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Y2 Nuclear Medicine - MPY8009 > Advanced Gamma Camera > Flashcards

Advanced Gamma Camera Flashcards

1
Q

What is the order of gamma camera components from closest to the patients?

A
  1. Collimator
  2. Scintillation crystal
  3. Light guide
  4. Photomultiplier tubes (PMTs)
  5. Pre-Amplifiers
  6. Analogue to Digital Conventers (ADCs)
  7. Anger arithmetic (to give position and energy signals)
  8. Pulse Height Analyser (PHA)
  9. Lead shielding
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2
Q

What risk does the weight of detectors pose to the gantry system?

A
  • The weight of detectors can cause sagging or drooping, requiring regular maintenance and periodic checks for mechanical performance.
  • The detectors are suspended above the patient during imaging, and so any mechanical defects can have fatal consequences.
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3
Q

What are the components of the imaging couch?

A
  • Made of low attenuating material
  • Curved with removable head rests, arm supports and pillows for patient “comfort”
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4
Q

What are the key characteristics of collimators?

A
  • Series of holes drilled/cast in lead or tunsten (highly attenuating material)
  • Septa are the walls between the holes
  • Designed to limit the directions from which the photons arrive at the crystal
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5
Q

What are the six different types of collimators?

A
  1. Parrallel hole
  2. Pinhole
  3. Slant hole
  4. Fan beam
  5. Diverging
  6. Converging
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6
Q

How does the size of the collimator hole effect image quality?

A
  • Size of holes denotes spatial resolution.
  • Small holes for high resolution collimators.
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7
Q

What is the effect of changing the thickness of septa?

A
  • Thickness of septa defines the septal penetration.
  • Relates to the energy of emission that collimators can be used with.
  • Thick septa for high energy collimators.
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8
Q

What is the effect of changing the length of the holes?

A
  • Length of the hole also has a big effect on the septal penetration.
  • Longer septa will provide improved resolution with distance and enable reduced septal thickness BUT they have reduced sensitivity.
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9
Q

What is the cause of star artefacts?

A

Caused by septal penetration.

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10
Q

What is the small angle approximation for septal penetration?

A

P = e^(-μ x T) x (D/2L)^2

  • T is the septal thickness
  • L is the length of hole
  • D is the hole diameter
  • μ is the attentuation coefficent of lead (23.0 cm^-1) for 140 keV photons
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11
Q

What is the equation for septal thickness?

A

T = 2DW/(L-W)

  • T is the septal thickness
  • L is the length of hole
  • D is the hole diameter
  • W is the actual path length through the septa
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12
Q

How is septal penetration calculated and what is an “acceptable” level of septal penetration used for collimator?

A

I/I_0 = e^(-μx)

  • Where 𝜇 is the attenuation coefficient (e.g. for 140 keV photons in lead) and 𝑥 is the thickness of lead the photon travels through.
  • An “acceptable” level of septal penetration (SP) often used for collimators is 5%
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13
Q

How can you design a collimator with SP < 0.05?

A
  • Consider the minimum distance a photon can travel completely through one septa without travelling through another (W)
  • SP = e^(-μW) = 0.05
  • -μW = ln(0.05)
  • W > 3/μ
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14
Q

What is the equation for the parallel hole collimator geometric resolution (mm)?

A

R_G = D +D/L(Z+G)

  • D is the hole diameter
  • T is the spetal thcikness
  • G is the distance form collimator to imaging plane
  • L is the hole length
  • Z is distance from point source to collimator
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15
Q

What is the definition of the geometric resolution?

A

The full width half maximum of the point spread function detected in the imaging plane.

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16
Q

What is the system spatial resolution (R_s)?

A

The overall spatial resolution of the system, which takes into account the geometric capability of the collimator, the effect of scatter from the patient and the intrinsic resolution at the detector level.

(R_s)^2 = (R_G)^2 + (R_I)^2

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17
Q

What is the intrinsic resolution (R_I)?

A

It is a measure of the spatial resolution at the detector level, without collimators.

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18
Q

What is R_sc?

A

The degradation of resolution due to scatter. While scatter has a significant effect on image contrast, the effect on resolution tends to be small and so this term can usually be ignored.

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19
Q

What is the definition and units of sensitivity?

A

Sensitivity is a measure of the camera efficiency in cps/MBq – i.e. how many gamma rays are detected vs how many disintegrations per second occur.

