Download - Computed Tomography
Computed TomographyComputed Tomography
Computed TomographyComputed Tomography
• Introduced in 70’s• Principle: Internal structures of an
object can be reconstructed from multiple projections of the object
Philips CTVision Philips CTVision SecuraSecura
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
DetectorsDetectors
• Scintillation crystals• Xenon-gas ionization chamber
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
Ionization ChamberIonization Chamber
• X-ray ionizes xenon gas
• Electrons move towards anode
• Generates small current
• Converted to electrical signal
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
Pixel - VoxelPixel - Voxel
• Pixel - picture element
• Voxel - volume element
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
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
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
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
Anatomic Landmarks on Anatomic Landmarks on CTCT
Axial CT SectionsAxial CT Sections
1. Zygomatic Arch2. Lat. Pterygoid plate3. Optic canal4. Sphenoid sinus5. Soft tissues of
nasopharynx
Coronal Coronal SectionsSections
1. Frontal bone (orbital plate)
2. Ethmoid air cells3. Middle concha4. Maxillary sinus5. Inferior concha
1. Vomer2. Ramus3. Follicle of molar4. Gr. wing of
Sphenoid5. Tongue6. Mylohyoid m
Magnetic Resonance Magnetic Resonance ImagingImaging
Magnetic Resonance Magnetic Resonance ImagingImaging• Three steps of MRI• MRR
– Magnetic Field– Radio-frequency Pulse– Relaxation
Magnetic Moment Magnetic Moment DirectionDirection
Application of RF PulseApplication of RF Pulse
Relaxation
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
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
Hydrogen NucleusHydrogen Nucleus
• Most abundant• Yields strongest MR signal
Radiofrequency PulseRadiofrequency Pulse
• RF pulse is an electromagnetic wave
• Caused by a brief application of an alternating electric current
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
Philips Gyroscan InteraPhilips Gyroscan Intera
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
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
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.
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)