12/9/2014
2014-2015 Residents' Core Physics Lectures
Mondays 7:00-8:00 am in VA Radiology and UCSDMC Lasser Conference Rooms
1
2
3
4
5
Topic
Introduction and Basic Physics
Interaction of Radiation and Matter
RSNA Week No Lecture
Computers
X-Ray Production
Christmas and New Year’s Holiday
Generators
Chapters
1, 2
3
4
5
5
Date
M 11/17
M 11/24
M 12/01
M 12/08
M 12/15
M 12/22,
12/29
M
01/05/2015
Faculty
Andre
Andre
Hall
Andre
Andre
Please!
LOG IN AND POST YOUR FACULTY
EVALUATIONS
This is an essential service to us.
Textbook: The Essential Physics of Medical Imaging, Bushberg, et al., Philadelphia: Lippincott
Williams & Wilkins, 2002, 2nd Edition
Course Web Site??: http://3dviz.ucsd.edu/~radiology_residents/Home.html
Recall from Prior Session
Measuring Attenuation of X- and Gamma-Rays
 = Rayleigh + Compton + Photoelectric + Pair Prod + Photodisint
 is function of: h , Z, 
/ = mass attenuation coefficient (cm2/g)
Probability of Absorption
•
•
•
• For monochromatic (single energy) radiation of intensity I0
– I = Io e-x or N = No e-x
–  = linear attenuation coefficient (cm-1)
–  = ln 2/HVL
– HVL = 0.693/ = thickness of absorber that attenuates
beam by 1/2
–  is function of: h , Z, 
Avg Energy (quality) and HVL increases
I
 e  x
I0
Beam Hardening
Photon intensity (quantity) decreases
Monochromatic X-Rays
1st HVL = 2nd HVL
Polyenergetic photon beam
e.g., Diagnostic x-ray beam
2nd HVL > 1st HVL
1
12/9/2014
An attenuation curve for a 120 kVp x-ray beam yields the following data:
100
75
50
25
0
0
1
2
3
4
5
Added filtration (mm Al)
0
0.5
1
2
3
4
5
The second half value layer
is approximately:
a. 1.0 mm
b. 1.7 mm
c. 2.0 mm
d. 2.2 mm
e. 3.0 mm
An attenuation curve for a 120 kVp x-ray beam yields the following data:
Relative Intensity
100%
50
40
27
20
15
12
100
75
50
25
0
0
Add 1 mm to the beam. What
is the HVL now?
a. 1.0 mm
b. 1.5 mm
c. 2.0 mm
d. 2.5 mm
e. 3.0 mm
1
2
3
4
5
Added filtration (mm Al)
0
0.5
1
2
3
4
5
The second half value layer
is approximately:
a. 1.0 mm
b. 1.7 mm
c. 2.0 mm
d. 2.2 mm
e. 3.0 mm
Relative Intensity
100%
50
40
27
20
15
12
Add 1 mm to the beam. What
is the HVL now?
a. 1.0 mm
b. 1.5 mm
c. 2.0 mm
d. 2.5 mm
e. 3.0 mm
An attenuation curve for a 120 kVp x-ray beam yields the following data:
100
75
50
25
0
0
1
2
3
4
5
Added filtration (mm Al)
0
0.5
1
2
3
4
5
The second half value layer
is approximately:
a. 1.0 mm
b. 1.7 mm
c. 2.0 mm
d. 2.2 mm
e. 3.0 mm
Relative Intensity
100%
50
40
27
20
15
12
Chapter 5. X-Ray Production,
X-Ray Tubes, and Generators
Add 1 mm to the beam. What
is the HVL now?
a. 1.0 mm
b. 1.5 mm
c. 2.0 mm
d. 2.5 mm
e. 3.0 mm
Michael P. Andre, Ph.D.
AAPM/ABR Syllabus
Module 5: Radiation Units
After completing this module, the resident should be able to apply the “Fundamental
Knowledge” and “Clinical Applications” learned from the module to example tasks,
such as those found in “Clinical Problem-Solving.”
