Labs: Gamma Absorption, Nuc Medicine, Eye Optics Flashcards
What was the goal of the optics of the eye experiment?
to determine the
- accommodation power
- visual acuity
- receptor density
- blind spot size
- distance from the blind spot to the yellow spot
What is the yellow spot?
Another name for it?
Macula Lutea
an small yellow area of high receptor density at the back of the retina
What controls the power of the eye’s lens? How?
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
What is accommodation?
a change in the curvature of the lens in order to focus on objects at different distances
What are near point, far point and accommodation power?
How is accommodation power calculated?
near point (Op)- nearest focusable distance
far point (Or)- farthest focusable distance
accommodation power (ΔD) - the difference between the far point and near point measured in diopters (1/m)
ΔD = 1/Op - 1/Or
What is visual acuity?
How is it calculated?
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%
What factors influence visual acuity?
- shape/reflectivity of the eye
- diffraction
- density of photoreceptors on the retina
How does diffraction affect acuity?
- 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
What is the limiting angle of view?
What is the normal limiting angle of view?
- the smallest angular view of two separated points that can be just distinguished
- normally one angular minute
What is normal sight called and what characteristics of the eye allow it?
Emmetropia
an elastic lens and normally round eyeball
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?
Myopia or nearsightedness
- caused by lengthening of the eyeball
- divergent or negative lenses correct myopia
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?
**Hyperopia **or farsightedness
- a **shortened **eyeball causes it
- a convergent **(positive)** lens corrects hyperopia
How many arc minutes is in one degree?
**60 arc minutes **per degree
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?
refractive index (n) = 1.34
distance K to retina = 17 mm
r = 5.1 mm
How can the actual limiting angle of view be calculated using data from a landolt broken ring measurement?
- 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)
How can receptor density be calculated?
- 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
- 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
How can the blind spot size be determined?
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
- Take two measurements for “o”: one for disappearance and reappearance of dot
- Use these measurements to determine “I” for each, giving you the distance from the macula to the blind spot’s edges
- Subtract smaller I from larger I to get size of blind spot
What was the goal of the nuclear medicine experiment?
- Familiarization with the scintillation counter and its use
- Determining the optimum integral discriminator setting to use for maximum signal-to-noise ratio
What does the scintillation counter do?
detects, counts and determines energy distribution of particles or photons of incident radiation
What are the 3 parts of the scintillation counter?
- Scintillator
- Photomultiplier Tube (PMT)
- Analysing/Counting Electronics
What does the scintillator of a scintillation counter do?
How?
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
- Ekin 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
What does the photomultiplier tube do and how?
PMT detects the individual scintillations and provides electric output pulses
- photocathode between PMT and scintillator converts light flashes to electron flow via photoelectric effect
- electrons are drawn toward 1st dynode in a series of 8-14 connected to increasing voltages
- secondary emission multiplies the number of electrons by 3-4 at each diode (creating gain of 3-410 electrons for a 10 dynode PMT)
- multiplied electron current is collected by an anode
What is secondary emission?
when a primary electron of high speed hits a metal surface and the surface emits 3-4 secondary electrons
What does secondary electron emission (and thus PMT gain) depend on?
voltage of the anode
What is noise and what are its sources in relation to the scintillation counter?
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
What is the feature of the scintillation counter which filters out internal noise?
And how?
**discriminator circuit **or integral discriminator (ID)
- lets through only pulses of a high enough amplitude, ignoring lower ones
What is the adjustable voltage level of the discriminator circuit called?
**discrimination level **or BASELINE
What is the measure of the successfulness of detection called?
How is it calculated?
signal-to-noise ratio
Ns/Nn
Ns = # signal pulses
Nn = # noise pulses
How is signal pulse number determined?
- First a noise pulse number is recorded without the isotope (Nn)
- Then a signal plus noise pulse number is recorded with the isotope (Ns+n)
- Then the noise pulse number is subtracted from the signal plus noise pulse number
Ns = Ns+n - Nn
In the experiment, how was the optimal discriminator level determined?
- 3 signal plus noise measurements were taken at 9 discriminator levels (100-900) and averaged
- 3 noise measurements (w/out isotope) were taken the same way
- signal pulse numbers and signal-to-noise ratios were determined with this data
What was graphed for the nuclear medicine experiment?
Signal-to-noise ratio (y-axis)
as a function of
Discriminator Level (Ud) 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
Draw the scintillator crystal/PMT set-up.
looks like the image below
also be sure to note:
- photoelectric effect in the crystal and photocathode
- increasing voltages connected to dynode series
What was the goal of the gamma absorption lab?
And how?
- 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
What is the photoeffect?
a gamma photon removes any electron from an atom and gives its complete energy to the removed electron
What is Compton scatter?
- 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
What is pair production?
- 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
What is elastic scatter?
a rare process in which gamma photons interact with an electron but only their direction changes
What is the attenuation law equation?
J = J0 * e-μ*x
What is μ in the attenuation law equation?
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
How are the linear attenuation coefficient and the mass attenuation coefficient different?
mass attenuation coefficient (μm) 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 (Xm, g/cm2 of surface area) replacing thickness (x)
What is the graphed in the gamma absorption lab?
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
