Transcript Computed Tomography RAD309 Dr. Eng. Sarah Hagi

Computed
Tomography
RAD309
Dr. Eng. Sarah Hagi MSc (USA) PhD (UK)
Course
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Dr. Mawya Khafaji
Book
Practical
TLD’s
Grading:
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Continuous Assessment (40%)
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Midterm, Quizzes, HM, Practical
Final Exams (60%)
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Written (40%)
Practical (20%)
Course Outline
Topic
Instructor
History, evolution of technology , process overview
S.Hagi
Computer Technology and the use of Computers in Radiography Physical Principles and instrumentation of
CT
S.Hagi
Principles of CT, Characteristics of X-radiation, CT beam attenuation, and linear attenuation coefficients
S.Hagi
Data Analysis
S.Hagi
CT generations and Spiral CT
S.Hagi
Components of CT Scanner, Gantry assembly (patient aperture, rotating frame,xray tube, collimator, and
detectors), patient table, operator console, CT computer and workstations
S.Hagi
Tissue Characteristics and Hounsfield attenuation numbers application
M.Khafaji
Data acquisition and manipulation, image reconstruction algorithms, such as filtered back projection and
transform
M.Khafaji
Image Quality and operating console parameters
M.Khafaji
Dose, technical parameters for dose measurement/possible reduction methods
M.Khafaji
Quality Assurance of computed tomography
M.Khafaji
WEEK
Begining Date
Lectures
Instructor
1
March 7th
Lect1&2
S.Hagi
2
March14th
Lect3&4
S.Hagi
3
March21st
Lect5&6
S.Hagi
4
March28th
Lect7&8
S.Hagi
5
April 4th
Lect9&10
S.Hagi
6
April 11th
revision
S.Hagi
7
April 18th
Midterm
S.Hagi
8
April 25th
Break
9
May 2nd
Lect11&12
M.Khafaji
10
May 9th
Lect13&14
M.Khafaji
11
May16th
Lect15&16
M.Khafaji
12
May23rd
lect17&18
M.Khafaji
13
May30th
Lect19&20
M.Khafaji
14
June6th
revision
M.Khafaji
15
June13th
revision
M.Khafaji
16
June20th
Final
M.Khafaji
Computed Tomography RAD309 Practical
Second Semester, Third Year
Conducted by Clinical Instructor
1. Visit to different hospitals to see components of available generations of
CT in the field of Medical Imaging
2. Group discussion-Physical principles and instrumentation involved in CT
3. Group discussion-characteristics of x radiation, CT beam attenuation, linear
attenuation coefficient
4. Tissue characteristics and Hounsfield attenuation numbers application,
demonstrated on display console-to show hounsfield numbers of different tissue
5. Data acquisition and manipulation, image reconstruction algorithms, such as
filtered back projection and transform
6. Group discussion on problem based learning of data acquisition and manipulation,
image reconstruction algothirms
7. Demonstration of components including functions, CT scanner, Gantry assembly,
patient table, operator console, CT computer and workstation
8. Demonstration -Operation of Scan console, to enter patient data, selection of scan
parameters
9. Demonstration -Operation of Display console manipulate scan data and post
processing such as Multiplanar reformatting....
Introduction
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A method of examining body organs by
scanning them with X rays and using a
computer to construct a series of crosssectional scans along a single axis
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Acquiring and reconstructing, thin cross
section on the basis of measurement of
attenuation
Tomography Since 1900s
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Several researchers were interested in a specific layer or section
to represent a single slice of the body on radiographic film
Tomo- section in Greek
Transverse section were developed by Watson (transverse= cross
section)
But with not enough details, clarity, not fully utilized as a clinical tool
Conventional Tomography
 Provides 2D view of a 3D distribution of an object (x,y,z) with
superimposition of all structures Disadvantages:
 depth information is lost
 overlapping structures may interfere with diagnosis, and small
differences in contrast is lost
 No quantitative
Method
The idea is based on simple principles of projective
geometry:
 x ray source on one side of the object and the film on
the other (diagonal)
 Source and detector move at constant rate opposite
directions
 Source and detector distance, rate of motion, adjusted
such that objects in the imaging plane project to the
same relative location on the film.
