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COMPUTED TOMOGRAPHY

Submitted By

B.Ramakanth


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INTRODUCTION

Computed Tomography (CT) imaging, also known as "CAT scanning"

(Computed Axial Tomography).

Tomography is f rom the Greekword "tomos" meaning "slice" or "section"

and graph meaning "describing". A technique to help locate a problem and confirm diagnosis in orderto give

the correct treatment.

It combines the use of a digital computer together with a rotating x-ray

device to create detailed cross sectional images or "slices" of the different

organs and body parts such as the lungs, liver, kidneys, pancreas, pelvis,extremities,brain, spine, and blood vessels.

The x-rays tube rotates around the body, the x-rays passes through the body

and cross sectional images (pictures) of tissues are formed


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Diagram showing relationship of x-ray tube, patient,detector, and image reconstruction computer anddisplay monitor


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Generations Of CT Scanner

First Generation: Parallel-Beam Geometry.

Second Generation: Fan Beam, Multiple Detectors.

Third Generation: Fan Beam, Rotating Detectors.

Fourth Generation: Fan Beam,Fixed Detectors.

Fifth Generation: Scanning Electron Beam.

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First Generation: Parallel-Beam Geometry.

In the first CT scannerdesign, a single X-ray source and a single X-ray

detectorcell collect all the data fora single slice .

The xray head and detectorwere rotated at arc of 180. The source and detectorare rigidly coupled and the pencil beam is

translated across the patient toobtain a set of parallel projection

measurements at one angle.

This process is repeated once for each projection angle.

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Second Generation: Fan Beam, MultipleDetectors

A fanshaped xray beam was projected onto a lineararray of

approximately 30 detectors.

The xray head and detectors could rotate 180 around the patient. The increased numberof detectors reduced the numberof linearscans

required during the 180 arc.

The time required fora scan was reduced to less than 90 seconds.

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Third Generation: Fan Beam, RotatingDetectors

Awiderfanshaped xray beam and a curved array of 250750 detectors.

The widerbeam and largerdetectorarray allowed the scannerto include

the entire body in a single exposure. Eliminated the need forlinearscanning to be combined with the xray head

rotation.

Third generation scanners would acquire approximately 700,000measurements peranatomical section.

Scan times reduced to less than 12 seconds.

360 scan arcs were possible and the scan arc could be varied.

Shorter scan times allowed sequential scans to approximateddynamic functions with approximately 4 scans per minute.

The single detector array made third generation scanners prone toring artifact.

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Fourth Generation: Fan Beam,FixedDetectors

A single projection fanshaped xray beam and 6002000 stationarydetectors.

The xray head rotates more than 360o around the patient.

The detectors are fixed in a circularring around the patient and xray head.Alignment of the xray beam to each detectoris essential.

The xray head needs to travel more than 360 in orderto provide anacceleration and deceleration zone.

Scanning takes place as the xray head travels in either direction,clockwise and counter clockwise.

Scan rates of approximately 15 scans per minute are achieved. Scanrates are limited by the inter scan time used to change the directionof the xray head travel.

Dynamic scanning modes are available.

Over scanning modes exist and use scanning arcs of greater than

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Components OfCT Scanner

Gantry

Patient Table

PowerDistribution Unit

OperatorConsole

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Gantry

The gantry maintains the alignment of the xray head and detectors while

moving the xray head through the scan arc.

A circularaperture in the center of the gantry frame allows the patient tobepositioned within the xray field by the table assembly.

The gantry aperture diameterlimits the maximum patient size.

Each scanner has a maximum table weight limit that is design to preventpatients that are too large forthe gantry from being forced into the scanner.

The entire gantry can be tipped orangled up to 30 to allow the acquisitionof partial coronal images.

Three positioning lights or lasers within the gantry assure centering of thepatient in the sagittal, coronal, and transverse planes.

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Gantry includes

The X-ray tube

The DetectorArray

The High-Voltage generator

The patient Support Couch

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Patient TablePatient Table

The table is a flat orslightly curved platform that the patient lies on during

the scan.

The table is responsible forpositioning the patient with the gantry. Up/down positioning movements are available as well as the longitudinal

positioningof the patient into the gantry aperture.

The tabletop is usually made of a carbon fiber composite to reducexray beam attenuation.

It is very important not to exceed the patient weight limit of thetable.

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Figure shows the numerous sub-systems that are

required fora CT Scanning Table


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PowerDistribution Unit

The power distribution unit is the power supply for the xray tube,

scanning movement drive motors, table, and the detectors.

The x-ray tube requires very high voltages, kV, supplied at very highcurrents. These high voltage, high current demands also require very fast

switching, sort rise and fall times.

The powerdistribution unit is housed in its own room orcabinet. Often the

high current, high voltage drivers are water-cooled.

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OperatorConsoleOperatorConsole

CT imaging systems can be equipped with two orthree consoles.

One consoles is used by the CT radiologic technologist to operate the

imagine system. Anotherconsole may be available fora technologist topost-process images

forfilming and filing.

A third console may be available for the physician to view the images and

manipulate image contrast,size, and general visual appearance.

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Application OfCT ImagingApplication OfCT Imaging

CT imaging is used f or studying the chest, abdomen and pelvis because itprovides detailed, cross-sectional views of all types of tissue.

CT imaging detect many different cancers, including lung, liver andpancreatic cancerand the tumors of brain and head.

CT shows blood clots and blood vessel defects.

It shows enlarged ventricles (caused by a build up of cerebrospinal fluid).

It also shows image and abnormalities such as those of the nerves ormuscles of the eye.

CT scanning help in diagnosis and treatment of vascular diseases that canlead to stroke, kidney failure oreven death.

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Benefits OfCT ImagingBenefits OfCT Imaging

CT scanning is painless, noninvasive and accurate.

A majoradvantage ofCT is its ability to image bone, soft tissue and blood

vessels all at the same time. Unlike conventional x-rays,CT scanning provides very detailed images ofmany types of tissue as well as the lungs, bones, and blood vessels.

CT has been shown to be a cost-effective imaging tool fora wide range ofclinical problems.

CT is less sensitive to patient movement than MRI.

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Risks OfCT ImagingRisks OfCT Imaging

There is always a slight chance of cancerfrom excessive exposure to

radiation.

However, the benefit of an accurate diagnosis faroutweighs the risk.

Women should always inform theirphysician and x-ray orCT technologist

if there is any possibility that they are pregnant.

CT scanning is, in general, not recommended forpregnant women unless

medically necessary because of potential risk to the baby.

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THANK U

ramakanth ct

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