Radiography

Question 1

What is radiograph imaging?

Answer 1

• Image modalities based on X-ray
• X-ray itself is a type of ionizing radiation.

Question 2

Who discovered X-rays?

Answer 2

• Discovered by Wilhelm Röntgen, 1895
• First Nobel Prize in Physics,1901
• First X-ray image: His wife’s hand

Question 3

What is ionization?

Answer 3

• Ionization is the ejection of an electron from an atom, creating a free electron and an ion.
• If radiation transfer energy to an orbiting electron which is equal to or greater than that electron’s binding energy, then the electron is ejected from the atom.
• Resulted in 1 electron + 1 ion (an ion pair).

Question 4

What is ionizing radiation?

Answer 4

• Radiation that carries enough energy (>13.6eV ) to ionize an atom is known as ionizing radiation.
• Examples:
  
  • X-rays
  • Gamma rays
• Two Forms of Ionizing Radiation
  
  • Particulate Radiation
  • Electromagnetic Radiation

Question 5

Describe Particulate Radiation.

Answer 5

Particulate Radiation (Electrons)

• Particles of direct consequence to the formation of medical images
  
  • Electrons
  • Positrons (solely in Nuclear Medicine)
• Energetic electrons interact and transfer energy to an absorbing medium by two modes:
  
  • Radiative Transfer
    
    • Characteristic Radiation and Bremsstrahlung X-ray
  • Collisional Transfer (No ionizing)

Question 6

Explain Characteristic Radiation

Answer 6

Characteristic Radiation

• In ionization, an electron shell is left with a “hole” that must be filled in order to return the atom to a lower energy state.
• The filling of these open holes comprises an important source of secondary radiation called characteristic radiation

Question 7

Explain Bremsstrahlung X-ray.

Answer 7

Bremsstrahlung X-ray

• Bremsen: brake, Strahlung: radiation
• Interactions of an energetic electron with the nucleus of an atom
• As electrons approaches the nucleus, the positive charge of the nucleus attracts the electron, causing it to bend.
• The electron decelerates around the nucleus, loss energy in the form of electromagnetic photon, results Bremsstrahlug radiation
• Primary source of X-ray tube!!

Question 8

Explain Collisional Transfer.

Answer 8

Collisional Transfer

• A fraction of the electron’s kinetic energy is transferred to another electron in the target medium with which it collides.
• Infra radiation and heat is generated.
• No ionization

Question 9

Explain Intensity vs Photon Energy.

Answer 9

Intensity vs Photon Energy

When energetic electrons bombard a target, both characteristic and Bremsstrahlung X-ray are produced.

Question 10

Explain Electromagnetic Radiation.

Answer 10

Electromagnetic Radiation

• Electromagnetic Radiation comprises an electric and magnetic wave traveling together at right angles to each other.
• Electromagnetic radiation is also conceptualized as “packets” of energy termed photons.
• Energy of photons: E= hv, where h=Planck’s constant and v=frequency.

Question 11

Which are the Primary Electromagnetic Radiation Interactions.

Answer 11

Primary Electromagnetic Radiation Interactions

1. Photoelectric effect
2. Compton scattering
3. Pair creation (photon to electron and positron), [MeV]

Question 12

Explain Photoelectric Effect

Answer 12

Photoelectric Effect

• In the photoelectric effect, a photon with energy hv interacts with the Coulomb field of the nucleus of an atom, causing ejection of an electron, usually a Kshell electron from an atom.
• The incident photon is completely absorbed by an atom.
• The ejected electron, called photoelectron, propagates away with energy EB =E_e-=hv-E_B
• E_B=binding energy of the ejected electron
• The remaining atom is now an ion.
• The hole is filled by electron transitions from higher-orbits which produce characteristic radiation.
• Sometimes the characteristic X-ray transfer its energy to the outer-orbit and ejected an electron known as Auger Electron

Question 13

Explain Compton Scattering.

Answer 13

Compton Scattering

• A photon with energy hv ejects a valence (outer-shell) electron, yielding a new energy electron called Compton electron.
• The incident Photon loses energy to the Compton Electron and changes its direction.
• The scattered Photon is known as Compton Photon.