Fundamental Knowledge:
1. Recognize that there are 2 different systems for units of measurement (i.e. SI and
Classical) used to describe physical quantities.
2. Describe the SI and Classical units for measuring the ionization resulting from
radiation interactions in air (e.g., exposure-related quantities).
3. Describe the concepts of dose‐related quantities and their SI and Classical units.
Clinical Application:
1. Discuss the appropriate use or applicability of radiation quantities in the health care
applications of imaging, therapy, and safety.
Units of Radiation
• Exposure (Roentgen)
• Absorbed Dose (Gray)
• Kerma (Gray)
• Equivalent Dose (Sievert)
• Effective Dose (Sievert)
• Activity (Becquerel)
1 R = 2.58 x 10-4 C/kg
1 Gy = 100 rad = 1 J/kg = 100 R • f-factor
K.E. transferred to charged particles
K = Ψ (tr/)E
1 Sv (H) = wR Gy = 100 rem
1 Sv (E) = ΣT wT HT
3.7x1010 Bq = 1 Ci
(Also known as Quality Factor, largely based on LET)
Clinical Problem-Solving:
1. Explain radiation exposure and dose quantities in lay language to a patient.
11
2
12/9/2014
Which of the following is not equal to one Gray?
a. 1.0 Joule/kg
b. 1000 rads
c. 1.0 Sv/Weighting Factor (wR)
d. (100 R) • (f-factor)
Which of the following is not equal to one Gray?
a. 1.0 Joule/kg
b. 1000 rads
1 Gy = 100 rads, 1 rad = 1 cGy
a. 1.0 Sv/Weighting Factor (wR)
b. (100 R) • (f-factor)
Chapter 5: X-Ray Production
Technologists must
Consider Many Factors
before taking Radiograph
1.
2.
3.
4.
5.
6.
7.
Patient positioning
Contrast media
Equipment type
Film + screen or
CR vs. DR
Body part thickness
Body part composition
Collimation
8.
9.
10.
11.
Focal spot size
Grid
Auto exposure control
Manual exposure
a) kVp
b) mA
c) Time
12. Many others
Quick Review of X-Ray Interactions
(ABR Syllabus)
Chapter 3
Chapter 5
or digital detector
Control Console
• What is the purpose of adding more Al or Cu filters in vascular
imaging?
• What makes a contrast agent radiolucent instead of radioopaque?
• Describe how the controls of an x-ray system affect the
technique factors used in diagnostic imaging.
3
12/9/2014
Electromagnetic Induction
CHANGING magnetic field induces electron flow:
# turns Primary
Step-Up Transformer
Isolation Transformer:
# turns Primary = # turns Secondary
Step-Down
Transformer
Step-Up
Transformer
# turns Primary
Power = I·V
1 watt = 1 J
sec
Autotransformer with Np = 440 turns
is connected to power source Vp = 440 V
If 10,000 turns (Ns) are enclosed by secondary taps,
what is voltage on secondary side of transformer?
Example Cont.
If primary current is 10 A, what is secondary current?
(Recall primary voltage is 440 V, secondary is 10kV.)
< # turns Secondary
Example:
Autotransformer with Np = 440 turns
connected to power source Vp = 440 V
Example:
> # turns Secondary
If 10,000 turns (Ns) enclosed by
secondary taps, what is voltage on
secondary side of transformer?
Vs  Vp
Ns
Np
10,000
440
Vs  10,000 V  10 kV
Vs  440 V
Example Cont.
If primary current is 10 A, what is secondary current?
(Recall primary voltage is 440 V, secondary is 10kV.)