 Objects out of the plane, blurred
Tomography had been one of the pillars of radiologic
diagnostics until the late 1970s
Goals of CT
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To overcome superimposition of structures
To improve contrast
To measure small differences in tissue
contrast
CT came to solve the problems of
tomography and conventional radiography,
image reconstruction from projections
Solution
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Transmitting a collimated beam through a
cross section of the body
Detectors, measuring small differences in
tissue contrast
Computer that allows data manipulation
and reconstruction
The Birth of CT
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1972
Nobel Prize in 1979, Sir Godfrey Hounsfield
& Alan Cormack “ computer assisted
tomography”
Solved the problem of conventional
Image Reconstruction History
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1917 Radon proved that it was possible to
mathematically build an image from large
number of its projections (different angles)
Has been widely used in many fields
Images of the body can be reconstructed
from a large number of projections from
diff. locations
Theory
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When radiation passes through an objects, some of it is
absorbed, scattered attenuation (which we will discuss in
details in another lecture)
Attenuation measurements is the basis of CT imaging
Radiation passes through each section in a specific way
(depending on the tissue properties/characteristics) onto a
detector that sends signal to a computer for processing
Computer produces clear, sharp image of internal structure of
the object
This doesn’t happen spontaneously, there are algorithms
mathematical computations to put the projections together
and produce the image data
Cont.
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All trials to make use of image
reconstruction techniques in radiology were
not successful
Technology barrier, computers
In 1967 Hounsfield applied reconstruction
techniques to produce the first clinically
useful scanner (used only for brain
imaging)
Who is He?
Production of X-Ray
“fast-moving electrons slam into a metal
object, x-rays are produced”
 Roentgen experimented with electron
beam in a vacuum tube surrounded
by cardboard
 He noticed light/glowing spots on
fluorescent screen
 Next, started to place objects b/w the
screen and the tube
 Finally he placed his wife’s hand in
front of the screen
Historical Developments
1895
1896-98
Wilhelm Roentgen
Henri Becqurel and the Curie’s
1946
Discovery of X-ray
Discovery of natural
radioactivity
Nobel Prize (1st physics)
Mathematical basis and
concepts of image
reconstruction
Discovery of NMR principles
1972
1973
Invention of CT
Producing MR imaging
Hounsfield and Cormack
Lauterber and P Mansfield
1979
2003
Nobel Prize in Medicine
Nobel in Physol.& Medicine
Hounsfield and Cormack
Lauterber and Mansfield
1901
1917
Roentgen
Radon
Flex Bloch and Edward Purcell
Hounsfields Invention
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Sir Godfrey N. Hounsfield, DSc, the father of computed tomography, died
on August 12, 2004 at the age of 84
First CT
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the "EMI-Scanner“
Limited to brain
acquired the image data in about 4 minutes (scanning two adjacent
slices)
images from these scans took 2.5 hours to be processed by
algebraic reconstruction techniques on a large computer
scanner required use of a water-filled tank with a pre-shaped rubber
"head-cap" at the front, which enclosed the patient's head
160 parallel readings through 180 angles, each 1° apart, with each
scan taking a little over five minutes
a single photomultiplier detector, and operated on the
Translate/Rotate principle
Basics of CT
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Measurement of attenuation of a cross section of
the body
System uses the data to reconstruct a digital image
of the cross section
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Each pixel in the image represents mean attenuation of a
voxel (boxlike element)
Attenuation measurements: quantifies the fraction
of radiation removed in passing through a given
mount of material of thickness x
Cont
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Different names in the past 30 years:
Computerized transverse axial scanning
(tomography)
Computer assisted tomography
Computerized axial tomography (CAT)
But the final name is CT
CT Process Overview
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The formation of CT image by CT scanner
three steps: data acquisition, image
reconstruction, and image display,
manipulation, storage, and recording.
