University of Basel
University of Basel
University of Basel

Basics of X-ray

Principles of Medical Imaging

Prof. Dr. Philippe Cattin
MIAC, University of Basel
Nov 11th, 2013
Contents Prof. Dr. Philippe Cattin: Basics of X-ray

Contents

Prof. Dr. Philippe Cattin: Basics of X-ray

(2) Abstract



Generation of X-ray

Outline (Generation of X-ray)

  1. Generation of X-ray (9 Slides)
  2. X-ray Spectra (5 Slides)
  3. Absorption of X-rays (5 Slides)
  4. Radiography (3 Slides)
  5. Perils of X-rays (1 Slides)
Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(4) X-ray

→ X-rays [http://en.wikipedia.org/wiki/X-ray] (or Röntgen rays) are Photons and thus part of the → electromagnetic spectrum [http://en.wikipedia.org/wiki/Electromagnetic_radiation] with a wavelength in the range of \unit{10-0.01}{nm}, corresponding to frequencies in the range of \unit{30-30\,000}{PHz}\(10^{15}).


Fig 1.1: The electromagnetic spectrum
As X-rays are ionising radiation they are potentially dangerous.


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(5) X-ray Tube

The X-ray tube is a vacuum tube (\unit{10^{-6}}{Torr}).

  • The emitter (either a filament or a cathode) emits electrons
  • The anode collects these electrons
  • The high voltage source connected to the cathode and anode, typically \unit{30-200}{keV}, accelerates the electrons

Fig 1.2: Ancient X-ray tube


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(6) Crookes Tube

The first X-ray tube was invented by → Sir William Crookes [http://en.wikipedia.org/wiki/William_Crookes] in the 19^{th} century. The → Crookes Tube [http://en.wikipedia.org/wiki/Crookes_tube] also known as discharge tube or cold cathode tube was used to make a visible fluorescence on minerals.

(A) Low voltage power supply to power the cathode (C), (B) energises the phosphor coated anode (P), the mask (M) is connected to the cathode.

By replacing the mask (M) with a beam focusing cylinder, the Crookes tube evolved into a electron gun that was later used for the oscilloscope.

It was also observed that the application of high voltage to the anode produces X-rays. The phosphor anode was later replaced with more effective metal targets which focused the beam on a small target.


Fig 1.3: Schematic


Fig 1.4: Crookes tube


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(7) Crookes Tube (2)


Fig 1.5: Crookes Maltese cross tube

Fig 1.6: Activated tube


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(8) Coolidge Tube

In 1913 the Crookes tube was improved by → William Coolidge [http://en.wikipedia.org/wiki/William_David_Coolidge]. In the Coolidge aka hot cathode tube the electrons are produced by a tungsten filament.

The high voltage between the cathode and the anode accelerates the electrons that then hit the anode and emit X-rays.

Only 1\% of the energy is emitted as X-rays, the rest is converted to heat.



Fig 1.7: Coolidge side-window tube (K) cathode filament, (A) anode, (Win, Wout) in- and outlet of the water cooling device (C), (Uh) cathode voltage ,(Ua) anode high voltage


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(9) Rotating Anode Tube

The → Rotating anode tube [http://en.wikipedia.org/wiki/X-ray_tube#Rotating_anode_tube] is an improvement of the Coolidge tube that improves the dissipation of the heat at the focal spot.

Due to the rotating anode, the focal spot is swept past the focal spot and the heat load spread over a larger area.

Typical anode materials are tungsten-rhenium target on a molybdenum core, backed with graphite.

With the exception of dental tubes, almost all medical X-ray tubes are of this type.


Fig 1.8: Scheme of a rotating anode tube


Fig 1.9: Rotating anode tube image


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(10) Focal Spot

The X-rays do not originate from a single point but from an area on the anode called Focal Spot.

  • The dimensions and angle are carefully calculated depending on the application
  • Anode angle determines the Effective focal spot size
  • Focal spot size is also influenced by tube current (heat)
  • Focal spot size influences the sharpness of the image

Fig. 1.10: An electron beam bombarding the target. The anode angle \theta determines the effective focal spot size


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(11) Focal Spot (2)

  • The smaller the anode angle \theta the wider the track → increases power rating
  • The angle also influences the field size (beam width) at a certain distance
  • Smaller field size ←→ better resolution
  • Larger field size ←→ lower resolution

Fig. 1.11: Increasing the anode angle reduces the real focal spot area


Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray

(12) X-ray Tube, Filter, Collimator

The X-ray tube enclosure is essential for proper operation:

  • Efficient cooling is required (not necessary for dental X-rays)
  • Removal of unwanted radiation
    • Al-Filter
    • Collimator

Fig. 1.12: Basic set up of the X-ray tube, filter and collimator


X-ray Spectra

Outline (X-ray Spectra)

  1. Generation of X-ray (9 Slides)
  2. X-ray Spectra (5 Slides)
  3. Absorption of X-rays (5 Slides)
  4. Radiography (3 Slides)
  5. Perils of X-rays (1 Slides)
X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray

(14) X-ray Spectra

Three different effects can be observed at the anode as they are hit by fast electrons:

  1. Heating of the anode (99\% of the energy is converted into heat)
  2. Bremsstrahlung
  3. Characteristic X-ray radiation


X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray

(15) Bremsstrahlung

→ Bremsstrahlung [http://en.wikipedia.org/wiki/Bremsstrahlung] is radiation produced by the deceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus.

The resulting Spectrum is continuous and looks similar for different (heavy) target materials, such as Tungsten.


Fig 1.13: Bremsstrahlung

Fig 1.14: Bremsstrahlung Spectrum


X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray

(16) Characteristic X-Ray Radiation

→ link [http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html] → X-ray Line Transitions [http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xterm.html#c1]

Characteristic X-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels.


