FRCR: Physics Lectures

Diagnostic Radiology

Lecture 2 The X-ray tube, the physics of X-ray

production and ‘exposure factors’

Dr Tim Wood

Clinical Scientist

Overview

• The X-ray tube

• Controlling the X-ray spectrum – Exposure factors– Filtration

• X-ray beam uniformity– The anode-heel effect

From last time…

X-ray Properties• Electromagnetic photons of radiation• Emitted with various energies & wavelengths

not detectable to the human senses• Travel radially from their source (in straight

lines) at the speed of light• Can travel in a vacuum• Display differential attenuation by matter• The shorter the wavelength, the higher the

energy and hence, more penetrating• Can cause ionisation in matter• Produce a ‘latent’ image on film/detector

Planar or three-dimensional?

• Planar imaging is the most common technique used in diagnostic radiology– General radiography e.g. PA chest– Mammography screening– Intra-oral dental radiography– Fluoroscopy (but some modern ones can do 3D)

• The anatomy that is in the path of the beam is all projected onto a single image plane– Tissues will overlap and may not be clearly visible –

contrast is generally poorer than in 3D imaging techniques

X-ray interactions with matter

• Contrast is generated by differential attenuation of the primary X-ray beam

• Attenuation is the result of both absorption and scatter interactions

• Scatter occurs in all directions, so conveys no information about where it originated – can degrade image quality, if it reaches film/detector

• Scatter increases with beam energy, and area irradiated

Interaction Processes

• Elastic scattering

• Photoelectric effect

• Compton effect

Photoelectric Effect

Compton Effect

The Mass Attenuation Interaction Coefficient

The Mass Attenuation Interaction Coefficient

Maximising Radiographic Contrast

• More Photoelectric absorption means higher patient dose

• Scatter rejection techniques attenuate the primary beam, so a higher patient dose is required for acceptable image receptor dose

• NEED TO BALANCE IMAGE QUALITY WITH PATIENT DOSE!!!

• Hence, the principle of ALARA (As Low As Reasonably Achievable)– Use the highest energy beam that gives acceptable

contrast, consistent with the clinical requirements

The X-ray tube

X-ray tube design - basic principles

• Electrons generated by thermionic emission from a heated filament (cathode)

• Accelerating voltage (kVp) displaces space charge towards a metal target (anode)

• X-rays are produced when fast-moving electrons are suddenly stopped by impact on the metal target

• The kinetic energy is converted into X-rays (~1%) and heat (~99%)

Filament(heats up on prep.)

Target

kV

+-

Electron production in the X-ray tubeApplied voltage chosen to give correct velocity to the electrons

mA

Bremsstrahlung

Characteristic X-rays

The X-ray spectrum

• Combination of these yields characteristic spectrum.

0.00E+00

5.00E+04

1.00E+05

1.50E+05

2.00E+05

2.50E+05

3.00E+05

3.50E+05

4.00E+05

0 20 40 60 80 100 120 140

Energy (keV)

Inte

nsi

ty

60 kVp80 kVp120 kVp

The X-ray spectrum

• The peak of the continuous spectrum is typically one third to one half of the maximum kV

• The average (or effective) energy is between 50% and 60% of the maximum– e.g. a 90 kVp beam can be thought of as effectively

emitting 45 keV X-rays (NOT 90 keV)

• Area of the spectrum = total output of tube– As kVp increases, width and height of spectrum

increases– For 60-120 kVp, intensity is approximately

proportional to kVp2 x mA

Controlling the X-ray spectrum -Exposure factors

• Increasing kVp shifts the spectrum up and to the right– Both maximum and effective energy increases, along

with the total number of photons

• Increasing mAs (the tube current multiplied by the exposure time) does not affect the shape of the spectrum, but increases the output of the tube proportionately

Quality & Intensity

Definitions:• Quality = the energy carried by the X-ray

photons (a description of the penetrating power)

• Intensity = the quantity of x-ray photons in the beam

• An x-ray beam may vary in both its intensity and quality

Back to tube design…

X-ray tube design

• Heat generation is a significant problem for X-ray tubes, and is generally the limiting factor upon their use

• Hence, it is necessary to:– Ensure efficient cooling mechanisms – take the heat

away so it doesn’t build up with multiple exposures– Have mechanisms to prevent over-heating – should it

get too hot, have mechanisms in place to stop further exposures

– Minimise heat generation on a single point of the anode (stop it melting!)

X-ray tube cooling• Generally, the tungsten target is mounted on a

copper block/rotor (either directly or indirectly) that extends out of the evacuated glass envelope

• Heat is transferred from the target to the surrounding coolant (most often oil, but very occasionally water) via conduction and/or radiation, which in turn gives up its heat to the atmosphere (possibly through a heat exchanger)

• Expansion bellows can detect when the coolant is getting too hot (or by other means) and prevent further exposures

• BUT, what about spreading the heat generating processes over a larger area?...

