Microwave imaging uses low-energy electromagnetic waves to create images of an object’s interior. The technology functions by emitting microwaves, a form of non-ionizing radiation, and capturing the signals as they are reflected or scattered. This process visualizes internal structures without invasive methods. Its versatility has led to its exploration in fields from medical diagnostics to industrial inspection, with prototype systems moving from research into commercial use.
How Microwave Imaging Works
The process of microwave imaging involves a system of antennas that transmit and receive low-energy microwave signals. One or more antennas send these electromagnetic waves, in the frequency range of 300 MHz to 300 GHz, toward the object being examined. These waves penetrate materials like human tissue or industrial composites.
As the microwave signals travel through the target, they are altered based on the material they encounter. This interaction is governed by the “dielectric properties” of the internal structures. Different materials, like water, fat, or muscle, absorb and reflect microwaves in unique ways, allowing the system to distinguish between them. For example, a tumor with higher water content than healthy tissue will interact with microwaves differently.
Other antennas receive the scattered signals that emerge from the object. These captured signals contain detailed information about the target’s internal composition. A computer then processes this data using sophisticated algorithms to reconstruct a 2D or 3D image. This process is conceptually similar to how sonar uses sound waves to map the ocean floor, using wave interactions to build a picture of an unseen environment.
The methods for creating an image can be categorized as either qualitative or quantitative. Qualitative techniques, like those used in synthetic aperture radar (SAR), produce a “reflectivity function” that shows the location of significant scatterers. Quantitative methods, also known as inverse scattering, are more complex and aim to solve a nonlinear problem to map the precise dielectric properties throughout the object.
Medical Applications of Microwave Imaging
One of the most researched medical applications is breast cancer detection. Because malignant tumors have different dielectric properties than healthy, fatty breast tissue, they create a strong contrast that microwave signals can detect. The higher water content in cancerous tissue is a primary reason for this contrast. Researchers are developing systems that offer a safe, non-invasive, and comfortable alternative to traditional mammography, which requires breast compression and uses ionizing radiation.
Stroke detection and monitoring is another promising area. The technology can differentiate between ischemic strokes, caused by a blood clot, and hemorrhagic strokes, caused by bleeding in the brain. This is possible because blood and brain tissue have different dielectric properties. This capability is important because treatment for the two types of strokes is very different, and portable systems could be used in ambulances for rapid diagnosis.
Microwave imaging is also explored for monitoring brain injuries. It can be used for continuous monitoring of patients with traumatic brain injuries to watch for signs of brain swelling or bleeding. The ability to track changes in the brain in real-time allows for quicker medical interventions. Research has also extended to bone imaging and detecting heart conditions like ischemia.
Industrial and Security Applications
Beyond the medical field, microwave imaging is a tool for non-destructive testing (NDT) in various industries. This technique allows for the inspection of materials like concrete, wood, and composites for internal flaws without causing damage. In civil engineering, it can detect corrosion on steel rebar within concrete or identify cracks in structures like bridges. The aerospace industry uses it for tasks such as measuring paint thickness on composite materials.
The technology’s ability to penetrate non-metallic materials makes it suitable for quality control applications. It can measure moisture content in materials, assess the curing state of composites, or inspect for delamination in layered pipelines. Unlike some other NDT methods, microwave imaging does not require a coupling agent and can be performed without direct contact with the object being tested. This flexibility makes it a practical choice for in-line inspections during manufacturing processes.
In security, microwave imaging is employed in personnel screening systems at airports. These body scanners use millimeter waves, a type of microwave, to detect concealed metallic and non-metallic objects under clothing. The waves pass through fabrics but are reflected by the body and any items on it, allowing security to identify potential threats. The technology is also used in geophysics for subsurface imaging to map soil moisture or locate buried objects.
Safety Profile and Technology Comparison
A primary advantage of microwave imaging is its safety profile, as it uses non-ionizing radiation. Unlike X-rays and CT scans, the low-power microwaves in this technology do not have enough energy to damage DNA. The power levels are very low, comparable to or lower than those from a cell phone. This makes the technology safe for repeated or continuous use, which is beneficial for ongoing medical monitoring.
Compared to X-ray and CT scans, microwave imaging’s main advantage is its use of non-ionizing radiation. However, current microwave systems have a lower spatial resolution, producing less detailed images than their X-ray-based counterparts. The technology is still evolving, with research focused on improving image quality through advanced algorithms and hardware.
Microwave imaging systems are less expensive and more portable than magnetic resonance imaging (MRI). They do not require the powerful magnets that make MRI machines large, stationary, and costly, allowing for bedside or ambulance-based systems. While MRI provides superior image detail for soft tissues, microwave imaging is a practical alternative where portability and cost are factors.
Both microwave imaging and ultrasound are safe, non-invasive, and can be used in portable devices. Ultrasound uses sound waves, while microwave imaging uses electromagnetic waves. A difference is that microwaves can penetrate materials that ultrasound cannot, such as bone. This gives microwave imaging an advantage in applications like brain imaging, where the skull can interfere with ultrasound signals. The two technologies offer complementary capabilities, each suited to different diagnostic challenges.