PHYS C6

Requisites for Generating X-rays

x-ray generator needs to:

▪ produce electrons

▪ accelerate electrons to high speed

▪ stop them abruptly

Diagnostic x-ray tubes

conventional Coolidge Tubes with Rotating Anodes

Thermionic Emission

Tungsten filament heated with low-voltage filament current (<10 A)

Electron excited, become loosely held

High enough temperature, electrons break free of nucleus

Increasing filament current, increases temperature, increases number of incident electrons, increases x-ray beam intensity

Space Charge

electron cloud just beyond filament where liberated electrons sit for a fraction of a second

Space charge effect

there is a limit to the number of electrons that can be in space charge before negative charge becomes strong enough to repel electrons back to filament, limiting emission

Focusing Cup

Used to concentrate emitted electrons into a tight beam towards anode target

Needed because electrons repel each other

Negatively charged concave molybdenum cup surrounding the cathode filament on 3 sides

Defines focal spot size which determines image sharpness

Dual-Focus X-ray Tube

Small and large filament parallel to each other

Only one filament energised at once

Small filament produces a narrow electron beam thus small focal spot, high spatial resolution, limited heat capacity

Large filament produces a less focused electron beam thus large focal spot, short-duration exposures, used for thicker body parts

Cold Carbon Nanotubes (CNT)

high voltage is applied to vertically aligned CNTs creating intense electric fields that extract electrons via quantum tunneling

releasing electrons without heat removes the need for cooling oil-bath and allows precise, instantaneous switching of the x-ray beam (start, stop, redirect)

have been used in portable systems, but lifetime, reliability, cost and integration challenges for universal adoption

Acceleration

Repulsion between incident electrons and focusing cups starts moving electron stream

During exposure focusing cup gets strong negative charge and anode gets strong positive charge, causing electrons to travel at high speed (high kinetic energy)

Voltage/electric potential difference

1V is the work required in Joules to move one Coulomb of charge from one point to another

Voltage formula

V = E/Q

eVs

1eV is the enrgy given to an electron accelerated through a potential difference of 1V

1eV = 1.602 × 10^-19 J

tube voltage of 100kV will give an electron 100keV of energy

Tube current

mA

number of electrons passing from cathode to anode

X-ray tube Energy Changes/Efficiency

Electrical potential energy converted to kinetic energy then at target surface 1% is converted to x-rays, 99% to heat

transformers

Step-up transformer creates kVp

Step-down transformer heats filament

Other X-ray Generator Circuit components

Rectifier ensures unidirectional electron flow

mA Selecter controls filament current and temperature which determines tube current

Exposure Timer sets exposure duration and is used to calculate mAs

kV_p

kilovoltage peak

maximum voltage applied across x-ray tube

equal to max kinetic energy of electrons

kV_p Selection

technologist sets maximum voltage on console

autotransformer determines voltage applied to step-up transformer

step-up transformer boosts voltage to kV range

Relationships

kVp controls x-ray energy (thus penetration) and quantity/intensity

mA contols x-ray quantity/intensity

Target

Higher atomic number increases x-ray intensity and energy

Tungsten used because of high Z, melting point, thermal conductivity

Tungsten vaporises and is deposited onto inside of glass enclosure over

time, upsetting electrics and eventually causing tube failure

Molybdenum used for mammograhpy, producing lower energy x-rays to maximise soft tissue contrast

Stationary or rotating targets

Stationary anode

used in some portable imaging systems

Rhenium-alloyed tungsten imbedded in end of a 45° angled copper rod

Rotating Anode

Promotes cooling between exposures by evenly dispersing beam across entire surface

Enables longer scans and higher doses

Improves lifetime

a thin rhenium-alloyed tungsten focal track on top acts as target, molybdenum in middle (lightweight so allows faster spinning and thermal conductivity that pulls heat away form focal spot), graphite on bottom to absorb heat

Rotor

Cylindrical copper structure surrounding a soft iron core

Attached to the anode stem, sits inside the rotating magnetic field

Stator

electromagnetic coils arranged as a three-phase winding

supplied AC power to create roating magnetic field around the rotor’s central axis

Faraday’s Law

time-varying magnetic flux passing through a circuit induces an electromotive force (EMF) directly proportional to the rate of change of that flux

Torque

eddy currents induced in the conductive rotor and produce their own magnetic field that opposes direction of stator’s magnetic fields (lenz’s law)

this produces a torque force causing rotor to rotate

Anode Operation

~ 3000 rpm

temp can be >1000°C

Glass envelope

contains components within vacuum enclosure

X-ray tube window

thin area of enclosure(~5 cm²) through which useful part of x-ray beam is emitted

Liquid Metal Bearing (LMB)

liquid metal alloy lubricants replace ball bearings allowinf for silent operation, higher rpm, better anode cooling, and increased lifespan

Rotating envelope tubes

entire vacuum tube rotates with anode acting as part of outer wall

cooling liquid oil circulates directly against heated anode allowing heat removal via conduction rather than radiation

more efficient cooling and bearings can be on outside for easy cooling and maintanince

Types of radiation generating x-rays

bremsstrahlung (braking radiation) - continuous curve on spectrum

characteristic radiation - peaks on spectrum

Bremsstrahlung Radiation

Incident electrons attracted to positive Coulomb field around atom’s nucleus

This field causes electron to slow down and change direction

Lost kinetic energy emitted as x-ray photon

X-ray photons have continuous energy ranging from 0 to max energy of the incident electrons but most is located at ~1/3 of max kV_p

Energy of x-ray photon depends on electron proximity to nucleus and energy of incident electron (tube voltage)

Major contibuter to x-ray spectrum in keV energies, sole contributer at MeV energies

Characteristic Radiation

Incident electrons eject target atom’s inner-shell electrons

incident electron keeps any excess energy that wasnt needed to overcome binding energy

outer-shell electrons fill the vacancy and emits a x-ray photon with an energy specific to energy difference between shells

Multiple peaks on x-ray spectrum because electrons that fill vacancy come from different outer shell

Instead of releasing an x-ray, energy can be transferred to another orbital electron, which is then ejected from atom as Auger electron (and can go on to cause x-ray emission)

X-ray Energy Spectrum

number of x-ray photons as a function of their energy

Focal Spot - def + link to image quality

Area of target where electrons strike and emit x-rays

Smaller focal spot = better spatial resolution

Line Focus Principle

Angling anode between 5 to 20 degrees (usually at 12 degrees) causes effective focal spot size to be much smaller than actual focal spot size Small effective focal spot improves spatial resolution

Large actual focal spot size allows heat to be dissipated over larger area to prevent tube melting

Smaller target angle = smaller effective focal spot size

Heel Effect

Electrons interact with target atoms at various depths in the target

x-rays emitted towards anode (target) must travel a greater distance through the target material, thus get attenuated more than x-rays emitted towards cathode

thus intensity of x-rays emitted through ‘heel’ of target is lower than (by up to 45%) compared to cathode side

Focal Effect

Effective focal spot is smaller, thus has lower radiation intensity, on anode side of x-ray field than on cathode side

So anode side produces shaper images but dissipates heat less effectively