Aspects of Medical Imaging and Technology

What does traditional MRI image?

proton content

What are specialised, newer MRI machines image?

noble gases

What is an analogue measurement?

When a quantity is continuous

What is a digital measurement?

When a quantity is discrete

What does ADC stand for?

Analogue-Digital Converter

What is the role of the transducer?

To convert the physical interaction into an analogue mA or mV signal that can be passed to the ADC

What is the shape of pretty much every transducer response curve?

What is noise?

about randomness - for the same input do you get the same output, how reproducible is it?

What is saturation?

The point at which a change in input solicits no change in output, often associated with overdriving the input

What is linearity?

The extent to which a doubling of the input provides a doubling of the output

What is the best place of the transducer response curve to operate the transducer?

The central, most linear part

What is dynamic range?

The range over which the response of the transducer is dynamic

What is dynamic range limited by?

Limited by ADC min/max input

What is clipping?

When the signal exceeds the min/max limit of the ADC

What is resolution?

how fine-grained the response is

What is the resolution of an analogue system limited by?

Noise

How do you maximise resolution (bit depth)?

Large num of samples

Steps as small as possible

What is the risk for medical imaging if the bit depth is too small?

Doctors cannot distinguish differences in image (e.g. differences in tissue density) for diagnosis

What is the digital acquisition chain?

Analogue interaction

Transducer(converts to mA or mV)

ADC (converts to binary)
Computer

What is the purpose of the latch?

The latch sits between the ADC and the computer and holds the most recent digital value

It helps to control the rate at which data is fed to the computer so it has adequate time to process it

What is the Nyquist sampling limit?

Nyquist sampling limit says that you must sample at twice the frequency of interest

What is the full flow chart of imaging from acquisition to image? (6)

What is an image?

A distribution of points of different intensities

What does the curve shifting principle do?

Moves the delta point away from the origin to any arbitrary position, in Dirac-Delta function

What is a line intensity function?

A quantitative description of the intensity distribution, provides a mechanism for describing the images as a general mathematical function

What is conjugacy?

The extent to which the object point is accurately reproduced in image space - imaging systems need good conjugacy to be effective

What does PSF stand for and what is it?

Point Spread Function = the function that represents the point object in image space

Denotes how much the point object is spread in image space by the imaging system

How do you derive the convolution?

• Energy of f at any point is defined by f(x)dx

• Image:  δ at each fdx is replaced by g 

• Sum up the energy (probability) at point z whilst shifting g

What is the convolution integral?

What are the three key properties of the convolution integral?

Commutativity

Associativity

Distributivity

What does commutativity mean for the convolution integral?

f*g=g*f

What does associativity mean for the convolution integral?

f*(g*h)=(f*g)*h

What does distributivity  mean for the convolution integral?

f*(g+h)=(f*g)+(f*h)

What is the equation for the Fourier series and what are the coefficients?

What is the equation for complex fourier coefficients?

What is the equation for a_n?

What is the equation for b_n?

What is the equation for the Fourier transform?

What does a phasor represent?

Amplitude and phase

What is the phasor for this general complex exponential?

What is the definition of attenuation of an EM wave?

The decreasing photon flux resulting from interaction processes as light travels through a medium

What does the notation A represent on the periodic table?

A is the mass number

What does the notation Z represent on the periodic table?

Z is the atomic number, proton number

What is the order of magnitude of the atomic radius?

10^-10m

What is the order of magnitude for the nuclear radius?

10^-15m

What is the relationship between charge and strength of interaction?

The stronger the charge, the stronger the interaction

What is the relationship between mass and strength of interaction?

The larger the mass, the stronger the interaction

What is the relationship between speed and strength of interaction?

The slower the speed, the stronger the interaction

What is the linear energy transfer coefficient?

The energy deposited per unit distance travelled

What are the characteristics of x-ray radiation? (7)

• Electromagnetic radiation

• Emitted as a result of charge reconfiguration

• A photon

• >1keV

• No charge

• No mass

• Not very interacting

What are the characteristics of gamma ray production?(2)

• Many nuclear transitions do not deexcite directly to the lowest energy state

• Gamma rays are an effective way of dealing with the remaining energy

Compare Maxwell, X-rays, and Gamma

• Maxwell 

  • An accelerating charge radiates (X-rays of many MeV is possible)

• X-rays

  • Atomic in origin (up to ~100keV)

• Gamma rays

  • Nuclear in origin (~8MeV)

What are the 3 electromagnetic interaction processes?

Absorption, scattering, and attenuation

What is absorption?

The transfer of a photon’s energy to surrounding material

What is scattering?

The random re-direction of a photon’s path

What is attenuation?

The decreasing photon flux resulting from interaction processes as EM waves travel through a medium

What is the attenuation coefficient of the absorber?

Probability of absorption per unit length, µ

What is mass attenuation coefficient represented by?

µ/ρ

What is Beer’s Law?

What does Beer’s Law show?

