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This page is a translation of the introduction page to the collection of Medische beeldvorming articles, tutorials, exercises etc. The links refer to the original articles with texts in Dutch − all languages are spoken by the images, flashlets, films and formulas.
The page was made on the occasion of GIREP 2006.
For more information, please contact Maarten Pieters.
Imagine you are a doctor and a patient comes to see you complaining about pain in the chest. About a hundred years ago you would not have many more options than to tap the patient, or to listen with a stethoscope. For the rest you would have had to guess what was causing the pain. Even though you would have liked to know what was wrong in the thoracic cavity, conducting surgery to take a look was usually not an option as the risks that came with it could well have been much worse than the problem itself.
Nowadays, doctors have a variety of possibilities to, so to speak, look into the body. Medical Imaging started in 1895 with the discovery of X-rays and was added with all sorts of imaging methods over the 20th century, in which the computer has played an indispensable part.
In this unit the following methods will be discussed (you can view the articles by clicking on the titles):
X-ray photograph
The eldest method, well-known for the use with bone fractures or examination of the teeth, but there are also less known computertechniques, like making the blood vessels visible.
CT-scan
CT stands for ‘computed tomography’. With the help of X-rays the computer creates an image of the cross-section of the body. The advantage of this is a much higher image quality, the disadvantage is a quite high dose of radiation for the patient.
Nuclear medical science
Here it is not so much the inside of the patient that is being looked at, but the physiology: where is the metabolism at its most active, how is the blood circulation of tissues, where could a tumor be forming etc. The patient is given a small amount of a radioactive substance and using a gamma-camera or other detectors we can see where in the body the substance ends up. A variation of this is the PET-scan (PET = positron emission tomography). When an electron and a positron are brought together, both particles disappear and energy appears in the form of two gammaphotons. With the use of detectors we can find out where that radiation came from.
MRI-scan
MRI stands for 'magnetic resonance imaging'. We make use of the fact that protons (hydrogen cores) behave like small magnets. They have 'spin'. Using a special method we make the hydrogen cores send out radiowaves. Using receptors and the computer we then construct an image. Tissues containing a lot of hydrogen can now be made out from the ones containing little hydrogen. The MRI-scan is especially useful to create images of soft tissue. An advantage is that no X-rays or radioactivity is used. The disadvantage is that the apparatus is very expensive, and that because of the strong magnetic field it cannot be used on patients that have electronic devices (like pacemakers) in their body.
Echography
Ultrasound is partly reflected at the border area of different tissues. Using the reflected sound, the computer can create an image of the tissue inside. The most known use is for check ups during pregnancies, but it can also be used to, for example, measure the velocity of the bloodflow. The advantage is that no ionizing radiation or radioactivity is involved in this method.
Medical imaging has grown to be an irreplaceable contribution to medicine. The average hospital (800 beds) annually has some 160 000 investigations for which more than a million recordings are made. One third of the investments a hospital makes is in apparatus for medical imaging.
It is a field which is still very much in development: refining the imaging, making it more efficient, bringing down the amount of radiation etc, and it still requires a lot of research involving doctors, engineers, phyisicists, information scientists and mathematicians.
What can be found in this unit:
Interactive extra lessons
Radioactivity
In this extra lesson you will find an overview of the construction of the atomic nucleus and the different sorts of radioactivity. About half life and activity.
Absorption of X-rays
This extra lesson discusses what X-rays are doing in the human body: in can go straight through it, be scattered, as the radiation is showing a particle character, or it can be absorbed (which will cause damage!). The term half-thickness will be discussed.
The origination of X-rays
This is not part of the exam matter, but does connect with nuclear physics: the construction of the X-ray tube, the originating of X-rays, bremsstrahlung and the X-ray spectrum. In a side text the synchrotron radiation will be dealt with: important in astronomy and for particle accelerators.
Antimatter About the origination of particle and antiparticle (pair production) and their disappearance (annihilation). How to calculate the energy needed at the least to create a particle and an antiparticle.
Grid Linking to the focusing of soundwaves in echography, an extra lesson deals with the grid. An animation shows what happens when waves interfere. What happens when you see one, two or eight sources of vibration working.
Articles
There are five articles in which the imaging methods and the physics behind them are discussed. The articles themselves are well comprehensible for N1 students. Occasionally there can be a reference to more detailed information, which is a little bit more complicated, and harder to understand.
This article tells you about the discovery of X-radiation and the use of it in medicine. Also described are modern similar methods like the digital screen, which enables us to create a good digital photograph using only little radiation. And the DSA-recording which makes it possible to make distinct pictures of the blood vessels. The dangers of X-rays will be discussed briefly.
Animations show you how this method works. How images of a cross-section of the body are possible and that we can also create 3D-images. In a more detailed discussion an 'image' of only 4 pixels shows us how the computer can calculate the grey values to create an image .
The patient is given a small amount of radioactive matter which emits gamma radiation. With a gamma camera we can find out where in the body the material ends up. A variation to this is the PET-scan. A radioactive isotope emits positrons that react with electrons after a short while. This reaction creates two gamma photons that move almost exactly in the opposite direction. Using a ring of detectors around the patient thoses pairs are registered enabling us to establish where the radiation came from.
Explained is why hydrogen nuclei start emitting radiowaves and how we can use the frequency of those waves to determine where the radiation is coming from. An often used and well working method, but all iron and sensitive electronics have to be kept away, due to the very strong magnetic field.
A transducer functions as a transmitter and receptor of ultrasoundwaves. A short pulse of sound is being transmitted and the echo is intercepted. From the difference in time between transmission and reception we can calculate how deep the reflecting surface is. The article also deals with creating 3D-images and measuring the velocity of the bloodflow using the Doppler effect.
Excercises
Several excersises about the physics behind imaging method can be found through the search menu, by entering the search terms 'medische beeldvorming' of 'nucleaire geneeskunde' (i.e. apos;Medical Imaging'and 'nuclear medicine').
A few examples:
Studies and Profession
Information about professions that involve medical imaging in the following articles:
