1. Basics of X-ray and Mammography Systems:
June 6, 2013
Basics of X-ray and Mammography Systems:
Techniques, Filtration, and System Configuration
David Zamora, MS
Medical Physicist
Kalpana Kanal, Ph.D., DABR
Associate Professor, Director of Resident Physics Education
University of Washington Medical Center, Department of Radiology
Diagnostic Imaging Section
A copy of this lecture is available at:

2. Outline Course Review Basics of X-ray, Continued Basics of Mammography
Intro to Radiation and Atom
Basic Interactions
Radiation Units
X-ray Tubes and Spectra
Radiation Biology
Radiation Effects
Radiation Protection
Basic Concepts in Radiography
Basics of X-ray, Continued
Techniques and Technique Trade-offs
Collimation / Filtration
Basics of Mammography

3. COURSE REVIEW

4. Introduction to Medical Imaging
Medical imaging requires some form of radiation capable of penetrating tissues
This radiation must interact with the body’s various tissues in some differential manner to provide contrast
The diagnostic utility of a medical image relates to both technical image quality and acquisition conditions
Image quality results from many trade-offs
Patient safety – levels of radiation utilized (ALARA)
Spatial resolution
Temporal resolution
Noise properties
© UW and Brent K. Stewart PhD, DABMP

5. Originate from nucleus Originate from electron interactions/states
Interactions – Big Picture
β+ - Positron
α2+ - Alpha Particle
Charged
Heavy
Light
Intermediate
Un-Charged
no - Neutron
Fast
Thermal
p+ - Proton
e- - Electron
· Low LET
· Path > Range
· High LET
· Path = Range
A) Collisional Losses
· Excitation
· Ionization
B) Radiative Losses
· Bremmstrahlung
Coulombic Interactions
Particle
· IN: photon + target outer shell e-
· OUT: scattered photon + ejected e-
· Energy shared b/w photon/e-
· Weak Ediagnostic
Rayleigh Scattering
Compton Scattering
Photoelectric Effect
Pair Production
· IN: photon + target inner shell e-
· OUT: ejected e- (photoelectron)
· OUT: characteristic x-ray or Auger e-
· Z3/E3 dependence
· Incident Ephoton must be > BEelec
“INTERACTIONS”
Gamma Ray
Originate from nucleus
X-ray
Originate from electron interactions/states
Photon
c.f.: Bushberg, et al, The Essential Physics of Medical Imaging, 3rd Ed, p46.

6. Effec tive Dose Equivalent Which correction factors?
Dose Terminology Workflow:
Air KERMA
Exposure – charge collected per mass air
(roentgens or Coulombs/kg)
KERMA – kinetic energy released in matter
Absorbed Dose – energy deposited per mass of material
(gray – J/kg)
WHY THE CONFUSION?
Correction Factor Updates
Terminology Changes
Change in Units Convention
Duplicate Unit Usage
Mis-use of definition
Over-stating or mis-stating utility of dose data
Dose Equivalent
Effec tive Dose Equivalent
rem or Sv???
Which correction factors?
rad or Gy???
*WEIGHT BY
RADIATION TYPE
Equivalent Dose
(sievert – J/kg)
*WEIGHT BY
TISSUE TYPE
Effective Dose
(sievert – J/kg)
APPLICATIONS: Stochastic Risk, Radiation Protection, Radiation Safety, Modality Comparison
Radiation Risk Estimation

8. Organ Systems Response: Skin
© UW and Kalpana Kanal, PhD, DABR
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 830.

