HSC Physics – Medical Physics notes – course summary

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HSC Physics – Medical Physics notes

This is a set of HSC Physics dot-point summary notes for Medical Physics. HSC Physics tutoring at Dux College provides students with the right support to achieve a band 6 result in HSC Physics.

Option Module: Medical Physics


Ultrasound is very high frequency sound waves above the hearing range (i.e. greater than 2000Hz). It can be refracted, scattered and superimposed like sound waves and light waves and these properties make it useful for medical diagnosis. It is reflected of parts of the body, detected and analysed to produce an image. These images are structural rather than functional and won’t show biochemical processes like other methods such as PET scans and radioisotopes.

In order to produce a clear image we must be able to distinguish between different body parts, some of which will be extremely close together. Images will be clearer and more detailed the higher the frequency of the sound is, i.e. the smaller the wavelength. Hence wavelength of frequencies between 3 MHz and 50 MHz is used. If the separation or size of objects is less than the wavelength then they will not detected.

However as frequency is increased and images become clearer the absorption of the sound wave increases and hence penetration is compromised. Hence the frequency must be suitable for the part of the body being imaged. 50MHz sound is used for skin and other superficial argons while 3MHz is used for the heart, liver and other abdominal organs.

An ultrasound transducer produces ultrasound of a specific frequency and the same transducer is capable of detecting the reflected sound. In order to produce sound waves of such high frequency it utilises a piezoelectric effect is the conversion of electrical to mechanical energy, resulting in the changing of the shape off a piezoelectric crystal when a potential difference is applied across it and vice versa. By applying a high frequency AC voltage across the piezoelectric crystal we can cause it to expand and contract rapidly and hence produce ultrasound frequency sound waves. The frequency of the sound can be controlled by changing the frequency of the AC voltage.

When the ultrasound returns after being reflected and hits the piezoelectric crystal there is a change in pressure on the surface of the crystal. This will use the crystal to be manipulated in the same way as when the sound was produced. However, in this case the changing shape will cause the production of a potential difference in the external circuit. This produced voltage that can be analysed by the computer to create an ultrasound image. The voltage produced will be greater when the amplitude of the returning wave is greater and hence the transducer provides information about the intensity of the returning wave.

Medical Physics - ultrasound is produced by vibrating a piezoelectric crystal

The backing block is used to dampen the vibrations. If it wasn’t there then the crystal would continue to vibrate even after the potential difference has removed and hence wouldn’t be ready to receive the reflected signal accurately.

A pulse of ultrasound is directed into the body and echoes from tissue boundaries are detected y the transducer. Some of the energy of the pulses is reflected to penetrate the body further.

Acoustic Impedance is a measure of how ready sound will pass through a material It is measured by:

Latex formula

Where Z = acoustic impedance (kgm-2s-1)

p = density (km-3)

v = velocity of sound (ms-1)

Different body tissues have different acoustic impedances. The amount of ultrasound that is reflected off a boundary between two substances depends on the acoustic impedance of each substance i.e. the difference in their acoustic impedances. It is measured by:

Latex formula

Where Latex formula = reflected intensity

Latex formula = initial intensity

Latex formula = acoustic impedance

From this equation we can see that when there is a large difference between acoustic impedance of two tissues a high portion of the ultrasound will be reflected. The very large difference in acoustic impedance between air and skin would cause most of the ultrasound to be reflected and not enter the body of a gel with the same acoustic impedance as skin wasn’t used to bridge the gap.

Ultrasound machines send ultrasound waves through the body and as these waves pass through different tissue layers each boundary gives off a partial reflection. By measuring, the time delay between each reflected sound wave and its intensity a picture inside the human body can be constructed.

