sign in sign up
<<
Home , Crookes tube

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

Basics of X-ray Contents

Abstract 2 Principles of Medical 1 Generation of X-ray X-ray 4 Imaging X-ray Tube 5 Crookes Tube 6 Crookes Tube (2) 7 Coolidge Tube 8 Prof. Dr. Philippe Cattin Rotating Tube 9 Focal Spot 10 Focal Spot (2) 11 MIAC, University of Basel X-ray Tube, Filter, Collimator 12 2 X-ray Spectra X-ray Spectra 14 Sep 19th, 20156 15 Characteristic X-Ray Radiation 16 Characteristic X-Ray Radiation (2) 17 Emitted X-ray Spectra 18 3 Absorption of X-rays Absorption of X-rays 20 Photoelectric Effect 21 Photoelectric Effect (2) 22 Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

1 of 27 26.09.2016 08:33 2 of 27 26.09.2016 08:33 Compton Effect 23 Prof. Dr. Philippe Cattin: Basics of X-ray Photoelectric vs. Compton Effect 24 4 Abstract (2) Radiography 26 Radiography Setup 27 Radiography Setup 28 5 Perils of X-rays "EDISON FEARS HIDDEN PERILS OF THE 30 X-RAYS"

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

3 of 27 26.09.2016 08:33 4 of 27 26.09.2016 08:33 Generation of Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray X-ray X-ray Tube (5) The X-ray tube is a tube ( ). X-ray (4) The emitter (either a filament or a ) emits → X-rays [http://en.wikipedia.org/wiki/X-ray] (or Röntgen rays) are The anode collects these Fig 1.2: Ancient X-ray tube Photons and thus part of the → electromagnetic spectrum electrons [http://en.wikipedia.org/wiki/Electromagnetic_radiation] with a The high source wavelength in the range of , corresponding to connected to the cathode and in the range of . anode, typically , accelerates the electrons

The electrons then collide with the tungsten (sometimes molybdenum) target and accelerate other electrons, , and nuclei within the target Roughly of the energy is emitted perpendicular to Fig 1.1: The electromagnetic spectrum the beam as X-ray photons → Bremsstrahlung

As X-rays are ionising radiation they are potentially dangerous.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

5 of 27 26.09.2016 08:33 6 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray

Crookes Tube (6) Crookes Tube (2) (7)

The first X-ray tube was invented by → Sir [http://en.wikipedia.org /wiki/William_Crookes] in the century. The → Crookes Tube [http://en.wikipedia.org /wiki/Crookes_tube] also known as discharge tube or tube was used to Fig 1.3: Schematic make a visible fluorescence on minerals.

(A) Low voltage power supply to power Fig 1.5: Crookes Maltese cross tube Fig 1.6: Activated tube the cathode (C), (B) energises the coated anode (P), the mask (M) is connected to the cathode. Fig 1.4: Crookes tube 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 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.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

7 of 27 26.09.2016 08:33 8 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray

Coolidge Tube (8) Rotating Anode Tube (9)

In 1913 the Crookes tube was The → Rotating anode tube improved by → William Coolidge [http://en.wikipedia.org/wiki/X- [http://en.wikipedia.org ray_tube#Rotating_anode_tube] is an /wiki/William_David_Coolidge]. In the improvement of the Coolidge tube Coolidge aka tube the that improves the dissipation of electrons are produced by a the heat at the focal spot. tungsten filament. Fig 1.8: Scheme of a Due to the rotating anode, the rotating anode tube The high voltage between the focal spot is swept past the focal cathode and the anode Fig 1.7: Coolidge spot and the heat load spread accelerates the electrons that side-window tube (K) over a larger area. then hit the anode and emit cathode filament, (A) X-rays. anode, (Win, Wout) in- and Typical anode materials are outlet of the water cooling tungsten-rhenium target on a Only of the energy is emitted device (C), (Uh) cathode molybdenum core, backed with Fig 1.9: Rotating anode tube image as X-rays, the rest is converted to voltage ,(Ua) anode high graphite. voltage heat. With the exception of dental tubes, almost all medical X-ray tubes are of this type.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

9 of 27 26.09.2016 08:33 10 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray

Focal Spot (10) Focal Spot (2) (11)

The X-rays do not originate The smaller the anode angle the from a single point but from an wider the track → increases power area on the anode called Focal rating Spot. The angle also influences the field size (beam width) at a certain The dimensions and angle distance are carefully calculated Smaller field size ←→ better depending on the resolution application Larger field size ←→ lower Anode angle determines resolution the Effective focal spot size Focal spot size is also Fig. 1.10: An electron beam bombarding the target. The influenced by tube current Fig. 1.11: Increasing anode angle determines the the anode angle (heat) effective focal spot size size influences reduces the real Focal spot focal spot area the sharpness of the image

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

11 of 27 26.09.2016 08:33 12 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Generation of X-ray X-ray Spectra X-ray Tube, Filter, (12) Collimator X-ray Spectra (14)

The X-ray tube enclosure is Three different effects can be observed at the anode as essential for proper operation: they are hit by fast electrons:

Efficient cooling is required 1. Heating of the anode ( of the energy is converted (not necessary for dental into heat) X-rays) 2. Bremsstrahlung Removal of unwanted 3. Characteristic X-ray radiation radiation Al-Filter Collimator

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

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

13 of 27 26.09.2016 08:33 14 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray X-ray Spectra

Bremsstrahlung (15) Characteristic X-Ray (16)

→ Bremsstrahlung [http://en.wikipedia.org/wiki/Bremsstrahlung] is Radiation radiation produced by the deceleration of a , such as an electron, when deflected by another Characteristic charged particle, such as an . X-rays are emitted from The resulting Spectrum is continuous and looks similar heavy elements for different (heavy) target materials, such as Tungsten. when their electrons make transitions between the lower atomic energy levels.

Fig 1.15: Characteristic K,L-Lines

Fig 1.13: Bremsstrahlung Fig 1.14: Bremsstrahlung Spectrum

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

15 of 27 26.09.2016 08:33 16 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray X-ray Spectra Prof. Dr. Philippe Cattin: Basics of X-ray X-ray Spectra

Characteristic X-Ray (17) Emitted X-ray Spectra (18)

Radiation (2) Spectrum of an X-ray tube with a Tungsten target. The characteristic X-rays emission are shown as sharp peaks in the spectrum. 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 Fig 1.17: X-ray spectrum with a contribute to the image but tungsten target and , , are just absorbed by the , and Fig 1.16: Continuous Bremsspectrum with characteristic lines 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.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

17 of 27 26.09.2016 08:33 18 of 27 26.09.2016 08:33 Absorption of Prof. Dr. Philippe Cattin: Basics of X-ray Absorption of X-rays X-rays Photoelectric Effect (21) The → Photoelectric Effect [http://en.wikipedia.org/wiki/Photoelectric_effect] is a quantum electronic phenomenon in Absorption of X-rays (20) which electrons are emitted from after the absorption of energy In the energy range used in diagnostic radiology ( from electromagnetic radiation such as Fig 1.18: Photoelectric effect ) two effects dominate the X-ray absorption: X-rays.

Photoelectric effect The energy of the photons are given by their or their wavelength Compton effect

Other X-ray weakening phenomenons such as (1.1) Fig 1.19: Photoelectric effect Scattering Reactions with the nucleus For a given material, there exists a certain minimum frequency (threshold play at the diagnostic energy levels either only a very frequency) of the incident radiation small or no role at all. below which no emission of electrons takes place.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

19 of 27 26.09.2016 08:33 20 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Absorption of X-rays Prof. Dr. Philippe Cattin: Basics of X-ray Absorption of X-rays

Photoelectric Effect (2) (22) Compton Effect (23)

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

(1.2) Fig 1.20: Photoelectric effect

where is Plank's constant, the minimum frequency (energy) required to remove the electron, and the resting mass and the velocity of the Fig 1.21: Fig 1.22: Principle of the Compton effect ejected electron. Photoelectric effect These secondary scattered photons (scattered radiation) are highly unwanted as they distort the image and irradiate medical personnel.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

21 of 27 26.09.2016 08:33 22 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Absorption of X-rays Radiography Photoelectric vs. (24) Compton Effect Radiography (26) For lower photon energies in the X-ray spectrum the Photoelectric effect dominates over Diagnostic radiography is the second the Compton effect. most commonly used medical test, after laboratory tests. The Compton effect is, however, the dominating physical principle for photon energies in the range Since the body is made up of various of . substances with differing densities, X-rays can be used to reveal the The location of the transition energy between the internal structure of the body on film by Photoelectric and the Compton Effect depends on the highlighting these differences using target material. 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

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

23 of 27 26.09.2016 08:33 24 of 27 26.09.2016 08:33 Prof. Dr. Philippe Cattin: Basics of X-ray Radiography Prof. Dr. Philippe Cattin: Basics of X-ray Radiography

Radiography Setup (27) Radiography Setup (28)

Figure 1.24 shows the Bucky addressed already in typical components common 1913 the problem of to every Radiography separating scatter from system: primary radiation with a grid of thin lead strips 1. X-Ray tube which collimated the 2. Object emerging radiation from the 3. Table patient allowing the 4. Scatter grid unscattered primary beam 5. Dosage meter to reach the film, blocking 6. Film most of the off-axis scatter radiation. Fig 1.24: Typical setup Fig 1.25: The design and operation The antiscatter grid of an antiscatter grid significantely improves image sharpness.

Principles of Medical Imaging Sep 19th, 20156 Principles of Medical Imaging Sep 19th, 20156

25 of 27 26.09.2016 08:33 26 of 27 26.09.2016 08:33 Perils of X-rays

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

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

Principles of Medical Imaging Sep 19th, 20156

27 of 27 26.09.2016 08:33