dig1 DRprinciples

Digital Radiography


Basic Concepts


Image Quality Concepts
– Spatial Resolution (limiting resolution)
– Noise: Quantum Mottle



Nature of the Digital Image
– Spatial Digitization
– Analog-to-Digital Conversion



Digital Radiography Factors
–
–
–

Spatial Digitization and Resolution
ADC and Noise
ADC and Dynamic Range


Basic Concepts: Limiting Resolution


Limiting Resolution (simplest form):

refers to the smallest, closely spaced objects
for which separate images can be seen


Measurement and Units: Bar pattern


Basic Concepts: Limiting Resolution
Limiting Resolution (simplest
form):
 Measurement and Units: Bar
pattern
Measured using bar pattern


(lead strips separated
by spaces) and
expressed as smallest visible bar size
or
highest
spatial frequency (line-pairs/mm)


Sources Blurring in radiography:
–
–
–

Focal spot (all types of radiography)
Motion (all types of radiography)
Receptor blur - depends on receptor


Radiography Image Receptor Blur


Dual Receptors (Screens)


Noise and Image Quality


Image Noise: Quantum mottle


Quantum mottle (QM) refers to the
“graininess” of x-ray images



QM is caused by using a limited number
of x-ray photons to make an image



QM interferes with ability to details



Using more photons (more mAs)
reduces noise but increases radiation
exposure


The Nature of the Digital Image
Basic Concepts: Resolution and Noise
 The Digitization Process


– Spatial Digitization
– Analog-to-Digital Conversion (ADC)


Radiation Dose, Noise and Resolution
– Resolution versus Dose: receptor thickness
– Dose versus Image Noise (Quantum mottle)



Dynamic Range


The Digitization Process


Every “image” starts out in analog
form:
–

–

“light” image emitted by screen
“light” image from intensifier output
phosphor
– TV camera voltages
– Stimulated light from computed radiography


Analog “image” must be converted
(digitized) to matrix of pixels stored
as binary numbers


Spatial Digitization (pixels): Sampling




Must “measure”
image along many rows
(512, 1024, etc) and at
many point along each
row
Sampling done by:
– detector with discrete
“elements” (eg, CCD
camera, flat panel
detector) or
– Raster scan process

Matrix Size, Resolution and Bytes
Regular Film/Screen: 5 line-pairs/mm
 To “Equal” with Digital Image :


– 5 lp/mm = 10 pixels/mm (to see 5 bars+5 spaces)
– 35 x 43 cm (14 x 17”) image = 350 x 430 mm
– 350 x 430 mm at 10 pixels/mm = 3500 x 4300
pixels
– 3500 x 4300 x 2 bytes/pixel (16 bits/pixel) = 30
MB


Digital Radiography
– Typically 2000 x 2500 pixels maximum (~3
lp/mm)


Digital Spatial Resolution


Spatial Digizitation: ADC


ADC and Noise: How many bits?


Contrast vs Latitude (Dyamic Range)

ADC and Dynamic Range


Suppose we have:
– 10 bit ADC: (1024 graylevels)
– 1000:1 dynamic range (e.g. we can measure and
record exposures from 1 mR to 1000 mR (1 R):



Need 1 mR difference for different
graylevel
– Differences between structures to see in image may
be < 1 mR in x-ray intensity reaching the receptor



Alternatives:
–
–

“throw out” some dynamic range (limit range)
Increase number of bits (still uncommon)


Digital Detectors
Cassette-based: Image Storage Phosphor
(CR)

 Image Intensifier
 Scanned Projection
 Direct Digitizing (Full Field)


–
–
–


CCD Camera
Selenium Flat Panel (“Direct” Digital Radiography)
Phosphor Flat Panel (“Indirect” Digital
Radiography)

Future Technology


Digital Detectors
Cassette based Image Storage Phosphor
(CR)
 Image Intensifier
 Scanned Projection
 Direct Digitizing (Full Field)


–
–
–


CCD Camera
Selenium Flat Panel (“Direct” Digital Radiography)
Phosphor Flat Panel (“Indirect” Digital
Radiography)

Future Technology


CR Clinical Use



Conventional CR Scanning


Flying Spot CR
Scan
In a conventional
flying spot CR
reader, stimulated
output exposure
(scan level) from
the IP is
proportional to the
laser intensity I
and dwell time Td


Absorption Efficiency

CR Blur