Three different kinds of scans can be constructed:

  • A-scans are range-measuring systems that measure the time taken for ultrasound to be reflected off tissue boundaries and return to the transducer. The intensity of the returning ultrasound is plotted on a graph as a function of time. Hence a one-dimension image is created which is seen or a CRO. Being only one-dimensional, A-scans are limited in their use. They are used when ultrasound is directed at a target to determine location, such as to determine the midline position to see if it has been compromised due to a tumour. However, this use has been outdate by better brain imaging methods. However, A-scans are still used for the diagnosis of eye disease as no interior image of the eye is sometimes needed and only distances must be known. They are also used for treatments such as to break kidney stones
  • B-scans are created by moving transducer so that the body is viewed from a range of angles. The intensities of the returning waves are represented as spots of varying brightness. Hence a two-dimensional cross-sectional image of the body can be created as each pulse at each angle creates a one-dimensional image that can be added together to create a two-dimensional image.
  • Sector scans are images in the shape of an arc made by a series of B scans produced by an array of transducers very close together in one head. Sector scans are useful in imaging through small spaces such as the space between the bones of an infant skull where other ultrasounds would be minimally penetrating due to the large difference in acoustic impedance created by the skull. They are also real-time scans and hence are useful to monitor foetal or heart movement.

Detecting Bone Density

The main purpose for checking bone density in patients is to diagnose osteoporosis, which is a disorder that reduces bone density. Other methods of checking density, such as x-rays are not used as they are only efficient when bone density has fallen by 30% and diagnosis must be done before this. Ultrasound measurement is more effective and is readily available.

A person must place their foot into a water bath in order to reduce the difference in acoustic impedance and ensure that most of the ultrasound is not reflected. A transducer will then send an ultrasound into the foot, targeting the knee area. When the ultrasound is received two factors will be checked: the speed of the ultrasound through the bone and the amount of attenuation (absorption). Once these figures are known they can be plotted against a known graph and the bone density can be measured. The healthier and denser a bone is the greater the attenuation will be.

This method is cheap and readily available in mobile units and pharmacies. It is also non-invasive (no surgery) and hence there is no risk of infection. Unlike alternatives such as x-rays, no damaging side effects are known. It is also more effective in producing results for osteoporosis than most other methods (excluding Dual Energy X-Ray Absorptiometry, which patients who have poor ultrasound density readings will undergo).

However it is not possible by this method to measures sites of fractures that are common for people with osteoporosis such as hip or spine. It also cannot pick up fractures even in the heel.

The Doppler Effect is the apparent change in frequency of a wave due to relative motion between the source and the observer. It is used in ultrasound to measure blood flow or the movement in the body.

An ultrasound pulse is directed into the body and some of the ultrasound I reflected off blood cells in the blood. If the blood cells are moving towards the transducer then they will experience the ultrasound at a higher frequency than was emitted from the transducer and hence ‘Doppler-shift’ will occur. However, when the blood cells reflect these sound waves they will act as a source of ultrasound that is moving relative to the transducer, i.e. they will also be moving into the waves they reflect. Hence ‘Doppler-shift’ will occur again. Therefore the transducer will pick up a reflected intensity much greater than the emitted intensity and by calculating the Doppler-shift the speed of the blood can be calculated.

Similarly blood cells moving away from the transducer will cause the reflected frequency to be less than the transmitted frequency. The amount of Doppler-shift also depends on the angle at which the ultrasound strikes the blood and hence this must be considered when calculating blood flow. Pulsed beams of ultrasound rather than a continuous stream are used to reduce scattering from tissues encountered when blood flow deep in the body is being tested.

High blood flow can indicate such as high blood pressure and blockage or narrowing of arteries while a low blood flow can indicate leaks around faulty heart valves.

Advantages of Ultrasound:

  • No damaging side effects
  • Non-invasive
  • More effective than x-rays at imaging soft tissues
  • Inexpensive and portable equipment
  • Real-time is possible

Disadvantages of Ultrasound:

  • Inadequate where bone/tissue or air/tissue boundaries exist such as head, stomach and lungs.
  • Images are not as clear as those obtained by other techniques.
  • Balance between depth and resolution.


X-Rays are extremely high frequency electromagnetic waves with a very short wavelength in the range of 0.001- 10 nm. They are often used in medical imaging to diagnose patients. But because of their high frequency, and hence energy, they may be absorbed by molecules in the body and cause them to lose their electrons. This is why it is considered harmful ionising radiations. The free radicals it creates may alter the base structure and sequences in DNA and chromosomes, hence causing mutations.

X rays are produced by colliding accelerated electrons on a tungsten block

The X-Ray Tube is highly evacuated and an extremely high potential difference of between 25kv – 250kv is applied.