VP IP = VS IS
440 V • 10 A = 10,000 V • IS
IS = 0.44 A = 440 mA
4
12/9/2014
Autotransformer
{Take a sip of coffee}
Complete Circuit for Simple X-Ray System
Single-Phase Full-Wave Rectified Transformer
kVp = Peak kilovoltage
Three-Phase Transformer
High-Frequency Inverter (Most Common)
Conventional analog input
Step-Up
Transformer
Step-Up
Transformer
Highly stable
peak voltage
and current
5
12/9/2014
Why is the Voltage Waveform important? Recall:
Voltage
Ripple
Avg Energy (quality) and HVL increases
Beam Hardening
Photon intensity (quantity) decreases
% Ripple =
Vmax-Vmin • 100
Vmax
Polyenergetic x-ray beam:
Spectrum of energies
Both Quality and Quantity of the
Output X-Ray Beam Spectrum
=
Are Important to Us
Spectrum Depends on:
• kVp
• Target material (e.g. W, Mo, Rh)
• Filtration
• Voltage Waveform
Increase kVp:
1. X-ray intensity : Exposure α (kVp)n, n=2-3.5
2. Maximum energy increases
3. Effective energy increases
High Z Target:
X-Ray Beam Spectrum Depends on:
•
•
•
•
kVp
Target material (e.g. W, Mo, Rh)
Filtration
Voltage Waveform
--More efficiently
produces
Bremmstrahlung X-rays
--Shift to higher E
--Produces higher E
characteristic x-rays
Bremmstrahlung
X-rays
Tungsten
Spectrum
Characteristic
X-rays
6
12/9/2014
X-Ray Beam Spectrum depends on:
•
•
•
•
kVp
Target material (e.g. W, Mo, Rh)
Filtration
Voltage Waveform
Reduces beam quantity, increases quality
~2.5 mm Al
Reduces beam quantity but increases quality
Why is this important?
Lower dose, better iodine image contrast,
but higher tube heat load
7
12/9/2014
Reducing ripple increases
beam quality and quantity.
X-Ray Beam Spectrum depends on:
•
•
•
•
Inverter generators are
preferred.
kVp
Target material (e.g. W, Mo, Rh)
Filtration
Voltage Waveform
X-Ray Beam Spectrum depends on:
•
•
•
•
kVp
Target material (e.g. W, Mo, Rh)
Filtration
Voltage Waveform
Energy Spectrum
Increase mA:
1. Intensity increases proportionately
2. Energy distribution remains the same
A change in ______ will change the ______ of an x-ray
beam, all other factors remaining the same.
A change in ______ will change the ______ of an x-ray
beam, all other factors remaining the same.
1.
2.
3.
4.
5.
1.
2.
3.
4.
5.
Tube current (mA)
Tube potential (kVp)
Rectification
Focal spot size
Exposure time
A. Quantity
B. Quality
C. Both quality, quantity
D. Neither
Tube current (mA) A
A. Quantity
Tube potential (kVp) C
B. Quality
Rectification C
C. Both quality, quantity
Focal spot size D
D. Neither
Exposure time A (best answer)
8
12/9/2014
Quick Review of X-Ray Interactions (ABR Syllabus)
• What is the purpose(s) of adding
more Al or Cu filters in vascular
imaging? Iodine K-edge=33 keV
• What makes a contrast agent
radiolucent instead of radioopaque?
• Describe how the controls of an
x-ray system affect the technique
factors used in diagnostic
imaging.
kVp, mA, Time
Technologists must
Consider Many Factors
before taking Radiograph
33
Ice
Air
Water
1.
2.
3.
4.
5.
6.
7.
Patient positioning
Contrast media
Equipment type
Film + screen or
CR vs. DR
Body part thickness
Body part composition
Collimation
8.
9.
10.
11.
Focal spot size
Grid
Auto exposure control
Manual exposure
a) kVp
b) mA
c) Time
12. Many others
Excitation
ABR: Describe the two mechanisms
by which energetic electrons
produce x rays and describe the
energy distribution for each
mechanism of x-ray production.
