Developing a novel positronium biomarker for cardiac myxoma imaging

Abstract

Purpose: Cardiac myxoma (CM), the most common cardiac tumor in adults, accounts for 50-75% of benign cardiac tumors. The diagnosis of CM is often elusive, especially in young stroke survivors and transthoracic echocardiography (TTE) is the initial technique for the differential diagnostics of CM. Less invasive cardiac computed tomography (CT) and magnetic resonance imaging (MRI) are not available for the majority of cardiac patients. Here, a robust imaging approach, ortho-Positronium (o-Ps) imaging, is presented to determine cardiac myxoma extracted from patients undergoing urgent cardiac surgery due to unexpected atrial masses. We aimed to assess if the o-Ps atom, produced copiously in intramolecular voids during the PET imaging, serves as a biomarker for CM diagnosing.

Methods: Six perioperative CM and normal (adipose) tissue samples from patients, with primary diagnosis confirmed by the histopathology examination, were examined using positron annihilation lifetime spectroscopy (PALS) and micro-CT. Additionally, cell cultures and confocal microscopy techniques were used to picture cell morphology and origin.

Results: We observed significant shortening in the mean o-Ps lifetime in tumor with compare to normal tissues: an average value of 1.92(02) ns and 2.72(05) ns for CM and the adipose tissue, respectively. Microscopic differences between tumor samples, confirmed in histopathology examination and micro-CT, did not influenced the major positronium imaging results.

Conclusions: Our findings, combined with o-Ps lifetime analysis, revealed the novel emerging positronium imaging marker (o-PS) for cardiovascular imaging. This method opens the new perspective to facilitate the quantitative in vivo assessment of intracardiac masses on a molecular (nanoscale) level.

Keywords: Biomarker; Myxoma; PET; Positronium.

Conflict of interest statement

The authors have no competing interests relevant to this study.

© 2023. The Author(s).

Figures

Fig. 1

A pictorial illustration of the basic processes leading to the formation and decay of positronium in the intramolecular voids of a hemoglobin molecule. We have used a sodium 22Na isotope as an emitter of positrons (e +), considering the role of sodium fluoride in cardiovascular imaging [34, 37]. a22Na radionuclide decays emitting a neutrino (brown arrow) and a positron (dark green arrow) (e +), and turns into an excited 22Ne* nucleus. It de-excites almost instantly (on an average in 3 ps) by the emission of the prompt photon (yellow arrow). b The positron thermalizes at a distance of about 1 mm [38], and annihilates into photons with one of the electrons (e−) in the surrounding molecules. Positron–electron annihilation in the tissue undergoes direct annihilation into two photons (solid light green arrows) in roughly 60% of the cases. However, it proceeds via the formation of positronium atom in 40% of the cases. The latter may be trapped in the tissue in the intramolecular voids [39]. Positronium atom can be created in two forms: (i) short-lived (125 ps) para-Positronium (p-Ps indicated in purple), which decays into two photons (dotted pink arrows) or (ii) long-lived (142 ns) ortho-Positronium (o-Ps indicated in mint), which decays into three photons (dashed red arrows). In the tissue, o-Ps predominantly annihilates either through an interaction with an electron (e−) from the surrounding molecule via pick-off process (dashed blue arrows) or through the conversion to p-Ps via an interaction with oxygen molecules, which subsequently decays into two photons (dashed black arrows) [16, 40]. These processes decrease the o-Ps lifetime, which becomes strongly dependent on the size of intramolecular voids and the concentration of bioactive molecules

Fig. 2

Myxoma cell culture isolated from the tumor of patient ID 2 and ID 3. a (Left) The workflow of myxoma cell culture isolation. Cells have been isolated by tissue digestion and cultured to obtain the highest possible number of cells for the experiment. A micrograph presenting the primary culture of cells isolated from cardiac myxoma, upon seeding, and 24 h later with erythrocyte contamination. The latter has been washed out during the primary culture. Secondary culture after the first passage has been established after 1 week. A scale bar of 100 μm. b (Left) The workflow of PALS measurement. The centrifuged cells have been placed in both parts of the aluminum chamber with a radioactive source (red dot) encapsulated between them. The chamber has been mounted between detectors in the temperature-controlled aluminum holder. The measurements have been taken at 37 °C. (Right) Positronium lifetime spectra with fitted components for the myxoma cell culture isolated from the given patient. The dark yellow, green, turquoise, and blue lines denote direct annihilation in the source material, p-Ps annihilation component, direct annihilation in the sample, and o-Ps annihilation component, respectively. The spectra are shifted by the offset coming from the detection system configuration (~ 5 ns). The patient ID annotations are described in Table 1

