L4 Digital Imaging

Introduction To Digital Imaging In Dentistry¹

Learning Objectives²

• Understand differences between analogue and digital imaging
• Know different types of imaging sensors
• What are advantages & disadvantages of digital imaging?

Recommended Textbooks³

• Essentials of dental radiography and radiology 5th edition by Whaites, Eric; Drage, Nicholas; Churchill Livingstone 2013
• Pocket atlas of dental radiology by Pasler, Friedrich A; Visser, Heiko; Flexibook, 2007
• Oral radiology: principles and interpretation 7th edition by White, Stuart C; Pharoah, M. J; Permalink 2014
  • 8th edition available now

Comparison Of Analogue And Digital Systems

Film vs Digital Receptors⁴

Processing

• Chemical vs Electronic, time, health & safety, …

Storage & Transfer

Image display

Image resolution

• Analogue = continuous density spectrum
• 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.

Digital Image Fundamentals And Pixels⁵

FIG. 4.1 A digital image is made up of a large number of discrete picture elements (pixels). The pixels are so small that the image appears smooth at normal magnification. The location of each pixel is uniquely identified by row and column coordinates within the image matrix. The value assigned to a pixel represents the intensity (gray level) of the image at that location.

pixel	350	351	352	353	354	355	356	357
261	228	222	184	107	76	92	90	98
262	227	218	186	110	90	104	103	98
263	222	219	181	107	97	107	102	104
264	225	217	176	107	98	100	100	107
265	221	204	159	107	105	101	107	102
266	217	196	157	114	105	104	106	100
267	209	190	154	114	107	103	97	100
268	202	195	166	118	102	102	92	94
269	197	196	168	122	98	102	90	94
270	195	190	166	130	104	105	92	97
271	199	190	172	144	111	107	100	106
272	201	193	177	160	120	103	112	106
273	203	195	181	166	129	102	111	106
274	201	200	186	172	133	110	112	102

Analog To Digital Conversion Process⁶

Analog-to-Digital Conversion (ADC)⁷

• Ie. From photons/voltages to digital Image
• Involves TWO steps
  1. Sampling
     • A range of voltages are grouped together
  2. Quantisation
     • Each sampled range is assigned a grey value
• Large sampling = ‘pixelated’ image ‘B’
• Narrow sampling = close to original signal ‘C’

1 2 3 4 5 6 7 8

8 pixel numbers

8 shades of grey

2 8 2 8 2
8 4 6 4 8
2 6 1 6 2
8 4 6 4 8
2 8 2 8 2

Each pixel allocated a number

Each pixel allocated a shade of grey

A B
C D

Digital Image Receptors⁸

• Two types
  • Solid-state detectors / sensors
    • Charged-coupled device (CCD)
    • Complementary metal oxide semiconductors (CMOS)
    • Flat panel detectors → extra-oral use
  • Photostimulable phosphor (PSP) plates

Solid State Detectors

CCD⁹

• charge-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)

A
B
C
Scintillation layer
CCD pixels
Scintillation layer
P-type silicon
N-type silicon
Insulating layer
Electrodes

Charge Coupled Device Technology

Photoelectric absorption in silicon:

Conduction band
-
Electron
e⁻
Hole
Valence band
+
e⁻

CCD Image Read-out

• “bucket brigade”
• As a charge reaches the end of its row, it is transferred to a readout amplifier and transmitted as a voltage to the ADC located within or connected to the computer

Complementary Metal Oxide Semiconductor Technology

CMOS (Complementary Metal Oxide Semiconductor) X-ray Detector

Explosion View of CMOS Sensor

Components:

• Back Housing + Cable
• Electronic Substrate
• CMOS Imaging Chip
• Fiber-optic Face Plate
• Scintillator Screen
• Front Housing

Direction of X-ray Beam:

CMOS Image Read-out

• CCD
• CMOS
• Photon to electron conversion
• Conversion: Charge to electrical signal
• Two different principles: CCD vs CMOS

Photostimulable Phosphor Plates¹⁰

PSP¹¹

• Photostimulable Phosphor
• 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)

Image Quality And Resolution

• Capacity for distinguishing fine detail in an image
• Measured and reported in units of line pairs per millimeter
• Eyes can detect 6 lp/mm.
  • Film > 20 lp/mm
  • SSD ~ 20 lp/mm
  • PSP ~ 10 lp/mm
• The smaller the size of the pixel, the higher the maximally attainable resolution

Spatial Resolution¹²

Contrast Resolution¹³

• Ability to distinguish different densities in the radiographic image
• Current digital detectors capture data at 8, 10, 12, or 16 bits.
• Conventional computer monitors are capable of displaying a grey scale of only 8 bits (2^8 = 256 shades of grey)
• The human eyes in darkroom / reporting room are capable of distinguishing 60 grey levels
  • Less than 30 grey levels in operatory dental clinics

Image Manipulation And Enhancement¹⁴¹⁵

• Magnification
• Brightness
• Contrast
• Inversion
• Sharpening
• Smoothing

5.9 Examples of digital image enhancement.

• A Original image.
• B Inverted/reversed.
• C Altered contrast.
• D Embossed/pseudo 3-D.
• E Automated measurement.
• F Magnified.
• G and H Pseudocoloured.

Clinical Comparison Of Solid State Detectors And Phosphor Plates¹⁶¹⁷

• SSD vs PSP (clinical factors)
• 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

END.

Footnotes

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17. Original PDF page 20: L4 Digital Imaging, p.20 ↩