Imaging methods are often crucial components of experimental studies, as they enable researchers to explore the kinematics of various substances. In the biomedical field, imaging is one of the most important diagnostic tools, as it physically reveals the tissues and organs.
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Thus, imaging methods are vital in the development of new medications and the study of different processes. By providing images of internal structures beneath the skin and bones, medical imaging aids in the diagnosis of abnormalities and the treatment of disease.1
High-resolution alpha particle imaging plays a crucial role in detecting alpha radionuclides within cells or small organs. This capability is crucial for developing radio compounds used in targeted alpha-particle therapy and other applications.
Alpha-particle imaging also provides valuable insights into the characteristics of alpha particles, which is beneficial in the study of nuclear and environmental processes.
What Are Alpha Particles?
Alpha particles are positively charged, consisting of two protons and two neutrons. These particles are almost identical to helium particles.2 As they are electrically charged, alpha particles can be easily accelerated to high energies using particle accelerators. This characteristic makes them valuable for studying nuclear phenomena.
Certain naturally occurring radioactive isotopes, like uranium and radium, emit alpha particles spontaneously. These particles can also be utilized to trigger nuclear transformations, providing neutrons for laboratory experiments.3
However, alpha particles have limited penetration ability; they can travel only about five to seven centimeters in air and can be stopped by a single sheet of paper. They cannot penetrate the skin, so the primary health risk associated with alpha particles arises from ingestion rather than external exposure.
Certain isotopes that emit alpha radiation, like uranium, can be managed safely with appropriate precautions against ingestion or inhalation. However, due to their biochemical properties, other alpha emitters, such as polonium and plutonium, are highly toxic to humans.
The interaction of alpha particles with matter involves their reaction with the electrons and nuclei of atoms in that matter, causing an ionization effect. During ionization, the atom interacting with the alpha particle loses an electron, forming an ion pair.
Each ionization event causes the alpha particle to lose energy and slow down. The electron stripped from the atom becomes a negatively charged ion, while the atom, now missing one of its electron shells, becomes a positively charged ion. This interaction leads to a gradual reduction in the velocity of the alpha particle as it traverses through the material.4
Alpha Particles: Detection Techniques and Emerging Challenges
The detection of alpha particles is essential both for research and in various industries, such as nuclear power plants, to ensure a safe working environment. Detection can be achieved using a Geiger–Müller tube or by employing a popular method involving a zinc sulfide scintillator.
In some cases, alpha particle imaging has also been conducted using an air-filled ionization chamber or another instrument known as the spark chamber. These detection methods rely on the interaction of ionizing radiation with matter.5
Scintillators, a popular choice for alpha particle imaging, are employed at nuclear sites. Specifically, a scintillator made of ZnS(Ag) has been extensively used by nuclear experts, as its accuracy is not affected by the harsh operational environment.
Silicon (Si) semiconductors are frequently utilized for energy spectroscopy purposes. However, they tend to be more expensive compared to scintillation detectors and are susceptible to noise interference due to their relatively small signal levels.6
Plastic scintillators have been utilized as transparent scintillators for alpha spectroscopy and imaging alpha particles. However, their light output is relatively low, typically around 10,000 photons per MeV. This limited light output can constrain both the energy and spatial resolutions in imaging systems.
Careful handling of alpha particle emitters is also a complex task that requires extremely strict regulatory frameworks due to the potential damage these particles can cause to humans.
The Diverse Applications of Alpha Particle Imaging
Alpha-particle emitters provide significant therapeutic benefits, especially for patients with limited treatment options and metastatic cancer. These radioisotopes can target small clusters of cancer cells effectively.
Due to their limited penetration depth in human tissue, typically less than 0.1 mm, alpha particles can specifically destroy tumor cells while sparing the surrounding healthy tissues..7
The high energy of alpha emitters and their strong linear energy transfer result in efficient cell killing through DNA damage. Thus, alpha radioisotopes offer a treatment alternative for individuals who do not respond to beta or gamma radiation therapy or chemotherapy.8 The most preferred isotopes for this type of therapy include 225Ac and its daughter nuclide 213Bi.
Targeted alpha therapy (TAT) is a promising approach in drug development, offering highly specific and potent treatment options for various late-stage cancers. The delivery of radiopharmaceuticals emitting alpha particles has proven effective in treating cancer.
However, the development of imaging techniques, such as radioactive decay imaging and alpha particle imaging, is desirable for the microdetection of alpha-emitting radiopharmaceuticals. This is because the range of emitted alpha particles is mostly confined within 100 μm, making precise imaging crucial for accurate targeting and the assessment of treatment efficacy.
This also enables the precise detection of therapeutic alpha particles to confirm they are targeting only the required area, thereby keeping healthy tissues safe during treatment.9
Alpha particle imaging is also essential for ensuring the safety of workers in nuclear facilities. Plutonium dioxide (PuO2) is utilized to make mixed oxide fuel (MOX). Leakage of PuO2 can be extremely harmful to workers in such facilities.
