Diagnostic Imaging Physics

MPHY-501

Bushberg Chapter 6

X-Ray Production, X-Ray Tubes and X-Ray Generators

During the next two years you will learn the basics of Diagnostic Imaging. Ultimately after this period you will enroll in a Medical Physics Residency Program and finally take the Board examination that, once you pass, will certify you as a medical physicist.

1

Basic Components

X-Ray Tube Insert

Electron Source

Medium for Acceleration (vacuum)

Electron Rich Target

X-ray Generator

Voltage Source to Create Potential Difference

Current

Exposure Timer

Tube Housing

Protective Shielding

Cooling

Tube Port

Filters (Beam Quality)

Collimators

X-Ray Production Process

Stage 1: Heat

Heat is required to boil electrons off of the Cathode

Stage 2: Acceleration

A potential difference (voltage) is placed across the Cathode and Anode

Stage 3: X-Ray production

Accelerated electron collides with the Anode (the “Target”)

Various atomic and nuclear interactions occur causing:

The initial accelerated electron to decelerate and give off x-rays

Electrons from target to be:

Ejected from target’s atoms giving off x-rays

Change energy levels (orbits) on the target’s atoms, giving off x-rays

Nuclear Collision

Electron capture (or similar event) causing total loss of electron’s kinetic energy, giving off a g-Ray

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

ANODE

+

Focusing Cup

Bias Voltage

Heater

+ High Voltage –

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

X-Ray

X-Ray

X-Ray

120,000 Volts

CATHODE

*Focusing Cup is Shaped like the letter “C” and Cathode starts with the Letter “C”

Concept

Kinetic Energy of Electrons are Transferred to X-Rays

Several types of electron interaction x-ray spectrum is continuous

0

20

40

60

30

50

70

80

90

10

Relative Output

Energy (keV)

Average Energy is 1/3 to 1/2 Maximum Energy

Filtered Bremsstrahlung Spectrum

Unfiltered Bremsstrahlung Spectrum

Filtration:

Objects like glass x-ray tube

Other objects purposely placed in beam like aluminum

Ratio of Radiative (Bremsstrahlung) energy loss to collisional energy loss

From 0 to 150keV

Several Levels of Energy Interactions 0 – >.5MeV

Bremsstrahlung radiation happens when electro changes direction this in order to balance the energy equation

Conduction band and Fluorescence band electrons

Inner shell electron interactions

Nuclear Attraction

Nuclear Absorption

Energetic Interactions

+

n

+

n

+

n

E0

l2

l1

Esc

q

Compton Electron

(Ee-)

Compton

e-

e-

e- + p = n + g

Non Schell Electrons

Bremsstrahlung

Increasing Energy

Decreasing Energy

e-

7

Characteristic X-ray generation

+

n

+

n

+

n

X-Ray emitted

Due to electron’s

energy shell change

Ejected K-Shell elecrton

Incoming Electron

Rebound Electron

Adjacent shell electron filling in

8

W, 90keV, Characteristic Spectrum

0

20

40

60

30

50

70

80

90

10

Relative Output

Energy (keV)

Filtered Bremsstrahlung Spectrum

Ka1

Ka2

Kb1

Extra Credit:

Name this peak

Shells and Subshells Filling

Shell name Subshell name Subshell max electrons Shell max electrons

K 1s 2 2

L 2s 2 2 + 6 = 8
2p 2p1/2 2p3/2 6

M 3s 2 2 + 6 + 10 = 18
3p 3p1/2 3p3/2 6
3d 3d1/2 3d3/2 10

N 4s 2 2 + 6 +10 + 14 = 32
4p 4p1/2 4p3/2 6
74 W

69.525

12.100

11.544

10.207

2.820

2.575

2.281

1.872

1.809

594.1†

490.4†

423.6†

Element

K 1s

L1 2s

L2 2p1/2

L3 2p3/2

M1 3s

M2 3p1/2

M3 3p3/2

M4 3d3/2

M5 3d5/2

N1 4s

N2 4p1/2

N3 4p3/2

10

Binding Energies and Characteristic Shell Edges

Electron Binding Energies keV of Common X-Ray Tube Targets

Electron Shell Tungsten Molybdenum Rhodium
K 69.5 20.0 23.2
L 12.1 11.5 10.2 2.8 2.6 2.5 3.4 3.1 3.0
M 2.8 1.9 0.5 0.4 0.6 0.2
K-Shell Characteristic X-Ray Energies (keV) of Common X-Ray Tube Targets

