Medical physics

What function does a piezoelectric crystal provide, how does it do so?

Used in production and detection of ultrasound waves as part of a piezo-electric transducer

• when a p.d. is applied across it, it changes shape

• when its shape changes, it generates an e.m.f.

Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures

• pulses of ultrasound are produced by piezo-electric crystals when they are deformed

• waves are reflected at boundaries between media

• reflected pulses are detected by ultrasound generator

• the time delay between transmission and receipt gives information about depth

• intensity of reflected pulse gives information about nature of the boundary

• gel is used to minimise reflection at the skin

• the degree of reflection depends on impedances of two media at the boundary

What is the specific acoustic impedance of a medium?

Z = ρc

p density of medium

c speed of sound in the medium

Z specific acoustic impedance

What is the intensity reflection coefficient. Give the equation to find it.

• the proportion of incident ultrasound signal that is reflected back

I reflected / I incident = (Z1 – Z2)² /(Z1 + Z2)²

If Z is the same, the wave is not reflected

If there is a greater difference between Z1 and Z2, more of the wave is reflected back

What is a coupling medium and why is it used in ultrasound

The intensity reflection coefficient between air and skin is very high. A coupling medium (e.g. gel) must be used between transducer and the body so ultrasound is not mostly reflected before entering the body.

• removes the air between the transducer and body

• usually oil or gel

• minimises the difference in acoustic impedances

What is attenuation.?

When ultrasound waves are absorbed and scattered, which decreases their amplitude.

What is the linear attenuation coefficient

• a measure of how easily an ultrasound wave can pass through a given material

• describes the rate of energy loss per unit thickness

The larger the coefficient, the quicker the intensity of the ultrasound will decrease as the wave passes through a medium

What equation gives the intensity of an ultrasound after passing a distance through a medium?

What is thermionic emission?

When is it used?

• a metal is heated until free electrons on the surface gain enough energy to be emitted

• used in the production of X-Rays

Describe the process of production of X-Rays

• electrons emitted from a filament by thermionic emission and are accelerated through a potential difference towards anode (metal target)

• once they collide, they decelerate and emit part of their energy as EM radiation

• this is in the form of X-ray photons - braking radiation. Forms a continuous spectrum of X-ray radiation

• some electrons collide with orbital electrons of the target atoms and ionise the atoms, causing electrons to de-excite, releasing X-ray photons (energy)

• this energy depends on the difference between energy levels of the metal’s atoms so will depend on the material

“Outline the principles of computed tomography (CT) scanning”. [5]

• X-rays are used

• object is scanned in sections

• scans taken at many angles, images of each section are 2-dimensional

• scans of many sections are combined

• to give 3-dimensional image of whole structure

“By reference to an ultrasound wave, explain what is meant by specific acoustic impedance'“ [2]

Product of density and speed

Speed of ultrasound in medium

“State what is meant by the attenuation of an ultrasound wave” [1]

Loss of intensity/amplitude/power of the wave

How to calculate the minimum wavelength of X-rays produced from the accelerating p.d.?

Product of charge of an electron and accelerating voltage = max energy because this is the value of the KE as the electrons hit the target.

Describe two methods of controlling beam intensity of X-ray

• increasing accelerating voltage. Electrons gain more KE, so photons will have higher energies

• increasing current passing through filament emitting electrons. More electrons released per second, therefore more X-ray photons produced per second

What is the meaning of contrast in X-ray scanning

• difference in degree of blackening between adjacent structures

• allows tissues to be differentiated

• impacted by how much of the X-ray is absorbed - linear attenuation coefficient is an indicator of how much will be absorbed

What is the equation to find the intensity of X-rays after they have passed through a medium?

In X-ray imaging, what is sharpness vs contrast?

Sharpness - ease with which edges can be distinguished

Contrast - difference in degrees of blackening

Define the intensity of an X-ray beam

The total energy emitted per second per unit area passing through a surface perpendicular

What are some advantages of a CT scanner

• non-invasive

• high quality image

  • full cross-sectional area

What are some disadvantages of a CT scanner

• patient exposed to large dose of ionising radiation

• expensive

• contrast between materials of similar densities is small

What is a tracer

A substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied

Decays by Beta-plus decay

Used in positron emission tomography

What is annihilation. When does it occur

• when a particle interacts with its anti-particle

• minimum energy of each photon emitted is equal to rest energy of electron/positron

• mass-energy and momentum are conserved

• energy of electron and positron is shared equally between gamma ray photons

Describe PET (Positron emission tomography) scanning

• used to form 3D and cross-sectional images of body

• patient injected with tracer

• patient left to allow the radionuclide to move to region of interest

• radionuclide will be absorbed and broken down, releasing positrons which collide with electrons present in the tissue and are annihilated

• this releases two gamma rays moving in opposite directions, recorded by detectors

• signals sent to computer

• an image of the radioactivity in that region is formed by processing the arrival times of the gamma-ray photons