Imaging of emitted alpha particles by plutonium using a zinc sulfide scintillator and polaroid film has been effective in determining plutonium concentrations to ensure the safety of the workers.10
Alpha Particle Imaging: Innovations and Technological Advances
Precise alpha-particle imaging is essential for accurately measuring distributions in cells or dissected animals, crucial for developing new radio-compounds and dosimetry in targeted alpha-particle therapy. It is also indispensable in mineralogical and nuclear studies.
Conventional imaging systems lack the spatial resolution needed to track alpha particle trajectories effectively in real-time. Researchers have devised a sophisticated system that combines a suitable scintillator plate with a highly sensitive optical camera alpha particle imaging.11
The selected scintillator plate, Gd3Al2Ga3O12 (GAGG), was paired with a magnifying unit and an EM-CCD camera. Alpha particles were directed toward the GAGG scintillator plate, positioned in front of the magnifying unit's lens.
The spatial resolution, determined by the width of the lateral profile of alpha particle trajectories in the images, was approximately 2 µm. This system enables the visualization of alpha particle trajectories emitted from individual cells containing alpha-emitting radionuclides.
It also has the potential to capture images of alpha particle trajectories displaying radial dispersion from emitting cells or particles. The system additionally facilitates real-time imaging, enabling high-resolution visualization of alpha particle trajectories at intervals as short as 500 ms.
A new imaging technique for alpha particles, developed by researchers from the University of Manchester, employs an ultrafast optical camera focused on a thin scintillator.12 This scintillator is placed 10 cm away from the input window of the intensifier within the Tpx3Cam, positioned orthogonally to the beam axis.
The Tpx3Cam, a single-photon sensitive camera with time stamping capabilities, employs a data-driven readout with a pixel dead time of approximately one microsecond. It achieves a temporal resolution of about 10 ns for detecting alpha particles, with the ability to resolve them spatially. This optical technique could be extended to register X-Rays or other forms of ionizing radiation that generate light flashes in a scintillator.
With the advent of modern imaging cameras, sensors, and highly optimized processing algorithms, significant advancements in alpha particle imaging have been made. There is a strong emphasis on integrating Machine Learning (ML) algorithms to obtain better optimization and post-processing mechanisms.
Alpha particle imaging is expected to lead to significant breakthroughs, playing a vital role in future studies in environmental sciences, nuclear studies, and biomedical applications.
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References and Further Reading
[1] Hussain, S., et al. (2022). Modern diagnostic imaging technique applications and risk factors in the medical field: a review. BioMed research international. doi.org/10.1155/2022/5164970
[2] United Nations Nuclear Regulatory Commission (2023). Alpha Particles. [Online] Office of Scientific and Technical Information. Available at: https://www.nrc.gov/reading-rm/basic-ref/glossary/alpha-particle.html [Accessed on 14 March 2024]
[3] U.S. Department of Energy. (2023). Alpha Particles. [Online] U.S. Department of Energy. Available at: https://www.osti.gov/opennet/manhattan-project-history/Science/Radioactivity/alpha.html [Accessed on 14 March 2024]
[4] Nuclear Regulatory Commission. (2011). Interaction of Charged Particles with Matter. [Online] Nuclear Regulatory Commission. Available at: https://www.nrc.gov/docs/ML1122/ML11229A666.pdf [Accessed on 15 March 2024]
[5] Bakhoum, G., et al. (2022). Direct Detection of Alpha Particles with Solid-State Electronics. The Physics Teacher. doi.org/10.1119/5.0037639
[6] Morishita, Y., et al. (2014). Performance comparison of scintillators for alpha particle detectors. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. doi.org/10.1016/j.nima.2014.07.046
[7] Johnson,.D., et al. (2023). Resonant laser ionization and mass separation of 225Ac. Sci Rep. doi.org/10.1038/s41598-023-28299-4
[8] Jalloul, W., et al. (2023). Targeted Alpha Therapy: All We Need to Know about 225Ac’s Physical Characteristics and Production as a Potential Theranostic Radionuclide. Pharmaceuticals. doi.org/10.3390/ph16121679
[9] Seo, Y. (2019). Quantitative Imaging of Alpha-Emitting Therapeutic Radiopharmaceuticals. Nucl Med Mol Imaging. doi.org/10.1007/s13139-019-00589-8
[10] Morishita, Y., (2020). Imaging of plutonium particles using a CCD-camera-based alpha-particle imaging system. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. doi.org/10.1016/j.nima.2018.12.023
[11] Yamamoto, S., et al. (2023). Development of an ultrahigh resolution real time alpha particle imaging system for observing the trajectories of alpha particles in a scintillator. Sci Rep. doi.org/10.1038/s41598-023-31748-9
[12] D'Amen, G., et al. (2021). Novel imaging technique for alpha-particles using a fast optical camera. Journal of Instrumentation. doi.org/10.1088/1748- 0221/16/02/P02006
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