(When I started writing this blog I was determined not to miss posting weekly, except in the event of nuclear war. Unfortunately, I had to miss last week due to emergency surgery. In this post I will try to extract some mileage from that event).
I have written before about my fascination with the interface of physics and biology in terms of animal behavior. Another direction of interest for me is the use of physics in medical imaging. I will list the major types below:
1. X-ray imaging: This corresponds to a famous discovery in physics, made by Wilhelm Roentgen. He was working with electron beams (cathode rays) and found they could generate X-rays (which are high frequency electromagnetic waves) when they bombarded solid substances.
X-rays, he found, are absorbed to different degrees by different materials. For example, human bone pretty much stops X-rays, while human tissue is less effective in doing so. So recording X-ray transmission through the human body (say on a photographic plate) allows us to see inside ourselves - the bones show up as white (revealing fractures, if any, as dark). Roentgen's first such image, of the bones of his wife's hand, was a sensation when it was published.
It should be noted that when X-rays are absorbed by atoms in the human body, electrons are often released. In other words, X-rays consist of ionizing radiation.
For his discovery, Roentgen was awarded the first Nobel prize in physics, in 1901.
2. CT or CAT scanning: Computed Tomography or Computer Aided Tomography is also an X-ray based technique. Normally X-rays provide two-dimensional images, typically recorded on photographic plates. CT scans involve X-ray images taken from multiple directions in three-dimensional space and assembled and displayed using computer programs. This gives a much more detailed image of the inside of the human body. A cool link that describes 3D visualization of CT scans.
The 1979 Nobel Prize for physiology or medicine was awarded to Allan M. Cormack and Godfrey Hounsfield for the development of computer aided tomography.
3. Ultrasound imaging: This technique uses high frequency sound waves, the idea being the same as that used by bats for echolocation. The way sound waves are transmitted and reflected off the internal organs in our bodies provides useful imaging information about them.
Ultrasound works in real time and is good, for example, at detecting stones (e.g. in the kidney and gall bladder), the presence of fluid (e.g. in the abdominal cavity), and obstetric information (e.g. the development of a fetus).
The history of development of ultrasound for medical purposes is fascinating. The first steps involved a collaboration between an engineer and an obstetrician.
4. MRI: Magnetic Resonance Imaging involves magnetic fields and radio waves. The magnetic fields magnetize atoms in our bodies, which then become capable of absorbing and emitting radio frequency waves. The emitted waves contain information about the location, density, etc. of these atoms. MRI basically addresses hydrogen atoms, and hence the tissue containing water or fat in our bodies.
MRI does not use ionizing radiation (as opposed to X-rays; and is therefore safer); is better at imaging tissue (as opposed to bones for X rays; and is therefore the technique of choice for diseases of the brain and spinal chord, for example); is slower and noisier (due to switching of the electromagnets) but more detailed (because of its ability to register tissue contrasts) than CT scanning; and is more uncomfortable for patients who are claustrophobic (guess who). Alas, the doctors were puzzled by my CT scan, and sent me off for an MRI.
Important advances in MRI are still ongoing. Earlier, patients with pacemakers or implanted defibrillators could not undergo MRIs as the magnetic fields would interfere with the electronics in these devices. Now some companies have started making pacemakers that allow for MRI scans.
The 2003 Nobel Prize for Physiology or Medicine was awarded to Paul Lauterbur and Peter Mansfield for the development of MRI.
*A bad pun on John Lennon's famous song.
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