advanced imaging techniques

7/28/2019 Advanced Imaging Techniques

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ADVANCED IMAGING

TECHNIQUES


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The most versatile of advanced imaging techniquesinclude:

Computed tomography(CT)

Magnetic resonance imaging(MRI)

Ultrasonography

Nuclear medicine.


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COMPUTED

TOMOGRAPHY

In 1972 Godfrey hounfield,an

engineer,announced the invention

of revolutionary imaging

techniques that used image

reconstruction mathamatics

developed by Alan cormade in

1950s & 1960s.


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Computed

tomographyscanners

This form of imaging is called as

computed tomography,abbreviated as

CT,Computed tomographic

scanning,axial tomography &

computerised transaxial tomography.


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CT scanner consists of an x-

ray tube that emits a finelycollimated,fan shaped x-ray

directed through a patient to

a series of scintillation

detectors or ionization

chambers.These detectorsmeasure the no of photons

that exit the patient.


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The intensity of the x-ray beam exiting the body is

determined by:

1)The energy of the x-ray tube source.

2)The distance between the source of x-ray & the

detector.

3)The attenuation of beam by materials in object being

scanned.


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TECHNIQUE

Patient would

lie on a

stationary

table while thex-ray source

rotates one

cycle around

them.The

table would

move 1-5mm

for the next

scan.

CT scan thatuses this type of

step &

shootmovement

for image

acquisition are

called

incrementalscanners.

The final image

set consists of

series ofoverlapping

axial

images,made up

of right angles

to the long axis

of the patients

body.


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These two dimensional

slices are cross-

sections,typically 1mm

thick.

In 1989 CT scannerswere introduced that

acquire image data in

helical fashion. In

helical CT

scanners,Pitch refers

to the amount of

patient movement

compared with the

width of the image

acquired.

Pitch=Table

travel per x-ray

rotations/imagethickness


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MULTIDETECTORHELICAL CT-

MDCT,multislice

ct,multirow CT.With

this method anywherefrom 4-64 adjacent

detectors arrays are

used in conjunction

with helical CT.

The time for the x-ray

tube to make a full

cycle around thepatients has been

reduced to as little as

0.35sec.


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ELECTRON BEAM CT

An electron gun generates an electron beam that is focused

electrostatically on a fixed tungstan target circling halfway

around the patient.

Because there are no moving parts an image may be

acquired in less than 100microsec.This technique is

primarily used for cardiac imaging to stop heart motion.


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CT TOMOGRAPHICSCANNER COMPUTEDASSEMBLYThe basic componentsof CT are:

1.Gantry

2.Patient supporting

couch

3.Computer


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GANTRY

The gantry is made up of detector array,patient supporting couch & the x-

ray tube or source.

The gantry can be tilted up to 30degree.

The facility to tilt helps in excluding structure from the scan that maydegrade the final image(eg metallic dental restoration).

Components of gantry are:

X-ray detectors

Detector array


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X-ray tubeCT scanners use x-ray tubes with rotating anodes.These tubes a high heat

capicity 8 million heat units.

Operate at-120-140(kvp)

Focal spot sizes ranges-0.5-2.0mm.

Xray beam is collimated before & after the patient.Prepatient collimation

adjusts patient exposure.Postpatient collimation controls slice thickness.

Slice thickness is typically between 1 &3mm.

Thinner slices results in higher spatial resolution & contrast less partial

volume effect & higher patient dose.


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DETECTORS

X-ray beam exiting the patient is captured by an array ofdetectors.

The detectors are usually gas filled or solid state.

Gas filled ion chamber detectors are usually made of high

pressure xenon.

Gas filled ion chamber respond quickly but only capture about50% of photons.

Solid state detectors are commonly used they are made up of

cadmium tungstate


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IMAGE

RECONSTRUCTION

Computer algorithms use these photons counts

to construct one or more often,many digital

crossectional images.The ct image is recorded

and displayed as a matrix of individual blocks

called voxels(volume elements).Each square of

the image matrix is a pixel.

Next an interpolator algorithm is used to

correct for the helical motion of the scanner

and to construct planer crossectional from the

helical information.


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PATIENT

SUPPORT COUCH

The patient support couch helps instabilising the position of the patient

during ct scan.The couch must be made

of low molecular weight material such as

carbon filter to ensure that the path of

the x ray beam is not altered before or

after it transverse the patient.


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COMPUTER

The rapidly of capturing the image,acquring data,and larger matrix

size(512 *512) necessitates the use of high speed computers.