Data Acquisition
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Collection of x ray transmission measurements
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After passing through patient they fall onto detectors
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Detector measures the attenuation value
Reconstruct an image, enough data needed (transmission
measurements)
Data collection scheme: Example
 tube & detector move in a straight line (translate) across
body part (from left to right); after collecting number of
transmission measurements they rotate 1 degree and start
again bt from right to left
Translate-rotate-stop-rotate (repeated 180 times) 180°
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More efficient scanning schemes were developed
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Image Reconstruction
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After enough transmission measurements (detector)
Sent to the computer for processing
Computer (uses mathematical techniques to reconstruct the
CT image)
Reconstruction algorithms (example: algebraic reconstruction
technique)
Need: minicomputer and microprocessors for performing the
function/ or array processors for calculations.
Image Display/Manipulation/Storage/Recording
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After image reconstruction
Image displayed on CRT (cathode ray tube), best for a gray
scale image
These monitors are on the console allowing the technologist
(operator console) and radiologist (doctors console)
Manipulation: transverse axial images can be reformatted
into coronal , sagital, and paraxial sections./ and three
dimensional processing
Storage: magnetic tapes and magnetic disks and optical
storage.
How CT Scanners Work
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Turn on power of scanner
Perform a quick test to make sure scanner
is working properly
Place the patient in the scanner opening/
setup depending on exam
Technical factors are setup by the
technologist at the console
What happens when x-ray passes
through the patient?
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Attenuated and measured by the detector
X ray tube and detectors are hidden inside the gantry of
scanner and rotate around patient during scanning
Detector converts x ray photons into electrical signals
(analog), which must be converted into digital (numerical) for
input on computer
Computer makes the image reconstruction process
Reconstructing an image is in numerical form and must be
converted into electrical signal so it can be displayed on a
television monitor for viewing
Image can be stored on magnetic tapes or optical disks and
recorded on x-ray film
Computers
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A computer performs wide range of tasks
Image reconstruction to storage, recording, digital
transmission to remote locations
Use in radiology, one of the reasons is film-less
hospitals
Advantage of computers: can process, store,
retrieve, and communicate information quickly
and accurately
What is a Computer System
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A machine for solving problems
High-speed electronic computational machine that
accepts information in the form of data and
instructions (though an input) and process the
information by performing arithmetic and logic
operations using a program stored in its memory
Results of the process can be
displayed/stored/recorded/transmitted
Three Components
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Hardware: physical component of the
machine, input devices, output devices,
processing hardware
Software: instructions to solve the problem
User/operator : design hardware/software
and operate the machine
Software
The hardware receives instruction from the
software ( instruction = written step by step how to
solve the problem/programs)
Three types:
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Systems software- start up, coordinate the activities
Applications software- programs we run or use on the
computer
Software development tool- programming languages
History of Computers
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1642 the abacus, counting machine
1694, calculating machine (multiplication and
division)
1890 – punch card machine
Rapid development till Howard Aiken’s MARK1 (large
electromechanical calculator)
1951, UNIVAC – universal automatic computer (first
commercially available)
Today's computers are fifth generations
Generation – a period of significant technical
developments in hardware and software
Computer Generations
1st: 1951-1958, large, slow, heat during operation, housed
in an air-conditioned room
2nd: 1959-1963, solid-state devices (transistors and
magnetic cores) for internal memory, less heat, smaller,
less power for operation
3rd: 1963-1970, integrated circuit, silicon chips/use of
magnetic disks for storage (several programs processed
at the same time) , faster, smaller, more reliable
Generations
4th: 1971-1987, 1000’s of integrated circuits on chip
5th: 1987-date, gallium based circuitry instead of silicon,
parallel processing, many processors are used to
operate on data at the same time
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These computer developments affected development in
other technologies and medical imaging is a important
example
Classification
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Depending on their processing capabilities
Supercomputers-large, high capacity,
processes data at extremely high speed
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Use: weather forecasting, oil exploration,
scientific modeling CRAY2, one of the fastest
computers available today
Cont.
