Medical Physics

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X-Rays

- thermionic emission

- high potential difference between the filament and the positive anode. producing an electric field.

- electrons are attracted + accelerated towards the anode

- on collision they decelerate, producing X-Rays, through them losing kinetic energy - which becomes an x-ray photon

- called the Bremsstrahlung (Braking) Radiation

- the system is evacuated so that electrons do not collide with air molecules

- it is cooled by water, so that it doesn't overheat and melt, this is because water has a high heat capacity, it is easy to obtain and it is cheap

Problems:

- 99% of the electrons energy becomes internal heat energy so only 1% is in the form of X-Rays

- to overcome this a rod of copper surrounds the target and is cooled by oil/water

- modern designs use rotating anodes

- this allows the anode to cool when it is not being hit by electrons, stopping it from overheating and allowing the heat energy to dissipate

- this is better than water, as water is not portable enough

X-Ray emission spectra:

- x-rays produce a continuous background by electrons colliding with the target and decelerating

- the most energetic X-Rays, with a high freq. and small wavelength, lose all of their energy at once

- when electrons lose less than their total energy this is due to X-Rays with a longer wavelength

- the line spectra is due to collisions within the atom levels of the metal

- these have a lot of energy and so are capable of knocking out electrons from deeper energy levels of the atom, so an outer electron may 'fall' into the 'vacant' area. they ionise the atom

- this results in a high energy quantum of EM radiation

Benefits:

- unobtrusive

- do not scatter significantly

- luggage monitoring

Limitations:

- they are ionising - high energy x-rays causes damage to cells

- low frequency X-Rays are easily absorbed by the body and must be filtered

Producing an image:

- X-ray images are negative

- lighter parts mean material has absorbed more radiation - denser areas e.g. bone

- darker parts mean little absorption - less dense areas e.g. skin

- lower energy x-rays are harmful so a filter is used to remove them

Taking an X-Ray of the stomach:

- usually give poor results

- so you can use a contrast medium

- a 'barium meal' will be ingested and show the stomach against the surrounding tissue

- the meal is more dense than the surrounding tissue and so shows up lighter

- Higher absorption coefficient

CT Scanners:

Advantages:

- 3D images

- more detailed

Disadvantages:

- exposed to more radiation

- more ionisation

- more expensive

Attenuation coefficient:

- a measure of the ability of a substance to reduce the intensity of radiation by absoption/diffusion

- heavier materials have a higher attenuation coefficient (μ)

- half value thickness can be more useful as this is the thickness required to half the intensity. this is equal to ln2 / μ

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Ultrasound

- it is a high frequency longitudinal sound wave

- between 1-5MHz (humans hear up to 20kHZ)

uses:

- cleaning teeth

- cleaning medical equipment

- scans

- breaking kidney stones

Making waves:

- an ultrasound transducer transforms electrical energy into high frequency longitudinal waves

- when a voltage is applied to a crystal it changes shape

when an alternating voltage is applied, the crystal shape will alternate at the same frequency as the voltage

- if the crystal size is chosen carefully so that the voltage frequency matches its natural frequency it will resonate

- this means that the amplitude of vibration is bigger and waves are produced in the medium that is contact with the crystal

Producing an image:

- the image is produced by waves reflected from parts of the body that are made of different tissue

- the darker parts mean that little/no reflection has taken place

- the lighter parts mean that more reflection has taken place

- the larger differences between the 'acoustic impendences' the more energy is reflected

- gel is placed between the patient and the transmitter so that no air stops the signal passing through

- it depends on transmission and reflection

- shorter wavelengths produce better detail

- but these have higher frequencies which are absorbed more easily and so less useful for deep investigations

Measuring blood flow:

- doppler ultrasound can be used from the change in frequency of an ultrasound wave when it is reflected by a moving blood cell

- the frequency shift is caused by the echo from a moving reflector and not a moving source/receiver

- volume of flow rate in an artery can be calculated from Av where A is the cross sectional area of the artery.

