Transcript Anode

ACVR Artifacts
Artifacts of Diagnostic Radiology
Clifford R. Berry, DVM
Adjunct Associate Professor, University of Tennessee
Veterinary Specialists Center, Maitland, FL
Special Thanks
Dr. Crispin Spencer
Veterinary Radiological Consultants
Dr. Mary Mahaffey
University of Georgia
Dr. Greg Daniel
University of Tennessee
Overview
Radiographic Artifacts - Definition
Overview of X-ray production
Review of Image Geometry, Magnification and Grids
Review of Image Formation
Artifacts Common to Exposure/Film Handling
Review of Image Development
Manual Processing (Artifacts)
Automatic Processing (Artifacts)
Overview of Approach to Artifact Problem Solving
Artifacts Section Review - 2002 Oral Boards
Artifacts
Definition:
“any appearance on a radiograph that is not
representative of a structure within the
patient being radiographed.”
Artifacts will:
Degrades image quality
Can mimic pathology
Cause visual distractions for radiologist
Render a radiographic study non-diagnostic
Artifacts - Overview
Descriptions of Artifacts:
Type of processing
Automatic
Manual - film holders/cut corners
Density of Artifact
Plus Density - (increased or positive optical
density)
Minus Density - (decreased or negative optical
density)
Potential Timing of Artifact
Before or after exposure
Artifacts - Overview
Descriptions of Artifacts:
Potential Location of Artifact
Exposure, Patient, Film-Screen, Grid
Processing Artifact
Appearance of Radiographic Film
Coloration
View the radiographic film in reflected/transmitted light
Emulsion torn or missing
X-ray Tube
X-rays were discovered by
Wilhelm Roentgen on
November 8, 1895
Modern X-ray Tube
Cathode
Anode
Target
Tube Housing
Focal Spot
Modern X-ray Tube
Cathode
Cathode
The filament heats like the electrical coils of an electric
stove
An electron cloud develops around the filament by a
Process called Thermionic Emission
Cathode
Focusing Cup
The focusing cup is in a metal shroud that focuses the electrons
To a specific point on the anode. Negative current (bias) will
help keep electron beam focused.
Cathode
Most x-ray tubes have two filaments
The small filament is used for low output exposures
where high detail is needed
The large filament is used for high output exposures
Modern X-ray Tube
Anode
Modern X-ray Tube
Rotating
Anode Target
Anode
The negatively charged electrons are accelerated
toward the positively charged anode
Anode
The electrons from the cathode interact with the
tungsten atoms of the anode to produce x-rays
Anode Design
Note the target of the
rotating anode of this
modern x-ray tube
•
Note the anode has
splits to allow for heat
expansion
Modern X-ray Tube
Window
X-ray Tube Housing
X-ray tube is encased in a metal housing
The outer casing contains lead to shield x-rays
produced in directions other than the patient
Anode Design
Rotating the anode will spread the energy of the
electron beam over a greater area but
maintain a small focal spot
Line Focus Principle
The smaller the focal
spot the better the
image resolution
The anode is angled
so the the affect
focal spot will be
small than the actual
focal spot
Anode Angle
Actual Focal Spot
Length
Effective Focal
Spot Length
Anode Angle
2.0 x 1.2
.68 x 1.2
20
2.0 x 1.2
.41 x 1.2
12
2.0 x 1.2
.35 x 1.2
10
Line Focus Principle - Heel Effect
The negative
consequence of
the line focus
principle is
intensity of the
beam varies
from the cathode
to anode end of
the tube
Line Focus Principle - Heel Effect
Note the x-rays on the
anode side must travel
a greater distance
through the target
before exiting.
This results in greater
absorption by the target
and this less intensity
on the anode side.
Line Focus Principle - Heel Effect
Anode
Cathode
Bremsstrahlung Radiation
The electrons from the cathode filament will
pass near the nucleus of the atom.
The positive charge of the nucleus will act on
the negative charge of the electron to
decelerate it from its original path.
As the electron slows and “bends” there is
release of it’s kinetic energy as a
Bremsstrahlung (braking radiation) x-rays.
Characteristic Radiation
X-rays are produced when an electron (from the
anode electron beam) directly hits an inner
shell orbital electron, ejecting it from orbit.
The excess energy is released in the form of an
x-ray (Characteristic x-ray).
The energy of the x-ray is the difference in the
binding energies between the two shells.
Polychromatic X-ray Beam
An x-ray tube produces a combination of both general
and characteristic x-rays.
The general x-rays are a variety of energy levels.
The characteristic x-rays are at specific energy levels
(dependent upon Z of anode material).
Electrical Current
Transfer of electrons along wires
The United States electrical current is alternating which
means the electrons change direction at 60 cycles per
second (60 Hz)
X-ray Generator - Single Phase
An x-ray unit producing 60 pulses of x-ray per
second is called half wave rectification
X-ray Generator - Single Phase
A circuit can be devised to so that the voltage potential applied to the
tube always has the anode (+) and the cathode (–).
This is called Full-Wave Rectification.
X-ray Generator
As the voltage potential changes of 0 to the
maximum so does the energy of the x-ray
beam.
The drop off in x-ray beam intensity is
referred to as ripple.
Single-phase generators have a 100%
ripple in x-ray beam intensity.
X-ray Generator
All voltage waveforms shown up to now are produced by
a single-phase electrical power (standard form of
power in the US)
X-ray machine using this type power are called singlephase generators and they produce:
 Half-wave rectified = 60 pulses of x-rays/sec
 Full-wave rectified = 120 pulses of x-rays/sec
Filtered X-ray Spectrum
The x-ray energies range of 0 to the kVp
The average energy will be equal to 1/3 of the kVp (single phase
generators).
kVp Selector
Increasing kVp will increase the average energy of the beam
Increasing kVp will also increase the number of x-rays produced
mAs Spectrum - Quantity
mAs control the number of x-rays
Note the energy spectrum and average x-ray photon is
not changed
X-ray Generator
Three-phase power is the result of sophisticated electrical
engineering that produces three simultaneous voltage
waveforms out of step with each other.
Single Phase - Motion
Three Images
Half wave
3/60 = 1/20 second time
Full-wave
3/120 = 1/40 second time
Three-Phase Generators
Electrons will continue to flow to the anode
during the entire time of exposure this
producing more x-rays per unit of time.
These generators require special heavy
duty wiring.
6 pulse or 12 pulse ratings.
High Frequency Generators
A high frequency generator increases the
frequency of the electrical wave form
from 60 Hz to between 400 to 2000 Hz.
These generators can operate off single
phase standard AC current.
High frequency generators are becoming
more common in veterinary practices.
The resulting wave form has less than a 1
% ripple.
High Frequency Generators
The resulting
wave form
has less
than a 1 %
ripple
High Frequency Generators
The result is more x-rays per unit time and higher
average beam energy than single phase and three
phase generators.
High Frequency Generators
Below is a graph show a comparison of a single phase
and a high frequency generator of the same mA