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300 Digital > Exam 1 > Flashcards

Exam 1 Flashcards

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1
Q

Direct vs Indirect

A

Direct
Directly digital read; i.e. x-ray to digitized signal
Uses a photoconductor.
X-ray to electrical signal

Indirect 
Produce analog signal that is digitized
Uses a phosphor/scintillator
Multiple conversions
*X-ray to light to electrical signal*
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2
Q

Basic steps of direct process

A 
Detector captures remnant radiation
Digital image goes to temporary storage
Image goes to monitor
Post processing
PACS
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3
Q

Advantages of DR technology

A

Almost immediate image viewing
Up to 5 AEC detectors (usually 3)
Shorter exam times
Less labor intensive

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4
Q

Disadvantages of DR technology

A

Large, bulky flat panel detector
Most exams use grid
Grid and AEC require accurate tube/plate centering
Some units: exam menu decides fixed field size

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5
Q

3 ways to describe/identify the various types of DR systems:

A
  1. Capture elements: capture/collect the x-rays incident upon the unit and convert them to light or electrons they include: PSP in CR, cesium iodide, gadolinium oxysulfide, amorphous selenium
  2. Coupling elements: transfer the x-ray generated signal to a collection element includes: lens, fiber-optic assembly, contact layer, or amorphous selenium
  3. Collection elements—photodiode, CCD, thin-film transistor (TFT). Photodiodes and CCDs are light sensitive and collect light photons. TFT is charge sensitive and collects electrons
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6
Q

Flat panel system components

A

Biggie: analog to digital converter inside the plate.

(Diagram)
Active matrix detector Switching electronics
Data electronics
Analog to digital converter
Internal memory Control electronics

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7
Q

Where do x-ray detection and digitization of signal take place?

A

In the flat panel detector

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8
Q

Host computer

A 
Detector linked directly or wirelessly to computer.
Host computer (brains) links:
X-ray machine
Detector
Image display monitors
Network communication
Storage i.e. PACS.
Controls image processing
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9
Q

2 varieties of flat panel detectors:

A

Scintillator- and non-scintillator based.

The non-scintillator based detector converts the x-ray beam into electrons to create an image. The steps are a little different, but both are what we call flat-panel image receptors. (ODIA)

A scintillator converts x-ray photons into light, which are emitted from the scintillator and interact with a photoconductive material typically made of amorphous silicon to convert the light photons into electrons. The electrons created in the amorphous silicon then migrate to thin film transistors and produce an electric signal.

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10
Q

Photoconductor

A

Absorb x-rays, emit electrons (electrical charge).

Used in direct.

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11
Q

Scintillator

A

A phosphor that emits light when exposed to ionizing radiation
Used in indirect
Absorb x-rays, emit light
A form of luminescence, only certain compounds scintillate
Can be used to detect radioactivity

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12
Q

Detector Array

A

Is the Image receptor/ radiation detector

Multi-functional:
Absorbs remnant radiation
Converts it to electrical signal
Sends signal to computer

Several layers, yet thin

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13
Q

Active Liquid Crystal Matrix Array

A 
Same technology as laptop screens, etc.
Glass substrate with:
Elements
Switches
Electrical connections

Detector elements and electronics microscopic
100 microns (1/10 mm)
All layered on top of the glass

Aka Active Matrix Array

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14
Q

Direct capture detector anatomy

A

Layers:
Top Layer: Electrode- Connected to high voltage

Dielectric layer

Photoconductor

  • Detector element
  • -Active Layer—x-ray interaction
  • -Capture element
  • -Amorphous selenium (a-Se)
  • -500 microns thick
  • Collection electrode
  • -TFT thin film transistor
  • -Storage capacitor
  • -Collects electrical charge
  • Glass substrate
  • -Backbone/support
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15
Q

Detector element (DEL)

A

In flat panel: Matrix of detector elements
Pixels (not quite accurate)
DEL: detector element, dexel (more accurate)

Rows and columns
Each pixel has an “address” in the array
Purpose: processing software knows where to put the data

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16
Q

Contained in each DEL

A

TFT (thin film transistor) switch
Storage capacitor
Capture (sensing) area
charge collector

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17
Q

DEL sensing area

A

Capture element / charge collector:
Active Layer: Amorphous selenium (a-Se)
-Semiconductor
-X-ray photons interact directly with this layer
-Highly sensitive to x-ray photons
-Convert x-ray photons to electric signal
-Layered on glass substrate with TFT and storage capacitor

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18
Q

Thin film transistor

A

Switching gate to release electrical charge when plate reads out
Switch/gate—electronics that open to send signal to ADC

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19
Q

Capacitor

A

Microscopic

Stores electrical charge created in the a-Se layer

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20
Q

CsI

A

Cesium iodide

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21
Q

CsI and charge coupled device is for direct or indirect?

