Digital Radiography

Question 1

What are some advantages to digital radiography?

Answer 1

Better image quality
Post processing simpler- more efficient
Easier to store and transport
More cost effective

Question 2

What are the basic characteristics of a digital image?

Answer 2

Recorded as multiple numeric values
Divided into small elements
Begin analogue, turned into electrical signals then digitised

Question 3

What are the basic characteristics of an analogue image?

Answer 3

X-ray is converted to light, recorded on film
Capture involves measurement of continuously changing signal

Question 4

What are some key features of a digital image?

Answer 4

Spatial resolution
Contrast resolution
Noise
Dose efficiency

Question 5

Describe pixel bit depth

Answer 5

Determined by number of bits used to define each pixel
Number of grey tones produced = bit depth to the power of 2

Question 6

Describe matrix

Answer 6

Matrix - square arrangement of numbers and columns
Pixel size = image size (mm)/ matrix size (pixels)

Question 7

Describe image display

Answer 7

When an image is processed it can be:
- Displayed on a monitor
- Printed on film
- Sent to PACS

Question 8

What are the basic parts of computerised radiography?

Answer 8

Cassette, photostimulable phosphor plate, computer workstation

Question 9

Describe the process of computerised radiography

Answer 9

Uses an imaging plate made of phosphor
Catches x-ray energy
Scanned with laser, emits light
Captured by plate reader to build image

Question 10

Describe how we use photostimulable phosphor

Answer 10

Creates and stores latent image
Scanned by laser of specific wavelength to view image
Scanning produces light proportional to image - photo stimulated luminescence

Question 11

What characteristics are required for photostimulable phosphor?

Answer 11

Good absorption
Capable of being laser stimulated
Luminescence compatible with photomultiplier tube
Luminescence time shorter than 1μs
Should be able to store latent image

Question 12

What is contained in phosphors?

Answer 12

Barium fluorohalide: europium
Halide can be chlorine, bromine, iodine or mix

Question 13

Describe the process of how the PSP responds to x-rays

Answer 13

Phosphor atoms ionised
~1/2 removed electrons trapped in conduction band
Quantity and distribution proportional to exposure
Represents latent image

Question 14

Describe the latent image sequence

Answer 14

Radiation interacts with imaging plate
Some absorbed energy released as light
Most energy absorbed, electrons trapped in colour holes
Number caught proportional to exposure

Question 15

Describe the function of a charged coupled device

Answer 15

Indirect conversion of x-ray photons into electric charge
Photons first converted to light via scintillating screen

Question 16

Describe Scintillators

Answer 16

Caesium iodide - coupled to CCD using fibreoptic bundle.
Light energy transmitted to CCD
Converted to electronic signal
Can absorb moisture

Question 17

Describe the structure of a charged coupling device

Answer 17

An array of CCDs
Arranged in tiling pattern, resulting in seams
Detectors are sealed to avoid this

Question 18

Describe the characteristics of a charged coupling device

Answer 18

Light sensitive
Wide dynamic range - wide range of light intensities

Question 19

Describe thin film transistors

Answer 19

Capture - scintillator
- caesium iodide
Coupling - contact layer
- amorphous silicon photodiode
Collection - Thin film transistors
- DELs

Question 20

Describe direct capture

Answer 20

No scintillator - no loss of resolution
Capture - Photoconductor
- Amorphous selenium
Collection - TFT

Question 21

Describe aquisition

Answer 21

Computer creates histogram
Digital pixel values vs. relative prevalence of values on image
Computer analyses and compares

Question 22

Describe exposure latitude

Answer 22

Range of exposure values detector can produce
Images require sufficient contrast resolution
Film has limited latitude

Question 23

Describe brightness

Answer 23

Amount of luminance on display monitor

Question 24

Describe contrast resolution

Answer 24

Systems ability to display subtle changes in grey tones

Question 25

Describe Detective quantum efficiency

Answer 25

How efficiently a system converts x-ray input signal into useful output

Question 26

How is detective quantum efficiency measured?

Answer 26

% of x-rays absorbed by detector

Question 27

Describe modulation transfer function

Answer 27

Ability to record available spatial frequencies
Quantifies contribution of each system component to overall efficiency

Question 28

Describe noise

Answer 28

Caused by non-uniform detector responses
Quantum noise is photon dependent
- appears as brightness

Question 29

Describe exposure indicators

Answer 29

Provides a numeric value indicating level of radiation exposure to receptor

Question 30

Describe sufficient DI & EI on Samsung machines

Answer 30

DI = -3.00 to 3.00
EI = ~400