Capturing the Digital Image: DR and CR

Question 1

What is Direct Radiography (DR)?

Answer 1

A flat panel detector built into the radiographic room

Question 2

What is a key advantage of Direct Radiography (DR) over Computed Radiography (CR)?

Answer 2

• High PACS compatability
• Increased department efficiency

Question 3

What is the main component of a Direct Radiography detector?

Answer 3

Hundreds of thousands of ‘dels’ (detector elements)

Question 4

What type of system is Computed Radiography (CR)?

Answer 4

Cassette-based system

Question 5

What technology does Computed Radiography (CR) use?

Answer 5

Photostimulable phosphors

Question 6

What is required for processing in Computed Radiography (CR)?

Answer 6

Processing in a plate reader

Question 7

How does the efficiency of Computed Radiography (CR) compare to Direct Radiography (DR)?

Answer 7

Less efficient compared to DR

Question 8

What is the size of detector elements (dels) in Direct Radiography?

Answer 8

Approximately 100 microns or 1/10 mm

Question 9

What are the dels part of?

Answer 9

Active Matrix Array

Question 10

What are the three components of a del?

Answer 10

• Semiconductor surface area
• Storage Capacitor
• Thin Film Transistor

Question 11

What is the semiconductor dectector surface area?

Answer 11

Phosphor where the photons land and where they are counted

Question 12

What is the amount of radiation that can fall on the surface called?

Answer 12

Fill factor

Question 13

What does a higher fill factor provide?

Answer 13

Higher signal to noise ratio and increased spatial resolution

Question 14

What does the storage capacitor do?

Answer 14

Stores the electrical charge

Question 15

What does the thin film transistor do?

Answer 15

Acts a gate to release the charge when the del is read out

Question 16

What is phosphor does Direct Conversion in DR use?

Answer 16

Uses Amorphous Selenium (a-Se) to convert X-rays directly to electrical charges

Question 17

What is the advantage of Direct Conversion?

Answer 17

Higher spatial resolution

Question 18

What phosphor does Indirect Conversion in DR use?

Answer 18

Amorphous Silicon (a-Si) with a phosphor screen

Question 19

What is the phosphor screen made up of?

Answer 19

Cesium iodide/gadolinium oxysulfide

Question 20

What process does the phosphor screen conduct?

Answer 20

It is a scintillator and gives off light when stimulated by x-rays

Question 21

What is Detective Quantum Efficiency (DQE)?

Answer 21

Measures detector’s ability to convert X-ray exposure to image

Question 22

What does a higher DQE indicate regarding radiation exposure?

Answer 22

Lower radiation needed for diagnostic image

Question 23

What factors influence Detective Quantum Efficiency (DQE)?

Answer 23

• Del surface area
• Fill factor

Question 24

Which conversion system has the higher detective quantum efficiency?

Answer 24

Indirect conversion

Question 25

What limits spatial resolution in imaging?

Answer 25

Focal spot size (0.5-1.2 mm)

Question 26

What determines the maximum spatial resolution?

Answer 26

Nyquist frequency

Question 27

What is the Nyquist frequency?

Answer 27

The sampling frequency expressed in LP/mm

Question 28

In a DR system what does the sampling frequency depend on?

Answer 28

Del/pixel pitch

Question 29

What is del/pixel pitch?

Answer 29

The distance from the center of one del to the center of the next

Question 30

What is the relationship between spatial resolution and del pitch?

Answer 30

SR= 1/2P

Question 31

What is the Nyquist frequency of a del pitch of 0.1mm?

Answer 31

SR = 1/2P SR=1/2(0.1) SR= 1/.2 SR=5 LP/mm

Question 32

What is the rule of thumb for counting the Nyquist frequency?

Answer 32

You need to sample twice per cycle

Question 33

What are the stages of CR Processing?

Answer 33

* Exposure * Laser Scanning * Signal Amplification * Image Conversion

Question 34

What happens during the Exposure stage of CR Processing?

Answer 34

Photostimulable phosphor stores latent image

Question 35

What phosphors are used in indirect conversion?

Answer 35

Barium fluorobromide and barium fluorochloride

Question 36

Where is the energy stored in the phosphors?

Answer 36

In F trap, which is a small defect that holds the electrons

Question 37

What happens during the Laser Scanning stage of CR Processing?

Answer 37

Red helium neon laser scans the plate and releases the energy stored in the F traps in the form of blue-violet UV light

Question 38

What happens during the Signal Amplification stage of CR Processing?

Answer 38

Blue-violet light is captured in the photomultiplier tube and amplifies signal

Question 39

What happens during the Image Conversion stage of CR Processing?

Answer 39

The amplified blue-violet light is converted to an electrical signal where it then goes to the ADC

Question 40

What is an important practice in both DR and CR?

Answer 40

- Proper cassette/detector handling - Understanding conversion mechanisms - Minimizing radiation exposure - Maintaining image quality

Question 41

Which system is more sensitive to scatter radiation?

Answer 41

CR cassettes, upwards to 10%