ct scanning report
TRANSCRIPT
CT Scanning Seminar Report ‘03
INTRODUCTION
There are two main limitations of using conventional x-rays to examine
internal structures of the body. Firstly superimpositions of the 3-dimensional
information onto a single plane make diagnosis confusing and often difficult.
Secondly the photographic film usually used for making radiographs has a limited
dynamic range and therefore only object that have large variation in the x-ray
absorption relative to their surroundings will cause sufficient contrast differences on
the film to be distinguished by the eye. Thus the details of bony structures can be
seen, it is difficult to discern the shape and composition of soft tissue organ
accurately.
CT uses special x-ray equipment to obtain image data from different angles
around a body and then shows a cross section of body tissues and organs. i.e., it can
show several types of tissue-lung,bone,soft tissue and blood vessel with great clarity.
CT of the body is a patient friendly exam that involves little radiation exposure.
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CT Scanning Seminar Report ‘03
BASIC PRINCIPLE
In CT scanning, the image is reconstructed from a large number of
absorption profiles taken at regular angular intervals around a slice, each profile being
made up from a parallel set of absorption values through the object. ie, CT also passes
x-rays through the body of the patient but the detection method is usually electronic
in nature, and the data is converted from analog signal to digital impulses in an AD
converter. This digital representation of the x-ray intensity is fed in to a computer,
which then reconstruct an image.
The method of doing of tomography uses an x-ray detector which
translates which translates linearly on a track across the x-ray beam, and when the
end of the scan is reached the x-ray tube and the detector are rotated to a new angle
and the linear motion is repeated. The latest generation of CT machines use a ‘fan-
beam’ geometry with an array of detectors which simultaneously detect x-rays on a
number of different paths through the patient.
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CT Scanning Seminar Report ‘03
CT SCANNER
CT scanner is a large square machine with a hole in the centre,
something like a doughnut. The patient lies still on a table that can move up/down
and slide in to and out from the centre of hole. With in the machine an X-ray tube on
a rotating gantry moves around the patient’s body to produce the images.
PHOTOGRAPH OF CT-SCANNER
PROCEDURE
In CT the film is replaced by an array of detectors which measures X-
ray profile. Inside the scanner, a rotating gantry that has an X-ray tube mounted on
one side an arc –shaped detector mounted on opposite side. An X-ray beam is emitted
in a fan beam as the rotating frame spins the X-ray tube and detector around the
patient. Each time the X-ray tube and detector make a 360 degree rotation and X-ray
passes through the patient’s body the image of a thin section is acquired. During each
rotation the detector records about 1000 images (profiles) of the expanded X-ray
beam. Each profile is then reconstructed by a dedicated computer into two time.
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CT Scanning Seminar Report ‘03
DIFFERENCE BETWEEN X-RAY IMAGE AND CT
SCANNED IMAGE
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XX--ray imageray image
Head cross-sections 3-D Head cross-section
Abdomen crossAbdomen cross--sectionssections
XX--ray imageray image
Head cross-sections 3-D Head cross-section
Abdomen crossAbdomen cross--sectionssections
XX--ray imageray image
Head cross-sections 3-D Head cross-section
Abdomen crossAbdomen cross--sectionssections
XX--ray imageray image
Head cross-sections 3-D Head cross-section
Abdomen crossAbdomen cross--sectionssections
CT Scanning Seminar Report ‘03
PHYSICS OF TOMOGRAPHY
X-ray photons interact with material in there principal ways: pair
production, photoelectric absorption and scattering .Pair production only occurs if the
photon energy is ›1.022mev,which is much higher than the energies used in medical
tomography. Photoelectric absorption occurs when the photon is completely absorbed
and transfers its energy to an electron .The electron then passes through the material
giving up its energy until it comes to rest.
Scattering has two components-coherent or Raleigh scattering in which
the direction of the photon is changed ,but it does not change frequency. The other is
that Compton or incoherent scattering, where the photon gives up some of its energy
to an electron and continues on in a different directions at lower energy. The
combined effects of scattering and absorption results in an exponential attenuation of
a beam of photons as it passes through a material. A mono energetic beam with an
input intensity of I0 photons passing through a length of material has an output
intensity of
I=I0℮(-µx)
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CT Scanning Seminar Report ‘03
CALIBRATION
The projection p(x) depends on measurements of both the transmitted X-
ray intensity I(0) and the incident X-ray intensity I0(x). The intensity variations with
time can be measured by putting a reference X-ray detector in a portion of beam
which does not intersect the patient, usually at the edge of the beam and sampling this
detector at the same time as the measurement of the beam transmitted through the
patient is sampled. The spatial fluctuations can be measured during an initial
calibration run using a known object, such as a water filled cylinder in the place of
patient.