The rotating anode

• Heat can be spread over a large area by rotating the anode during exposure

• Tungsten annulus set in a Molybdenum disk attached to a copper rotor

• The assembly is rotated via an induction motor• Full rotation ~20 ms• Takes about 1 s to get up to speed

– The prep phase (push the exposure switch down to the first stop until you can hear it whirring) before pushing down all the way to expose

X-ray tube design – The rotating anode

Geometric unsharpness and the focal spot

• Spatial resolution is dependent upon (more on this next time…):– Geometrical unsharpness– Motion unsharpness– Absorption unsharpness

• Geometric unsharpness is related to the fact that we cannot (and in fact do not want to) produce an ideal point source of X-rays– The focal spot of the X-ray tube has a finite size that

results in blurring across the edge of structures– Can be reduced by using a smaller focal spot,

decreasing the object-film distance (OFD) or using a longer focus-to-film distance (FFD)

Geometric unsharpness –The ideal point source

Object

Film/detector

Ideal point source of X-rays

FFD

OFD

Geometric unsharpness –A ‘real’ focal spot

Object

Film/detector

Focal spot of finite size, f

FFD

OFD

Penumbra

Geometric unsharpness and the focal spot

• So, to minimise geometric unsharpness, the smallest focal spot should be used…

• BUT, this would be at the expense of excess heating and reduced tube life

• The solution is to use an angled target as the source of X-rays– Angle allows broad beam of electrons to give a

smaller apparent focal spot– Have multiple filaments for focal spot size selection –

large focal spot for general use (tube lasts longer), and small focal spot where better resolution is required

X-ray tube design – dual focus

The focal spot

Actual focal spot size

The focal spot• Apparent focal spot size will vary across the film

– Elongated on cathode side, contracted on anode side

• Target angles vary between 7 and 20°– Steeper angles allow greater tube loading– BUT, the useful X-ray field is smaller due to the anode-

heel effect (more on this later)– Hence, steep target angles suitable for applications with

limited fields of view e.g. mammography, cardiology

• Typical focal spot sizes are– 0.15-0.3 mm for mammography– 0.6-1.2 mm for general radiography– 0.6 mm for fluoroscopy– 0.6-1.0 mm for CT

Heat rating

• kV, mA and exposure time should be such that the temperature of the anode does not exceed its safe limit– The control system is designed to prevent exposures

that exceed the tube rating

• Require much higher tube ratings for CT and interventional fluoroscopy units

Shielding

• X-rays are emitted from the target in all directions, not just towards the patient

• Hence, Lead shielding is used in the tube housing to absorb X-rays not required for imaging of the patient

• Legal requirements on how much ‘leakage’ radiation is emitted from the tube during operation– Medical Physics testing checks this during the Critical

Examination of new installations

The diagnostic X-ray tube

The diagnostic X-ray tube

• Tube Current (mA)/Exposure Factor (mAs)– Double the mA/mAs, double the intensity– Beam quality not affected

• Tube Voltage (kVp)– Intensity α kVp2

– Penetrating power increases with kVp– Higher kVp reduces skin dose

• Filtration (mm Al)• Focus-to-skin distance

Factors Affecting Patient Dose

Patient dose reduction – Filtration and beam hardening

• ‘Soft x-rays’ contribute to patient dose without contributing to image production

• Placing Al filters in the beam will increase beam quality – this is known as ‘Beam Hardening’– Alternative materials may be used for filtration in

specialised applications e.g. mammography (Mo, Rh, Ag) and fluoroscopy (Cu)

• Lowest energy photons are most readily absorbed as photoelectric absorption dominates (proportional to the E3)• As the beam passes the Al, the proportion of low

energy photons is reduced, and the average photon energy increases

Filtration

Filtration

0 20 40 60 80 100

keV

Ou

tpu

t

0.5 mm Al

2.5 mm Al

7.5 mm Al

Patient dose reduction – Filtration and beam hardening

• Hence, Patient dose is reduced with little affect on the radiation reaching the detector

• However;• Radiographic contrast is reduced due to the higher

mean energy of the beam• Greater exposure factors required to yield satisfactory

dose at film/detector (have to drive the tube harder, and hence tube life may be reduced)

• The X-ray beam is also filtered by the target that they are produced in, the coolant oil and the window of the housing• ‘Inherent filtration’ equivalent to about 1 mm Al

Focus-to-skin Distance:The Inverse Square Law

• For a point source, and in the absence of attenuation, intensity decreases as the inverse of the square of the distance

• This is a statement of the conservation of energy

21

22

2

1

r

r

D

D

The inverse square law

• Patient dose can be significantly reduced by increasing the distance to the X-ray tube– FSD < 45 cm should not be

used (<60 cm for chests – 180 cm used in practice)

X-ray beam uniformity

• The X-ray tube emits X-rays in all directions

• A collimator system is used to adjust the beam to the required size– Two pairs of parallel blades

of high attenuation material that can be adjusted to define the required rectangular field size (has a light beam system for visualisation)

The anode heel effect

• Ideally, the X-ray beam defined by the jaws would be uniform across the whole image

• However, this is not the case due to the anode heel effect, which results from the combination of the angled anode and the depth at which X-rays are generated– The steeper the target angle, the worse the effect– Filtration differences at the edge of the field, the

inverse square law and apparent focal spot size also influence beam uniformity, but to a lesser extent

The anode heel effect

The anode heel effect• The electrons penetrate a few micrometres

below the surface of the anode before generating X-rays

• Hence, the X-rays that are generated in the target may be attenuated on their way out

• X-rays travelling towards the anode edge of the field (A) will have to pass through more of the target before exiting the tube– Hence, attenuation will be greater on this edge, and

beam intensity will be lower than on the Cathode side of the field (B)

– Roughening of the anode surface as the tube ages make this effect worse

The anode heel effect

• Generally not noticeable on most films– This effect is corrected for in digital imaging through

‘flat-fielding’ the detector• The anode heel effect is actively exploited in

some modalities e.g. mammography (more on this later)

• Can be minimised by using greater focus-to-film distances, smaller fields and shallower target angles