CT made easy

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15 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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CT made easy




Introduction


The computed tomograpic (CT) scanner is
revolutionary.


It does not use an ordinary image reseptor, but
instead a well
-
collimated x
-
ray beam directed on the
patient, and the attenuated image response is
transmitted to a computer.


The computer reconstructs the image and displays it
on a monitor.


The reconstructions are accomplished with
algorithms adapted for computer processing.

History of CT


The first demonstration of the technique was done in
1970, but the components to construct a CT scanner
was avalible 20 years before this.


In 1982, Godfrey Houndsfield shared the Nobel prize
in physics with Alan Cormack. Cormack had earlier
developed the mathematics used to reconstruct CT
images.


No other x
-
ray equipment development are as
important as the CT. Some say that MRI and UL are
as equally important, but they are, however, not x
-
ray equipments.

Principles of use


CT is a tranaxional/transverese image. That
means that you get axial pictures of the body.


It is extremly complicated to understand the
precise method on how the CT makes these
images. You need to have good knowlegde
of physics, engineering and computer
science.


The basic principles can be demonstrated if
you consider the simplest CT systems.

Principles of use


The x
-
ray source and and detector are connected so
that they move at the same time.


When the machine makes one sweep over the
patient the internal structures of the body attenuate
the x
-
ray beam according to their mass density and
effective atomic number.


The machine takes several sweeps of the body and
collects this in a computer, this computer then
reconstructs the images of the anatomic structures in
that slice.

Principles of use


The detector signal during each sweep is
registered in increments with values as high
as 1000. The value og each increment is
related to the x
-
ray attenuation coefficient of
the total path trough the tissue.


Through the use of simultaneous equations,
a matrix of values is obtained that represents
a cross section of anatomy.

The different generations of CT


There are four/five generations of CT
scanners. The fifth is still under development.


The first generation: translate
-
rotate
configuration, pencil
-

shaped beam, single
detector, 5
-
min scan time


Second generation: translate
-

rotate
configuration, fan
-
shaped beam, detector
array, 30 s
-

scan time

The different generations of CT


Third generation: rotate
-

rotate configuration,
fan
-
shaped beam, detector array, 1
-
s scan
time, disadvantage: ring artefacts


Fourth generation: rotate
-
stationary
configuration, fan
-
shaped beam, detector
array, 1
-
s scan time


Third generation scanner


In these scanners the x
-
ray tube and detector array
are rotated concentrically about the patient.


They can produce an image in one sec.


It uses a curvilinear array containing many detectors
and a fan beam.


The curvilinear detector array results in a constant
source
-
to
-
detector path length, which was an
advantage for good image reconstruction.


This also allows for better x
-
ray beam collimation to
reduce the effect of scatter radiation.

Third generation scanner


One disadvantage is the ring artefacts. They
occur for several reasons.


Each detector views a ring of anatomy , so if
any single detector malfunctions, the aquired
signal will result in a ring on the
reconstructed image.


Software
-
corrected image reconstruction
algorithms minimize such artefacts.

Fourth generation scanners


Radiation detection is accomplished through a fixed
circular array, which contains as many as 8000
individual elements.


The fixed detector array does not result in a constant
beam path from the source to all the detectors, but it
allowes each detector to be calibrated and its signal
normalized during a scan.


They are generally without ring artefacts


Disadvantage: patient dose and cost of buying


Fifth generation scanners


Development of CT is always going on. The
producers wish to make a CT scanner with
improved image quality at a lesser patient
dose.


Rotate
-
nutate scanners: Toshiba has
produced a novel extension of the fourth
generation. To maintain the x
-
ray source at
the same distance from the patient as the
detectors, the detector array nutates, as the
x
-
ray source rotates.

Fifth generation scanners


Electron
-
beam CT (EBCT) is a fundamentally
different way to produce CT images. Imatron
came up with the idea for scanning the heart.


Currently , EBCT is used to scan all tissues,
but especially when ultrafast imaging is
helpful.


EBCT images are produced in 50 ms.

System components


The gantry


The computer


The operating console

The gantry


Includes the x
-
ray tube, the detector array,
the high
-
voltage generator, the patient
support couch and the mechanical support
for each.


X
-
ray tube: it has special requirements. The
power capacity must be high. The anode
heating capacity must be atleast several
million heat units (MHU).

The gantry


High speed rotors are used in most tubes for the
best heat dissipation.


Focal
-
spot size is important. CT scanners designed
for imaging using high spatial resolution incorporate
x
-
ray tubes with small focus
-
spot.


