Chapter 6.5 - Medical Imaging

Question 1

How an x-ray tube works

Answer 1

Electrons are emitted by the hot filament cathode, which is heated by a current. The electrons are directed at a tungsten anode and are accelerated across the p.d. and smash into it. When they collide, some of the electrons kinetic energy is converted into x rays.

Question 2

Maximum energy of an emitted x ray

Answer 2

W=QV

Energy is equal to the charge of an electron * the p.d. of the x ray tube

Question 3

The other way that x rays appear in an x ray tube

Answer 3

An x ray knocks a low energy electron out of the atom and a high energy electron will then take its place and emit its lost energy as an x ray

Question 4

Intensity of an x ray beam

Answer 4

Power per unit area

Question 5

X ray attenuation

Answer 5

When x rays pass through matter they are absorbed and scattered

Question 6

How are X rays attenuated (3)

Answer 6

Photoelectric effect - Photon absorbed by electron which is ejected and gap is filled by another electron which emits a photon
Compton scattering - A photon knocks an electron out of an atom which causes the photon to lose energy and be scattered
Pair production - High energy photon decays into an electron-positron pair

Question 7

How to distinguish materials with similar attenuation coefficients using x rays

Answer 7

Artificial contrast media with high attenuation coefficients are used (often swallowed)

Question 8

Examples of artificial contrast media (2)

Answer 8

• Barium

- Iodine

Question 9

How a CAT scan works

Answer 9

A x ray tube creates a fan shaped tube. This is then rotated around a patient and will therefore read a thin ‘slice’. The x-rays are detected by a ring of detectors and visualised with computer software

Question 10

Advantage of CAT scan over normal x-rays

Answer 10

It provides a much more accurate picture and can be used to build up a 3d image from which a doctor can observe and remove certain densities. They are also more sensitive to changes in density

Question 11

Collimated

Answer 11

When waves are made to be parallel

Question 12

Medical tracer

Answer 12

A radioactive substance that is injected/ingested into the patient

Question 13

Examples of medical tracers (2)

Answer 13

• Technetium-99m (Tc-99m)

- Fluorine-18

Question 14

How a gamma camera works

Answer 14

Gamma rays emitted by the tracer pass through the collimater, which is made of lead and stops photons that have been scattered from being detected. The photons then hit the scintillator, which is a large crystal of sodium iodide. This flashes whenever a photon hits it (it emits many photons of visible light). This visible light is then incident on an array of photomultiplier tubes which emit an electron for every photon via the photoelectric effect. These electrons cause a current which can be detected and stored and viewed on a computer

Question 15

Use of x rays vs medical tracers

Answer 15

x rays show the structure of organs whereas tracers show the function of organs

Question 16

How a PET scan works

Answer 16

A beta-plus tracer is injected into the body and allowed to move through it. The emitted positrons annihilate with electrons in the body and emit high frequency gamma radiation. This is then detected with a gamma camera. Detected photons are only counted if another photon is detected in the opposite direction at the same time

Question 17

Diagnosis using a PET scan

Answer 17

The medical tracer can be bound to, for instance, sugar. This will then build up in cells that are more active, such as cancer cells, and therefore these will appear brighter on the picture

Question 18

Ultrasound

Answer 18

Longitudinal wave with frequency > 20kHz

Question 19

How piezoelectric crystals work

Answer 19

They deform when a p.d. is put across them and they created a p.d. when they are deformed.

Question 20

How ultrasound is emitted and received

Answer 20

An alternating current is run through piezoelectric crystals causing them to extend and compress in quick succession, creating an ultrasound wave. This wave is then detected by other piezoelectric crystals which produce a p.d. when the ultrasound hits them. They do this because the ultrasound causes them to resonate.

Question 21

Thickness of the piezoelectric crystal

Answer 21

Half the wavelength of the ultrasound

Question 22

Ultrasound transducer

Answer 22

Device that emits and absorbs ultrasound

Question 23

Advantages of ultrasound (4)

Answer 23

• No hazards
• Can obtain real time images
• Cheap and portable
• Quick

Question 24

Disadvantages of ultrasound (3)

Answer 24

• Doesn’t penetrate bone
• Cannot pass through air spaces (e.g. lungs)
• Can’t give detail on solid masses

Question 25

Impedance matching and an example of it

Answer 25

The use of a coupling medium between the transducer and the body so that less ultrasound is reflected back. Example is gel used.

Question 26

A scan

Answer 26

A short pulse of ultrasound is sent into the body and the returning intensity is plotted against time on a CRO screen

Question 27

B scan

Answer 27

The reflected intensity maps to brightness in an image and a linear array of transducers can be used to build up a 2d image

Question 28

Acoustic impedance of a medium

Answer 28

Z = pc
where
p is density
c is the speed of sound in the material

Question 29

Reflection of ultrasound at a boundary

Answer 29

A function of the acoustic impedances given in formula book

Question 30

How you can measure the speed of blood flow using ultrasound

Answer 30

Using the doppler effect and given equation