**3. Practical tomograph implementation example**

58 Microwave Materials Characterization

must be taken as narrow as possible.

**Figure 5.** (a) Top and (b) side views of the SUT.

surrounding the investigation area.

specifications. In practical measurements, many things must be taken into account. According to the inversion algorithm needs, it is possible to select the optimal antenna characteristics that allow obtaining the best performance. In order to enhance the inversion accuracy many algorithms use multiple frequencies, some others use pulsed broadband signals. This requirement, for example, restricts the antenna choice to broadband types only. In the case of a two dimensional inversion algorithm, the SUT is assumed to be homogeneous and uniform along the z axis. In other words, the scatterer is divided into slices along this axis. Looking at the top view (Fig. 5(a)) the antenna radiation angle must be large enough to "illuminate" all the SUT surface. On the other hand, because of the properties of the inversion algorithm, the vertical radiation pattern (side view, Fig. 5(b))

During the measurement setup design, it is also very important to consider the space surrounding the inspection area. The inversion algorithms usually assume this space to be perfectly homogeneous with known characteristics. In a real implementation this assumption is not valid. The measurement setup is composed by many mechanical components that could affect the measurements quality. A little variation of the surrounding volume can modify the measurements repeatability. The use of antennas with a limited back lobe can drastically decrease the influence of this problem. Another possibility to decrease the influence of the surrounding volume is the use of microwave absorbing materials

(a) (b)

The choices of the number of antennas and their positions are extremely important. In this context there are many possible implementation options depending on the application needs and the inversion algorithm topology. One can distinguish mainly between two kinds of measurement setup: static or dynamic one. Each one brings advantages or disadvantages in relation to the specific application and the designer must therefore consider the properties of the object to be imaged. Depending on the SUT, it is generally possible to exclude right away some configurations. For example, in the case of a single piece that can fit entirely into the investigation domain, it is possible to perform a multiview inspection by rotating the SUT on its own axis instead of moving the illuminating antenna. On the other hand, if the goal is to measure only a part of a bigger object (e.g., parts of the human body, such as arms, legs or breast) the system has to perform a multiple view scanning without the SUT rotation.

This section describes the developed tomograph prototype (Fig. 6). In order to be as flexible as possible, a linearly polarized antenna with a large bandwidth has been chosen. Fig. 7 shows the considered antennas.

**Figure 6.** Prototype of the microwave tomograph (developed at SUPSI).


**Figure 7.** Rohde & Schwarz HL050 Antenna.

The generation, measurement and pre-processing of the microwave signal is carried out with a VNA, able to perform measurements from 10 MHz to 20 GHz. A managing software running on an external PC gathers all the measurement data through an Ethernet connection. The VNA generally requires a calibration. However, in a typical tomograph application, an empty (or void) reference measure is taken first and compared against subsequent measures with the SUT, therefore eliminating static errors due to reflections, phase adjustment errors, fixed interferers, and so on. Fig. 8 shows an example of the used VNA.

**Figure 8.** Agilent VNA.

For mechanical reasons, some parts of the tomograph (in particular the basement) are composed by an aluminum structure. Therefore, microwave absorber material (Fig. 9 and Fig. 10) are used to prevent reflections.


**Figure 9.** Eccosorb AN79

During the development of the measurement system, the planning of the illuminating signal power and receiver sensitivity play, as previously mentioned, an important role. A good dynamic range is the key for reliable and accurate measurements. The entire measurement chain can be represented with the block diagram reported in Fig. 11.

The main elements are:


**Figure 10.** Absorber mounting.

**Figure 8.** Agilent VNA.

**Figure 9.** Eccosorb AN79

The main elements are:

quality and length.

Fig. 10) are used to prevent reflections.

For mechanical reasons, some parts of the tomograph (in particular the basement) are composed by an aluminum structure. Therefore, microwave absorber material (Fig. 9 and

During the development of the measurement system, the planning of the illuminating signal power and receiver sensitivity play, as previously mentioned, an important role. A good dynamic range is the key for reliable and accurate measurements. The entire measurement

 **VNA output power (typically -10 – 20 dBm):** usually the transmitted power level can be easily adjusted on the instrument. If needed, there is the possibility to add an

 **RF cable loss (typically 1 - 10 dB):** the signal amplitude loss due to the cable attenuation is variable in function of the investigation frequency band, of the cable

chain can be represented with the block diagram reported in Fig. 11.

external amplifier to increase the power level.

**Figure 11.** Measurement system block diagram

The most critical case is probably when the antennas are at a large distance and the SUT is relatively large. The specific example represents the attenuation of a SUT composed by wood (�� = 2.2 and � = �.�������).