Detector assembly: Early scanners had one
detector. Modern scanners have up to 8000, devided
in to groups; scintillation detectors and gas
-
filled
detectors.

The gantry


Scintillation detectors:


Containes scintillation crystal
-
photodiode
assemblies. They convert light into electronic
signals. They are highly efficient at detecting x
-
rays,
almost 90 % of the x
-
rays are absorbed and
contribute to the output signal.


But the space between each detector is big, so the
overall detection efficiency may only be 50 %. They
give dose to patient but do not contribute to the
image.

The gantry


Gas
-
filled detectors:


Contructed of a large metallic chamber with baffles
spaced with 1 mm intervals.


The baffles are like grid stripes and devide the large
chambers into small ones.


Each small chamber is one detector.


It is sealed and filled under preassure with an inert
gas with high atomic number (xenon/xenon
-
krypton
mixture)


The overall total detetction efficiency is 45 %, almost
the same as scintillation detectors.

The gantry


Collimators: Required for the same reason as
conventional x
-
ray. Correct collimation reduces
patient dose and improves image quality due to less
scattered radiation.


In CT there are normally two collimators.


One is the prepatient collimator; on the x
-
ray tube
housing/adjacent to it.


It limits the area of the patient that intercepts the
useful beam and thereby the slice thickness and the
patient dose.

The gantry


Improper adjustment of this collimator is the cause of
most of the un
-
necessary dose to patient.


The predetector collimator; located under the patient,
over the detector array.


Reduces scatter radiation


improves image quality


When coupled correctly with the prepatient detector,
it defines the slice thickness.


Has nothing to do with patient dose.

The gantry


High
-
voltage generator;


All CT scanners operate on three
-
phase or
high
-
frequenzy power.


Most manufactors built them into the gantry
or by mounting on the rotating wheel of the
gantry. It reduces the amount of space
needed, and winding and unwinding a power
cable is unnecessary.

The gantry


Patient positioning and support couch;


It has to be made of a material with a low
atomic number (carbon fiber) so that is does
not interfere with x
-
ray beam transmission
and patient imaging.


It should move smoothly for accurate patient
positioning, and is especially important for
spiral CT

Computer


It is unique for the CT and a must! A ultra
-
high speed
digital computer is needed for making CT images.


Depending on the format the computer has to do up
to 250 000 equations at the same time!


In the computer there is a microprocessor and a
primary memory. These determine the
reconstruction time= the time from end of scanning
to image appearance.


Array processors are becoming more common. They
are faster than the microprocessor and can
reconstruct an image in less than 1 s.

Operating console


Many CT scanners have 2 or 3 consoles.


One for the CT radiologic technologist to operate the
scanner.


One for an other technologist to postprocess images.


One for the physician to view the image, manipulate
contrast, size and general visual appearance.


A typical operating console contains controls and
monitors for the various technique factors.

Image characteristics


With CT, the x
-
rays form a stored electronic
image that is displayed as a matrix of
intensities.


The CT scan format consists of many cells
with its own number which is shown as a
brightness level.


A matrix of 512 x 512 = 262 144 cells of
information.


Image characteristics


Each cell is a pixel (picture element)


The numerical information in each pixel is a
CT number/ Houndsfield Unit (HU)


It is a two dimensional representation of a
corresponding tissue volume.


The diameter of image reconstruction is
called the field of view (FOV)


Image characteristics


When the FOV is increased for a fixed matrix ( for
example: from 12 to 20 cm) the size of each pixel is
increased proportionately.


When the matrix size is increased for a fixed FOV
(for example 512 x 512 to 1024 x1024)the pixel size
grows smaller.


Pixel size = FOV/matrix size


The tissue volume is known as a voxel (volume
element)


Voxel size= Pixel size x slice thickness

CT numbers


Each pixel is displayed on the video monitor
as a level of brightness and on the
photographic image as a level of optical
density.


The levels correspond to a range of CT
numbers from
-
1000 up to +1000 for each
pixel.


-
1000 is air, +1000 is dense bone and 0 is
water.



CT numbers


The CT number is related to the x
-
ray
attenuation coefficient of the tissue contained
in the voxel.


Remember: the degree of x
-
ray attenuation
is determined by the avarage energy of the
x
-
ray beam and the effective atomic number
of the absorber and is expressed by the
attenuation coefficient.

CT numbers


By the scale of HU there is a range of 2000
different gray scales with imformation, but
most of it goes ”lost”.


