A2 Unit 24 Medical Physics

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What happens when a piezo-electric crystal has its shape changed?

Generates an e.m.f.

What happens when a piezo-electric crystal has a p.d. applied across it?

It changes shape

How is a piezo-electric crystal used to generate and detect ultrasound waves?

In a piezo-electric transducer, an alternating p.d. is applied to produce ultrasound waves and sent into the patient’s body. The returning ultrasound waves induce a p.d. in the transducer for analysis by a healthcare professional.

Define an ultrasound

A high frequency sound above the range of human hearing. Above 20kHz.

What happens when an ultrasound meets a boundary?

Some of the wave is reflected back to the transducer

What can the reflection of an ultrasound tell us?

The depth - due to the time delay between transmitting the signal and receiving the reflection.

The nature of the tissue - due to the intensity of the wave reflected.

Define acoustic impedance

The product of the speed of the ultrasound in the medium and the density of the medium

Give the equation to calculate acoustic impedance

Z = ρc

Where c is the speed of the ultrasound in the medium

What happens to the wave if the two mediums have a large difference in acoustic impedance?

Large reflection

Why do we need to use a coupling gel when taking ultrasounds?

Air and skin have a large difference in acoustic impedance so a large portion of the wave would be reflected back and no image would be seen. A coupling gel has a similar acoustic impedance to the skin so the wave is transmitted.

What is the intensity reflection coefficient?

The ratio of the intensity of the reflected wave relative to the incident (transmitted) wave

What is attenuation of ultrasound?

The reduction of energy due to the absorption of ultrasound as it travels through a material

What is the equation for attenuation of ultrasounds?

I = I₀e^-μx

How are x-rays produced?

A metal plate is bombarded with accelerated electrons. These electrons collide with electrons in the atom and cause them to be ejected from the atom. Outer shell electrons move down the energy shells to replace the lost electrons, releasing an x-ray photon.

Give the equation to calculate the maximum energy an x-ray photon can have

E_max = eV

Where V is the potential difference accelerating the electrons

Why is a continuous spectrum of wavelengths produced?

The electrons hitting the metal plate have a distribution of accelerations. Therefore a continuous spectrum is observed.

Why is there a sharp cut-off at shorter wavelengths produced?

Electrons have a maximum energy which corresponds to a minimum wavelength

Define contrast in x-ray imaging

The difference in degree of blackening between structures

How can contrast of an image be improved?

Using the correct level of x-ray hardness: hard x-rays for bones, soft x-rays for tissue. Using a contrast media.

Define sharpness in x-ray imaging

How well-defined the edges of structures are

How can sharpness of an image be improved?

Using a narrower x-ray beam. Reducing x-ray scattering by using a collimator or lead grid. Smaller pixel size.

Give the equation for attenuation of x-rays

I = I₀e^-μx

When will an x-ray image have good contrast?

When there is a large difference in intensities of wave from the different mediums

How does a CT scan work?

An x-ray tube rotates around the stationary patient. A CT scanner takes x-ray images of the same slice, at many different angles. This process is repeated, then images of successive slices are combined. A computer pieces the images together to build a 3D image. This 3D image can be rotated and viewed from different angles.

Give 3 advantages of a CT scan

Produces much more detailed images. Can distinguish between tissues with similar attenuation coefficients. Produces a 3D image of the body by combining the images at each direction.

Give 2 disadvantages of a CT scan

The patient receives a much higher dose than a normal x-ray. Possible side effects from the contrast media.

Define a radioactive tracer

A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body

Why do radioactive tracers usually have a short half-life?

So the patient is not exposed to the radiation for a significant amount of time

What is a PET scan?

A type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues

What type of radioactive tracer is used in PET scans?

A tracer that decays by β+ emitting a positron

What happens to emitted positrons in PET scans?

They travel less than a mm before annihilating with an electron releasing a pair of photons

What is annihilation?

When a particle meets its equivalent antiparticle they are both destroyed and their mass is converted into energy

How does a PET scan work?

Positrons are emitted by the decay of the tracer. They travel a small distance and annihilate when they interact with electrons in the tissue. This annihilation produces a pair of gamma-ray photons which travel in opposite directions. These are detected. An image can be formed by processing the arrival time of the gamma-ray photons.

What 2 parts does a PET scanner contain and what does each part do?

Crystal scintillator - detects the gamma photons by absorbing them.

Photomultiplier - amplifies the signal.