Labs: Gamma Absorption, Nuc Medicine, Eye Optics

Question 1

What was the goal of the optics of the eye experiment?

Answer 1

to determine the

• accommodation power
• visual acuity
• receptor density
• blind spot size
• distance from the blind spot to the yellow spot

Question 2

What is the yellow spot?

Another name for it?

Answer 2

Macula Lutea

an small yellow area of high receptor density at the back of the retina

Question 3

What controls the power of the eye’s lens? How?

Answer 3

Ciliary muscles connect to the lens via suspensory ligaments

• When C muscles contract, ligaments relax and lens has higher curvature and greater power
• When C muscles relax, ligaments contract, and lens is flattened, decreasing power

Question 4

What is accommodation?

Answer 4

a change in the curvature of the lens in order to focus on objects at different distances

Question 5

What are near point, far point and accommodation power?

How is accommodation power calculated?

Answer 5

near point (O_p)- nearest focusable distance

far point (O_r)- farthest focusable distance

accommodation power (ΔD) - the difference between the far point and near point measured in diopters (1/m)

ΔD = 1/O_p - 1/O_r

Question 6

What is visual acuity?

How is it calculated?

Answer 6

Visual acuity (AKA resolution or visus) is a measure of the eye’s ability to distinguish between two points

• can be expressed as ratio of the normal *limiting angle of view *(1 minute) to the actual limiting angle of view in percents

1’ / α’ x 100%

Question 7

What factors influence visual acuity?

Answer 7

1. shape/reflectivity of the eye
2. diffraction
3. density of photoreceptors on the retina

Question 8

How does diffraction affect acuity?

Answer 8

• Due to light’s wave nature, light entering the eye is projected onto the retina in diffraction patterns called Airy disks
• these disks are not small sharp, points of light, but rather larger and more blurred and thus tend to sometimes spread out over multiple photoreceptors, decreasing resolution

Question 9

What is the limiting angle of view?

What is the normal limiting angle of view?

Answer 9

• the smallest angular view of two separated points that can be just distinguished
• normally one angular minute

Question 10

What is normal sight called and what characteristics of the eye allow it?

Answer 10

Emmetropia

an elastic lens and normally round eyeball

Question 11

What is the name of the condition when only close objects can be focused on?

What characteristics of the eye cause it?

What kind of lens corrects it?

Answer 11

Myopia or nearsightedness

• caused by lengthening of the eyeball
• divergent or negative lenses correct myopia

Question 12

What is the name of the condition which only allows far objects to be in focus?

What characteristic of the eye causes it?

What kinds of lens corrects it?

Answer 12

**Hyperopia **or farsightedness

• a **shortened **eyeball causes it
• a convergent **(positive)** lens corrects hyperopia

Question 13

How many arc minutes is in one degree?

Answer 13

**60 arc minutes **per degree

Question 14

Draw the reduced eye model.

What is the refractice index (n) of the eye in this model?

And the distance between the nodal point and the retina, where the image is formed?

And the radius of curvature between the nodal point and the eye surface?

Answer 14

refractive index (n) = 1.34

distance K to retina = 17 mm

r = 5.1 mm

Question 15

How can the actual limiting angle of view be calculated using data from a landolt broken ring measurement?

Answer 15

• take the inverse tangent of the size of the break in the ring over the distance from the eye to the ring (this gives you angle in degrees)
• convert to minutes by multiplying by 60

α = tan^-1r/d

remember tangent is opposite/adjacent

r is ring break size (opposite)

d is distance from eye to ring (adjacent)

Question 16

How can receptor density be calculated?

Answer 16

1. determine the size of the ring break image on the retina via: **a’ = 17a/x ** a’ = ring break image a = actual ring break size (0.4 mm) x = distance btwn eye and ring break
2. this image size is approx. the size of one dimension of the receptor so to get receptor density use: **r.d. = 1/(a’)² **for an answer in 1/mm² or recepters per mm²

Question 17

How can the blind spot size be determined?

Answer 17

Considering the reduced model of the eye:

o / O = i / I

In the case of this lab:

• object distance o is the distance at which the blind spot causes the dot to disappear/reappear
• object size O is the distance between the cross and dot (60 mm here)
• image distance i is 17 mm (nodal point to retina)
• image size I is the distance from the macula lutea to the near or far edge of the blind spot

1. Take two measurements for “o”: one for disappearance and reappearance of dot
2. Use these measurements to determine “I” for each, giving you the distance from the macula to the blind spot’s edges
3. Subtract smaller I from larger I to get size of blind spot

Question 18

What was the goal of the nuclear medicine experiment?

Answer 18

• Familiarization with the scintillation counter and its use
• Determining the optimum integral discriminator setting to use for maximum signal-to-noise ratio

Question 19

What does the scintillation counter do?

Answer 19

detects, counts and determines energy distribution of particles or photons of incident radiation

Question 20

What are the 3 parts of the scintillation counter?

Answer 20

1. Scintillator
2. Photomultiplier Tube (PMT)
3. Analysing/Counting Electronics

Question 21

What does the scintillator of a scintillation counter do?

