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Y2 Nuclear Medicine - MPY8009 > Non-Imaging Nuclear Medicine > Flashcards

Non-Imaging Nuclear Medicine Flashcards

1
Q

What are the two methods for converting high energy electromagnetic radiation into a useable signal?

A
  1. Scintillation and PMT detection
  2. Solid state semiconductor
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2
Q

How do scintillator detectors work?

A
  • Scintillators detect radiation by the process of luminescence, which is the emission of light from a material in which electronic excitations have occurred.
  • Detectors made of solid materials with high atomic numbers are preferred, due to their high counting efficiency.
  • Inorganic scintillators fit these requirements well, among which NaI(Tl) is the best suited for gamma sample counters
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3
Q

What are the physical properties of NaI(TI) which make them advantageous for radiation detection?

A
  • High density and effectice atomic number makes it an efficient absorber of low and medium gamma rays
  • Signal is proportional to the nergy lost in the crytal so can be used for energy-selective counting
  • High yeild at room temperature giving adequate energy resolution
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4
Q

What are the physical properties of NaI(TI) which make them disadvantageous for radiation detection?

A
  • Fragile and can be fractured by mechanical pressure or temperature change
  • Hygroscopic, produces yellow discoloration when exposed to the atmosphere which attenuates the light output
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5
Q

What are the two main electromagnetic interactions in the crystal?

A
  • Photoelctric effect
  • Compton scattering
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6
Q

What does the pulse size spectrum show?

A

A spectrum as a result of different interaction process occuring in the crystal, with a photo peak which contains contributions from both photoelectric and compton processes.

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7
Q

How does a solid state detector work?

A
  • Crystaline structure with bandgaps of a few electron volts
  • Radiation absorbed causing ionisation which moves the bound electrons out of the valance band into the conduction band
  • Conduction electrons can flow freely
  • The holes left behiind behave as positive charge carriers
  • Electric field causes the free electrons and holes to drift in opposit directions
  • The number of electron-hole pairs formed is propotional to teh radiation energy which is proportional to the amplitude of the electric pulse generated
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8
Q

What type of detecter is used by a gamma counter?

A

Sodium Iodide Scintilation Detector

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9
Q

What are the clinical applications of gamma counters?

A
  • Glomerular filtration rate assessment
  • Blood and urine excretion rates
  • Quality control of radioactive compounds to assess binding efficiency
  • Wipe/leak testing of sealed sources and surfaces
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10
Q

What is the typical design for a gamma camera?

Draw the diagram

A
  • Well type variant
  • The sealed unit houses a sodium iodide crystal(s) and photomultiplier tube(s)
  • The detector is sheilded from external radiation sources
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11
Q

What is the gamma camera sensitivity dependant on?

A
  • Detector characterstics
  • Source geometry
  • Source Isotope
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12
Q

What formula is used to calculate sensitivity (S), describe the parameters in the equation?

A

S= C−B/A⋅e(−λΔt)

  • C is the gross sample count rate
  • B is the background count rate
  • A is the activity measured un kBq
  • λ is the decay constant for the isotope
  • Δt is the time between the activity measurement and the start of counting
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13
Q

What are the units for sensitivity?

A

Counts per minute (CPM) per kBq

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14
Q

In practice how do you measure sensitivty?

A
  • Start with a known high activity concentration with a similar energy to the samples that are usually counted allowing it to decay to an appropriate level
  • Measuremenst are repeated over the cours of a few days to establish the expected variation and tolerance for daily constancy measurements
  • These daily constancy of sensitivity measurements should be reviewed on a quarterly basis to look for any longer-term trends
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15
Q

How should the energy resolution and peak channel for the photopeak be measured?

A

The full width at half maximum of the photo peak using a reference source e.g. Co-57, Ge-68, I-129 or Cs-137

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16
Q

How are repeatability measurements made?

A
  • A single sample should be measured using the intended clinical protocol at least 20 times in succession
  • The repeatabilty is reported as the standard deviation or coefficient of variation of the measurement
  • To avoid issues with decay, a long-lived source can be used.
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17
Q

How can a chi-squared (χ²) test be used in repeatability testing, and what does a small p-value indicate?

A

The χ² goodness of fit test checks for instrument instabilities by comparing observed variations to a Poisson distribution. A small p-value (⩽0.05) indicates that random variations are unlikely to follow a Poisson distribution, suggesting other sources of random variation are present, and the null hypothesis is rejected.

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18
Q

What is the purpose of the count-rate performance (linearity) test?

A

The test evaluates count-rate performance across a range of activities and determines the upper limit of activities that can be counted before dead-time effects cause significant errors.

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19
Q

How is the count rate error determined in the linearity test?

A

The measured count rate is plotted as a function of activity concentration. The expected count rate is determined by extrapolating the linear least-squares fit from lower activities (where dead time is negligible) to higher activities, and the error is calculated from the difference between the measured and expected count rates.

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20
Q

How can errors due to sample volume effects be minimised in gamma counting?

A

Sample volumes and vials should be kept identical when counting matching samples. Sample volume effects can be assessed during commissioning to select a volume that balances efficiency and pipetting errors, particularly for well-type detectors.