Shell Transition Tungsten Molybdenum Rhodium
Ka1 59.32 17.48 20.22
Ka2 57.98 17.37 20.07
Kb1 67.24 19.61 22.72
Basic Components

X-Ray Tube Insert

Electron Source (Cathode)

Medium for Acceleration (vacuum)

Electron Rich Target (Anode, can be a rotor/stator or fixed)s

X-ray Generator

Voltage Source

Potential Difference,

Mammography 25-40keV,

Other imaging 40-150keV

Current

Fluoroscopy 1-5 mA in continuous mode, 10-50mA in pulsed mode

Projection Radiography 50-1,2000 mA,

1mA = 6.25 X1015 electrons/sec

Exposure Timer

In projection Radiography typically < 10ms Tube Housing Protective Shielding Cooling Tube Port Filters (Beam Quality) Collimators Many times the product of Tube current and exposure time are combined into one unit: milliampere Seconds (mAs) Cathode The Cathode is the negatively charged electrode in the x-ray tube Combination of: filament focusing cup Filament receives ~10-Volts and up 7 ampere Most x-ray tubes have two filaments, one small, one large Each filament is placed into its own machined focusing cup One end of filament may be connected to or insulated from the focusing cup The other end of the filament is definitely insulated from the focusing cup Only one of the two filaments is energized at any given time Current from the filament circuit heats a filament, which releases electrons by thermionic emission Focusing Cups At the top are typical electron distributions incident on the target anode (the focal spot) for the unbiased and biased focusing cups. Application of ?4,000 V on an isolated focusing cup completely stops electron flow, even with high voltage applied on the tube; this is known as a grid biased or grid pulsed tube The focusing cup shapes the electron distribution when it is at the same voltage as the filament Isolation of the focusing cup from the filament and application of a negative bias voltage (? .100 V) reduces electron distribution further by increasing the repelling electric fields surrounding the filament and modifying the electron trajectories Tube Current Vs. Filament Current Dependence of approximately kV1.5 For tube voltages 40 kV and lower, a space charge cloud shields the electric field so that further increases in filament current do not increase the tube current. This is known as “space charge-limited” operation Above 40 kV, the filament current limits the tube current; this is known as “emission-limited” operation. Anodes Heat destroys anodes Damaged Anode produce fewer x-rays This requires more current that then causes more heat https://www.spellmanhv.com/en/Technical-Resources/Application-Notes-X-Ray-Generators/AN-02 Anodes Tungsten 90%/Rhenium10% alloy is good choice due to heat profile W Melting Point = 3,422C = 3,695K = 6,192F = (darn hot) To prevent heat damage the duration of x-ray production is limited Other Anodes are used to take advantage of characteristic x-rays Molybdenum Rhodium See Chapter 8 Essentials of Medical Physics (Third Edition), Bushberg, et.al. Anode Configurations Fixed Dental Mobile devices Some Portable Fluoroscopy Rotating Comprises most devices for heat management reasons Can withstand higher intensity production Rotation speeds 3,000 – 3,600 RPM low speed 9,000 – 10,000 RPM High Speed System is designed to not allow electron flow until anode is at full speed This is why these systems require one or more seconds before exposure button is pushed For an excellent description of rotating-anode failure modes see: https://www.spellmanhv.com/en/Technical-Resources/Application-Notes-X-Ray-Generators/AN-02 Simple Fixed Anode The anode of a fixed anode x-ray tube consists of a tungsten insert mounted in a copper block. Heat is removed from the tungsten target by conduction into the copper block. Rotating Anode The anode of a rotating anode x-ray tube is a tungsten disk mounted on a bearing-supported rotor assembly. The rotor consists of a copper and iron laminated core and forms part of an induction motor. Stator, which exists outside of the insert. A molybdenum (poor heat conductor metal) stem connects the rotor to the anode to reduce heat transfer to the rotor bearings (bottom) Focal Spot The anode (target) angle, ?, is angle of the target surface wrt the central ray. The focal spot length, as projected down the central axis, is foreshortened, according to the line focus principle (lower right). Effective Focal length = Actual Focal Length X sin q Diagnostic Imaging: 7o < q < 20o Commonly 12o < q < 15o Focal Length Examples EXAMPLE 1: Actual anode focal area is 4 mm (length) by 1.2 mm (width). Anode angle is 20-degrees, What is projected focal spot size at central axis position? Answer: Effective length = actual length × sin ? = 4 mm × sin 20 degrees = 4 mm × 0.34 = 1.36 mm; therefore, the projected focal spot size is 1.36 mm (length) by 1.2 mm (width). EXAMPLE 2: If Example 1’s anode angle is reduced to 10 degrees and actual focal spot size remains same, what is projected focal spot size at central axis position? Answer: Effective length = 4 mm × sin 10 degrees = 4 mm × 0.174 = 0.69 mm; thus, the smaller anode angle results in a projected size of 0.69 mm (length) by 1.2 mm (width) for the same actual target area. CATHODE e- e- e- e- e- e- e- e- e- e- e- e- Heal Effect Effective Focal Spot: Terminology Anode Side Cathode Side Heal Effect Anode Anode Side of the Anode Cathode Side of the Anode Cathode Side of Imaging Plane Anode Side of Imaging Plane More Photons Fewer Photons Heal Effect is Less Prominent at large Source Image Distance (SID) Source distance is measured from focal spot center of anode Image distance measured from center of imaging plane SID Differences in Projected Focal Spot Size Within the Imaging Field Anode Side Cathode Side NEMA Standards for Focal Spot Sizes There are three limits, small, medium and large focal spots Focal spots can be larger than nominal size but NOT SMALLER. For focal spots Must be done at 74keV and 50% of max mA for each spot size (remember that many cathodes have two elements) fs < 0.8 mm + 50% 1.5mm ? fs ? 0.8mm +40% fs > 1.5mm +30%