The cathode is a filament that is heated by circuit and hence emits electrons through thermionic emission. These electrons are accelerated to extremely high velocities towards the anode due to the large voltage. The anode is a metal target usually made of tungsten metal. When electrons strike this tungsten target there are two mechanisms by which x-rays are produced:

  • The electrons hit the tungsten atoms and are slowed down due to the repulsion from the tungsten’s electron cloud. Hence their kinetic energy is converted into x-rays, the frequency of which depends on the amount that the electron was slowed. As the electrons are travelling at different speeds and are slowed different amounts a range of frequencies is emitted. This radiation is called ‘Bremsstrahlung (braking) radiation’.
  • The second method is due to the ionisation of the target atom so the frequency of the x-rays depends on the type of atom used. As the accelerated electrons hit the tungsten target they may ionise it by knocking out one of its inner shell electrons. In this an outer shell electron will jump down an energy shell to take its place and hence in doing so it will release energy in the form of EMR, the frequency of which depends on the energy difference between the two shells. In a tungsten atom the difference in energy between shells means that x-rays will be produced. This is known as the ‘k-shell’ method of producing radiation.

By placing the tungsten target at an angle to the incoming electron beam, the x-rays can be sent off at a predetermined angle. The production of x-rays is not very efficient as only 1% of the energy input is converted into xo-rays with 99% being turned into heat. Therefore temperatures at the anode can get extremely hot. This is why tungsten is used as it has a melting point of 3400oC. It is also mounted on a copper anode mounting as copper is a good conductor of heat and helps dissipate the heat. Coolant is also circulated through the anode mounting to help carry some of the heat away. The target is also rotated at 3600 RPM to distribute the heat over a greater area.

X-Ray images are shadows of objects placed in the beam. The image is clearest when the x-rays are in a narrow beam which can be achieved by the angle of the tungsten target. X-rays can be recorded on photographic film when a record is needed or projected onto a screen using an image intensifier.

As mentioned above, x-rays cause between 0.001 and 10 nm. Hard x-rays are those that are at the high energy (low wavelength) part of this spectrum while soft x-rays are those that are at the low energy (longer wavelength) part of this spectrum. Hard x-rays are preferred in imaging as they are more penetrating and are more easily absorbed by tissue and hence create a sharper image whereas soft x-rays are not as penetrating and simply expose the patient to useless and dangerous excess radiation while not contributing much towards the image. Hence a thin sheet of aluminium is placed in the path of the x-rays and absorbs any soft x-rays, hence creating a hard beam.

X-rays images are formed due to x-ray attenuation, which is the amount an x-ray is absorbed and its intensity is reduced as it passes through material. The extent of attenuation depends on the density of protons. Hard tissues such as bone which have a high atom density tend to attenuate x-rays more while soft tissues which are less dense attenuate less. Therefore x-rays are particularly useful for measuring imaging skeleton. Bones will only let limited and small amount of x-ray pass and hence will show up white on photographic film. This also makes x-rays useful for imaging tooth decay and wisdom teeth.

Soft tissues can be imaged to a limited extent by introducing a contrast medium that all absorb the x-rays. To image the circulatory system iodine is introduced which will absorb the x-rays and to image the gastrointestinal tract the patient drink a solution of barium sulphate so that the barium can absorb x-rays. However, there are much better and more accurate methods of imaging soft tissue.

Computerised axial tomography uses x-rays to obtain a more detailed and cross-sectional. It measures very slight differences in x-ray attenuation unlike x-ray imaging and so has the additional advantage of imaging soft-tissue rather than only skeletal components.

In a CAT scan a patient is placed in a tube which is a donut shape and contains a detection system. It is all mounted on a frame called a gantry. At any moment during the scan, a part of the patient’s body will be positioned in a gap in the gantry. Hence a cross-section of the part of the body will be produced. Therefore to gain a complete image of the body the patient is moved slowly through the gantry so that ‘slices’ can be taken at regular intervals and added together to form a complete scan of the body.