The screen only shows 32 grayscales.

Image reconstruction


Filtered back projection = all the projections
during on CT examination is stored in the
computers memory, and the reconstructions
are made by these.


With filter we do not mean a metal filter as in
the tube of the x
-
ray, but it is a mathematical
function. A
difficult

one!


Image reconstruction


In CT there over 250 000 pixels to
reconstruct from, that means that the
machine has to solve 250 000 equations to
find the solutions for the images.

Image quality


Spatial resolution


Contrast resolution


Noise


Linearity


Uniformity

Spatial resolution


If you scan a regular geometric structure that has a
sharp interface,the image at the interface will be
blurred.


The degree of blurring is a measure of spatial
resolution of the system and is controlled by several
factors.


If you take a scan over an area that has a high
contrast interface, for example the brain and the
skull, the image will be blurred.


The system will fix some of the blurring, and
smoothen the picture.

Spatial resolution


This, however, reduses the spatial resolution
because of some features of the scanner.


The larger the pixel size and the lower the subject
contrast, the poorer the spatial resolution will be.


The detector size and design of prepatient and
postpatient collimation affect the level of scatter
radiation and influence the spatial resolution by
affecting the contrast of the system.


Also the x
-
rays’ focal spot has influence on spatial
resolution.

Contrast resolution


Contrast resolution = the ability to distinguish
one soft tissue from another without regard
for size or shape.


Contrast resolution is superior in CT,
principally because of x
-
ray beam
collimation.

Contrast resolution


Imagine a scan over abdomen, where you
have spine, liver and fat. The atomic
numbers are different, but in conventional x
-

ray it is difficult to seperate them. With CT
and the CT numbers it makes it easy! With
HU the CT can amplify these contrast
differences, and make the contrast high.
Then we can cleary see differences between
tissue.



Noise


Noise= the precentage of standard deviation of a
large number of pixels obtained with a water
-
bath
scan.


Noise depends on the following factors:

1: kilovolt peak filtration

2: Pixel size

3: Slice thickness

4: Detector efficiency

5: Patient dose

Noise


Example:

If you scan a homogeneous medium like water
the pixel value should be zero. But because
the system is not perfect some pixel values
will be both higher and lower than zero.
These variations in HU will show in the
image as graininess, and is what we call
noise. The larger the variations in pixel value,
the more noise you get in the image.

Linearity


The CT must be calibrated frequently so that the HU are
correct.


There is a test you can do with a phantom and a water bucket.


The result from this test should show a linear line passing
through the CT number of water (0)


If the test shows deviation from linearity it’s a sign of
malfunction of the CT.


It may not show on the visual image, but could greatly affect
quantitative analysis of tissue, the determination of tissue
composition based on CT number.

Uniformity


When you scan a uniform object (water) the
pixel value should be zero (for water!).But
since the machine is very complicated
mechanically this does not happen. The
value may drift from day to day/hour by hour.


If it is scanned, and the pixel value is
constant in all regions of the reconstructed
image, this is called
Spatial uniformity


Uniformity


There is also a test for this, where you scan a
bucket of water and plot the numbers along
an axis of the image. If this axis is within 2
standard deviations of the mean value, the
system has acceptable spatial uniformity.


Because of the x
-
ray beam hardening, there
may be a decrease of CT numbers, so the
middle of the image is darker than it should.


This is called ”cupping” artefact.

Summary


The collimated x
-
ray beam is directed to the
patient.


The attenuated image
-
forming x
-
ray beam is
measured by a detector array.


The signal from the detector array is
measured by a computer.


The image is reconstructed in the computer.


The image is displayed on a TV monitor.

Summary


CT makes transverse images (axial images)


The internal structures of the body attenuate the x
-
ray beam according to their mass density and atomic
number.


All data are processed in digital form.


The resulting computer image is an electronic matrix
of intensities.


Matrix size is generally 512x512 individual cells or
pixels.

Summary


In each pixel is numerical information called a CT
number or HU.


The pixel is a two
-
dimensional representation of a
corrensponding tissue volume.


The voxel (volume element) is determined by
multiplying the square of the pixel size by the
thickness of the CT scan slice.


HU


-
1000=air, 0= water, 1000= dense bone

Summary


The CT scanner has exellent contrast
resolution because of the reduction of scatter
radiation by the x
-
ray beam collimators.


The ability to scan low
-
contrast anatomic
structures is limited by the noise of the
system.


System noise is determined by the numbers
of x
-
rays used by the detector array to
produce the image.


Questions...

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