GCSE Physics P3- Medical Physics

X-Rays are elctromagnetic waves. They have a high frequenency and carry a lot of energy, making them a form of ionising radiation. 

X-Rays are absorbed by high density materials such as bone, so leave white areas on the photographic plate. As X-Rays can pass through soft tissue, the rest of the plate is a darker grey or back. 

Risks

- X-Rays are a form of ionising radiation. Ionising radiation can kill a cell completely or damage it so it can't divide, causing tissue damage. Radiation can also alter the DNA in a cell, causing mutations. This can cause the cell to divide uncontrollably, causing cancerous tumours. 

Advantages 

- Using X-rays to properly diagnose an injury is better than risking using the wrong treatment

- X-rays are non-invasive; you don't have to perform surgery to diagnose an injury or disease. This means that the process is quick and you don't have to recover from surgery or risk infection etc. 

- Hospitals limit the amount of ionising radiation you are exposed to by only using X-rays when necessary, and limiting the dosage and the time exposed. 

Medical personnel limit their and the patient's exposure to ionising radiation in a number of ways. This includes: 

- Medical staff standing a distance away from the source, normally in a remote control room. This is becauase the intensity of radiation decreases with distance. 

- Limiting exposure of staff and patients to the source - this might mean only prescribing a scan if the benefits outweigh the risks, and monitoring how much radiation personnel are recieving. There are laws in place to make sure staff do not recieve more than 5mSv/year averaged over five years. 

- Limiting the dosage of ionising radiation. Lower doses of X-rays and gamma rays can be used to reduce cell tissue damage. 

- Shielding from physical barriers. Gamma rays cannot penetrate thick lead, so personnel and patients may wear lead aprons or other shielding to stop the ionising radiation reaching their bodies. The remote control room which the radiograoher operates from is lead and steel lined. 

Computerised axial tomography (CAT) scans use X-rays to produce an image of two-dimensional slices through the body. 

In a CAT scan, an X-ray source is moved around the patient in a circle. X-ray detectors are positioned opposite the X-ray source, to capture the pattern of waves which have not been absorbed and translate these into an image. Sometimes a computer is used to build up a 3-D image. 

CAT scanners are often used to find cancerous tumours - CAT scanners show up the different shades of soft tissue a lot better than X-rays, so it is easier to see areas which are darker or lighter than usual. The image is also higher quality. 

Ultrasound waves are waves with a frequency higher than we can hear - over 20,000 Hz. Ultrasound waves are non-ionising so are a safer option for medical treatment. Here are two medical uses of ultrasound: 

Breaking down kidney stones

Ultrasound can firstly be used to break down kidney stones. These are hard masses which can block the urinary tract and are very painful. High energy ultrasound waves break down the stones into small enough particles to pass out in urine, preventing the need for surgery.

Measuring the speed of blood flow 

Ultrasound works in real-time, so you can track changes in the body as they occur. This makes it useful for looking at how blood flows through organs such as the heart and liver, where special ultrasound machines can measure the speed of blood flow and identify blockages in the veins and arteries. 

The major use of ultrasound is in pre-natal scans, which show how a foetus is developing.

When waves enter a material of a different density, waves are reflected and refracted at the boundary between the materials. Ultrasound machines use this principle. When the waves reach a different medium such as soft tissue, waves are reflected back and recorded by a computer. This produces an image on the screen. 

Endoscopes use visible light, a non-ionising EM wave, to produce an image of the inside of the body. 

An endoscope is a long tube containing two bundles of optical fibres, one to carry light and the other to carry the image back. The optical fibres are very thin so that the angle is greater than the critical angle, and total internal reflection occurs: 

The image can be seen through an eyepiece or displayed as a full-colour moving image on a TV screen. 

The major advantage of endoscopes is that operations can be performed with a very small incision (keyhole surgery), using the light and images from the endoscope. 

Flouroscopes use X-rays to create moving images of the inside of a patient.

Basic flouroscopes work by puttin the patient in between an X-ray source and a flourescent screen. Different amounts of X-rays are absorbed when they pass through a patients body, and a the intensity of the X-rays vary depending on what they have passed through. The X-rays hit a flourescent screen which absorbs them and flouresces (gives off light) to show a live image on the screen. The images can be recorded using a computer. 

