Unit Two: Indirect and Direct Imaging Flashcards
Two types of Indirect Imaging
- Charged Coupling Device (CCD)
- Thin Film Transistor (TFT)
Indirect w/ CCD steps
- x-ray photons strike scintillator (CSI)
- Scintillator produces light
- light energy is transmitted to CCD through fiber optics
- CCD converts light to electrical signal
- Electrical signal is sent to the ADC
- ADC converts electric signal to digital signal, then sends to the computer.
Two types of scintillators
Cesium Iodide
Gadolinium Oxysulfide
Why is CSI used instead of gadolinium?
Higher resolution due to less light spread
(Higher dose, more expensive)
What makes up a matrix?
DELs (pixels)
Each DEL contains:
CSI, fiber optics, and CCD
Indirect w/ TFT steps
- xray strikes scintillator
- Scintillator produces light
- light gets absorbed by photodetector/photodiode (amorphous silicon)
- Amorphous silicon converts photons into electrons
- Electrons (electric charges) are captured and transmitted by TFT array to the computer
CCD and Amorphous Silicon job?
Converting light into electrical signal
Does indirect w/TFT use fiber optics?
No
Why is direct conversion better for SR than indirect?
No light step (less light spread)
Direct doesn’t use a scintillator but uses a semi-conductor and TFT array
Underneath each DEL is:
TFT and Storage Capacitor
Indirect capture flat panel IR:
3000x3000 matrix that contains 9 million DELs within a glass substrate
Direct Conversion steps
- exposure takes place
- xray absorbed by amorphous selenium
- AS converts photons into an electric signal
- Electric signal migrate to the TFT array
- Storage capacitor stores and amplifies signal
- TFT reads and releases signal to the ADC
- ADC converts to digital signal - sends to computer
Radiosensitive portion of the DEL
Fill Factor
What contains the Amorphous Selenium?
Fill Factor
SR list of 8
OID
SID
Filament Size
Anode Angle Size
Pt. Motion
Shape Distortion
Size Distortion
Image Processing and Display
Spatial Resolution influenced by (non-geometric factors)
- Size of matrix (monitor)
- Pixel size (monitor)
- Pixel pitch (monitor)
- FOV
- DEL
- Sampling Frequency
How efficiently a system converts the xray input signal into a useful output image.
Detective Quantum Efficiency (DQE)
Measurement of xray absorption efficiency within the IR
DQE
Describes how effectively an xray imaging system can produce an image from the output signal (remnant beam) with a high signal-to-noise ratio (SNR)
DQE
Noise
Mottle/Blur
Mottle
Underexposure
Hardware:
Storage capacitor, TFT, DEL
Hardware produces:
Noise - not signal
Higher SNR
DESIRABLE
Electronic noise is inherent in all systems
Each DEL contains hardware and fill factor
Larger Fill factor = ____ radiation detected
More
larger FF = ____ SNR = ___ DQE
larger FF = more SNR = higher DQE
A larger FF is ____ efficient at absorbing radiation
more
100% of the transmitted remnant beam =
Signal
Edges of the DEL produce
Noise
Every photon that creates signal (interacts with the fill factor) ____ the SNR
increases
An increase in kVp = a ____ in DQE
Why?
An increase in kVp = a decrease in DQE
every photon that transmits through the IR lowers SNR ratio
DQE is dependent on:
radiation exposure, spatial frequency, MTF, and detector material
too little exposure = mottle = ____ SNR
decreased
2 reasons why direct has a higher DQE
Amorphous Selenium has a higher quantum efficiency
No light step (less light spread)
Highest to lowest DQE
Direct, indirect, CR, film
What is SR determined by?
Pixel pitch
How is pixel pitch measured?
center to center
Small DEL = __ SR
Increased
Referred to as a pixel within the detector
Detector Element (DEL)
The detector is made up of
a matrix of many DELs (pixels)
More pixels (DELs) within a matrix = __ SR
better
larger fill factor = ___ radiation required
less (less hardware kinda)
Why does dose increase with a smaller DEL?
