Ch. 2 Digital Imaging Characteristics Flashcards
Digital images are recorded as ___ ___ ____ and are divided into an array of____ _____ that can be processed in many different ways.
multiple numeric values; small elements
Analog refers to…
a device or system that captures or measures a continuously changing signal
An analog signal wave is used in its…
original form
In DR analog signals are converted into…
numbers that are recorded
Digital images are formed through…
multiple samplings of a signal
A digital image begins as a….
analog signal
What are the critical characteristics of a digital image?
spatial resolution, contrast resolution, noise, and dose efficiency
What is a pixel?
a picture element which is the smallest element in a digital image
What is the size of a pixel directly related to?
the amount of spatial resolution or detail in the image
smaller pixel =
better detail
When can pixel size change?
when the size of the matrix or FOV changes
What does FOV stand for?
Field of view
What is pixel depth?
the number of bits within a pixel
In regards to pixels, what is a factor in determining the image contrast resolution?
The gray level
Define matrix
a square arrangement of numbers in columns and rows, and in digital imaging, the numbers correspond to discrete pixel values
Each box within the matrix also corresponds to a specific …
location in the image and corresponds to a specific area of the patient’s tissue
How is the image digitized/
both by position and intensity
What are other terms for position and intensity?
position=spatial location; intensity=gray level
What is the typical range in the number of pixels in a matrix?
512 x 512 to 1024 x 1024 and can be as large as 2500 x 2500
What does the size of the matrix determine?
the size of the pixels
Which has the smaller pixels? a 10x12 or a 14x17 and both have a 512 x 512 matrix
the 10x12 will have smaller pixels
The term field of view is synonymous with what?
the x-ray field
What does FOV stand for?
field of view
What is the field of view?
the amount of body part or patient included in the image
What would happen if the field of view changes?
it will not affect the size of the matrix, but if the matrix changes, the pixel size will be affected
Why is the pixel size affected if the matrix changes?
because as the matrix increases and the FOV remains the same size, the pixel must decrease to fit into the matrix
What is the relationship between the pixel size, matrix size and FOV?
The matrix size can be changed without affecting the FOV and the FOV can be changed without affecting the matrix size, but a change in either the matrix size and/or the FOV changes the size of the pixels.
What does the exposure index number refer to?
the amount of exposure received by the image receptor (IR), not by the patient.
How is air kerma measured?
in J/kg or Gy
What is air kerma?
(kinetic energy) the measurement of radiation energy(J) absorbed in a unit of air(kg)
What is the exposure typical of a imaging receptor system?
the standardized radiation exposure
The indicated equivalent air kerma is the measurement of…
the radiation that was incident on the IR for the particular exposure.
How is the indicated equivalent air kerma derived?
from reading the pixel values produced by the exposure on an IR.
What are for-processing pixel values (Q)?
pixel values produced by the exposure on an IR
The indicated equivalent air kerma simply stated is…
the amount of exposure on the IR
What does the indicated equivalent air kerma help determine?
whether the IR has been overexposed or underexposed for that particular body part.
What is the target equivalent air kerma value?
a set of values, established by either the system manufacturer or the system user, that represents a optimal exposure for each specific body part and view.
What is the deviation index?
the difference between the actual exposure and the target exposure, except it is expressed in logarithmic fashion.
What does the DI do?
help the tech determine whether the image has been underexposed or overexposed.
What number would indicate a perfect image according to DI?
0.0
What if the DI number is negative?
then the image is underexposed
What if the DI number is positive?
then the image is overexposed
If you had to repeat, how would you raise the DI number +1?
increase the technique by 20%
If you had to repeat, how would you decrease the DI number by -1?
decrease technique by 25%
What are some of the reasons that could cause the reading of the pixel values to be off?
- a prostheses within the image
- gonadal shielding within the image
- failure of the system to recognize the collimated border
- an unexpected body part in the image
What does brightness in the digital terms refer to?
its appearance on the display monitor of the computer
How can brightness be controlled?
there are controls on the monitor
What is window leveling?
when you adjust the brightness on the computer monitor
Contrast resolution is…
it refers to the ability of the digital system to display subtle changes in the shade of gray
What does higher contrast resolution mean?
the differences between adjacent densities are enhanced–more shades of gray
contrast resolution in digital imaging is directly related to what?
the bit depth of the pixels in the image
How is contrast resolution adjusted in digital imaging?
higher kVp values and lower mAs values
What happens to the patient if you raise kVp and lower mAs?
patient dose goes down
What contrast resolution in digital imaging DOES depend on is…
the amount of scatter
How are we able to use higher kVp values without compromising contrast resolution in digital imaging?
With the use of tight collimation and the correct grid
How is screen resolution controlled?
by a technique called window width
what is spatial resolution?
the ability of the imaging system to demonstrate small details of an object
What determines resolution in film/screen?
the crystal size and thickness of the phosphor layer
What determines resolution in photostimulable phosphor systems?
phosphor layer thickness and pixel size
As far as resolution goes, the thinner the layer of phosphor, the….
higher the resolution
In film/screen resolution at its best is limited to..
about 10 line pairs per millimeter
In digital receptors, resolution is about…
2.55 lp/mm up to 10 lp/mmin PSP systems–giving less detail
what is dynamic range?
the ability to respond to varying levels of exposure
In digital, what would cause more tissue densities on the digital image to be seen, giving the appearance of more detail?
dynamic range
Why are we able to see more soft tissue (like in a knee radiograph) in digital than we are able to see in film/screen?
because there is a wider dynamic recording range but does not mean there is additional detail
Spacial resolution: the smaller the pixels…
the higher the spacial resolution
What is modulation transfer function?
the ability of a system to record available spatial frequencies
What does MTF stand for?
modulation transfer function
MTF is a …
ratio
The sum of the components in a recording system cannot be…
greater than the system as a whole
What does MTF quantify?
the contribution of each system component to the overall efficiency of the entire system
A perfect system would have a MTF of…
1 or 100%
Anything that interferes with the formation of the image is considered..
noise
What is anatomic noise?
if superimposition of body parts occurs
What does radiographic noise consist of?
equipment noise and quantum noise
Where does equipment noise come from?
noise in the detector elements and non-uniform detector responses
Can the tech control equipment noise?
no
How do we put a value on noise?
with the noise power spectrum
What is NPS?
noise power spectrum
What is the NPS?
it describes the spatial frequency content of the noise as well as spatial characteristics
What does SNR stand for?
signal-to-noise ratio
define SNR
how much noise can be tolerated in the image
As SNR increases, the noise…
decreases
What does the term exposure latitude refer to?
the range of exposure diagnostic image values the image detector is able to produce.
exposure latitude is dependant on…
the image detector
detective quantum efficiency:
how efficiently a system converts the x-ray input signal into a useful output image
What does DQE stand for?
detective quantum efficiency
What is DQE a measurement of?
the percentage of x-rays that is absorbed when they hit the detector
Systems with higher quantum efficiency can produce…
higher quality images at lower doses
What have increased DQE over PSP?
amorphous selenium, amorphous silicon thin film transistor (TFT), charge couple device (CCD), and complementary metal oxide semiconductor detector technology
What has the highest DQE? Why?
amorphous selenium detectors because they do not have the light conversion step and consequently not light spread.
Why would the newer complementary metal oxide semiconductor capture system be equal to direct image acquisition?
because of the crystal light tubes, which also prevent light spread
The DQE of detectors changes with…
kVp
The greater the area of the TFT array, the…
higher the DQE
