intro

Question 1

purpose of collimator

Answer 1

sharp image by only accepting gamma rays aligned with holes

allowing each point in the image to correspond to a point within the source

Question 2

what ensures only gamma rays from the patient are detected

Answer 2

lead housing

Question 3

what affects sensitivity, resolution and magnification of the image

Answer 3

hole width, length and geometry

Question 4

what to the hole width, length and geometry affect

Answer 4

sensitivity, resolution and magnification

Question 5

what can collimator performance be characterised by

Answer 5

sensitivity
resolution

Question 6

what is the meaning of sensitivity

Answer 6

the number of counts for a stated activity

Question 6

what is the affect of pinhole collimators

Answer 6

magnifying the image

Question 7

purpose of scintillator crystal

Answer 7

to absorb the gamma radiation and convert into light photons

Question 8

what makes sodium iodide an efficient absorber of gamma rays

Answer 8

its density and atomic number

Question 9

what is the signal collected by a sodium iodide crystal proportional to

Answer 9

the energy deposited within the crystal

Question 10

what gives sodium iodide a good energy resolution

Answer 10

high output of photons per KeV at room temperature

Question 11

what is the lifetime of the excited state of the sodium iodide crystal and what does it mean

Answer 11

0.23 micro s - allows high count-rate

because only detects one gamma photon at a time

Question 12

what does the transparency of sodium iodide allow

Answer 12

large detectors can be constructed without significant light loss

Question 13

what are two disadvantages of sodium iodide as scintillator

Answer 13

1. fragile
2. hygroscopic (attracts and holds water molecules)

Question 14

how to avoid damage to sodium iodide crystal

Answer 14

avoid mechanical and thermal stresses

Question 15

how to help with hygroscopic property and how does it limit lifetime

Answer 15

seal in an aluminium case
yellowing limits lifetime

Question 16

why is multiple compton scatter unlikely

Answer 16

because PE effect is more likely at low energies

Question 17

advantage of increasing crystal thickness

Answer 17

increased stopping efficiency

Question 18

disadvantage of increasing crystal thickness

Answer 18

decreased spatial resolution

Question 19

why does increasing thickness decrease spatial resolution (2)

Answer 19

more potential multiple scatters

and fewer photons for tubes nearest event

Question 20

what is a good feature of scintillator detector

Answer 20

sensitivity

Question 21

scintillation: what is proportional to the detected gamma photon’s energy

Answer 21

the number of light photons

Question 22

PM tubes: what is the energy of a photoelectron

Answer 22

(hv - w) eV

where v = frequency, w = work function

Question 23

PMT: definition of quantum efficiency

Answer 23

QE = no. photoelectrons / no. of incident photons

Question 24

PMT: what is typical quantum efficiency

Answer 24

20 - 30 %

Question 25

PMT: energy of photoelectron

Answer 25

= (hv-w) e