FRCR: Physics Lectures

Diagnostic Radiology

Lecture 1 An introduction to radiography with X-rays

and the X-ray tube

Dr Tim Wood

Clinical Scientist

Learning Objectives

5.2: Distinguish between different types of diagnostic medical image and understand how such images are created, reconstructed, processed, transmitted, stored and displayed

5.3: Describe the construction and function of medical imaging equipment including the radiation or ultrasound source, image-

forming components and image or signal receptor

5.4: Indicate how imaging equipment is operated and describe the imaging techniques that are performed with such equipment

Learning Objectives

5.5: Identify the type of information contained in images from different modalities

5.6: Distinguish between different indices or image quality, explain how they are inter-related and indicate how they are affected by changing the operating factors of imaging equipment

5.7: Identify agents that are used to enhance image contrast and explain their action

5.8: Explain how the performance of imaging equipment is measured and expressed

Learning Objectives

5.9: Describe the principles of quality assurance and outline how quality control tests of imaging equipment are performed and interpreted

A little bit of history…

• Wilhelm Röntgen discovered X-rays on 8th Nov 1895

• Took first medical X-ray of wife’s hand (22nd Dec 1895)

• Used to diagnose Eddie McCarthy’s fractured left wrist on 3rd Feb 1896 (20 min exposure)

• Awarded first Nobel Prize in Physics in 1901 for his discovery of ‘Röntgen rays’

A little bit of history…

Thankfully, things improved!…

What is diagnostic radiology?

ra·di·ol·o·gy  

The science dealing with X-rays and other high-energy radiation, especially the use of such radiation for the diagnosis and treatment of disease

Origin: 1895–1900; radio-  + -logy

Related forms:ra·di·ol·o·gist, noun

What is diagnostic radiology?

• The underlying principle of the majority of diagnostic radiological techniques is that X-rays display differential attenuation in matter– When the X-ray beam is targeted at a patient, the

different tissues in the body will remove a different number of X-rays from the beam

• The resulting modified X-ray flux can then be ‘captured’ by some form of detector to produce a latent image or radiation measurement– Detection may be through film, phosphor screens,

digital detectors, etc

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

Planar or three-dimensional?

Planar or three-dimensional?

1 11 7

11

1 1 1

3 9 32D detector

Subject contrast7:1

2D image contrast3:1

Planar or three-dimensional?• 3D imaging offers superior contrast to 2D• More techniques are becoming available

– Computed Tomography (CT), Cone beam CT, Tomosynthesis, etc

• Compromise is that doses tend to be much higher than the planar image– e.g. CT chest = 6.6 mSv c.f. PA chest = 0.02 mSv (a

factor of 330 difference!)

• Hence, despite being less common, they account for a significant proportion of the UK populations exposure to medical radiation– CT accounts for 11% of examinations, but 68% of

dose (HPA 2008 review)

X-ray interactions with matter

• It is the physics of the interactions with matter that determine how each imaging technique works, and how it is used in clinical practice

• So, a bit of revision…

X-ray interactions with matter (revision)

• 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

Pass throughAbsorptionScatter Attenuation

Attenuation

• For a mono-energetic photon beam:

where, I = final intensity, I0 = incident intensity, µ = attenuation coefficient, x = thickness

• Equal thicknesses of material reduce the intensity by the same fraction (half-value thickness).

Attenuation

• Attenuation coefficient, µ, decreases with increasing photon energy (except for absorption edges)

• Increases with atomic number of material, Z• Increases with density of material, ρ• Transmission of radiation @ 70 kVp;

– 1 cm of soft tissue 66% transmitted– 1 cm bone 17% transmitted– 1 cm tooth 6% transmitted

Forward vs. Back-scatter

• Forward scatter is most likely, but ...• Forward scatter is attenuated by the patient, and• Deeper layers receive a smaller intensity, so there

are fewer scattering events

• Overall, see more back scatter.• Advantage for image quality (less scatter, but more

attenuation at the detector), but may pose a risk in terms of radiation protection

Forward vs. Back-scatter

Interaction Processes

• Elastic scattering

• Photoelectric effect

• Compton effect

Elastic Scatter

• Photon energy smaller than BE• Causes e- to vibrate – re-radiates energy• No absorption, only scatter• < 10% of total interactions in diagnostic range

i.e. not significant

E

Z 2yProbabilit

Photoelectric Effect• Process of complete absorption• ~30% of interactions in diagnostic range• Energy is transferred to bound e-, which is

ejected at a velocity determined by difference in photon and BE

• e- dissipates energy locally, and is responsible for biological damage

• Hence, main source of radiographic contrast (and dose), and why Lead is used in protection

3

3

E

ZyProbabilit

Photoelectric Effect

Photoelectric Effect

• Leaves atom in unstable state – electronic reconfiguration results in emission of X-ray or Auger electron

• Auger emission more probable for low Z material – short range in tissue (= more biological damage)

