Microwave Imaging

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

The dominant application of imaging in the microwave band is radar. The unique feature of imaging radar is its ability to collect data over virtually any region at any time, regardless of weather or ambient light conditions.

An imaging radar works like a flash camera in that it provides its own illumination (microwave pulses) to illuminate an area on the ground and take a snapshot image. Instead of camera lens, a radar uses an antenna and digital computer processing to records its images. In radar image, one can see only the microwave energy that was reflected back towards the radar antenna.

Microwave imaging techniques.

• Quantitative imaging techniques

Quantitative imaging techniques 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.

• Qualitative imaging techniques

Qualitative microwave imaging methods calculate a qualitative profile (which is called as reflectivity function or qualitative image) to represent the hidden object.

How it happens?

A microwave imaging system has two parts.

• Hardware; responsible to collect data from the sample under test
• Software (or post-processing)

figure 1: A microwave imaging system

A transmitter antenna sends EM waves toward sample under test. (E.g. human body for medical imaging)

If the sample is made of only homogeneous material which extends to infinity, no EM wave will be reflected.

If the sample has any anomaly, in comparison with the surrendering homogeneous medium may reflect a portion of EM wave.

This reflection will be collected and measured by the same antenna (in monostatic case) or a different receiver antenna. (in bistatic case)

When the antenna finishes scanning the entire sampling area, one can put together collected data all over the sampling area and process them together, simultaneously.

The collected data which is sometimes referred as raw data is fed into the software part to get processed.

Domains which are used Microwave imaging

Health

 1. Nondestructive testing and evaluation

Flaw detection in spray-on foam insulation and the acreage heat tiles of the Space Shuttle. Corrosion and precursor pitting detection in painted aluminum and steel substrates, Disband detection in strengthened concrete bridge members retrofitted with carbon fiber reinforced (CFRP) composite laminates

2. Structural health monitoring

Electromagnetic waves in low frequency (e.g., <10 GHz) can easily penetrate inside concrete and reach to object of interest which is usually radar. Microwave imaging also can be used to detect any embedded anomaly inside concrete

3. Medical imaging

Microwave imaging is a method for the detection of early stage breast cancer.

Security

1. Concealed weapon detection at security check points

These security systems employ methods derived from microwave holography techniques that utilize phase and amplitude information recorded over a two-dimensional aperture to reconstruct a focused image of the target.

2. Through-the-wall imaging

Through-the-Wall Microwave Imaging (TWMI) radar system facilitates us to detect and localize objects behind the wall. Through wall imaging of moving targets.

3. Radar

This is the major application of microwave band. Its unique ability is to collect data over virtually any region at any time without regarding the weather or ambient light condition.

Contributed by

Group Jaspers

I.A.S Lakmali

R.A.K.I Ranasinghe

W.B.P.N Herath