CONTROL CONSOLE

The control console allows the operator to select the parameters of

the ct scan and view the image as they are been generated.

Image is stored in the computer so that it can be manipulated in no.

of ways(eg. Magnetic tapes or discs).


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COMPUTED TOMOGRAPHIC IMAGE

CT number representing tissue density.

The no. is propotional to the degree to which the materialwith in the voxel has attenuated the xray beam.

Ct no. also called as hounsfield units range from -1000 to

+1000.

An image optimised for viewing bone-a bone window,may

have a range of 700 units and mean of 5000units.


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TYPICAL HOUNSFIELD UNITS FOR AIR AND TISSUES

TISSUE HOUNSFIELD UNITS

BONE +400-+1000

SOFT TISSUE +40-+80

WATER 0

FAT -60_-100

LUNG -400_-600

AIR -1000


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ARTIFACTS

Different types of

artifacts may degrade

CT images.

Some are: Partial volumeartifacts

Beam hardening

artifacts

Metal artifacts


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CONTRASTAGENTS

Are the substances

used to improvevisualization of

structure. CT

imaging frequencyuses

iodine,administered

i/v to enhance soft

tissue and vascular

image details.


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ADVANTAGES

Multiplaner

images.

Greater geometric

projections.

Manipulation ofacquired images.

Soft tissue

imaging.


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DISADVANTAGES

Expensive.

Patients exposure to

radiations.

Production

of

artifacts.


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USES IN DENTISTRY

Evaluation of presence & extent of clinically suspected pathology in the

head & neck.

Detection of extention of disease process into the paranasal sinus,baseof skull& orbit.

Determination of location,extent & displacement of maxillofacial skeletal

fractures,including direction of subdural & epidural haematomas.

Salivary gland imaging.


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Evaluvation of potential implants sites

using 3D image.

Evaluation of components of TMJ.

CT guided FNAC biopsies.

Virtual surgeries.


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NUCLEAR MEDICINE

Radionuclide imaging uses radioactive atoms or

molecules that emit gamma rays.

Radionuclides allow measurement of tissue

funtions in vivo & provide early marker of

disease through measurement of biochemical

changes.

After the radionuclides are administered in the

body they distribute according to there chemical

properties.


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RADIONUCLITIDES

The ideal radionuclide has a short half life,emits gamma rays and is capable of binding to a variety of

pharmaceuticals.Although many gamma emitting isotopes are used in imaging,including

iodine,gallium,selenium.The most commonly used is Technetium.

Technetium has a half life of 6hr and emits primarily 140 kiloelectron volt photon.As Tc is injected i/v itgets concentrated in thyroid gland and gastric mucosa.


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TO IMAGE A BONE

Tc is typically bound to methylene diphosphonate and a dose of 20 to

30mlci is injected i/v.immediately after injection the tracer distributes

intravascularly.

Images made during this flow phase,the first 60-90sec are called as

radionuclide angiography.

In the second,or blood pool phase,the tracer moves quickly into the

extracellular spaces.

The third or bone scintigraphy phase,is made 2-3hrs after injection show

most of the tracer activity in skelton,kidney,and bladder.


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GAMMASCINTILLSTION

CAMERA

Gamma

sclintillation

camera alsocalled as anger

camera are most

common means

of forming an

image.

These camera

capture photon

and convertthem to light

and then to

voltage signal.

These signal is

constructed to a

planer imagethat shows the

distribution of

the radionuclide

in the patient.


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SINGLE

PHOTON

EMISSION

COMPUTED

TOMOGRAPHY

SPECT is the

method of

acquiring

tromographic

slices through

the patient.

In this

techniqueeither a single

or multiple

gamma camera

is rotated

360degree

about thepatient.

Image

acquisition

takes about 30-

45min.

Tomography

enhances

contrast and

removes

superimposed

activity.


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APPLICATIONS

Investigate

abnormal metabolic

bone activity.helps

in assessing growthactivity in case of

condylar hyperplasia

and presence of

metabolic lesions.


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POSITRON EMISSION

TOMOGRAPHY

PET is most advanced

imaging modality in

nuclear medicine.

100 times sensitive

than gamma camera.

Relies on positron

emitting radionuclide

generated in cyclotron.

Radionuclide used are

glucose,aminoacid


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USES

The PET/CTcombination has

been helpful in

staging andtreatment of

squamous cell

carcinoma in

head and neck.


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PRESENTED

BY

SAKEENA

ASSAD

ROLL

NO:75

advanced imaging techniques

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