Unit 20: Medical Physics — Short Questions

Gamma radiation is produced. The radioactive tracer injected into the body emits positrons as it decays; each positron travels a short distance and annihilates with a nearby electron, and this annihilation converts their mass into two gamma-ray photons travelling in opposite directions, which are detected by the ring of detectors.

The piezoelectric effect. When an electric field is applied to a piezoelectric crystal (e.g. quartz or PZT) in the transducer, it changes shape slightly — this converse piezoelectric effect generates the mechanical vibrations that become ultrasound waves.

The abdomen (liver, kidneys, gallbladder, pancreas, bladder) is commonly examined using ultrasound. (The fetus during pregnancy, in obstetrics, is another valid example.)

Different tissues have different densities and thicknesses, so they absorb (attenuate) X-rays by different amounts, following I = I₀e⁻^(μx), where μ is the attenuation coefficient. This difference in absorbed/transmitted intensity between tissues is what produces contrast in the image.

A standard X-ray gives a single flat, two-dimensional image in which organs and bones overlap each other, while a CT scan combines many X-ray images taken from different angles into cross-sectional slices and stacks these to build a detailed three-dimensional image.

Part of the ultrasound wave is reflected back as an echo because the two tissues have different acoustic impedances (determined by their density and the speed of sound in them); the remainder of the wave continues into the second tissue.

CT stands for Computed Tomography — "Computed" because a computer processes the data, and "Tomography" from the Greek tomos meaning slice. It improves imaging by combining many X-ray images of the same cross-section taken from different angles into a detailed 2D slice, then repeating this along the body and stacking the slices to build a 3D image, removing the overlap seen in a single plain X-ray.

Air has a very different acoustic impedance from soft tissue. At a tissue–air boundary this large impedance mismatch causes almost all of the ultrasound wave to be reflected, so very little sound energy penetrates into or beyond the air-filled region, and no useful echo pattern can be built up to form an image of structures like the lungs.

When two tissues attenuate X-rays by almost the same amount, their boundary cannot be clearly distinguished in the image — the structures appear blended together with poor visibility. The solution is to use a contrast medium, such as iodine (Z = 53) or barium (Z = 56), injected or swallowed; because of their high atomic number these substances absorb X-rays strongly, increasing the difference in attenuation between the structure of interest and surrounding tissue and making it stand out clearly.

In a plain X-ray, all structures along the beam path are projected onto a single flat image, so organs at different depths overlap. In CT, the X-ray tube rotates around the patient, capturing many images of the same cross-section from different angles; a computer combines these to reconstruct a single cross-sectional slice showing structures at their correct depth without overlap, and this is repeated along the body to build up a full 3D picture.

Since brighter regions of a PET image correspond to higher tracer uptake and therefore higher tissue activity, an area of unusually high concentration could indicate a tumour or cancerous growth with elevated metabolic activity, while an area of unusually low concentration could indicate damaged or underactive brain tissue, for example following a stroke or in a degenerative condition.