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202 MEDIMAGE > 7. Digital Imaging: Image Receptors > Flashcards

7. Digital Imaging: Image Receptors Flashcards

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1
Q

what are the 2 types of digital IRs

what are their differences and similarities

A

computed and direct

different construction techniques and acquire latent images differently however how this digitized and processed is the same regardless of technique

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2
Q

what is the latent image

A

image on an exposed film or print that has nor yet been made visible by developing

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3
Q

for CR what is done to the exposed image plate and processed

A

placed/sent to a reader and converted from analog plate to digital data for processing

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4
Q

for CR the reader units have drive mechanisms what does these do

A

move the plates through the scanner

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5
Q

for CR the reader units have an optical system
what components does it consist of
what do these do

A

laser, laser scanner and erasure unit

laser scans the exposed image plate to release the stored energy via visible light

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6
Q

for CR the reader units have photomultiplier tube what does these do

A

collect, amplifies and converts the light to a electrical signal

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7
Q

for CR reader units the readers have analog to digital converter what does these do

what 2 characteristic of the ADC?

A

convert analog to digital data

sampling frequency and pitch

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8
Q

in a CR reader units what are the 4 main components

A

drive mechanisms
optical system
photomultiplier tube
analog to digital converter

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9
Q

in CR the cassette holds what

A

the image plate

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10
Q

the image plate of CR what interactions occur at the Image plate

A

exiting radiation interacts with the IP

photon intensities are absorbed by the phosphor

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11
Q

what is luminesence and what does it create

A

emission of light when stimulated by radiation

creates latent image

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12
Q

what 3 components make up the image plate in CR

what are the 2 types of phosphor

A

phospor, protective layers and support

photostimulatble phosphor
photostimulatable luminescence

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13
Q

what do the image plates do in terms of the radiation and photon absorption

A

stores images from exit radiation via photon intensities absorbed by the phosphor layer

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14
Q

how are images erased off the CR image plate

A

expose to bright white light

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15
Q

what is the photostimulatable phosphor made of

what is its important feature

A

It is composed of barium flourohalide crystals doped with europium

it’s luminescent so when stimulated by a high-intensity laser beam it emits a visible light. (absorbed energy released as light)

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16
Q

in CR imaging what are the 2 steps to creating the image

A

1/ Image capture

2/ image readout

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17
Q

in CR what happens in the 1st step of creating an image when the image is captured

what happens in terms of exit x-rays and atom excitation

A

latent image is formed in the photostimulable phosphor when the exit x-ray is obsorbed.

exit x-ray is energy which excites the atoms in the phosphor layer

When an atom is excited it gets a higher energy level and that allows it to move. Spme atoms retun to normal state but some atoms get trapped in their new place until released by the processing laser and this is what gives us the tissue differential from the x-ray absorption.

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18
Q

the image plates in the CR imaging should be processed quickly why

A

(within an hour) as latent images dissipate over time.

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19
Q

what is exit radiation

A

photons that made it through tissue

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20
Q

what are the 3 stages when digitizing the CR latent image

A

scanning
sampling
quantization

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21
Q

what is the purpose of the scanning stage when digitizing the CR latent image

A

convert latent image into an electrical signal - voltage - that can be digitized and displayed as an image

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22
Q

once in the reader unit what has happened to the IP

A

IP removed from cassette and scanned with a He neon laser beam to release the stored energy as visible light

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23
Q

what does absorption of the laser beam for a CR digitization of image do to the e- in the IP

A

releases trapped e- and they return to a lower energy state

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24
Q

the scanning of the IP in CR digitization of the image results in what happening to the light intensities and where is it sent to

A

continuous pattern of light intensities being sent to the PMT which is then directed to the ADC for sampling and quantization