Start your Spread Sheets!!!!!!

Focal Spot Size Focal Spot Size
Nominal Size Tested Size PASS/FAIL Passing Criteria
fs < 0.8 mm + 50% 1.5mm ? fs ? 0.8mm +40% fs > 1.5mm +30%
4 – Ways to test focal spot size

Pin hole

Typically 10micrometer to 30micrometer diameter hole (highly attenuating)

Slit

Must be placed in two different directions

Along the anode cathode direction

Perpendicular to the anode cathode direction

Star

Bar

How many lines/mm is a resolution of 0.6mm?

A line pair consists of a dark line and a bright line.

So if one line is 5 microns wide, then a line pair will be 10 microns wide and there would be 1 mm/10 microns = 100 line pairs per millimeter.

Off-Focal Radiation

Electrons scatter off or anode are re-accelerated to anode but not to the focal spot

Give low intensity x-rays about the face of the anode

Many off-focal radiation are caught by the collimator

Grounded metal X-ray tube enclosures attract many of the scattered electrons thus keeping them from returning to anode

Off-Focal Radiation Distribution

Hot Spots

Description: X-Ray Tube Insert

Contains

Cathode

Anode

rotor assembly

sealed support structures

glass or

metal

enclosure under a high vacuum

Gas molecules leach from internal structures

”Getter” eliminates those molecules

Note1: “Getter” is a deposit of reactive material placed inside a vacuum system, for completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by absorption. Thus the getter removes small amounts of gas from the evacuated space.

https://en.wikipedia.org/wiki/Getter#:~:text=A%20getter%20is%20a%20deposit,gas%20from%20the%20evacuated%20space.

Note 2:

Mammography Inserts include a beryllium (z=4) window in the port specifically to minimize the the absorption of low energy x-rays.

Other ports (non mammography) use the same material as the tube itself.

Description: X-Ray Tube Housing

Between Insert an Housing is Oil, provides:

Electrical insulation

Heat conduction (cooling)

Expansion bellows (for the expansion of oil as it heats up)

Overtemperature (senses oil temperature) switch stops x-ray production

CT and Interventional Fluoroscopy (high duty cycle) Housings

may have built-in heat exchangers (for cooling)

Shielding for scattered “leakage” (x-rays not in direction of imaging)

21 CFR (code of federal regulations) 1020.30 requires

leakage less than 100 mR/hr (0.88 mGy/hr) @ one meter from focal spot

Leakage must be tested at maximum rated kV (typically 125kV – 150kV) and highest possible current typically 3mA to 5mA

Equipment software/hardware should not allow operation at above max rated kV

Testing with

Area survey dose rate meter

Collimators

Adjust size and shape of x-ray field from tube port

Parallel opposed shutters

rectangular FOV for “plain film”

other shapes available (Fluoroscopy, hexagonal Iris…)

Collimator Assembly

Lead insert Shield

Collimator

Assembly

Collimation must be within 2% of SID

Example: SID = 100cm

Collimation must be:

100cm ± 2cm, 40” ± 0.8”

20CFR1020.31

Positive Beam Limitation (PBL)

Automatically senses film cassette size and adjusts so that beam cannot exceed cassette size

Changeable Filter

Al, Cu, Moly, Rho,…

Special Tube Designs

Pulsed Fluoroscopy

Pro: 4kV grid bias keeps electrons in cloud by cathode when off electrons rapidly fly toward anode allowing rapid pulse times.

