Page 49 - ARNM-2-2

Advances in Radiotherapy
& Nuclear Medicine PET and Compton Camera CZT based system

Table 1. Summary of various non‑pure positron‑emitting isotope properties and applications
Isotope Half‑life (h) β yield (%) β + range Prompt γ yield (%) Application
+
ave
(mm) γ (keV)
18 F 1.83 97 0.62 NA NA Pure PET emitter used in FDG for use in oncology
68 Ga 1.13 89 3.56 1,077 3.2 Aid for radiotherapy in PSMA-PET
72 As 26.00 88 5.19 693 8.07 Used as an imager for radiotherapy
834 81.0
89 Zr 18.40 23 1.27 909 99.0 Used in immuno-PET and multi-isotope imaging in head
and neck squamous cell carcinoma
44 Sc 4.04 94 2.46 1,157 99.4 Aid for radiotherapy in PSMA-PET
124 I 100.32 23 3.37 603 62.9 Used in imaging pharmacokinetics of radiopharmaceuticals
723 10.4
1,691 11.2
Abbreviations: FDG: F-fluorodeoxyglucose; PET: Positron emission tomography; PSMA-PET: Prostate-specific membrane antigen positron emission
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tomography.
coincidence reconstruction techniques using its liquid
xenon time projection chamber technology through a
pseudo-time-of-flight (TOF) technique. Although not the
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purpose of this work, a potential 3-γ imaging technique is
visualized in Figure 2. This method relies on the coincidence
detection of a prompt-gamma with annihilation photons.
While non-TOF PET back projection techniques will
assign equal probability along the line of response (LOR),
utilizing LORs with the additional back projection of a
cone of response (COR) created by detecting the Compton
scattering of higher energy prompt-gammas, it is then Figure 2. Triple-gamma coincidence techniques for pseudo-time-of-flight
possible to localize the source distribution to a smaller image reconstruction
segment of the LOR through the LOR-COR intersection. Abbreviations: COR: Cone of response; LOR: Line of response.
Addressing the engineering challenge of creating and head and neck PET imaging. 28-42 The CZT detectors
systems for the simultaneous detection of LOR and in our system are 40 × 40 × 5 mm crystals arranged in an
COR information requires the selection of a scattering edge-on orientation. The edge-on orientation allows a 4 cm
detector material with high energy resolution and spatial thickness of CZT with a density of 5.78 g cm (Z = 48.2),
-3
CZT
resolution, as well as the development of an electronic enabling attenuation of high-energy gammas with energy
readout scheme that can operate in conjunction with resolution as low as 5%. Notably, the energy resolution
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the PET detector layer. Traditionally, in CC systems, two of CZT differentiates itself from other comparable pre-
layers of detectors are necessary to obtain CORs, known as clinical scanners that utilize common scintillation crystals,
the scattering and absorption layers. The whole gamma offering energy resolutions as good as 14%. 43,44 High-
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imaging system adopts this dual-detector layer approach to energy resolution crystals are crucial for capturing the
induce the scattering of high-energy gammas for detection Compton scattering of gammas, as the angular resolution
in coincidence with 511 keV annihilation photons. 23,24 depends on energy. The cross-strip electrode design
However, this method presents drawbacks in terms of reduces the number of channels for a pixelated detector
hardware and electronics complexity for synchronizing from n to 2n channels, with the capability of providing
2
two separate devices. In addition, this approach can be spatial resolutions of up to 1 × 5 mm in the x-y plane and
2
costly to implement, as it requires constructing a second 1 mm in the z-direction. 31
detector to be inserted between the radiation source and Our system offers advantages over conventional hybrid
the PET device.
systems in terms of hardware simplicity by eliminating
We propose a dedicated head and neck dual-panel the need for synchronization of separate scattering
system with an edge-on orientation of detectors, which and absorption layer detector electronics. In theory,
builds upon extensive research in cross-strip pixelized this approach could reduce the cost of designing and
cadmium zinc telluride (CZT) detectors for small animals constructing separate scattering and absorption layer

Volume 2 Issue 2 (2024) 3 doi: 10.36922/arnm.3330