Transcript Compute Tomography

Computed
Tomography
NOREEN MARWAT
SENIOR SCIENTIST
NORI, Islamabad.
Introduction
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4th decade of clinical use
Invaluable diagnostic tool for many clinical
applications
Cancer diagnosis to trauma to
Osteoporosis screening
Introduction
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Computed tomography (CT) scanning, also
called computerized axial tomography (CAT)
scanning, is a medical imaging procedure
that uses x-rays to show cross-sectional
images of the body.
Introduction
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A CT imaging system produces crosssectional images or "slices" of areas of the
body, like the slices in a loaf of bread. These
cross-sectional images are used for a variety
of diagnostic and therapeutic purposes
Axial Tomography
Computed Axial Tomography
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A motorized table moves the patient through a circular
opening in the CT imaging system.
While the patient is inside the opening of the CT imaging
system, an x-ray source and detector within the housing
rotate around the patient. A single rotation takes about 1
second. The x-ray source produces a narrow, fan-shaped
beam of x-rays that passes through a section of the
patient's body.
A detector opposite from the x-ray source records the xrays passing through the patient's body as a "snapshot"
image. Many different "snapshots" (at many angles
through the patient) are collected during one complete
rotation.
For each rotation of the x-ray source and detector, the
image data are sent to a computer to reconstruct all of the
individual "snapshots" into one or multiple crosssectional images (slices) of the internal organs and
tissues.
Invention
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Godfrey Newbold Hounsfield
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1972
EMI Central Research Laboratories
Allan McLeod Cormack
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1963
Tufts University
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Nobel Prize in medicine in 1979.
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CT Scan machine is “the great legacy” of
the Beatles
Invention
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The first CT scanner EMI Mark 1, produced
images with 80×80 pixel resolution.
Each pair of slices required 4.5 minutes of
scan time and 1.5 minutes of reconstruction
time.
Basic principles (cont.)
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Plain film imaging reduces the 3D patient
anatomy to a 2D projection image
Density at a given point on an image
represents the x-ray attenuation properties
within the patient along a line between the xray focal spot and the point on the detector
corresponding to the point on the image
Basic principles (cont.)
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With a conventional radiograph, information with
respect to the dimension parallel to the x-ray beam
is lost
Limitation can be overcome, to some degree, by
acquiring two images at an angle of 90 degrees to
one another
For objects that can be identified in both images, the
two films provide location information
Basic principles- Conventional
Radiograph
Basic principles- Conventional
Radiograph
Basic principle of operation
When the abdomen is imaged with
conventional radiographic techniques, the
image is created directly on the film image
receptor and is low in contrast principally
because of scatter radiation.
The image is also degraded because of
superposition of all the anatomic structures
in the abdomen.
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For better visualization of an abdominal
structure,
such as the kidneys, conventional
tomography can be used.
Basic principles
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Mathematical principles of CT was first
developed by Radon in 1917.
Radon’s treaties proved that an image of an
unknown object could be produced if one had
an infinite number of projections through the
object.
Basic principle
Intensity profile/ Projection
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The intensity of
radiation detected
varies
according to this
attenuation pattern and
forms an intensity
profile, or projection
Intensity profile/ Projection
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If this process is repeated many times, a large
number
of projections is generated. These projections are
not displayed visually but are stored in digital form in
the computer.
The computer processing of these projections
involves the effective superimposition of each
projection
to reconstruct an image of the anatomic structures
in that slice.
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The tomographic is a picture of a slab of the
patient’s anatomy.
The 2D image corresponds to 3D section of
patient.
So that even with CT a 3D image is
compressed into 2D image.
2D array of pixel in CT image corresponds to
3D voxels in the patient.
Pixels
and
Voxels
1st Generation Data Collection
Hounsfield’s CT Formulation
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Measurement Ni
written as sum
of attenuation of
pixel along path
Solve simultaneous equations
from data at
many positions
and angles
Experiments
achieved 0.5%
accuracy.
Hounsfield’s Experimental CT
Device
1st Generation Data Collection
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1 Pencil Beam and
1 NaI detector
160
samples/traverse
1o increms over 180o
28,800 samples
Solved simultaneous
equations (Fortran)
1602 image matrix
but reduced to 802
for practical clinical
use
EMI Mark 1
What are we measuring?
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The average linear
attenuation coefficient
“µ” between tube and
detectors
Attenuation coefficient
reflects the degree to
which x ray intensity is
reduced by a material.
Tomographic acquisition
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Single transmission measurement through the
patient made by a single detector at a given moment
in time is called a ray
A series of rays that pass through the patient at the
same orientation is called a projection or view
Two projection geometries have been used in CT
imaging:
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Parallel beam geometry with all rays in a projection parallel
to one another
Fan beam geometry, in which the rays at a given projection
angle diverge
Acquisition (cont.)