How?

Answer 21

Using a Thallium-doped NaI crystal, the scintillator converts gamma photons into light flashes via the photoeffect.

• the incident gamma photon removes an electron from an atom of the crystal via photoeffect
• E_kin of removed electron dissipates via ionizations and excites scintillation material
• Scintillator material fluoresces
• Because scintillator crystal is transparent to the emitted fluorescence (in the case of NaI(Tl), this is blue light), these fluorescent flashses can enter the PMT

Question 22

What does the photomultiplier tube do and how?

Answer 22

PMT detects the individual scintillations and provides electric output pulses

1. photocathode between PMT and scintillator converts light flashes to electron flow via photoelectric effect
2. electrons are drawn toward 1st dynode in a series of 8-14 connected to increasing voltages
3. secondary emission multiplies the number of electrons by 3-4 at each diode (creating gain of 3-4¹⁰ electrons for a 10 dynode PMT)
4. multiplied electron current is collected by an anode

Question 23

What is secondary emission?

Answer 23

when a primary electron of high speed hits a metal surface and the surface emits 3-4 secondary electrons

Question 24

What does secondary electron emission (and thus PMT gain) depend on?

Answer 24

voltage of the anode

Question 25

What is noise and what are its sources in relation to the scintillation counter?

Answer 25

useless scintillation pulses coming from sources that are:

• external - background radiation from radioactive stain, furniture, equipment, etc.
• internal - noise from the electronics, mostly electrons emitted by the dynodes just due to their strong electric field

Question 26

What is the feature of the scintillation counter which filters out internal noise?

And how?

Answer 26

**discriminator circuit **or integral discriminator (ID)

• lets through only pulses of a high enough amplitude, ignoring lower ones

Question 27

What is the adjustable voltage level of the discriminator circuit called?

Answer 27

**discrimination level **or BASELINE

Question 28

What is the measure of the successfulness of detection called?

How is it calculated?

Answer 28

signal-to-noise ratio

N_s/N_n

N_s = # signal pulses

N_n = # noise pulses

Question 29

How is signal pulse number determined?

Answer 29

1. First a noise pulse number is recorded without the isotope (N_n)
2. Then a signal plus noise pulse number is recorded with the isotope (N_s+n)
3. Then the noise pulse number is subtracted from the signal plus noise pulse number

N_s = N_s+n - Nn

Question 30

In the experiment, how was the optimal discriminator level determined?

Answer 30

1. 3 signal plus noise measurements were taken at 9 discriminator levels (100-900) and averaged
2. 3 noise measurements (w/out isotope) were taken the same way
3. signal pulse numbers and signal-to-noise ratios were determined with this data

Question 31

What was graphed for the nuclear medicine experiment?

Answer 31

Signal-to-noise ratio (y-axis)

as a function of

Discriminator Level (U_d) in volts (x-axis)

• should look like a clear peak at the optimal discrimator level, with diminished s-to-n ratio values on either side

Question 32

Draw the scintillator crystal/PMT set-up.

Answer 32

looks like the image below

also be sure to note:

• photoelectric effect in the crystal and photocathode
• increasing voltages connected to dynode series

Question 33

What was the goal of the gamma absorption lab?

And how?

Answer 33

• measuring the attenuative capacity of different metals
• different thicknesses of Al, Pb and Fe were placed between the isotope and the scintillation detector
• the data was used to determine attenuation coefficients and half value thicknesses

Question 34

What is the photoeffect?

Answer 34

a gamma photon removes any electron from an atom and gives its complete energy to the removed electron

Question 35

What is Compton scatter?

Answer 35

• a gamma photon strikes an outer shell eectron of an atom
• energy is split between the removed “Compton electron” and a “Compton photon” of lower wavelength than the original gamma photon

Question 36

What is pair production?

Answer 36

• a gamma photon of at least 1.022 MeV interacts with an atom’s nucleus, producing an electron and positron
• positron goes on to merge with another electron of the absorptive matieral and two 511 KeV photons are formed
• higher energy process than Compton scatter and photoeffect

Question 37

What is elastic scatter?

Answer 37

a rare process in which gamma photons interact with an electron but only their direction changes

Question 38

What is the attenuation law equation?

Answer 38

J = J₀ * e^-μ*x

Question 39

What is μ in the attenuation law equation?

Answer 39

linear attenuation coefficient

• a constant based on the type and density of material as well as the nature of the absorbed radiation
• unit is 1/cm

Question 40

How are the linear attenuation coefficient and the mass attenuation coefficient different?

Answer 40

mass attenuation coefficient (μ_m) is determined by dividing the linear attenuation coeff. by the density of the material

• has a unit of cm²/g
• can be used in an altered attenuation law equation with surface density (X_m, g/cm² of surface area) replacing thickness (x)

Question 41

What is the graphed in the gamma absorption lab?

Answer 41

Four lines, one each for: lead, aluminum, iron and lead rubber

Scintillations Counted (y)

as a function of

Material Thickness (x)

• graphed on normal x-axis and logarithmic y-axis