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CT Scanning Seminar Report ‘03
SYSTEM COMPONENTS
All computed tomography system consists of four major subsystems.
Scanning System – takes suitable reading for a picture to be reconstructed. This
includes x-ray source and detectors.
Processing Unit – converts these readings into intelligible picture information.
Viewing System – presents this information in visual form and includes other
manipulative aids to assist diagnosis.
Storage Unit – here picture is stored in digital form.
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CT Scanning Seminar Report ‘03
SCANNING SYSTEM
The purpose of the scanning is to acquire enough information to
reconstruct a picture for an accurate diagnosis. In basic scanning process, a collimated
x-ray beam passes through the body and its attenuation is detected by a sensor that
moves on a gantry along with the x-ray tube. The tube and the detector moves in a
straight line.
Inorder to get a clear image, rotation machines have been designed in
which only the x-ray source rotates within a full circle of stationary detectors
arranged around a patient. The individual detectors are lined up practically without
gaps so that the radiation which has penetrated the patient is optimally used. The
system permits calibration during scanning, which eliminates the problem of detector
drift.
X-RAY SOURCE
In CT scanners, the highest image quality free from disturbing blurring
effects is obtained with the aid of pulsed x-ray radiation. During rotation, high
voltage is applied at all times. A grid tube prevents the electron current from striking
the anode except when desired allowing the x-rays to be emitted in bursts. As the
gantry rotates an electric signal is generated at certain positions of rotating system.
DETECTORS
For a good image quality, it is important to have a stable system response
and in that detectors play a significant role. There are three types of detectors
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CT Scanning Seminar Report ‘03
commonly used in CT scanning. They are xenon gas ionization detector, scintillation
crystal and photomultiplier and scintillarc. A good detector is a pre-requisite to obtain
optimal image quality, the measuring electronics must have a large dynamic range to
backup the detector.
PROCESSING UNIT
The information received by the computer from the scanning gantry
needs processing for reconstructing the pictures. The data from the gantry
contains information on the following parameters.
Positional information-such as which traverse is being performed and how
far the scanning frame is along its traverse.
Absorption information-the values of attenuation coefficient from the
detectors.
Reference information-obtained from the reference detector that monitors the
X-ray tube.
Calibration information-Obtained at the end of each traverse.
The first stage of computation is to analyze and convert all the
collected data in to a set of profiles. However the main part is of processing
the profiles to convert the information which can be displayed as a picture and
used for diagnosis. In general the reconstruction method can be classified in to
three major techniques.
Back projection-which is analogous to graphic reconstruction.
Iterative methods-which implement some form of algebraic solution.
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CT Scanning Seminar Report ‘03
Analytical methods-where an exact formula is used. Two of these are
filtered back projection, which incoperates the convolution of the
data and fourier filtering of the image, and two dimensional fourier
reconstruction technique.
The method of back projection without any further processing is
simple and direct. In This method each of the measured profiles is projected
back over the image area at same angle from which it was taken. At the same
time each projection not only contributes to the point that originally formed
the profile but also to all the other points in its paths. The technique in fact
produces starred images and blurring and this makes it totally unsuitable for
providing pictures of adequate clarity for medical diagnosis.
The earlier scanners used iterative technique which took a succession
of back projection correcting at each stage until an accurate reconstruction was
achieved. The method requires several steps to modify the original profiles in
to a set of profiles which can be projected back to give an un blurred image.
This technique however tends require long computation time.
Current commercial scanners use a mathematical technique known as
convolution of filtering. This technique employes a spatial filter to remove the
artifacts.
VIEWING SYSTEM
In most of the CT system the final picture is available on a
television type picture tube. The picture is constructed by a number of
elements in a square matrix wherein each element has a value representative
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CT Scanning Seminar Report ‘03
of the absorption value of the point in the body which it represents. This
technique enables to have a much larger dynamic range than the eye can
possibly have.
STORING AND DOCUMENTATION
For subsequent processing or evaluation of a CT picture, various
methods of storage are used. The picture is stored in the digital form so that
the evaluation is convenient on a computer assisted program. For this purpose
the data carries generally employed are magnetic disc ,magnetic tape and
floppy disc. The magnetic disc normally hold a small number of pictures. So
it cannot be employed as a long term storage medium. Most manufactures of
CT units use magnetic tape and floppy disc and floppy disc provide medium
storage range. For long term storage magnetic tapes are performed.
