Chapter 3 Ct

7/29/2019 Chapter 3 Ct

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PRINCIPLE OFCOMPUTED

TOMOGRAPHY

(CT) IMAGING

Lecturer : Siti Afifah Mohshim

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INTRODUCTION

A CT scan, takes pictures of the body anduses a computer to put them together.

CT stands for computerized tomography.A CT scanner uses X-rays and is apainless procedure.

A series of X-rays are taken of your body

at slightly different angles, to produce verydetailed pictures of the inside of yourbody.


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INTRODUCTION

The pictures produced by CT scans arecalled tomograms and they providedoctors with information to help themreach a diagnosis about a variety ofconditions.

The CT scanner is a large machine.

The pictures are taken while you lie on acouch, which moves backwards andforwards through the hole of the machinethat is shaped rather like a giant doughnut.


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HISTORY PERSPECTIVE

Computed tomography (CT) has also beenidentified as computerized axialtomography (CAT), computerize transaxialtomography (CTAT), and digital axialtomography (DAT).

Computed tomography results in a digitalimage - transverse (transaxial) image.

The principal advantage of CT imagingover other x-ray imaging is improvedcontrast resolution.


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HISTORY PERSPECTIVE

Tomography is from the Greek "tomos,"meaning section.

Unfortunately, we identify an imagesection as a "slice.

Emission CT involves nuclear medicineand 1-ray emission from a patientadministered a radionuclide.

Computed tomography utilizes x-raytransmission through a patient.

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CONVENTIONAL TOMOGRAPHY Conventional tomography is a radiograph obtained

with a moving source image receptor assembly.

Conventional tomography results in an image ofsuperimposed tissues.

There is no superimposition of tissues in CT.

Scatter radiation reduces radiographic contrastresolution.

Conventional tomography improves contrast

resolution by blurring tissues above and below thefocal plane.

Conventional tomography does not improve spatialresolution.


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CONVENTIONAL TOMOGRAPHY

Equipment arrangement for obtaining :

A) Conventional Radiograph

B) Conventional Tomograph

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1st Generation of CT Scan Finely collimated x-ray beam (pencil

beam) was used in first-generation CT

imagers.

Fan-shaped x-ray beam (fan beam) isused in all current CT imagers.

Single radiation detector.

Translate-rotate motion.

180 translations with 1o rotationbetween translations.

Single image projection per

translation.

Five minute imaging time.

Head imager only, not capable of body

imaging


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1st Generation of CT Scan

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2nd

Generation of CT Scan Fan-shape x-ray beam.

Multiple radiationdetectors-a detectorarray.

Translate-rotate motion.

Usually 18 translationswith l0o rotation betweentranslations.

Multiple image projections

per translation. Approximately, 30 s

imaging time.

Head and body imager


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3rd Generation of CT Scan

A fan beam x-ray source is used and it views theentire patient during imaging.

As many as several hundred radiation detectorsare incorporated into the curvilinear detectorarray.

The curvilinear detector array provides constantdistance between source and each detector,resulting in good image reconstruction.

This development is based on 360o

rotate-rotatemotion. Both the x-ray source and the detectorarray rotate about the same axis.

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3rd

Generation of CT Scan Hundreds of imageprojections are acquiredduring each rotation,resulting in better contrastresolution and spatialresolution.

Imaging time is reduced to1s or less.

Various arc scans are

possible in order toimprove motion blur-halfscan, full scan.

Ring artifacts arecharacteristic thirdgeneration imagers.


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4th Generation of CT Scan Fourth generation was developed principally to suppress ring

artifacts.

The x-ray source is collimated to a fan beam as in thirdgeneration.

The detector array can contain several thousand individualdetectors.

The mechanical motion is rotation of the x-ray source arounda fixed detector array (rotate stationary).

There is a modest sacrifice in geometry; however, the un-attenuated leading edge and un-attenuated trailing edge of

the fan beam allows for individual detector calibration duringeach scan.