10. Protection Global Perspective TIME
Minimize the amount of time that you spend in the presence of radiation +
DISTANCE
Increase your physical separation between sources of radiation (Inverse Square Law) +
SHIELDING
Position yourself such that there is shielding between you and a radiation source
ALARA – ‘As low as reasonably achievable’ – this is a guiding principle in radiation protection that takes into account social and economic factors… be reasonable, basically

11. Global Perspective Radiography X-Ray Tube Patient Detector
Image Receptor:
- A) Analog Detector
* Screen-film
- B) Digital Detector
* Computed Radiography
* In-direct Digital
* Direct Digital
X-ray Source:
We (or the system) control:
- photon energy (kVp)
- # of photons produced (mA)
- exposure time (sec)
X-ray Interactions in Body:
- Photo-electric ϵ Z3/E3
- Compton Scatter ϵ 1/E
We take advantage of these photon/tissue interactions to create a usable image.
• This is classified as transmission imaging (i.e. photons transmit through patient)
• This is a 2D PROJECTION of 3D anatomy!
- Anatomical overlay (we lack some spatial information)
- SOLUTION: Different views (or a different modality)
E – photon energy
Z – atomic number

12. BASICS OF RADIOGRAPHY
Alright, let’s move on to aim 3, we’ll identify some of the reasons for the volume differences that we observed.

13. Radiography X-ray Tube Trade-offs
This is a summary of the parameters discussed in the module, along with how changing those parameters affects image resolution, field coverage, heat handling, and cost:
MAIN IDEA: Geometry matters, pay attention to heat deposition, understand the system config.
c.f.: Bushberg, et al, The Essential Physics of Medical Imaging, 3rd Ed, p 181.

14. More energy imparted to e-
Peak KiloVoltage - kVp
inc kVp
kVp Rules-of-Thumb
kVp is related to beam penetrability - higher kVp leads to higher energy x-rays, which can penetrate further into a given medium
Modifying the kVp changes the voltage applied between the anode and cathode:
Increasing kVp is associated with worsening of
low-contrast detectability
(see photon interactions as a function of photon energy)
Higher Voltage
More energy imparted to e-
Changing the kVp changes the maximum photon energy that can be created (i.e. most energy transfer to photon is via direct collision of e- with nucleus) –
check for moving x-intercept on your spectrum
ALL ELSE EQUAL, exposure is proportional to kVp2
We can produce higher energy photons (x-rays), higher yield of characteristic x-rays
c.f.: (center) Bushberg, et al, The Essential Physics of Medical Imaging, 2nd Ed, p 98.
c.f.: (right) Bushberg, et al, The Essential Physics of Medical Imaging, 3rd Ed, p 203.

15. Tube Amperage - mA
mA Rules-of-Thumb
inc mAs
mA is related to number of photons. Higher mA leads to more electrons and more x-rays
Modifying the mA changes the current applied to the filament:
Increasing mA is associated with the improvement of noise, low-contrast detectability , and better statistics (more counts) in our image
Higher current
Higher filament temperature
This is not a perfect example, but a decrease in mA (for a given exposure time) leads to a loss in x-ray intensity. Notice that the x-intercept is identical at the kVp.
More e- boiled off (thermionic emission)
ALL ELSE EQUAL, exposure
is proportional to mA
More photons created
c.f.: (center) Bushberg, et al, The Essential Physics of Medical Imaging, 2nd Ed, p 98.
c.f.: (right) Bushberg, et al, The Essential Physics of Medical Imaging, 3rd Ed, p 203.

16. Radiography Phototimers
Phototimers (aka Automatic Exposure Control - AEC) are an alternative to manual technique:
Measure actual amount of radiation incident on image receptor
Signal measured in one of photocells (ion chamber / solid state)
Match photocell selection to imaged anatomy
Terminates X-ray exposure when proper amount is obtained
Radiation  Signal in Cell  Amplified  Comparator for Cutoff (also backup timer)
GOAL: provide consistent exposure to image receptor, independent of the particular patient you are imaging
FEEDBACK
GRID
AEC
DETECTOR
c.f.: Bushberg, et al, The Essential Physics of Medical Imaging, 2nd Ed, p 134.