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Purpose of CT scanner hardware is to acquire a
large number of transmission measurements
through the patient at different positions
Single CT image may involve approximately 800
rays taken at 1,000 different projection angles
Before the acquisition of the next slice, the table that
the patient lies on is moved slightly in the cranialcaudal direction (the “z-axis” of the scanner)
Tomographic reconstruction
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Each ray acquired in CT is a transmission
measurement through the patient along a line
The unattenuated intensity of the x-ray beam is also
measured during the scan by a reference detector
I t  I0e
 t
ln(I 0 / I t )  t
Image Reconstuction
Image Reconstuction
Backprojectio
n
Backprojection (con’t)
Backprojection
Back Projection
Backprojection
Backprojection is formed by smearing each
view back through the image in the direction
it was originally acquired. The final
backprojected image is then taken as the
sum of all the backprojected views
backprojected image is very blurry
Filtered Backprojection
Filtered Backprojection
Convolution
Filtered Backprojection
Filtered Backprojection
CT Image Characteristic
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Image matrix: Original EMI format was 80 x
80 so there were 6400 cells of information
called pixels.
Today the format is 512 x 512 resulting in
262,144 pixels.
The numerical number in each pixel is a CT
number or Hounsfeld Number.
CT Image Characteristic
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CT number or Hounsfeld Number represents
the tissue volume in the pixel.
Field of View (FOV)is the diameter of the
reconstructed image. As the FOV increases,
the size of the pixel increases.
Voxel: is the square of the matrix times
the thickness of the slice.
Hounsfeld or CT Number
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The precise CT number is related to the
attenuation of the tissue contained in the
voxel.
Bone = +1000
Muscle= +50
Lung= -200
Air = -1000
CT Numbers: Hounsfield
Units
Example 1: voxel contains water (up=
uw):
CT# = 1000 x (uw - uw)/ uw = 0
Example: voxel contains air (up≈ 0):
CT# = 1000 x (0 - uw)/ uw = 1000 x (-1) = 1000
CT Numbers
Generations of CT machines
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First generation
Second generation
Third generation
Fourth generation
Spiral CT
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Difference in all above generation is speed of
scanning and movements of gantry.
In first two generation there are
1) Linear
&
2) Rotary movements
In next two generation there is only one movement
i.e.
rotary movement.
1st generation: rotate/translate,
pencil beam
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Only 2 x-ray detectors used (two different slices)
Parallel ray geometry
Translated linearly to acquire 160 rays across a 24
cm FOV
Rotated slightly between translations to acquire 180
projections at 1-degree intervals
About 4.5 minutes/scan with 1.5 minutes to
reconstruct slice
1st generation (cont.)
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Large change in signal due to increased x-ray flux
outside of head
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Solved by pressing patient’s head into a flexible membrane
surrounded by a water bath
NaI detector signal decayed slowly, affecting
measurements made temporally too close together
Pencil beam geometry allowed very efficient scatter
reduction, best of all scanner generations
First Generation CT Scanner
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Pencil Beam
Translate-Rotate
Design
180 one degree images
or translations.
One or two detectors.
5 minutes scan time
2nd generation:
rotate/translate, narrow fan
beam
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Incorporated linear array of 30 detectors
More data acquired to improve image quality
(600 rays x 540 views)
Shortest scan time was 18 seconds/slice
Narrow fan beam allows more scattered
radiation to be detected
3rd generation: rotate/rotate,
wide fan beam
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Number of detectors increased substantially
(to more than 800 detectors)
Angle of fan beam increased to cover entire
patient
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Eliminated need for translational motion
Mechanically joined x-ray tube and detector
array rotate together
Newer systems have scan times of ½ second
Ring artifacts
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The rotate/rotate geometry of 3rd generation
scanners leads to a situation in which each
detector is responsible for the data
corresponding to a ring in the image
Drift in the signal levels of the detectors over
time affects the t values that are
backprojected to produce the CT image,
causing ring artifacts
4th generation:
rotate/stationary
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Designed to overcome the problem of ring
artifacts
Stationary ring of about 4,800 detectors
3rd vs. 4th generation
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3rd generation fan beam geometry has the x-ray
tube as the apex of the fan; 4th generation has the
individual detector as the apex
5th generation:
stationary/stationary
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Developed specifically for cardiac tomographic
imaging
No conventional x-ray tube; large arc of tungsten
encircles patient and lies directly opposite to the
detector ring
Electron beam steered around the patient to strike
the annular tungsten target
Capable of 50-msec scan times; can produce fastframe-rate CT movies of the beating heart
6th generation: helical
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Helical CT scanners acquire data while the table is
moving
By avoiding the time required to translate the patient
table, the total scan time required to image the
patient can be much shorter
Allows the use of less contrast agent and increases
patient throughput
In some instances the entire scan be done within a
single breath-hold of the patient
7th generation: multiple
detector array
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When using multiple detector arrays, the collimator
spacing is wider and more of the x-rays that are
produced by the tube are used in producing image
data
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Opening up the collimator in a single array scanner
increases the slice thickness, reducing spatial resolution in
the slice thickness dimension
With multiple detector array scanners, slice thickness is
determined by detector size, not by the collimator