The feasibility of an MRI compatible ultrasound transducer

The feasibility of an MRI compatible

ultrasound transducer

A.H. (Anne) Schrader

BSc Report

C e

Ir. F.S. Farimani Dr.ir. J.R. Buitenweg Dr.ir. J.F. Broenink

July 2016 021RAM2016 Robotics and Mechatronics

EE-Math-CS University of Twente

P.O. Box 217

7500 AE Enschede

The Netherlands

Abstract

The goal of this thesis is to determine the feasibility of an MRI compatible ultrasound trans- ducer. Combining the capabilities of MRI and ultrasound allows us to create a superior imaging method which has a fast frame rate and a high quality image. A literature study was combined with prototype testing to determine which materials were suitable for use in an MRI compat- ible ultrasound transducer. The presence of conductive materials does not seem to be a di- rect problem for the safety of the patient and the imaging quality, while ferromagnetic metals should be avoided. Image degradation occurred when a signal was applied to the prototype.

Despite expectations set by the literature study, aluminium did not provide sufficient shield- ing, as an increase in zipper artefacts could be seen in the MRI image. This study showed that it should possible to make an MRI compatible ultrasound transducer if shielding is taken into account.

Keywords: Ultrasound imaging, MRI compatibility, Magnetic Resonance Imaging

Acknowledgements

This report is the final step in my study Biomedical Technology at the University of Twente.

Without the help of several people, this thesis would not have existed. First of all, my thanks go to Foad S. Farimani, my daily supervisor, for supporting me during the 10 weeks writing this thesis took. Also I would like to thank all the people at RaM for their support and of course my friends outside of RaM, for putting up with me at the stressful moments and standing by me.

Lastly I would like to thank Ferroperm Piezoelectrics A/S for sending me their piezoelectric elements free of charge.

Anne Schrader

Enschede, July 2016

Abbreviations

McRobot MRI compatible Robot

MURAB MRI and Ultrasound Robotic Assisted Biopsy MIS Minimally Invasive Surgery

CT Computed Tomography

RF Radio Frequent

MRI Magnetic Resonance Imaging

MHz MegaHertz

RaM Robotics and Mechatronics department

Port Port Plastics

Ciba Ciba-Araldite Products

Li Li Tungsten Co.

Si Sigri Corporation, Carbon and Graphite

Rohm Rohm and Haas

H Hysol Divison, Dexter Corp.

Contents

1 Introduction 1

1.1 Problem statement . . . . 1 1.2 Background . . . . 3

2 Overview 9

2.1 State of the Art . . . . 9 2.2 Materials . . . . 13

3 Requirements 21

4 Conceptual design 23

5 Prototyping & Testing 26

6 Results 28

7 Evaluation 30

7.1 Discussion . . . . 30 7.2 Conclusion . . . . 30 7.3 Future work . . . . 31

A Appendix 1 35

A.1 Phantom model . . . . 35

List of Figures

1.1 Several possible crystal geometries. A: Thickness-expander rectangular plate.

B: Thickness-expander circular plate disk. C: Length-expander bar. D: Width-

extensional bar or beam plate. [2] . . . . 3

1.2 Artefacts resulting from the presence of metal inside the MRI. . . . 5

1.3 Effect of paramagnetic and diamagnetic metals on a magnetic field. M denotes the metal side, while S is the surrounding material. [14] . . . . 6

1.4 Effect of paramagnetic, ferromagnetic and diamagnetic metals on a magnetic field, with µ

the permeability of the metal and µ

the permeability of the sur- rounding material. . . . 7

1.5 Zipper artefacts shown along the vertical axis. . . . 7

1.6 Schematic view of a coax cable including charges and an AC current. S is the shielding, while c is the core. . . . 8

2.1 Section view of the MARIUS transducer. [15] . . . . 9

2.2 Shielded ultrasound transducer with fiducial markers. [16] . . . . 9

2.3 Piezoelectric PZT-4 covered in Cu-Epoxy composite. [17] . . . . 10

2.4 Setup of an optical transducer with two lasers. [8] . . . . 11

2.5 Schematic view of an optical transducer. [8] . . . . 11

2.6 Transducer with two matching layers. Zp is the piezoelement, Zt is the tissue and Z1 and Z2 show the matching layers and Z is the width of both matching layers. . 16