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20
Q

What is the parrallel hole collimator sensitivity?

A

The product of the fraction of gamma rays that pass through the central hole and the number of holes that fall within the FWHM of the point spread function

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21
Q

How is sensitivity increased?

A
  • Larger hole diameter
  • Thinner septal walls
  • Shorter collimator holes
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22
Q

What is the equation for the geometric efficiency (%)?

A

η_G = k (D/L)^2 x (1+T/D)^-2x 100%

  • Where “k” is the collimator arrangement constant, usually k =0.063 for hexagonal holes in a close packing arrangement.
  • D is the hole diameter
  • T is the septal thickness
  • L is the length of teh holes
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23
Q

What is the equation for sensitivity (cps/MBq)?

A

sensitivity = η_crystal x η_G x 10000

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24
Q

What are the purposes of pinhole collimators?

A
  • Magnify the object into large planar image
  • The image is inverted as well as magnified
  • Usually used for imaging small organs i.e. thyroid
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25
Q

What is the magnification equation for pinhole collimators?

A

M = (G+L)/Z

  • G is the distance from collimator to the imaging plane
  • L is the hole length
  • Z is the distance from object to pinhole aperture
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26
Q

What is the geometric resolution of the image for a pinhole collimators?

A

R_G = (D x (G+L+Z))/Z

  • G is the distance from collimator to the imaging plane
  • L is the hole length
  • Z is the distance from object to pinhole aperture
  • D is the hole diameter
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27
Q

What is the geometric resolution of the object for a pinhole collimators?

A

The effective resolution is the geomeric resolution divided by the magnification:

R_G(effective) = D + D/(L+G) x Z

  • G is the distance from collimator to the imaging plane
  • L is the hole length
  • Z is the distance from object to pinhole aperture
  • D is the hole diameter
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28
Q

What is the pinhole efficiency equation?

A

η_G = 1/16 x (D/Z)^2

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29
Q

What are the ideal characteristics of a scintillation detector?

A
  • High stopping efficiency
  • High probability of photoelectric effect (not Compton effect)
  • High conversion efficiency of gamma rays to light photons
  • Transparent to own emissions
  • Wavelength of emissions matched to PMT response
  • Short scintillation time
  • Mechanically robust
30
Q

What is the scintillation process?

A
  • Gamma ray interacts with the crystal and gives up some (CE) or all (PE) of its energy
  • Creating a secondary electron which then causes ionisation in the crystal
  • When an atom is ionised, an electron is excited from the valence band into the conduction band
  • If a bound electron–hole pair migrates through the crystal to an activation centre and falls back down to the valence band via a radiative transition
  • The energy is emitted as a scintillation (visible light) photon
31
Q

What is the effect of doping NaI crystal with thallium?

A

Provides more activation centres and so increases the likelihood of radiative transmissions

32
Q

What factors affect the number of photons detected?

A
  • Scatter and aabsorption of photons in the patient, by the couch/positioning materials
  • Geometric efficiency of the collimator
  • Crystal scintillation efficiency
  • Light guide losses
  • Photomultiplier tubes - low quantum efficiency
  • Electronic noise and signal losses/degradation while signal is processed through preamplifiers and pulse height analyser/digitiser
33
Q

What is the purpose of the light guide?

A
  • The light guide is the interface between the crystal and the photomultipliers, with the same refractive index of the crystal so that internal reflection is minimised.
34
Q

What is the effect of a thicker light guide?

A
  • A thicker light guide will improve the uniformity of the image, but would also make the spatial resolution worse (because the gap, G, between collimator and image plane will be increased).
  • Most modern cameras will use a thin crystal and thin light guide to get the best possible resolution; choosing to compensate for losses in sensitivity and uniformity elsewhere (e.g. using software corrections).
35
Q

What is the purpose of photomultiplier tubes?

A
  • Converts the light photpn into a photoelectron,each dynode multiplies the number of electrons and amplifies the electronic signal
  • They are covered in optical grease to couple them directly to the light guide
36
Q

How is a digital signal created?

A
  • Each photomultiplier tube has a pre-amplifier and an individual analogue to digital converter (ADC)
  • Pre-amplifiers convert the small charge pulse generated by the photomultiplier into a measurable voltage pulse.
  • Analog to digital converters digitise the signal from each PMT
37
Q

What is the purpose of a pulse height analyser?

