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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
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