Nuclear Magnetic resonance (NMR)

Nuclear Magnetic Resonance

NMR works by getting hydrogen nuclei in the body to emit radio waves. Analysis of this radiation enables a very detailed image to be built.

Magnetic nuclei

• Nuclei are charged and spin. This means nuclei with an odd number of protons or neutrons behave like mini magnets.

Magnetic nuclei

• This means in an applied magnetic field the nuclei will line up like compass needles but either with (low energy) or against (high energy) the field

Magnetic nuclei

• The stronger the applied field, the bigger the difference between the two energy states

Resonating

• Just like the compass needle in a field, the nuclei can oscillate

Resonating• If a hydrogen nuclei is placed in a strong

magnetic field and is “pushed” by radio waves of the correct frequency (60 MHz) it will resonate

Resonating• The resonating hydrogen nuclei will emit

radio waves as it resonates, which can be detected and used to build up an image.

Magnetic resonance imaging (MRI)

• The radio waves emitted by the resonating nuclei are detected by a coil of wire and analysed by a computer. There are useful aspects of the radiation that can be used to build the image.

MRI - Frequency

• The frequency of the resonance depends on the strength of the field (stronger field = higher frequency) If the field is varied from one place to another, the position of the source can be found.

MRI - Frequency

• The frequency of the resonance depends on the strength of the field (stronger field = higher frequency) If the field is varied from one place to another, the position of the source can be found.

Radio transmitter

Radio receiver

MRI – Relaxation time

• The time taken for the oscillation to die away is different for different tissue. This can be added to the image.

Use of lasers in medicine

Light from a normal bulb

• Spreads out on all directions and is normally many wavelengths (and phases)

Light from a normal bulb

• When focussed by a lens not all light hits the lens

Light from a normal bulb• The light is not parallel so it is not all

focused on the same point – a diffuse spot is formed

Laser light

• The light from a laser IS parallel and consists of the same wavelength (and in phase). This is called coherent light.

Laser light

• When focussed by a lens, laser light can produce a very intense point of light

Laser light

• Energy from beam can be focused on a very small piece of matter, causing it to be vaporised.

Cloud of vaporised dog

Advantages of cutting with a laser?

• Finer cut

• Vaporising action seals small blood vessels causing less bleeding

Used with optical fibres

• The laser light can be fed along optical fibres enabling surgeon to performs operations inside the body without large incisions.

Laser angioplasty

• An optical fibre is fed along a blood vessel from the arm to the heart where it is used to cut away unwanted material.

Measuring oxygen content of blood

Pulse oxiometer

• Laser light will pass through thin body parts like a finger of baby’s foot

Pulse oximeter

• Two lasers are used, one giving out red light and the other infra red light.

Pulse oximeter

• Red light is absorbed most by blood with no oxygen and IR most by blood with oxygen.

Pulse oximeter

• By measuring the relative absorption of the two wavelengths it is possible to calculate the amount of oxygen in the blood (as well as monitoring the pulse.