artifacts of diagnostic radiology clifford r. berry, dvm adjunct associate professor, university of...

Artifacts of Diagnostic Radiology

Clifford R. Berry, DVMAdjunct Associate Professor, University of Tennessee

Veterinary Specialists Center, Maitland, FL

ACVR Artifacts

Special Thanks

Dr. Crispin SpencerVeterinary Radiological Consultants

Dr. Mary MahaffeyUniversity of Georgia

Dr. Greg DanielUniversity of Tennessee

Overview

Radiographic Artifacts - DefinitionOverview of X-ray productionReview of Image Geometry, Magnification and GridsReview of Image FormationArtifacts Common to Exposure/Film HandlingReview of Image DevelopmentManual Processing (Artifacts)Automatic Processing (Artifacts)Overview of Approach to Artifact Problem SolvingArtifacts 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 aProcess called Thermionic Emission

Cathode

The focusing cup is in a metal shroud that focuses the electronsTo a specific point on the anode. Negative current (bias) will help keep electron beam focused.

Focusing Cup

Cathode

Most x-ray tubes have two filamentsThe small filament is used for low output exposures

where high detail is neededThe large filament is used for high output exposures

Modern X-ray Tube

Anode

Modern X-ray TubeRotating

Anode Target

AnodeThe 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 DesignNote 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

CathodeAnode

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 single-phase 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 SelectorIncreasing 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-raysNote 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 wave3/60 = 1/20 second time

Full-wave3/120 = 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