Medical Imaging Dr. Hugh Blanton ENTC 4390

X-rays How does an x-ray machine work? We first accelerate electrons with a high voltage (several thousand volts). We then allow the high speed electrons to smash into a target. As the electrons slow down on collision, they can emit photons - via photoelectric effect or Compton scattering. Dr. Blanton - ENTC 4307 - X-Ray 2

An electron gun inside the tube shoots high energy electrons at a target made of heavy atoms, such as tungsten. Dr. Blanton - ENTC 4307 - X-Ray 3

X-rays However, the maximum energy of the electrons limits the maximum energy of any photon emitted. In general glancing collisions will give less than the full energy to any photons created. This gives rise to the continuous spectrum for x-ray production. Dr. Blanton - ENTC 4307 - X-Ray 4

X-rays If an electron knocks out an inner shell electron, then the atom will refill that missing electron via normal falling of electrons to lower levels. This provides a characteristic emission of photons, and depends on the target material. For the inner most shell, we can use a formula similar to the Bohr atom formula: Dr. Blanton - ENTC 4307 - X-Ray 5

X-rays For the inner most shell, we can use a formula similar to the Bohr atom formula: ionization = 13.6 eV * (Z-1)2 where the -1 comes from the other inner shell electron. Dr. Blanton - ENTC 4307 - X-Ray 6

X-rays If the electrons have this ionization energy, then they can knock out this inner electron, and we can see the characteristic spectrum for this target material. For iron, the ionization energy is: 13.6 eV * (26-1)2 = 1e * 8500 volts. Dr. Blanton - ENTC 4307 - X-Ray 7

X and  ray penetration High energy photons interact with material in three ways: the photoelectric effect (which dominates at low energies), Compton scattering, and pair production (which dominates at high energies). Dr. Blanton - ENTC 4307 - X-Ray 8

X and  ray penetration But whether one photon interacts with one atom is a probablistic event. I = Io e-x where  depends on the material the x-ray is going through. Dr. Blanton - ENTC 4307 - X-Ray 9

X and  ray penetration  1 MeV Energy pair production total Compton Scattering photoelectric effect Dr. Blanton - ENTC 4307 - X-Ray 10

Measuring Health Effects Gamma rays (high energy photons) are very penetrating, and so generally spread out their ionizations (damage). Beta rays (high speed electrons) are less penetrating, and so their ionizations are more concentrated. Alphas (high speed helium nuclei) do not penetrate very far since their two positive charges interact strongly with the electrons of the atoms in the material through which they go. Dr. Blanton - ENTC 4307 - X-Ray 11

Bremsstrahlung When the electrons strike the dense metal target, strong Coulomb forces are created between the negative electron particles and the strongly positive nuclei of the metal. This interaction causes the electron to slow down (brake), or change directions, very quickly. Thus, bremsstrahlung. Dr. Blanton - ENTC 4307 - X-Ray 12

Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when the velocity of the electron shot at the tungsten changes. The electron slows down after swinging around the nucleus of a tungsten atom and loses energy by radiating x-rays. Dr. Blanton - ENTC 4307 - X-Ray 13

Due to the conservation of energy principle, this loss of kinetic energy has to be compensated for and is done so by the production of a photon of electromagnetic energy. We call this photon an X-ray. Dr. Blanton - ENTC 4307 - X-Ray 14

X-rays are just like any other kind of electromagnetic radiation. They can be produced in parcels of energy called photons, just like light. Dr. Blanton - ENTC 4307 - X-Ray 15

There are two different atomic processes that can produce x-ray photons. One is called Bremsstrahlung, which is a fancy German name meaning "braking radiation." The other is called K-shell emission. They can both occur in heavy atoms like tungsten. Dr. Blanton - ENTC 4307 - X-Ray 16

In the quantum picture, a lot of photons of different wavelengths are produced, but none of the photons has more energy than the electron had to begin with. After emitting the spectrum of x-ray radiation the original electron is slowed down or stopped. Dr. Blanton - ENTC 4307 - X-Ray 17

The K-shell is the lowest energy state of an atom. The incoming electron from the electron gun can give a K-shell electron in a tungsten target atom enough energy to knock it out of its energy state. Then, a tungsten electron of higher energy (from an outer shell) can fall into the K-shell. The energy lost by the falling electron shows up in an emitted x-ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, and so on. K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray photon comes out at a single wavelength. Dr. Blanton - ENTC 4307 - X-Ray 18

The energy lost by the falling electron shows up in an emitted x-ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, and so on. K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray photon comes out at a single wavelength. Dr. Blanton - ENTC 4307 - X-Ray 19

In the same way that x-rays are deflected in the target crystal, they are deflected by atoms in our body. When radiation strikes an atom it has the ability to knock electrons out of the orbiting shells. Once these atoms are ionized, they seek out other atomic particles or ions to make themselves more stable. Dr. Blanton - ENTC 4307 - X-Ray 20