Multiplexed PET-CT

Imaging Two PET-Tracers Simultaneously

Introduction:

Medical imaging techniques such as Positron Emission Tomography (PET) are extensively employed in preclinical and clinical settings. Fluorine-18-fluorodeoxyglucose, or 18F-FDG, is a radiotracer that is used in PET/CT scans as part of cancer care to detect elevated glucose intake, a characteristic of cancer cells. Additionally, a large number of different radiotracers have been created by researchers to target other disease-specific markers.

The way PET operates is by identifying two 511 keV annihilation photons that are produced when an electron in the body and a positron released by the radiotracer annihilate. One radiotracer can only be imaged at a time, though, as all PET isotopes emit the same two 511 keV photons. In order to identify signatures from several tracer, successive PET scanning is necessary. However, this approach is expensive, contingent on the tracer degrading enough over time, and exposes the patient to higher radiation levels from the corresponding CT scans.

a novel technique for image reconstruction that permits the simultaneous in vivo imaging of two distinct PET tracers. The multiplexed PET (mPET) technology was described by the researchers.

PET images are created using lines of response (LORs) between detector pairs that detect two annihilation photons (“double” events) within a coincidence-timing window of about 3.5 ns. Some positron-emitting isotopes also emit an additional prompt gamma photon. If this is detected within the coincidence window it gives rise to a “triple” event, which is usually considered spurious and not reconstructed. Often, such isotopes are avoided in medical scans.

The main idea behind the multiplexed PET (mPET)

1)   Use Standard (β+) & Non-Standard (β+γ) Radionuclides

For dual isotope imaging in Multiplexed PET-CT:

The standard radioisotope emits a high fraction of Positron in each decay process (standard positron emitter), while the non-standard radioisotope emits positron and gamma photon for each decay process.

2)   Detection of Doubles and Triple Coincidences

a.   The standard radioisotope causes double coincidence as two gamma photons 511 Kev will be received by two opposite detectors.

b.   The non-standard radioisotope causes triple coincidence because of the two 511 Kev and the single gamma photon will be detected by three detectors at the ring of PET-Scanner.

3) Image Reconstruction & Separation

A.   Traditional PET Scanner favor double coincident and remove the gamma photon by narrowing the energy window (350-550 Kev).

B.   The narrowing of energy window reduces the scattered radiation and enhances the image quality.

C.  Multiplexed PET(mPET) maximize the counting of triple coincidence by widening the energy window .

D.  Multiplexed PET opens the energy window to be able to scan dual isotopes simultaneously. The image of double coincidence for the standard radioisotope, while the triple coincidence for the non-standard radioisotope.

E.   The scattered radiation will be high, since the energy window is wide, so the iteration method can be used to reduce the scattered radiation and enhance the image quality.

F.   As an example, we can use Zr89 as a standard radioisotope, while 124I as a non-standard isotope.

G.  Zr-89 is positron emitter, while I -124 is emitting Positron and Gamma (602 Kev) .

H.  By using the list mode, we can detect the double and triple coincidences, then double and triple reconstruction can be done separately.