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25
what does the PMT do in terms of CR digitization of an image
collects, amplifies and converts the visible light to an electrical signal proportional to the energies stored in the IP
26
why is sampling needed in the digitization of the image in CR
to digitize the analog signal from the PMT
27
what is the sampling frequency and related to in the process of image digitization in CR
ADC
28
what is the sampling frequency and what does it determine
determines how often the analog signal is reproduced in its discrete digitized form
29
increasing the sampling frequency of an analog signal has what effect on the pixel density and spatial resolution
increases pixel density and improves the spatial resolution
30
what is the sampling pitch
the distance between 2 sampling points
31
increasing the sampling frequency will have what effect on the sampling pitch what relationship do they have with eachother
increase sf = decrease sp and results in smaller sized pixels inverse relationship
32
the larger the sampling pitch the ___ the pixel size
larger
33
what is the pixel pitch
the distance between the midpoint of one pixel to the midpoint of an adjacent pixel
34
in terms of the size of pixels and the number of pixels what improves the spatial resolution
increasing number and smaller sized pixels
35
increasing the sampling frequency results in what effect on the sampling pitch and pixel pitch and spatial resolution
the smaller the gap between pixels so smaller for both pitches improves spatial resolution
36
why would fixed matrix size system improve the spatial resolution
to maintain the same matrix size and number of pixel the pixels must be smaller in size
37
what does it mean when the spatial resolution for an IP is fixed in terms of matrix and IP size what is fixed
matrix size is proportional to the IP size (larger IP size means it will have a larger matrix to maintain the same pixel size) sampling frequency is fixed Fixed sampling keep pixels the same size
38
what does it mean when the matrix size for an IP is fixed
change the sampling frequency to maintain the matrix size changing the size of the IP would affect the spatial resolution of the digital image as the pixels in a smaller image plate will be diff size/smaller than those in a large one. a larger IP size will result in a larger pixel size and decreased spatial resolution
39
what is a matrix
Number of pixels = matrix
40
for a fixed matrix size CR system using a smaller IP (decreasing the IP) for a given field of view has what effect on the spatial resolution
improves the spatial resolution of the digital image
41
what does it mean when the degree of CR quantization is mentioned
the pixel bit depth
42
what does CR quantization control
number of grey shades/contrast of the image the bigger the pixel bit depth = more values of grey
43
what does CR quantization reflect
precision at which each sampled point is recorded
44
before CR plates are reused/returned to service what must be done and why
exposed to intense white light to release residual energy that could affect future exposures
45
what is the DR's scanning system like
self-scanning
46
DR systems convert x-ray intensities into proportional ______
electronic signals for digitization
47
what does DR allow in terms of image acquisition and readout
reduces the delay between the 2 processes
48
what is the difference between DR and CR in terms of the image acquisition process
CR requires 2 step process and results in a delay between image acquisition and readout DR combines the 2 processes so images are available directly after exposure
49
what are flat panel detectors and which type of radiography are they used in
DR solid state IRs using a large area active matrix array of electronic components
50
what are the 3 processes integrated into the flat panel detector
signal storage, readout and digitizing
51
in a DR flat panel detector do the first, second and third layers composed of
1st layer = x-ray converter 2nd layer = thin film transistor array 3rd layer = glass substrate
52
what is the thin film transistor in the flat panel detector divided into what do these divisions do
square detector elements (DEL) they store electrical charges and a switching transistor for readout
53
what does the electronic signal get sent to after being processed by the flat panel detector in DR
ADC for digitization
54
what is the limitation of the DEL in the flat panel detector in DR in terms of the pixel size
the DEL has a x-ray sensitive area representing each pixel in the image matrix the pixel is therefore smaller than the DEL and can only capture a % of the x-rays reaching the detector
55
what is the fill factors for DELs of DR what is its value normally
the percentage of x-ray capture by the DEL normally around 80%
56
flat panel detectors with smaller DELs have what pro and con
``` pro = smaller pixel size so improved spatial resolution con = fill factor decreased ``` Efforts to further decrease the DEL for improved spatial resolution would require more radiation exposure to reach the IR to create the digital image as smaller pixel size means more radiation is needed as there is more sampling
57
what are 3 advantages of using flat panel detectors DR in terms of dose, spatial resolution and pixel size/pitch
highly dose efficient better spatial resolution than CR fixed pixel size and pitch so limits spatial resolution to square detector elements
58
what are the 2 ways that flat panel detectors create electrical charges proportional to the x-ray exposure
indirect and direct
59
indirect conversion detectors use a ___ to convert ___ radiation into ___ ___
scintillator exit radiation into visible light
60
in indirect conversion detectors the visible light is converted into electrical charges by ____ and the electrical charge is stored by __ in the __ ___ before being digitized and processed in the computer
converted to electrical charges by photodetectors stored in the capacitor in the TFT array
61
what are the 3 steps in indirect conversion detectors
scintillators detectors converts exit radiation into visible light photodetector converts visible light to proportional electrical charges electrical signals sent to the ADC for digitization
62
what are the 2 stage processes in indirect conversion detectors
convert x-ray intensities first to visible light and then to electrical charges during image acquisition
63
are the 2 steps in the direct conversion detectors
amorphous selenium coated detector converts exit radiation directly into electrical charges electrical signals are sent to the ADC for digitization
64
why is an electrical field applied across the selenium later of a direct conversion detector
limit the lateral diffusion of e- as they migrate towards the thin film transistor array
65
what are the similarities between the indirect and direct conversion detectors in the process of charge storage
electrical charge is stored in a TFT array before its amplified, digitized and processed in the computer
66
what stage does the direct conversion detector skip when compared to the indirect
skips the stage of needing to create physical light
67
what is the dynamic range
ability of a detector to accurately capture the range of photon intensities that exit the patient
68
what does a wider dynamic range mean for the contrast and exposure
more contrast and risk over exposure
69
what is the detective quantum efficiency
measurement of the efficiency of an image receptor in converting the x-ray exposure it receives to a quality radiograph image
70
what does 100% detective quantum efficiency mean
no info is lost
71
the higher the detective quantum efficiency what happens to the radiation exposure to produce a quality image and patient exposure
lowers the radiation exposure required to produce a quality image decreases the patient exposure
72
which 2 factors is the detective quantum efficiency affected by
the type of material used in the IR to capture the exit radiation the energy of the x-ray
73
what is the signal to noise ratio
describes the strength of the radiation exposure compared with the amount of noise apparent in a digital image
74
what is the signal in the signal to noise ratio
the signal is the strength of the radiation exposure
75
what is the noise in a digital image from
sources include e- that capture, process and display the image
76
what is quantum noise
too few x-ray photons captured by the image receptor to create a latent image
77
what does increasing the signal to noise ratio do for the quality of the digital image
improve the quality by improving the visibility to anatomical details
78
what is the contrast to noise ratio
describes the contrast resolution compared to the amount of noise apparent in a digital image
79
increasing the contrast to noise ratio does what to the visibility of anatomic details
increases visibility

202 MEDIMAGE (18 decks)

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