Physics of pure and non-pure positron emitters for PET: a review and a discussion

Abstract

With the increased interest in new PET tracers, gene-targeted therapy, immunoPET, and theranostics, other radioisotopes will be increasingly used in clinical PET scanners, in addition to (18)F. Some of the most interesting radioisotopes with prospective use in the new fields are not pure short-range β(+) emitters but can be associated with gamma emissions in coincidence with the annihilation radiation (prompt gamma), gamma-gamma cascades, intense Bremsstrahlung radiation, high-energy positrons that may escape out of the patient skin, and high-energy gamma rays that result in some e (+)/e (-) pair production. The high level of sophistication in data correction and excellent quantitative accuracy that has been reached for (18)F in recent years can be questioned by these effects. In this work, we review the physics and the scientific literature and evaluate the effect of these additional phenomena on the PET data for each of a series of radioisotopes: (11)C, (13)N, (15)O, (18)F, (64)Cu, (68)Ga, (76)Br, (82)Rb, (86)Y, (89)Zr, (90)Y, and (124)I. In particular, we discuss the present complications arising from the prompt gammas, and we review the scientific literature on prompt gamma correction. For some of the radioisotopes considered in this work, prompt gamma correction is definitely needed to assure acceptable image quality, and several approaches have been proposed in recent years. Bremsstrahlung photons and (176)Lu background were also evaluated.

Keywords: Non-conventional PET isotopes; PET; Positron emitter; Prompt gamma; Radioisotopes.

Fig. 1

Three mechanisms associated with prompt gamma spurious coincidences: a ¹²⁴I has a prompt gamma of energy within or close to the PET energy window (603 keV); b ⁶⁸Ga has a prompt gamma of energy that can enter the PET energy window if it undergoes Compton scatter (1077 keV); and c ⁷⁶Br has several high-energy gammas that can reach the detectors and generate e ⁺/e ⁻ pairs

Fig. 2

Radial net true sinogram profiles (all angles added) of a NEMA image quality phantom, filled with water solutions of several radioisotopes used for PET imaging: ⁶⁸Ga (thick black line), ⁹⁰Y (thin red line), ¹²⁴I (thick red line), and ¹¹C, ¹⁸F, ⁶⁴Cu, and ⁸⁹Zr (all in black dotted lines). The phantom was scanned on a Siemens mCT PET/CT scanner. The profile is shown after normalization to maximum, a in linear scale and b log scale

Fig. 3

Radial net true sinogram profiles (all angles added) of a NEMA image quality phantom, filled with water solutions of ¹²⁴I. A second-order polynomial is used to fit the background: the ¹²⁴I profile (black line), the second-order polynomial fit (red line), and the tail region used for the fit (thick red dotted line) are shown. The phantom was scanned on a Siemens mCT PET/CT scanner

Fig. 4

Additional mechanisms associated with spurious coincidences: a high-energy positrons, escaping the patient skin, can travel in air and reach the PET scanner internal tunnel, where they annihilate and produce two 511-keV gammas in coincidence; b in LSO- or LYSO-based PET scanners, a small background of ¹⁷⁶Lu radiation can generate coincidences between β⁻ in a detector and two photons (307 keV + 202 keV) in another; and c high-energy Bremsstrahlung X-rays can be emitted by slowing down of high-energy β⁻ or β⁺ and create e ⁺/e ⁻ pairs in the detectors

Fig. 5

Transaxial planes of the reconstructed images of ⁶⁸Ge/⁶⁸Ga cylindrical source in the scanner, a without and b with a copper shield. The positrons escaping the phantom annihilate in the copper, which becomes a source of 511 gamma pairs, and are visible in the image as a shell surrounding the phantom

Fig. 6

a Drawing of a ⁶⁸Ge/⁶⁸Ga phantom on a foam support inside a PET scanner tunnel. b Net true sinogram radial profile in the top-view projection, or at 0° (black line), and in the side-view projection, or 90° (red line)

Fig. 7

Net true sinogram radial profile of an image quality phantom with 3785 MBq ⁹⁰Y (thin black line), 613 MBq ⁹⁰Y (thick black line), background acquisition with no source (red line), and a polynomial fit of the background (broken red line)