Download Computed Tomography: An Overview

Computed Tomography:
An Overview
CLRS 408: Introduction to CT
Rebecca Keith, MS, RT(R)(CT)
Outline:
1. Meaning
2. Process
3. How CT Scanners Work
4. Historical Perspective
5. Digital Image Processing
6. Applications of Volume
Scanning
CT is a Key Diagnostic and Imaging Tool
Cross-sectional imaging
Cancer treatment planning
Trauma / Emergency
Vascular imaging
Cardiology
‘Fusion’ imaging
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In the beginning, there was
Radiography …
Discovered by Wilhelm Roentgen in 1895
◦ 2D photo of a 3D structure
◦ Great for internal anatomy
X-ray tube
Image receptor (IR)
Limitations of Radiography
◦Superimposed anatomic structures
◦Hard to differentiate similar tissues
(poor contrast resolution)
◦Not quantitative
Tomography
“Process of recording a section”
◦ Body Section Radiography and Stratigraphy  1920s
◦ Tomography - 1935
Uses motion of x-ray tube and IR to blur out structures
above & below level set by technologist
◦ Contrast resolution better than radiography
but not as good as CT imaging
◦ Limited use today
◦ Nephrotomograms
◦ Digital Tomosynthesis (breast, chest, bone)
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Conventional Tomography
Beginning of exposure
End of exposure
Anatomy at this level will appear sharp on radiograph
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Anatomy above and below will be blurred
End of exposure
Beginning of exposure
Simulation
Patient with Right Flank Pain
◦Radiography order of KUB
(Kidneys, Ureters, Bladder)
◦Radiopague structure seen
overlying right transverse
process of L3 and psoas muscle
◦Stone in the kidney? Ureter?
Artifact from GI tract?
◦Poor Contrast Resolution
Patient with Right Flank Pain
◦Patient injected with iodinated contrast media for
Intravenous Pyelogram study  conventional tomography
◦Image blurred above & below level of kidneys and ureters
◦Strictures in right ureter,
but is stone inside?
◦Still has poor
Contrast Resolution
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Patient with Right Flank Pain
CT overcomes the limitations in detail and clarity by using
computerized reconstruction
Based on CT, we can tell the
location of the stone is in the
right UPJ
Image Reconstruction from Projections
Image Reconstruction from Projections:
1917  Radon
1980  Herman
1960s  Oldendorf, Kuhl, Edwards (NM)
1963  Cormack
1967  Hounsfield
2 Types of Computed Tomography
Emission CT
◦ To view function
◦ Receive emissions from radioactive agents that
have been introduced into the patient’s body
◦ Single Photon Emission
Computed Tomography (SPECT)
◦ Gamma emissions
◦ Positron Emission Tomography (PET)
◦ Positron emissions
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2 Types of Computed Tomography
Transmission CT
◦ To view anatomical structures
◦ X-ray tube travels around patient at least 360˚
◦ X-ray beam pulses on and off
◦ Transmitted x-rays are detected by detectors
◦ Computer reconstructs data to produce
cross-sectional slices of body
◦ Image reconstructions are made…
Anatomic vs Functional Imaging
Patient is Alive
CT Scan
Patient is Dead
Anatomical information
PET Scan
Functional information
A Little History…
Johann Radon (1917)
◦ Austrian mathematician who developed theory of image reconstruction
◦ 2D or 3D object from large number of projections from different directions
◦ The Radon Transform
From the AAPM
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A Little History…
Dr. Allan Cormack –
◦ Solved mathematical issues of CT
◦ 1963 & 1964 – publishes mathematical analyses of tomographic image
reconstruction in Journal of Applied Physics, but received no interest at the time
◦ Unaware of Radon’s work
◦ Read more >>
From Journal of Applied Physics
A Little History…
Sir Godfrey Hounsfield
◦ 1967 – applied reconstruction techniques to produce
world’s first clinically useful CT scanner:
◦ If x-ray beam were passed thru an object from all directions, and
measurements were made of all x-ray transmissions, information
about the internal structures of that body could be obtained
◦ This info could be shown in form of images showing 3D representations
