Conventional and Computed Tomography
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Transcript Conventional and Computed Tomography
Conventional and
Computed Tomography
Introduction
Is
a radiographic technique that employs
motion to show anatomical structures lying
in the a plane tissue while blurring or
eliminating the detail in images of
structures above and below the plane of
the interest
Principle
The
principle is based on synchronous
movement of 2 or 3 elements in a
tomographic system
Tomographic units synchronize the
movements of the x-ray tube and the
image receptor in opposite directions
around stationary fulcrum (pivot point)
during the exposure
The fulcrum area is sharp
Principle
The
longer the blurring the less
opportunity to create a sharp image
The greater distance to the fulcrum the
greater the blurring
The further the object is from the fulcrum
the greater difference between its
projected motion of the image receptor
and the motion of the image receptor
This cause image more sharp
Tomographic Quality
The
tomographic amplitude is the total
distance the tube travels
The tomographic amplitude is equal or
greater to the exposure amplitude
Exposure amplitude is the total distance
the tube travels during the exposure
Blur is the smearing that results in the
loss of nearly all recorded detail of objects
outside the focal plane
Tomographic Quality
Increased
blurring causes decreased
density
It is effected by tomographic amplitude,
distance from the fulcrum, distance from
the image receptor, and orientation of tube
motion
Distance from the fulcrum has a direct
relationship to blur width
Distance from the image receptor has a
direct relationship to blur width
Tomographic Quality
Orientation
of tube motion has a direct
relationship to blur width
Fulcrum controls the section level, and it
may be fixed and patient can move up and
down to change section level
Focal plane is the section
Section thickness is the width of the focal
and its control by exposure angle
Tomographic Quality
Exposure
angle inversely proportional to
section thickness
The tomographic images exhibit less
contrast than static image
Section Interval is the distance between
fulcrum levels
Types of Motion
Linear
it moving along a straight line
The major quality problem is SID and OID
Curvilinear reduce magnification and
maintain SID and OID
Circular
Elliptical
Figure eight
Trispiral
Hypocyclodial
Exposure Factors
Time
it must match the time of the x-ray
tube to complete
Complex tomographic motion often require
3-6 seconds
mA usually used below 100 to set long
time
30% more mAs is needed for wider angle
tomography
Zonography tomographs usually require
the same mAs as static radiograph
Exposure Factors
kV
15% rule is a critical tool to determine
the adjustment
5% change in kVp is required to produce
visible density
Zonography is a narrow angle tomography
exposure amplitudes less than 10o
Computed Tomography
Scanner Generations
1-
First generation
2- Second generation
3- Third generation
4- Fourth generation
Gantry
Is
the movable frame of the CT unit
It contains the x-ray tube and the detectors
It maintain the alignment of the tube and
the detectors and contains the equipment
necessary to perform the scanning
movement
The aperture for the patient is 50-58 cm
Do not force the obese patient into the
aperture
Gantry
Most
gantrys can be angled up to 30o
Table angulation can be sometimes be
used in place of gantry angulation
Positioning lights are usually mounted on
the gantry intense white halogen lights and
low power laser lights
There are often 3 positioning lights for
accurate sagittal, coronal, and transverse
centering
X-ray Tubes
CT
images produce massive amounts of
heat in the x-ray tube
Focal spots size use 0.6 mm and 1.2 mm
0.6 mm use a pulsed beam to reduce the
heat load
Modern CT pulsed scanner tubes operate
at 120 kVp, 1-5 msec pulses
X-ray tube produce 0.5-5 million heat unit
Liquid and air-cooled tube housing design
have been developed
X-ray Tubes
The
radiation beam is double collimated to
assist in eliminating scatter information
Slight misalignment can create ring artifact
image
The anode is aligned with their long axis
perpendicular to the scanner plane, this to
prevent heel effect
Collimation controls voxel length
Can be vary between 1mm to 13 mm
usually control by the software program
The dimension width determine by section
thickness or voxel length
Detectors
CT
detectors should have:
(a) high capture efficiency i.e. how well the
detectors receive the photon from the
patient and that can be control by
detectors size and the distance between
detectors
(b) high absorption efficiency i.e. how well
the detectors convert incoming photons, it
determine by the material used as well the
size and thickness of the detector
Detectors
(c)
high conversion efficiency i.e. how well
the detector convert the absorb photon to
analog or digital signal
CT detectors should have also high
stability, fast response time, and wide
dynamic range which is the ratio between
the largest signal to the smallest that can
be measured
Typical modern scanners are capable of
dynamic range of 1,000,000:1
Computer
Is
design to control the data acquisition,
process and display, and storage
The computer should be in enclosed room
with controlled temperature and humidity
CT console provide the radiographer
access to the software program that data
acquisition, controls data processing and
display and storage functions
Computer
A
system program is used to start up the
CT unit, this program turns on and perform
quality assurance, and record various
problems
The CT console operate from the menu
simply uses a keyboard, light pen
Data Acquisition
Controls
the tube and detector collimation
(pixel size), matrix size, gantry angle, table
top entrance, section increment
movement, kVp, mA, scan speed
Display Console
Controls
the digital image production
process, that compile the image and
display parameters, such as window width
and level
Exposure Factor
Most
CT performed at 120 kVp
Time is not a factor as it must be
controlled by scanning program
mA should be setup
Dual energy scanning units required
usually 80 kVp and 140 kVp
Artifacts
Motion
Metal
or Star
Beam Hardening
Partial Volume Effect
Ring artifacts