Chapter 32: Medical Imaging

X-ray Production

X-rays are produced when charged particles are rapidly decelerated or accelerated and their kinetic energy is transformed into high frequency photons of electromagnetic radiation. X-rays and gamma rays have overlapping frequency spectra. X-rays are produced by Bremsstrahlung or braking radiation. X-rays used in medical imaging are often referred as soft X-rays as they have energies generally lower than gamma rays.

X-ray Tube

The X-ray tube produces X-ray by accelerating electrons in a high voltage electric field then rapidly decelerating them through collision with hard metal anode. External power supply produces a voltage between the two electrodes, resulting to the acceleration of a beam of electrons across the gap between the cathode and anode, bombarding the anode at high speed, stopping the electrons, hence losing a small amount of their kinetic energy in the form of x-ray photons which emerge in all direction.

X-ray Spectrum

X-ray photons produced when electrons is accelerated have range of acceleration when hitting the target metal hence forming braking radiation, the characteristic radiation is due to the de-excitation of orbital electrons in the target metal (anode). The sharp cut off at the short wavelength is because an electron gives all of its energy to a single photon and is stopped in a single collision,

Fmax = eV/h or λmin = hc/Emax

X-ray Attenuation

The gradual decrease in the intensity of x-rays as it passes through matter is called attenuation. The intensity is given by;

I = P/A where P is power and A is the unit cross sectional area.

The attenuation of X-ray is given by;

I = I0 eꙠx

Improving X-ray images

  1. By reducing the exposer of patient to the harmful X-rays.
  2. By improving the sharpness if the image, so that finer details can be seen.
  3. By improving the contrast of the image, so the different tissues can be clearly investigated. 

Computerized axial tomography (CAT scan)

A CAT scan is an effective way of examining the internal three-dimensional structure of a patient using x-ray imaging. The CAT scanner records a large number of 2D X-ray images then assembles them into 3D image with the help of computer software.

Advantage of CAT scanner is that it scan over the standard X-ray image. The image is of higher resolution.

Disadvantages of CAT scanner is that it provides greater exposure to X-ray and it is more expensive.

The image of 8-voxel cube can be developed using CAT scanner.

Ultrasound

Ultrasound is a longitudinal sound wave with the frequency greater than human hearing range. The sound wave can be refracted, reflected and diffracted. Using these wave behaviors, it is used for medical purpose.

Ultrasound is a non-ionizing and non-invasive technique that is quick and affordable. A transducer in the ultrasound device is used to produce an electrical signal from soundwaves, this can then be generated into image by computer software.

Piezoelectric Effect

A piezoelectric material generates a voltage when it is contracted or expanded or will contract and expand if a voltage is applied, therefore, by applying a voltage to a piezoelectric crystal we can produce ultrasound vibration. An ultrasound transducer has an alternating potential difference that causes repetitive compression and stretching of the crystal. A resonant frequency of the crystal is chosen to increase the intensity. Ultra sound wave are pulsed to allow time for the reflected wave to be received. Smaller wavelength give more detailed image.

Magnetic Resonance Imaging (MRI)

It is a diagnostic technique used in medicine. It can provide image of inside of a patient and it does not rely on exposing the patient to ionizing radiation such as X-rays. It is also known as nuclear magnetic resonance imaging.

The principle of use of MRI are;

  • Strong uniform magnetic field is applied
  • Radio frequency pulse is applied, at the Larmor frequency, causing resonance
  • Non-uniform field superimposed on uniform field.
  • Allows for the position of detection to be changed.

The function of non-uniform magnetic field, superimposed on the large constant magnetic field, in diagnosis using MRI:

  1. Strong uniform magnetic field aligns the nuclei and gives rise to Larmor frequency in the r.f. region
  2. Non-uniform magnetic field enables the nuclei to be located or changes the Larmor frequency.