1
Q
What is radiograph imaging?
A
- Image modalities based on X-ray
- X-ray itself is a type of ionizing radiation.
2
Q
Who discovered X-rays?
A
- Discovered by Wilhelm Röntgen, 1895
- First Nobel Prize in Physics,1901
- First X-ray image: His wife’s hand
3
Q
What is ionization?
A
- 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).
4
Q
What is ionizing radiation?
A
- 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
5
Q
Describe Particulate Radiation.
A
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)
- Radiative Transfer
6
Q
Explain Characteristic Radiation
A
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
7
Q
Explain Bremsstrahlung X-ray.
A
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!!
8
Q
Explain Collisional Transfer.
A
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
9
Q
Explain Intensity vs Photon Energy.
A
Intensity vs Photon Energy
When energetic electrons bombard a target, both characteristic and Bremsstrahlung X-ray are produced.
10
Q
Explain Electromagnetic Radiation.
A
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.
11
Q
Which are the Primary Electromagnetic Radiation Interactions.
A
Primary Electromagnetic Radiation Interactions
- Photoelectric effect
- Compton scattering
- Pair creation (photon to electron and positron), [MeV]
12
Q
Explain Photoelectric Effect
A
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
13
Q
Explain Compton Scattering.
A
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.
14
Q
Particulate vs Electromagnetic Radiation
A
Particulate vs Electromagnetic
- Particulate Radiation
- X-ray generation
- Electromagnetic Radiation
- Interactions with Human bodies
- Photoelectric Effect vs Compton Scattering
15
Q
Explain Ionization VS Excitation.
A
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
16
Q
Which are the Ionization Effects?
A
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
17
Q
What is Attenuation of X-ray Radiation?
A
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!!
18
Q
What is Monoenergetic?
A
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
- I=I0e(-\mu \delta x), where:
- Fundamental photon attenuation law:
- N=N0e(-\mu \delta x)
19
Q
What happens if the slab is not homogenous? (Monoenergic)
A
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’)
20
Q
What does Polyenergic mean?
A
Narrow Beam, Polyenergeric
- The linear attenuation varies as a function of Photon energy level.
21
Q
Explain the term Broad Beam.
A
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.
22
Q
What is a collimator?
A
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).
23
Q
How are X-rays generated? Which are the two main sources?
A
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.
24
Q
Explain Field emission X-ray tube
A
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.
25
Which are the four emerging technologies in X-ray imaging?
1. Field emission X-ray tube
2. Micro focus X-ray tube
3. Phase contrast imaging
4. Color CT
26
Benefits of Field Emission X-ray tubes?
* Low power consumption
* Low dose
* Portability
* Fast switching
* Multiple tube CT scanner without gantry rotation
27
What is Projection Radiography?
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
28
What are the advantages and limitations of projection radiography?
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.
29
Clinical applications for Chest X-ray?
* Airways
* Breast shadows
* Bones, e.g. rib fractures
* Cardiac enlargement
* Diaphragm (evidence of free air)
* Extrathoracic tissues (thorax)
30
Clinical applications for AXR?
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
31
Clinical applications Angiography?
Vascular Imaging (Angiography)
* Inject Iodine-based contrast agent to study the compromised blood flow
* Mainly brain and heart
32
Explain Mammography Conventional Radiography.
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.
33
Explain Beam Hardening.
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
34
Explain the concept of Beam Restriction.
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.
35
Explain the concept of Scatter Reduction.
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)
36
How are the limitations of radiographic film handled?
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
37
What are Contrast Agents?
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)
38
Give examples of Contrast Agents.
Contrast Agents
Examples:
* Iodine
* Blood Vessels
* Heart Chambers
* Tumours
* Kidneys
* Bladder
* Barium
* Stomach
* Lung (together with air)
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?
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.
40
Which are the Geometric effects.
Geometric Effects
* Inverse Square Law
* Obliquity
* Beam Divergence and Flat Detector
* Path Length
* Depth-dependent Magnification
41
Explain Inverse Square Law.
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=Is/(4\*\pi\*d)* where *I0* is the intensity at the origin of the detector plane, *Is* 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:
* *Ir=Is/(4\*\pi\*r2)*
* Without compensation, this effect could falsely interpreted as object attenuation in a circular pattern around the detector origin
42
What is Obliquity?
Obliquity
* Reduction in beam intensity due to obliquity:
* *Id= I1cos(\theta)*
* *Id(x,y)=I0cos3(\theta)*
43
What is Path Length?
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)
* *Id(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.
44
Explain Anode Heel Effect.
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
45
Explain Depth-dependent magnification.
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.
46
What is the Imaging Equation with Geometric Effects?
Imaging Equation with Geometric Effects
47
What is the refractive index for light and x-ray?
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
48
What are the noise associated with X-rays?
**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)
49
What is Quantum Efficiency?
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
50
Diagnostic Imaging Exam
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