Gamma Cameras
What does a photomultiplier tube do?
-Converts visible light (scinitillation) into an electronic pulse. -Electron amplification through a series of dynodes Overall gain 10^6
Spatial Linearity Correction
-Energy correction provides stability but does not improve uniformity -Events between PMTs are mis-positioned and contribute to poor uniformity -Corrections for mis-positioned events (Spatial linearity correction) produces improvement in uniformity
What are Gamma Camera Corrections are necessary to an imaging system?
-Energy corrections -Spatial linearity correction -Uniformity correction -Pulse pile up correction -PMT stablization
What are the current gamma camera parameters?
-Field Uniformity <5% -Intrinsic Spatial resolution (3.5-4.5 mm) -System spatial resolution (8-12 mm) -Energy resolution (9-10%) -Multi-window spatial registration <2 mm -Temporaral resolution 1-2 usec
What are advantages to NaI(Tl) crystal?
-High sensitivity for photons >85% at 140 keV -Moderate energy resolution 9-10% at 140 keV -Low cost relative to other scintillators (easy to maufacture)
What are disadvantages of NaI(Tl) crystal?
-Hygroscopic -Poor count rate performance relative to other scintillators (200 nanosecond scintillation decay time) -Sensitive to mechanical and thermal stresses
What is Intrinsic Spatial Resolution limited by?
-Multiple compton scattering interactions of gamma rays in the crystal -Statisitical ditribution of scintillation light photons among the photomultiplier tubes.
Why are energy corrections necessary?
-Non-uniformities in the crystal -Variations in light collection efficiency -Differeneces in PMT gains In a certain amount of scintillation light the response from one area of the detector to another will differ. To correct fo the differences the photopeak window is shifted depending on the local response (because the photopeak will be shifted locallY)
The total signals combined from all PMT's determine the energy of the scintillation event. The energy is used to what?
-Normalize the position signals -Determine wif the event falls within the discrimination setting (whether or not the event will be accepted or rejected)
A pulse height analyzer allows selectrion of energy range...
-Scatter discrimination -Multiple energy acquisition
What are two reasons PMT do not respond ideally?
-The response in front of the PMT is broader than ideal because the tube is not a point. -The response as the scintillation event moves further away is not leinear due to light reflections in the crystal.
What are two methods that the amount of adjustment a PMT needs is determined?
1. LED method 2. Split window method
What are two negative effects of pulse pile up?
1. Loss of counts 2. Image distortion/ resolution degradation
FWHM is proportional to what?
1/square root of E
What is the thickness of a medium energy collimator?
10 mm
What is the thickness of a high energy collimator?
11 mm
What percentage of the window should be centered around the photopeak?
15-20%
A pusle height analyzer typically has a window centered at what percentage of the photopeak?
15-20% window
How thick is a NaI(Tl) scintillator?
3/8 inch or 9.5mm
What is the typical amount of PMT in a gamma camera?
37-75 PMT 16-17" crystal
What is the electron gain at each dynode?
4
How large is a NaI(Tl) Scintiallator?
40x50 cm
What is the thickness of an ultra-high resolution collimator?
6.5 mm
CFOV (Central field of view) emcompasses what percentage of the detector?
75% only the inner portion
What is the thickness of a high resolution collimator?
8 mm
What is the thickness of a general purpose collimator?
9.5 mm
UFOV (Useful field of view) encompasses what percentage of the detector in intrinsic flood field uniformity?
95% the inner and outter except for remaining 5% that are the edges
What is MWSR a measure of?
A cameras ability to maintain the same image magnification regardless of the energy deposited in the crystal. -Magnification is the distance between two points in the image divided by their actual distance.
How does the Split Window method work for PMT stabilization?
A flood source is imaged an two narrow windows are set with energies above the photopeak to ensure no scatter is detected. PMT adjustment is based on the ration of counts in the two windows, which should be constant over time unless drift has occurred. The preamplifier on te PMT is adjusted until the expected ration is restored.
What does too much septal penetration lead to?