Question 14

Particulate vs Electromagnetic Radiation

Answer 14

Particulate vs Electromagnetic

• Particulate Radiation
  
  • X-ray generation
• Electromagnetic Radiation
  
  • Interactions with Human bodies
  • Photoelectric Effect vs Compton Scattering

Question 15

Explain Ionization VS Excitation.

Answer 15

Ionization VS Excitation

• If an ionizing particle transfer some energy to a bound electron but less than the electron’s binding energy, then the electron is raised to a higher energy state – e.g. more outer orbit – but not ejected.
• This is known as excitation

Question 16

Which are the Ionization Effects?

Answer 16

Ionization Effects

• Biological Effects
  
  • Damage cell structure (DNA), bone and skin
  • Measure the Dose (Important for clinicians)
• Disadvantages:
  
  • Cancerous and damages
• Advantages:
  
  • Damage cancer cell via ionizing and heating (therapy)
  • Strong penetration – imaging (medical and non-medical, e.g. non-destructive testing)
  • Cutting materials, e.g. IC fabrication, eye operation, etc

Question 17

What is Attenuation of X-ray Radiation?

Answer 17

Attenuation of X-ray Radiation

• Attenuation: the loss of strength of a beam of electromagnetic radiation.
• Different tissue has different attenuation to Xray radiation, which forms the primary mechanism for radiography modalities.
• Contrast for Radiographic Imaging!!

Question 18

What is Monoenergetic?

Answer 18

Narrow Beam, Monoenergeric

• Monoenergetic: all photons have same energy level.
• Assume the slab is homogenous, the intensity measured at the detector becomes:
  
  • I=I₀e^{(-\mu \delta x)}, where:
    
    • I₀=intensity of incident beam
    • \mu=linear attenuation coefficiant
• Fundamental photon attenuation law:
  
  • N=N₀e^{(-\mu \delta x)}

Question 19

What happens if the slab is not homogenous? (Monoenergic)

Answer 19

Narrow Beam, Monoenergeric

• If the slab is not homogenous, i.e. the attenuation coefficient varies as a function of x, the problem becomes
• I(x)=I0e-(\int(0->x) \mu(x’)dx’)

Question 20

What does Polyenergic mean?

Answer 20

Narrow Beam, Polyenergeric

• The linear attenuation varies as a function of Photon energy level.

Question 21

Explain the term Broad Beam.

Answer 21

Broad Beam

• Compared to the narrow beam case
  
  • Photons outside the detector’s line-of-sight geometry might get scattered toward the detector by compton interactions
  • The general attenuation law does not hold.
  • Straight line propagation is violated.
  • Even for monoenergetic beam, the beam detected is no longer monoenergetic because the Compton scattering process reduces photon energy.
• Fortunately, most X-ray imaging modalities use detector collimation which reduces the number if X-ray from non-normal directions that can hit the detectors
• The narrow beam assumption is fairly accurate.

Question 22

What is a collimator?

Answer 22

A collimator is a device that narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spatial cross section of the beam to become smaller (beam limiting device).

Question 23

How are X-rays generated? Which are the two main sources?

Answer 23

X-Ray Tube

• Generates both characteristic and Bremsstrahlung X-ray
• 1% of energy is transferred to x-ray
• 99% of energy is dissipated as heat during the bombardment
• As a result, the anode is set into rotation to avoid melting the anode target (rotates 3200 - 3600rpm).

X-ray are generated in X-rays tubes with rotating anode and stationary cathode. Electric current in the cathode heats up and generates electrons. High voltage is applied in the anode which attracts the generated electrons from the cathode. As electrons collide with anode, X-rays are emitted at perpendicular direction in a cone beam form.

Two main sources of X-ray radiation are Characteristic radiation and Bremstrahlung radiation.

• Characteristic radiation: Is generated when energetic electrons ionizes an atom, leaving a hole by the ejected electron. This hole is filled by another electron from higher energy level. As this electron jump from higher to lower energy level, it emits charactersitic radiation.
• Bremstrahlung radiation: Energetic electron approaches nucleus and is attracted by the nucleis positive charge. It then bends and decelerates which results in loosing energy. Emitted radiation is called Bremstrahlung radiation.

