Lecture 1 Flashcards
To learn for the first coursework exam.
BASICS –ELECTROMAGNETIC SPECTRUM
Range of frequencies in X-ray spectrum 10^17 – 10^20Hz
Range of wavelengths? Wavelength 10^-8 to 10^-13 meters (short) (c = fʎ), f = frequency, ʎ = wavelength, c = speed of light 3 x 10^8 m/s
Range of photon energies in eV? Photon energies in the range 100 eV to 100 keV (electron volts) (E = hf= hc/ʎ), h = planks constant (6.626 ×10-34 joule/second) , f = frequency
X-RAY 1901
Wilhelm Röntgen
X-RAY TUBE
An early Crookes x-ray tube from a museum dedicated to Wilhelm Conrad Roentgen in Würzburg, Germany-First Nobel prize Physics December 10th 1901, Philipp Lenard-Nobel prize Physics December 10th 1905
X-RAY
What do you know?
• How are x-rays produced?
• What do you need to produce x-rays?
• What are the 3 main parts of the imaging system?
The three main components of an x-ray imaging system
The three main components of an x-ray imaging system
- The x-ray tube
- The operating console
- The high-voltage generator
X-RAY RADIOGRAPHIC TABLE
What do you know about a radiographic table?
X-RAY Tilting Table
Tilting tables are designed for both diagnostic and fluoroscopic work • Tilting models usually tilt to 90 degrees in one direction and 15-30 degrees in the other direction • Tilting models include ancillary equipment; footboard, shoulder support, handgrips, compression bands.
X-RAY FLUROSCOPY TABLES
The tube is under the table, image capture is above the patient. Monitor>Video camera>Optical coupling>Image Intensifier>Grid>Patient>Table>Filtration>Collimator>X-ray tube>X-ray generator
BASICS –THERMIONIC EMISSION
Thermionic emission is a process of emission of charged particles (known as thermions)from the surface of a heated metal. • The charged particles normally are electrons.
BASICS -X-RAY TUBE TUNGSTEN ANODE
• Energy of X-rays are produced at the anode when the beam of electrons strike the anode in the X-ray tube.
• Two types of X-rays are produced: • Braking X-rays (Bremsstrahlung radiation) • Characteristic X-rays (from K-shell transitions)
Characteristic X-rays (Spikes)
Braking X-rays (Curve)
ELECTRON ANODE INTERACTION -BRAKING X-RAYS
• Imagine the energy needed to propel electron from 0 to half the speed of light in one to three centimeters. • The electrons that travel from the cathode to the anode are called projectile electrons. • When they strike the heavy metal atoms of the anode they interact with the atoms and transfer their kinetic energy to the target. • These interactions happen at a very small depth of penetration into the target. • If the incident electron passes close to the nucleus of an atom in the target anode, it will decelerate as it passes the nucleus, and its kinetic energy will decrease. The excess kinetic energy is given off as an X-ray photon. • More than 99% of the kinetic energy of the projectile electron is converted to thermal energy.
ELECTRON ANODE INTERACTION – CHARACTERISTIC X-RAYS
If an incident electron ejects an electron in the innermost K-shell of an atom, other electrons will transition downwards into the K shell. The energy lost by these electrons in transition is released as X-ray photons.
Characteristic vs. Bremsstrahlung X-rays
- Characteristic X-ray require 70 kVp or higher. Based upon the energy of the k-shell electron. • Characteristic x-rays have a precisely fixed or discrete energies. • These energies are characteristic of the differences between electron binding energies of a particular element. • For tungsten you can have one of 15 energies.
- Bremsstrahlung X-rays can be produced at any projectile electron energy. In diagnostic radiography most of the x-rays are bremsstrahlung x-rays.
Four Factors Influencing the X-ray Emission Spectrum
- The electrons accelerated from the cathode do not all have the peak kinetic energy. Depending upon the type of rectification and high voltage circuits, many electrons will have very low energy that produces low energy x-rays. 2. The target is relatively thick. Many of the bremsstrahlung x-ray emitted result from multiple interactions of the projectile electrons. Each successive interaction results in less energy. 3. Low energy x-rays are more likely absorbed by the target. 4. External filtration is always added to the tube assembly. This added filtration serves to selectively remove the lower energy photon.
Factors affecting the size and relative position of the x-ray emission spectra.
- Tube Current (mA)affects the amplitude 2. Tube Voltage affects the amplitude and position. 3. Added Filtration affects Amplitude most effective at low energies. 4. Target material affects spectrum and position of the line spectrum. 5. Voltage waveform affects the amplitude, most effective at high energies
Influence of Tube Current
A change in mA or mAs results in a proportional change in the amplitude of the x-ray emission spectrum at all energies and the intensity of the output.
Influence of Tube Potential
•Unlike tube current, a change in kVp affects both the amplitude and the position of the x-ray emission spectrum. •When kVp increases the relative distribution of emitted photons shifts to the right or to higher energies. •15% increase in kVp is equivalent to doubling the mAs.
Influence of Added Filtration
•Adding filtration to an x-ray machine has an effect on the relative shape of the spectrum similar to that of increasing the kVp. •Added filtration effectively absorbs more low energy x-ray than high energy x-rays, therefore the spectrum is reduced more to the left.
X-ray Filtration
- Filtration of the x-ray beam has two components: •Inherent Filtration •Added Filtration
- Filtration is required by law.
- Aluminum is most common material
Filtration Affects the Beam Spectrum
•Filtration removes the lower energy photons that do not contribute to image production. •Added filtration results in an increased half value layer or higher quality beam. •The overall result is an increase in the effective energy of the beam •The discrete and maximum energy of the x-ray spectrum is not effected.