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CT Scanning Seminar Report ‘03
IMAGE RECONSTRUCTION
Computed tomography scans are a very powerful tool in medicine. X-rays
passing through an object can be absorbed or scattered and the resulting loss in
intensity is given by
I=Iexp(-µx)
Where µ is the linear attenuation coefficient
X is the distance the X-ray has traveled.
The initial ground work for computed tomography was laid by Radon,
and he demonstracted that an object could be reconstructerd from an infinite number
of projections through that object .In a modern CT scanner an X-ray fan beam and
dector sweep around the patient obtaining thousands of projections at different angles
.The CT scanner measures the intensity of the X-ray beam which pass through the
object .The average linear attenuation coefficient along the projected line through the
object is given by
µ=ln(I/I0)/-Nt(delta t)
where µ is the average attenuation coefficient
I/I0 is the normalized intensity
Delta t is the product of step size
Nt is the number of steps
RECONSTRUCTION
There have been many different algorithms developed to accomplish this
task and while they have all been shown to be fundamentally identical the actual
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CT Scanning Seminar Report ‘03
techniques appear quite different. One of the most popular algorithm is the filtered
back projection technique .As its name implies this technique involves two parts.
Back projecting along the projection lines used ,and filtering the image.
BACK PROJECTION
Back projection is a relatively elementary process .One simply assigns the
mean attenuation coefficient given by the equation
ln(I/I0)/Nt(delta t) to each point along that line.
This back projection is repeated for all angles .The attenuation coefficient
for a particular point will be built up from all projection passing through that
point .In imaging jargon, each of these points pixel ,that is an element of the final
image or picture.
In reality ,this process is not quite trival .The image under
reconstruction is not continuous ,but is composed of discrete pixels. The projection
lines will not pass perfectly through the centre of each pixels in their path and it is
necessary to establish a method for describing the projection lines in terms of
individual pixel with in a matrix.
By establishing an N/N matrix reconstruction matrix –g(x,y)-where N is
the number of translation pixels in the t axis. For a given angle in measurement space
a value at can be calculated for each pixel in the N/N matrix as follows.
t=xcos+ysin
This pixels in the N/N matrix can be assigned attenuation coefficient values
from measurement
G(x,y)=µ(t,)
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CT Scanning Seminar Report ‘03
This process is repeated for each angle.
Ie g(x,y)=(1/N)εg(x,y)
And the matrices are summed and divided by the number of angles to
obtain the final back projected image.
FILTERING
Back projection alone results in a blurred reconstruction image .Filtering
must be applied to correct for this and obtain an accurate image of the object.
There are a number of choices in the type of filter to use .The simplest and
most rigorous one is the ramp filter.
H()=()
The most commonly used filter is the Shepp-Logan filter, which combines
a sine function with ramp filter.
Hsl()=||sine(/max)
This filter results in a small amount of blurring, but is much less sensitive to noise.
Filtering and back projections are both linear operations.
Filtering is performed by multiplying the fourier transform of a wave form by the
filter function ,the result is then inverse fourier transformed to produce the filtered
waveform. In fourier transforms, multiplication in Fourier space is equivalent to a
convolution in normal space.
G()=F()*H()
g(t)=f(t)*h(t)
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CT Scanning Seminar Report ‘03
where G() and H() are fourier transforms of g(t),f(t) and h(t)
respectively and * represents a convolution integral. The convolution integral is
defined by
f(u)h(t-u)du.
CT Numbers
Once both filtering and back projections have been performed ,the
result is a two dimensional array of attenuation coefficient.
For historical reasons ,the attenuation coefficients are converted into CT
numbers in units of Houvsfield.
CT Number=1000(µ-µw)/µw
Where μw is the attenuation coefficient for water.
The main problem with CT has been the potential danger it represents
because of radiation exposure .The developments in CT imaging have made marked
improvements in its technological capabilities ,the radiation effects problem has not
received the same degree.
The new processing data method reduces the amount of radiation
exposure needed while maintaining ,CT’s high resolution. The method is based on an
algorithm that reconstructs the wavelet coefficients of an image from the radon
transform data. The properties of wavelets are used to localize the radon transform
and reconstruct a local region of the cross section of a body, using almost completely
local data. This significantly reduces the radiation exposure and less computation
time. The variance of the elements of the null space is negligible in the locally
reconstructed image. An upper bound for the reconstruction error in terms of data
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CT Scanning Seminar Report ‘03
used is also determined by the algorithm, which for example requires 2% of full
exposure data to reconstruct a local region 16 pixels in radius in a 256*256 pixel
image.