2.7 Effect of different percentages of filling on acoustic impedance (l) and attenuation (r) at 30 Mhz. [27] . . . . 17

2.8 Attenuation of different composites at frequencies from 3 to 7 MHz in Spurr epoxy. From top to bottom: tungsten, PZT, alumina and pure Spurr epoxy. [28] . 18 5.1 The copper prototype(left) and piezo prototype(right). . . . 26

5.2 Phantom with the copper prototype inserted and wrapped in plastic foil. . . . 27

6.1 Results of the MRI-scans. A: Phantom B: Phantom + piezo C: Phantom + Copper prototype D: Phantom + transducer prototype . . . . 28

6.2 MRI-scan of phantom and piezoelectric element, deeper slice. . . . 28

6.3 Results of the MRI-scans with power on the prototype. The green square shows the location of the prototype, while the yellow square denotes roughly the inner square in the phantom. A: Copper prototype with aluminium B: Copper proto- type without aluminium. C: Copper prototype with aluminium and power. D: Copper prototype without aluminium and power. . . . 29

A.1 Drawing of the structure of the phantom used in the images. Measurements are

in mm. . . . 35

1 Introduction

In recent years image-guided interventions are used increasingly in the medical field, with one of the imaging techniques being magnetic resonance imaging (MRI). MRI is able to provide medical personnel with high resolution images of human tissues, without exposing the patient to damaging radiation. The downside of MRI is that it takes a long time to complete and the costs are much higher than those of other imaging modalities. Also, MRI is not real-time and the images are susceptible to movement artefacts.

In this thesis the main focus will be on interventions in the form of an image-guided biopsy, as it is essential to know if the needle is in the correct position during a biopsy. Using MRI to determine the position of the needle takes a long time as the patient has to be taken out of the MRI scanner repeatedly to adjust the needle and later check the position of the needle again inside the MRI. On the other hand, using another imaging modality like ultrasound has its downsides too. While ultrasound is real-time and cheap to perform, the resulting images are of poor quality. If there are similar tissues next to each other it becomes increasingly difficult to determine the correct location of the needle.

In general combining ultrasound and MRI increases the susceptibility of imaging, as each modality can differentiate between different tissues and materials. The combination also al- lows real-time images to be taken with the resolution of an MRI image. For RaM the usefulness lies with the Murab and McRobot project.

Murab aims to create a robot that can perform biopsies, by means of mechanical imaging and image fusion outside of the MRI. This does not only speed up the process, but will also be able to detect more lesions as ultrasound and MRI both have the ability to detect different lesions. By making a transducer which can be present during an MRI scan, we can link the MRI image to the ultrasound image. This MRI compatible transducer enables merging of the separate images later on.

McRobot on the other hand aims to create a fully MRI compatible robot. Ultrasound will be used to guide the robot to the correct positions inside the body while the MRI is switched on. Because of this, the patients do not have to be removed from the MRI at any point during the biopsy. To achieve this, it has to be shown that it is feasible to create an MRI compatible ultrasound transducer within the department at a reasonable cost.

1.1 Problem statement

While MRI is used more often, the MRI is both time consuming and costly. A breast biopsy with MRI takes around 1 hour, but can take even longer if the needle placement has to be rechecked.

This makes MRI waiting times very long, up to a 100 days for routine scans.