**Figure 12.** Simplified signal amplitude budget.

In order to acquire multiview data, in the developed tomograph prototype both the Rx antenna and the SUT are rotating and the Tx antenna is static (see the previous section). This choice leads to obtain data with the angular increments needed by the inversion algorithms. Future developments will be devoted to the implementation of a fully static design. The developed tomographic system (Fig. 6) is the result of various optimization steps. One of the major problems was the positioning accuracy of the servo motors in charge of moving the object and the Rx antenna. This accuracy should be an order of magnitude better than the required angular increments.

**Figure 11.** Measurement system block diagram

**Figure 12.** Simplified signal amplitude budget.

required angular increments.

wood (�� = 2.2 and � = �.�������).

The most critical case is probably when the antennas are at a large distance and the SUT is relatively large. The specific example represents the attenuation of a SUT composed by

In order to acquire multiview data, in the developed tomograph prototype both the Rx antenna and the SUT are rotating and the Tx antenna is static (see the previous section). This choice leads to obtain data with the angular increments needed by the inversion algorithms. Future developments will be devoted to the implementation of a fully static design. The developed tomographic system (Fig. 6) is the result of various optimization steps. One of the major problems was the positioning accuracy of the servo motors in charge of moving the object and the Rx antenna. This accuracy should be an order of magnitude better than the




**Figure 13.** (a) Horizontal circular movement (*b*) and horizontal and vertical linear movement of the developed prototype.

As mentioned, in order to keep the potential reflections as low as possible, the body of the tomograph was developed in such way that all the metallic parts can easily be hidden from the inspection area using absorbers (Fig. 10). Moreover, since covering the vertical arms with absorber was unpractical, these parts have been manufactured in glass resin. These precautions have made it possible to reduce the impact of the tomograph construction on the measured samples to a strict minimum at the benefit of better results.

A major drawback of this type of configuration is the time required to obtain a single image. First of all, each motor movement (summarized in Fig. 13) requires some travel and settling time. Consequently, as the measurement points increase, the number of time intervals needed for the motors to stabilize and for the network analyzer to acquire the signals and send the data to the computer increases, too. Table 1 shows the total time required for a full series of mechanical displacements of the Rx antenna corresponding to one view of the SUT. This time needs to be multiplied by the number of different views (angular positions) of the SUT.


**Table 1.** Time consumption based on the mechanical positioning parameters.

The acquisition software developed for the management of the tomograph and running on a PC has to communicate with two different devices:


All steps involved in the acquisition procedure, i.e., a precise mechanical positioning, an accurate microwave measure, a lossless data transfer and storage need to be performed sequentially because an error free execution of each of them is necessary for a successful overall result. To address these requirements a simple but robust software has been developed (Fig. 14). It consists of a linear flowchart without parallelisms, which is flexible enough to allow adding checks after every step. The simplicity of the flowchart makes it possible to easily add processing blocks or functionalities in case of future needs. For a system prototype to be used during the design and validation of new imaging algorithms this was considered an important feature.

Nondestructive Evaluations by Using a Prototype of a Microwave Tomograph 65

**Figure 14.** Acquisition software flowchart.

64 Microwave Materials Characterization

**Number of measurement points** 

information;

SUT.

As mentioned, in order to keep the potential reflections as low as possible, the body of the tomograph was developed in such way that all the metallic parts can easily be hidden from the inspection area using absorbers (Fig. 10). Moreover, since covering the vertical arms with absorber was unpractical, these parts have been manufactured in glass resin. These precautions have made it possible to reduce the impact of the tomograph construction on

A major drawback of this type of configuration is the time required to obtain a single image. First of all, each motor movement (summarized in Fig. 13) requires some travel and settling time. Consequently, as the measurement points increase, the number of time intervals needed for the motors to stabilize and for the network analyzer to acquire the signals and send the data to the computer increases, too. Table 1 shows the total time required for a full series of mechanical displacements of the Rx antenna corresponding to one view of the SUT. This time needs to be multiplied by the number of different views (angular positions) of the

**Angular increment** 

3 90 0.5 11 27 1.5 16 18 2 31 9 3.5 46 6 5 91 3 10

The acquisition software developed for the management of the tomograph and running on a

the servo motor control units to provide position inputs and to read back status

All steps involved in the acquisition procedure, i.e., a precise mechanical positioning, an accurate microwave measure, a lossless data transfer and storage need to be performed sequentially because an error free execution of each of them is necessary for a successful overall result. To address these requirements a simple but robust software has been developed (Fig. 14). It consists of a linear flowchart without parallelisms, which is flexible enough to allow adding checks after every step. The simplicity of the flowchart makes it possible to easily add processing blocks or functionalities in case of future needs. For a system prototype to be used during the design and validation of new imaging algorithms

**(in degrees) Time (in minutes)** 

the measured samples to a strict minimum at the benefit of better results.

**Table 1.** Time consumption based on the mechanical positioning parameters.

the VNA to input acquisition settings and to transfer the measures.

PC has to communicate with two different devices:

this was considered an important feature.

### **4. Measurement and simulation results**

The measured data are stored and a processing method is applied in order to produce rough images of the reconstructed distributions of the dielectric parameters of the SUT cross section. The reconstruction procedure is based on the iterative application of the so-called distorted-Born approximation (Chew et al., 1994). Details concerning the implementation of the iterative procedures can be found in (Salvadè et al., 2010; Pastorino, 2010; Pastorino et al., 2006; Pastorino, 2004).

The capabilities of the proposed imaging systems have been preliminary assessed by means of comparisons with numerical results in terms of the measured electromagnetic field in the presence of scatters. The scatterer is a wood slab whose cross section has dimensions ��.�� � ��.����. A void circular hollow of radius of 2��� has been drilled in the sample. The values of the dielectric parameters of the wood slab are about 2.2 (relative dielectric permittivity) and 0.04 S/m (electric conductivity). Fig. 15 and Fig. 16 provides the values of the measured and simulated samples. In particular, the figure plots refer to the amplitudes and phases of the incident (Fig. 15) and total (Fig. 16) electric fields, respectively. As can be seen, the measurements are in good agreement with the simulated values.

A lot of wood and plastic slabs have been reconstructed by using the proposed prototype. The capabilities of the approach in detecting voids and defects inside these structures have been also evaluated, even in the presence of noise and interfering signals. The reader can refer to papers (Salvadè et al., 2010; Monleone et al., 2012; Pastorino et al., 2009) and the reference therein. An example is reported in the following. The inspected target is composed by a hollow wood beam with rectangular cross section of ��.����� � ��.���� and height of �0��� (with a rectangular hole of size �.����� � ��.����) and a plastic object of 11 cm × 9 cm and having the same height (with a rectangular hole of size �.����� � ��.0���) containing sand. The nominal values of the relative dielectric permittivities of wood, plastic, and sand are 1.8, 2.7, and 3, respectively. An example of the reconstruction results obtained with a frequency of �.����� is reported in Fig. 17. As can be seen, the reconstruction is fairly good and both the hollow cylinders can be quite accurately located (inside the test area) and shaped. The values of the relative dielectric permittivity are also quite similar to the actual ones, confirming that the approach is able to provide a so-called "quantitative" imaging.

More recently, the presence of metallic inclusions inside a dielectric structure has been also considered (Salvadè et al., 2008; Maffongelli et al., 2012). In this case, the imaginary part of the contrast function is retrieved, since it is related to the electric conductivity distribution of the structure. The possibility of locating metallic inclusions inside dielectric objects is clearly very appealing in several industrial application, e.g., in the wood industry, where undesired metallic object can compromise the industrial process (the row material can be not usable if foreign bodies are present). Moreover, these metallic inclusions may also damage the cutting machines used in that application.

Two examples are provided. In the first case, a wood slab of rectangular cross section of dimensions ������ � ����� is assumed. Its relative dielectric permittivity is equal to � � 2.2 and its electric conductivity is equal to � � 0.0������. For the considered scenario, 3 frequencies in the range ��, ������ are assumed Moreover, ��� sources, and � � �� measurement points are considered. A single inclusion, modeled as a perfect electric conducting (PEC) metallic inclusion (i.e., ���) with circular cross section of radius � � ���� and located at �2.0, 0.0����, has been considered.

The distribution of the electric conductivity [���] provided by the inversion algorithm are shown in Fig. 18. Although, largely underestimated (no a priori information about the metallic nature of the scatterers have been included into the electromagnetic model), the distribution of the conductivity allows one to correctly identify the inclusion present in the wood slab.

machines used in that application.

� � ���� and located at �2.0, 0.0����, has been considered.

