P3 Topic 2 - X-rays And ECGs Flashcards
How can you calculate the kinetic energy of an electron as it accelerates towards something?
Kinetic Energy = Electronic Charge x Accelerating Potential Difference
Joules = Coulombs x Volts
How would you calculate the size of the current produced from the beam of electrons?
Current = No. of particles per second x Charge on each particle Amps = No./seconds x Coulombs
What is the charge on one electron?
-1.6 x10^-19 coulombs
How does the intensity of X-ray radiation depend on the distance from the source?
The further away it is, the lower the intensity.
I = P/distance^2 I = P/area
How does thickness affect the intensity of X-ray radiation?
The thicker and denser a material is, the more it absorbs X-rays. For example, lead and concrete are used to shield people from X-rays in hospitals.
How does fluoroscopy work with X-rays to produce moving images of the insides of a patient?
- The patient is placed in-between an X-ray source and a fluorescent screen. Different amounts of X-rays are absorbed by different types of tissue in the body, so different intensities will come out of the body.
- The fluorescent screen absorbs the x-rays and gives off light to show a live image of the patient. The higher the intensity of the x-ray, the brighter the screen.
- Modern methods of fluoroscopy intensify the image so that lower doses of x-rays can be used.
- The screen is attached to a computer so that they can be recorded.
- Fluoroscopy is used to diagnose problems with the way in which organs function.
How do you produce X-rays through thermionic emission?
- Inside a glass tube, a circuit with a filament, which acts as a cathode, is heated until it the electrons have enough energy to escape. This is known as thermionic emission.
- The electrons then accelerate towards a metal target, which acts as an anode.
- When the electrons collide with the metal, some of the kinetic energy is converted into X-rays.
- There is a vacuum inside the glass tube so that the electrons do not collide with anything and get knocked off target.
- A lead casing is placed around the glass tube so that X-rays that are not aimed at the target, e.g. an arm, get absorbed.
How do CAT scans work with X-rays to produce 2D slices of a patient?
Computerised Axial Tomography (CAT)
- An x-ray beam rotates around the body and is detected by thousands of detectors on the other side.
- A computer works out how many x-rays are absorbed and produces a high quality image.
- CAT scans can also be used to make 3D images by stacking individual slices together.
- They are often used to find tumours and cancer.
What is an electrocardiograph (ECG) and how does it work?
An electrocardiograph records the weak electrical signals of the heart using electrodes. The results are displayed on a screen or printed out on a graph called an electrocardiogram. It is used to look at the action of the heart.
How can you work out the heart rate from an ECG?
Frequency = 1/Time Period Hz = 1/Seconds
What is a pacemaker and how does it work?
The heart has natural pacemakers which produce electrical signals that pulse around 70 times a minute. They cause the heart to contract and pump blood around the body. Occasionally, these natural pacemakers aren’t fast enough or have an irregular pace.
People with these problems can be fitted with an artificial pacemaker that stimulates these electrical impulses in the heart. They use a battery to power them and eventually run out of charge. Many can be programmed externally and even charged with out the need of surgery.
What are pulse oximeters and how do they work?
Pulse oximeters shine red light and infrared light through a patient’s finger to measure the oxygen concentration in the blood. The other side has a photo detector to measure the amount of light that has passed through the finger. The amount that gets through depends on the colour of the blood. Oxygen rich blood is bright red, while blood low in oxygen is usually more purple. The normal ratio of oxyhaemoglobin and reduced haemoglobin is 19:1. Readings are then shown on a screen as a percentage.