In this thesis the feasibility of an MRI compatible ultrasound transducer will be determined, which could enable MRI and ultrasound image fusion to take place. For Murab the aim is that the full biopsy can be done outside the MRI after one scan, as the ultrasound and MRI image will be fused which creates a high-quality real-time image of the biopsy area. In com- bination with the McRobot the full biopsy can take place inside the MRI. Because of this the patient does not have to be moved in and out the MRI and as a result the biopsy time is reduced.

Currently ultrasound transducers cannot be used within an MRI because they contain metals,

cannot be tracked inside the MRI and do not contain shielding to block generated radio fre- quent(RF) waves. Ferromagnetic metals can be dangerous for the patient and cause damage to the machine. Non-ferromagnetic metals are a danger because of inductive heating, eddy currents and deformation of the magnetic field, possibly burning the patient and decreasing image quality of both the MRI and the ultrasound image.

During this study the following questions will be answered in depth:

1. Is it possible to have an MRI compatible ultrasound?

2. Why are current ultrasound transducers not compatible with MRI?

3. What has been done before in regard to MRI compatible ultrasound?

4. How is an ultrasound transducer made?

5. What materials can be used inside the MRI?

1.2 Background

To answer the questions posed above, it is important to know how MRI and ultrasound works.

This will be covered in the following sections.

Ultrasound imaging

Ultrasound is one of the most used medical imaging technologies [1]. Ultrasound waves are generated by transducers, which are reflected by tissue and again received by the transducer.

The elements that are used nowadays to generate and receive these ultrasound waves are piezo- electric elements. When the material is excitated electrically it expands or contracts, which in turn produces ultrasound waves. While receiving ultrasound, the piezoelectric material is de- formed slightly, which creates an electrical pulse. This pulse can be detected by machines and later transformed into an image. The ratio of this conversion from the electrical domain to the mechanical domain is dependent on the coupling coefficient, which is in turn dependent on the inherent material properties and geometry [2]. The different crystal geometries that can be used for piezoelectric elements can be seen in figure 1.1.

Figure 1.1: Several possible crystal geometries. A: Thickness-expander rectangular plate. B: Thickness- expander circular plate disk. C: Length-expander bar. D: Width-extensional bar or beam plate. [2]

In the past thickness expander elements, which have good conversion in the thickness direc- tion, were mostly use in single element transducers. Now with the existence of array transduc- ers, width and thickness expander transducers are used more often.

The wavelength of a wave determines the resolution and imaging depth. Higher frequencies create a higher resolution, but they cannot penetrate as deep into the body [3]. Lower frequen- cies can penetrate deeper into the body. Because of this, broadband transducers, which are transducers that span several frequencies, are needed to correctly image several layers of the body. The downside of a broadband transducer is that the broader the signal, the lower the sensitivity of the transducer [4].

The frequency of an ultrasound wave with wavelength λ and speed v is given by equation 1.1 [5].

F r equenc y = v

λ (1.1)

The wavelength and amount of periods per pulse (ppp) can be used to calculate the spatial pulse length (SPL) as seen in equation 1.2.

SP L = ppp • λ (1.2)

The axial resolution of a transducer is equal to half the SPL and a shorter SPL creates a broader

bandwidth, which is the range of frequencies that a transducer outputs. This in turn tells us

that a broader bandwidth gives a smaller axial resolution. The bandwidth is often given as a

percentage of the central frequency, so a 5 MHz transducer with a bandwidth of 50% has signal ranging from 2.5 MHz to 7.5 MHz. Bandwidth is important for imaging transducers, as without bandwidth only one layer of tissue can be imaged.

Approximate imaging frequencies in medical imaging for different areas can be seen in table 1.1.

