Transcript Body-Section Radiography

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
Types of Tomographic Motions
Linear
Circular
Elliptical
Figure 8
Hypocycloidal
Trispiral
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
Digital Imaging
Digital Imaging
• Conventional film-intensifying screen
radiograph - analog image.
• Digital radiograph—film-less.
• Conventional films can be digitized, with
a likelihood of loss of information.
Photostimulable Phosphor (PSP)
• Storage phosphor
• Indirect Digital Radiography
• Similar to the intensifying screen
phosphors
• Difference - PSP traps a significant
number of electrons in its phosphor,
which is later read by a laser beam
Photostimulable Phosphor (PSP)
Charge-coupled Devices
• An amorphous silicon wafer containing an
array of pixels (picture elements)
• Each pixel acts as a capacitor storing
charge
• On radiation exposure, electric charge is
deposited in the pixels proportional to the
intensity of the beam
• The variation in charge deposition can be
digitally converted to an image
Pixels
• Each square
(pixel) has its
own color
(numerical
value)
Pixel Array
CMOS
• Complimentary Metal Oxide
Semiconductor
• Principle similar to CCD
• Simpler circuit design
CCD, CMOS
Resolution
• Ability to identify two different objects
that are close to each other
• D-speed films: >10 line pair/mm
• Digital images:7-10 line pair/mm
• Eyes: 4-6 line pair/mm
Digital Subtraction
• Shows changes in
an area over time
• Requires two films
exposed with similar
geometry
Digital Subtraction
• Two radiographs are obtained
• Identical position
• One superimposed over another
• Differences in two images
identified digitally
• Allows identifying changes in hard
tissue that occurred between the two
examinations
Radiation Dose
Full Mouth Series
Receptor Collimation
D
E
Digital
D
E
Digital
Round
Round
Round
Rectangular
Rectangular
Rectangular
Background equivalent
8
4
2
4
2
1
days
days
days
days
days
day
Image
Processing/Reconstruction
• To improve diagnostic accuracy
• May improve the diagnosis of one
disease, while obscure another
• Fraud
Digital Radiography: Advantages
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Instant images
Consistent quality
High signal/noise ratio
Image Manipulation
Lower radiation dose
Disadvantages
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Lower resolution
Quality depends on monitor and printer
Print quality often not optimal
Higher initial cost
Unwanted manipulation of images