Con: Costs much more $$$$$

Mammography

Special tube output port (Beryllium instead of glass or other metal)

Potentially Different Targets (molybdenum, rhodium, tungsten)

Smaller Focal Spot (0.3mm, 0.1mm)

Grounded Anodes

CT

High instantaneous output, high heat load, Rapid Cooling

Rapid Tube Rotation about Gantry. More than 200RPM!!! https://www.youtube.com/watch?v=aBlJebipLgM

CT Advanced X-Ray Tube Features:

Anode & planar cathode within rotating vacuum enclosure.

Bearings mounted outside vacuum enclosure.

Deflection coils magnetically direct electron beam to specific areas on the target.

Circulating oil rapidly removes excess heat from the anode.

Flying Focal Spot: Electron beam rapidly deflects between two focal spots

How to Maximize X-Ray tube life

Hold ready state only as long as needed (dead man 2-stage switch)

Holding on keeps current in filament … damages filament

Evaporated tungsten deposits on inside of x-ray tube (not good)

Use lower tube current and longer exposure times (when reasonable)

Avoid extended repeat operation at high technique

Can etch anode and cause several problems lower photon output etc.

Do not make high mA exposure on cold anode

follow manufacture’s recommendations

Limit Rotor start & Stop operations

Allow 30 to 40 seconds between exposures

Filtration

Inherent Filtration

X-ray Tube

Glass (SiO2) and Aluminum have similar attenuations, ZSiO2=14, Zal = 13

Glass and Aluminum attenuate all x-rays below 15 keV

Port

Housing Oil

Collimator Assembly

Mirror

Added (beam quality filters)

Intended to eliminate low energy x-rays that add to dose but not image

Metal

Plastic

X-Ray Generator’s Voltage Needs

Np = 5

Ns = 10

Vs = 2Vp

Transformer Nomenclature| Ideal Transformer

Ideal Transformer: Power Input = Power Output

Power is measured in watts

1watt = 1ampre * 1volt

P = I * V

Where:

P = power

I = current

V = Volts

Remember:

Pin = Pout

Therefore:

Vin * Iin = Vout * Iout

In Reality:

Pin = efficiency constant * Pout

Some number between zero and one

Loss due to heat?

Rectifiers

https://www.electronics-tutorials.ws/diode/diode_6.html

Full Wave Rectifier Circuit

Full-wave Rectifier with Smoothing Capacitor

5uF Smoothing Capacitor

50uF Smoothing Capacitor

Note Capacitor value’s impact on Smoothing

V=Q*C

Volts = charge (coulombs) * capacitance (Farads)

High Frequency Production and Rectification

5uF Smoothing Capacitor

50uF Smoothing Capacitor

Step 5

Step 1

Step 6

Step 2

Step 7

Step 4

Step 3

Step 8

Low Frequency

Rectification

Hi Frequency

Rectification

Production of

Hi Frequency

Voltage Ripple

Switches

High-Frequency Inverter on primary side of HV transformer because of low Voltage

Alternately “Grid Control” can stop x-ray production through biasing the grid (-4,000 V) to stop electron acceleration to target anode.

Grid Control is the fastest switching method

Timer(s)

Old equipment used mechanical timers

New equipment uses:

digital timers

Automatic Brightness Control (ABC, used in spot film devices)

Automatic Exposure Control (AEC, Fluoroscopy devices) also called “Photo timer”

Back-up ”Count Down Timer”

Used in the case that the ABC or digital timer fail so exposure is terminated

Fluoroscopy Systems have another timer to warn user that the tube on time has increased (more on this in Fluoroscopy section)

ABC and AEC automatically adjust kV and mAs to achieve correct exposure to detector for optimal image

Phototimer/Automatic Exposure Control (AEC)

AEC detectors measure the radiation incident on the detector and terminate the exposure according to a preset optical density or signal-to-noise ratio achieved in the analog or digital image.

Chest cassette stand and the locations of ionization chamber detectors are shown. The desired signal to the image receptor and thus the signal-to-noise ratio may be selected at the operator’s console with respect to a normalized reference voltage.