Fig. 3

Cardiac myxoma experiment workflow. a The clinical examination of a symptomatic patient by transthoracic echocardiography and surgical excision of cardiac myxoma (CM). b Sample preparation and examination of myxoma, adipose tissues and CM cells: one piece of CM was fixed for histopathology examination (H&E), while other piece of CM and adipose tissue samples have been used for studying their positronium properties and later fixed for the X-ray imaging (micro-CT). CM cell culture has been derived from the part of tissue designated for PALS experiment before fixation. c Micro-CT imaging: staining in Lugol solution for 5 days and micro-CT scanning; gray and blue arrows represent a workflow for two different scanning runs: before and after positronium measurement (PALS). d A chamber for ortho-Positronium lifetime measurement comprising CM or adipose sample with the 22Na radionuclide (red dot) emits (green arrow) a positron (+) and a prompt photon (yellow arrow). Positron and electron from a sample create a positronium atom (bound state of electron and positron), indicated pictorially by a white dotted circle. An example annihilation of positronium into two photons (blue arrows). Figure 4 contains a detailed description of positronium lifetime measurement

Fig. 4

A scheme showing positronium lifetime measurements in cardiac myxoma and adipose tissues. a Photographs of non-fixed cardiac myxoma (CM) and adipose tissue samples. The numbers indicate patient ID (Table 1). Each sectioned tissue has been cut in halves with a 22Na radionuclide placed between and inserted into the aluminum measurement chamber. b The left part of the panel depicts the scheme of the detection system: scintillators (S), photomultipliers (PM), attenuators (A), discriminators (D), coincidence units (C), digitizer, and a data acquisition system (DAQ). The photograph displays a part of the system together with the plastic rod localized between the scintillators. The superimposed scheme indicates an aluminum chamber inserted inside the rod. 22Na (red dot) emits (green arrow) positron (+), which annihilates (predominantly into two photons indicated in blue) with electrons (−) in the tissue. Following the positron emission, 22Na changes into an excited nucleus of 22Ne, which de-excites almost instantly by the emission of the de-excitation photon (indicated in yellow). The PALS detection system, enables the measurement of the positronium lifetime by registering the time of emission of the de-excitation photon (corresponding to the time of positronium formation) and the time of creating the annihilation photons (corresponding to the time of the positronium decay). c For each sample, 1 × 106 coincidences between annihilation and deexcitation gamma quanta have been registered, resulting in the lifetime spectrum (example for the adipose tissue—patient ID 2). The analysis enables the extraction of the mean lifetime and intensities of para-Positronium (green line) and ortho-Positronium (blue line) atoms trapped in the intramolecular voids [42]. The dark yellow, turquoise, and purple lines denote direct annihilation in the source material, direct annihilation in the sample, and the background because of the accidental coincidences, respectively. The spectra are shifted by the delay coming from detection system configuration (~ 5 ns). d Results of the mean ortho-Positronium (o-Ps) lifetime (upper) and intensity (lower panel) for CM (black squares) and mediastinal adipose (red circle) tissues

Fig. 5

Comparing cardiac myxoma (CM) tissues and the isolated cell line. a Micrograph showing exemplary histopathology findings of cardiac myxoma (CM), for patient ID 2 and ID 3 in H&E staining. CM cells stained in purple (blue arrow) can have stellate (ID 3) or globular (ID 2) shape. The red/orange structures (white arrow) correspond to the blood vessels with erythrocytes. The surrounding myxoid matrix is stained in pink (green arrow). b Confocal microscopy image with the CM cells stained for F-Actin (red), nucleus (blue), and VE-cadherin (green). The scale bar is 50 μm. c Micro-computed tomography results for CM (upper row) and adipose tissues (lower row). Histograms on the left side present normalized X-ray attenuation within the sample: (i) the mineral deposits range from 0.6 to 1.0 in the CM samples; (ii) the blood vessels range from 0.75 to 1.0 in the adipose tissue samples. These attenuation ranges have been binarized to extract the mineral deposits and blood vessels for further analysis and visualization. The right side contains volume-rendered 3-D models of the two most representative samples, namely CM and adipose tissues. The internal mineral deposits have been highlighted in the CM model. Its diameter has been color-coded using a heat map. The blood vessels have been colored red in the adipose tissue. d Results of the mean ortho-Positronium (o-Ps) lifetime for CM tissue (red circles), isolated from the same patient myxoma cell line (black squares). Patients ID annotations are described in Table 1