Medical Physics

Kayleigh Martin 12WV

Medical Physics

Introduction

Physics has become an important part of medicine allowing specialist doctors and radiographers to rapidly access a patient’s condition and to help in long-term diagnosis. This enables doctor’s to treat the patient before their condition deteriorates. This would not be possible without the use of X-rays, CAT scans, MRI scans, ultrasound and endoscopes, which allow doctors to see inside the body with little or no surgery. Without such equipment doctors would be forced to use invasive techniques, which could cause patients more harm as it increases the risk of infection.

Within my project I hope to explain what the different pieces of medical equipment, which use physics are and how they work.

X-ray machines

What is an X-ray?

X-rays are virtually the same thing as visible light rays. They are both wavelike forms of electromagnetic energy carried by particles called photons. The main difference between the two is the wavelength of the rays, which is also expressed as the energy level of the individual photons.

The movement of electrons in atoms produces visible light photons and x-ray photons. Electrons reside in different orbitals around an atoms nucleus. When an electron drops to a lower orbital, energy is released in the form of a photon. The photons energy level depends on how far the electron is dropped between orbitals. When a photon collides with another atom, the atom may absorb the energy level of the photon boosting an electron to a higher level.

In order for this to happen, the energy level of the photon has to be the same as the energy difference between the two electron positions. If this is not the case the photon is unable to shift electrons between orbitals.

How the x-ray machine works

The heart of an x-ray machine is an electrode pair (a cathode and an anode) that sits inside a glass vacuum tube. The cathode is a heated filament; the machine passes a current through the filament in order to do this. The heat sputters electrons off the filament surface. The anode, which is a positively charged flat disc made of tungsten, draws the electrons across the tube.

There is an extremely high voltage difference between the electrode pair, so the electrons fly across the tube with a great deal of force. When a speeding electron collides with tungsten, it knocks loose an electron from one of the atoms lower orbitals. An electron from a higher orbital instantly falls to the lower energy level. Releasing its extra energy in the form of a photon. As it is a large drop the photon has a high energy level (it is an X-ray photon).

As there are high-impact collisions involved in X-ray production a lot of heat is generated, so a motor rotates the anode to keep it from melting as the electron beam is not always focused on the same area. A thick lead shield to keep the X-rays from escaping in all directions surrounds the entire mechanism. A small window in this shield lets some of the X-ray photons through a narrow beam. The beam passes though a series of filters on its way to the patient. A camera on the other side of the patient records the pattern of X-ray light that passes all the way through the patient’s body. The x-ray light sets off chemical reactions so it uses the same film technology as an ordinary camera.

Normally, doctors will keep the film as a negative, so that areas that are exposed to, more light appear darker and the areas that are exposed to less light appear lighter. Harder materials such as bone, appear white, whereas softer materials appear black or grey. This means doctors can bring different materials into focus by varying the intensity of the X-ray beam.

Contrast media are used in order for soft tissues to appear more clearly. They are often used in conjunction with a fluoroscope. The fluoroscope allows x-rays to pass through the body onto a fluorescent screen ...