on a digital monitor
contrast resolution
Ability to differentiate between shades of gray, controlled by Bit depth, Bit range, Pixel depth
spatial resolution is measured in
lp/mm
maximum spatial resolution is the result of
a large matrix and small pixels
small pixels =
Better spatial resolution
Image matrix
contributes to spatial resolution, the layout of cells in rows and columns; the larger the matrix, the better the spatial resolution
pixels, voxels, hounsfield units (CT)
pixel density
the number of pixels per given area
The human eye is capable of seeing ? shades of grey
30-32
the higher the contrast resolution, the more distinct adjacent structures are
with similar densities
digital imaging systems have significantly higher
contrast resolution than film screen
contrast resolution aka
pixel depth
bit range
dynamic range
contrast resolution is rated in
bit depth
latitude
margin for technique error
thousands of shades of gray
low contrast
Bit range
1 bit = 2 shades 2 bit = 4 shades 3 bit = 8 shades 4 bit = 16 shades 5 bit = 32 shades 6 bit = 64 shades 7 bit = 128 shades etc...
Window width
selecting which of the available grays are to represented to the eye upon the monitor
controls digital image contrast
window level
the exact center of the window selected
controls digital image brightness (density)
