Physics in Medicine

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? Introduction Physics is about understanding the laws that govern the world around us. Most people know about the problems solved and the discoveries made by physicists in the past, such as the orbits of planets found by Galileo, the law of gravity first unravelled by Newton and later extended by Einstein. It is not as widely known how diverse the subjects investigated by physicists are now, and how much their discoveries change our everyday life. Modern hospitals are among the best showcases of the remarkable and often unexpected ways in which Physics is used.

For more then a century, Physics has been the key to new treatments and more and more accurate diagnostics. X rays, ultrasounds, optical fibres, ion and neutron beams are all examples of how breakthroughs in Physics become practical tools used in medicine. Physics enables clinicians to cure diseases such as cancer and kidney stones by destroying unhealthy tissues by delivering energy in the most appropriate form and in the most precise way exactly where it is needed. ? The Medical Physicist Medical physics is the use of physics principles in medical diagnosis and treatment.

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Many medical physicists are heavily involved with responsibilities in these areas, often with specific patients. They are responsible for selecting and specifying the types of equipment that are used in radiation therapy. They also play a vital role on the medical research and development team in key areas such as cancer, heart disease, and mental illness. A Medical Physicist at work The medical physicist is called upon to contribute clinical and scientific advice and resources to solve the numerous and diverse physical problems that arise continually in many specialized medical areas.

In radiation oncology departments, one important example is the planning of radiation treatments for cancer patients, using either external radiation beams or internal radioactive sources. ? Physics of the Body Medicine is full of biological and chemical science, but physics is rapidly taking on a greater role. In the human body, physics is involved in mechanics and movement as well as the pressure, optical and electrical systems of the body. Radiotherapy is used to treat tumours, and scans such as MRI and PET scans are used to look inside the body and check for diseases and disorders related to these areas of medicine which involve physics.?

MRI Scans Magnetic Resonance Imaging (MRI) uses a circular magnet, strong enough to pick up a car, to produce detailed images of internal organs. Doctors can use MRI to see which parts of the brain are active when you perform certain tasks or feel certain emotions and sensations. A large proportion of the human body is made up of fat and water, both of which contain lots of hydrogen atoms. In fact, you are made up of approximately 60% hydrogen atoms. MRI works by measuring the way in which these hydrogen atoms absorb and give off electromagnetic energy. When you have an MRI scan, you lie inside a machine that contains a powerful magnet.

The nucleus of a hydrogen atom is like a tiny magnet, so by lying in line with the strong magnetic field inside the scanner, all of your hydrogen nuclei line up too. Photograph of an MRI scanning machine The scanner sends out a pulse of radio waves which gives enough energy to the hydrogen nuclei for them to change direction. When the pulse is switched off the nuclei revert back to their original position and each nucleus gives off energy in the form of a radio wave. The frequency of these waves depends on the strength of the magnetic field where each nucleus is and this means that the scanner can work out the location of each nucleus.

Functional Magnetic Resonance Imaging (fMRI) is a type of MRI that allows you to see which parts of your brain are active when you perform different tasks or feel certain emotions. Brain activity requires energy and a good supply of oxygen-rich blood. The scanner can see the increase in blood flow to the most active parts of the brain because it can detect the difference between hydrogen nuclei in oxygenated blood and those in de-oxygenated blood. In this way the scanner builds-up a 3D map of which parts of the brain are working particularly hard.

fMRI mapping of the brain is used to find out how the brain carries out mental tasks and what parts of the brain are responsible for different brain disorders. ? PET Scans Positron Emission Tomography (PET) is used to look at how the body uses substances such as glucose, ammonia, water and oxygen. Seeing how these molecules move through the body, and where they are being used, allows a doctor to check for anything unusual that might suggest the presence of disease. By tracking how the body uses glucose, a PET scan can produce images of cancerous tumours and help the doctor work out what treatment is best.

We use glucose for energy in our body so cancerous tissue, which uses more glucose than normal body tissue, will show up as a bright area on the PET image. If oxygen is used as the tracking molecule, PET scans can be used to image brain activity and look at blood flow in the heart to detect coronary heart disease and other heart problems. Photograph of a PET scanning machine To allow the molecules to be tracked they have radioactive isotopes attached to them, before being injected into the body. The dose of radiation involved is about the same as you would be exposed to naturally over two or three years.

This means that your body isn’t damaged, although you wouldn’t want to have a lot of scans over a short period of time! ? Radiotherapy Radiotherapy is a method of treating cancerous tumours using targeted beams of radiation. Not all cancers are the same and different tumours need different treatment plans. The three main forms of treatment are surgery, chemotherapy (using drugs) and radiotherapy (using radiation). Radiotherapy uses precisely targeted beams of high energy photons to damage the cells of a cancerous tumour, making them unable to reproduce and spread.

The high energy photon beams are created and delivered using a clinical linear accelerator which can be rotated around the patient to deliver the radiation from any direction. The radiation can damage healthy cells as it passes through normal tissue on its way to the tumour. To reduce this damage, the radiation is fired at the tumour from a series of different directions. This ensures that the cancerous tumour will receive a full dose whilst the surrounding healthy tissue receives a much lower dose. Photograph of a patient on a linear accelerator ? Medical Physics Moments in History.

Some of the greatest medical advances in the history of medicine occurred in the past couple of centuries and came from the minds and laboratories of physicists. X-rays were discovered by Wilhelm Conrad Roentgen in 1895. In 1977, the Nobel Prize in Physiology and Medicine was awarded to Rosalyn Yalow for her development of radioimmunoassay, an extremely sensitive diagnostic technique that can quantify tiny amounts of biological substances in the body. In 1979, Allan M Cormack and Godfrey Newbold Hounsfield developed CT. In 2003, the Nobel Prize in Physiology and Medicine was awarded to Paul Lauterbur and Peter Mansfield for their work in MRI.

? Conclusion Healthcare has changed a lot in the past 62 years. People now live on average at least ten years longer than they did in 1950, and medical advances have brought many breakthroughs and improvements in patient care. The world is fast changing and I believe that Physics remains an essential driving force, especially in medicine, behind the progress and innovation as it was centuries ago in the times of Galileo, Newton and Einstein. ? Bibliography http://nd. edu/~nsl/Lectures/mphysics/index. htm http://www. radonc. jhmi. edu/html/medical_physics. html http://www. enotes. com/medical-physics-reference/medical-physics.

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htm http://my. clevelandclinic. org/services/pet_scan/hic_pet_scan. aspx http://www. cancer. ie/cancer-information/treatments/radiotherapy Physics of the Human Body – Irving P. Herman Essential Physics of Medical Imaging – Jerrold T. Bushberg