TeraHertz Imaging: Portable plant imaging
Published: 2023-11-20
Working on plant biology? Want to detect leaf water stress or assess the quality of dry fruits like chestnuts and hazelnuts? TeraHertz Imaging could be the right approach for you. We spoke with Dr. Alessandra Toncelli, Associate Professor at the University of Pisa, part of our Molecular Imaging Italian Node, to learn more about this exciting technology that is offered to external users via Euro-BioImaging as part of the Proof-of-Concept study.
We are today talking about TeraHertz Imaging. Please provide a short summary of this type of imaging and list some applications:
Alessandra Toncelli: TeraHertz (THz) Imaging consists in illuminating an object with a pulsed terahertz radiation and capturing the response through an array of sensors in the same spectral region.
In the THz region, a photon carries a very small energy when compared with the ionization energy of atoms, which is why it is a totally safe technology that poses no risk to biological organisms. In addition, the very long wavelength (a fraction of a millimeter) makes it possible to strongly limit scattering processes due to the presence of small particles in the sample.
In contrast, this radiation is strongly absorbed by some molecules and, in particular, by water. This is why it is an ideal technology for observing both dry objects and very thin objects. Plant leaves are ideal elements to study in this region, as are dried fruits. In fact, it has been demonstrated that Thz imaging can provide valuable information on the hydration status of plants, and it has also been used to automatically select healthy dried fruit from those with internal defects.
The current state of technology allows us to observe relatively thin objects with lateral dimensions of the order of some centimeters. Image acquisition is quite simple and fast so much so that it could also be easily implemented for real-time acquisition even within an industrial production line.
Tell us a bit more about a specific project that was done in your facility using this technology? What scientific questions were you addressing?
Alessandra Toncelli: This method makes it possible to monitor the amount of water in the leaves or major leaf districts, even on the same leaf as a function of time. This allows, for example, early detection of any signs of leaf water stress to enable precision irrigation. See for example: “Non-invasive absolute measurement of leaf water content using terahertz quantum cascade lasers”
In addition, the method is suitable, for example, to study the effect of the presence of ozone in the air on the plants or the interaction with hormones such as abscisic acid (ABA).
Moreover, it has been used for the quality assessment of dry fruits like chestnuts and hazelnuts. See for example “Detection of fungal infections in chestnuts: a terahertz imaging-based approach"
What are some advantages of this technique that make it suited to addressing this type of question?
Alessandra Toncelli: Most of all, THz imaging offers a unique advantage in moisture and liquid detection. The high contrast between wet and dry regions, facilitated by the strong absorbance of water in the THz range, has enabled the mapping of moisture content in diverse materials. This principle has been applied not only to the detection and diffusion mapping of water but also to other liquids, provided their permittivity significantly differs from that of the surrounding materials. Such moisture mapping has found applications in monitoring hydration levels in plant leaves, which can have implications for water resource allocation in agriculture. Furthermore, the high THz reflectivity due to water content in items like grapes has led to the development of THz imaging as a non-invasive methodology for predicting crop yields. This focus on water content detection highlights the central role of water in biological functions and supports the use of THz imaging in both medical applications and non-destructive evaluation of agricultural products such as grain and pecans.
What other services do you provide in your facility that would be useful in combination with this type of imaging?
Alessandra Toncelli: The technology presented in this context offers great versatility, addressing various research needs. It is designed to be adaptable for on-site use, provided there is access to a standard electrical outlet. The system consists of two primary components: a source and a camera, both of which are compact, operate at room temperature, and can be used even in fields.
The lightweight, transportable TeraHertz Imaging system offered by the University of Pisa can easily be deployed in the field as long as a standard electrical socket is available.
Within the Physics Department in Pisa, the setup is permanently installed on a fixed stand, carefully designed to optimize uniform camera illumination. The camera is connected to a computer for real-time data acquisition and download. The acquired images are available in both monochromatic and false color representations, and they can be saved in different formats, such as ascii data matrix or png, with the flexibility to accommodate other formats upon request.
What's particularly convenient is the portability of the entire apparatus, which can be mounted on a compact stand for easy transportation beyond the laboratory setting. In terms of sample preparation, the only requirement is that the samples have a side area within the physical dimensions of the sensor (48 x 48 mm) and do not exhibit exceptionally high absorption, preventing radiation from passing through.
Notably, the research group provides access to this technology and comprehensive technical support for data acquisition and processing entirely free of charge. This support underscores the research group commitment to facilitating scientific exploration and advancing research in this field, both within their institution and for external collaborations.
Other applications of TeraHertz Imaging:
Terahertz Imaging technology has made significant advances in addressing a range of critical biological and biomedical questions since its introduction in the late 1990s. One of the key applications lies in medical imaging, where THz technology has been employed for skin and breast cancer margin detection, burn wound imaging, skin hydration monitoring, and corneal hydration measurement. The presence of water in physiological tissue and the high THz absorption of water make reflective THz imaging especially advantageous in in vivo applications. THz imaging allows for the detection of small changes in hydration, serving as an effective contrast mechanism for differentiating between tissue types, including cancerous and healthy tissues, and for assessing burn injuries.
These capabilities position THz imaging as a valuable tool for in-vivo imaging of skin cancers, including melanoma and carcinoma, as well as corneal pathologies.
The versatility of THz imaging is not limited to these areas; it extends to fields like chemistry, pharmaceuticals, and security. THz waves, located between the microwave and infrared regions, have unique properties that enable chemical component analysis through spectroscopic fingerprinting. They are transparent to various dry dielectric materials, making them suitable for the detection of concealed items in bags or packaging. Additionally, THz waves exhibit extreme sensitivity to water, making them an attractive choice for in vivo inspection due to their non-ionizing nature and minimal impact on living organisms. The terahertz region, often referred to as the "terahertz gap," has vast potential, but its exploration has been constrained by the lack of high-efficiency sources and sensitive detectors. As technology continues to advance, THz imaging promises to play a crucial role in addressing a wide array of biological and biomedical questions, as well as in various other scientific and practical domains.