Table 1.1: Approximate imaging frequencies used in medical ultrasound. [6]

Frequency Imaging area

2.5 MHz Deep abdomen, obstetric and gynaecological imaging 3.5 MHz General abdomen, obstetric and gynaecological imaging 5.0 MHz Vascular, breast, pelvic imaging

7.5 MHz Breast, thyroid

10.0 MHz Breast, thyroid, superficial veins, superficial masses, musculoskeletal imaging 15.0 MHz Superficial structures, musculoskeletal imaging

To generate the ultrasound waves needed for imaging, two principles can be used. The most common option is the use of piezoelectric elements, for which the generated wavelength is approximately equal to twice the thickness of the element [7]. Higher frequencies thus mean thinner piezoelectric elements, which in turn limits the highest frequency possible as materials become increasingly fragile. Most piezoelectric medical transducers operate between a range of 1-15 MHz.

Another option for ultrasound generation is an optical transducer. Optical transducers can be made by using the thermoelastic effect to generate ultrasound. The downside of this is that most optical transducers work in the frequency range of 20-50 MHz [8] and cannot be focused on a central frequency correctly. As an optical transducer does not contain electrical parts, it requires less shielding than a piezoelectric transducer.

Magnetic Resonance Imaging

The second imaging modality that will be covered is MRI. In MRI, the spin of the nuclei of hy- drogen atoms is used to image different tissues bases on their hydrogen content. A full expla- nation is beyond the scope of this thesis, but in short the scanner sends out an radio frequent (RF) pulse, which causes the hydrogen atoms to move slightly and thus create a change in the magnetic field of the which can be detected [9].

There are several scans that can be used, but in this thesis only the Spin Echo T1 will be used for testing. In a spin echo a 90 degree RF excitation pulse is followed by a 180 degree RF refocus- ing pulse. When a scan is T1 weighted, as the Spin Echo T1 is, fat appears bright, while water appears darker [10]. While taking an scan there are several parameters that affect the image quality [10].

• Repetition time (TR) is defined as the time between two consecutive scans. TR influ- ences T1 contrast strongly.

• Echo time (TE) is the time between the RF excitation pulse and the pulse echo.

• Slice thickness is the depth of each slice and thus determines the details that can be seen in the MR image.

• Number of acquisitions shows the amount of times that an RF pulse is applied to the same voxel. More acquisitions increase the signal to noise ratio.

• Field of View (FOV) determines the dimensions of the imaging area.

For each scanning method different settings are required. Besides the options discussed above, there is also the acquisition matrix, which is the total number of independent data samples in the image and depends on the settings used in the scan. When a scan is taken, it has to be made sure that any object present inside the while testing is MRI compatible.

MRI compatibility

Before MRI compatibility can fully be taken into account, a definition has to be known. In 2005 ASTM International [11] published new definitions for MRI compatibility, which included the terms MRI-safe, MRI-unsafe and MRI-conditional.

• MRI-Safe: it poses no known hazards in de MRI environment.

• MRI-Conditional: it poses no known hazards in the MRI environment with specific con- ditions of use.

• MRI-Unsafe: it poses known hazards in all MRI environment.

As can be seen an MRI-Safe or MRI-conditional material does guarentee safety of the patient, but not quarantee good image quality. Besides ASTM other companies have tried to define MRI compatibility. GE guidelines [12] define four zones of MRI compatibility as follows:

Zone 1 If it may remain in the imaging volume and in contact with the patient throughout MRI scanning.

Zone 2 If it can remain in the imaging volume and in contact with the patient throughout the procedure and scanning, but is not located in the imaging field.

Zone 3 If it is used within the imaging volume, but will be removed during scanning or when not in use.

Zone 4 If it is suitable for use in the magnet room during procedure when kept at least one meter from the isocenter or beyond the 20 mTesla line.

The aim is to design a transducer that fits in zone 1 according to GE guidelines and a MRI-Safe or MRI-Conditional transducer according to ASTM definition. Besides these formal guidelines, image quality also has to be sufficient for any solution to work.