X-Rays:

- highly penetrating, produce images by transmission

- ionising radiation so risk damage to healthy cells

- limitations on how often it can be used for examining a patient

- short wavelength producs more detail than ultrasound

- permanent images obtained using photographic plates

- careful precautions necessary to protect the operator

- can be used for imaging in security screening

Ultrasound:

- images produced by reflection

- non ionising so can be used for lengthy investigations

- can be used to provide real time images of organ functions

- can be readily moved to image different parts of the body

- can monitor blood flow rates

- no danger to the operator

- readily absorbed so obtaining images may be difficult with obese patients

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MRI

Precession:

- protons have charge and they spin so they behave like tiny magnets

- when they are put in a strong magnetic field they protons in the nuclei of hydrogen atoms line up in the direction of the magnetic field

- when the magnetic field is removed each one precesses around the direction of the Earth's magnetic field

- lamor frequency is the frequency of precession and this is proportional to the Earth's magnetic flux (at that point)

- they are detectable as they induce an emf

How they work:

- depends on the density of hydrogen nuclei in various parts of the body

- a strong magnetic field is applied to the patient

- protons get aligned in this strong magnetic field

- and they precess at the lamor frequency

- variable magnetic/gradient fields are applied to the patient in all 3 dimension

- this coupled with the original magnetic field changes the size + direction of the magnetic field on each proton

- as the lamor frequency is proportional to the magnetic field, protons in different places have different lamor frequencies

- and so there is a patient full of protons precessing at different lamor frequencies

Getting an image:

- a radio signal is transmitted throughout the body

- protons that have a lamor frequency equal to the rf resonate and get more excited

- the radio frequency is stopped which causes the protons to radiate energy

- the energy is detected

- a new frequency is applied and the process repeats

- computers analyse the data and produce an image from the layers of images

Advantages:

- can look at regions that are normally blocked

- only radio waves used: not ionising

- doctors have control over which areas can be investigates and which plane to 'look' at

Disadvantages:

- very expensive: over £1m

- scans take a long time, have a long waiting list

- claustrophobia and noise can be a problem

- patients containing metal can distort the magnetic field

- diameter of the core is limited so plus size people may not be able to get one

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Endoscopy

Optical Fibres:

- these use a process called total internal reflection to transmit information

- past a certain angel you get TIR and this is when all the light inside the block is reflected

- at the critical angle this happens

- the information is normally in the form of light or infrared waves

- these are part of the EM spectrum

- they carry a lot of information e.g. in digital transmissions

- light is more secure as it is harder to hack

- higher frequencies carry more information

- the image formed by the endoscope can be viewed using a magnifying eyepiece or a CCD camera

Advantages for medicine:

- endocopy

- HD image

- true colour

- real time moving image

- can be magnified

- allows surgeons to carry out internal operations without major surgery

Channels:

- air/water (cleaning + drying)

- controls

- vacuum

- illumination channel (for torch light by TIR)

- image channel

Coherent and incoherent:

- in coherent the picture would stay the same as the arrangements of fibres are the same throughout

- in incoherent the fibres are randomly arranged and so picture would be muddled and so these are used for illumination, but they are cheaper

- coherent fibres eventually become incoherent.

Problems:

- multipath dispersion: creates blurry images

-> can use thinner fibres to fix

- fibres get misaligned

- light leakage

-> coating each fibre with dark glass can reduce this effect

- light intensity is reduced

-> can be improved electronically

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CCDs:

Advantages:

- more sensitive to light than film

- image can be built up over a long time - long exposure

- signals are electrical and can be enhanced

- moving images can be produced

Workings:

- Metal-Oxide-Silicon semi-conductor capacitors (MOS)

- each photoside is a photodiode which has the ability to store the charge that is generated

- photons of light fall onto MOS capacitors called photo sites

- each photo site is one element/pixel

Detailed:

- photons falling on the photo site liberate electrons from atoms

- brighter light means more photons which means more liberated electrons

- the variation in liberated electrons between photo sites gives the final image

- the detail depends on number of pixels/photo sites

Quantum efficiency:

- eye uses about 1% of received photons

- film uses 4%

- CCDs use 70%

- each photo site must have the same efficiency or the intensity of the image will not be accurately recorded

Transferring the image information to a screen:

1- a clock pulse shifts the charge in each row downwards so that the charge on each photo site is transferred to the photo site below it

2- the bottom row is fed into a shift register where the charge is measured and another clock pulse empties the row by shifting each charge horizontally

3- the the output of the shift register is a series of pulses that represent the charge level in each photo site

4- when it has been emptied, a clock pulse moves each row downwards again and the process is repeated until all the sites have been processed

5- the system can identify which charge level in the sereial stream of information goes with which pixel on the screen and the image can be built up

Coloured Images:

- the final colour depends on the relative intensity of each of the primary colours

- expensive cameras use three CCDs (RGB)

- normal cameras use one CCD array

- Red, blue and green filters are used over the array.