SPECT, PET, small animal scanners

Question 1

most commonly used gamma detector

Answer 1

Scintillation detectors

Question 2

main components of Scintillation detectors

Answer 2

scintillation crystal and photodetector

Question 3

photodetector types

Answer 3

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)

Question 4

Gamma Camera and Planar Scintigraphy Main components

Answer 4

collimator + scintillation crystal + photomultiplier tube

Question 5

Requirement for 3D tomography

Answer 5

A set of projections at equally spaced
angular intervals

Question 6

– Filtered Backprojection (FBP) steps

Answer 6

• 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

Question 7

Iterative Algorithms steps

Answer 7

1. Start from an initial simple estimate (blank or uniform image)
2. Forward projection: compute the projections for the estimated image by summing up the intensities along all lines of response
3. Compare projections of the estimated image with the actually recorded projections from measurement data
4. Use the difference to update the estimate
5. Repeat the compare-and-update process until the difference is small enough

Question 8

Complication of iterative algorithms

Answer 8

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

Question 9

Conjugate counting

Answer 9

Acquiring data (or image profiles) for directly opposing views and then combining these data into a single dataset or line of response (LOR)

Question 10

advantage of using geometric mean of opposing detectors in conjugate counting

Answer 10

Both the amplitude and the width of the profile remains nearly constant at all distances and depths in the water phantom.

Question 11

Attenuation Correction

Answer 11

Even with geometric mean of conjugate counting, there are residual scaling factors caused by attenuation that needs correction for quantitative accuracy

Question 12

Attenuation Correction – Chang‘s multiplicative method

Answer 12

Contours of the image are used to estimate the attenuation path length for each projection

Question 13

Transmission Scans and Attenuation Map

Answer 13

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

Question 14

Scatter Correction

Answer 14

Scattered events should be removed before attenuation correction, otherwise they will be amplified

Question 15

which one has higher sensitivity? pet or spect?

Answer 15

PET, because of the absence of collimators

Question 16

The ability to localize events on the basis of coincidence timing, without the need for absorptive collimation, is referred to as

Answer 16

electronic collimation

Question 17

explain time-of-flight (TOF) PET

Answer 17

annihilation location can be determined by the difference in the time at which the two annihilation photons are detected

Question 18

PET – Spatial Resolution
Depth of Interaction (DOI)

Answer 18

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)

Question 19

Two physical limitations on PET spatial resolution are

Answer 19

positron range and noncolinearity of annihilation photons

Question 20

PET – Detectors

Answer 20

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)

Question 21

Photomultiplier tubes cannot be used
here, instead APDs or SiPMs are used

Answer 21

PET/MRI Scanners

Question 22

For accurate quantification in PET, several data corrections required:

Answer 22

• Normalization correction
• Attenuation correction
• Random Correction
• Scatter Correction

Question 23

most commonly used radiotracer in PET

Answer 23

18F-FDG (flurodeoxyflucose)

Question 24

Extract physiological parameters from image

Answer 24

SUV = Activity concentration in image (kBq/ml) /
(Injected activity (kBq) / body weight (kg))