CT

1
CT

• Seeram Chapter 1
• Computed Tomography
• - An Overview

2
Early History

• tomos
• Greek word meaning section
• Sectional imaging methods first developed in
  1920s

3
Early HistoryConventional 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

4
Conventional vs Axial Tomography
Conventional Cut
CT Axial Cut
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Conventional Tomography Blurring

• Image produced on film
• Objects above or below fulcrum plane change
  position on film thus blur

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CT Image

• Not produced on film
• Mathematically reconstructed from many projection
  measurements of radiation intensity
• Digital Image calculated

Acme Mini- Compu- ter
Digital Image
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How Did We Go From
8
The story concerns these men. What was their
Link?
???
Geoff
Paul, Ringo, George, John
9
It Was the Late 1960s
10
A lot of the money was going here
11
Follow the Money
12
Measure Intensity of a Pencil Beam
X-Ray Source
Radiation Detector
13
CT Image

• Measure a bunch of pencil beam intensities

14
CT Image

• Now make measurements from every angle

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CT Image

• When you get done, multiple pencil beams have
  gone through every point in body

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Digital Image

• 2-dimensional array of individual image points
  calculated
• each point called a pixel
• picture element
• each pixel has a value
• value represents x-ray transmission (attenuation)

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Digital Image Matrix
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Numbers / Gray Shades

• Each number of a digital image corresponds to a
  gray shade for one pixel

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Image Reconstruction
X-Ray Source
Acme Mini- Computer
Pixel (calculated) Data
Radiation Detector
Projection (raw) Data
20
Image Reconstruction

• CT math developed in 1910s
• Other Applications
• astronomy (sun spot mapping)
• electron microscope imaging
• Nuclear medicine emission tomography
• MRI

21
CT History

• First test images in 1967
• First clinical images 1971
• First commercial scanner 1972

22
CT History

• CT math developed in 1910s
• First commercial scanner 1972
• What took so long?

23
CT History

• CT made possible by high speed minicomputer

24
CT Computers

• Old mainframe computers too expensive bulky to
  be dedicated to CT

25
The 1st Computer Bug
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CT history - Obsolete Terminology

• CTAT
• computerized transverse axial tomography
• CAT
• computerized axial tomography
• CTTRT
• computerized transaxial transmission
  reconstructive tomography
• RT
• reconstructive tomography

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Data Acquisition

• cross sectional image reconstructed from many
  straight line transmission measurements made in
  different directions

Tube
Detector
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Translate / Rotate
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CT Early Units

• 4 minute scans
• 5 minute reconstruction
• 80 X 80 matrix
• head only
• water bag fit tightly around head
• Original computer software couldnt deal with
  transition from skull to air

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Beam Translation

• beam collimated to small round spot
• collimated at tube and collimator

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Beam Translation

• Tube/detector translates left to right
• Entire assembly rotates 1o to right
• Tube/detector translates right to left

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Translate - Rotate

• 180 translations in alternate directions
• 1 degree rotational increments between
  translations

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Projection Measurements

• Radiation detector generates a voltage
  proportional to radiation intensity

34
Image Reconstruction

• Minicomputer does its thing

Analog to Digital (A to D) conversion
35
Digital Image Matrix

• Digital Matrix contains many numbers which may be
• Displayed on CRT
• Manipulated
• Stored

36
Digital Image Manipulation

• Window
• Level
• Smoothing
• Edge enhancement
• Slice reformatting
• 3D
• derived from multiple axial slices

37
Digital Image Storage

• Magnetic Disk
• CD
• Tape
• Optical Disk
• PACS archive
• picture archival and communications system
• not part of CT
• contains images from many modalities
• allows viewing on connected computers

38
CT - Improvements

• all CT generations measure same multi-line
  transmission intensities in many directions
• Improvements
• Protocol for obtaining many line transmissions
• of line transmissions obtained simultaneously
• detector location
• Overall acquisition speed

39
2nd Generation CT

• arc beam used instead of pencil beam
• several detectors instead of just one
• detectors intercepted arc
• radiation absorbent septa between detectors
• reduced scatter
• acted like grid

Tube
Detectors
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2nd Generation CT
10o

• arc beam allowed 10 degree rotational increments
• scan times reduced
• 20 sec - 2 min
• 2 slices obtained simultaneously
• double row of detectors

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3rd Generation CT

• Wide angle fan beam
• rotational motion only / no translation
• detectors rotate with tube
• 30o beam
• Many more detectors
• scan times lt 10 seconds

42
3rd Generation CT
Z-axis orientation perpendicular to page
Patient
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4th Generation CT

• Fixed annulus of detectors
• tube rotates (no translation) inside stationary
  detector ring
• only a fraction of detectors active at once

44
3rd 4th 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

45
3rd / 4th Generation Image Quality Improvements

• Faster scan times
• reduces motion artifacts
• Improved spatial resolution
• Improved contrast resolution
• Increased tube heat capacity
• less wait between scans / patients
• better throughput

46
Spiral CT

• Continuous rotation of gantry
• Patient moves slowly through gantry
• cables of old scanners allowed only 360o rotation
  (or just a little more)
• tube had to stop and reverse direction
• no imaging done during this time
• no delay between slices
• dynamic studies now limited only by tube heating
  considerations

47
Spiral CT
Z-axis orientation perpendicular to page
Patient
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Multi-slice CT

• Multiple rows of fan beam detectors
• Wider fan beam in axial direction
• Table moves much faster
• Substantially greater throughput

49
Computer Improvements

• Reconstruction time
• Auto-printing protocols
• Image manipulation
• Backup time
• Slice reformatting
• 3D reconstruction

And the ability to do it all simultaneously
50
Cine CT (Imatron)

• four tungsten target rings surround patient
• replaces conventional x-ray tube
• no moving parts
• like 4 moving focal spots
• electron beam sweeps over each annular target
  ring
• can be done at electronic speeds
• 2 detector rings
• 2 slices detected
• maximum scan rate
• 24 frames per second

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Imatron Cine CT (scanned from Medical Imaging
Physics, Hendee)
53
CT Patient Dose

• In theory only image plane exposed
• In reality adjacent slices get some exposure
  because
• x-ray beam diverges
• interslice scatter

54
Dose Protocols

• Plain X-ray
• entrance skin exposure
• Mammography
• mean glandular dose
• CT
• Computer tomography dose index (CTDI)
• Multiple-scan average dose (MSAD)

55
CT Dose depends on

• kVp
• mA
• time
• slice thickness
• filtration

• Noise
• detector efficiency
• collimation
• matrix resolution
• reconstruction algorithm

56
CT Patient Dose

• Typically 2 - 4 rad
• AAPM has a single slice protocol for measuring
  head body doses
• More dose required to improve spatial resolution
  for same noise level
• smaller pixels
• need to increase does to get same dose / pixel
• More dose required to improve noise for same
  spatial resolution

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New Stuff

• CT Angiography
• CT fluoroscopy
• CT virtual endoscopy / colonoscopy / ??scopy