X-Ray Production

BMP 205Lecture 3

Mike McNitt-Gray Ph.D.

Some images scanned from A.B. Wolbarst, Physics of RadiologyBushberg et. al., Essential Physics of Medical Imaging

OutlineCh 5 of Bushberg

• X-ray Production– Tube

• Anatomy• Operation

• Generator– Function

• Waveform

• Beam Production– Quality– Quantity– Heat

X-Rays

• Discovered by Wilhelm Roentgen 1895

X-Rays

• Do occur naturally• All medical X-ray generated by machine …• With X-ray Tube and High Voltage

Generator• Conversion of Electrical Energy to

Electromagnetic Radiation

X-ray Production

• Fancy lightbulb – high voltage vacuum tube• Cathode e- source and Anode target• electrons accelerated (high voltage) across vacuum• Suddenly decelerated (smacked) into high Z target• Conservation of Energy: Electron kinetic energy (1/2mv2)

converted into heat and E-M Radiation – (1% efficient)

X-Ray production (e- bombardment of high Z target)

X-Rays

• Bremsstrahlung• Characteristic

Bremsstrahlung Production

Bremsstrahlung Spectrum

Characteristic Production

Binding Energies

Electron Shell Tungsten Molybdenum RhodiumK 69.5 20.0 23.2L 12.1/11.5/10.5 2.8/2.6/2.5 3.4/3.1/3.0M 2.8-1.9 0.5-0.4 0.6-0.2

X-rays Tungsten Molybdenum RhodiumK1 59.32 17.48 20.22

K2 57.98 17.37 20.07K1 67.24 19.61 22.72

units: keV

Total Spectrum

X-ray Tube

X-Ray Tubes (Inserts)

Vacuum enclosure

• Vacuum enclosure – pyrex glass or grounded Al• High vacuum – no air molecules to impede or

cause secondary ionizations• insulator and or grounded for high voltage

applications• able to handle high temperatures and thermal

expansion

Siemens Straton Tube for CT

Siemens Straton Tube for CT

Siemens Straton Tube for CT

kV, mA, mAs

• kV: a measure of the voltage applied across the tube (from the anode to the cathode)

• mA: the measure of electron flow from cathode to anode (e.g., tube current)

• mAs: time integrated tube current

Generator Console

X-ray Beam: kV and mAs

• mA or mAs– affects quantity of x-rays

• kV– Affects x-ray beam energy and penetrability

(quality)– Also affects efficiency of production (quantity)

Basic Operation• Cathode is source of electrons• Heat up a filament, which emits electrons

(thermionic emission)• Electrons liberated from filament flow through the

vacuum of tube when a positive voltage is applied to anode (wrt cathode)

• Adjustments in filament current change temperature of filament to control tube current

• Electrons hitting anode produce bremmstrahlung and …..????

Cathode

Focusing Cup Bias

Grid controlled tubes can pinch off the electron flow.

Space Charge Effect

• thermionic emission results in an electron cloud (space charge)

• sufficiently large cloud => repel further emissions

Space Charge Effect

• Applied high voltage across the tube – tube current flows: no cloud buildup– no repelling of further emissions– Accelerates electrons from cathode to anode

• Emission versus space charge limited output– voltage dependent

Space Charge Limited Output

X-Ray Tube Anode• High Z – related to efficiency of X-ray

production• Tungsten (W) Z=74

– high melting point – 3370 C– reasonably good heat conductor– alloy w/ Rhenium (10%) for structural strength

Anode

• Two Types– Stationary– Rotating

• Rotating anodes prevent heat buildup– ~1% energy converted into x-rays (heat)– rotating anode prevents heating 1 spot

continuously

Stationary Anode

Anode: Rotating

• Rotating – higher heat capacity– greater surface area– 3600 & 10000 rpm– stator/rotor induction motor– Molybdenum stem

Anode and Focal Spot

Anode Angle and Field Coverage

Line Focus Principle

• Problem of competing needs• Want small focal spot for high

resolution (penumbra)• Want large focal spot for high

heat capacity – due to low efficiency of X-ray

production

Line Focus Principle

• Angled anode face (12 - 20 degrees) allows:– larger actual focal spot size– smaller effective focal spot size

• Decreasing angle – decreases heat capacity but – increases resolution

Anode Heel Effect

Anode Heel Effect

e-

Higher Intensity“Softer” Beamlarger apparent size

Lower Intensity“Harder” Beamsmaller apparent size

cathode anode

Anode Heel Effect• Intensity gradient from self absorbtion of

anode (heel)• As great as 30% along anode-cathode• Anode side: smaller apparent fs, harder

beam, lower intensity • put thicker/denser anatomy at cathode end• becomes more pronounced with:

– short SID– large field size– small anode angle

Adjustable Collimator with light localizer

Coincidence of light & x-ray field

Generator

• Heart and Brain of X-ray System

X-Ray Generators• Converts electrical power

from building electrical grid into form that can be used by X-ray Tube

• All grid regulated to 60Hz Alternating Current

• Single phase supply 110 Volt AC

• Three phase 220 Volt

Tube requirements

• Needs DC (is in fact a vacuum tube diode)• Can only conduct in one direction:

– cathode negative with respect to anode– Because of thermionic emission– Therefore need rectifiers; convert AC to DC

• Need high voltage for X-ray production– ½ mv2 into h– 110 Volts vs 110 thousand volts– Therefore need transformers (changes voltage)

Transformer

sspp

s

p

s

p

iViVNN

VV

Single Phase

3 Phase

Voltage Ripple

I kVp2

One vs. Three Phases

Other generators

• Battery storage• Capacitor discharge• Constant potential gradient (CPG)

– Tetrodes (high voltage vacuum tubes) control kV and exposure time directly on high voltage side

– Flat waveform but expensive– High freq nearly as good

Summary

• Production– Bremsstrahlung and Characteristic– Quality and Quantity

• Tube and Generator– Operation– Imaging

Additional Detail Slides

Transformers• Two separate coils of wire

wrapped around closed core

• Many configurations• Electrical supply

connected to 1• Output device to 2• Step up or step down

Laws of Transformers• 1) Voltage in two circuits

proportional to number of turns in the two coils

• 2) Power (Energy) is conserved:– As Power (watts) is voltage x

current:– Therefore as voltage increases by

turns ratio, current decreases

sspp IVIV

s

p

s

p

VV

NN

p

s

s

p

II

NN

Bushberg

Autotransformer• Unique single winding design

– Self inductive• 1 & 2 defined by number of turns

enclosed by taps• Variable number of turns from taps

allows voltage control at relatively low potential

• Feeds primary of high voltage transformer and filament transformer

• Can be both step up and down

Filament circuit• Step down transformer drops voltage

– 10 V @ 3-5 A• Filament current (A) indirectly controls tube

current (and output X-ray intensity)

High Voltage Circuit• Step up transformer • > 500 fold voltage increase• Immersed in dielectric• Secondary side of autotransformer• Fixed # of transformer windings• Grounded at center (mA meter)

– So for 100 kVp, potential on one side is +50,000 V & other is –50,000 V

– Less of an insulation problem

Rectification• Converts AC (needed by

transformer) to DC (needed by tube)• Conduct current in one direction

only• Vacuum tubes (old style) large,

bulky, and burnout• Solid state semiconductor diodes

– Made of N-P semiconductors• Conduct only on forward bias

Diode Bridge (Wheatstone bridge)

• Four diode arrangement to allow current to flow in one direction through tube regardless of polarity of secondary side of high tension transformer

• Full wave rectified generator• 2x as efficient as self (half) wave

rectified• But inefficient compared to high

freq & CPG generators

Generator Efficiency• Single Phase – 100% ripple w/

half or full wave rectified• High voltage varies between 0

and max• For single phase, average

voltage is R.M.S.

peakpeakSMR 707.02

...

Three phase generators• Recall AC power avail. in 3• 3 voltage peaks per 1/60 sec• 3, 6 pulse

– High volt transform & rectify– 13.5% ripple

• 3, 12 pulse– 2 different winding config on 2°

• Delta and wye– Another 30° phase shift for 2 halves of

output, peaks fill troughs– 3.5% voltage ripple

Medium/High Frequency• Transformer efficiency: V ~ NA• By increasing frequency, cross sectional area reduced for same power (50kW in tube head!)• Frequency of invertor ranges from 5-100 kHz!• Feedback loop controlled – during exposure if kV drops off, increase invertor frequency & kV increases• Timer accuracy• Shorter exposures

– (<10 ms)

Generator Type / High Generator Type / High Voltage WaveformVoltage Waveform

Tube Limits & Rating Charts

• Tube insert has power/load limit• Function of heat produced in exposure• HU = kVp x mA x time x correction factor

– single phase generator – less efficient– Correction factor cpg generator =1.4– 70 kVp x 100 mA x 0.1 sec = 700 HU (single phase)

• Joules = watts x seconds– 1 W = 1 V x 1 A = 1000 V x 0.001 A = keV x mA!– assume constant voltage, so divide by correction factor!– 70 kVp / 1.4 x 100 mA x 0.1 sec = 500 J (single phase)– For cpg is 700 Joules

Question:

What is highest kVp can safely use to get 35 mAs

(350 mA & 100ms)?

Question:

What is highest kVp can safely use to get 35 mAs

(350 mA & 100ms)?

Answer:

Should not exceed 100 kVp

Falling Load• Integrates area under tube

rating curve• Applies highest mA in

shortest time, reduces mA as exposure continues

• Expensive, not used as much with today’s high output tubes

Generator Efficiency Implications

• Single phase seldom at peak voltage, so set higher kVp

• Three phase higher average kVp• Less ripple means more mR/mAs (shorter

exposure time)– 5 mR/mAs single vs. 10 mR/mAs three phase

• Ripple based on some multiple of 60 Hz• High frequency more common now, smaller and

cheaper than CPG

Generator Power Rating• Tube power handling should match generator output• Rated in kilowatts under load (kVp x mA) @ 100 kVp• 80 kW generator can produce 800 mA at 100 kVp

(simultaneously)– Polydoros 80s, Medio CP80

• Small clinic may have 20kW, 200 mA at most• Angio/Cardio generators 100 kW and greater• CT not necessarily high instantaneous, but tube and generator

sustain for long periods