Option I: Medical physics

Question 1

Sate and explain that a change in observed loudness is response of the ear to a change in intensity:

Answer 1

• Different people can describe different intensity sounds as appearing to have the same loudness – the frequency of the sound is an important factor.
• Depends on the amount of energy sound waves bring to a unit area every second.
• Depends on the amplitude of the sound. A more intense sound (louder sound) must have a larger amplitude.
• The relationship between amplitude and intensity is true for all waves.

Question 2

State and explain that there is a logarithmic response of the ear to intensity:

Answer 2

• Sound intensity levels are measured on the decibel scale (dB).
• The decibel unit is one tenth of a base unit and is called bel (B).
• The human ear can respond to a huge range of different sound intensities.
• The decibel scale is logarithmic.
• The scale compares any given sound intensity with intensity at the threshold of hearing (the weakest sound that a person is able to hear)

Question 3

Intensity

Answer 3

is the amount of energy a sound wave bings to a unit area every second

Question 4

Intensity level

Answer 4

the amount of energy transmitted

Question 5

Attenuation coefficient

Answer 5

a constant that mathematically allows us to calculate the intensity of the X-rays given any thickness of material

Question 6

Half-value thickness

Answer 6

For a given energy X-rays and given material there will be a certain thickness that reduces the intensity of the X-ray by 50%

Question 7

Acoustic impedance

Answer 7

the product of the density of a substance and the speed of sound in that substance.

Question 8

Solve problems involving acoustic impedance

Answer 8

Z=ρc

• Very strong reflections take place when the boundary is between the two substances that have very different acoustic impedance. This can cause some difficulties.
• In order for the ultrasound to enter the body in the first place, there need to be no air gap between the probe and the patient’s skin.
• An air gap would cause almost all of the ultrasound to be reflected straight back. The transmission of ultrasound is achieved by putting a gel or oil between the probe and the skin.
• Very dense objects (such as bones) can cause a strong reflection and multiple images can be created. These need to be recognized and eliminated.

Question 9

Pulse oximetry

Answer 9

• Red and infrared laser light is shone through a thin part of a patient´s anatomy, e.g. fingertips
• The amount of absorbance of each wavelength will depend on many factors but a changing relative absorbance between the two wavelengths can be used to determine the ratio of blood cells and oxygen and those without oxygen.
• This allows the overall oxygen content of the blood to be determines without having to take a sample of blood.

Question 10

Endoscopes

Answer 10

• An endoscope provides the ability to look inside the human body without invasive surgery.
• A tube is inserted into the body. Within the tube there is a collection of optical fibers that allows illumination from a outside source to reach the end of the tube and illuminate the region under investigation.
• Reflected light is collected using a lens system and further optical fibres are used allow an image to be viewed.
• Many endoscopes include additional instruments including a laser.

Question 11

Laser as a scalpel and as a coagulator

Answer 11

• A laser focused on a small region can increase its temperature so high that cuts through tissue like a scalpel.
• The laser heating ensures the site id kept free of germs and thus the risk of infections is reduced.
• Also blood vessels and nerves are automatically sealed off.
• A defocused laser beam can stop bleeding by stimulating the blood to form a clot.

Question 12

Exposure

Answer 12

the amount of electric charge per unit mass, produced in a body due to ionizing radiation.

Question 13

Absorbed dose

Answer 13

the amount of radiation energy absorbed per unit mass; units are the gray (1Gy = 1 J kg-1)

Question 14

Quality factor (relative biological effectiveness)

Answer 14

a dimensionless factor that takes into account the fact that different radiations have different effects even when they deposit the same energy in a body.

Question 15

Dose equivalent

Answer 15

the product of absorbed dose times the quality factor for the radiation involved. It is measured in Sieverts (1Sv = 1 J kg-1)

Question 16

Physical half-life

Answer 16

the time after which the activity of a radioactive sample decrease by a factor of two.

Question 17

Biological half-life

Answer 17

the time needed for the activity in the body to be reduced by half natural bodily functions that physically remove the isotope from the body.

Question 18

Effective half-life

Answer 18

the time needed for the activity in the body to reduce to half taking into account both the decay of the isotope as well as its removal by natural bodily functions.

Question 19

Cobalt-60

Answer 19

Source of high-energy gamma rays used for radiotherapy. Long half-life so does not need to be replaced often.

Question 20

Technetium-99m

Answer 20

This is a gamma emitter with a short half-life that can be produced relatively easily. It can be used in different forms to study blood flow, liver function, and bone growth.

Question 21

Iodine-123

Answer 21

A gamma emitter that is readily taken up by the thyroid. It can also be used to study the function of the liver.

Question 22

Iodine-131

Answer 22

A gamma emitter that is sometimes used to treat cancer of the thyroid

Question 23

Xenon-133

Answer 23

A gamma emitter in gaseous form which can be inhaled and used to study lung function

Question 24

Solve problems involving particular diagnostic applications.

Answer 24

A possible procedure to determine the total red blood cell volume would be as follow:
•Sample of a patient´s blood taken
•The red blood cells in the sample are radioactively labelled with chromium-51
•This sample is re-injected into the patient
•Radioactively labelled blood is allowed to circulate and mix with the rest of the patient´s blood.
•After a suitable time interval, further blood samples are taken
•The radioactive levels of chromium-51 in the new samples are measured using gamma counter
•A comparison between the original sample and the new samples allows the dilution level to be calculated and hence the total volume can be calculated