Medical Imaging Physics Final 118
complete solutions.
Medical Imaging Physics Final 118
complete solutions.
Difference between mammography and projection radiography - ANSWER - 2 filaments with different
sized focal spots (one has high resolution for magnification which causes nonuniform blurring)
- much lower energy range (40 keV; more attenuation at lower energy)
- different target material (Mo or Rh)
Hardware features in mammogram - ANSWER -Dedicated xray equipment
-Specialized xray tubes
-Breast compression devices
-More uniform field of view
-Antiscatter grids
-Xray detectors (screen film or digital)
-Phototimer detector systems
Why do mammography systems use dual filaments in the cathode andMo and Rh targets in anode -
ANSWER - dual filaments are in the focusing cup and are used to produce 0.3 and 0.1 mm focal spot
sizes
- the latter is used for magnification and reduces geometric blurring
Why do mammography systems use Mo and Rh targets in anode - ANSWER - their characteristic xray
production is the main reason
- Mo K shell energies: 17.5 and 19.6 keV
- Rh K shell energies: 20.2 and 22.7 keV
these energies fit in the energy range for mammography and the number of xrays in the OPTIMAL
energy range for breast imaging are significantly increased by characteristic xray emission
,Medical Imaging Physics Final 118
complete solutions.
Explain how the 'heel effect' impacts breast imaging - ANSWER Intensity of xrays emitted from the focal
spot varies within the beam, with the greatest intensity on the cathode side of the projected field and
the lowest intensity on the anode side
Dropoff in fluence
identify one technique for minimizing the effect during a mammogram. - ANSWER position the cathode
over the chest wall of patient and the anode over the anterior portion (nipple)
Actual vs Projected focal spot - ANSWER -The projected focal spot size varies along the cathode-anode
axis
-The actual focal area is the electron distribution area on the anode
-Actual focal spot appears on anode
explain why/how the projected focal spot dimensions change with location. - ANSWER if you go to closer
to the cathode the focal spot length increases due to the heel effect, but the width remains fairly the
same
Calculate dimensions based on the relationship between effective focal spot, projected focal spot, and
target angle - ANSWER
What are tradeoffs with decreasing focal spot size - ANSWER - produces less magnification dependent
blur
- tradeoff: projected image is a lot bigger, so theres more radiation exposure and magnification
dependent resolution loss
What is the preferred energy spectrum for mamography - ANSWER 15-25 keV
What types of xrays contribute to that spectrum - ANSWER bremsstrahlung and characteristic radiation
how does the system reach its preferred energy spectrum - ANSWER use xray tube filters made of Mo or
Rh that have K absorption edge between 20 and 27 keV
, Medical Imaging Physics Final 118
complete solutions.
Explain how the energy spectrum is shaped in a mammography system. - ANSWER the spectrum is
described by
- at higher xray energies, attenuation decreases because of selective transmission of xrays in a narrow
band of energies from 15 keV-K absorption edge
- characteristic xrays produced by Mo target occur at lowest attenuation of the filter in the energy range
To shape this..
- use different filter materials to absorb unwanted energies
fluence - ANSWER number of photons that pass through a region of space
unit: photons/cm^2
flux - ANSWER rate of photon fluence
unit: cm^-2*s^-1
energy fluence - ANSWER flux*energy
KERMA - ANSWER kinetic energy released in matter
energy transferred to the material associated with indirect ionization
unit: Gray
Absorbed Dose - ANSWER energy imparted by ionizing radiation/mass
unit: Gray
equivalent dose - ANSWER characterizes effectiveness of radiation causing biological damage
unit: Sievert
complete solutions.
Medical Imaging Physics Final 118
complete solutions.
Difference between mammography and projection radiography - ANSWER - 2 filaments with different
sized focal spots (one has high resolution for magnification which causes nonuniform blurring)
- much lower energy range (40 keV; more attenuation at lower energy)
- different target material (Mo or Rh)
Hardware features in mammogram - ANSWER -Dedicated xray equipment
-Specialized xray tubes
-Breast compression devices
-More uniform field of view
-Antiscatter grids
-Xray detectors (screen film or digital)
-Phototimer detector systems
Why do mammography systems use dual filaments in the cathode andMo and Rh targets in anode -
ANSWER - dual filaments are in the focusing cup and are used to produce 0.3 and 0.1 mm focal spot
sizes
- the latter is used for magnification and reduces geometric blurring
Why do mammography systems use Mo and Rh targets in anode - ANSWER - their characteristic xray
production is the main reason
- Mo K shell energies: 17.5 and 19.6 keV
- Rh K shell energies: 20.2 and 22.7 keV
these energies fit in the energy range for mammography and the number of xrays in the OPTIMAL
energy range for breast imaging are significantly increased by characteristic xray emission
,Medical Imaging Physics Final 118
complete solutions.
Explain how the 'heel effect' impacts breast imaging - ANSWER Intensity of xrays emitted from the focal
spot varies within the beam, with the greatest intensity on the cathode side of the projected field and
the lowest intensity on the anode side
Dropoff in fluence
identify one technique for minimizing the effect during a mammogram. - ANSWER position the cathode
over the chest wall of patient and the anode over the anterior portion (nipple)
Actual vs Projected focal spot - ANSWER -The projected focal spot size varies along the cathode-anode
axis
-The actual focal area is the electron distribution area on the anode
-Actual focal spot appears on anode
explain why/how the projected focal spot dimensions change with location. - ANSWER if you go to closer
to the cathode the focal spot length increases due to the heel effect, but the width remains fairly the
same
Calculate dimensions based on the relationship between effective focal spot, projected focal spot, and
target angle - ANSWER
What are tradeoffs with decreasing focal spot size - ANSWER - produces less magnification dependent
blur
- tradeoff: projected image is a lot bigger, so theres more radiation exposure and magnification
dependent resolution loss
What is the preferred energy spectrum for mamography - ANSWER 15-25 keV
What types of xrays contribute to that spectrum - ANSWER bremsstrahlung and characteristic radiation
how does the system reach its preferred energy spectrum - ANSWER use xray tube filters made of Mo or
Rh that have K absorption edge between 20 and 27 keV
, Medical Imaging Physics Final 118
complete solutions.
Explain how the energy spectrum is shaped in a mammography system. - ANSWER the spectrum is
described by
- at higher xray energies, attenuation decreases because of selective transmission of xrays in a narrow
band of energies from 15 keV-K absorption edge
- characteristic xrays produced by Mo target occur at lowest attenuation of the filter in the energy range
To shape this..
- use different filter materials to absorb unwanted energies
fluence - ANSWER number of photons that pass through a region of space
unit: photons/cm^2
flux - ANSWER rate of photon fluence
unit: cm^-2*s^-1
energy fluence - ANSWER flux*energy
KERMA - ANSWER kinetic energy released in matter
energy transferred to the material associated with indirect ionization
unit: Gray
Absorbed Dose - ANSWER energy imparted by ionizing radiation/mass
unit: Gray
equivalent dose - ANSWER characterizes effectiveness of radiation causing biological damage
unit: Sievert
