Thermionic emission can be used to produce X-rays.
- Thermionic emission involves heating a filament (cathode) to give more energy to electrons. This enables the electrons to boil off.
- A potential difference accelerates the electrons towards the metal anode.
- When they collide, some of their kinetic energy is converted into X-rays which are released through a window in the casing.
- A vacuum is needed to prevent the electrons colliding with air particles.
- A lead casing is needed to absorb some X-rays so that they're only aimed at a certain point.
Calculating the kinetic energy of an electron:
- Kinetic energy (J) = electronic charge (C) x accelerating potential difference (V)
KE (1/2 x m x v2) = e x v
Calculating the size of current produced:
- Current (A) = number of particles per second (1/s) x charge on each particle (C)
I = N x q
X-Ray Intensity and Absorption
- X-rays are ionising enough to be able to damage or destroy body cells.
- X-ray intensity follows an inverse square relationship.
- i.e. when you double the distance from the source, the strength decreases to a quarter.
- Generally, the more dense the material is, the more radiation it absorbs.
- A thicker material will absorb more X-rays.
- Lead or concrete can be used to limit exposure to X-rays
Computerised axial tomography (CAT) scans
- An X-ray source is moved around a patient in a circle. X-ray detectors positioned opposite the source pick up the rays.
- A computer can produce many cross sectional, high quality images of the body by working out how many X-rays are absorbed.
- CAT scans can also produce 3-D images by stacking up the individual slices.
- Unusual darker or brighter areas in images can indicate tumours.
- A patient is placed between an X-ray source and a fluorescent screen.
- Different amounts of X-rays are absorbed as they pass through the body.
- X-rays hit the screen which absorbs them and fluoresces to show a live image.
- The moving image can show whether an organ is working or not.
Risks and Benefits of X-Ray Imaging
- X-rays are ionising so they can damage or destroy unhealthy cells
- Gives a very high dosage that can increase the risk of cancer.
- A painless, quick and non-invasive procedure.
- Produced a better quality image than safer methods such as ultrasound.
- It is better to diagnose an injury with X-ray imaging than to risk using the wrong treatment.
- Risk of damaging other tissues is reduced by firing X-rays at different directions so that only the tumour gets a high dosage.
- Action potentials produce weak electrical signals on the skin.
- An ECG records the a.p. of the heart using electrodes
- An action potential passes through the atria, making them contract. (P)
- Another a.p. passes through the ventricles, making them contract too. The atria relaxes. (QRS)
- The ventricles relax once the a.p. has passed. (T)
Frequency (hertz) = 1 / time period (seconds)
Pulse Oximetry and Pacemakers
- A pulse oximeter has a transmitter which emits red and infrared light. A photo detector measures light.
- Beams of light pass through the finger tissue and some is absorbed by the blood.
- Oxyhaemoglobin absorbs more infrared than reduced haemoglobin.
- Looking at peaks of infrared absorption can be used to calculate a pulse.
- Pulse oximetry can also be used to work out how much oxygen the blood is carrying.
- A pacemaker detects action potential and applies electrical signals to other parts of the heart so that the heart chambers contract correctly.