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Mainframe Computers- large, high speed
computation, large memories, terminals enable
multiple users access to primary memory (use in
banks, universities) Millions of Instructions Per
Second
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multiple operating systems, and thereby operate not as a
single computer but as a number of virtual machines. In
this role, a single mainframe can replace dozens or even
hundreds of smaller servers.
Cont.
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Minicomputers- mid level computer built to
perform complex computations while
dealing with high level of input and output
for users connected via terminals (multi
user computer) much smaller than
mainframes (fill a room)
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minicomputers will be discussed more when we
talk about CT components
Cont.
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Microcomputers- small digital computers/
personal computers, built so all circuitry is placed
in a single chip or multiple circuit boards
(microprocessor, central processing unit CPU)
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Microprocessor is a digital integrated circuit that
processes data, controls work of microcomputer
The processing capability is related to number of bits,
binary digits(0 and 1)used to represent data
Cont.
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8-bit (28)processor = represents 256 numbers
16-bit (216) processor = 65,536 numbers can be represented
32-bit(232)
2 types of computers Digital and Analog
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Digital operate on digital data (discrete units) and analog
operate on continuous physical quantities
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Digital computes are most common, they operate on digital
data through arithmetic and logic operations, used in
radiological applications, its important that we understand the
nature of digital systems
Numbering Systems
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Decimal numbers system = based on ten
0,1,2,3,4,5,6,7,8,9
Any number written must be a sum of these
digits multiplied by 10x
Example 321
Unit/tens/hundreds/thousands/ten
thousands/hundreds thousands/million
Binary numbers system
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based on factors of 2
Only two values 0 and 1
1 2
4 8
16 32
Writing 7 and 10 in binary
0111 and 1010
64
128
Converting
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Decimal to Binary
Example 133 , list 1 to 128
Binary to Decimal
Example 01010110 , count 8
Put 0’s and 1’s under numbers then add
Other Numbers
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Since binary numbers can be long, octal
and hexadecimal systems
Octal 8 digits 0 1 2 3 4 5 6 7
Hexadecimal 16
Example: convert binary to octal
010110100, group them
010, 110, 100 100=4; 110=6; 010=2 so
the octal number is 264
Why
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A binary digit, a bit, which is a single binary
number,
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4 binary bits (0.5byte)
8 binary bits (1byte)
16 binary bits (2 bytes)
32 binary bits (4 bytes)
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Binary numbers, grouped in 8 digits called bytes
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Byte is a location in the memory, memory
capacity is measured in bytes
Why
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When we enter information on computer,
the characters we use are converted into
binary codes, two famous ones are ASCII
and EBCDIC
Data Communications
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Hardware: modem a device that converts
digital data to analog signals and converts
analog signals to digital data to be
transmitted and received
Multiplexer allows many computers to
share communication line
Network , more on communication
Computers in Radiology
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1955, to calculate radiation dose
distribution in cancer patients
Mathematical approaches in radiology
Two categories: imaging and no imaging
Imaging
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Information from the patient needs
processing
Digital image processing techniques
Digital images: digital radiography, digital
fluoroscopy, MRI, CT
Non-imaging
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Radiology information system (RIS), like patient
admission, billing, film library, word processing..
An electronic system for archiving, transmitting,
viewing, and manipulating images
(Picture Archiving and Communication Systems
PACS) HIS (hospital information system)
Mechanical view boxes are being replaced by
workstations
workstation = powerful stand alone computer with
high graphic capability