Anti-Scatter Grid

Power Ratings, Heat Loading & Cooling

Power = Volts times Amperes (P= VI)

The Power rating (two ways to achieve)

= maximum power an x-ray generator can deliver

or

= maximum power a focal spot can accept

Testing is done for 0.1 seconds @ 100 kV

Example: a generator that can deliver 800 mA of current @ 100kV in 0.1 seconds has a power rating of:

0.8 Amperes X 100000 volts = 80,000 Watts or 80kW

When recommending an x-ray generator type the intended use should be taken into consideration. Power ratings can help. Duty cycle is many times required before recommendation can be made.

Heat Loading

The “Heat Unit”

The Heat Unit is labeled “HU” this is an unfortunate label because the CT x-ray linear attenuation units are translated into Hounsfield Units also labeled “HU”

Energy is in Joules where 1 Joule = 1 Watt * 1 Second

HU (Single Phase) = Voltspeak * Amperes * Seconds

HU (Single Phase) = kVpeak * mA * Seconds

HU (Three Phase) = 1.35 * (kVpeak * mA * Seconds)

HU (High Frequency) = 1.4 * (kVpeak * mA * Seconds)

Energy (Joules) = VoltsRMS * Amperes * Seconds

Energy (HU) = 1.4 * Energy (Joules)

Danger Ambiguation Situation

Danger Notice Vpeak & VRMS are used here

Anode Heating and Cooling Chart

Energy (Watts), HU (Joules), Joules = Watt Seconds

Therefore: Energy = HU/s = Volt Amps = Watts

CT scanner anode heating and cooling curves. Power input curves (2 kW – 28kW) determined by kV and mA settings during continuous x-ray tube operation over time. Cool down curve shows the rate of cooling and indicates faster cooling at higher anode heat loads (temperature). In this example the maximum capacity is 5.7 MJ. For low power inputs, heating and cooling rates eventually equilibrate and reach a steady state, as shown for the 2, 4, and 8 kW curves.

4kW

Quesiton:

5.7 MJ = how many MHU?

Factors Effecting X-Ray Emissions

Anode Target Material

Tube Voltage

Tube Current

Beam Filtration

Generator Waveform

Anode Target Material

Higher Z = more bremsstrahlung radiation

Characteristic x-rays depend on target material

Tube Voltage

Tube Current

Beam Filtration: 21 CFR 1020.30

Designed Operating Range Measured x-ray tube voltage Minimum HVL (mm of Aluminum)
<51kV 30 0.3 40 0.4 50 0.5 51-70 kV 51 1.3 60 1.5 70 1.8 >70 kV 71 2.5
80 2.9
90 3.2
100 3.6
110 3.9
120 4.3
130 4.7
140 5.0
150 5.4
Spread Sheet Lookup table

Tube Filtration and Skin Entrance Dose

10cm PMMA (60kV) 20cm PMMA (80k) 30cm PMMA (100kV)
TUBE Current DOSE (mGy) TUBE Current DOSE (mGy) TUBE Current DOSE (mGy)
mAs %D Dose %D mAs %D Dose %D mAs %D Dose %D
3.8 0 264 0 6.8 0 1,153 0 14.5 0 4,827 0
5.0 32 188 -29 8.2 21 839 -27 16.5 14 3,613 -25
6.2 63 150 -44 9.3 37 680 -41 17.6 21 2,960 -39
8.8 132 123 -53 11.2 65 557 -52 19.8 37 2,459 -49
Filtration
0 mm Al
2 mm Al
0.1 mm Cu + 1 mm Al
0.2 mm Cu + 1 mm Al
OUTPUT DIGITAL IMAGE EQUIVALENT SIGNAL . TUBE FILTRATION, MEASURED CHANGES IN mAs, AND SURFACE DOSE (?Gy)

Filtration Causes:

1) acquisition technique much higher,

2) entrance dose much lower

3) no loss of image quality for digital imaging

Used Photo Timed Mode

Generator Wave Form

Affects the quality of the emitted x-ray spectrum. For the same kV, a single-phase generator provides a lower average potential difference than does a three-phase or high-frequency generator. Both the quality and quantity of the x-ray spectrum are affected

Output intensity (bremsstrahlung) spectra for the same tube voltage (kV) and the same tube current and exposure time (mAs) demonstrate the higher effective energy and greater output of a three-phase or high-frequency generator voltage waveform (?5% voltage ripple), compared with a single-phase generator waveform (100% voltage ripple)

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