Question 24

Explain Field emission X-ray tube

Answer 24

Field emission X-ray tube

Disruptive technology, changes the way x-ray was generated in one hundred years

Cathode: A material that emits electrons under external stimulation, heating or electric field

Field Emission: is a electron emission process whereby electrons tunnel through an energy barrier under the influence of a high electric field

Cold Cathode or field emission cathode: An electron emitter that emits electrons without being heated.

Question 25

Which are the four emerging technologies in X-ray imaging?

Answer 25

1. Field emission X-ray tube 2. Micro focus X-ray tube 3. Phase contrast imaging 4. Color CT

Question 26

Benefits of Field Emission X-ray tubes?

Answer 26

* Low power consumption * Low dose * Portability * Fast switching * Multiple tube CT scanner without gantry rotation

Question 27

What is Projection Radiography?

Answer 27

Projection Radiography * The most commonly used method of medical imaging utilizing X-ray. * Projection of the 3D volume of the body onto a 2D surface (3D → 2D) * Represents the transmitted X-ray beam through the patient, weighted by the integrated loss of beam energy due to scattering and absorption in the body * Also known as conventional radiography

Question 28

What are the advantages and limitations of projection radiography?

Answer 28

Advantages and Limitations * Advantages * Short exposure time (0.1second) * Production of large area image (14 x 17 inch) * Low cost * Low radiation exposure * Excellent contrast and spatial resolution * Limitation * Lack of depth resolution- superimpositions of shadows from overlying and underlying tissues sometimes “hide” important lesions, which limits contrast.

Question 29

Clinical applications for Chest X-ray?

Answer 29

* Airways * Breast shadows * Bones, e.g. rib fractures * Cardiac enlargement * Diaphragm (evidence of free air) * Extrathoracic tissues (thorax)

Question 30

Clinical applications for AXR?

Answer 30

Abdominal X-ray (AXR) * Covers liver, spleen, stomach, intestines, pancreas, kidneys and bladder * Bowel obstructions (intestinal obstruction), volvulus and malrotations * Renal, urethral and bladder stones

Question 31

Clinical applications Angiography?

Answer 31

Vascular Imaging (Angiography) * Inject Iodine-based contrast agent to study the compromised blood flow * Mainly brain and heart

Question 32

Explain Mammography Conventional Radiography.

Answer 32

Mammography * Each breast is compressed horizontally (stable, avoid motion artifacts) * X-ray is then illuminated and image is taken on the film plate. * Around 10% of False Alarm rate.

Question 33

Explain Beam Hardening.

Answer 33

Filtration – Beam Hardening * Beam hardening = increasing the beam’s ”effective energy” * Undesirable for low-energy photons to enter the body (almost entirely absorbed within the body – high dose, no contribution to the image) * -\> Filter low energy photons by * anode absorbs LE photons * x-tube glass/oil housing * extra aluminium filter

Question 34

Explain the concept of Beam Restriction.

Answer 34

Beam Restriction * X-rays that exit from the tube form a cone that is ordinarily much larger than the desired body region to be imaged. * The exiting beam must be further restricted * To avoid exposing body parts of the patient that need not to be imaged * To help reducing the effect of Compton scatter * Diaphragms and Collimators are used. * Compensation filters may also be used.

Question 35

Explain the concept of Scatter Reduction.

Answer 35

Scatter Reduction * X-ray that are not absorbed by the body will arrive at the detector from the line segment originate from the x-ray source * If the photon is scattered, it will still reach the detector, which will reduce the contrast of the image. * Three methods * Grid * Air gaps * Scanning Slits (in front of patient)

Question 36

How are the limitations of radiographic film handled?

Answer 36

Film Screen Detector * X-ray exposes on today’s radiographic film. * Only 1 to 2% of X-ray are stopped by the film. * Inefficient! * As a result, modern x-ray units always have intensifying screens on both sides of the radiographic film. **Intensifying Screen:** Stop most of the x-ray, converts them into light and then exposes the film

Question 37

What are Contrast Agents?

Answer 37

Contrast Agents * Contrast agents are chemical compounds that are introduced into the body in order to increase x-ray absorption (attenuation) within the anatomical regions. * With the agents, X-ray contrast is enhanced (compared with neighboring regions without such agents)

Question 38

Give examples of Contrast Agents.