•Bremsstrahlung is the principal
source of x-ray production in radiology
•Characteristic x-rays (photoelectric
effect) are secondary
X-Ray Tube
{Take a sip of coffee}
Focal Spot
9
12/9/2014
Tube Rating Charts
A complicated topic but they serve a very practical purpose
Massive Overheating
Rotor failed to rotate
Tube Rating Charts
1 Watt = 1 V • 1 A
Power rating (kW) ≡ Avg power at:
Amax , 100 kVp, 0.1 sec (by convention)
Power = 100 kVp • RMS • Amax for 0.1 sec exposure
Bad bearings—erratic rotation
Heat Loading
Heat Unit (HU) = kVp • mA • time
for single-phase generator
Heat Unit (HU) = kVp • mA • time • 1.35
for 3-phase, high frequency, constant potential generators
For fluoroscopy (continuous x-rays)
HU/sec = kVp • mA
0.3 mm 3000 rpm
0.3 mm 10,000 rpm
Allowed? T or F
1.2 mm 3000 rpm
1. 3Φ, 75 kVp, 500 mA, 0.1 sec
2. 3Φ, 75 kVp, 450 mA, 0.1 sec
3. 1Φ, 120 kVp, 200 mA, 0.3 sec
1.2 mm 10,000 rpm
10
12/9/2014
The advantages of a 3-phase generator (Hi-F)
compared to a single-phase unit are:
Allowed? T or F
1. 3Φ, 75 kVp, 500 mA, 0.1 sec F
2. 3Φ, 75 kVp, 450 mA, 0.1 sec T
3. 1Φ, 120 kVp, 200 mA, 0.3 sec T
A.
B.
C.
D.
E.
1, 2
1 , 2, 5
3, 4, 5
1, 2, 3
1, 2, 3, 4
1. Less kV ripple
2. Higher effective E at same kVp
3. Shorter exp time at same kVp
4. Lower tube heat input rate
5. Lower cost
X-Ray Tube Cooling Curves
The advantages of a 3-phase generator (Hi-F)
compared to a single-phase unit are:
A.
B.
C.
D.
E.
1, 2
1 , 2, 5
3, 4, 5
1, 2, 3
1, 2, 3, 4
1. Less kV ripple
2. Higher effective E at same kVp
3. Shorter exp time at same kVp
4. Lower tube heat input rate
5. Lower cost
Can you perform a 20-exposure series of 100 kVp, 75 mAs each if
tube has accumulated 200 KHU? How long to wait?
X-Ray Tube Cooling Curves
Active Cooling of X-Ray
Tube and Housing
Can you perform a 20-exposure series of 100 kVp, 75 mAs each if
tube has accumulated 200 KHU? How long to wait?
20x100x75=150 KHU: NO! Wait to cool to 150 KHU: 2.5 min
11
12/9/2014
Technologists must
Consider Many Factors
before taking Radiograph
1.
2.
3.
4.
5.
6.
7.
Patient positioning
Contrast media
Equipment type
Film + screen or
CR vs. DR
Body part thickness
Body part composition
Collimation
8.
9.
10.
11.
Focal spot size
Grid
Auto exposure control
Manual exposure
a) kVp
b) mA
c) Time
12. Many others
Line Focus Principle
{Take a sip of coffee}
Focal Spot Size: Pinhole Camera
Apparent focal spot size
varies with projection angle.
Electron beam
In practice, limited by FOV
(beam dimensions).
Small focal spot
Large focal spot
Focal Spot Size depends on:
•
•
•
•
Filament size
Electron focusing
Tube loading
Anode Angle
Electron beam
Small Focal Spot
0.6 mm
Large Focal Spot
1.2 mm
12
12/9/2014
Heel Effect
Tradeoffs of Focal Spot Size and Anode Angle
Anode
Cathode
Anode end
Cathode
Anode
The Heel Effect
is exploited in
radiography,
fluoroscopy and
mammography
Cathode
Anode
Anode
Cathode
Cathode end
Technologists must
Consider Many Factors
before taking Radiograph
1.
2.
3.
4.
5.
6.
7.
Patient positioning
Contrast media
Equipment type
Film + screen or
CR vs. DR
Body part thickness
Body part composition
Collimation
8.
9.
10.
11.
Focal spot size
Grid
Auto exposure control (AEC)
Manual exposure
a) kVp
b) mA
c) Time
12. Many others
Automatic Exposure Control
Fini
SEND IN YOUR FACULTY
EVALUATIONS, PLEASE!
Image Receptor
Happy Holidays
See you January 15
AEC must be calibrated for specific image receptor and clinical protocol
(kVp, patient thickness, body part, etc.)
13