Medical Physics
Table of Contents
I.
Introduction
II. MRI
III. Radiology
IV. EMG

I. Introduction
Medical Physics is a branch of physics that devotes itself to applying ideas from physics to the study, diagnosis, and treatment of human disease. Usually involved in radiology-related work, medicial physicists give consultations, work in clinics, do research, and sometimes teach. Most of the research is in radiology; however, some physics discoveries in particle physics have led to innovations and research in the treatment and possible cure for cancer.


II. Magnetic Resonance Imaging (MRI)
History and Introduction of MRI
Magnetic Resonance Imaging is a tomagraphic imaging technique based on principles used by phsycicist to study the structure and properties of atoms and molecules called Nuclear magnetic Resonance (NMR). Two Physicists, Felix Bloch from Stanford University, and Edwards Purcell from Harvard discovered that when certain nuclei absorbed energy in the radiofrequency range of th electromagnetic spectrum when placed in a magnetic field. The nuclei then re-emitted this energy once it returned to its original state. The strength of the magnetic field and the radiogrequency matched one another as discoverd by the Irish physicist Sir Joseph Larmor (2857-1942), thus known as the Larmor relationship. This incident was termed NMR:
"Nuclear"-only the nuclei of certain atoms reacted this way
"Magnetic"-a magnetic field was required
"Resonance"-because of the direct frequency dependence of the magnetic and radiofrequency fields
With this discovery NMR spectroscopy was established and became an important method for the studying the composition of chemical compounds. in 1952, Bloch and Purcell were awarded the Nobel Prize for their physics accomplishments.
Felix Bloch
Felix Bloch

Edward Purcell
Edward Purcell

MRI Timeline

1946
MR phenomenon - Bloch & Purcell
1952
Nobel Prize - Bloch & Purcell
1950
NMR developed as analytical tool
1960
1970
1972
Computerized Tomography
1973
Backprojection MRI - Lauterbur
1975
Fourier Imaging - Ernst
1977
Echo-planar imaging - Mansfield
1980
FT MRI demonstrated - Edelstein
1986
Gradient Echo Imaging
NMR Microscope
1987
MR Angiography - Dumoulin
1991
Nobel Prize - Ernst
1992
Functional MRI
1994
Hyperpolarized 129Xe Imaging
2003
Nobel Prize - Lauterbur & Mansfield

After the Bloch and Purcell phenomenon here is a timeline of some developments and discoveries to follow. Bloch and Purcell’s discovery eventually led to the development of the modern MRI.


How the MRI Works
When the nuclei of some hydrogen atoms that are spinning randomly get caught in a magnetic field, they line up. If the protons are then hit with a small burst of radio waves, they flip around momentarily. Then while they attempt to return to their original position, they let off a small radio wave of their own. The intensity of their radio wave emission reflects the amount of matter in that given “slice” of matter.
Below is a more detailed diagram that goes through this process.


This “slice” of matter is a topographic image.





This “slice” of matter is a topographic image.
external image scanning-small.JPG


IV. EMG (Electromygraphy)
EMG, or Electromyography, is used to test nerve and muscle function. It is usually performed along with a NCS, or nerve conduction study. After the NCS a needle examination may also be used to test the responses of muscles. In the EMG a current is sent through the body from a paticular point, and the sensors on the skin receive signals about the frequency changes2. An example of the monitor and changes is shown below:

external image emg-arv.jpg1

EMG’s are usually performed by an Electromyographer, who can also be a specialized neurologist or physiatrist. Technicians also help with the process, which can be done in a clinic. These tests can be used to detect nerve compressions, nerve injuries, or even muscular dystrophy.



Are these tests painful?
Naturally, most doctors claim that patients will experience “slight discomfort” and “minor soreness” after the test is conducted. Certainly the test provides useful information and is necessary to detect serious diseases like muscular dystrophy before they get out of hand. However, the shocks definitely have an element of surprise and slight pain to them. It’s somewhat similar to being shocked from static electricity, but many more times through whichever part of the body that is being tested. While the electric shocks are uncomfortable, the needle portion is infinitely more painful. The needles are approximately 1.5 inches long and are inserted in different sections of the muscles near the tested area. Once the needles are inserted, the patient is asked to contract his/her muscle to test the reactivity. This is more than a “slight discomfort;” tightening a muscle while it has needles in it is most definitely painful. As the needles are moved around the testing sight, the contractions of muscles are repeated and the results are recorded. After the testing is finished about an hour and a half later, the muscles will feel quite sore from being poked and prodded. In a few days the discomfort fades away.



Sources
1 http://www.emedicinehealth.com/electromyography_emg/article_em.htm
2 http://www.biopac.com/ApplicationImages/emg-arv.jpg