Transcript Body-Section Radiography

Computed Tomography
Computed Tomography
• Introduced in 70’s
• Principle: Internal structures of an
object can be reconstructed from
multiple projections of the object
Philips CTVision Secura
Mechanism of CT
Detectors
• 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
• Scintillation crystals
• Xenon-gas ionization chamber
Scintillation Crystals
• Materials that produce light
(scintillate) when x-rays interact
• Similar to intensifying screen
• Number of light photons produced
a energy of incident x-ray beam
• Light photons need to be
converted to electrical signal
Ionization Chamber
• X-ray ionizes
xenon gas
• Electrons move
towards anode
• Generates small
current
• Converted to
electrical signal
Attenuation
• Reduction in the intensity of an xray beam as it traverses matter, by
either the absorption or deflection
of photons from the beam
Pixel - Voxel
• Pixel - picture
element
• Voxel - volume
element
CT Number
Tissues
Air
Lung
Fat
Water
Muscle
Bone
Typical CT values
Range (Hounsfield unit)
-1000
-200 to –500
-50 to –200
0
+25 to +45
+200 to +1000
Image 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 CT
• Uses cone shaped xray beam.
• Beam scans the head
in 360 degrees.
• Raw data are
reformatted to make
images
Benefits of Cone Beam
Imaging
• 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 CT
Axial CT Sections
Coronal Sections
1.
2.
3.
4.
5.
Zygomatic Arch
Lat. Pterygoid plate
Optic canal
Sphenoid sinus
Soft tissues of
nasopharynx
1. Frontal bone
(orbital plate)
2. Ethmoid air cells
3. Middle concha
4. Maxillary sinus
5. Inferior concha
1.
2.
3.
4.
Vomer
Ramus
Follicle of molar
Gr. wing of
Sphenoid
5. Tongue
6. Mylohyoid m
Magnetic Resonance Imaging
Magnetic Resonance Imaging
• Three steps of MRI
• MRR
– Magnetic Field
– Radio-frequency Pulse
– Relaxation
Magnetic Moment Direction
Application of RF Pulse
Relaxation
Spin 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 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 Nucleus
• Most abundant
• Yields strongest MR signal
Radiofrequency Pulse
• RF pulse is an electromagnetic wave
• Caused by a brief application of an
alternating electric current
Receiver 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 Intera
Relaxation
• 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 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 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 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)