After scanning the patient the operator can go straight to the wavelet
transform without having to first reconstruct the image. To obtain wavelet transform
the algorithm can be applied to full data or local data. Local image reconstruction is
achieved with superior definitions in shortest time and with less radiation exposure to
the patient.
That is in summary
Reconstructs with high accuracy and with few computations the wavelet
transform of an image directly from the tomographic measurements.
Computes to high accuracy a small region of the image from measurements
on line passing only through the region reducing computation time and
radiation exposure.
Reconstructs the density at a point using only line integral data on lines that
pass through a small region containing that point ,achieving reduced radiation
exposure.
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CT Scanning Seminar Report ‘03
BENEFITS AND RISKS
BENEFITS
Unlike other imaging methods CT scanning offers detailed view of many
types of tissues , including lungs, bones, soft tissues and blood vessels.
CT scanning is painless , noninvasive and accurate.
CT examinations are fast and simple.
Diagnosis made with the assistance of CT scan eliminate the need for
invasive exploratory surgery and surgical biopsy.
CT scanning can identify both normal and abnormal structures, making it a
useful tool to guide radiotherapy , needle biopsies and other minimally
producers.
CT has been shown to be a cost-effective imaging tool for a wide range of
clinical problems.
RISKS
CT does involve exposure to radiation in the form of X-rays , but benefits of
an accurate diagnosis far outweighs the risks .The effective radiation does
from this procedure is about 10mv , which is about the same as the average
person receives from background radiation in 3 years.
Special care is taken during X-ray examination to ensure maximum safety for
the patient by shielding the abdomen and pelvis being imaged.
The risk of serious allergic reaction to iodine containing contrast material is
rare and radiology departments are well equipped to deal them.
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Limitations of CT scanning of the body
Very fine soft tissues details in areas such as a shoulder or knee can be
more readily and clearly seen with MRI. In some situations soft tissues are may be
unclear by near by bone structures. The exam is not generally indicated for pregnant
women.
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FUTURE DEVELOPMENTS
The current trends in the industry appear to be improve picture quality
primarily resedation improvement and artifact reduction, and to lower the cost with
existing quality. In the future, there is much interest in developing a real time heart
tomography machine, allowing radiologists to observe sequences of heat functioning
throughout its cycle of operation.
Industrial tomography is another future direction in which tomography is
heading. Tomography allows detailed inspection of complex and critical parts. Two
important potential applications are inspection of jet engines and machine parts such
as blades and disks & inspection of rods in nuclear reactives.
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CONCLUSION
When CT scanners first appeared it used to take four minutes to scan a
section, thus making it impossible to image moving organs like the heart. But the
machines of today can complete the scan in few seconds. Special machines are being
developed for heart scanning which completes scans in milliseconds. Also the
developments in CT imaging have made marked improvements in its technological
capabilities the radiation effects problem has not received the same degree. The
properties of wavelets are used here. This significantly reduces the radiation exposure
and less computation time.
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REFERENCES
Hand book of biomedical instrumentation – R.S. Khandpur, Tata Mc Graw
Hill
Medical Instrumentation – John G. Webster
Biomedical Instrumentation and Measurements- Lesliecromwell
www.colorado.edu|physics|2000|index.pl
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CONTENTS
1 INTRODUCTION 1
2 BASIC PRINCIPLE 2
3 CT SCANNER 3
5 PHYSICS OF TOMOGRAPHY 5
6 CALIBRATION 6
4 SYSTEM COMPONENTS 7
7 RECONSTRUCTION METHOD 12
8 FUTURE DEVELOPMENTS 19
9 CONCLUSION 20
10 REFERENCES 21
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ABSTRACT
CT scanning – computed tomography is a mechanism of getting the
internal details of a section. It is a diagonostic imaging procedure in which anatomical
information is digitally reconstructed from X-ray transmission data obtained by
scanning an area from many directions in the same plane to visualize information in
that plane. CT is a fast patient friendly and has the unique ability to image a
combination of soft tissue, bone, lungs and blood vessels.
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ACKNOWLEDGEMENT
I express my sincere gratitude to Dr.Nambissan, Prof. & Head,
Department of Electrical and Electronics Engineering, MES College of
Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminar guide Mrs. Haseena.P.Y
(Lecturer, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge,
Department of EEE) for their invaluable advice and wholehearted cooperation
without which this seminar would not have seen the light of day.
Gracious gratitude to all the faculty of the department of EEE &
friends for their valuable advice and encouragement.
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