Download 04 Basic Concepts of Other Imaging Modalities 08

Basic Concepts of Other
Imaging Modalities
Dent 5101
Body-section Radiography
• A special radiographic technique that
blurs out the shadows of superimposed
structures
• Object of interest less blurred
• Does not improve the sharpness
Tube and Film Move in Opposite Direction
• Tube and film
move in
opposite
direction, and
rotate about a
fulcrum
• The level of
the fulcrum is
the focal plain
Blurring
• Determined by:
– Distance of the tube travel
– Distance from the focal plain
– Distance from the film
– Orientation of tube travel
Panoramic Radiography
Panoramic Radiography
• Obtained by rotating a narrow beam of
radiation in the horizontal plane
• The film is rotated in the opposite
direction while the object (jaws) is
stationary
Focal Trough
• A 3-dimensional
curved zone or
image layer in
which structures
are reasonably
well defined.
Types of Panoramic Machines
• Panorex – Two centers of rotation.
Interruption of exposure in the midline
• Orthopantomogram – Three centers
of rotation. Continuous image
Panorex Image
Orthopantpmograph
Image Intensification
Early Fluoroscopy
• Early fluoroscopy done
by direct observation
• Screen was poorly
illuminated - image
perception inadequate
Image Intensification
• Image intensifier improved viewing
of fluoroscopy
Intensifier Tube
• Four parts:
– Input phosphor and
photocathode
– Electrostatic
focusing lens
– Accelerating anode
– Output phosphor
Intensifier Tube (Cont.)
• Input phosphor: cesium iodide (CsI) or zinccadmium-sulfide.
• Photocathode: A photo-emissive metal.
• Electrostatic focusing lens: series of
negatively charged electrodes—focuses the
electron beam.
• Output phosphor: Provides thousand-fold
more light photons.
Intensifier Tube
• Used in:
– Sialography
– Arthrography
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)
Advantages of MRI
• Higher resolution of tissues
• No ionizing radiation
• Multiplanar imaging
Disadvantages of MRI
• Long imaging time
• Hazards with ferromagnetic metals
(pacemakers, vascular clips, etc)
• Claustrophobia
• Higher cost
Relative Brightness of Tissues
Fat
Marrow
Brain
Muscle
Body Fluid
TMJ Disk
Cortical Bone
Air
White
Gray
Black
Nuclear Medicine
Nuclear Medicine
• Radioactive compounds
• Target tissues
• Radioactive agents pools in the
target tissues
• Detected and imaged by external
detectors (gamma camera).
Nuclear Medicine
• Shows structure and function of the
target tissues
• Static and dynamic conditions
• Scintigraphy scans or RN
(radionuclide) scans
• Bone scans or salivary gland scans
Technetium
• 99mTcO4- - thyroid and salivary
gland scan
• 99Tc phosphate - bone scan
• Is this an active
disease?
Phases of Salivary Gland Scan
• Flow phase:
– Five to 10 mCi of 99mTcO4
– first 30 to 120 seconds
– shows flow of blood
• Concentration phase:
– next 30 to 45 minutes
– demonstrate the anatomy and function
• Washout Phase:
– administer sialagogue
– demonstrates secretory capabilities
Cephalometric Radiography
• Reproducible and standardized views
• For measurements and assess growth
• Fixed source to film distance – 60
inches
• Cephalostats and earplugs help in
reproducible positions
Cephalometric Radiography
Contrast Agents
Contrast Agents
• Radiopaque materials
• Water soluble
• Fat soluble
• 28 – 38% iodine
Phases of Sialography
• Ductal
• Acinar
• Evacuation
Indications of Sialography
• Acute swelling secondary to ductal
obstruction
• Recurrent Inflammation
• Palpable salivary gland mass
• Autoimmune Sialadenitis
Contraindications of
Sialography
• Sensitivity to contrast agents
• Acute Sialadenitis
• Limited use in tumor diagnosis
Scintigraphy
Sialography
Radioactive material Radiopaque material
Through blood
stream
All glands imaged at
the same time
Imaged by gamma
camera
Through duct
One gland at a time
Imaged by
fluoroscopy
Contrast Studies:
Arthrography
Arthrography
• Contrast media is introduced in joint
spaces
• Upper vs. lower joint space
• Viewed by Image Intensifier Fluoroscopy
• Video recording allows study of joint
movement
Contrast Material Injection
Open Position
• Translation of
condyle
• Reduction of disk