Computed Tomography

Presented by :

120130103071-Nandasana Jay

120130103084-Prajapati Jay1

Introduction

Computed tomography (CT) is a medical imaging method employing tomography.

The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).

A large series of two-dimensional X-ray images (slices) of the inside of an object are taken around a single axis of rotation.

Digital geometry processing is used to generate three-dimensional images of the object from those slices.

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History

The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967. and it was publicly announced in 1972.

Allan McLeod Cormack of Tufts University in U.S. independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.

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CT Scan

CT scan produces axial sections/cuts /Slices

The CT image is recorded through a SCAN.

Scan?

A scan is made up of multiple X-Ray attenuation measurements around an objects periphery

X-ray tube

Detector

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Understanding Basic factors

Absorption :-stopping of x-rays with transfer of energy

Scatter:- deflection of x-rays

Incident Intensity :- No. of x-ray photons falling on an object

Transmitted Intensity:-No. of photons passing through

Incident x-

ray beam

Transmitted

X-ray beam

Scattered x-rays

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Attenuation

The reduction of the beam intensity on passing through the material due to absorption plus scatter

The degree of attenuation is obtained by measuring and comparing the incident and transmitted intensities

More dense

material

Less dense

material

Less

transmitted

x-rays

More

transmitted

x-rays

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Slice / Cut

The cross sectional portion of the body which is scanned for the production of CT image is called a slice.

The slice has width and therefore volume.

The width is determined by the width of the x-ray beam

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Matrix

The image is represented as a MATRIX of numbers.

Matrix :- A two dimensional array of numbers arranged in rows and columns.

Each number represents the value of the image at that location

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VOXEL

Each individual element or number in the image matrix represents a three dimensional volume element in the object, called a VOXEL

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PIXEL

The VOXEL is represented in the image as a two-dimensional element called PIXEL - (picture element)

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CT numbers

The numbers in the image matrix are called CT numbers.

Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object

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Visual image & Gray Scale

To obtain a visual image, the CT numbers are assigned different shades of gray on a gray scale.

Each shade of gray represents the x-ray attenuation within the corresponding voxel

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Tissue Types And Appearance

*Cerebrospinal fluid (CSF)

CT Image

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Phases of CT imaging

1. Scanning the patient

2. Data Acquisition

I. Tube or tube and detector move

II. Multiple attenuation measurements are taken around the object

3. Image reconstruction

4. Image Display

5. Image archival (recording)

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DATA ACQUISITION

Basic components X-ray tube

Collimators

Detector/s

Collimated x-ray beam traverses the object and enters the detector.

X-ray

tubeCollimators

Detector

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Practical method of Generation of Attenuation profiles

Attenuation profiles are a recording of X-ray attenuation verses position in the object.

Attenuation is related to the ratio between incident

intensity (I0) and transmitted intensity (It)

I0It

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X-ray intensity is measured by a DETECTOR which converts x-ray photons to electrical current

The reference (Input) detector measures the incident intensity (I0)

The output detector measures the transmitted intensity (It )

I0= incident intensity = tube output

It

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I0 should remain constant

It will change depending on the attenuation of the object scanned (e.g. centered, round object of uniform density)

Io=1000

It=1000It

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Calibration profile

If the object scanned is a centered, round water bath each attenuation profile is called a CALIBRATION PROFILE.

and the set of profiles are called the Calibration File or Cal file

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DATA Calibration Subtraction of

calibration file from the attenuation profiles of the object called data calibration

(consider an object containing two areas of different densities as shown)

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Scan Data File

The difference profile are stored as numerical values as a function of position in the profile.

This data is used in the image reconstruction process

The set of difference profiles for a complete scan is called SCAN DATA FILE or raw data

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Back projection of scan data

1. Back projection of the corrected attenuation profiles is accomplished by feeding the numerical values of each point along the profile into a matrix. (Since the scanning motion is circular, the matrix is usually round)

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Formation of Star artifact and streaks

Consider a scan of a single high density object suspended in air

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The attenuation profile for this object has a single impulse signal

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Back projections are crated for each profile

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Addition of the attenuation profiles create an image with star and streak artifacts

To be continued – CT Complementary 327

Generations of CT Scan

1st generation: rotate/translate, pencil beam

2nd generation: rotate/translate, narrow fan beam

3rd generation: rotate/rotate, wide fan beam

4th generation: rotate/stationary

5th generation: stationary/stationary

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1st generation: rotate/translate, pencil beam

• Only 2 x-ray detectors used (two different slices)

• Parallel ray geometry

• About 4.5 minutes/scan with 1.5 minutes to reconstruct slice

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2nd generation: rotate/translate, narrow fan beam

• Incorporated linear array of 30 detectors

• More data acquired to improve image quality

• Shortest scan time was 18 seconds/slice

• Narrow fan beam allows more scattered radiation to be detected

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3rd generation: rotate/rotate, wide fan beam

• Number of detectors increased substantially (to more than 800 detectors)

• Angle of fan beam increased to cover entire patient.

• Mechanically joined x-ray tube and detector array rotate together

• Newer systems have scan times of ½ second

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4th generation: rotate/stationary

• Designed to overcome the problem of ring artifacts.

• Stationary ring of about 4,800 detectors

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5th generation: stationary/stationary

• 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.

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Latest update:-

6th generation: helical

7th generation: multiple detector array

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Modern CT scanner

1. gantry aperture (720mm diameter)

2. microphone

3. sagittal laser alignment light

4. patient guide lights

5. x-ray exposure indicator light

6. emergency stop buttons

7. gantry control panels

8. external laser alignment lights

9. Patient couch

10. ECG gating monitor 36

Advantages

•Quick and painless

•Can help diagnose and guide treatment for a wider range of conditions than plain X-rays

•Can detect or exclude the presence of more serious problems

•Can be used to check if a previously treated disease has recurred

Disadvantages

•Small increased risk of cancer in future from exposure to ionisingradiation (X-rays). Risk is greater for children•Uses higher doses of radiation, so the risks (while still small) are in general greater than other imaging types•Injection of a contrast medium (dye) can cause kidney problems or result in allergic or injection-site reactions in some people•Some procedures require anaesthesia

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Questions?

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Thank You……………

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