Digital Imaging

Question 1

True or False?

A digital image has a continuous density across it.

Answer 1

False, a digital image has discrete pixels with numerical density values.

Question 2

What are the two types of digital radiography?

Answer 2

Computed radiography (CR) and Digital radiography (DR)

Question 3

What is the difference between CR and DR?

Answer 3

CR uses imaging plates that need developing in a processor whereas DR acquires an image directly without using a processor.

Question 4

True or False?

In digital radiography, the film is the detector and storage medium

Answer 4

False, this is screen-film radiography. In digital radiography, the detector only generates the image which is then stored in a digital medium.

Question 5

List the 4 steps of the digital imaging series

Answer 5

• Generation
• Processing
• Archiving
• Presentation

Question 6

Name 4 types of DR (Direct Radiography) digital detectors

Answer 6

• Photoconductor FPD (Direct Conversion)
• Scintillator-TFT (Indirect Conversion)
• Image intensified (Indirect)
• Selenium-drum (Direct)
• Scintillator-CCD (Indirect)

Question 7

What are the 2 types of digital detectors?

Answer 7

Indirect conversion & Storage phosphors

Question 8

Name and explain the main functions of a digital image detector

Answer 8

Capture- Absorbing X-rays
Coupling- Linking between the capture element and the converter
Conversion- Change of X-ray energy into electrical signal which can be read (can be direct or indirect)

Question 9

What photoconductor absorbs X-rays?

Answer 9

Amorphous Selenium

Question 10

What happens to the electrical charge generated by the DR photoconductor?

Answer 10

It is held in storage elements (e.g. capacitors) on a 2 dimensional detector array, which produces a stored charge distribution

Question 11

What are the types of CCD indirect radiography systems?

Answer 11

• Lens-coupled CCD based system

- Slot-scan CCD based system

Question 12

What is the function of Scintillator materials?

Answer 12

To produce a brief burst of light when struck by high energy rays like X-rays

Question 13

Finish the sentence.

“In direct digital radiography (DDR), X-ray energy is converted directly into..”

Answer 13

..electrical signal.

Question 14

List 1 similarity and 1 difference between a Selenium-drum detector system and a Flat-panel detector system

Answer 14

• They both use Amorphous Selenium Photoconductors
• A flat-panel system uses TFT detector arrays to convert X-ray energy into electrical charge whereas selenium-drum detector systems use A/D converters to do so.

Question 15

What does a photoconductor made of a-Se do?

Answer 15

Absorbs X-ray energy and generates electrical charge

Question 16

True or False?
In indirect DR, each cell in the detector has a top layer of a scintillation crystal made of TI (Thallium) activated by CsI (Caesium Iodide), which produces light when hit by X-rays.

Answer 16

False. It is a layer of a scintillation crystal made of CsI activated by TI on top of the detector cells.

Question 17

List 3 similarities between Direct and Indirect DR.

Answer 17

• Quick reading (about 1 second)
• The digital image is sent to an output device
• A 2D array of electrodes containing switching elements in a thin film transistor (TFT) reads the stored charge distribution
• Sampled electrical charge values are counted to produce a digital image

Question 18

Give 2 features of digital imaging

Answer 18

• Wide exposure latitude which enable a wide range of exposures that give an acceptable image
• Wide “dynamic range” that records a lot of densities in one exposure
• CR plates that are sensitive to X-ray fogging
• Good resolution

Question 19

True or False?

There is a greyscale of 10,000+ in CR and DR imaging as opposed to film imaging where there is a greyscale of 30

Answer 19

True.

Question 20

What value is recorded that indicates whether or not the X-ray exposure of the digital image is within the recommended range?

Answer 20

Detector dose index (DDI)

Question 21

What do the following terms mean and on which systems are they found on?
EI, logM, S value, REX, DEI, DI

Answer 21

• EI: Exposure Index (Kodak/Siemens/Philips)
• logM: Log median exposure (Agfa)
• S value: Sensitivity number (Fuji)
• REX: Reached exposure value (Canon)
• DEI: Detector exposure index (GE)
• DI: Deviation Index (Toshiba)

Question 22

How many mAs are equivalent to a dI value of +3 (excessive)?

Answer 22

60

Question 23

What causes a grainy (noise) image?

Answer 23

Under-exposure

Question 24

Define what is meant by the term “Quality vs. Dose”?

Answer 24

“Quality versus dose” refers to radiographers over-exposing in order to get high quality images and avoid image noise.

Question 25

When does the effect of variations in numbers of individual X-ray photons become noticeable?

Answer 25

At very low mAs values, where random fluctuations in X-ray beam intensity starts to show (Quantum Mottle)

Question 26

Apart from Spatial Resolution, what 2 other factors affect digital image quality?

Answer 26

Contrast & Noise (SNR)

Question 27

What does MTF mean?

Answer 27

MTF (modulation transfer function) is the capacity of the input signal at a given spatial signal to its output

Question 28

List 4 sources of noise

Answer 28

• Random electrical fluctuations in circuits
• Detector faults
• Quantum Mottle (occurs with small numbers of X-rays and very sensitive plates
• Pressure on digital plates

Question 29

What is the difference between signal from anatomical detail and signal from noise?

Answer 29

Signal from anatomical detail is constant over time whereas with signal from noise it is random and variable over time.

Question 30

What decreases as the DQE is increased for the image SNR and exposure conditions?

Answer 30

The radiation exposure that the patient requires.