General medical imaging topics - Medical physics training

General medical imaging topics

1. General
medical imaging
topics

2. kV X-ray Generation
General
medical imaging
topics

3. Kilovoltage X-ray Generation
•First, a cathode containing a heated filament emits electrons via thermionic
emission.
•Next, a high voltage is applied between the cathode and the anode which
accelerates the electrons toward toward the anode
•Finally, kilovoltage x-rays are generated by Bremsstruahlung (aka Braking)
radiation when the accelerated electrons interact with the nucleus in the x-ray
tube anode/target. A minority of x-ray fluence is produced as characteristic x-rays
resultant of ejection of orbital electrons in the anode.
•Key Point: Mean energy of a clinical kV photon beam is approximately 1/2 - 2/3
of the maximum energy.

4. X-ray Tube Design
Anode
•A positively charged anode serves as the target for
the accelerated electrons.
•The anode must be made of a high atomic number
material in order to achieve reasonable efficiency in
Bremsstrauhlung photon production.
•Anode must be able to handle high heat loads
because of inefficient Bremsstrahlung production in
this energy range (<1% efficiency).
Cathode
•The cathode consists of a wire filament and a
negatively charged focusing cup.
•When heated by an applied current, the filament
emits a cloud of electrons.
•The focusing cup concentrates the electrons for
acceleration which improves penumbra.
Envelope
•Maintains a vacuum within the X-ray tube.
oA vacuum is important to prevent air scattering of
the electrons which would increase focal size
increase photon beam penumbra.
•Historically, a glass envelope was used to ensure a
vacuum within the x-ray tube. Today, many kilovoltage
x-ray tubes utilize a metal-ceramic design which offers
several benefits which, taken together allow for smaller
x-ray tubes that produce kV photons at higher efficiency
and requiring less current.
Key Point: Tungsten is a superior target/anode material to lead because of its high melting
point (Tungsten: 6192F vs Lead: 612.4F).

5. X-ray Tube Design

6. X-ray Spectrum
•The X-ray spectrum is primarily
comprised of Bremsstrahlung
produced photons which, in
absence of filtering and self-
attenuation, would yield a
distribution of electron energies
that is highest at low energy and
decreases approximately linearly
to the maximum energy equal to
the applied kVp.
•Bremsstrahlung production
efficiency is low (<1%) but
increases with increased energy.
•Z is the atomic number of the
target.
•E is the accelerating potential
in volts.

7. Characteristic X Rays: Electron-Electron
interaction
•Starts with ejection of e- mainly from k
shell (also possible for L, M,…) by
ionization 
•e- from L or M shell fall into the
vacancy created in the k shell
•Energy difference is emitted as
photons
•A sequence of successive electron
transitions between energy levels 
•Energy of emitted photons is
characteristic of the atom

8. What is beam filtration?
Absorber placed between
Source and object
Will preferably absorb
the lower energy photons
Or absorb parts of spectrum
(K-edge filters)
Filtration Filtering attenuates the low energy photons
more than higher energy photons. This causes the
"hump" shape of the fluence energy spectrum

10. X-ray
Spectrum
•Characteristic X-rays Characteristic X-rays are
emitted when a photon interacts with the target
such that an inner shell (usually k shell) orbital
electron is ejected. To fill the orbital vacancy, an
outer shell electron descends to the lower
energy inner shells by emitting an X-ray. These
X-rays are called characteristic X-rays because
they are emitted with energies corresponding to
the difference between outer shell and inner
shell energies.

11. Power Generation
Power is supplied to the X-ray tube in the form of a (nearly) constant direct current (DC) voltage. Although
U.S. power distribution voltages are in the kV region (110kV and above are standard), the power supplied
to buildings is typically 120V or 240V. Further, power is delivered in alternating current (AC). A transformer
and a rectifier are required to convert the VAC outlet power to kVDC power
Rectifiers
A full-wave rectifier is used to convert alternating current (AC) to
direct current (DC) for use in X-ray generation. A rectifier bridge
is formed by connecting four diodes, each of which allows
current to flow in only one direction. The rectifier bridge inverts
the negative portion of the AC waveform creating a waveform
that oscillates between the maximum voltage and 0. A
smoothing capacitor is then used to convert the waveform
exiting the rectifier bridge into an approximately DC source.

12. Power Generation
Transformers
Transformers use electromagnetic induction to transfer
energy from one wire to another. In a transformer, a wire
supplying power (known as the primary winding) is
wrapped around a metallic transformer core. A second wire
is wound around the transformer core and is induced to
transmit power. By varying the ratio of the number of
winding of the primary winding to the secondary winding,
the voltage may either be increased (a step-up
transformer), decreased (a step-down transformer), or
remain unchanged (an isolation transformer).
Transformer law: The ratio of the voltages between the
secondary winding (V
s
) and the primary winding  (V
p
) is equal
to the ratio of the number of turns in the secondary winding
(N
s
) to the number of turns in the primary winding (N
p
)

13. Factors Impacting Photon Fluence
kVp
Kilovoltage peak (or peak kilovoltage) is the
maximum voltage across the X-ray tube.
Because voltage across the tube is not
constant, kVp is higher than mean kV.
mA
Tube current (mA) is the electron flow (i.e.
number of electrons per second) from
cathode to anode within an X-ray tube.
Tube current is related to electric power as
in the below equation:
mAs is the product of tube current (mA)
and time in seconds (s). mAs is related to
electric energy as in the below equation:

14. Factors Impacting Photon Fluence
Heel Effect
Heel effect is the reduction in X-ray beam
intensity toward the anode side of an X-ray
field. Heel effect is caused by greater
attenuation of X-rays inside the anode.

