Transcript receptor digital

Fundamentals
of Digital
Radiology
(year 1)
George David, MS, FAAPM, FACR
Medical College of Georgia
Digital Image Formation
 Place mesh over image
Digital Image Formation
 Assign each square
(pixel) a number based
on density
 Numbers form the
digital image
194
73
22
Digital Image Formation
 The finer the mesh, the better the digital rendering
Numbers / Gray Shades
 Each number of a digital image corresponds to a
gray shade for one picture element or pixel
So what is a digital image?
 Image stored as 2D array of #’s representing
some image attribute such as
 optical density
 x-ray attenuation
 echo intensity
 magnetization
125
25
311
111
182
222
176
199
192
85
69
133
149
112
77
103
118
139
154
125
120
145
301
256
223
287
256
225
178
322
325
299
353
333
300
Computer Storage
125
25
311
111
182
222
176
199
192
85
69
133
149
112
77
103
118
139
154
125
120
145
301
256
223
287
256
225
178
322
325
299
353
333
300
125, 25, 311, 111, 182, 222,
176, 199, 192, 85, 69,
133, 149, 112, 77, 103,
118, 139, 154, 125, 120,
145, 301, 256, 223, 287,
256, 225, 178, 322, 325,
299, 353, 333, 300
Digital Copies
 Digital copies are identical
 All digital images are originals
Image Matrix
 Doubling the matrix dimension quadruples
the # pixels
111
118
125
25
311
111
199
192
85
69
77
103
118
139
145
301
256
223
87
155
2 X 2 Matrix
4 pixels
4 X 4 Matrix
16 pixels
Matrix Size & Resolution
More pixels = better spatial resolution
The Bit
 Fundamental unit of
computer storage
# of unique values which can be
represented by 1 bit
2 unique combinations / values
1
2
# of unique values which can be
represented by 2 bits
1
2
4 unique combinations / values
3
4
# of unique values which can be
represented by 3 bits
1
5
2
6
3
7
4
8
8 unique combinations / values
Bit Depth & Contrast Resolution
 bit depth indicates # of possible brightness levels for a
pixel
 The more bits per pixel the more possible gray shades
and the better contrast resolution.
2 bit; 4 grade shades
8 bits; 256 grade shades
Computed Radiography (CR)
 Re-usable metal imaging plates replace film &
cassette
 Uses conventional bucky & x-ray equipment
 No electronics in receptor
 Requires external reader
CR Exposure & Readout
CR Operation
 after read-out, plate erased using a bright light
 plate can be erased virtually without limit
 Plate life defined not by erasure cycles but by
physical wear
Film Screen vs. CR Latitude
CR Latitude:
.01 – 100 mR
100
Digital Radiography (DR)
 Digital bucky
 Electronic
 Incorporated
into x-ray
equipment
Digital Radiography (DR)
 Receptor provides direct digital output
 No processor / reader required
 Images available in ~ 5 seconds
 Much faster throughput

Eliminates many steps for technologist
Raw Data Image
 Unprocessed image as read from
receptor
 Not a readable diagnostic image
 Requires computer post-processing
 Specific software algorithms must be applied
to image prior to presenting it as finished
radiograph
Enhancing Raw Image (Image
Segmentation)
*
Identify collimated image border
Separate raw radiation from anatomy
Apply appropriate tone-scale to image
1.
2.
3.

Done with look-up table (LUT)
This process is
specific to a
particular body
part and
projection
Image Segmentation
 Computer establishes collimated
border of image
• Computer defines anatomic
region
• Finished image produced by
tone scaling
Requires histogram analysis of
anatomic region
Image Post-Processing
 Histogram
 Graph showing how much
of image is exposed at
various levels
 Tone scaling
 Body part & projectionspecific algorithms
 Must correctly identify
anatomical region
 Histogram used to display
image with proper
 Density
 Contrast
Film/Screen Limited Latitude
 Film use has
little ambiguity
about proper
radiation
exposure
CR / DR Latitude
DANGER
Will
Robinson!!!
 Almost impossible to under or
overexpose CR / DR
 Underexposures look noisy
 Overexposures look GOOD!!!
So how do I know if exposure is optimum by
looking at my image?
Exposure Index
 Each manufacturer provides feedback to technologist on
exposure to digital receptor
 Displayed on PACS monitor
Calculated Exposure Index
Affected by
 X-Ray technique selection
 Improper centering of image on cassette
 Improper selection of study or projection
 Placing two or more views on same cassette
 Can cause image to appear dark
Shifting Gears:
Fluoroscopy Issues
Digital Video Sources
 DR type image receptor
 Conventional Image Intensifier with Video
Signal Digitized (“Frame Grabber”)
I
Image
T m
u a
b g
e e
Tube
X-Ray
Input
TV
Lens System
Amplfier
Analog
to
Digital
Convert
er
Digital
Memory
(Computer)
Digital Spot Film
 Frame grabber digitizes image
 Digital image saved by computer
Last Image Hold
 Computer displays last fluoro image
 Allows operator to review static processes
without keeping beam on
Fluoro Frame Averaging
 Conventional fluoro only displays current frame
 Frame averaging allows computer to average
current with user-selectable number of previous
frames
 Averages current frame & history
Fluoro Frame Averaging
Tradeoff
 Advantage:
 Reduces quantum noise
 Disadvantage
 Because history frames are averaged with current
frame, any motion can result in lag
Other Fluoro Features
 Real-time Edge Enhancement / Image Filtering
 Option of using lower frame rates (15, 7.5, 3.75
fps rather than 30)
 computer displays last frame until next one

reduces flicker
 Lowers patient and scatter exposure

Exposure proportional to frame rate
 dynamic studies may be jumpy
Digital Possibilities
 Multi-modality
imaging / Image
fusion
 PET/CT
DR & Energy Subtraction
 2 images taken milliseconds apart at 2
kVp’s
 Combine / subtract images
Soft Tissue Image
Bone Image