At each interval a separate image is created by beams which rotates by 1o until an 180o angle around the patient’s body has been swept out. It gathers the data at each degree of movement and adds them together to form the cross-sectional ‘slice’. These slices can then be added together to create a 3D image of the body.

In order to create these slices a very narrow x-ray beam is required. To create these narrow slices very high voltages are used in the creation of x-rays. Once this have passed through the body they are picked by electrons on the other side of the tube and are converted into electrical signals to be analysed by computer.

The reason that CAT scans can differentiate between soft tissues is because x-ray attenuation is measured from different angles and hence by adding these figures together attenuation at each point in the body can be calculated. This allows us to determine what kind of tissue is present at each point and hence differentiates the different kinds of soft tissue.

CAT scans are a superior diagnostic tool to ultrasound and conventional x-rays however they are also more expensive. The high voltages required in the production of x-ray beams cause high level of heat to be produced and in addition the requirement to move the tubes during scanning means that the tubes often need to be replaced and hence this causes expense.

CAT scans are more useful in imaging areas such as the brain that are shielded by dense material. Ultrasound cannot be used as most of the sound energy wouldn’t be able to penetrate the skull and normal x-ray imaging would show an image of skull but not the brain as most of the x-ray would be attenuated before it would reach the brain. However by taking x-rays at many angles in the CAT scan small differences in different density brains tissue even if most of the x-ray is attenuated by the skull and hence it is more useful for this purpose.

CAT scans also tend to provide greater detail and resolution and hence are preferred for areas such as the lungs. X-rays have lesser detail and resolution than CAT scans and also have trouble imaging areas such as the lungs due to the rib cage and other attenuating parts nearby. Ultrasound cannot image lungs due to the presence of air which causes it to be reflected.

CAT scans are more harmful to the patient as their dose of electromagnetic radiation is much higher than x-rays. Therefore, techniques such as ultrasound and x-ray may be used first before prescribing a CAT scan.


Total internal reflection occurs when a light ray meets a boundary at an angle of incidence greater or equal to the critical angle required for the light to be refracted away from the normal so much that it is trapped in the medium. For this to happen the refractive index of the medium carrying the incident beam must be greater than the refractive index of the medium into which light attempts to escape into.

Endoscopes used in medical imaging

An optical fibre is a glass core surrounded by a cladding of lower refractive index. Light is trapped inside optical fibres and through total internal reflection and hence can be transferred over distances, even if there are bends in the optical fibre.

At their most basic endoscopes are simply bundles of up to 10,000 optical fibres. Endoscopes are instruments that use light to view inside the body. Each optical fibre provides one small point of light (pixel) and hence a bundle of them is needed to create a 2D image. Each optical fibre is between 5-25 µm.

The optic fibre bundles can be constructed into two different ways. A coherent bundle is where the optic fibres keep the same position relative to each other at each end. These bundles are used to carry light from the inside of the body as the pixels of light will exit the bundle in the same order that they entered, hence allowing the inside of the body to be imaged accurately. The fibres in these bundles tend to be narrower as a better quality image is produced with better resolution. However, these bundles tend to be more expensive to make. The other type of bundle used in a non-coherent bundle in which the fibres are not kept in the same position relative to one another. Therefore, this is used to carry light to open area being examined as it is not necessary for the light to exit the fibre in the same order with which it entered and it is less expensive to produce. Unlike coherent bundles, it is advantageous to have thicker fibres as they are more efficient at carrying light without loss.

Endoscopes also have the ability to take small samples called biopsy through the use of small biopsy forceps, scissors and other folding instruments. Sometimes lasers are used to carry out keyhole surgery as they can act without causing bleeding or distorting the area around the endoscope. An air/water nozzle is also present as it allows cleaning and washing if needed inside the body.

Endoscopes rely on coherent bundles of fibre optics

The endoscope is inserted through a natural orifice in the body or through a small incision. This allows doctors to see inside the body using keyhole surgery rather than open surgery which reduces risks such as infection for the patient and is less invasive. It also allows patients to recover quicker and reduces the cost of healthcare as open surgery can be expensive.

A knee orthoscopy use endoscope and keyhole surgery to view inside the knee joint and carry out repairs. Two small 4mm incisions are made on each side of the knee, one for endoscopes and one for surgical instruments. Fluid is pumped into the area to enlarge it and allow the surgeon to operate.

