Digital Films

Question 1

What are the 2 types of digital image receptors?

Answer 1

1. Solid-state detectors / sensors

• Charged-coupled device (CCD)
• Complementary metal oxide semiconductors (CMOS)
• Flat panel detectors = extra-oral use

2. Photostimulable phosphor (PSP) plates

Question 2

What are the differences between analogue and digital films?

Answer 2

• Processing
  
  • Chemical vs Electronic, time, health & safety, …
• Storage & Transfer
• Image display
• Image resolution
  
  • technically analogue is better with this
  • Analogue = continuous density spectrum – endless shades of grey
  • Digital = numeric format of the image content and its discreteness
• Pixels
  
  • At each pixel of an electronic detector, the absorption of x-rays generates a small voltage
• Dosage
• Others
  
  • Cost, image enhancement, etc.

Question 3

What is CCD?

Answer 3

Charged Coupled Device

• first digital image receptor to be adapted for intraoral imaging
• thin wafer of silicon pixels consisting of n-type silicon and p-type silicon covered with a scintillation layer
• Light and silicon interaction creates charge packet for each pixel (latent image)

Question 4

What is CMOS?

Answer 4

Complementary metal oxide semiconductor

• Basis for typical consumer-grade digital cameras
• Similar in construction to CCDs; consist of an array of pixels covered with a scintillation layer

CMOS Image Read-out

• Each CMOS pixel is isolated from its neighbour and directly connected to a transistor.
• The charge packet from each pixel is transferred to the transistor as a voltage enabling each individual pixel to be assessed separately.

Question 5

What is PSP?

Answer 5

Photostimulable Phosphor Plate

• Europium-doped barium fluorohalide
• Image processing is NOT instant
• Absorb and store energy from x-rays –> latent image
• Release this energy as light (phosphorescence) when stimulated by another light of an appropriate wavelength (= scanner)

Question 6

What is spatial resolution?

Answer 6

the capacity for distinguishing fine detail in an image

Measured and reported in units of line pairs per millimeter

Question 7

What is the spatial resolution for eyes, film, SSD & PSP?

Answer 7

Eyes can detect 6 lp/mm.

Film > 20 lp/mm

SSD ~20 lp/mm

PSP ~ 10 lp/mm

Question 8

What is contrast resolution?

Answer 8

Ability to distinguish different densities in the radiographic image

Question 9

What are the differences between SSD and PSP in clinical factors?

Answer 9

• Patient comfort
  
  • Intraoral SSDs are bulky (5-7mm) & often corded
  • PSPs are similar to films – flexible and thin
• Intra-oral holders
  
  • SSD holders are different
• Exposure
  
  • Computer & software must be ready before exposure
• Processing
  
  • SSDs are immediate
  • PSPs need a scanner
• Sterilisation