A

Indirect

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22
Q

CsI

A

Cesium iodide-used as a capture element of an image intensifier tube

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23
Q

a-Si

A

Amorphous silicon

Used in indirect

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24
Q

CsI phosphor in DR IR

A

The cesium iodide (CsI) phosphor in digital radiography image receptors is available in the form of filaments to improve x-ray absorption and reduce light dispersion. (Results in low dose to patients)

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25
GdOS
Gadolinium oxysulfide
26
a-Se
Amorphous selenium Used in direct-NO scintillation phosphor is involved Is both the capture element and the coupling element.
27
Active layer of capture element for direct
a-Se Semiconductor X-ray photons interact directly with this layer Highly sensitive to x-ray photons Convert x-ray photons to electric signal Layered on glass substrate with TFT & capacitor
28
Fill Factor
The percentage of the DEL that is sensitive to radiation vs. the non sensitive area. Higher fill factor means more sensitivity. (Sensitive area in direct is a-Se, minus the areas for the TFT and capacitor)
29
Capacitor in direct
Microscopic | Stores electrical charge created in the a-Se layer
30
TFT in direct
Switching gate to release electrical charge when plate reads out Switch/gate—electronics that open to send signal to analog digital converter (ADC)
31
How plate works with direct
Pre-exposure: High volt positive charge applied to top electrode Negative charge applied to del electrode (bottom) Makes plate sensitive to x-ray When x-ray photons ionize a-Se: Photoelectric interaction occurs Number of ionized atoms = number of photons that reach plate Very sensitive=High DQE Ionized a-Se atoms release electrons: Electrons (-) migrate to top electrode (+) Leaves an e- hole in a-Se atom (+) which migrates down to the del electrode (bottom electrode) (-)
32
Latent image on direct
Is the positively charged aSe atoms held at the bottom of the plate
33
Voltage of direct
When circuit closed: Voltage along gate lines changes: -5v to +10v Stored charge flows along data lines to external electronics Voltage for that line changes back to -5v Stored charge flows down data line to amplifier Signal enhanced Sent to ADC—on detector array edge ADC to computer to be processed
34
What happens with a higher fill factor?
Higher contrast resolution and higher spatial resolution
35
Of the two categories of flat panel detectors, which one converts x-rays to an electrical signal?
Direct
36
Name the top layer of a flat panel detector
Electrode
37
What is a TFT?
An electrical component , the switch/gate that allows the stored charge to flow
38
What is the x-ray sensitive component of the del made of for direct?
a-Se
39
How are the DEL’s discharged?
Voltage changes along data lines attached to the TFT’s, circuit closed, charge moves thru TFT, out of the DEL along data lines, then on to amplifier, then ADC.
40
What makes up the electron hole pair?
aSe+ atom and the electron freed when x-ray photon interacts with aSe atom
41
The charge at the bottom of the semiconductor post x-ray exposure is positive or negative for direct?
Positive
42
The address driver controls?
The order in which DEL’s read out
43
The bias voltage in the FPD changes from __ to __.
-5 to +10 volts*
44
What happens to the scintillator when exposed to x-radiation?
Fluoresces (lights up)
45
What are the two types of scintillator phosphors?
Cesium iodide Columnar needle formation* Less light spread-Higher resolution Higher DQE (ability to detect x-rays) Gadolinium oxysulfide Turbid crystals* Lower resolution- more light spread
46
How to reduce light spread in indirect?
CsI formed as needle shaped crystals, 10 – 20 micrometers in diameter, channels light toward aSi photodiode. Thin screen to minimize light spread. Once the light reaches the aSi, it is converted into an electric signal.
47
X-rays interact with phosphor. Name them:
CsI and GdOS
48
What does the phosphor do?
Emits light—fluoresces, or fluorescent light
49
Light photons activate the amorphous silicon layer to emit __?
Electrons
