Medical

Question 1

What’s an X-ray tube

Answer 1

A glass tube w/ an electric circuit, w/ a cathode (e emitted via hot filament) + anode (target metal which e targeted to)

Question 2

Why are X-ray tubes surrounded by lead?

Answer 2

To keep x rays contained

Question 3

Why is the X-ray tube evacuated

Answer 3

To prevent e from colliding w/gas particles

Question 4

How is the filament heated?

Answer 4

Running I through it (different I from in anode)

Question 5

Why is the cathode filament contained in cup-shaped housing?

Answer 5

Focuses the beam of e onto target metal (tungsten)

Question 6

What happens when e meets anode?

Answer 6

Decelerate + some eK -> electromagnetic E in form of X-ray photons

Question 7

Why is tungsten rotated + mounted on copper

Answer 7

To avoid overheating via spreading of heat

Copper conducts heat away

Question 8

How else are x-rays emitted?

Answer 8

When outer e (of tungsten) fill vacancies of an inner e that had been knocked out by the beam of e

Question 9

What are the 2 ways to increase intensity of X-ray beam

Answer 9

1) Increase tube V = increases eK = more E able to be converted to protons
2) increase I to filament = frees more e per sec from cathode

Question 10

What are the 4 ways x-rays are attenuated?

Answer 10

1) simple scattering - Low E P = elastically deflected
2) the photoelectric effect - 30keV P = absorbed be e -> e ejected from atom. e from outer shell -> fills vacancies -> photon emitted
3) Compton effect - 0.5-5MeV loses E via interaction w/ e in an atom. e -> ejected + P deflected
4) pair production - > 1.1MeV -> decays into e and positron

Question 11

What affects the amount of E absorbed by a matter?

Answer 11

Atomic number

Question 12

How can you distinguish between tissues of similar attenuation coefficient?

Answer 12

Artificial contrast media
E.g. barium meals or iodine injected into blood
Both = high atomic number = shows up more

Question 13

CAT scan

Computed axial tomography

Answer 13

• 2-D image
• Thin x-rag beams in shape of fan = Emitted by rotation of tube around the body
• detectors move along the body
• Computer produces images of slices of the body

Question 14

Advantages of CAT over normal X-Ray scans

Answer 14

• more detailed image

* can produce 3D images

Question 15

Medical tracers

Answer 15

• = radioactive substances = show function and structure of tissues/organs
▪= F-18/technetium-99m bound to a bodily substance (e.g. H2O/glucose) -> allows specific location
▪Injected/swallowed
▪Radiation is emitted – detected via gamma camera /PET scanner = produces image

Question 16

Uses of medical tracers

Answer 16

1) Identifies damaged tissue in heart via areas w/decreased blood flow (e.g. coronary artery disease)
2) Identifies cancer tumours – cancers cells have high metabolism = uses more tracer
3) Identifies blood flow & activity in brain – aids research in mental illness

Question 17

Why technetium-99m

Answer 17

Technetium-99m – Gamma Radiation that it emits = passes through body easily + half life = six hours (long enough for data to be to recorded and short enough that patient exposure is acceptable).
• decays -> much more stable isotope

Question 18

Why F-18?

Answer 18

▪F-18 – undergoes beta-plus decay = necessary of PET scan. Short half life(110 minutes) = Patient exposure to radioactivity limited

Question 19

5 main parts to Gamma cameras

Answer 19

1) Lead shield – stops radiation from other sources being detected.
2) Lead collimator – a piece of lead with thousand of vertical holes = allow only gamma rays travelling to pass through.
3) Scintillator – a NaI crystal that emits a flash of light whenever a gamma ray emits it
4) Photomultiplier tube – detects the flashes of light from the scintillator and converts them to pulses of electricity
5) Electronic circuits – collect signals from the tubes and sends them to the computer to process into an image

Question 20

Benefits of gamma cameras

Answer 20

1) allows diagnosis without surgery
2) cheaper than PET scan( still expensive)

BUT they require exposure to ionising radiation

Question 21

How do Gamma cameras make an image?

Answer 21

computer uses distribution of gamma ray emissions -> create 2D image ( i.e. an image showing where the medical tracer has been taken up)

gamma camera is often rotated = multiple images from different angles = better image

Question 22

PET scans

Answer 22

• A Positron emitting radiotracer emitted into body
• Positron emitted + e in organ annihilates = 2 high E gamma rays emitted in opposites directions.
• patient surrounded by ring of gamma detectors
• 3D image is created
• arrival times/delay times of photons are used to pinpoint areas of increased activity

Question 23

Adv + disad of PET

Answer 23

• Brain activity can be investigated
▪Radiotracers have short half life do exposure to radiation is limited

▪Short half lives = limited time can be scanned ( unlike gamma cameras)
▪Extremely expensive

Question 24

What happens when an ultrasound meets a boundary between 2 materials ?

Answer 24

Partially reflected + partially transmitted (undergoing refraction if i doesn’t = 0

Question 25

Most common frequencies used in medical ultrasound?

Answer 25

1-15MHz

Question 26

How is ultrasound used to determine the speed of the blood in the artery?

Answer 26

• transducer placed at an angle to the artery
• ultrasounds are reflected by moving blood cells
• Hz/wavelength of US is changed
• ΔHz/Δwavelength is related to speed by eqt…

Question 27

Describe the principles of ultrasound scanning

Answer 27

• piezoelectric transducer sends PUSLES of US into patient
• waves are reflected back
• Intensity of reflected wave depends on acoustic impedances + used to identify tissues
• delay time determines depth

Question 28

How does having similar acoustic impedance help

Answer 28

Allows maximum transmission of ultrasound/Reduces reflection of waves

Question 29

Difference between A scan and B scan

Answer 29

A scan is in one direction only and determines depth/distance

B-scan uses number of sensors in diff positions to build up 2D/3D image

Question 30

Piezoelectric effect

Answer 30

The application of a PD across a material when deformed

Question 31

Principles of B scan

Answer 31

1) US reflected at boundary
2) B scan takes place in many direction
3) I of reflected US depends on acoustic impedance between materials

Question 32

Describe how the piezoelectric effect = emit and receive us

Answer 32

Emission: alternating emf -> crystal resonates/contracts and expands -> emissions

Reception: US causes resonance