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21
Q

What are the clinical applications of gamma probes?

A
  • Sentinal node localisation for breast, head and neck, vulvar and penile cancers
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22
Q

What is the primary function of a gamma probe system?

A

Gamma probe systems are based around a small detection probe sensitive to gamma photons, showing either the counts detected or the count rate in a given energy window.

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23
Q

How is the data from gamma probes processed and displayed?

A

Detection events generate electrical signals transmitted via Bluetooth or cable to a base unit, which displays count rate or total counts. Some systems process and display counts directly on the handheld probe with additional audible signals for instantaneous count rate.

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24
Q

How do gamma probe systems detect gamma rays?

Give examples of the types of gamma probes.

A

Gamma probe systems use a small sensitive detector at the end of a probe, which may be a small inorganic scintillator crystal or semiconductor-based detector.
E.g.
* Scintillation crystal (NaI(TI)/ CsI(Na)/ BGO)
* Seminconductor (CdTe/ CdZnTe)

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25
Q

What are the differences between semiconductor probes and inorganic crystal-based detectors in gamma probes?

A

Semiconductor probes have a smaller diameter, are sensitive to a narrower energy range (allowing for better scatter rejection), but have lower sensitivity for medium- and high-energy gamma radiation. Reusable probes use sterile sheaths for cleaning between patients.

26
Q

What are collimators in gamma probes, and why are they recommended?

A

Probes usually have removable collimators made of lead or tungsten. Clinically, it is recommended to always use an appropriate collimator depending on the application to improve spatial and angular resolution and scatter rejection.

27
Q

How do collimators affect probe performance?

A

Collimators reduce the apparent sensitivity of the probe but improve its ability to accurately localize areas of interest, increasing confidence that a maximum in detected counts indicates a region of increased activity.

28
Q

How do gamma probes compare to well-type gamma counters, and why are they suited for surgical use?

A

Gamma probes are less sensitive than well-type gamma counters due to their smaller detector size and reduced geometric efficiency. However, they are ideal for surgery because they are small, light, maneuverable, easy to clean, and feature an audible signal proportional to the count rate, enabling easy monitoring without needing to look at the display.

29
Q

What is squelch in a gamma probe?

A

Squelch effectively removes a set number of background counts from those detected. The user finds an area of background radiation and if at that point they activate the squelch function, this background count rate is then removed from the subsequent count rates detected. Useful for high background regions close to primary injection site.

30
Q

What energy window options do gamma probe systems typically offer, and what are the trade-offs of narrower energy windows?

A

Most systems provide manufacturer-defined selectable energy windows, and some allow user-defined windows. Narrower energy windows reduce counts from scattered and background radiation but may decrease sensitivity to true emissions depending on the probe’s energy resolution and the window’s width.

31
Q

How do gamma probe systems display gamma-ray detections, and how are these modes used?

A

Gamma probe systems display detections either as an instantaneous count rate or as the total integrated count over a set time period (typically 1–60 seconds, with ~10 seconds standard). Instantaneous count rate is used to locate nodes, while integrated mode is used to confirm radiation presence.

32
Q

What are common faults that can occur in gamma probes?

A

Faults may include a cracked detector crystal due to physical trauma or issues in the wiring and connectors linking the probe to the base unit.

33
Q

What are the different gamma probe tests and what are their frequency?

A
  • Cleaning/decontamination (Daily/weekly)
  • Power check and visual inspection (Prior to use)
  • Background checks (Commissioning or prior to each use)
  • Constancy of sensitivity and long-term stability (Tolerances set at commissioning)
  • Short-term stability (Commisioning/quarterly)
  • Energy window check (Commisioning/quarterly)
  • Sensitivity in air (Commisioning/quarterly)
  • Linearity (Commisioning/repair)
  • Sensitiviy in scatter (Commissioning)
  • Sheilding and background characterisation (Commissioning)
34
Q

What security is required for the gamma probe sealed source?

A
  • Lockable source store
  • Logbook to record access
  • Training for staff
  • Leak testing performed by the physics team
35
Q

What are the daily quality control tests?

A
  • Power, check the status of the battery charge before surgical use
  • Visual inspection: The probe casing, tip, and outer surfaces should be inspected for cracks or chips that could compromise sterility or indicate internal damage. The base unit, visual and audible displays, and all cabling and connectors should be checked for damage to ensure proper operation and maintain electrical safety.
  • Background check, the total counts should be very low and stable
36
Q

Why is measuring probe sensitivity the most important daily test?

A

It ensures sensitivity reproducibility over time and can indicate faults in subsystems such as detector damage, electrical gain changes, energy window drift, or electronics faults.

37
Q

How is probe sensitivity expressed and measured?

A

Sensitivity is expressed in counts per second per unit activity (e.g., cps MBq⁻¹) and is measured using a long-lived sealed source in a reproducible geometry. A fixed period of counting is used to measure a minimum of 1000 counts to give an acceptable inherent statistical variation. Its advisable to us eth esame count interval as is used clinically. Measured readings should not deviate by more that 2 SD from the expected value.

