Specialized Imaging Techniques 2010

8/6/2019 Specialized Imaging Techniques 2010

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Basic Concepts of OtherImaging Modalities

Dr. Anita Gohel


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Imaging the maxillofacialcomplex

Conventional Tomography CT

Cone Beam CT MRI


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Conventional Tomography

Tomo = cut, section or layer slice Image which provides just a slice of the

whole structure

Focused image without superimpositionof structures with thick image layer


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Conventional Tomography

Film based tomography Blurring of structures outside plane of

interest

X-ray tube moves about a fulcrum

Tube and film opposite sides of fulcrum

Tube and film move in oppositedirections

Angle determines thickness of the slice


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Conventional FilmTomography

Wider angle = thinner section Narrow angle = thicker section

Objective is to blur objects not ofinterest

Accomplished by many different typesof movements Linear, Circular, Trispiral, Elliptical,

Hypocycloidal

More complex movement = less likely toimage an unwanted structure


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Tomography Movements


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Linear Tomography


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Spiral Tomography


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Hypocycloidal Tomography


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Computed Tomography

Introduced in 70s Principle: Internal structures of an

object can be reconstructed frommultiple projections of the object


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CT Scanner


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CT Movement/Geometry


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CT Geometry

Early scanners involved few detectorswith the tube and the detectors moving

Newer scanners use spiral or helicalmovement to create overlapping images


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Mechanism of CT

X-ray tube is rotatedaround the patient

Radiation transmitted

through the patient isabsorbed by a ring ofdetectors

Absorbed radiation isconverted to an image

Detectors


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Image construction

X-ray hits detector which consists of anionization chamber or scintillation crystals

Scintillation crystals

Cadmium tungstate most common Crystals convert x-rays to light

Photodiode converts light to electric current

Ionization chamber detector w/ Xe gascreates current

Analog information digitized and sent to thecomputer for reconstruction


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Ionization Chamber

X-ray ionizes xenongas

Electrons move

towards anode Generates small

current

Converted toelectrical signal


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Image Construction


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CT Image Construction


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Pixel - Voxel

Pixel - pictureelement

Voxel - volume

element


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Image construction CT

The image consists of a matrix of individualpixels representing the face of a volumecalled a voxel

A single-plane image is constructed anddisplayed as differences in optical density

Range is from 1000 to +1000 Hounsfield units

-1000 is air, 0 is water and +1000 is bone


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Windowing

Usual CRT can display ~256 gray levelsApprox. 60 shades are visually

discernible

2000 CT numbers

Assign 8 CT numbers to one shade

Or, select the CT number of the tissueof interest, then range of 128 shades


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Windowing

Center CT number is the window level Range of CT number above and below

the window level is window width

Window level determines brightness

Window width determines contrast


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Window Width & Level

Center CT number is thewindow level

Range of CT number above andbelow the window level iswindow width

Window level determinesbrightness

Window width determinescontrast


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Windowing

Manipulation allows customization ofvisibility

soft tissues

brain

dense structures

bone


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WL = 0

WW = 400

200

-200

400 0


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CT Number

+200 to +1000Bone

+25 to +45Muscle0Water

-50 to 200Fat

-200 to 500Lung

-1000Air

Range (Hounsfield unit)TissuesTypical CT values


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CT


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Advantages of CT overFilm Radiography

Elimination of superimposition ofstructures outside area of interest

High contrast resolution

Differences of


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1. Temporalis m

2. Masseter m3. Lat pterygoid m

4. Med pterygoid m

5. Carotid canal


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1. Superior rectus m.

2. Optic nerve

3. Inferior rectus m.

4. Lateral rectus m.

5. Zygomatic bone


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Magnetic Resonance Imaging

Three steps of MRI MRR

Magnetic Field

Radio-frequency Pulse

Relaxation


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Philips Gyroscan Intera


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Philips Gyroscan Intera


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Spin or Angular Moment

1H, 14N, 31P, 13C, and 23Na has nuclearspin

They spin around their axes similar to

earth spinning around its axis

Elements with nuclear spin has odd

number of protons, neutrons


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Magnetic Moment

When a nucleus spins, it has angularmomentum

When the spinning nucleus has a

charge, it has magnetic dipole moment

Moving charges produce magnetic fields


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Hydrogen Nucleus

Most abundantYields strongest MR signal


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Spinning Hydrogen NucleusApplied magnetic

field

S

N

Spin


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Precession of a Top


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Radiofrequency Pulse

RF pulse is an electromagnetic wave Caused by a brief application of an

alternating electric current


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Receiver Coils

Send or broadcast the RF pulse Receive or pick up the MR signals

Types: Body coils, head coils, and avariety of surface coils


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Magnetic Field Strengths

Measured in Tesla or Gauss Usual MRI field strength ranges from

0.02 to 2.0 tesla


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Advantages of MRI

Higher resolution of tissues No ionizing radiation

Multiplanar imaging


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Disadvantages of MRI

Long imaging time Hazards with ferromagnetic metals

(pacemakers, vascular clips, etc)

Claustrophobia

Higher cost


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Relative Brightness of Tissues

Fat WhiteMarrow

Brain

Muscle GrayBody Fluid

TMJ Disk

Cortical Bone

Air Black

1 Tensor veli


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1. Tensor velipalatini

2. Mastoid air cells

3. Temporalis m.

4. Medulla oblongata

1 Infratemporal sp


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1. Infratemporal sp.

2.

Masticator space3. Parotid space

4. Parapharyngeal sp

5. Int. carotid

6. Carotid space


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Nuclear Medicine

radioactive compounds target tissues

radioactive agents pools in the targettissues

detected and imaged by external

detectors


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Nuclear Medicine

Shows structure and function of the targettissues

Static and dynamic conditions

Scintigraphy scans or RN (radionuclide)

scans

Bone scans or salivary gland scans


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Technetium

99m

TcO4-

- thyroid and salivary gland scan99Tc phosphate - bone scan

99m

Tc with sulfur-colloid - liver and spleenscan


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Case 1

Is this an activedisease?


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TMJ Implant Bony

Use

Simple Technique LoConventional

AdvantageModality


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Bone scan

TMJ, Soft tissues,Pathology, Sinus

TMJ, Implant, BonyPathology

TMJ, Implant, BonyPathology, Some

soft tissue

TMJ, Implant, Bony

Pathology

Detecting Metastases,Active infection

Nuclear Med.

Soft tissuevisualization,Nonionizing radiation

MRI

Low Radiation,resolution,images

many structures

Cone BeamCT

Great resolution,images many

structures

CT

Simple Technique, Lo

radiation

Conventional

Tomography

Documents

Specialized Imaging Techniques 2010