A
  • A pulse height analyser (PHA) is used to discriminate between direct interactions and scattered events.
  • The PHA produces a pulse-height histogram, which shows the number of events at each energy
38
Q

How is the pulse arithmetic used to determine the position of the signal?

Write out the equation

A
  • Pulse arithmetic in gamma cameras involves determining the interaction position of a gamma ray (X) using signals from photomultiplier tubes (PMTs)
  • Each PMT at a known location (x_1, x_2, x_3, …) generates a signal (V_1, V_2, V_3, …), where the largest signal indicates the PMT closest to the event
  • The position (X) is calculated as the weighted sum of the PMT signals (V_n . x_n) divided by the total sum of signals ( E = ∑V_n)
39
Q

What is the purpose of the photomultiplier tube gain?

A

The gain (amplification) across each PMT will determine the size of the energy signal (pulse) from each detected event.

40
Q

How is the photomultiplier tube gain stabilised?

A
  • The gain of the PMTs can be stabilised by exposing the PMT to a light source and compared to a fixed reference voltage
  • If the output is not equal to the reference voltage, the gain is adjusted
  • This optimises the PMT gain and can prevent gain fluctuation caused by changes in temperature, high voltage, magnetic field and by ageing PMTs.
41
Q

How is the non-uniform output of a PMT imporved?

A
  • Use hexagonal PMTs to minimise gaps between PMTs
  • Light guide to artifically decrease the efficiency at the centre of each photocathode
42
Q

What are the causes of non-uniformities in gamma camera images?

A

Non-uniformities in gamma camera images can be caused by:

  • Non-uniform light output of the crystal,
  • Non-uniform light transmission,
  • Light collection variation with position (e.g., light loss at edges of the PMT and crystal),
  • PMT efficiency variation across the tube face.
43
Q

How is energy correction performed in gamma cameras?

A

Energy correction involves:

  • Irradiating the detector with a mono-energetic source.
  • Comparing the photopeak pulse height at each X,Y location to the mean photopeak.
  • Adjusting the signal for each pixel, similar to tuning/peaking PMTs but applied per pixel, not per PMT.
44
Q

What is a linearity correction matrix in gamma cameras used for?

A

A linearity correction matrix is used to correct for spatial non-linearities in the initial acquisition of gamma camera images.

45
Q

How is the linearity correction matrix generated?

A

The matrix is generated by acquiring an (un-collimated) image of parallel lines in each direction, then analyzing the image to measure deviations of each line from a perfect straight line.

46
Q

How does the linearity correction affect detected counts?

A

The linearity correction does not change the number of detected counts; it applies pixel-by-pixel corrections to reposition them accurately.

47
Q

What is sensitivity correction in gamma cameras?

A

Sensitivity correction, often called a “uniformity map,” involves uniformly irradiating the detector and scaling up or down the counts in each pixel to produce a uniform image.

48
Q

What assumption do energy and linearity corrections make about the depth of interaction in the crystal?

A

Energy and linearity corrections assume a fixed depth of interaction in the crystal.

49
Q

How does the actual depth of interaction in the crystal affect linearity?

A

If the interaction happens further from the PMT (shallow interaction), the light spreads over a wider range of tubes, resulting in worse linearity for deep interactions.

50
Q

What are the different acquisition types?

A
  • Static
  • Wholebody
  • Dynamic
  • Gated
  • Single Photon Emission Computed Tomography (SPECT)
  • Frame Mode
  • List Mode
51
Q

What is the difference between the frame mode versus list mode?

A

Frame Mode:
* The image parameters are pre-defined and stored as an imaging protocol

List Mode:
* Data stored as X, Y, Z and t
* Flexible
* Greater post-processing time
* No check imagr
* Can rebin/frame

52
Q

How is a step and shoot whole body scan aquired?

A
  • Series of static acquisitions
  • Time-per-view pre-selected
  • Overlapping to minimise edge effects
  • Digitally “Stitched” together
  • Can introduce stitching artefacts
53
Q

How is a scanning whole body aquired?

A
  • Acquired as a single sweep
  • Scan speed pre-selected
  • Counts stored in correct position of matrix for camera position
  • Electronic “shutter” to correct exposure times
54
Q

What is gated imaging?

A

Several frames acquired covering the cardiac cycle (typically 16 bins)
Acquired over many cycles / beats
Summed together to give individual images for each stage of the cycle

55
Q

What is automated planar motion correction?

