Medical Physics

What provides the energy of the X-rays in an X-ray tube?

The kinetic energy of the accelerated electrons.

What is the typical p.d. of an X-ray tube for medical imaging?

30-100kV

What causes the emission of electrons from the cathode in an X-ray tube?

It is heated to produce thermionic emission.

What is an essential property for the target metal in an X-ray tube?

High melting point

What causes the production of X-rays in an X-ray tube?

The deceleration of electrons when they hit the target metal/anode.

In an X-ray tube what happens to most of the kinetic energy lost by the electrons when they hit the anode?

It is transferred to thermal energy of the anode.

What measures can be taken to prevent an anode melting in an X-ray tube?

It is cooled by oil or rotated

What determines the maximum frequency of X-rays produced in an X-ray tube?

The maximum kinetic energy of a single electron

What happens to the position of the X-ray tube during a CAT scan?

It rotates around the patient

What are the key advantages of CAT scans over conventional X-rays?

They produce a 3D image and can distinguish between tissues with similar attenuation coefficients

What are the disadvantages of CAT scans compared to conventional X-rays?

The radiation dose is greater and they are more expensive and take more time

How does a CAT scan produce a 3D image?

A series of slices are imaged and processed via a computer

What is X-ray attenuation?

The decrease in intensity as X-rays pass through matter

What are the four mechanisms of X-ray attenuation?

Simple scatter, Photoelectric Effect, Compton Scattering, Pair Production

What occurs during simple scatter of X-rays?

A low-energy X-ray photon encounters an electron in an atom causing it to be scattered (change direction) without a change in energy

Which method of attenuation is most significant with X-rays with energy in the range 1-20keV?

Simple scatter

What occurs during the photoelectric effect attenuation of X-rays?

An X-ray photon is absorbed by an inner shell electron causing it to be ejected from the atom as a photoelectron

Which method of attenuation is significant with X-rays with energies up to 100keV?

Photoelectric effect

Which method of attenuation is most significant to medical X-ray imaging?

The photoelectric effect

What occurs during Compton scattering attenuation of X-rays?

An X-ray photon is deflected by an interaction with an orbital electron causing the wavelength of the photon to increase and the ejection of the electron from the atom at a high speed

Which method of attenuation is significant with X-rays with energy in the range 0.5-5MeV?

Compton scattering

What occurs during pair production with X-rays?

A high energy X-ray photon interacts with the nucleus of an atom causing the production of an electron-positron pair

Which method of attenuation occurs with X-rays with energies 1.02MeV?

Pair production

What is a contrast medium in X-rays?

A material with a high attenuation coefficient used to image soft tissue

Why are high energy X-rays prefereable to gamma radiation for cancer therapy?

An X-ray source can be turned off

Which radionuclide is used for imaging with a gamma camera?

Technitium-99m

Which radionuclide is used in PET scanning?

Fluorine-18

Why are gamma emitting sources useful for imaging inside the body?

They are the least ionising and can penetrate through a patient to be detected externally

Why are short half-lives necessary for the radioisotopes used for medical imaging?

To ensure a large activity from a small amount and to minimise radiation dose after the procedure

What is a medical tracer?

A compound containing a radioisotope that is put into the patient’s body

What is the function of a collimator in a gamma camera?

To absorb any photons not travelling along the axis of the tubes so the source location of gamma radiation can be idenitifed

What is the function of a scintillator in a gamma camera?

It produces many photons of visible light when a gamma photon interacts with it

What is the function of the photomultiplier tubes in a gamma camera?

They convert visible light photons to an electrical pulse

What type of radiation is detected in a PET scan?

Gamma

What type of radiation is emitted by Fluorine-18 nucleus in a PET scan?

Beta plus/positron

What process produces gamma photons in a PET scan?

Electron and positron annihilation

How is the the source of the gamma photons located in a PET scan?

By the time delay between the arrival of diametrically opposite gamma photons

Why is medical imaging using gamma emitting tracers useful?

It can measure the function of organs in the body rather than just the structure

What are the disadvantages of a PET scan?

They are expensive and require facilities to prepare tracers local to the scanner

What is the minimum frequency of ultrasound?

20 kHz

What is the piezoelectric effect?

When a crystal produces an EMF when it is distorted

What happens to a piezoelectric crystal when p.d. is applied across opposite faces?

It is compressed or stretched

What are the main advantages of ultrasound imaging?

It is non-invasive and does not require ionising radiation

What range of frequencies are typically used for ultrasound scans?

1-15MHz

How is an ultrasound pulse generated by a transducer?

A high frequency p.d. is applied at the natural frequency of the piezoelectric crystal causing the crystal to resonate

How is an ultrasound pulse detected by a transducer?

Sound incident on the crystal causes it to vibrate generating an EMF across its faces

What is the purpose of the damping material in an ultrasound scanner?

It prevents reflection of the pulse from behind the piezoelectric crystal

What is an ultrasound A-scan?

A single transducer produces pulses that travel along a single path through the patient (allows you to measure the length of body parts e.g. eye) which are reflected back at the boundaries between surfaces

What is an ultrasound B scan?

The signal from an array of transducers is processed by computer to build up a 2D image

What determines the brightness of the dots on a B scan display?

The amplitude of the reflected ultrasound from that point

What determines the acoustic impedence of a substance?

The speed of sound in the substance and its density

What factor determines the reflection intensity coefficient for ultrasound at a boundary?

The difference between the acoustic impedances of the two substances.

Why does a coupling gel need to be used when performing an ultrasound?

Air pockets between the transducer and skin cause reflection of nearly all of the ultrasound at the skin-air boundary.

What is an ultrasound coupling gel?

A substance applied to the skin and transducer with an impedance similar to skin.

What is impedance/acoustic matching?

When two substances havce similar acoustic impedances so negligible reflection of ultrasound occurs.

What is doppler ultrasound used for?

Measuring the speed (and volume) of blood flow.

What is measured in doppler ultrasound to determine the velocity of the blood?

Change in observed frequency of the reflected ultrasound

Why does the transducer have to placed away from the normal during a doppler ultrasound?

To ensure blood is moving towards (and away from) the ultrasound pulse