15. Image Quality Metrics
General
medical imaging
topics

16. Fundamental Image Metrics
Noise (Quantum Mottle)
Image noise is the fluctuation in pixel
intensity due to statistical uncertainty.
Noise is typically measured as the standard
deviation (σ) of pixel intensity over a
uniform area. Reducing image noise is
important as noise limits the ability to
distinguish low contrast objects within
an image. Image noise can arrive from the
random fluctuations in photon distribution
(photon statistics), digitization and electron
noise sources such as dark currents
•σ is the standard deviation
•xi is an event
• is the average number of events
x̅
•N is the number of measurements

17. Fundamental Image Metrics
Statistics of Image Noise
Image noise may be statistically modeled as a Gaussian Distribution which, for
the large number of information carriers collected, may be approximated as a
Poisson Distribution. The probability of observing x events over a given interval
for a Poisson distribution may be computed using the below equation where m is
the average number of events collected:
Standard deviation of a Poisson distribution may be simplified
as:
This allows simple computation of signal-to-noise ratio (SNR) for
Poisson distributions as:
Rose Criteria
The Rose Criteria states that a signal-to-noise ratio (SNR) of 5
or greater is required for reliable detection of an object within an
image

18. Contrast
Contrast is the difference in intensity, I, between two
objects in an image. Contrast allows differentiation between
objects
Factors influencing contrast
•Inherent contrast between subject and background
•Display contrast
  Window
  Level
•Physical perturbations
  Fogging
  Scatter
 Non-uniform detector efficiency

19. Spatial Resolution
Spatial resolution (R) is the
minimum separation
between two high contrast
objects such that they
appear as two separate
objects, often measured in
line-pairs per millimeter
(lp/mm).

20. Factors influencing spatial
resolution
•Geometric penumbra
 Unsharpness originating from
finite source size in
radiographic systems.

21. •Subject unsharpness
 Rather than a sharp
differentiation between
subject and background the
edges may blur together.
•Detector unsharpness
 Unsharpness arising from
scatter or diffusion in the
detector
•Motion (subject or detector)

22. Quantitative Image Analysis
Contrast-to-Noise Ratio (CNR)
Contrast-to-noise ratio (CNR) is a metric that relates how
easily low contrast objects may be distinguished from their
surroundings. Higher CNR indicates a more readily
distinguishable object.

24. Detective Quantum Efficiency
(DQE)
Detector Quantum Efficiency (DQE) is
a measure of the imaging system’s
efficiency in converting data carriers
into an image. DQE is influenced by
every component of the imaging
system and is a good metric of overall
dose efficiency for an imaging system.

25. Quantum Detection Efficiency (QDE)
Quantum Detection Efficiency (QDE) is a measure of
how efficient the detection system is at collecting
information carriers. DQE is impacted by a number of
factors including detector construction and geometry.

26. Imaging System Shielding
General
medical imaging
topics

27. Overview
Diagnostic X-ray system
shielding is covered by NCRP-
147 and AAPM TG-108. 
The methodology used for
shielding imaging systems
shares many similarities
with linac vault shielding and
NCRP-151.

28. What are
we
looking
for?

29. Objetive

31. Determining the thickness
of shielding material
required to reach a given
transmission factor is more
complicated than in MV
shielding calculations
because the attenuation
coefficients of materials
vary strongly with energy
spectrum for kV photons.
The below equation gives
the minimum required
barrier thickness. α, β, and
γ are fitting parameters for
broad beam x-ray source
attenuation of a given
shielding material.

32. CT Shielding (NCRP 147 Methodology)

33. Typical
Shielding
Requirements

34. Fundamentals
of Shielding
for Medical X-
Ray Imaging
Facilities 
(NCRP-147)
Primary and secondary radiation
exposure to individuals depends primarily
on the following factors:
•The amount of radiation produced by the
source.
•The distance between the exposed person
and the source of the radiation.
•The amount of time that an individual
spends in the irradiated area
•The amount of protective shielding
between the individual and the radiation
source.

35. Methods
for
determinin
g Air
Kerma per
patient
(K1)

36. Isodose Map Method
The simplest method for
determining
K1 is to simply use a vendor
supplied isodose map for the
scanner.
If using this technique, care must
be taken to correct for
differences in acquisition
technique used to create the
map such as slice thickness, mAs,
and kVp.

37. Types of Medical X-Ray Imaging
Facilities
•Radiographic Installations
•Fluoroscopic Installations
•Interventional Facilities
•Dedicated Chest Installations
•Mammographic Installations (Permanent and Mobile)
•Computed Tomography Installations
•Mobile Radiography and Fluoroscopy X-Ray Units
•Dental X-Ray Facilities
•Bone Mineral Measurement Equipment
•Veterinary X-Ray Facilities
•Other X-Ray Imaging Systems

38. Gracias