Modern machines often use an image intensifier so lower dosages of X-rays can be used. 

Flouroscopy diagnoses problems in the way organs are functioning, by looking at movement of substances down the gut or by looking at bloodflow. 

A contrast dye is often used to improve the clarity of the image, as it makes soft tissue more visible. 

Electrocardiograms (ECGs) measure the action potential of the heart by using electrodes stuck onto the chest, arms and legs to record small electrical impulses. 

When the heart beats, an action potential passes throught the atria, making them contract. Very soon after, another action potential causes the ventricle to contract. Once the action potential has passed, the muscle relaxes. These action potentials are small electrical impulses on the skin. The results are displayed on a screen. 

ECGs are used to find abnormalities in the heart rate of an individual. They can diagnose conditions such as an abnormaly fast or slow heart rate. 

A typical ECG looks like this:         

P wave: impulses from the SAN (sino-atrial node) cause the atria to contract, and stimulate the AVN (atrio-ventricular node).

QRS wave: impulses from the AVN (atrio-ventricular node) cause the ventricles to contract. An odd shape is formed as the atria are also relaxing. 

T wave: the ventricles relax 

You can work out heart rate from an ECG using this formula:

frequency (hertz) =   1 / time period (seconds) 

In this ECG, the time from peak to peak is around 0.7 seconds. So the frequency is 1/0.7 = 1.43 Hz. 

Multiply the frequency by 60 converts this into a heart rate in beats per minute. 1.43 x 60 = 86 beats per minute 

Pacemakers regulate the heart beat. 

The heart has a natural pacemaker which controls the rate of contraction and relaxation. This is controlled by electrical signals. Sometimes these signals aren't fast enough, or they are irregular. 

Sometimes, an artificial pacemaker can be fitted. The device keeps the heart beating at a steady rate using small electrical signals to regulate the heart beat. These record and amplify the natural action potentials of the heart.  These electrical signals are sent via electrodes - thin wires are attached to the heart. 

Pacemakers are surgically inserted with a small incision. They do need replacing every so often, as they are battery-powered. Many modern pacemakers can now record and transmit information about to the heart to external sources, and the settings can be changed without surgery. Some pacemakers monitor breathing and temperature as well so they can adapt to different activities such as exercise. 

Pulse oximeters measure the amount of oxygen carried by haemoglobin in the blood. They are used for measuring a patients health before and after surgery.                                            

A pulse oximeter has a transmitter, which emits two beams of red light- visible red light and infrared. It also has a photodetector to measure light. These are placed either side of the finger, with the emmiters above and the detector below the finger. 

The beams of light pass through the tissue. On the way, some of the light is absorbed by the blood, reducing the amount of light reaching the detector. 

By comparing the absorbance between the infrared and visible red light the pulse oximeter works out how much oxygen the blood is carrying, and it displays this figure as a percentage on the display. By counting at the peaks of infrared absorbance as oxygenated blood pases, the p.oximenter can also work out pulse.

Radiation is any form of energy originating from a source, including both waves and particles. Radiation doesn't have to be ionising - light waves and sound waves are both forms of radiation. 

The intensity of radiation decreases the further way you get from a source. The medium it is travelling through will affect the intensity- the denser the medium, the more radiation will be absorbed and the intensity decreases. 

There is an equation for this:

Intensity = Power/Area 

The unit for intensity is Watts/metres squared.

The intensity is inversely proportional to the square of the distance from the source. This is known as the 'inverse sqaure law' - If the distance is doubled, the intensity is reduced by four times (two squared). 

Some radioactive isotopes are used as tracers to diagnose some medical conditions. The tracer is injected into the patient or swallowed. An external detector follows its progress as it moves around the body. A computer uses these readings to create an image showing where the strongest areas of radiation are. 

One of the radioactive isotopes used in medicine is iodine-131, which is absorbed by the thyroid gland. The amount of radiation it gives off indicates if the thyroid is functioning correctly. Another isotope commonly used is flourodeoxyglucose (FDG), which is taken up by the body and treated as glucose. As cancer tumours take in a lot of glucose to grow quickly, tumour cells glow brightly on a PET scan as the radiation from the isotope is all coming from one place. This helps the diagnosis of cancer. 