A
  • Acquire dynamic sequence
  • Compress (sum) each frame along rows and columns
  • Stack resultant line images and estimate x and y motion from deviations in position of maximum intensity (often easier if images are pre-filtered)
  • Translate each image frame according to the displacement co-ordinates
  • Sum the translated frames to get resultant static
56
Q

How do you do planar dynamic motion correction?

A

If you’re expecting motion (i.e., young children) but only want a static image, it is possible to acquire a dynamic sequence, motion correct and then sum all frames

57
Q

How do you do automated motion correction for SPECT?

A
  • Sinograms and Linograms reviewed for discontinuities that indicate there has been motion
  • The shift required to re-align is then calculated and projections translated
58
Q

What are the limitations of motion correction?

A

Spatial resolution degraded by need to interpolate to perform the translations
Motion parallel to detector face can be estimated
Out of plane motion cannot be accurately corrected
Motion within a frame cannot be corrected
Changes in distribution can confound correction algorithms

59
Q

What are the advantages of SPECT/CT?

A
  1. 3D localisation
  2. Improved contrast
  3. Improved quantification
  4. Gives patient specific maps for AC
  5. Can correct for some causes of image quality degredation:
    * Attenuation
    * Scatter
    * Collimator Detector Response (CDR)
60
Q

What are the disadvantges of planar imaging?

A
  • 2D representation of 3D Distribution of activity
  • No depth information
  • Structures at different depths are superimposed
  • Loss of contrast
61
Q

How is a SPECT image acquired?

A

Camera rotates around a set axis
Planar images (projections) acquired at regular angles around patient

62
Q

What is a radon transfom?

A
  • The integration of 2D sice thorugh 3D object to give 1D profile
  • In SPECT it can be a row of pixels in projection

n(s, θ) = ∬f(x, y) δ(xcosθ + ysinθ - s) dx dy
* n(s, θ) is the no of counts at point s along the profile aquired at angle θ from origin
* f(x, y) is the distribution of the tracer within the object

63
Q

What do you need fine angular sampling and what is the minimum angle?

A
  • To avoid streaking
  • Δθ <= d/(1/2D)
  • The rule of thumb is the the number of views = matrix size
64
Q

What are the steps to SPECT image reconstruction?

A
  1. Projection data is acquired by rotating gamma cameras around the patient, collecting counts from multiple angles.
  2. The projections are mathmatically described by the Radon Transform which maps the 2D spatial distribution into 1D projections
  3. Attenuation, detector response and scatter correction
  4. Use the inverse Radon Transform for reconstruction: Perform filtered back projection to transform 2D projection data into a 3D image matrix
  5. Apply filters (e.g., ramp, Butterworth, or Hanning filters) to reduce noise and improve image clarity. Filtering is often done during the back projection process.
65
Q

What is the effect of increasing the number of views?

A
  • More views > better reconstruction
  • 1/r blurring even with an infinite number of views
66
Q

What are the two common attenuation correction approaches for filterback projection?

A
  • Chang
  • Sorenson
67
Q

What are the cautions to be aware of when doing AC?

A
  • Inaccurate AC can introduce artefacts
  • Usually used without scatter correction
    μ values should be broad beam not narrow beam
  • Homogeneous attenuation OK for soft tissue and bone
  • Does not work for lungs and not OK for thorax
68
Q

What are the issues with iterative reconstruction?

A
  • Computationally Intensive
  • Long Reconstruction Times
  • Requires fast computers for reconstruction
  • Each iteration takes twice as long as FBP reconstruction – typically 16 iterations
69
Q

What are the most common iterative reconstuction algorithms?

A
  • MLEM
  • OSEM
70
Q

What are the limitations of image quality?

A
  • Spatial resolution
  • Image Noise
  • Non-uniformities in camera response
  • Attenuation and scatter
  • Technical errors or scanning difficulties
71
Q

What are the different causes of image artefacts?

A
  • Pharmaceutical; Labelling problems
  • Equipment; Image non-uniformity, COR errors
  • Patient; Attenuation, Movement, Containation, Preparation Non-compliance
  • Operator; Extravasation, External attenuation, Aquisition erros, Display errors
72
Q

What are the causes of statistical variation?

A
  • Radioactive decay
  • Number of scintillation photons in crystal
  • Number of electrons at photocathode and dynodes

Y2 Nuclear Medicine - MPY8009 (11 decks)

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