How Everything Works
How Everything Works How Everything Works

Medical Imaging and Radiation
Page 1 of 2 (17 Questions and Answers)

463. How do the "user-friendly" MRI machines work vs. the old catacomb type? (the opened vs. closed types)
The shape of the MRI machine is dictated primarily by the strong magnetic field it uses to record information about protons in a person's tissues. This field needs to be very uniform over a large region of space and the simplest way of producing such a uniform field is with a huge coil of current-carrying wire. The person would go inside the coil, in the uniform field and other parts of the MRI machine would record the information. While the coil could be dressed up to look more like a tubular hole than a coil of wire, it was still very confining. Newer MRI machines use two smaller coils of current carrying wire, one above the other, to create a uniform field for imaging. This arrangement is trickier because the two coils must be shaped very carefully to ensure that the field is appropriately uniform. Moreover, most MRI machines use superconducting wires in these coils to achieve very high magnetic fields. Since superconducting coils must be cooled to very low temperatures, they require liquid helium coolants and sophisticated thermal insulation. While the single coil magnets required only a single refrigerator and insulating chamber, those with two coil magnets required two refrigerators and insulating chambers. That increases the expense of the magnet and its operation, but produces a more open imaging region.

464. If bones "stop" electrons, then why do we see a skeletal image on an X-ray? Would we get a negative image?
The bones cast shadows on the film; wherever there is bone, few X-rays strike the film. When the film is developed, it turns black wherever X-rays hit it. Thus the areas that were shadowed by the bone appear white.

465. On an X-ray result picture, why is the film in the background blue? Is this the only way it will show up? If so why?
The X-ray image itself is formed by tiny black silver particles, just as in a normal black and white photographic negative. If those particles were supported by a clear plastic sheet, then the X-ray should appear either clear or black and have no color. The blue you are referring to must be caused either by a colored pigment in the plastic X-ray film sheet or by a colored light used to illuminate the X-ray. I suspect the later. Fluorescent lamps tend to be bluish and the ones used to view X-rays are probably particular blue. It probably increases the apparent contrast in the image so that small variations in density become visible.

466. What exactly does the bone do with the X-rays that the skin doesn't?
The skin's atoms are too small to experience the photoelectric effect with X-rays. Most X-rays go right through skin and soft tissue. However calcium atoms are large enough to experience the photoelectric effect and thus absorb many of the X-rays. Bones cast a shadow on film, which is how an image of your bones is formed.

629. I read recently that scientists at CERN produced some form of antimatter, but that it could not be stored. Why can't it be stored and, if it could, would it be a viable method of propulsion? — BC, Ottawa, Ontario
The antimatter that was formed at CERN was an antihydrogen atom, which consisted of an antiproton and an antielectron (often called a positron). Antiprotons and positrons have been available for a long time, but it has been a challenge to bring them together gently enough for them to stick to one another and form a bound system. An antihydrogen atom is hard to store because, like a normal hydrogen atom, it moves or falls so quickly that it soon collides with its container. For a normal hydrogen atom, that collision is likely to cause a chemical reaction. But for an antihydrogen atom, that collision is likely to cause annihilation. When an antiproton touches a proton, the two can destroy one another and convert their mass into energy. The same is true for a positron and an electron. To store an antihydrogen atom, you must keep it from touching any normal matter. That's not an easy task. Because of its ability to emit its entire mass and that of the normal matter it encounters into energy, antimatter is the most potent "fuel" imaginable. But don't expect it to show up in a rocket ship any time soon.

644. How can MRI pictures show slices through an object? And how do you get an image from using a magnet?
MRI images show where hydrogen nuclei (protons) are located in a person's body. Protons are magnetic particles that have only two possible states in a magnetic field: aligned with the field or aligned against the field (also called "anti-aligned"). This limited range of alignments is the result of quantum physics. Normally, the protons in a person's body are equally divided between aligned one way and aligned in the opposite way. But when a person is placed in a strong magnetic field, the protons in their body tend to align with the magnetic field and the distribution of aligned and anti-aligned protons shifts. There are then somewhat more aligned protons than anti-aligned protons.