50
What do scintillators, PSP’s, & amorphous selenium have in common?
They are all capture elements, are sensitive to x-rays
51
Name some coupling elements
Lens, fiber optics, aSe, aSi
52
What device do all flat panel detectors use to collect the image signal and send it to the computer?
TFT (or DEL)
53
A photodiode is sensitive to:
Light
54
The photodiode used for indirect capture FPD’s is:
aSi
55
Describe the energy conversion process for indirect:
X-ray to light to electrons
56
How is a latent image created with CCD?
1. X-ray interacts with scintillator 2. Scintillator emits light-in proportion to amount of x-ray energy it received 3. Lenses/fiber optics capture light-reduce/compress light to fit CCD chip 4. Photodetector/photosensitive portion of CCD chip absorbs light. Electrons ejected in response. * Spectrally matched to light from scintillator*** (has to be sensitive to the light it emits, or it will not take the light and eject electrons) 5. Electrons held in capacitors 6. Charge released line by line, sent to ADC
57
How does CCD read out?
Like bucket-brigade. Pixel by pixel, row by row.
58
CCD advantages
Very light sensitive-responds to low levels of light Wide dynamic range Good spatial resolution- 5 lp/mm max Fast image construction Less expensive than flat panel detectors Easier to repair, replace individual chips (where as have to replace whole FPD if 'chip' breaks) Small size
59
CCD disadvantages
De-magnification of image-lenses focus light onto CCD chip Results in: Minimizing image Lower detective quantum efficiency (DQE) Loss of spatial resolution with all the energy conversions
60
Major difference in CCD and CMOS
Complimentary metal oxide semiconductor All conversions happen in each pixel. Each del (pixel) has transistor, amplifier, noise correction & digitization circuits
61
CMOS semiconductors
Silicon Semiconductor: element that can act as conductor or insulator depending upon conditions Uses 2 metal oxide semiconductor field effect transistors (MOSFET) * N type transistor: extra electrons, negative charge** * P type transistor: carry positive charge** * Form electronic logic gate** Low power use when not being used Heat up quickly when used
62
CCD vs CMOS
``` CCD: More light sensitive Uniform signal 110x energy use More expensive to manufacture Higher quality pixels Technology more refined ``` ``` CMOS: More transistors between dels = less sensitive More image noise (from lower signal) Uses less energy Less expensive Old, revised technology, still improving ```
63
Fluoro-conventional vs digital
Con: Capable of continuous & pulsed operation 30 frames/sec 3-10 msec/exposure Low mA, High kVp 65-120 kVp Spot film uses radiographic exposure factors ``` Dig: Capable of continuous & pulsed operation- pulsed mostly 60 frames/sec High mA 65-120 kVp Grid-controlled x-ray tube High frequency generator Uses radiographic exposures ```
64
Fluoro- flat panel
Latest development in DF Rectangular image receptor, coupled to same shape monitor Indirect or direct capture Same as DR plates Smaller, lighter Easier to maneuver Replaces the CCD/II combination *Operates in radiographic, continuous & pulsed fluoro modes Pixel size can be adjusted electronically: binning, reduces image mottle- but less spatial resolution 60 frames/sec
65
What is pulse width in fluoro?
Exposure time. Takes 3-10 msec 3 msec @ 20 mA can halve pt. dose compared to conventional fluoro Up to 30 frames/sec Caveat: Operating unit at high mA and longer pulse width Potential for excess dose
66
What is Interrogation time?
Time required for machine to turn on & reach selected mA and kVp. Must be less than 1 ms.
67
What is Extinction time?
Time required for tube to be switched off. Must be less than 1 ms.
68
Dose reduction features for fluoro?
Pulse progressive fluoroscopy Copper filtration-removes low energy photons Automatic dose rate control (ADRC)-Modulate mA, kVp, pulse width, filtration to reduce dose while maintain good quality images. Software that optimizes imaging certain anatomy Automatic positioning systems DAP meters-measures entrance dose for entire procedure

300 Digital (2 decks)

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