Fig 1.15: Characteristic K,L-Lines


X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray

(17) Characteristic X-Ray Radiation (2)

The characteristic X-rays emission are shown as sharp peaks in the spectrum.


Fig 1.16: Continuous Bremsspectrum with characteristic lines K_\alpha,K_\beta


X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray

(18) Emitted X-ray Spectra

Spectrum of an X-ray tube with a Tungsten target.

The dashed lines in Fig 1.17 show the theoretical spectrum of the → Bremsstrahlung [http://en.wikipedia.org/wiki/Bremsstrahlung]. The lower energy (soft) rays are unwanted as they don't contribute to the image but are just absorbed by the tissue and thus increase the dosage.

Thin metallic sheets (aluminium, copper) are placed between the X-ray tube and the target to harden the X-rays by effectively filtering out the lower energy rays.


Fig 1.17: X-ray spectrum with a tungsten target and \unit{80}{kV}, \unit{100}{kV}, \unit{120}{kV}, and \unit{140}{kV}


Absorption of X-rays

Outline (Absorption of X-rays)

  1. Generation of X-ray (9 Slides)
  2. X-ray Spectra (5 Slides)
  3. Absorption of X-rays (5 Slides)
  4. Radiography (3 Slides)
  5. Perils of X-rays (1 Slides)
Absorption of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray

(20) Absorption of X-rays

In the energy range used in diagnostic radiology (\unit{30-200}{keV}) two effects dominate the X-ray absorption:

Other X-ray weakening phenomenons such as

play at the diagnostic energy levels either only a very small or no role at all.



Absorption of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray

(21) Photoelectric Effect

The → Photoelectric Effect [http://en.wikipedia.org/wiki/Photoelectric_effect] is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as X-rays.

The energy of the photons are given by their frequency \nu or their wavelength \lambda

\[E_{Photon}=h\nu=\frac{hc}{\lambda}\](1.1)

For a given material, there exists a certain minimum frequency (threshold frequency) of the incident radiation below which no emission of electrons takes place.


Fig 1.18: Photoelectric effect

Fig 1.19: Photoelectric effect


Absorption of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray

(22) Photoelectric Effect (2)

The energy of the photon is split into the escaping energy E_0 and the kinetic energy E_{kin} of the electron, thus

\begin{eqnarray*}E_{Photon}&=&E_0+E_{kin}\\&=&hf_0+\frac{1}{2}mv^2\\\end{eqnarray*}(1.2)

where h is Plank's constant, f_0 the minimum frequency (energy) required to remove the electron, and m,v the resting mass and the velocity of the ejected electron.


Fig 1.20: Photoelectric effect

Fig 1.21: Photoelectric effect


Absorption of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray

(23) Compton Effect

In physics, → Compton effect [http://en.wikipedia.org/wiki/Compton_effect] or the Compton scattering, is the decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, when it interacts with matter.


Fig 1.22: Principle of the Compton effect

These secondary scattered photons (scattered radiation) are highly unwanted as they distort the image and irradiate medical personnel.



Absorption of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray

(24) Photoelectric vs. Compton Effect

For lower photon energies <\unit{100}{keV} in the X-ray spectrum the Photoelectric effect dominates over the Compton effect.
The Compton effect is, however, the dominating physical principle for photon energies in the range of \unit{100}{keV}-\unit{100}{MeV}.

The location of the transition energy between the Photoelectric and the Compton Effect depends on the target material.



Radiography

Outline (Radiography)

  1. Generation of X-ray (9 Slides)
  2. X-ray Spectra (5 Slides)
  3. Absorption of X-rays (5 Slides)
  4. Radiography (3 Slides)
  5. Perils of X-rays (1 Slides)
Radiography Prof. Dr. Philippe Cattin: Basics of X-ray

(26) Radiography

Diagnostic radiography is the second most commonly used medical test, after laboratory tests.

Since the body is made up of various substances with differing densities, X-rays can be used to reveal the internal structure of the body on film by highlighting these differences using attenuation, or the absorption of X-ray photons by the denser substances


Fig 1.23: Roentgen's X-ray picture of the hand of Alfred von Kolliker, taken 23 Jan 1896


Radiography Prof. Dr. Philippe Cattin: Basics of X-ray

(27) Radiography Setup

Figure 1.24 shows the typical components common to every Radiography system:

  1. X-Ray tube
  2. Object
  3. Table
  4. Scatter grid
  5. Dosage meter
  6. Film

Fig 1.24: Typical setup


Radiography Prof. Dr. Philippe Cattin: Basics of X-ray

(28) Radiography Setup

Bucky addressed already in 1913 the problem of separating scatter from primary radiation with a grid of thin lead strips which collimated the emerging radiation from the patient allowing the unscattered primary beam to reach the film, blocking most of the off-axis scatter radiation.

The antiscatter grid significantely improves image sharpness.


Fig 1.25: The design and operation of an antiscatter grid


Perils of X-rays

Outline (Perils of X-rays)

  1. Generation of X-ray (9 Slides)
  2. X-ray Spectra (5 Slides)
  3. Absorption of X-rays (5 Slides)
  4. Radiography (3 Slides)
  5. Perils of X-rays (1 Slides)
Perils of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray

(30) "EDISON FEARS HIDDEN PERILS OF THE X-RAYS"

Clarence Dally one of Edisons workers lost a hand and an arm to his research with the Fluoroscope. Edison himself reported focusing problem with on eye.
Wizard Edison and Employee injured by X-rays and Fluoroscope, which almost cost Dally's life.



→ New York World [http://home.gwi.net/~dnb/read/edison/edison_xrays.htm], Monday, August 3, 1903, page 1

Nov 11th, 2013 Principles of Medical Imaging