Radioactive isotopes emit beta or gamma, which can pass out the body without damaging too many internal cells. They have short half lives so they stop being dangerously radioactive soon after the scan. Isotopes for medicines are usually made in cyclotrons near the hospital for this reason. 

Positron-emission tomography (PET) scanners is a medical technique used to show tissue or organ function. 

PET scans show areas of damaged tissue in the heart by detecting areas which have a lower blood flow than usual. This is useful for diagnosing coronary artery disease, or damage caused by a heart attack. They can also look at how blood flows through the brain, diagnosing conditions such as alzheimers and epilepsy. 

PET scans can identify cancerous tumours by showing metabolic activity in tissue - cancer cells are growing quickly so take up a lot of glucose, which means glucose-based tracers such as FDG gather in the affected area and emit gamma radiation, which is detected by the computer screen. 

PET scanners work on the principle of annihilation. When a particle meets its antiparticle, the two particles annihilate and produce two gamma rays in oppsoite directions: 

This is true for electrons and positrons. When a positron-emmiting radio isotope is injected into a patient, the emmited positron collide with electrons in the organs, causing them to annihiliate and produce a pair of gamma rays travelling in opposite directions. There will be a higher uptake of the radio isotope in cancer cells, so more gamma radiation comes from these areas. Detectors around the body detect each pair of radiowaves. Three pairs of gamma radiation give an accurate loactation of the tumour by using triangluation. 

Momentum is always conserved in the annihilation. The particles have the same mass and opposite velocities, so the total momentum before the collision is zero. When the collision occurs, the two gamma rays have the same energy but are travelling at exactly opposite velocities, meaning their overall momentum is also zero. 

In any particle reaction, the charge before the reaction equals the charge after the reaction. The total charge before the positron-electron annihiliation is zero, as the charge on the positron of +1 is balanced by the -1 charge on the electron. Gamma rays have no charge so the charge of the reaction at the end is still zero. 

Lastly, mass energy is always conserved in a positron/electron annihilation. Einstein said that mass is a form of energy - mass can be converted to other forms of energy. Mass energy is conserved because all the mass of the electron and positron have been converted into energy. 

Energy (joules, J) = Mass (kilograms, kg) x Speed of light (3x10*8 m/s)*2 

Ionising radiation can be used internally and externally in the treatement of cancerous tumours. 

Internal radiation therapy 

With internal radiation therapy, a radioactive source is placed inside the body into or near a tumour. This can be done in many ways, but is normally completed by injecting or implanting a small amount of radioactive substance. 

Brachytherapy 

Brachytherapy refers to the treatment of prostate cancer. Small radioactive seeds are inserted into the prostate, and these release alpha radiation which kill the cancer cells. An alpha source is used because it has a very low penetrative ability, so shouldn't affect normal cells. The alpha source has a short half life so it decays to safe radioactive levels after the treatment is complete. 

Adv: loacalised killing of cells should mean healthy cells are unaffected 

External radiotheraphy uses external radiation to treat cancer cells. Gamma rays can kill cancer cells if the intensity is high enough: 

To minimize damage to healthy cells, many gamma rays of weak intensity are directed at the tumour from different angles. Where the gamma rays meet, the intensity is high enough to kill all the cells there. As shown in the image, protection such as lead clothing and helments prevent damage to healthy cells. 

Disadvantages: Damage to normal cells leads to many side affects, both short and long term. This includes sickness, loss of appetite, burns, bleeding gums, tiredness, bowel damage and even secondary cancer. It is a long and unpleasent process over weeks of treatment. 

Sometimes, radiotheraphy is used to reduce suffering of someone affected by cancer. Palliative care will not cure the cancer but can shrink it or stop growth to alleviate symptoms. 

Some people will opt to not have palliative care because of the risk of side affects - radiotheraphy can seriously affect quality of life if you are in pain as a result of the treatment. Many will prefer to live whilst they can in relative health with friends and family than prolong their lives unhappily.