Radioactivity in Diagnosis

Isotopes of an element are varying number of neutrons in different atoms of the same element, i.e. with the same number of protons. A radioactive isotope or radioisotope is an isotope that is radioactive and has an unstable nucleus and will emit radiation from its nucleus in order to make it stable.

There are three main types of radiation:

  • Alpha particles which are small helium nuclei. Its large mass means that if cannot penetrate more than a few millimetres inside the body and its charge of +2 means that it is extremely ionising and dangerous. Hence it is not used for medical diagnosis.
  • Beta particles are electrons released when a neutron in the nucleus decays to form a proton and a fast moving electron. This usually occurs in nuclei with excess neutrons. Though it is more penetrating than alpha particles and has only a -1 charge, it is still not suitable in medical imaging.
  • Gamma rays are very high energy electromagnetic waves and emitted alongside other forms of radiation such as alpha and beta. They have a much greater penetrating power than both alpha and beta and as they are EM waves they have no charge, making them much less dangerous than ionising radiation. However, their high energy can still ionise atoms inside the body and so they are still dangerous to some extent. Hence they are used for medical diagnosis.

A radioisotope’s half-life is the time taken for half of the amount of the radioactive substance to decay and turn into something else. Half-life differs for all radioisotopes.

Radioisotopes with a long half-life are not used in medical diagnoses as it remains within the parent long after diagnosis. Imaging hence leads to a large amount of radiation exposure for patients and those around them, causing potential harm. Similarly, isotopes with a short half-life are not used as the radioisotope mostly decays before the images for diagnosis can be formed. It is also difficult to transport such isotopes from where they are made in nuclear reactors and particle accelerator to where they are to be used. Radioisotopes used for medical imaging have a half-life between a few minutes and a few days.

The radioisotope that is used in medical imaging depends on what element or compound is taken up by the target organ. A radioisotope f that element or a compound labelled with that isotope can then be administered to the patient, either through an injection, inhaling it or in a liquid and it will build up in the target organ. When a compound is labelled with a radioisotope it is called a radiopharmaceutical. The body is unable to differentiate between radioactive and non-radioactive forms of an element and so the radiopharmaceutical will undergo the normal chemical process that occurs.

For example, the thyroid gland metabolises iodine and so the patient is made to drink a dilute solution of sodium iodide labelled with iodine-123 which is a gamma emitting radioisotope and its accumulation is measured from 10 minutes since it is administered.

Technetium-99m is a very common isotope used in radioactive diagnoses and imaging due to its conveniently short half-life of 6 hours which means that it eaves the patient quite quickly and it is easily attached onto molecules used in biochemical reactions. When imaging the bones Technetium-99m is attached to blood serum and used as a tracer to measure brain blood flow, allowing stroke of dementia damage to be identified.

Once the radiopharmaceutical is inside the body it will begin to accumulate in the target organs and emit gamma rays which can be detected outside the body by a gamma camera. The images from the gamma camera can be taken after several hours when the radioisotope has accumulated or else multiple images over several hours to show the distribution and the rate of absorption.

The gamma rays emitted from the body travel towards the gamma camera. They first must pass through a lead collimator which is a slab of lead with many holes perpendicular to the face. This ensures that only gamma rays travelling at the right angles to the sodium iodide crystal behind the collimator are the only ones to be used in imaging as they produce the highest quality image. When the gamma rays hit the sodium iodide crystal light is produced in a quick flash. Photomultiplier tubes above the sodium iodide detect these flashes of light and turn them into an electric signal which a computer then constructs into an image.

When analysing images radiologists look for ‘hot spots’ where there is an excess of biochemical activity or ‘cold spots’ where there isn’t enough biochemical activity as these are usually the area where abnormalities occur.

To image brain function the patient may inhale small quantities of CO which accumulate quickly in the brain and is labelled with Carbon-II. Oxygen is used to label water and study blood flow in the brain.

β-D-glucose is a chemical that is used exclusively by the brain and is like its ‘fuel’. Then it can be labelled to study brain function for certain diseases such as Parkinson’s disease and schizophrenia. β-D-glucose is labelled with positron emitting fluorine-18 to produce 2-fluoro-2-deoxy-D-glucose (FDG) and hence can be used to monitor brain function in a PET scan.

Advantages of radioisotopes:

  • Can image areas that other techniques such as x–ray and ultrasound cannot
  • It provides functional information of biochemical processes in the body rather than simply structural images from other techniques
  • Lower radiation dose generally than x-rays and much less than CAT scans as only a very small quantity of radiopharmaceutical is administered

Disadvantages of radioisotopes:

  • Resolution is much poorer than other methods of imaging
  • Slightly invasive as radioisotope must be injected, often disliked by patients
  • Costs much more than ultrasound or x-ray (similar cost to MRI)
  • Can only be carried out in some hospitals that are in very close proximity to a nuclear reactor or cyclotron which are extremely expensive.

Radioisotopes are often used in conjunction with MRI or CAT scans to show where functional and chemical activity is occurring relative to structural features.

Magnetic Resonance Imaging (MRI)

Our bodies are made of a range of compounds. One of the most common elements in these compounds is hydrogen which makes up 1/10 of the mass of our bodies, 70% of which is found in water molecules and 20% in fats. Hence it is the nuclei most commonly used in MRI.

All nuclei used in MRI must have net spin which is a quantum property of nuclei. If a nuclei has net spin it may behave as a small magnet. Most hydrogen atoms consist of only one proton in the nucleus with a single orbiting electron and hence have a net spin, meaning that they have an associated magnetic field and this is why they are used in MRI. Other nuclei apart from hydrogen-1 such as fluorine-19 and phosphorous-31 also have net spin and can be used in MRI but they are not as abundant.

Hydrogen atoms have nuclear spin

Usually the magnetic field associated with hydrogen nuclei are not aligned and randomly oriented. However, by applying a strong external magnetic field we can align these hydrogen nuclei so that those in a low energy state point parallel to the direction of the external magnetic field where those in a higher energy state point anti-parallel to the external field.

Precession of hydrogen spin can have different states

The magnetic field in the gantry is extremely strong, up to 5T (more than 10,000 times the Earth’s field). This magnetic field is usually produced using superconductors. The huge costs of MRI are for this reason. Huge amounts of energy are required to cool helium into its liquid form in order to keep the superconductor below its critical temperature. Helium is also costly as there is a limited supply of it on Earth. Installing the magnet is also costly as it weighs 6 tonnes and hence is difficult to transport and rooms must be supported before it is placed in them. The magnetic field produced also extends for outside the room as it is so strong and hence magnetic shielding is also required.

When the hydrogen nuclei align due to the external magnetic field created by superconductors they don’t stay in a steady positron and point in the direction of the magnetic field (or against it). Precession is the movement in a conical path of the axis of a spinning object. The frequency with which the nuclei precess around the direction of the magnetic field is known as the Larmor frequency. It differs for different nuclei in the same magnetic field and is directly proportional to the strength of the magnetic field.

Once the patient is inside the gantry and the protons are precessing around the magnetic field lines we beam a pulse of radiowaves into the patient. These radio waves are produced using radiofrequency coils (RF coils) and they are produced so that their frequency corresponds with the Larmor frequency of the precessing hydrogen nuclei. The nuclei resonate with the radiowaves i.e. absorb energy when it is applied at a frequency that matches the natural frequency of an object; hence moving into a higher energy state and begin to precess in that phase with each other.

When the hydrogen nuclei absorb the radiowaves and begin to precess in phase with each other. They move into a higher energy state which means that their precessional orbit tilts and no longer is around the direction of magnetic field lines. However when the radio wave pulse is switched off the protons cease to precess in phase and their precessional bit returns to being around the direction of magnetic field lines. This means that they have moved back to their lower energy state and in turn have released the energy that they originally gained in the form of radio waves. These radio waves are picked up by receiver RF coils ad sent as electrical signals to a computer which converts them into an image to display on a screen.

