Microwave imaging

Microwave imaging is a science which has been evolved from older detecting/locating techniques (e.g., radar) in order to evaluate hidden or embedded objects in a structure (or media) using electromagnetic (EM) waves in microwave regime (i.e., ~300 MHz-300 GHz).

Quantitative imaging techniques (are also known as inverse scattering methods) give the electrical (i.e., electrical and magnetic property distribution) and geometrical parameters (i.e., shape, size and location) of an imaged object by solving a nonlinear inverse problem.

A transmitting antenna sends EM waves towards the sample under test (e.g., human body for medical imaging).

If the sample is made of only homogeneous material and is of infinite size, theoretically no EM wave will be reflected.

Introduction of any anomaly which has different properties (i.e., electrical/magnetic) in comparison with the surrounding homogeneous medium may reflect a portion of the EM wave.

The bigger the difference between the properties of the anomaly and the surrounding medium is, the stronger the reflected wave will be.

Depending on the applied processing algorithm, microwave imaging techniques can be categorized as quantitative and qualitative.

As one example, there are several research groups all around the world working on developing efficient microwave imaging techniques for early detection of breast cancer.

Recently, microwave imaging has shown great potential to be used for structural health monitoring.

Lower frequency microwaves (e.g., <10 GHz) can easily penetrate through concrete and reach objects of interest such as reinforcement bars (rebars).

[citation needed] Microwave imaging also can be used to detect any embedded anomaly inside concrete (e.g., crack or air void).

These applications of microwave imaging are part of non-destructive (NDT) testing in civil engineering.

Changes of the dielectric properties at surfaces (e. g. shrinkage cavities, pores, foreign material inclusion, or cracks) within the interior of the device under test reflect the incident microwave and send a part of it back to the test probe, which acts as a transmitter and as a receiver.

A test probe attached to the DUT's surface gives information about the material distribution below the point of contact.