Answer 38

Contrast Agents Examples: * Iodine * Blood Vessels * Heart Chambers * Tumours * Kidneys * Bladder * Barium * Stomach * Lung (together with air)

Question 39

Assume a *35 keV* (monochromatic) x-ray source. The K-shell energies of iodine and barium are 33.2 *keV* and 37.4 *keV*. Assuming that either agent could be made into a compund that would go to the tumor, what would be the best agent to use and why?

Answer 39

The best agent to use would be iodine because binding energy of iodine (33.2 keV) is less than that of X-ray energy (35 keV). Abrupt increase in attenuation coefficient is observed at binding energy level. Therefore, if X-ray has more energy than the binding energy, increased attenuation coefficient is attained. Otherwise no change in attenuation coefficient is observed.

Question 40

Which are the Geometric effects.

Answer 40

Geometric Effects * Inverse Square Law * Obliquity * Beam Divergence and Flat Detector * Path Length * Depth-dependent Magnification

Question 41

Explain Inverse Square Law.

Answer 41

Inverse Square Law. * It states that the net flux of photons (i.e. photons per unit area) decreases as 1/r² , where r is the distance from x-ray origin: * *I₀=I_s/(4\*\pi\*d)* where *I₀* is the intensity at the origin of the detector plane, *I_s* is the intensity of the source and *d* is the distance between the source and the detector plane. * Intensity at arbitrary point *r(x,y)* on the detector plane is given by: * *I_r=I_s/(4\*\pi\*r²)* * Without compensation, this effect could falsely interpreted as object attenuation in a circular pattern around the detector origin

Question 42

What is Obliquity?

Answer 42

Obliquity * Reduction in beam intensity due to obliquity: * *I_d= I₁cos(\theta)* * *I_d(x,y)=I₀cos³(\theta)*

Question 43

What is Path Length?

Answer 43

Path Length * Consider imaging a slab of material with constant linear attenuation and thickness L. * *L'=L/cos(\theta)* * Intensity at the detector (without considering obliquity and inverse square law) * *I_d(x,y)=I₀e^{(-\mu*L/cos(\theta)}* * If not compensated, it will be interpreted as different attenuation within the object or different object thickness. * Ambiguous situation * Radiologist are trained to study radiographic locally * Not a desirable situation for computer-based image analysis.

Question 44

Explain Anode Heel Effect.

Answer 44

Anode Heel Effect * Generation of x-rays within the anode is isotropic at the atomic level, but the geometry of the anode makes that the x-rays beam’s intensity is **not uniform.** * X-rays travelling in the directions having more anode material to go through will be attenuated -\> non uniform beam intensity * This effect can outweight the effects of obliquity and inv. sq. law -\> should be compensated by filters

Question 45

Explain Depth-dependent magnification.

Answer 45

Depth-dependent magnification * Consequence: * Two objects within the body of the same size may appear to have different sizes on the radiograph. * Comparison of the radiograph of the same patient taken over months or years can only be made if the same radiographic conditions and patient position is used.

Question 46

What is the Imaging Equation with Geometric Effects?

Answer 46

Imaging Equation with Geometric Effects

Question 47

What is the refractive index for light and x-ray?

Answer 47

Refraction * Refractive index * Light; n ≈ 1.2-2 * X-ray; n\<1, or more specifically, n=1-\delta where \delta ≈ 10^-5 =\> We can neglect x-ray refraction in medical imaging

Question 48

What are the noise associated with X-rays?

Answer 48

**Noise** * **Two situations:** * Low X-ray Energy (keV) * Contrast is high as the difference between the attenuation of different tissue increases as energy decreases * Less transparent to the body (absorbed, high dose) * As a result, Nb is low (Nb – number of photons per burst per area A) * High X-ray Energy * Low Contrast – attenuation to different tissue is similar * Nb is high * Extreme high or low energy results low SNR (Tradeoff)

Question 49

What is Quantum Efficiency?

Answer 49

Quantum Efficiency * In order to be detected, an incident photon needs to interact with the detector. * However, not all the photons will interact with the detector. * Quantum Efficiency is the probability that a single photon incident upon the detector will be detected. * A basic property of the detector. * To better characterize detector performance, detective quantum efficient (DQE) considers the transformation of SNR from a detector’s input to its output