A smaller DEL has less fill factor which means that more photons are needed because they hit the hardware - which produces noise not signal
Dose ___ w/ large DEL = ___ SR = ___ MTF
Dose decreases w/ large DEL = decreased SR = increased MTF
Dose ___ w/small DEL = __ SR = ____ MTF
Dose increases w/ small DEL = increased SR = decreased MTF
Smaller pixel has ___ SR
Better
Smaller DEL = ___ SR = ___ MTF
higher SR and higher MTF
If FOV remains constant, matrix size increases, and pixel size and pitch decrease, then SR ____
Increases
How accurately a system converts xray input signal into a useful output image
(Modular Transfer Function) MTF
A subcategory of DQE that quantifies spatial frequency/SR capabilities of the IR
MTF
Smaller DEL = ___ MTF
increased
A measure of the ability of an imaging system to preserve signal contrast as a function of the SR
MTF
Often regarded as the ideal expression of image quality provided by a detector
MTF
Blur, mottle, and penumbra ___ MTF
Decrease
The frequency that a data sample is acquired from the exposed detector
(how many samples are we taking)
Sampling Frequency
How is sampling frequency expressed
Pixel Pitch and Pixels/mm
Determined by receptor size depending on the vendor
Sampling Frequency
KODAK 8x10 has better detail than 14x17
To get the highest sampling rate, use the
smallest imaging plate possible for each exam
The ___ the spatial frequency (smaller an object) = the harder to image with accurate resolution
Higher
Smaller DEL = more DELs on board = ___ sampling freq.
Higher
Small DEL = smaller objects can be imaged
When sampling a signal, the sampling freq, must be greater than twice the bandwidth of the input signal because half of the info will be lost in translation
Nyquist Theorem
If too few DELs are sampled = ____ of resolution
loss
there must be ____ DELs as there are pixels in the monitor matrix
2x
The highest spatial freq. (smallest object) that can be recorded by a digital detector is determined by:
Pixel pitch
The distance from the center of one pixel to the center of an adjacent pixel
Pixel Pitch
DEL/pixel density of 10 pixels/mm = Nyquist frequency of
5 pixels/mm
SNR (signal-to-noise ratio)
Background electronic noise
Always a very small electric current flowing in any circut
Higher DQE = ___ SNR
higher
Measure for assessing the ability of an imaging system to generate clinically useful image contrast
CNR (contrast-to-noise) ratio
Gives objective measure of useful contrast
CNR
Why is a bigger pixel size better?
More signal than noise
Why is a smaller pixel worse?
Smaller pixel = less signal = more noise
Lines of the matrix produce
Noise
Useful image info
Signal
Accumulation of quantum noise and electronic interference
Noise
Decreased pixel size = ____ signal capture = ___ SNR
Decreased
Decreased
Must balance the pixel size and SNR to provide optimal SR
Pixel size is known by the system so that the software can measure image structures based on
pixel size
Can give an angle measurement between 2 structures, and is commonly used when reading spine studies
Angle Measurement
Gross overexposure of the IR
Saturation Effect
When does saturation effect happen?
When you expose past the allowed 200% of overexposure
Saturation effect cannot be corrected through
Rescaling
Have to retake it
Underexpose 50% or overexpose 200%
dynamic range
Photoconductive material/layer
Fill factor
Where an electrical charge/signal is created in proportion to xrays received
Photoconductive Layer
Processing methods used to:
- Reduce image noise
- Increase visibility of detail
- Adjust and optimize the image contrast characteristics.
Performed to plot the raw data
Histogram Analysis
Used to form the histogram are compared with a “normal” histogram of the same body part by the computer.
Raw data
Histogram Y axis
how many pixels in the image represent that gray level
VOI
Values of interest
Each pixel has a luminance value
Amount of light coming off a monitor
Luminance
The acquired histogram containing the VOI is compared to a default histogram =
Rescaled image produced
A set of anatomic specific algorithms that determine the appropriate amount of contrast and brightness for a displayed image.
Look Up Table (LUT)
Window level is for
Brightness
Window Width for
Contrast
adjusts pixel display for the area of interest, regardless of the amount of exposure unless the exposure is too low or too high. In those cases, quantum mottle or contrast loss occurs.
Automatic rescaling
anatomic region specific and remains fairly constant from patient to patient.
histogram
determined by the number of bits used to define each pixel
bit depth
A grayscale image is composed of pixels represented by multiple bits of information, typically ranging from _ to _ bits or more
2 to 8
The greater the bit depth, the ____ the number of tones (grayscale or color) that can be represented.
greater
Determines the number of grays
bit depth
LUT has a box of crayons: how many crayons in box is controlled by bit depth (because bd controls number of grays)
Controlling factor of image contrast
LUT
1 bit (2^1)= 2 tones
2 bits (2^2) = 4 tones
3 bits = 8 tones
the amount of error that still results in a quality image at an appropriate dose.
exposure latitude
The total range of exposures that can be captured by the detector.
dynamic range
Histograms show a wide range of exposure because of
automatic rescaling of the pixels
What has the highest dynamic range
DR
smallest amount of radiation that can be detected
dynamic range
Sharpness control is referred to as
spatial/frequency resolution
Factors such as FSS,OID, SID , matrix size, pixel pitch, processing speed class, light spread during image acquisition, processing, and computer algorithms will ALL affect
SR
SR measured in
lp/mm
used to adjust where on the scale the window is set (brightness of the image).