Image quality is dependent on the presence of image artefacts in the final image. As MRI makes use of a magnetic field to put the hydrogen atoms in their starting position and then sends out RF pulses to change the position of the hydrogen atoms, it is important for the magnetic field to be the same in all places. When there is a change in the magnetic field of the MRI, an artefact can be seen at the place of the inhomogeneity. This can be seen in figure 1.2 and is called a local inhomogeneity of the magnetic field [10].

Figure 1.2: Artefacts resulting from the presence of metal inside the MRI.

These changes in the magnetic field due to metal pieces can be explained by Maxwell’s equa- tions, which can be seen in equations 1.3 and 1.4 [13].

B

= B

(1.3)

1

µ

• B

= 1

µ

• B

(1.4)

Here B is the magnetic field strength, with B

and B

signifying the strength in different di- rections as can be seen in figure 1.3. µ is the magnetic permeability, which shows how the material affects the surrounding magnetic field. Ferromagnetism is basically the same as para- magnetism, in the sense that it pulls in the magnetic field, but for ferromagnetism the effect is much stronger, which is shown by the fact that a force acts upon ferromagnetic materials inside a magnetic field. Comparing the magnetic permeability relative to the magnetic permeability of the vacuum of both materials, that of paramagnetic materials is only slightly higher than 1, while ferromagnetic materials can have a magnetic permeability up to the hundreds.

Figure 1.3: Effect of paramagnetic and diamagnetic metals on a magnetic field. M denotes the metal side, while S is the surrounding material. [14]

From the above equations we obtain the following equation [13], 1

µ

• B

B

= 1

µ

• B

B

(1.5)

With this equation it can be concluded that when µ

≈ µ

the magnetic field is not affected

much, as B

/B

≈ B

/B

. But if µ

>> µ

, then B

/B

>> B

/B

, which means that α

is

almost 90

. This results in B being perpendicular to the surface, which can be seen in figure

1.4 as ferromagnetism. Paramagnetism has the same effect as ferromagnetism, except much

weaker. Diamagnetism has the opposite affect, thus pushing the magnetic field away and this

happens when µ

<< µ

.

Figure 1.4: Effect of paramagnetic, ferromagnetic and diamagnetic metals on a magnetic field, with µ

the permeability of the metal and µ

the permeability of the surrounding material.

Currently there is no method to fully eliminate this type of artefacts, but the effect can be de- creased. A shorter TE, larger acquisition matrix and smaller FOV may decrease the size of the artefact [10]. Another artefact that can show up in the MRI image is the zipper artefact, which looks like lines of alternating bright and dark pixels running through the image as can be seen in figure 1.5 They are caused by RF noise from outside and can thus be reduced by decreasing the amount of RF noise [9].

Figure 1.5: Zipper artefacts shown along the vertical axis.

Reducing RF noise can be done with shielding, for which a good example is a coax cable. Fig-

ure 1.6 shows a schematic view, with I being the current, which changes over time and L the

circumference of the cable.

Figure 1.6: Schematic view of a coax cable including charges and an AC current. S is the shielding, while c is the core.

The shield will generate a charge opposite to the charge of the core, so when the charges are exactly the same in magnitude the electric fields cancel each other. To avoid an electric charge taking place on the outside of the shielding, it should be grounded, allowing the charge to be dispersed.

For the magnetic field we know that;

I

C

B • dl = µ

Z

S

I d A (1.6)

For a wire with circular symmetry this means that equation 1.7 is true.

I

C

B • dl = 2πr • B = µ

(I

+ I

) (1.7)

Equation 1.7 shows that when the shielding always has the opposite charge to the core, there

will not be a resulting magnetic field outside of the wire caused by the current through the

cable, as the currents will be opposites of each other.

2 Overview

2.1 State of the Art

Research has been done to create MRI compatible ultrasound transducers, with many different approaches. In this section, several methods will be explained to give an overview of what has been done and what is possible.

