OCR A Physics A level Topic 6.5: Medical Imaging

Flashcards on Medical Imaging

How are X-rays produced?

X-rays are produced when charged particles are rapidly decelerated (or accelerated), transforming their kinetic energy into high-frequency photons of electromagnetic radiation.

What is the primary difference between gamma rays and X-rays?

The distinction lies in their origin; gamma rays result from radioactive decay or particle collisions, while X-rays are produced by Bremsstrahlung or braking radiation.

What is thermionic emission?

The process by which electrons are emitted from a heated source, such as a high-resistance coiled wire in a vacuum tube.

What is the purpose of the vacuum tube in X-ray production?

To prevent electrons from colliding with air molecules before they gain sufficient energy to emit X-rays.

Why is the anode often rotated or cooled in an X-ray tube?

To prevent the anode from overheating due to the thermal energy generated during electron collisions.

What is the function of a collimator in X-ray equipment?

A collimator is used to create a straight and parallel beam of X-rays, directing it at specific areas and minimizing exposure to other parts of the patient's anatomy.

Name four mechanisms by which X-rays are absorbed and beams are attenuated.

Simple Scattering, Photoelectric Effect, Compton Effect, and Pair Production.

How do contrast media enhance X-ray imaging?

Contrast media contain heavy atoms with high proton numbers, enhancing X-ray absorption and making specific tissues or blood vessels more visible.

What is a 'barium meal,' and how is it used in X-ray imaging?

A 'barium meal' is a white liquid mixture containing barium sulphate, used as a contrast medium to outline the intestines and locate blockages in the digestive system.

What are the advantages and disadvantages of CAT scans compared to conventional X-rays?

CAT scans offer a 3D image with greater resolution and the ability to distinguish soft tissues but expose the patient to a higher dose of radiation and take longer.

Why are gamma-emitters preferred for medical tracers?

Gamma-emitters are less ionizing and more penetrative, making them suitable for non-invasive diagnosis where emissions need to be detected from outside the body.

Why do radioisotopes used in medicine tend to have short half-lives?

To minimize the patient's exposure to harmful radiation and allow for quick imaging.

How does a gamma camera work?

It uses a collimator to direct photons towards a scintillation crystal, which emits visible photons. These photons are converted to electrons, amplified by photomultiplier tubes, and the position of impact is used to locate the emission site.

How does Positron Emission Tomography (PET) work?

PET uses positron-emitting tracers. The emitted positrons annihilate with electrons, producing gamma photons that are detected by a ring of gamma cameras. The arrival times are used to calculate the location of annihilation.

What is the key advantage of Ultrasound?

Ultrasound is a non-ionizing and non-invasive technique that is quick and affordable.

How does ultrasound work?

A voltage is applied to a piezoelectric crystal to produce ultrasound vibrations, returning the alternating voltage as the ultrasound is absorped.

How is the acoustic impedance of a medium defined?

Acoustic impedance (Z) is defined as the product of the medium's density (𝜌𝜌) and the speed of sound in that medium (𝑐𝑐): 𝑍𝑍 = 𝜌𝜌𝜌𝜌.

What is the Doppler Effect used for in medical imaging?

The Doppler Effect is used to measure the speed of blood flow non-invasively by analyzing the frequency shift of ultrasound waves reflected from moving blood.