window level
used to adjust the contrast of an image
Window width
allows text to be added to an image.
annotation
allows for flipping of an image for proper interpretation.
image flip
allows for the changing of the image from negative (bone is white) to positive (bone is dark)
image inversion
used to better view smaller objects in an image
magnification
Increases the contrast along the edge of a structure through a software function.
edge enhancement
A function to suppress noise that results from averaging of the frequency of each pixel with surrounding pixel values to remove high-frequency noise. Results in a reduction of noise and contrast
Smoothing
software function that allows underexposed areas (light areas) to be made darker and overexposed areas (dark areas) to be make lighter
equalization
A quantitative function of digital imaging that allows for the pixel value of a selected area of interest to be calculated. This value can help characterize disease
region of interest
Protocol is defined as how a set of images will be displayed on the monitor
hanging protocol
Removal of the white unexposed borders results in an overall smaller number of pixels.
Reduces the amount of info to be stored.
Masking
Tool is used to orient the image in the correct anatomic hanging position.
Tool is usually a left-to-right flip and a 90-degree clockwise and counterclockwise icon.
Flip and Rotate
Digital R and L may not be upheld in court during a legal case because of the ability to mark anywhere on the image and flip and rotate the image into any layout on the screen.
used for anatomy or areas of interest too large to fit on one image receptor.
image stitching
Sometimes, special cassette holders are used and positioned vertically, corresponding to foot to hip or entire spine radiography, scoliosis, or long bone measurement (orthoroentgenography).
Image Stitching
Annotations used to:
indicate prone or supine, 30 minutes, upright or flat.
Allows for the changing of the image from negative (bone is white) to positive (bone is dark). Some pathologic conditions are better identified in this way.
image inversion
Annotations are NOT to be used to label left or right to indicate the patient’s side. Will NOT hold up in court!!!
Most common measurement function on PACS
distance measurement
light coming from the monitor.
luminance
light coming from sources other than monitor.
ambient
matrix and pixel size will contribute to the sharpness of structural edges recorded in the image.
SR
Exposure to DEL (IR) will determine
brightness
overall lightness or darkness in the radiographic image due to level of exposure to the IR
brightness
Replaced the term “density“, when the transition was made from film to digital.
image brightness
unit of measurement for luminance
candela/sq. meter
brighter image = ____ image
darker
An underexposed image receptor will produce a
mottled image
An overexposed image will produce
darker image
Increasing window/level will increase brightness and create an overall
darker image
Decreasing window/level will decrease brightness and create a
lighter image
Why do digital systems havesignificantly greater latitude?
Area receiving little radiation can be enhanced by the computer
Higher exposure levels can be separated and brought down to the visible brightness ranges
(more room for error- dynamic range)
Describes the ability of an imaging system to distinguish between similar objects that attenuate the x-ray beam similarly in digital imaging.
contrast resolution
is the smallest exposure change or signal difference that can be detected by the IR.
contrast resolution
which is the range of exposures that can be captured by a detector.
dynamic range
a product of the bit depth of each pixel.
dynamic range
pixel pitch depth
2^2 = 4 shades of gray
2^8 = 256 shades of gray
Appearance of more detail is due to the
wider dynamic range/contrast resolution
Because so many more brightness levels are recorded in DR (wide dynamic range), images appear to have greater
SR
a basic picture element on a display.
A pixel is “any of the small discrete elements that together constitute an image.”
pixel
the process or capability of distinguishing between individual parts of an image that are adjacent.
resolution
The smallest area represented in a digital image.
pixel
A term that describes the number of pixels/mm in an image. Pixel density is determined by the pixel pitch.
pixel density
smaller pitch = ___ density
smaller
The typical number of pixels in a matrix range from about 512 × 512 to 1024 × 1024 and can be as large as 2500 × 2500
larger matrix = ___ pixels = ____ resolution
more
better
Direct conversion converts xrays into
Electrons
Direct uses TFT, which functions to
Collect electrical charges
Which devices collect and convert light into an electronic signal?
Photodetector and CCD
Direct capture DR means no
Light step to form the latent image
The measure of the ability of an imaging system to preserve contrast as a function of SR
MTF
The purpose of the photodetector in indirect
Absorb light
Can prevent a repeat if a mistake in technique is made
Rescaling
Individual components comprising the matrix of a digital detector
DEL
- the smallest structures that may be seen in an image measured in LP/mm.
- the ability to image adjacent objects as being separate.
- refers to the distinctness or sharpness of structural lines that represent an image.
SR
the amount of error that still results in a quality image at an appropriate dose.
exposure latitude
Adjusts the pixel values to fit the normal histogram of the body part and adjust for small exposure errors.
rescaling