The dependence of transmitted flux on absorber thickness

How does flux, I, change with absorbing thickness x?

Changes exponentially, shown by Beer’s law

What is fluence?

The flow concentration per unit area

What is HVL?

Half Value thickness - it is 0.693/µ

What is reaction cross section?

The probability of an interaction interpreted as an effective area of interaction per particle

What is differential cross section?

A refined version of the reaction cross section to account for dependence on other parameters 

What is photoelectric absorption proportional to?

Z⁴/E³

What happens in photoelectric absorption?

Incoming photon interacts with an electron, resulting in disappearance of photon and ejection of electron

What are the three attenuation processes?

Photoelectric absorption, compton scattering, pair production

What happens in photoelectric absorption?

Resonant coupling between radiation and inner electron shell

Electron ejected with excess kinetic energy

What is photoelectric absorption used for?

Diagnostic imaging, produces contrast in an x-ray image

What is photoelectric absorption proportional to?

Z⁴/E³

What is RBE?

Relative biological effectiveness

What is the equation for relative biological effectiveness?

What is the equation for Dose equivalent? and what does Q represent?

Dose Equivalent = Absorbed dose x Q x modifying factor

where Q is the radiation weighting factor

What are the principles for an acceptable dose?

Minimise time of exposure

Maximise distance from source

Shielding and effective working protocols

What is an early effect?

Cell damage may lead to cell death

Sometimes compromise the genetic data sufficiently

→ subsequent and immediate generations are non viable

What is a late effect?

Cell imperfectly repairing itself with errors encoded into the DNA

Can lead to generations of cells (years) before the error visibly manifests itself into e.g. cancer

What are stochastic effects?

Probabilistic, no safe dose

What are deterministic effects?

Threshold, severity related to dose

In a GM tube, what are the four modes A-D?

A - low voltage region

B - ionisation chamber region

C - proportional counter region

D - Geiger counter region

What happens in the low voltage region of the graph of log of detected current against voltage(??)?

(Mode A)

Gamma ray ionises gas

Ions drift towards cathode/anode

Ions recombine before reaching anode/cathode

-

ionise, drift, recombine

What happens in the ionisation chamber region of the graph of log of detected current against voltage(??)?

(Mode B)

Gamma ray ionises gas

No time for ion recombination

Ions attracted to anode/cathode

What happens in the proportional counter region of the graph of log of detected current against voltage(??)?

(Mode C)

Gamma ray ionises gas

Ions accelerated to cathode/anode

Collisions along their path produces further ionisation

Single gamma ionisation produces a many-ion current pulse

Gas amplification factor up to 10⁶

-

ionises, accelerated, further ionisation, many-ion current pulse, amplification

What happens in the Geiger counter region of the graph of log of detected current against voltage(??)?

HT set to place tube just below the threshold of spontaneous discharge

Gamma ray ionises gas

Ions greatly accelerated to cathode/anode

Collisions produce ion avalanche

Gas amplification factor 10¹⁰

-

threshold, ionises, greatly accelerated, avalanche, amplification

Describe the process of Compton scattering (4)

Billiard ball type interaction (relativistic)

Between photon and free electron

Incident photon deflected with energy transfer to electron

Produces scatter/noise in diagnostic image

What is Compton scattering proportional to?

Z/E²

Describe pair production(2)

Photon in excess of 1.02MeV passes close to nucleus of an atom

Spontaneous conversion into electron positron pair

What is the process that happens in the GM tube? (3)

Gas in glass envelope, includes two electrodes

ionisation of gas by incoming photon generates ion pair, both are attached to electrodes responsible for generating an electric field

Result is pulse of charge down electrode wire that signifies an ionisation has occurred

What is the efficiency of the GM tube at few MeV?

<2%

What does TLD stand for?

Thermoluminescence Dosimetry

What is thermoluminescent dosimetry? (6)

Energetic structure of crystal includes energy traps

Ionisation causes some electrons to be caught by the traps

Has regular crystalline structure but when impurities are added imperfections arise in the lattice which act as energy traps

Following an interaction with radiation most excited electrons rapidly return to the ground state, but some remain trapped in the impurity levels

Upon heating, the trapped electrons are released and de-excite with an emission of light

These de-excitations occur in a well defined “glow curve”

What is the advantage of TLD?

Can measure accurately (2%) a dose over a very wide range (10^-5 to 10^3 Gray)

What is Kerma?

Kinetic Energy Released in the Material

dE/dm

What is absorbed dose?

Distributed energy transfer

What is the difference between KERMA and absorbed dose?

KERMA is at the point of interaction, absorbed dose is the effect

What does Bremsstrahlung mean?

Radiation that decelerates

What is LET?

Linear Energy Transfer coefficient

LET = dE/dm

What is a direct mechanism of damage?

Refers to an incident in which ionising radiation strikes the DNA directly

Why are direct mechanisms of damage unlikely?

DNA is a very small target

When do indirect mechanisms of damage occur?

When water molecules inside the cell become ionised and then can go onto cause damage to DNA