Medical Imaging

Question 1

What are xrays?

Answer 1

electromagnetic waves with a wavelength in the range 10^-8 -10^-13m

Question 2

Which has greater energy, an x ray photon or a visible light photon

Answer 2

x ray

Question 3

How are x-rays produced?

Answer 3

fast moving electrons decelerated by the atoms of a metal and the kinetic energy of the electrons are transformed into x ray photons

Question 4

Describe the process of the formation of x ray photons:

Answer 4

• electrons fired through high pd
• travel from cathode to anode (cooled by water)
• 1% of kinetic energy of electrons transformed into x ray photons

Question 5

What are the 4 attenuation mechanisms?

Answer 5

simple scatter, photoelectric effect, compton scattering, pair production

Question 6

How does the simple scatter work?

Answer 6

x ray photons scattered elastically by an electron

Question 7

How does the photoelectric effect occur?

Answer 7

x ray photon disappears and removes an electron from the atom

Question 8

How does compton scattering work?

Answer 8

the x ray photon is scattered by an electron, energy decreases, then ejected from the atom

Question 9

How does pair production work?

Answer 9

x ray photon disappears to produce an electron positron pair

Question 10

What is the relationship between intensity of x rays and thickness of materials?

Answer 10

intensity of parallel beams of x rays decreases exponentially with the thickness of the material

Question 11

Why are x rays used for medical imaging?

Answer 11

the attenuation coefficient of bone is greater than that of soft tissue

Question 12

What is a contrast material?

Answer 12

a material ingested/ injected into a patient to improve visibility of soft tissues on xrays

Question 13

What is the main attenuation mechanism used in hospitals?

Answer 13

photoelectric effect

Question 14

What needs to be proportional for the photoelectric effect to work?

Answer 14

atomic number cubed and attenuation coefficient

Question 15

Why are barium and iodine used as contrast materials?

Answer 15

they have larger attenuation coefficients than soft tissue

Question 16

In what medical situations is barium used?

Answer 16

showing the digestive system in xrays

Question 17

In what medical situations is iodine used?

Answer 17

showing circulatory problems

Question 18

What is a CAT scan?

Answer 18

Computerised Axial Tomography: records cross sectional area scans

Question 19

How does a CAT scan work?

Answer 19

• patient lies horizontally in gantry ring
• fan shape of x rays to get cross sectional image
• detectors move down patient to produce 2D and 3D image

Question 20

What isotope is used in the gamma camera?

Answer 20

technetium 99m

Question 21

What is injected into a patient in order to show specific cells when using the gamma camera?

Answer 21

technetium 99m

Question 22

What is a collimator?

Answer 22

long thin cylindrical tubes made from lead

Question 23

How can gamma photons hit the scintillator?

Answer 23

travelling along the axis of the collimator

Question 24

What is a scintillator?

Answer 24

material that allows gamma photons to hit it and produce thousands of photons of visible light

Question 25

What are photomultiplier tubes?

Answer 25

electrical devices that produce electrical pulses whenever a photon of visible light is incident on it

Question 26

What do the computer and display of a gamma camera do?

Answer 26

pulses used to pin origin of the gamma photon and show the concentration of Tc99m

Question 27

What does PET stand for?

Answer 27

Positron Emission Tomography

Question 28

How do PET scans work?

Answer 28

use fluorodeoxyglucose which has radioactive atoms of fluorine 18, injected and gathers in tissues with high rates of respiration

Question 29

What is a disadvantage of PET scans?

Answer 29

expensive

Question 30

What can PET scans do that other scans can’t?

Answer 30

show functions of organs on site

Question 31

How do PET scans work?

Answer 31

• use fluorodeoxyglucose which has radioactive atoms and gathers in tissues with high rates of respiration
• computers determine the exact point of annihilation of FDG positron and electron from the arrival times of photons at detectors and the speed of the photons
• image of the FDG concentration levels displayed

Question 32

What is an ultrasound?

Answer 32

any sound wave with a frequency greater than 20kHz

Question 33

What is an ultrasound transducer?

Answer 33

device used to generate and receive ultrasound

Question 34

What is the piezoelectric effect?

Answer 34

when materials produce emf, or when pd applied, causing material to expand or compress

Question 35

What are common piezoelectric materials?

Answer 35

lead zirconate titanate and polyvinylidene fluoride

Question 36

How are ultrasounds produced?

Answer 36

piezoelectric material vibrates when high frequency AC are applied on opposite ends, the vibration in the air produces US

Question 37

How does an A scan work?

Answer 37

ultrasound partially reflected in boundaries between soft tissues

Question 38

How does a B scan work?

Answer 38

US transducer moved across patients, brightness of dot corresponds to the intensity of the pulse

Question 39

Define acoustic impedance:

Answer 39

product of its density and the speed of ultrasound in a material

Question 40

What does coupling gel do?

Answer 40

reduces reflection of ultrasound on skin as has a similar impedance to that of of skin (impedance matching/ acoustic matching)

Question 41

What is directly proportional to the speed of blood?

Answer 41

change in frequency

Question 42

What happens to low energy xrays?

Answer 42

absorbed by the skin