Hybrid Imaging PET/CT/MRI

1.22 MeV, 511 KeV

PET tracers utilize two separate positron emissions that need to meet the minimum energy requirement of or separately.

higher spatial resolution, scatter, penetrating power

Having a higher energy threshold allows us to have even , without as much from the body due to of the positron emission.

511 KeV, 180 degrees

PET emission occurs when photons of equal energy are emitted simultaneously, in in opposing directions, usually within a few tenths of a mm to a few mm of the location where the positron was produced.

back-to-back annihilations

PET is detected in photons that are produced when a positron interacts with an ordinary electron in the detector material

This is done with various directional detectors that allow for the localization of individual positron emissions from the patient or source.

True

What is annihilation coincidence detection (ACD)?

Is the PET detectors are able to function without absorptive collimators

Detection of a pair of annihilation photons in opposing detectors actually defines the volume from which they were emitted

True

square or rectangular cross sections

Most ACD detectors have , with dimensions equal to those of the detectors.

Why does the ACD detectors have a square or rectangular cross sections?

the volume of the tissue being examined is essentially a box of square or rectangular cross sections with dimensions equal to those of the detectors

Pixel size can vary greatly from camera to camera.

True

How do we calculate the pixel size of the scan?

display field of view needs to be divided by the size of the matrix.

How can we convert the DFOV needs to be converted from 30.2 cm into mm?

can be done by moving the decimal one place to the right making it 302 mm.

Then how we calculate the display view and the matrix values?

While taking the data of the pixel size and image displays how do we convert that in a voxel?

would need DFOV & the detector thickness from the acquisition

What is coincidence logic?

Due to the nature of two separate 180 degree photons that need to be counted, coincidence logic is employed to help analyze the signals from opposing detectors.

Time stamp, 1-2 nanoseconds

This is done by having a digital placed on each recorded event within a window of

comparison

The coincidence processor examines the time stamp for each event in with events recorded in the opposing detectors

How the coincidence logic reads a match up?

Match up within the specified coincidence timing window which is generally 6-12 nanoseconds

Is there potential differences in travel lenght for the source of the coincidence time stamp?

yes, there is a timing window that tries to account for these variances

What is the timing resolution?

explains the finite limit in accounting for Positron emission with the temporal window that the camera allots before it counts the emissions and not related.

What is the timing resolution range?

0.5 to 5 nanoseconds, depending on the scintillator material and photodetector used

Does the finite window width permits other types of events?

Yes, coincidence with slight variations to the degree ± 5% from the 180 degrees we would normally expect from the positron emissions

What is electronic collimation?

The ability of ACD to localize events on the basis of coincidence timing without the need for absorptive collimation

What is the purpose of the electronic collimation?

PET imaging has much higher sensitivity (which relates to the number of events detected per unit of activity within the object) is much higher that what is clinically obtainable with general nuclear/SPECT imaging

Why the PET Imahing is so enhanced?

incorporating multiple opposing detectors in an array in coincidence with multiple detectors on each side of the array. Allowing for multiple projection angles to be acquired simultaneously

Does the PET detectors move?

No, are comprised of a stationary ring that does not move or rotate around the patient.

What is the good qualities of the PET detectors?

• Speeding up the scan rather than rotating around the patient

• fast dynamic studies

• reduction of artifacts caused by patient motion

What is the time of flight PET?

determine the location along the line between two ACD detectors at which the signal is collected by determine in time at which they are detected by their separate detectors

What does the time of flight technique allows to do what?

us to generate tomographic images without mathematical reconstruction algorithms;

What is the first factor that hinders saptial resolution of positron emission?

is the finite range of positron travel before it undergoes annihilation

What is the effective positron range?

is the average distance from the emitting nucleus to the end of the positron range.

How is the positron effective range measured?

perpendicular to a line defined by direction of the annihilation photons

What does noncolinearity of annihilation photons result?

from residual moment of the electron and positron at annihilation

What does noncolinearity leads to?

to positioning errors, that are roughly +/- 0.25 degrees, centered around 180 degrees

What are the typical scintillator detector crystal size for gamma cameras?

1.25 cm or less

What are the PET detector scinillators crystal size?

2-3 cm with greater stopping power

bismuth germanate (BGO), or Lutetium Orthosilicate (LSO)

However for PET systems they generally employ 2-3 cm thick scintillators with greater stopping power like or or even (LYSO, if you remember

What is the depth of interaction effect (DOI)?

Due to the thickness of the detector elements, there is a degradation of resolution

What only occurs when there is a depth of interaction effect?

axial direction from cross plane coincidence detections they still are present and are seen in the radial direction