The capabilities of the proposed imaging systems have been preliminary assessed by means of comparisons with numerical results in terms of the measured electromagnetic field in the presence of scatters. The scatterer is a wood slab whose cross section has dimensions ��.�� � ��.����. A void circular hollow of radius of 2��� has been drilled in the sample. The values of the dielectric parameters of the wood slab are about 2.2 (relative dielectric permittivity) and 0.04 S/m (electric conductivity). Fig. 15 and Fig. 16 provides the values of the measured and simulated samples. In particular, the figure plots refer to the amplitudes and phases of the incident (Fig. 15) and total (Fig. 16) electric fields, respectively. As can be

A lot of wood and plastic slabs have been reconstructed by using the proposed prototype. The capabilities of the approach in detecting voids and defects inside these structures have been also evaluated, even in the presence of noise and interfering signals. The reader can refer to papers (Salvadè et al., 2010; Monleone et al., 2012; Pastorino et al., 2009) and the reference therein. An example is reported in the following. The inspected target is composed by a hollow wood beam with rectangular cross section of ��.����� � ��.���� and height of �0��� (with a rectangular hole of size �.����� � ��.����) and a plastic object of 11 cm × 9 cm and having the same height (with a rectangular hole of size �.����� � ��.0���) containing sand. The nominal values of the relative dielectric permittivities of wood, plastic, and sand are 1.8, 2.7, and 3, respectively. An example of the reconstruction results obtained with a frequency of �.����� is reported in Fig. 17. As can be seen, the reconstruction is fairly good and both the hollow cylinders can be quite accurately located (inside the test area) and shaped. The values of the relative dielectric permittivity are also quite similar to the actual ones, confirming that the approach is able to provide a so-called "quantitative" imaging.

More recently, the presence of metallic inclusions inside a dielectric structure has been also considered (Salvadè et al., 2008; Maffongelli et al., 2012). In this case, the imaginary part of the contrast function is retrieved, since it is related to the electric conductivity distribution of the structure. The possibility of locating metallic inclusions inside dielectric objects is clearly very appealing in several industrial application, e.g., in the wood industry, where undesired metallic object can compromise the industrial process (the row material can be not usable if foreign bodies are present). Moreover, these metallic inclusions may also damage the cutting

Two examples are provided. In the first case, a wood slab of rectangular cross section of dimensions ������ � ����� is assumed. Its relative dielectric permittivity is equal to � � 2.2 and its electric conductivity is equal to � � 0.0������. For the considered scenario, 3 frequencies in the range ��, ������ are assumed Moreover, ��� sources, and � � �� measurement points are considered. A single inclusion, modeled as a perfect electric conducting (PEC) metallic inclusion (i.e., ���) with circular cross section of radius

The distribution of the electric conductivity [���] provided by the inversion algorithm are shown in Fig. 18. Although, largely underestimated (no a priori information about the metallic nature of the scatterers have been included into the electromagnetic model), the distribution of

the conductivity allows one to correctly identify the inclusion present in the wood slab.

seen, the measurements are in good agreement with the simulated values.

**Figure 15.** Comparison between the measured and simulated incident electric fields at the measurement points. (a) Amplitude (in volts per meter). (b) Phase (in degrees). © [2009] IEEE. Reprinted, with permission, from M. Pastorino, A. Salvadè, R. Monleone, G. Bozza, and A. Randazzo, "A new microwave axial tomograph for the inspection of dielectric materials," IEEE Trans. Instrum. Meas., vol. 58, no. 7, pp. 2072-2079, 2009.

(b)

In the second case, two buried objects, modeled as metallic inclusions with circular cross section of radius � � ���� and located at �−2.0, 0.0���� and �2.0, 0.0����, respectively, are considered. In this case, too, the reconstructed distribution of the electric conductivity, shown in Fig. 19, allows a correct identification of the two inclusions.

**Figure 16.** Comparison between the measured and simulated total electric fields at the measurement points. (a) Amplitude (in volts per meter). (b) Phase (in degrees). © [2009] IEEE. Reprinted, with permission, from M. Pastorino, A. Salvadè, R. Monleone, G. Bozza, and A. Randazzo, "A new microwave axial tomograph for the inspection of dielectric materials," IEEE Trans. Instrum. Meas., vol. 58, no. 7, pp. 2072-2079, 2009.

### **5. Conclusions**

68 Microwave Materials Characterization

58, no. 7, pp. 2072-2079, 2009.

considered. In this case, too, the reconstructed distribution of the electric conductivity,

(a)

**Figure 16.** Comparison between the measured and simulated total electric fields at the measurement points. (a) Amplitude (in volts per meter). (b) Phase (in degrees). © [2009] IEEE. Reprinted, with permission, from M. Pastorino, A. Salvadè, R. Monleone, G. Bozza, and A. Randazzo, "A new

(b)

microwave axial tomograph for the inspection of dielectric materials," IEEE Trans. Instrum. Meas., vol.

shown in Fig. 19, allows a correct identification of the two inclusions.