A loss of resolution and/ or contrast.
What is intrinsic flood field uniformity?
A quantitative measure of the uniformity of the detector.
What is an analog signal?
A signal that continuously varies and can take any possible value at a given time The signal from the crystal and the PMT is analog. In an analog camera the signal that arises from the detector is digitized by an ADC (Analog to digital converter) then passed to the computer.
Pulse Pile up corrections do what?
Account for count rate losses
What is Intrinsic spatial resolution?
Affect of the crystal and electronics on resolution
What is the LED method for PMT stabilitzation?
An LED emits a constant light on the photocathode. The LED produces light that generates a large number of electrons in the PMT which reduces the influence of statistical noise. The amount of light produced by the LED is relatively stable over time and the amount of generated signal in the PMT can be compared ot a reference value for that PMT. If there is significant deviation from the reference valuse, the gain of the PMT is adjusted.
Correction offsets-
Are stored in a look up table. After an event location is determined the position circuit looks up the corrections to be applied to the determined location and the event location is adjusted. Correcting the mis-positioning of events (spatial linearity) has a profound effect on field uniformity.
Misplaced events add to the __________ of the image, which lowers the _______________.
Background, contrast
How is PMT stabilization performed?
By adjusting the pre-amplifiers for each PMT as needed to ensure a stable output.
How can the ideal PMT response be aproximated?
By interposing a light pipe between the crystal and PMT Channeling of light towards PMTs improves accurate positioning of events. There is some degradation of spatial resolution through light losses. Light pipes have been eliminated in many systems using digital non-linear mapping
How does a parallel hole collimator operate?
By selective absorption of photons that are not travelling perpendicular to the crystal.
How is spatial resolution quantified?
By the FWHM of the point or line spread funciton
Spatial linearity corrections do what?
Compensate for mis-positioning of events
Uniformity corrections do what?
Compensate for regional count differences
Energy corrections do what?
Compensate for regional spectra shifts
The amount of scintillation light produced by the crystal is proportional to the amount of energy deposited in the _____________
Crystal
What is the formula for Current?
Current = Voltage / Resistance
How are Differential uniformity values calculated?
DU= High - Low/ High + Low
Using a thinner crystal has a negative effect by what?
Decreasing the senstivity
How can you calculate a rough estimate of intrinsic spatial resolution?
FWHM = 1.75 x bar pattern Most systems have an intrinsic spatial resolution of 4 mm
Correcting the mispositioning of events have a profound effect on?
Field uniformity
Integral values in intrinsic flood field uniformity will be ____________ than differential values.
Greater
Useful field of view UFOV values will be ________ than CFOV central field of view.
Greater
What does the matrix describe in image formation?
How many pixels there are in the x and y direction (64x64, 128x128, 256x256)
How are Integral Uniformity values calculated?
IU= Max pixel cont - Min pixel count/ Max pixel count + Min pixel count
In what ways can MWSR be tested?
Imaging a point source from an isotope that emits multiple photons such as Tl-201 or Ga-67 or imaging a bar phantom.
A thinner crystal has a positive effect by what?
Improving resolution
What is a digital camera?
In a digital camera each PMT contains an ADC so that the signals arising from each PMT are immediately digitized before leaving the detector. Event positioning and all corections are performed on the digital signals in the software. Digital cameras have the ability to perform more advanced corrections and therefore more accurate position scinitllation events.
How is intrinisc uniformity measured?
In terms of the Integral value and the Differential value. Each camera manufacturer will specify quantitative uniforminty values that their camera is expected to meet. If a cameras uniforminty values are out of the specified range, service of the camera may be necessary.
As count rate increases, the probability of recording two events at the same time (pulse pile up) _______.
Increases
As photon energy increases, the number of electrons produced __________.
Increases
As the photon energy increases, the number of electrons produced _______________. More elctrons allows us to decrease the ________________ ___________ of measurement.
Increases./Statistical error
FWHM is inversley proportional to what in d x a/ L?
L - The length of the collimator hole
More electrons allows us to decrease th estastical error of measurements by?