• Low energy X-rays reabsorbed locally• Rapid fall-off with increasing energy

Compton Effect

• Process of scatter and partial absorption – inelastic scattering

• Photon collides with a free electron (photon energy >> BE)

• Loses small proportion of its energy and changes direction

• Energy loss depends on scattering angle and initial photon energy

• Photon free to undergo further interactions until completely absorbed (Photoelectric)

Compton Effect

Compton Effect

• Compton scatter mass attenuation coefficient almost independent of energy over diagnostic range

• Ratio of Z/A similar for most elements of biological interest (~0.5) – offers little in terms of radiographic contrast

A

ZyProbabilit

• Each process is independent – can add the interaction coefficients to give the total mass attenuation coefficient

• Z dependence is the source of contrast in radiographic imaging

The Mass Attenuation Interaction Coefficient

The Mass Attenuation Interaction Coefficient

The Mass Attenuation Interaction Coefficient

Maximising Radiographic Contrast

• Maximise contrast due to Photoelectric absorption – use lower energy photon beams (note, it is the mean energy of the beam, not kVp that is important)

• Use scatter rejection techniques such as scatter grids and air gaps

• Limit beam to smallest area consistent with diagnostic task to minimise amount of scatter generated

• BUT…

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%)

X-ray tube designStationary anode – dental X-ray tube

Rotating anode – general X-ray tube

X-ray tube design• Evacuated glass envelope (allow electrons to

reach the target)• Filament (cathode) is source of electrons, with a

focussing cup around it to generate a narrow beam of electrons– Often dual focus to offer finer resolution on diagnostic

sets

Thermionic emission

• Applying a current to the filament causes it to heat up to ~2200°C (‘white hot’ like a light bulb)

• ‘Free’ electrons in the metal gain enough energy to overcome the binding potential – Can overcome the forces holding

them in the metal and escape from the surface

• Tungsten metal is ideal material

Thermionic emission

• Require two sources of electrical energy to generate X-rays– Filament heating current (~10

V, ~10 A)– Accelerating voltage of

between 30-150 kV (30,000-150,000 V); this results in a current of electrons between the anode and cathode (0.5-1000 mA)

Filament(heats up on prep.)

Target

kV

+-

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

mA

The physics of X-ray production

• Electron reaches the anode with kinetic energy equivalent to the accelerating potential (kVp)

• Electrons penetrate several micrometres below the surface of the target and lose energy by a combination of processes– Large number of small energy losses to outer

electrons of the atoms = heat– Relatively few, but large energy loss X-ray producing

interactions with inner shell electrons or the nucleus

Heat generating processes

• When an electron (e-) strikes the target, most likely interaction is with loosely bound e-s that surround nuclei

• Relatively weak interactions – slight deflection, ionisation or excitation

• Small amount of energy transfer (per interaction) – observed as heat

• However, accounts for ~99% of all energy dissipated from e- beam in the diagnostic range

Bremsstrahlung

Bremsstrahlung• If e- passes close to nucleus, strong electromagnetic

interaction – decelerates, and deflected• Radiates energy in all directions as X-ray photons,

up to a maximum equivalent to kVp = continuous spectrum

• High energy cut-off (≡ kVp) due to release of all energy in head on collision with heavy nucleus

• Low energy cut-off due to self-attenuation by target, X-ray window and additional filtration

• >80% of X-rays produced are Bremsstrahlung (except for mammography)

Bremsstrahlung

Characteristic X-rays

Characteristic X-rays• Interactions with tightly bound e- (typically K-shell)• If energy of e- exceeds binding energy (BE) of bound

state → ionisation• Vacancy leaves atom unstable• e- from higher state drops down (most often from L- or

M-shell), releasing X-ray photon (energy = difference in BE)

• Gives characteristic peaks on X-ray spectrum that are specific to the target material (BE Z2)

• For Tungsten target, Kα = 58 keV and Kβ = 68 keV– Not observed below 70 kVp

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

• kV waveform – three-phase or high frequency generators will have more high energy photons than single phase. Hence, output and effective energy are higher

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

ty60 kVp80 kVp120 kVp

The X-ray spectrum

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

Quality

• Describes the penetrating power of the X-ray beam, and is governed by the kilo-voltage (kVp)

• Usually described by the Half-Value Thickness– i.e. the thickness (in mm) of Al required to half the

beam intensity for a given kVp

• Typically >2.5 mm Al for general radiography• Changing the quality of the beam will change the

contrast between different types of tissue.• A highly penetrating beam is referred to as

‘Hard’ and a poorly penetrating beam as ‘Soft’

Intensity

• Intensity - is the quantity of energy flow onto a given area over a given time; the ‘brightness’ of an x-ray beam

• The tube current (mA) is a measure of X-ray beam intensity

• Intensity is directly proportional to mA.– i.e. Double the mA, double the dose (quality not

affected)

• Intensity is also affected by kVp