SPECT, PET, small animal scanners Flashcards
most commonly used gamma detector
Scintillation detectors
main components of Scintillation detectors
scintillation crystal and photodetector
photodetector types
1Photomultiplier tubes
2Semiconductor-based photodetectors
a. Avalanche Photodiodes (APDs)
b. Silicon Photomultipliers (SiPMs)
c. Cadmium zinc telluride (CZT) -> (Direct
conversion of gamma-rays to charge)
Gamma Camera and Planar Scintigraphy Main components
collimator + scintillation crystal + photomultiplier tube
Requirement for 3D tomography
A set of projections at equally spaced
angular intervals
– Filtered Backprojection (FBP) steps
- Calculate 1D FT of projection profiles and take them to k-space
- Apply a ramp filter to k-space profile to enhance high spatial frequencies and suppress low spatial frequencies
- Backproject the filtered profile
Iterative Algorithms steps
- Start from an initial simple estimate (blank or uniform image)
- Forward projection: compute the projections for the estimated image by summing up the intensities along all lines of response
- Compare projections of the estimated image with the actually recorded projections from measurement data
- Use the difference to update the estimate
- Repeat the compare-and-update process until the difference is small enough
Complication of iterative algorithms
Backprojection is the most time consuming part, that needs to be done once in FBP, but here it has to be performed at every iteration
Conjugate counting
Acquiring data (or image profiles) for directly opposing views and then combining these data into a single dataset or line of response (LOR)
advantage of using geometric mean of opposing detectors in conjugate counting
Both the amplitude and the width of the profile remains nearly constant at all distances and depths in the water phantom.
Attenuation Correction
Even with geometric mean of conjugate counting, there are residual scaling factors caused by attenuation that needs correction for quantitative accuracy
Attenuation Correction – Chang‘s multiplicative method
Contours of the image are used to estimate the attenuation path length for each projection
Transmission Scans and Attenuation Map
When attenuation coefficient cannot be assumed to be constant (thorax or pelvic regions)
* An external source of radiation used to acquire transmission data to generate an attenuation map of the object
Scatter Correction
Scattered events should be removed before attenuation correction, otherwise they will be amplified
which one has higher sensitivity? pet or spect?
PET, because of the absence of collimators
The ability to localize events on the basis of coincidence timing, without the need for absorptive collimation, is referred to as
electronic collimation
explain time-of-flight (TOF) PET
annihilation location can be determined by the difference in the time at which the two annihilation photons are detected
PET – Spatial Resolution
Depth of Interaction (DOI)
Depths at which the gamma rays interact within the scintillation crystal are unknown
The DOI effect depends on
* source location
* diameter of the scanner (D)
* length of the crystal elements (x)
* width of the detector elements (d)
Two physical limitations on PET spatial resolution are
positron range and noncolinearity of annihilation photons
PET – Detectors
Discrete detectors:
* More finely cut elements, higher spatial resolution, but more expensive, more complicated, and more dead space
Continuous detectors:
* As crystal thickness increases, the sensitivity increases, but the accuracy of X-Y position degrades (worse spatial resolution)
Photomultiplier tubes cannot be used
here, instead APDs or SiPMs are used
PET/MRI Scanners
For accurate quantification in PET, several data corrections required:
- Normalization correction
- Attenuation correction
- Random Correction
- Scatter Correction
most commonly used radiotracer in PET
18F-FDG (flurodeoxyflucose)
Extract physiological parameters from image
SUV = Activity concentration in image (kBq/ml) /
(Injected activity (kBq) / body weight (kg))