38
Q

Why is ⁵⁷Co commonly used for sensitivity testing of gamma probes?

A

⁵⁷Co has a long half-life (270 days) and emits gamma rays at 122 keV, which closely matches the photopeak energy of ⁹⁹ᵐTc (141 keV), making it suitable for probes optimised for detecting low-energy gamma emitters.

39
Q

How is the repeatability test performed?

A

Counts are recorded for a fixed time period (e.g., 10s) and repeated at least 20 times using a long-lived check source in a fixed geometry. The total counts collected in each counting period should be in excess of 1000.
The subsequent spread of total counts should lie within the 95% confidence interval.

40
Q

How is repeatability reported and how often should it be performed?

A
  • The repeatability is reported as the standard deviation or coefficient of variation of the measurements, and the chi-squared value reported.
  • This test should be performed quarterly or following any probe system repairs
41
Q

How is the spatial resolution in air gamma probe check performed?

A

The source (A 57Co or 22Na pen source, or a small 99mTc or 18F point source) position is fixed, and the probe is placed a given distance (e.g., 3 cm) from the source. Counts are acquired at a range of lateral distances from the probe’s central axis.

42
Q

What is done with the data from the spatial resolution in air gamma probe check?

A

The data is plotted to show the spatial resolution profile, and the interpolated full width at half maximum (FWHM) and full width at tenth maximum (FWTM) values are reported. The results should typically be within 10% of those measured at commissioning.

43
Q

How is the linearity check typically performed?

A

Start with a high-activity source (≥10 MBq) of 99mTc or 18F and repeat measurements as the source decays over ~10 or more half-lives. The fixed time count results should be plotted against activity, and linearity should be assessed using the correlation coefficient, with a typical tolerance level of r > 0.95.

44
Q

What alternative can be used if the decay-based method is impractical?

A

A series of different activity sources can be used, covering a sufficient activity range, with care taken to ensure consistent geometry.

45
Q

When does quenching occur?

A

When the energy emitted by the radioisotope is not caught entirely by the PMT of the counting instrument.

46
Q

What are the three different types of quenching and when do thy occur?

A
  • Physical Quench: the radioisotope is not coupled with the scintillator so doesnt reach the solvent part
  • Chemical Quench: when the energy of the beta particle is absorbed by compound thar will not re-emit the energy during teh transfer to the solvent molecules.
  • Color Quench: When the light emitted is absorbed by the color in the sample.
47
Q

What is dead time?

What is the dead time of NaI?

A

It is related to the time required to process individual detected events.

0.5-5 μs

48
Q

What is a non paralyzable system?

A

One for which if an event occurs during the dead time of a preceding event then the second event is simply ignored with no effect on subsequently occurring events.

49
Q

What is a paralyzable system?

A

One for which each event introduces a dead time whether or not that event actually was counted. Thus an event occurring during the dead time of a preceding event would not be counted but still would introduce its own dead time during which subsequent events would not be recorded.

50
Q

What is the equation for the observed counting rate for a paralyzable system?

A

Ro=Rt x e(-Rt x t)

  • Ro observed counting rate (cps)
  • Rt true counting rate (cps)
  • t dead time (tau)
51
Q

How are percentage dead time losses calculated?

When Rt x tau is small how are percentgae dead time losses calculated?

A

Percentage losses = (Rt - Ro/Rt) x 100

Percentage losses = (Rt x tau) x 100%

52
Q
  • How is the standard deviation
    σ of a radioactive sample calculated if the average count is n?
  • How is the mean of measurements expressed?
A
  • σ = √n(bar)
  • n(bar) ± σ
53
Q
  • What distribution law does radioactive decay follow?
  • How can the Poisson distribution be approximated if the number of measurements is large?
A
  • Radioactive decay follows the Poisson distribution law.
  • It can be approximated by a Gaussian distribution.
54
Q

What percentage of measurements fall within one standard deviation (n ± σ) of the mean in a Gaussian distribution?

A

68% of measurements fall within this range.

55
Q

What percentage of measurements fall within one standard deviation (n ± 2σ) of the mean in a Gaussian distribution?

A

95% of measurements fall within this range.

56
Q

What percentage of measurements fall within one standard deviation (n ± 3σ) of the mean in a Gaussian distribution?

A

99% of measurements fall within this range.

57
Q

How can a single count n of a radioactive sample be estimated when n is large?

A

It can be estimated as close to the mean n(bar), such that n = n(bar) and σ = √n

58
Q

How is the percent standard deviation calculated?

A

%σ = σ/n x 100 = 100/√n

59
Q

What is the equation for the standard deviation of count rates?

A

σc = σ/t = √c/t

60
Q

Write out the propagation of errors equations for addition, subtraction, multiplication and division

A
  1. σ(x+y or x-y) = √σx^2 + σy^2
  2. σ(xy) = xy √(σx/x)^2+ (σy/y)^2
  3. σ(x/y) = x/y √(σx/x)^2+ (σy/y)^2

Slide 85

61
Q

How is the variance adjusted when additional random errors (Δn) are present?

A

σ^2 = n + Δn^2

Y2 Nuclear Medicine - MPY8009 (11 decks)

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