Mammography (Diagnostic Radiology) Flashcards
What is the ideal energy range of x-rays? Why?
15-20 keV
Offers the best balance between contrast and dose
Why is molybdenum target often used?
(two reasons)
Have a kEdge of ~20 keV
Also have an L BE of 2.5, so dropping down means 17.5 keV photons which are also in the ideal range
What is the purpose of applying a target filter?
To filter out the low and high energy photons
Remember we want it in the middle range between 15 and 20 keV
What two components are added together to make the effective anode angle?
What degree should it be greater than or equal to?
Anode angle + physical tube tilt
Should be atleast 22 degrees
True or false
0 degree anode angle tilt with a 24 physical tube tilt has a greater field coverage as 16 degree anode angle with a 6 degree tube tilt.
False, they’re both above 22 degrees total. Above that angle the field coverage will be approximately the same.
What is the cathode side of the tube adjacent to? Why is this important?
(Hint: think Heel effect)
Adjacent to patient’s chest wall
It’s the thickest part of the breast, so you want the most intensity. Cathode side is unaffected by heel effect
Equation for nominal focal spot size.
NEED TO MEMORIZE
aref = achest wall (1- tan(theta - Ø)/tan(theta))
achest wall = sin(theta)
theta is the the effective anode angle
phi is half of that
Regarding breast compression in mammo, the following effects will occur. Which contribute to contrast enhancement, which to resolution?
- Reduction of overlapping anatomy
- Decrease thickness - fewer scatter X-rays
- Decrease thickness - less geometry blur
- Reduces motion
- Contrast
- Contrast
- Resolution
- Resolution
Will breast compression cause lower or higher dose?
Lower
Increases contrast and resolution naturally. So you can reduce the mAs
Fill out the following flow logic,
Scatter –> Noise –> Lower contrast and SNR –> ______ —> Higher dose
AEC
Automatic Exposure Control
True or false
Scatter radiation is largely independent of kV in the mammography energy range
True
Which of the following will lead to better detailed images? Why?
- Magnification
- Smaller focal spot size
- Target/Filter changes
- Compression
- Air Gap increase
- Magnification
- Smaller focal spot size - Less Geo blur
- Compression - Reduces overlapping anatomy, motion blur and geometry blur
Which of the following will lead to better contrast images? Why?
- Magnification
- Smaller focal spot size
- Target/Filter changes
- Compression
- Air Gap increase
- Magnification - Air Gap (less scatter)
- Target/Filter changes - Ideal Energy Range
- Compression - Less scatter
- Air Gap increase - Less Scatter
Which of the following will lead to lower dose? Why?
- Magnification
- Smaller focal spot size
- Target/Filter changes
- Compression
- Air Gap increase
- Target/Filter changes - Idea E Range (less high and low photons)
- Compression - AEC decreased thickness
Which of the following will lead to higher dose? Why?
- Magnification
- Smaller focal spot size
- Target/Filter changes
- Compression
- Air Gap increase
Only air gap
AEC will hold for longer
Main benefit of screen-film mammography over digital systems?
Better resolution
Why would you position your screen behind film?
X-ray interations are greater near the entrance surface, by having screen behind the film there’s minimal light spread at the film surface
(better spatial resolution)
Fill out the following flow chart of events for Digital Mammorgraphy (CR)
X-ray –> _____ –> Lights —> Trapped electrons in film —> _______ —-> Digital signal
Screen
CR Reader
Main benefits of digital mammography (there’s 2)
Wider dynamic range
Better contrast
Why does magnification mode need a smaller focal spot?
To minimize geometric blur
Otherwise blurred edges would be too exaggerated
Which of these does not lead to better resolution?
- Smaller focal spot size
- Larger SOD
- Smaller OID/breast thickness
- Higher mA/less imaging time
- Less light spread
- Smaller Grain/pixel size
None, they all do
- Smaller focal spot size - Less geometric blur
- Larger SOD - Less geometric blur
- Smaller OID/breast thickness - Less geometric blur
- Higher mA/less imaging time - Less motion blur
- Less light spread - Less detector blur
- Smaller Grain/pixel size - Less detector blur
What is the range of input signal over which a desirable contrast could be formed called?
What has this better range, Screen film or digital system?
Dynamic Range
Digital system
Equations for Noise, relative noise and SNR
Noise: N1/2
Relative Noise: N1/2/N = 1/N1/2
SNR: N/N1/2 = N1/2
How do you account for scatter in Digital breast tomosynthesis?
You can’t use an anti-scatter grid
Need to make a correction algorithm in the system
Formula for Mean Glandular Dose (MGD)
Dg = DgN x XESAK
DgN = some conversion factor you look up
XESAK = entrance skin air kerma
What type of breast tissue is the site of carcinogensis?
Glandular tissue
What is the fraction of “average breast” tissue that is glandular. What about fat?
50/50
Which of the following cause a higher MGD? Why?
- Thicker Breast
- Higher Adipose Fraction
- Grid
- Thicker Breast - Need more penetrating beam, higher kVp, higher deposited energy
- Grid - Less radiation, AEC holds for longer
Which of the following cause a lower MGD? Why?
- Thicker Breast
- Higher Adipose Fraction
- Grid
- Higher Adipose Fraction - More x-ray attenuation in the adipose tissue, less elsewhere since thickness is same
Equation for detective quantum efficiency (DQE)
DQE(f) = SNR2out / SNR2in
Ideally, DQE = 1
DQE is a function of spectrum, spatial frequency and exposure
