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tomographytomographytomographytomography

Principle

It is a complex application of Law of Tangent,

image quality improves with number of image

forming rays passing tangentially through the

object.

It can be re-constructed from multiple projection

of object.

X-ray projection formed by

scanning a thin cross-section

of the body with narrow x-

ray beam & measuring the

transmitted radiation with

radiation sensitive

detectors.

Detector adds up all the

energy of transmitted

photons and is processed by

computer to form an image

CT consist of high thermal capacity x-ray tube and generator mounted on rigid circular support called Gantry, which rotates around the patients.

Detectors are also mounted on the gantry opposite to x-ray tube.

Images are received as counts.

Large Data acquired is processed by computers to reveal an image or tomogram

Generations of CT machines

First generation

Second generation

Third generation

Fourth generation

Spiral CT

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.

Original scanner developed by Hounsfield.

It had

1) X-ray tube,

2) Pair of detector,

3) Head enclosed in water-bath,

4) Third detector between the x-ray tube and patient,

5) Rigid gantry maintaining the

relative position of tube &

detectors.

It had linear & rotary movements

X-ray tube used to be on during linear movement & off during rotary movements

First Generation

It had single pencil like x-ray beam with single

detector.

It had both linear & rotary movements

Rotary movements was of 1 degree at end of linear

scan.

Each tomogram required 5 min & total study time

of 25-30 min.

Second Generation

It had fan like x-ray beam with multiple

detectors, max up to 30 in some machines.

It had both linear & rotary movements

Rotary movements was of 30 degree arc thus

cutting down the scan time

Each tomographic section required 10 to 90 sec.

Third Generation

This type has only rotation movements-

Rotate rotate type, with fan shaped collimated X-ray tube with multiple (over 700) detectors loaded on Gantry which moves completely around the patients.

Fan beam should completely cover the object

Single image is computed from 1000 images in short time as less as 1 sec.

Fourth generation

In this type the detectors form a ring that completely

surrounds the patient and are fixed.

X-ray tube rotates around in circle inside the detector ring

and has fan shaped collimated x-ray beam.

It is rotate-fixed type with patient advancing in gantry.

Scan time is lesser than 1 sec for each section.

AdvantageAdvantageAdvantageAdvantage–––– speed

DisadvantageDisadvantageDisadvantageDisadvantage– increased amount of scattered radiations

Types of detectors

Two types

1. Scintillation crystals- Bismuth germanate crystals coupled with photomultiplier or sodium iodide crystals, are more efficient and has less afterglow .

2. Ionization chamber containing xenon gas under pressure, they have no afterglow but are slightly less efficient.

Image Reconstruction

Image is first converted to digital image and then to

conventional gray scale image

Image of the section is broken down in ,

Tiny picture elements called pixelspixelspixelspixels,

Projection of solid cores of tissues called voxels.voxels.voxels.voxels.

Computer counts the light flashes in solid detectors &

ions in ionization detectors and gives them a CT number

according to there attenuation coefficients

Reconstruction of whole image of the section by

multiple projection is a complex and is done by

computer. One of the method is backbackbackback----projectionprojectionprojectionprojection.

Contrast can be adjusted by operators

Spiral CT

Newer method

X-ray tube moves continuously with slip-ring technique,

the outer fixed detector ring and inner rotating x-ray tube

ring having brushes that remain in electrical contact with

the inside of outer ring.

X-ray tube moves continuously with patient moving

cephalad with constant rate producing a spiral or helix

around the patient.

Tube is energized for as much as 30 sec at a time as compared to conventional which is energized for 1 sec.

Tube has a very high thermal anode.

Cadmium tungstate is used preferred crystal photoiodides used.

Reconstruction is by interpolation I.e calculation of intermediate values in a series.

It has 3 dimensional image constructed

Advantages of spiral over

conventional CT

Shorter scanning time

Reconstruction along z axis with overlapping of

successive images leading to detection of small

lesions

Fewer motion artifacts

3D reconstruction of image