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4th

Generationof CT Scan

Patient dose may be

somewhat higher with fourth-generation scanners becauseof inter-space betweendetectors.

When there is an inter-spacebetween detectors, some x-radiation falls on the inter-space, resulting in wasteddose.

As the fan beam passes

across each detector, animage projection is acquired.

Imaging time is 1s or less.

Various arc scans areavailable-half scan, full scan,over scan.


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Electron Beam CT (EBCT) This CT imager was developed specifically for fast

imaging.

Images can be obtained in less than 100ms, aboutthe time of a radiograph.

The x-ray source is not an x-ray tube but

rather a focused, steered, and microwaveaccelerated electron beam incident on a tungstentarget.

The target covers one-half of the imaging circle;

the detector array covers the other half. The electron beam is steered along the curved

tungsten target creating a moving source.

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Electron Beam CT (EBCT)


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Electron Beam CT (EBCT)

There are four targets, or focal tracks, and fourdetector arrays, resulting in four contiguousimages simultaneously.

Electron beam CT is principally applied to

cardiac imaging and frequently advertised as aheart scan.

Electron beam CT has no moving parts.

Electron beam CT uses a focused electronbeam on a tungsten target ring as an x-raysource.

Heat dissipation is no problem in EBCT.

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Electron beam CT can produce up to eight slicessimultaneously.

Electron beam CT scan times as short as 50msare possible.

Principal application for EBCT is cardiac imaging.


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Spiral CT

Spiral CT was introduced to clinical practicein 1989 and is now the standard CT imager.

If a third- or fourth-generation CT imager iscaused to continually rotate while thepatient couch is moved through the imagingplane, spiral CT results.

The development of slip rings was the

technology breakthrough that made spiralCT possible.

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Spiral CT

Spiral CT requires slip ring technology for datatransfer from the rotating gantry.

Spiral CT requires either an on-board highvoltage supply so that coiled high-voltage cables

are unnecessary or slip rings for high voltagetransfer.

The principal advantage to spiral CT is the abilityto image large volumes of anatomy in less time.

Single breath hold imaging of the entire torso ispossible with spiral CT.


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1st Generation of CT vs. Spiral CT

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The CT Every computed tomography (CT) imager hasthree distinguishing components-theoperating console, the computer, and thegantry.


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CT Gantry

The operating console performs two majorfunctions-imaging control with pre-selectedtechnique conditions and image viewing andmanipulation.

Multiprocessing allows a computer to performseveral functions at the same time, which reducesreconstruction time and increases capacity.

The gantry is special to CT. It houses the x-raysource, the detector array, the collimator assembly,

and, maybe, also the high-voltage generator. The patient aperture of a CT gantry has maximum

diameter of approximately 70 cm.

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X-ray Source

X-ray tubes developed for CT have veryhigh heat capacity.

Rapid heat dissipation is provided by largediameter, thick anode disks rotating at10,000 rpm.

Anode heat capacity of 6 MHU (millionheat units) are common.

That compares to less than 1 MHU forgeneral radiography.


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Heat Units (HU)

Heat units (HU) and joules (J) areequivalent measures of energy.

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Detector Array

Detector efficiency is important because itdetermines maximum tube loading andcontrols patient dose.

Three important features of the detectorarray are efficiency, number of detectors,and detector concentration.

Early CT imagers used a scintillationcrystal, photomultiplier tube as a singleelement detector.


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DetectorArray

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Detector Array

A grouping of detectors is called a detector array.

There are two types of detector array; gas-filled and solidstate.

Gas-filled detectors-high-pressure xenon have very fastresponse and no afterglow but only about 50% detectionefficiency.

Gas-filled detectors can be packed more tightly than

solid state detectors with less inter-space septa. Most solid state detectors today use a scintillator,

cadmium tungstate (CdWO4), optically coupled to aphotodiode.


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Chapter 3 Ct

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