Life & Death of a Photon (part 2)
In part one of this article, we left off with the process of x-ray creation in the tube. Through the process of creating Brems and Characteristic radiation, we have an isotropic beam of x-rays being created at the rotating tungsten anode. Since the beam travels in all directions from the source, the x-ray tube housing is lined with lead to absorb x-ray photons that are traveling in directions that aren’t useful for diagnostics and to prevent unwanted and harmful leakage. The diagnostic x-ray beam is allowed to leave the tube housing through the window of the tube housing where it is directed at a patient subject. As the x-ray photons leave the tube housing through the window, there is some special beam filtration that takes place. When we discussed Brems radiation, we learned that the x-ray photos generated through this process are heterogenous, meaning that they have different levels of energy. Some of the photos do not have enough energy to be useful as diagnostic x-rays. These photons would simply be absorbed by the patient, adding to patient radiation dose and not contribute to diagnostic imaging in any way. Thin aluminum filters are added at this stage to absorb the low-energy photons. The size and shape of the beam leaving the x-ray tube is also controlled by collimators. These lead leaves allow the technician to modify the size and shape of the beam that interacts with the patient. The collimator leaves are made of lead, so they simply absorb additional radiation.
Primary Beam
At this stage of the x-ray process, we have the primary beam. The primary beam consists of x-ray photos that have left the tube housing but have not had any interactions with the patient. The primary beam has two important characteristics which are quantity of photons and the amount of energy they carry.
mAs
The quantity of x-ray photons in the primary beam is controlled by the mAs setting on the controller. mAs stands for milliamperes x time (in seconds). This refers back to the thermionic emission process at the filaments when electrons are boiled off in the x-ray tube. The milliamperage number refers to the number of electrons per second that flow from cathode to anode within the tube, and the time is the duration of that flow. You can consider the mAs setting to represent the total volume of photons that will be produced
kVp
Those photons that are produced at the cathode are pushed from the cathode to the anode by kilovoltage that is applied to the tube. The potential difference between the negatively charged cathode and the positively charged anode determines the velocity of the electron flow within the tube. Higher velocity electrons produce higher energy x-ray photons during their interaction with the tungsten atoms in the anode target. These higher energy photons have more penetrating ability than lower energy photons.
In brief summary of mAs and kVp, the mAs represents the volume of x-ray photons in the beam, and the kVp represents their penetrating ability.
Patient Interaction
Once our x-ray photons in the primary beam interact with the patient, one of three things will happen…
Transmission
The incoming x-ray photon may pass through the anatomy without interacting with any atoms in the body. When this happens, the photon passes directly through to the image receptor and becomes part of the diagnostic x-ray image.
Scatter
The incoming x-ray photon may interact with electrons in the anatomy. If the photon interacts with outer-shell electrons in the anatomy, the electrons will be ejected from their orbit and the photon will change direction and continue with less energy. The photon may or may not find its way to the image receptor after changing direction. If it does hit the image receptor, the exposure that it creates is unwanted and contributes to a phenomenon known as fog. The photon may also scatter in directions other than the image receptor. This scattered photon may also interact with other electrons in the body several times before it either becomes absorbed or leaves the body. At any rate, the scattered photon is not useful in the diagnostic imaging process. This process of scattering photons via interaction with outer shell electrons in the anatomy is known as the Compton Effect.
Absorption
The incoming x-ray photon may be completely absorbed within the body. Complete absorption occurs when the incoming photon ejects an inner-shell (K-shell) electron within the anatomy from its orbit. The incoming photon gives up all of its energy during this process. The ejected electron is called a photoelectron and the photon’s ability to eject this electron is known as ionization. This process is known as the photoelectric effect. When the inner shell electron is ejected from its orbit, an outer shell electron will cascade into the open space. This process gives off a second x-ray photon. This secondary photon is in the form of scatter radiation which may leave the patient or interact with other electrons.
The Secondary Beam
The secondary beam is the radiation that passes through the patient and strikes the image receptor. This beam consists of both transmitted and scatter radiation. Transmitted radiation passes directly through the patient and strikes the image receptor, exposing the film or digital sensor. This exposure creates dark areas on the film or digital image. The unexposed areas on the film or digital sensor are created by absorption of photons in the anatomy. When a photon is absorbed, that photon will not strike and expose the image receptor. The white or light colored areas on the radiograph represent areas of the anatomy that absorbed x-ray photons.
The secondary beam is responsible for creating the radiograph. There are a lot more details to be discussed about the radiation that exists in the secondary beam and how we manipulate that radiation through various technique changes. Contrast and density are the two primary photographic qualities of a radiograph. My next essay on this site will dig in to the processes of manipulating contrast and density…
Until then…
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