17. MANUAL EXPOSURE (mA-s)
System Power On/Off
Detector Selection
Table Bucky
Wall Stand*
Free Cassette
Protocol Selection
Focal Spot Size Selection
Small Focal Spot (SFS)
Medium Focal Spot (MFS)
Large Focal Spot (LFS)*

18. MANUAL EXPOSURE (mAs-s)
Acquisition Mode
Manual (mA-s)
Manual (mAs-s)
AEC

19. AEC EXPOSURE Density Steps (Fine Tune) -3 / -2 / -1 / 0 / 1 / 2 / 3*
AEC Sensitivity (Coarse Tune)
Chest*
CR200
CR300
CR400
Active AEC Photocell Selection
(aka - cells, phototimers, chambers)
Left
Center*
Right

20. Techniques Trade-offs
This chart summarizes what the module told us. This is just a first-line representation, DON’T rely on this in all circumstances, a lot depends on other factors that we will keep learning. Notice that for a constant detector exposure, kVp and mAs move in opposite directions.
This is why we collectively refer to these parameters as mAs (i.e. product of mA and time), as an indicator of number of photons!
Doubling exposure time (s) and halving mA results in:
- 1) same number of e- hitting target
- 2) same number of x-ray photons
- 3) same x-ray spectrum

21. Techniques Trade-offs
Now that we understand mAs, we can further simplify the prior chart. Just remember to account for the potential for patient motion and its relation to exposure time along with tube output limits.

22. Techniques Collimation / Filtration
Filtration – attenuation of x-ray beam as it passes through a layer of material
Inherent filtration – via x-ray system components (glass, oil, port, etc…)
Added filtration – intentionally added to harden beam (Cu, Al, Rh, Mo)
Lowers beam intensity, increases avg beam energy (slight contrast loss), may have to increase technique to get more photons
Collimator – used to shape radiation field
Blades move in tandem (‘upper set’ pair, ‘lower set’ pair)
Recall, smaller radiation field means less scatter, collimate when you can!
Smaller exposed area, better for patient dose
c.f.: Bushberg, et al, The Essential Physics of Medical Imaging, 2nd Ed, p 115.

23. MAMMOGRAPHY

24. Mammography X-ray Tube Configuration
Cathode side of tube closest to patient (why?)
kVp below 40 (high contrast)
1-mm thick Beryllium tube port
mA fixed for large and small focal spot
phototimers

25. Lower grid ratios used, fiber interspaces
Mammography
X-ray Tube Configuration
Breast compression is necessary
reduces overlapping anatomy and decreases tissue thickness of the breast
less scatter, more contrast, less geometric blurring of the anatomic structures, less motion and lower radiation dose to the tissues
Lower grid ratios used, fiber interspaces
phototimers

26. Mammography X-ray Tube Configuration
Optimal x-ray energy is achieved by use of specific target materials and filters to remove the low- and high-energy x-rays
Molybdenum (Mo) and Rhodium (Rh) are used for mammography targets and produce characteristic x-ray peaks at 17.5 and 19.6 keV (Mo) and 20.2 and 22.7 keV (Rh)
Added tube filters of the same element as the target reduce the low- and high-energy x-rays in the x-ray spectrum and allow transmission of characteristic x-ray energies
Mo/Mo
Rh/Rh
Mo/Rh
W/Rh
W/Al

27. Mammography
X-ray Tube Configuration

28. Mammography Magnification Magnification of 1.5x to 2.0x
Increased effective resolution
Small focal spot size used (lower mA, longer exposure times)
Dose increased
Reduction of scatter

29. Radiography
GRID
DETECTOR
AEC

30. Mammography AEC MAG STAND GRID DETECTOR DETECTOR AEC GRID
Magnification
MAG STAND
AEC
GRID
Breast Tissue
DETECTOR
SPOT COMPRESSION PADDLE
DETECTOR
AEC
COMPRESSION PADDLE
GRID
Breast Tissue

31. Questions
• Raphex 2000: The decision is made to add 1 mm Al permanently to the filtration of an x-ray beam. This is done in order to reduce the:
a) Load on the x-ray tube
b) Scatter into the detection system
c) Maximum field size
d) Overall system latitude
e) Patient skin dose
Most of the soft (i.e. low energy) radiation in an x-ray beam is absorbed in the patient and does not contribute to the image. Hardening the beam, by adding filtration, reduces the patient’s skin dose for the same radiation reaching the detector.