Less blurring at higher energies Fractional error = SD/mean
What is Intrinsic spatial resolution limited by?
Multiple compton scattering interaction sof gamma rays in the crystal Statistical distribution of scintillation light photons among the photmultiplier tubes
Spatial Non-linearities
Non-linearities are due to the position signals not changing linearly as you move across the face of the detector. As a source moves towards a PMT the light collection efficiency of the PMT increases more rapidly than the distance the source is moved. This results in an over-response as you move towards a PMT and an under-response as you move away from a PMT.
How often is a should bar phantom be performed?
Once a week
How is intrinsic uniformity assessed?
Over the usedful field of view UFOV and the central field of view CFOV. UFOV encompasses 95% of the detector area and the CFOV encompasses 75% of the detector area.
What doe energy correction circuits adjust?
PMT gains to compensate for variations in point source sensitvity- can be monitored and adjusted on the fly (PMT stabilation)
What are diferent types of gamma camera collimators?
Parallel hole Diverging Converging Pinhole Fan beam
What is the role of the collimator on a gamma camera?
Projects the gamma ray image of the radionuclide source distribution onto the scintillation crystal. Operates by selective absorption of photons that are not travelling perpendicular rto the crystal.
If the signal from the energy circuit is within the energy window (discriminators) of the PHA, then the X and Y positions of the event will be _________________
Recorded
What is the order in a Digital Event Processor?
Signal weights, Position calculations, Energy summation, Normalization and PHA are performed in software.
What does spatial resolution depend on?
Source to collimator distance
What are offset values?
Spatial linearity corrections
A smaller value for the FWHM = better ___________________ __________________
Spatial resolution
What does energy normalization improve?
Spatial resolution Multi-window spatial registration
What do digitial positions point to?
Specific locations in the matrix. Each time a count is detected in a pixel, the location in the matrix corresponding to that pixel is incremented by 1. The image is complete when a specified amount of time elapses or when a total number of counts in the image are attained.
What is an effect of septal penetration on an image?
Star artifact
What does it mean if your bar phantom appears wavey?
That the linearity corrections are not being applied properly.
System spatial reolution considers what?
The affect of the crystal and electronics as well as the collimator and patient scatter on resolution
What is System spatial resolution?
The affect of the crystal and electronics as well as the collimator and patient scatter on resolution
What is spatial resolution?
The amount of blur/sharpness/detail
How are scintillation events within the crystal determined?
Through the use of a weihted average of the signals coming from the PMTs.
What is the purspose of a bar phantom?
To asses how well spatial linearity correction is being applied.
What is one of the most useful purposes of imaging a bar phantom?
To assess how well the spatial linearity correction is being applied.
Why must the position signals be normalzed by the energy signal Z?
To ensure that the determined position is not dependent on the deposited energy
What is for PMT stabilization for?
To ensure uniform response from PMTs
What areas are affected by performance degradation?
Uniformity Spatial resolution Energy resolution Contrast
What is septal penetration?
When a photon is emitted at a non perpendicular angle towards the detector and is not absorbed by the collimator.
When will uniformity degrade with a PHA?
With off peak windows
What are the formulas for X and Y in ratio circuit?
X = (+X) + (-X)/ Z Y= (+Y) + (-Y)/Z
What happens if you double the matrix?
You quadruple the pixels.
What does a stand for in d x a /L?
a - distance from image to collimator
FWHM is proportional to what in d x a / L?
d - width / diameter of collimator hole
The PMT that is closest to the event will receive the greatest amount of scintillation light and will therefore produce the ______________ _______________ signal.
greatest, output
Resolution increases as L _____________
increases
As PMT's get further from the scinitillation event, they will recieve ________________ scintillation light and therefore put out _________ signal.
less, less
Without normalization, greater photon energies would result in greater __________ from the PMTs and would __________ the determined positions of the events.
outputs, shift
The number of photons is proportional to _________
the energy deposited
FWHM is inversley proportional to what?
the square root of E