In this paper, the development of a prototype of a tomographic imaging systems working at microwave frequencies has been reported. Starting with a brief review of microwave measurement concepts, the details of the designed system have been reported. Morever, some measurements of the incident and total electric field have been provided, together with comparisons with simulated data. Finally, some reconstruction results concerning the

**Figure 17.** Reconstructed distribution of the dielectric permittivity of an inhomogeneous target (constituted by two hollow cylinders with rectangular cross sections, made of wood and plastic materials; one of them is filled by sand).

**Figure 18.** Reconstructed image of a wood slab with a metallic inclusion. © [2008] IEEE. Reprinted, with permission, from A. Salvadè, M. Pastorino, R. Monleone, A. Randazzo, T. Bartesaghi, G. Bozza, and S. Poretti, "Microwave imaging of foreign bodies inside wood trunks," *Proc. 2008 IEEE International Workshop on Imaging Systems and Techniques (IEEE IST08)*, Chania, Crete, Greece, Sept. 10-12, 2008.

inspection of dielectric material have been shown. The case of wood material with metallic inclusion has also been reported. Although quite preliminary, these results demonstrated that the inspection of these materials with microwave tomography is feasible. The developed system is quite simple and relatively inexpensive and can be further considered for application at several industrial levels, e.g., for nondestructive estimations in wood industry.

**Figure 19.** Reconstructed image of a wood slab with two metallic inclusions. © [2008] IEEE. Reprinted, with permission, from A. Salvadè, M. Pastorino, R. Monleone, A. Randazzo, T. Bartesaghi, G. Bozza, and S. Poretti, "Microwave imaging of foreign bodies inside wood trunks," *Proc. 2008 IEEE International Workshop on Imaging Systems and Techniques (IEEE IST08)*, Chania, Crete, Greece, Sept. 10-12, 2008.

### **Author details**

R. Monleone, S. Poretti, A. Massimini andA. Salvadè

*Department of Technology and Innovation, University of Applied Sciences of Southern Switzerland, Switzerland* 

M. Pastorino and A. Randazzo

*Department of Naval, Electrical, Electronic and Telecommunication Engineering, University of Genoa, Italy* 

### **6. References**

Agilent (2005). Appl. Note 5989-2589EN: Basic of measuring the Dielectric properties of Materials.

Boughriet, A. K.; Legrand, C. & Chapoton, A. (1997). Noniterative stable transmission/reflection method for low-loss material complex permittivity determination, IEEE Trans. Microwave Theory Tech., vol. 45 (1997), pp. 52-57.

Balanis, C. A. (1989). Advanced Engineering Electromagnetics, Wiley, NewYork, 1989.

Bertero, M.; Miyakawa, M.; Boccacci, P.; Conte, F.; Orikasa, K. & Furutani, M. (2000). Image restoration in Chirp-Pulse Microwave CT (CP-MCT), IEEE Trans. Biomed. Eng., vol. 47 (2000), pp. 690-699.

70 Microwave Materials Characterization

**Author details** 

M. Pastorino and A. Randazzo

*University of Genoa, Italy* 

**6. References** 

Materials.

*Switzerland* 

R. Monleone, S. Poretti, A. Massimini andA. Salvadè

inspection of dielectric material have been shown. The case of wood material with metallic inclusion has also been reported. Although quite preliminary, these results demonstrated that the inspection of these materials with microwave tomography is feasible. The developed system is quite simple and relatively inexpensive and can be further considered for application at several industrial levels, e.g., for nondestructive estimations in wood industry.

**Figure 19.** Reconstructed image of a wood slab with two metallic inclusions. © [2008] IEEE. Reprinted, with permission, from A. Salvadè, M. Pastorino, R. Monleone, A. Randazzo, T. Bartesaghi, G. Bozza, and S. Poretti, "Microwave imaging of foreign bodies inside wood trunks," *Proc. 2008 IEEE International Workshop on Imaging Systems and Techniques (IEEE IST08)*, Chania, Crete, Greece, Sept. 10-12, 2008.

*Department of Technology and Innovation, University of Applied Sciences of Southern Switzerland,* 

Agilent (2005). Appl. Note 5989-2589EN: Basic of measuring the Dielectric properties of

Boughriet, A. K.; Legrand, C. & Chapoton, A. (1997). Noniterative stable transmission/reflection method for low-loss material complex permittivity

determination, IEEE Trans. Microwave Theory Tech., vol. 45 (1997), pp. 52-57. Balanis, C. A. (1989). Advanced Engineering Electromagnetics, Wiley, NewYork, 1989.

*Department of Naval, Electrical, Electronic and Telecommunication Engineering,* 


microwave imaging, Proc. Advanced Electromagnetics Symposium 2012 (AES'2012), Paris, France, April 16-19, 2012.


**Chapter 0 Chapter 5**