Once there are more aligned protons than anti-aligned protons, it becomes possible to flip them about. Flipping these protons from aligned to anti-aligned takes energy and this energy can be provided by a radio wave. But not just any radio wave will do: its frequency must be just right in order to provide the proper amount of energy or the proton won't flip. When the right radio wave is provided, some of the aligned protons will flip to become anti-aligned. This flipping of protons can be detected by a sensitive radio receiver.

By placing the person in a non-uniform magnetic field and by adjusting the frequencies and timings of the radio waves, an MRI device can determine where protons are located in the person's body to with a few millimeters. A computer records where the protons are and then displays information about them as cross sectional images. For example, the computer can display a dense concentration of protons as white and a region with few protons as dark. MRI is particularly good at imaging tissue because tissue contains lots of hydrogen atoms and their protons.

678. What are the risks of occupational exposure to "black" fluorescent lamps? - MB
By "black" lamps, you mean ultraviolet lamps. Since ultraviolet light is able to cause chemical damage to biological tissue, long-term exposure to this light isn't so good. How much risk there is depends on how much ultraviolet light they produce and how near you are to them. Sunlight contains a considerable amount of ultraviolet, so long exposure to sunlight burns and ages skin. The photons of ultraviolet light contain enough energy to cause changes in molecules and thus upset the cellular machinery that keeps us healthy. Ultraviolet lamps will do the same thing, given enough intensity and time.

793. Radioactive elements' half-lives are fixed and they decay at a constant rate. Their decay rates have been determined thanks in part to our nuclear weapons research. Under what circumstances can a radioactive element have its decay rate changed? Can the element's radioactivity be destroyed (cancelled) by applying high temperatures? If so, how high would the temperature have to go to achieve this? — RD, Humble, TX
Since radioactivity is a feature of atomic nuclei, the only way to alter radioactivity is to alter atomic nuclei. But there aren't many ways to change atomic nuclei. Of various atomic and subatomic particles, only a neutron can enter a nucleus easily and cause it to rearrange. However, it's more common for a neutron to increase radioactivity than to destroy it, so that's not a good approach. Furthermore, the only practical way to obtain neutrons is with radioactivity.

Heating a collection of nuclei can cause them to collide and rearrange. However, this process is also fraught with problems. The products of the fusion and fission events that occur when nuclei collide will probably be radioactive themselves, so that it's unlikely that heating radioactive materials will make them less radioactive. Instead, it's likely that heating radioactive materials will make them more radioactive. Furthermore, the temperatures at which nuclei will begin to collide are extraordinarily high. Even the smallest nuclei repel one another fiercely so that they need temperatures of 100 million degrees C or more to begin colliding effectively. Larger nuclei, such as those common in nuclear wastes, won't collide until their temperatures exceed 1 billion degrees C. The only way to reach these temperatures is with nuclear weapons and they certainly don't reduce the radioactivity of nearby materials. In short, the only way to get rid of radioactivity is by waiting patiently.

832. How can we measure magnetic fields or magnetic potentials of solvent atoms that reside interstitially inside solid solutes? — DR, Tampa, FL
You can measure the magnetic fields in which certain atoms reside with the help of nuclear magnetic resonance (NMR). This technique examines the magnetic environment of the atom's nucleus by determining how much energy it takes to change the orientation of the nucleus. Since the nucleus is itself magnetic, it tends to align with any magnetic field—like a compass. The stronger that magnetic field, the harder it is to flip the nucleus into the wrong direction.

833. What happens when matter and anti-matter collide? Do they just destroy each other? I thought that matter couldn't be created or destroyed? - S
As Einstein's famous formula points out, mass and energy are equivalent in many respects. In most situations, mass is conserved and so is energy. But at the deepest level, it's actually the sum of those two quantities that's conserved. When matter and anti-matter collide, they often annihilate one another and their mass/energy is converted into other forms. For example, when an electron and an anti-electron (a positron) collide, they can annihilate to produce two or more photons of light. There is no fundamental law that prevents matter from being created or destroyed but there is a fundamental law that mass/energy must be conserved. In this case, the masses of the electron and positron become energy in the massless photons. Overall, mass/energy has been conserved but what was originally mass has become energy. The fact that when matter and anti-matter annihilate, the product is usually energy, makes this mixture attractive as a possible super-rocket fuel. But don't hold your breath; anti-matter is incredibly difficult to make or store.
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