specialized imaging techniques 2010
TRANSCRIPT
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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