◦ Unaware of either Radon or Cormack’s work
◦ Read more >>
A Little History…
Sir Godfrey Hounsfield
1967  1st scan
◦ 9 days  2.5 hours to process 28,000 measurements collected by gamma detector
◦ Replaced by x-ray tube, which was more accurate but still took 1 day to produce an image
◦ Worked with Dr. James Ambrose (radiologist)
1971  1st clinical prototype CT brain scanner (EMI Mark 1)
◦ Required water bath
◦ Processing time reduced to 20 mins
◦ Addition of Microcomputers – reduced to 4.5 mins
1972  1st patient scanned
◦
Suspected brain lesion
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A Little History…
Robert Ledley from Georgetown University
◦ Dentist turned biomedical researcher and computing trailblazer
1974 – 1st whole body CT scanner
◦ Automatic Computerized Transverse Axial (ACTA) scanner
◦ No water bath
◦ Original prototype of the ACTA
scanner is at the
Smithsonian Institution
A Little History…
Dr. Willi Kalendar – German Medical Physicist
◦ 1989 – 1st to report on a practical spiral CT scanner at RSNA
◦ Dr. Kalender has made significant contributions to the technical
development and practical implementation of
spiral/volume CT
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Evolution of Terms
◦ Hounsfield called his new technique “Computerized Transverse Axial
Scanning (Tomography)” in British Journal of Radiology in 1973
◦ Other terms seen in journals:
◦
◦
◦
◦
◦
“Computerized Transverse Axial Tomography”
“Computer-Assisted Tomography”
“Computerized Axial Tomography”
“Computerized Transaxial Transmission Reconstructive Tomography”
Patients say they are coming for a “CAT Scan”
◦ “Computed Tomography” established by RSNA in its journal Radiology
◦ This is what we use today!
Overview of CT Imaging Process
1.
2.
3.
Data acquisition
Image Reconstruction
Image Display, Manipulation, Storage, Recording
and Communication
Data Acquisition
How we collect information from the patient!
X-rays pass through patient
◦ Some are absorbed by the patient
◦ Some are scattered
◦ Some are transmitted onto detectors that measure
the transmission values (attenuation values)
◦ Enough x-rays must be transmitted to meet
requirements of recon process!
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Image Reconstruction
Image reconstruction algorithms  transmission
data sent to computer for processing
Hounsfield used algebraic reconstruction technique
Today we use:
Filtered Back Projection (FBP)
Adaptive Statistic Iterative Reconstruction (ASIR)
Image Display, Storage, and Communication
After processing, reconstructed image is displayed
◦ Text says CRT monitors but …LCD and Flat panel being used 
high-res is key!
Post-processing occurs here
◦ Reformattes to coronal, sagittal, oblique, para-axial, etc
◦ 3D, edge enhancements, other manipulations
Storage
◦ PACS  has to be DICOM image
◦ Magnetic Tapes / Laser Disks?
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First Generation
Pencil beam
Single detector
Translate-Rotate
180 /1 degree
5 minutes
Second Generation
Multiple pencil beams
Multiple detectors
Translate-Rotate
180/10 degree
30 sec
Third Generation
Fan beam
Multiple detectors
360 rotate
1 sec
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Fourth Generation
Stationary detectors
Moving tube
Fan beam
Thousands of detectors
1 sec or less
Rapid Advances
1990 - Spiral CT
1994 - Sub-second spiral
1998 – Multislice (Multidetector) Spiral
2005 – Dual source CT
Volume Scanning
Lots of data
Quick acquisition
Leads to:
◦ CT angiography/cardiac CT
◦ 3D/4D imaging
◦ Virtual reality imaging
◦ PET/CT; SPECT/CT
◦ CT simulation
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Advances or Generations?
• Electron Beam CT (EBCT)
• Cardiac Uses
• 5th Generation?
Advances or Generations?
• Dual Source CT
• Multiple Sources/Detectors?!
• Sixth Generation?
Advances or Generations?
• Flat Panel Technology
• High spatial resolution volumetric imaging and dynamic CT scanning
• Diagnostic and interventional clinical purposes
• 7th Generation?
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Uses outside Medicine
Lumber industry
Paleoanthropology
Stock breeding
Explosives detection
The End
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