Chapter 2 Digital imaging
+ or - 1 changes the KIND by how much?
+25 or - 20%
What is considered a perfect DI?
0.0, negative is under exposed and positive is overexposed
What would be the MTF of a perfect image?
1 or 100%
What is the range for bit depth?
1-6536 (2 to the 0 or 2 to the 8th)
+1 is approx. how much of the intended exposure?
125%
What is the typical number of pixels in a matrix?
512x512 to 1024x1024 or 2500 x2500
-1 is approx. how much of the intended exposure?
80%
What group posted "An Exposure Indicator for Digital Radiography?" in 2009?
American Association of Physicists in Medicine
How does a digital image begin?
As an analog
How is contrast resolution related to bit depth?
Contrast resolution is directly related to the bit depth of the pixels in the image
What is different about the dynamic range of digital and film/screen?
Digital has a wider dynamic range
How is an Analog signal recorded?
In its original form (signal wave)
What group posted "Medical electrical equipment- Exposure of digital x-ray imaging systems- Part 1: Definitions and requirements for general radiography" in 2008?
International Electrotechnical Commission (IEC)
What did the article AAPM posted include?
It expressed the need to clinically have this standardization, saying that it will give technologists more confidence in adjusting technical factors while following ALARA
What are the units for Air Kerma?
J/kg or Gy
Indicated Air Kerma
KIND is the measurement of the radiation that was incident on the image receptor for that particular exposure, read from pixel values
Target Equivalent air kerma value
KTGT is a set of values that have been established by either the system manufacturer or by the system user that represents an optimal exposure for each specific body part and view (b,v)
Standard Radiation Exposure
Kstd is a standard exposure typical of that imaging receptor system, tests functionality
What is the true determining factor of image acceptance?
Noise
Digital
Recorded as multiple numeric values and are divided into an array of small elements that can be processed in many different ways
What are the characteristics of digital imaging?
Spatial resolution, contrast resolution, Noise, and Dose efficiency
Pixel size is related to....
The size of the pixel is directly related to the spatial resolution of the image (small pixel, higher spatial resolution)
Modulation Transfer Function
ability of a system to record available spatial frequencies, quanitfies the contribution of the system to efficiency
Spatial Resolution
ability of the imaging system to demonstate small details of an object
How is the median found? (Q)
after data correction has taken place and then the value is compared to the KSTD in order to derive the KIND
What has a higher DQE than PSP?
amorphous selenium, TFT, CCD, and CMOS
Which system has the highest DQE?
amorphous selenium, because no light conversion is present and newer CMOS due to crystal light tubes
noise
anything that interferes with the formation of the image
Brightness
appearance of the image on a display monitor
How do pixels affect contrast resolution?
by giving pixels whole number or discrete values, max contrast resolution is reached
What is true about systems with higher DQE?
can produce higher quality images at a lower dose
Equipment noise
caused by the detector elements or nonuniform detector responses
What will a change in either the FOV or the matrix size do?
change the size of the pixels in order to fit within the matrix within the field of view
In order to detect exposure latitudue
contrast resolution has to be present
How do you decrease the DI by 1?
decrease the technique by 25%
Increase in light spread does what to the MTF?
decreases it
DQE
detective quantum efficiency, how efficiently the system converts x-ray input signal inot a useful output image
Deviation Index
difference in KIND and the target value for that body part (KTGT)
When does radiographic noise occur?
during the image acquisition
When is Air Kerma read?
during the processing of the exposure
KIND
exposure on the IR, helps determine whether the image is under or over exposed
FOV
field of view, x-ray field
What factor determines contrast resolution?
gray level
High NPS means what for the noise?
higher noise for a specific detector
Indicated Equivalent air kerma
iindicator of x-ray beam incident on the digital detector
smaller pixels do what to spatial resolution?
increase
How do you raise the DI by 1?
increase the technique by 20%
matrix
is a square arragnement of numbers in columns and rows. In digital imaging, the numbers correspond to discrete values
Does changing the FOV affect the matrix?
it doesn't affect the matrix size
What is recommended of manufacturers when testing systems?
it is recommended that they provide methods to remove the anti-scatter grid without otherwise changing the detector's response or provide grid attenuation factors to be used in calibration
Air Kerma
kinetic energy released per mass of air, measurement of radiation in juoles absorbed in air
Why is it difficult to measure exposure amounts?
maufacturers differ in the way exposure is numerically represented
The higher the dynamic range of the detector the?
more values can be detected
Will technical adjustments change the brightness?
no technical adjustment will, it's strictly a display quality (winow level)
What happens when SNR increases?
noise decreases, but pt exposure increases (mAs in relation to quantum mottle)
NPS
noise power spectrum, describes the spatial frequency of the noise as well as spatial characteristics
Bit depth
number of bits or pieces of information within each pixel
What happens in digital detectors when an x-ray photon excites a phosphor/scintillation layer?
phosphor produces light, spreading of ligth occurs, ligth spread reduces efficiency
Pixel
picture element, the smallest in digital imaging.
How is the image digitized?
position or spatial location and intensity or gray level
What could cause the DI to be off?
prostheses, gonadal shielding, faliure to recognize collimation, unexpected body part
Exposure latitude
range of exposure values the image detector is able to produce
Analog
refers to a device or system that captures or measures a continuously changing signal
Contrast Resolution
refers to the ability of the digital system to display subtle changes in the shades of gray
exposure index
refers to the amount of exposure recieved by the image receptor, not the patient
what is required for tests of system performance?
relationship between QK values and the standardized radiation exposure incident to the DR detector
SNR
signal to noise ratio, how much noise is tolerated by the radiologist
How is digital imaging separated?
spatially into pixels with discrete (whole numbers)
How do you find the number of gray tones?
the bit depth is the exponent over 2, 2 to the power of 8 (256 shades of gray)
If FOV is constant and the matrix size is increased, what happens to the pixels?
the greater the number of smaller individual pixels would appear. This improves image quality
What determines the sixe of the pixels?
the matrix size
DQE is a measurement of
the percentage of x-rays that is abosrbed when they hit the detector
The larger the matrix...
the smaller the pixels, which means more shades of gray and higher contrast resolution
What is recommended of systems to provide functions?
to be able to display images without processing and report the mean, median, mode, standard deviation, and pixel count values within graphically defined regions
During KSTD why is the exposure made with added filtration?
to harden the beam, simulating patient tissue
What is a recommendation for Exposure Indicator?
to provide appropriate analytical tools and allow for-processing image data or exposure values to be displayed and analyzed on the console
Can matrix size be changed without affecting the field of view?
yes
Can the FOV be changed without affecting the matrix size?
yes
