University of Basel
University of Basel
University of Basel

# Basics of X-ray

### Principles of Medical Imaging

##### Nov 11th, 2013
Contents Basics of X-ray

# Contents

• 2Abstract
• 1 Generation of X-ray
• 4X-ray
• 5X-ray Tube
• 6Crookes Tube
• 7Crookes Tube (2)
• 8Coolidge Tube
• 9Rotating Anode Tube
• 10Focal Spot
• 11Focal Spot (2)
• 12X-ray Tube, Filter, Collimator
• 2 X-ray Spectra
• 14X-ray Spectra
• 15Bremsstrahlung
• 18Emitted X-ray Spectra
• 3 Absorption of X-rays
• 20Absorption of X-rays
• 21Photoelectric Effect
• 22Photoelectric Effect (2)
• 23Compton Effect
• 24Photoelectric vs. Compton Effect
• 5 Perils of X-rays
• 30"EDISON FEARS HIDDEN PERILS OF THE X-RAYS"
Basics 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)
5. Perils of X-rays (1 Slides)
Generation of X-ray 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 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 electronsThe anode collects these electronsThe high voltage source connected to the cathode and anode, typically $\unit{30-200}{keV}$, accelerates the electrons Fig 1.2: Ancient X-ray tube
• The electrons then collide with the tungsten (sometimes molybdenum) target and accelerate other electrons, ions, and nuclei within the target
• Roughly $\unit{1}{\%}$ of the energy is emitted perpendicular to the electron beam as X-ray photons → Bremsstrahlung

Generation of X-ray 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 Basics of X-ray

# (7) Crookes Tube (2)

 Fig 1.5: Crookes Maltese cross tube Fig 1.6: Activated tube

Generation of X-ray 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 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 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 applicationAnode angle determines the Effective focal spot sizeFocal 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 Basics of X-ray

# (11) Focal Spot (2)

 The smaller the anode angle $\theta$ the wider the track → increases power ratingThe angle also influences the field size (beam width) at a certain distanceSmaller field size ←→ better resolutionLarger field size ←→ lower resolution Fig. 1.11: Increasing the anode angle reduces the real focal spot area

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

# Outline (X-ray Spectra)

1. Generation of X-ray (9 Slides)
2. X-ray Spectra (5 Slides)
3. Absorption of X-rays (5 Slides)
5. Perils of X-rays (1 Slides)
X-ray Spectra 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

X-ray Spectra 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 Basics of X-ray

→ 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 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 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 $\unit{140}{kV}$

# 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)
5. Perils of X-rays (1 Slides)
Absorption of X-rays 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:

• Photoelectric effect
• Compton effect

Other X-ray weakening phenomenons such as

• Scattering
• Reactions with the nucleus

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

Absorption of X-rays 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 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 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 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.

1. Generation of X-ray (9 Slides)
2. X-ray Spectra (5 Slides)
3. Absorption of X-rays (5 Slides)
5. Perils of X-rays (1 Slides)

 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

 Figure 1.24 shows the typical components common to every Radiography system: X-Ray tubeObjectTableScatter gridDosage meterFilm Fig 1.24: Typical 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

# 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)