Transcript Advanced Physics PCS 436

CT
 Seeram: Chapter 1
Computed
Tomography
- An
Overview
Early History
 “tomos”
 Greek word meaning section
 Sectional imaging methods first developed in
1920’s
Early History:
Conventional Tomography
 first used in 1935
 image produced on film
 Image plane oriented
parallel to film
 Anatomy in plane of
fulcrum stays in focus
 anatomy outside of
fulcrum plane
mechanically blurred
Conventional Tomography Blurring
 Image produced on film
 Objects above or below
fulcrum plane change position
on film & thus blur
Conventional vs Axial
Tomography
Conventional Cut
CT Axial Cut
CT Image
 Not produced on film
 Mathematically reconstructed from many
projection measurements of radiation intensity
 Digital Image calculated
Acme
MiniComputer
Digital Image
How Did We Go From…
The story concerns these men.
What was their Link?
???
Godfrey
Paul, Ringo, George, & John
It Was the Late 1960’s
A lot of the money was going here
Follow the Money
Electronic and Musical
Industries LTD
Measure Intensity of a Pencil Beam
X-Ray
Source
Radiation
Detector
CT Image
 Measure a bunch of pencil beam intensities
CT Image
 Now make measurements from every angle
CT Image
 When you get done, multiple pencil
beams have gone through every point
in body
Image Reconstruction
X-Ray
Source
Acme
MiniComputer
Radiation
Detector
Projection
(raw)
Data
Pixel
(calculated)
Data
Digital Image
 2-dimensional array of image points
 each point called a pixel
 picture element
 each pixel has a value
 value represents x-ray transmission
(attenuation)
Digital Image Matrix
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Numbers / Gray Shades
 Each number of a digital image corresponds to
a gray shade for one pixel
Image Reconstruction
 Math developed in 1910’s
 Other Applications
 Astronomy (sun spot mapping)
 Electron microscope imaging
 Nuclear medicine emission tomography
 MRI
Acme
MiniComputer
Digital Image
CT History
 First test images in 1967
 First clinical images ~ 1971
 First commercial scanner 1972
CT History
 CT math developed in 1910’s
 First commercial scanner 1972
 What took so long?
CT History
 CT made possible by high speed minicomputer
CT Computers
 Old mainframe computers too expensive & bulky to be
dedicated to CT
The
st
1
Computer Bug
Data Acquisition
 cross sectional image reconstructed from
many line transmission measurements
made in different directions
Tube
Detector
Translate / Rotate
CT Early Units
 4 minute scans
 1 slice
 5 minute reconstruction
 80 X 80 matrix
 head only
 water bag fit tightly around head
Beam Translation
 beam collimated to small round spot
 collimated at tube and collimator
X-ray
Tube
Detector
Beam Translation
 Tube/detector translates left to right
 Entire assembly rotates 1o to right
 Tube/detector translates right to left
X-ray
Tube
Detector
Translate - Rotate
 180 translations in alternate directions
 1 degree rotational increments between translations
Projection Measurements
 Radiation detector generates a voltage
proportional to radiation intensity
Image Reconstruction
 Minicomputer does its thing
Analog to Digital
(A to D)
conversion
Digital Image Matrix
 Digital Matrix contains many numbers
which may be
 Displayed on monitor
 Manipulated
 Stored
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Digital Image Manipulation
 Window
 Level
 Smoothing
 Edge enhancement
 Slice reformatting
 3D
 derived from multiple axial slices
Digital Image Storage
 Magnetic Disk
 CD
 Tape
 Optical Disk
 PACS archive
 picture archival and communications system
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not part of CT
contains images from many modalities
allows viewing on connected computers
CT - Improvements
 All CT generations measure same multi-line
transmission intensities in many directions
 Improvements
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Protocol for obtaining many line transmissions
# of line transmissions obtained simultaneously
Detector location
Overall acquisition speed
2nd Generation CT
 arc beam allowed 10 degree
rotational increments
 scan times reduced
 20 sec - 2 min
 2 slices obtained
simultaneously
 double row of detectors
10o
3rd Generation CT
 Wide angle fan beam
 Rotational motion only
/ no translation
 detectors rotate with tube
 30o beam
 Many more detectors
 Scan times < 10 seconds
3rd Generation CT
Z-axis orientation
perpendicular to page
Patient
3rd Generation (Non-spiral) CT
 Tube rotates once around patient
 Table stationary
 data for one slice collected
 Table increments one slice thickness
 Repeat
 Tube rotates opposite direction
3rd Generation Image Quality
Improvements
 Faster scan times
 reduces motion artifacts
 Improved spatial resolution
 Improved contrast resolution
 Increased tube heat capacity
 less delay between scans / patients
 Increased throughput
Spiral CT (late 1980’s)
 Continuous rotation of gantry
 Slip ring technology
 Patient moves slowly but continuously
through gantry
 No dead time as gantry
reverses
 Much faster
Spiral CT
Z-axis orientation
perpendicular to page
Patient
Multi-slice CT (2000’s)
 Multiple rows of fan beam detectors
 Wider fan beam in axial direction
 Table moves much faster
 Substantially greater throughput
Computer Improvements
 Virtually instantaneous reconstruction
time
 Auto
 Window protocols
 Transmission to PACS
 Backup
 Image manipulation
 Slice reformatting
 3D reconstruction
And the ability to
do it all
simultaneously
Fundamental CT Tradeoff
 Typically phantom dose: 1-2 rad (10-20 mSv)
To improve one requires
compromise on another
Noise
Resolution
Dose
CT Usage
 16% of imaging procedures
 23% of total per capita exposure
 49% of medical exposure
CT Usage
 Annual growth
 U.S. Population: <1%
 CT Procedures: >10%
 ~ 67,000,000 procedures in
2006
 about 10% pediatric CT
Computed Tomography — An Increasing Source of Radiation Exposure
David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.
New England Journal of Medicine, 2007
How many children’s lives are
saved by CT?
6/19/2001
“Each year, about 1.6 million children in
the USA get CT scans to the head and
abdomen — and about 1,500 of those
will die later in life of radiationinduced cancer …
•Medical imaging procedures should be
appropriate & conducted at the lowest
radiation dose consistent with acquisition of
desired information
•Discussion of dose risks should be
accompanied by acknowledgement of
procedure benefits
•Risks of medical imaging at effective doses <
50 mSv (5 rad) for single procedures or 100
mSv(10 rad) for multiple procedures over
short time periods are too low to be detectable
& may be nonexistent.
• Predictions of hypothetical cancer incidence
and deaths in patients exposed to such low
doses are
• highly speculative
• should be discouraged
•These predictions are harmful because
they lead to sensationalistic articles …
that cause some patients & parents to
refuse imaging procedures, placing them
at substantial risk by not receiving the
clinical benefits of the prescribed
procedures