Diagnosis in Physics

How the Heart Works

The heart produces action potentials to make the heart beat. The impulses begins at the sino-atrial node and moves across the atria to the atrio-ventricular node. This then renews the impulse, contracting again and sending the electrical signal down the septum wall and then up through the apex of the heart via the purkinje fibres.

How an ECG is Carried Out

This impulse is then transmitted throughout the body, as it contains dissolved salts and water hence it can conduct electricity to the skin surface. Pads are placed on the surface of the skin where they detect the action potentials, hence monitoring the heartbeat. This helps to show irregularities of the heart.

The PQRST  of an ECG shows the heartbeat.

P wave= Activation of the atria.

QRS complex = Activation of the ventricles.

T wave= Recovery wave.

Pulse Oximetry

• Pulse oximetry measures pulse rate and saturation of blood.

A pulse oximeter consists of two LED's which is placed on the end of a finger. One emits red light and the other infrared radiation. Each heartbeat causes a surge of blood in the arteries.

Oxygenated blood absorbs more infrared radiation than deoxgenated blood. This then can work out the saturation of the blood by seeing how much infrared is absorbed. The pulse is calculated by looking at the peaks of infrared absorbance, the machine working out the pulse.

PET scan - Positron Emission Tomography scan.

PET scans produce an internal image of the body. This is done by injecting a tracer into the patient which is an radioactive isotope that has a short half-life so it will decay close to where it is used. The tracer is the radioactive isotope binded to compounds like glucose. Depending on the rate of division of the cells dictates how much glucose is absorbed. Hence areas with higher radiation from the absobed tracer suggests a tumour or rapid healing. The radiation is then detected by the PET scanner.

The PET scanner detects the positrons relased by the radioactive isotope undergoing beta + decay. This is done by the annihilation of matter and anti-matter. The positron is released and collides with electrons surrounding nuclei. This produces gamma radiation which is released at opposite angles to which the matter and anti-mater have collided. The PET scanner detects this and creates an internal image.

X-Rays

Ionising Radiation - the turning of atoms into ions by removing electrons from their outer shell.

An x-ray machine consists of an evacuated tube that contains two electrodes. The cathode is a wire filament wihch when heated become an electron gun (beam of electrons) through a process called thermoinic emission (the emission of an electron from a heated surface). The anode is postive and attracts the negative electrons. The large potential difference between the anode and cathode accelerates the electrons to the metal anode. The electrons collide with the metal anode and slow down very quickley. Here the kinetic energy of the electron is transferred into x-rays and thermal energy.

• The tube is evacuated so the electrons do not collide with other particles in the air as they accelerate towards the anode.
• A higher potential difference will produce x-rays with greater energy.
• The higher the frequency of an x-ray the more energy they have.
• If the temperature of the cathode is increased the number of electrons emitted increases, increasing the number of x-rays produced.
• The acceleration of electrons to the anode completes the circuit.

X-rays

• The denser the material the more x-rays it absorbs.
• Bone is denser than issues in organs so it absorbs more x-rays.
• The dark areas on an x-ray is where the x-rays has been detected as the tissue was not dense enough to absorb a lot of it. Hence bone shows up as white as it absorbs a lot of x-rays so they cant be detected.

CAT Scans

In a CAT scan the source of gamma radiation is moved around the patient in a circle and detectors are postitioned opposite the source. The multiple x-rays are used to build up a 3D image of the body, thorugh computer generation. CAT scans can be used in the diagnosis of tumours and areas where blood supply has been obstructed as it shows up unusual dark and light areas.

Fluroscopes

• This is used to show a real time image of the internal workings of the organs.
• It consisits of an x-ray source and detector attached to a digital video camera. The patient is placed between the source and detector.

Endoscopes

• Endoscopes use light radiation to create a real time image of the inner workings of the organs.

Optical fibres are glass fibres that transmits information through total internal reflection. TIR occurs when light is approaching the medium at an angle greater than the critical it will reflect back into the medium. In this case, when light enters the optical fibres at an angle greater than the critical TIR occurs, however the light is constantly being reflected along the fibres.

Light enters the endoscope and is reflected down the optical fibres to the site of the problem, where then end of the endoscope is. The light then reflects off of the tissue and re-enters the endoscope travelling back up to the eye-piece through TIR where it is focused and forms an image so the patient diagnosed.

Ultrasounds

Ultrasounds are not a form of radiation for medical diagnosis and treatment. Ultrasounds are waves that are beyond a frequency we can hear.

Ultrasound waves travel through objects but are sometimes reflected. Ultrasounds can be used to create a real time image of the internal organs of a patient. This includes the detection of kidney stones and pregnancy scans. The waves are at a high frequency which is directed at an area of the body. These waves pass through the first layers of tissue but as they reach denser mediums they are reflected back. Depending on how far the waves were before they were reflected depend on the tissues. Hence the reflections are detected and create an on-screen, real-time image of the internal workings.

The frequency of the wave is ususally at 20,000 hertz.

• There is an increasing trend in ultrasound scans and a decrease in birth weight.