32. Questions
• Raphex 2000: Increasing the kVp of an x-ray generator without changing any other control setting will increase the:
1) Amount of heat produced at the anode.
2) Intensity of the x-ray beam.
3) Exposure to a person in the room out of the direct beam.
4) Contrast in a film taken at the higher kVp.
5) Quantum noise in a radiography taken at the higher kVp.
a) 1, 3
b) 1, 4
c) 1, 2, 3
d) 2, 3, 4
e) 3, 4, 5
Increasing the kVp increases the kinetic energy of the electrons colliding with the target. This leads to greater x-ray production as well as additional heat load for the target. Contrast is reduced at higher kV settings as is quantum noise, which is inversely related to the number of photons produced.

33. Questions
• Raphex 2001: The purpose of an x-ray tube filament found in an x-ray circuit is to:
a) Allow the current to flow in one direction only
b) Increase or decrease voltage
c) Create thermionic emission
d) Measure the time of the exposure
e) Measure tube current
The process by which a filament is heated to release electrons is called thermionic emission.

34. Questions
• Raphex 2000: To increase the heat handling ability of an x-ray tube while maintaining the same focal spot size, the manufacturer can:
a) Decrease the diameter of the rotating anode
b) Decrease the rotation speed of the anode
c) Decrease the target angle
d) Decrease the anode mass
e) None of the above
A decreased target angle allows a larger physical focal spot for the same effective focal spot size. Decreasing the diameter or rotation speed decreases the kW rating. The anode mass has no influence on the kW rating.

35. Questions
• The ratio of heat to x-rays produced in a typical diagnostic target is about:
a) 1:99
b) 10:90
c) 50:50
d) 90:10
e) 99:1
Generating x-rays is a highly inefficient process, about 99% of the energy produces heat, which does not help us.

36. Questions
• The effective photon energy of an x-ray beam can be increased by __________:
a) Increasing the tube current
b) Decreasing the filtration
c) Increasing the mAs
d) Increasing the tube voltage
e) All of the above
Decreasing the filtration will increase the intensity, but decrease the effective energy and HVL. Tube current (mA) and mAs have no energy effect.

37. Questions
• In mammography, a fiber interspaced grid is preferred over aluminum because it :
a) Reduces the dose
b) Improves resolution
c) Removes more scatter
d) Reduces image mottle
e) Improves contrast
Aluminum attenuates more of the primary rays than fiber, leading to higher patient doses.

38. Questions
• What is the effect of magnification on mammography patient dose?
1) increases
2) decreases
3) has no effect
Entrance skin exposure and breast dose are reduced because no grid is used, but are increased due to the shorter focus to breast distance.

39. Questions
• What change in focal spot and mA occurs when switching from non-magnification mammography to a spot magnification view?
a) a smaller focal spot and lower mA
b) a larger focal spot and lower mA
c) a smaller focal spot and higher mA
d) a larger focal spot and higher mA
e) no change
Magnification uses small focal spot size which improves resolution but has decreased tube heat loading characteristics thus mA is reduced.

40. Questions
• The tube port (window) for a mammography x-ray tube is typically made of :
a) glass
b) lead
c) tungsten
d) beryllium
Using beryllium instead of glass reduces attenuation of the low-energy x-rays in mammography

41. Questions
• Which part of a mammography x-ray tube is positioned closest to the chest wall of the patient?
a) anode end
b) cathode end
c) side or long axis
Continued……

42. Questions
• The reason the mammography tube is positioned as it is in the previous question is:
a) collimation
b) half value layer
c) heel effect
d) penumbra
e) technique
Due to heel effect - Highest radiation output to thickest part of breast (chest wall)
Anode-cathode axis oriented in anterior-posterior direction or Cathode side of tube closest to patient

43. Questions • Compression in mammography results in increased:
a) breast dose
b) geometric blurring
c) patient motion
d) scatter
e) none of the above
Patient motion, blurring, scatter and dose all reduce due to compression of breast.

44. Questions • The low voltage used in screen/film mammography reduces :
a) Subject contrast
b) Dose
c) Microcalcification visibility
d) Scatter
e) Film processing time
Low kV needed to optimize contrast, photoelectric interaction, not Compton interaction.