Download - Computed Tomography
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Computed TomographyComputed Tomography
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Computed TomographyComputed Tomography
• Introduced in 70’s• Principle: Internal structures of an
object can be reconstructed from multiple projections of the object
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Philips CTVision Philips CTVision SecuraSecura
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Mechanism of CTMechanism of CT
• X-ray tube is rotated around the patient
• Radiation transmitted through the patient is absorbed by a ring of detectors
• Absorbed radiation is converted to an image
Detectors
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DetectorsDetectors
• Scintillation crystals• Xenon-gas ionization chamber
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Scintillation CrystalsScintillation Crystals
• Materials that produce light (scintillate) when x-rays interact
• Similar to intensifying screen• Number of light photons produced
energy ofincident x-ray beam• Light photons need to be
converted to electrical signal
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Ionization ChamberIonization Chamber
• X-ray ionizes xenon gas
• Electrons move towards anode
• Generates small current
• Converted to electrical signal
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AttenuationAttenuation
• Reduction in the intensity of an x-ray beam as it traverses matter, by either the absorption or deflection of photons from the beam
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Pixel - VoxelPixel - Voxel
• Pixel - picture element
• Voxel - volume element
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CT NumberCT Number
Typical CT values
Tissues Range (Hounsfield unit)
Air -1000
Lung -200 to –500
Fat -50 to –200
Water 0
Muscle +25 to +45
Bone +200 to +1000
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Image Display: WindowingImage Display: Windowing
• Usual CRT can display ~256 gray levels
• 2000 CT numbers• Select the CT
number of the tissue of interest, then range of ±128 shades
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Cone Beam CTCone Beam CT
• Uses cone shaped x-ray beam.
• Beam scans the head in 360 degrees.
• Raw data are reformatted to make images
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Benefits of Cone Beam Benefits of Cone Beam ImagingImaging
• Less radiation than multi-detector CT due to focused X-rays (less scatter)
• Fast and comfortable for the patient (9 to 60s)
• Procedure specific to head and neck applications
• One scan yields multiple 2D and 3D images
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Anatomic Landmarks on Anatomic Landmarks on CTCT
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Axial CT SectionsAxial CT Sections
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1. Zygomatic Arch2. Lat. Pterygoid plate3. Optic canal4. Sphenoid sinus5. Soft tissues of
nasopharynx
Coronal Coronal SectionsSections
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1. Frontal bone (orbital plate)
2. Ethmoid air cells3. Middle concha4. Maxillary sinus5. Inferior concha
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1. Vomer2. Ramus3. Follicle of molar4. Gr. wing of
Sphenoid5. Tongue6. Mylohyoid m
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Magnetic Resonance Magnetic Resonance ImagingImaging
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Magnetic Resonance Magnetic Resonance ImagingImaging• Three steps of MRI• MRR
– Magnetic Field– Radio-frequency Pulse– Relaxation
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Magnetic Moment Magnetic Moment DirectionDirection
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Application of RF PulseApplication of RF Pulse
Relaxation
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Spin or Angular MomentSpin or Angular Moment
• 1H, 14N, 31P, 13C, and 23Na has nuclear spin
• 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 MomentMagnetic Moment
• When a nucleus spins, it has angular momentum
• When the spinning nucleus has a charge, it has magnetic dipole moment
• Moving charges produce magnetic fields
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Hydrogen NucleusHydrogen Nucleus
• Most abundant• Yields strongest MR signal
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Radiofrequency PulseRadiofrequency Pulse
• RF pulse is an electromagnetic wave
• Caused by a brief application of an alternating electric current
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Receiver CoilsReceiver Coils
• Send or “broadcast” the RF pulse• Receive or “pick up” the MR
signals• Types: Body coils, head coils, and
a variety of surface coils
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Philips Gyroscan InteraPhilips Gyroscan Intera
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RelaxationRelaxation
• This is the process that occurs after terminating the RF pulse
• The physical changes caused by the RF pulse revert back to original state
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T1- Spin Lattice RelaxationT1- Spin Lattice Relaxation
• At the end of RF pulse, transversely aligned nuclei tend to return back to equilibrium
• This return to equilibrium results in the transfer of energy
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T2- Spin-spin RelaxationT2- Spin-spin Relaxation
• While the nuclei are in transverse phase, their magnetization interfere with each other.
• This interference leads to the loss of transverse magnetization.
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Magnetic Field StrengthsMagnetic Field Strengths
• Measured in Tesla or Gauss• Usual MRI field strength ranges
from 0.5 to 2.0 Tesla• Earth’s magnetic field is about
0.00005 Tesla (0.5 Gauss)