This might seem a strange subject to discuss when the overall subject matter is energy. However there are some interesting energy interactions involved in medical scanning and so below I explain how some such machines work but using my views and explanations of energy.
X Ray Machines:
The production of x rays is illustrated in the diagram. A voltage of about 100, 000 volts between anode and cathode means there are more electrons at the cathode than are needed to supply proton energy needs. They are already jostling for position and when we input photon energy by heating this cathode they become even more agitated and some become loosely linked to the cathode protons.
At the anode there are electron shortages. The proton desires for photons are not being met by their surround electron and photons are being drawn from any emitting structure like the glass tube and cathode. The loose linked electrons respond to the anode photon flows sensing a need for their energy gathering services and use their energy to accelerate toward the anode.
The electrons are heading for electron “holes” in the tungsten structure but their speed causes them to overshoot and increase photon energy exchanges with other electrons that don’t want their close approach. The high speed electrons may at this stage be releasing many x rays. Many anode electrons are involved in resisting the high speed electron approaches. They are issuing photons that oppose their motions and that gives them reactionary motions. Even electrons in positions close to the nucleus are set in motion leaving “holes” that the tungsten proton desires for energy want to fill. The photon flows to them encourage local electrons to accelerate into these “holes” but they overshoot them and exchange photon energies intent on positioning them into these more stable space locations.
Lots of local electrons are being encouraged to move into more desirable “holes”. Each such move is associated with exchange kinetic energies as described above. Some released energies are in the x ray range and some pass through a small window in the lead shield. Most released energies are lower photon energies that deliver considerable heat. The rotating anode is one of a number of measures intent on dispersing this heat.
The intensity of the x ray beam can be controlled and directed through a patient’s body to a camera that records the pattern of X-ray light that is passed through it. Bone, fat, muscle, tumors, etc all absorb X-rays at different levels. The denser particle structures of bone for example are more likely to absorb an X-ray photon. X ray photons are dangerous because they can remove electrons from atom structures upsetting their proper working in our bodies. However, our bodies have a repair capability and can cope with limited exposure to x rays.
Computed Tomography (CT or Cat Scan)
This type of scan also uses x rays but now the x ray tube can rotate around the patient’s body along with a detector panel on the opposite side of the patient. In addition the bed on which the patient lies can be be moved in and out of the machine. To improve the imaging and depending on the part of the body being investigated the patient may be asked to drink or be injected with a contrast medium.
Clearly with such an arrangement x rays can be taken from several angles and through many sections of the body. A computer program mathematically interprets all the data and produces a tomogram of the area under investigation.
Magnetic Resonance Imaging (MRI Scan)
These machines use magnetism to scan human bodies. The electro magnets used are very powerful. Several miles of wire are immersed in liquid helium reducing its resistance to zero and creating a super conductor magnet.
Bodies contain much water and water contains hydrogen atoms with their single protons. Protons, like electrons, have a spin property. We saw when looking at magnetism how spin delivers a rotating magnetic field. Zillions of tiny proton magnetic fields are randomly orientated until we switch on the powerful magnet. When we do so protons have little choice but to turn and best align their mini fields with the main field that runs from head to toe.
Gradient coils and radio frequency coils are also a part of an MRI machine. They only come into play during the scan process. The gradient coils vary the powerful magnet field along the tube length whilst radio frequency coils deliver a pulsing magnetic field perpendicular to the main field. The pulsed magnetic fields are of short duration and interact with the gradient main field so that their combination acts on the protons, truing to deflect them from their their aligned state.
The photons emanating from the main and gradient field not only align protons but also mildly influence their structure positions relative to other particles. They thereby have a say in the frequency at which the protons at a specific section will resonate. Resonance occurs when the energy desires and motions of the protons match the frequency of supply photons. When the radio frequency delivers that resonant frequency appropriate to a section the protons absorb much such photon energy and move to and fro in line with it, flipping away from the aligned state.
When the radio frequency is switched off the flipped protons are intent on ceasing their resonant energy interactions and try to re align with the main magnet field. However different structure tissues relax at different rates and so the intensity of release of the photon interactions is different for bones, soft tissues, blood vessels and organs such as brain and heart. Receiver coils act as aerials, detecting and collecting the emitted signals which are used to create a sectional image. The images can reveal unhealthy tissue.
Positron Emission Tomography (PET Scan)
In discussing nuclear energy we saw how some isotopes in undergoing radioactive decay emitted positrons. They have near same energy as an electron but unlike an electron energy that seeks to maintain a distance from other electrons a positron energy will seek out and be attracted to an electron energy to the extent of annihilation outputting two gamma ray energies in the process. PET scans make use of such beta positive radiation.
In the lab an isotope with a short half life is most commonly added via a cyclotron to glucose (sugar) molecules producing what is called a radioactive tracer. For some investigations the isotope may be added to a protein or a hormone. The tracer is usually injected into the blood stream of a patient who then lies on a bed that passes in and out of a circular detector. All this has to be done quickly because of the short half life of the isotope. The isotope may be Carbon 11, Nitrogen 13, Oxygen 15, Fluorine 18, Gallium 68, Zirconium 89 or Rubidium 82.
The blood has the role of taking glucose sugar to every cell in the body for the purpose of energy production and so all cells are soon radiating positrons that annihilate close by electrons and emit gamma rays but at a level related to the energy activity of the cells. This is particularly useful as cancer cells are noted for being high users of energy. PET scans do not so much deliver a picture of the human body as a picture of its levels of activity. They are therefore most used in conjunction with the machines earlier described.
The gamma rays are emitted in pairs but in near opposite directions. They are detected by a scintillator material that fluoresces when struck by high energy photons. The light output passes through photo multipliers and along with details of bed position enable the establishment of a 3d picture of body activity levels.