ct scanning

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.

Dept. of EEE MESCE Kuttippuram1


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.



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.



DIFFERENCE BETWEEN X-RAY IMAGE AND CT

SCANNED IMAGE


XX--ray imageray image

Head cross-sections 3-D Head cross-section

Abdomen crossAbdomen cross--sectionssections











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)



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.



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.



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



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.



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



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.



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



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,)



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)



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



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.



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.



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.



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.



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.



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



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



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.



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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