9. Digital imaging part 2 Flashcards
What is dynamic range?
- Number of brightness values (grey shades) that can be represented.
What is the dynamic range identified by?
-Identified by the bit depth of each pixel (associated with each individual pixel)
As you increase dynamic range, what does it does it do to Contrast resolution and bit depth?
Increases it
How many shades does a 2 bit system have the capability of showing?
four shades
What is the range in mAs for optimal dynamic range?
Between 4 different values
What is automatic rescaling in DDR
During processing, the system recognizes an inconsistency and automatically rescales the pixel values to a proper contrast/brightness value through pre programmed algorithms
What is spatial resolution?
Is the ability to image small objects
What is more important in DDR, SR, or CR?
CR
What controls SR?
Acquisition pixel size
As you decrease the pixel size, does the SR increase or decrease?
It increases
What are 3 ways to describe pixel size?
- Matrix size/FOV (3 by 3)
- Pixel Pitch (the distance between the middle of one pixel to another)
- Pixel Density (For the same FOV, the greater the pixel density-numbers of pixels)
How does the matrix size affect SR?
- For the same field of view (FOV) (size of IR), spatial resolution increases with a larger matrix
- Matrix size by itself doesn’t effect SR, however to fit more pixels in a smaller field of view, we have to decrease the size of the pixels which increases the SR
Describe the general process of how the CR captures an image:
General
- Excite electrons to a higher energy level and traps them in that level
- Produces latent image
- During processing, the e- drop down and release the energy in the form of light
- We turn the light into an electric signal
What does the PSP do?
- Captures the latent image
What is the PSP composed of?
General
Composed of extremely small particles (pixels) which store and release energy
What captures and stores the image in the PSP?
F centers trap and store the electrons
What is the relationship between the e- in the PSP to the photons?
excited electrons trapped is proportional to the # x-ray photons
What is the image called before it is processed in CR
Latent image
When is the PSL used?
Happens when scanning CR cassette
What is PSL in terms of the e- in the PSP?
-Causes e- to drop down to their ground state
-Electrons return to ground state and release energy in form of light
-Emits light when exposed to a infrared (lazer light)
What happens during the read process in CR?
-The PMT(photomultiplier tube) converts the varying light intensities into varying electrical signals.
-Electrical signals are amplified & sent to an A/D converter. A/D converter produces the binary values for the brightness of each pixel
In DDR, how does direct digital acquisition work?
- X-rays interact with the photoconductor (amprphous se)
Creates an electronic signal (charge) - Storage capacitors in the DEL’s collect the charge
- The charge is released to the ADC -analog to digital converter
- ADC converts the charge to a digital signal which is used to produce the digital image
What is the advantage of direct digital acquisition?
Better SR
What is the disadvantage of DD acquisition as opposed to indirect?
More expensive to purchase and repair
In DDR, how does indirect digital acquisition work with CCD system?
- X rays hit Sinsilatior
- Scintillation layer is ‘coupled’ to each CCD chip (pixel) by lenses or fiber optics
- Converts x-rays to light and causes blur from light spread
- CCD chips convert the light photons to an electronic signal
- ADC converts the charge to a digital signal which is used to produce the digital image during processing
What is the Scintillation layer made of?
Scintillation layer – cesium iodide
How does the scintillation layer help light focus?
Cesium iodide can be formed into small needles like columns to helps to focus the light
What is advantage and disadvantage of indirect digital acquisition?
A: Better contrast resolution compared to direct acquisition
D: Less spatial resolution compared to direct acquisition
How does Indirect Acquisition-Thin film Transistor (TFT) work?
- Has Scintillation layer
- Converts x-rays to light
- Photodiode layer converts the light into an electronic signal 4.Transferred to the TFT array
- TFT sends the signal to the ADC
- ADC converts the charge to a digital signal which is used to produce image
What modalities is TFT used in?
TFT used for angiography and fluoroscopy and general radiography
