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.
Comments
Post a Comment