When the pulse is switched off we can analyse the returning radio waves so that a ‘slice’ through the patient’s body is determined. The ability to distinguish between ‘slices’ of the patient’s body is achieved by changing he strength of the strong magnetic field generated by the superconductor slightly and uniformly along the length of the patient’s body through the use of gradient coils. Hence the Larmor frequency changes along the body and RF transmitter coils can change the frequency of the radio pulses to match the Larmor frequency at a particular ‘slice’ of the body.

The gradient magnetic field is generated by a coil which has a single uniform current going through I but varies in how tightly the coil is wound, hence generating a different strength magnetic field for each slice.

MRI uses an external magnetic field to align hydrogen atom spins

In order to determine exactly where the radio pulse is coming from within each ‘slice’, two or more gradient magnetic fields are applied at perpendicular angles to the slices. One of these two fields changes the frequency of precessing protons in the slice while one makes the protons in the slice slightly out of phase with one another. This allows each slice to be divided into 256 x 256 voxels. The responding radio waves rom protons in each voxel will be different and hence we can determine exactly where the radio signal came from.

The radio signal returned is influenced by many factors:

  • The greater the density of protons the larger the signal and the brighter the image. Air and outer bone have no hydrogen nuclei and hence appear dark in MRI images.
  • The type of tissue determines how easily protons can relax and release their energy and hence the type of tissue influences the intensity of the signal. Hence by examining the time taken to ‘relax’ by each nuclei the type of tissue can be intensified.
  • The rate at which radio frequency pulses are switched on and off. Different tissues take different times to relax and by changing the frequency of radio pulses (on/off) we can emphasise different aspects of the image.

During T1 weighted imaging the radio pulses are turned on and off extremely quickly. Hence only substances such as fat, liver and spleen with short T1 values (i.e. they release energy quickly) are able to send radio pulses to be detected. On the other hand water is dark as its hydrogen nuclei release energy slower (hence a longer T1). This provides good contrast between the two and hence is used for body structure and soft tissue detail. In this method protons release their energy to the surrounding molecules and hence these are called spin-lattice interactions. T1 weighted MRI is good at differentiating between grey and white tissue in the brain which have hydrogen bonded differently and hence different T1 times.

In T2 weighted images we are measuring the time for protons to go out of phase with each other by exchanging energy with each other rather than with the other molecules. T2 is short for solids and larger molecules found in areas such as tendons, muscles and liver but is long for watery tissues. In T2 weighted images we turn the radiofrequency beam on and off slowly so that watery tissues with long T2 can relax. This is hence used to image diseased tissues such as cancer and tumours as these are areas of high biochemical activity due to their fast growth and hence form watery tissues.

If a great enough contrast between tissues cannot be gained using T1 and T2 weighted images then a contrast medium can be introduced to the blood stream. These have the potential to make the signal stronger from certain tissues and areas and hence allow better imaging.

Impact of Medical Physics on Society

Ultrasound has allowed doctors to gain information about patients without the need for harmful radiation or any level of invasiveness. Hence it has allowed diagnosis to become safer and risk free which has increased safety and health in society. It is also good for patients as they are made more comfortable with the prospect of having ionising radiation. It has especially helped in analysing foetal development and blood flow rate, things that were very difficult without the use of harmful radiation, hence reducing problems associated with pregnancy and blood pressure. Procedures such as MRI and CAT scans have allowed the construction of 3D images of the body with extreme accuracy. This has allowed doctors to see within patients without the need for invasive surgery and the early detection of life threatening diseases such as cancer.

MRI and PET scans have made viewing biochemical processes possible, something that couldn’t be done before. This has allowed research into diseases such as schizophrenia and Parkinson’s disease which are abnormalities in brain function rather than physical deformation; hence increasing our knowledge of medicine and allowing us to produce medicine and pharmaceuticals to counter these diseases.

The use of many technologies in medical physics has increased research into their uses and properties and allowed them to be used elsewhere. Increased research into radioisotopes for PET scans has allowed them to be used elsewhere such as sodium-24 in plumbing, cobalt-60 for food production and americium-241 in smoke alarms. The development of superconductors for MRIs has led to their use in maglev trains and the CERN supercollider.

Overall the advancement of medical physics has increased the practicality and safety of medicine. It has increased life expectancy in society. The only drawback has been harmful radiation inevitably causing cellular damage in some patients.

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