MARIUS transducer

MARIUS (Magnetic Resonance Imaging Using Ultrasound) aims to combine ultrasound and MRI to make biopsies quicker and gentler and is developed by the Fraunhofer Institute for Biomedical Engineering [15]. This project has created a new transducer based on existing transducers, which makes no use of ferromagnetic materials and uses multi-layered metal shielding to make it MRI compatible. As the wires do not disturb the imaging significantly, these were left unshielded [15].

Figure 2.1: Section view of the MARIUS transducer. [15]

To increase the imaging area of the transducer an MRI compatible motor is used, allowing the transducer to rotate 180 degrees. The MRI compatible motor is made of non-ferromagnetic components and driven by piezoelectric elements. Figure 2.1 shows the final result, which is a patient mounted transducer that is fixed to the breast with adhesive pads. During testing no noticeable difference between a turned off and turned on state can be seen and the signal-to- noise (SNR) ratios are the same for the multi-layered shielded transducer.

Tang transducer

A.M. Tang et al. [16] attempted to create an MRI compatible transducer by adapting an existing one. The ultrasound transducer used for this is a Terason T3000. To limit the RF interference, the transducer is completely wrapped in aluminium foil. Braided shielding, which basically are metal fibers wrapped around the cable with a braiding technique, is put around the cable and both braid and aluminium are grounded to the room.

Figure 2.2: Shielded ultrasound transducer with fiducial markers. [16]

The transducer is tracked while in the MRI. Passive tracking is done with four fiducial markers attached to the head, as can be seen in Figure 2.2. Active tracking is done with an electro- magnetic tracking system from EndoScout, Robin Medical. The system measures the induced voltage on the sensor coils by the MRI and with calibration the location and orientation of the transducers can then be determined.

No broadband noise, which is noise whose energy is spread over a large section of the ultra- sound range, was observed when de ultrasound probe was placed in the isocenter without any shielding when switched off. The introduction of aluminium foil to the probe strongly reduced the amount of broadband noise. Covering the ultrasound windows with a single layer of foil reduced zipper noise effectively, but this coverage of foil caused some degradation in the ultra- sound images, which was remedied by turning up the time gain compensation.

The results of this experiment show that it is possible to adapt an already existing ultrasound transducer in such a way that it is MRI compatible.

Cu-Epoxy transducer

B. Gerold et al. [17] attempted to create a piezoelectric transducer by examining each compo- nent for MRI compatibility. The main invention is a Cu-Epoxy Composite material that can be used as shielding, but at the same time will function as a matching layer for ultrasound.

The piezoelectric transducer used, which can be seen in 2.3, was made of Lead Zirconate Titanate(PZT-4) and covered in the Cu-Epoxy mixture. No compatibility issues arose with this material.

Figure 2.3: Piezoelectric PZT-4 covered in Cu-Epoxy composite. [17]

The Cu-Epoxy material is made 55% epoxy and of 45% copper which in turn is compromised of 30% 200 mesh powder and 70% 325 mesh powder. The results show an MRI-conditional transducer, which depending on the settings can be used inside the MRI without problems.

Optical transducer

S. Ashkenazi et al. [8] aimed to create a fully operational optical transducer. This is done by combining an etalon and thermoelastic material. An etalon works by changing its thickness when an ultrasound wave passes through and thus changes the optical path of the etalon. Fig- ure 2.5 shows a schematic view.

Thermoelastic material is used for its ability to emit ultrasound waves when heated up. S.

Ashkenazi et al. [8] used a black PDMS film as thermoelastic layer. Two lasers are focused on

this thermoelastic/etalon structure to generate and detect ultrasound, as can be seen in fig- ure 2.4. The pulsed laser is used for heating the PDMS film, causing the ultrasound waves to be emitted in pulses. These ultrasound waves are reflected in the tissue and will return to the structure over time. When the ultrasound waves return they will change the cavity size of the etalon and in turn the optical path of the probing laser changes, which can be detected.

Figure 2.4: Setup of an optical transducer with two lasers. [8]

In Figure 2.4 a full setup is shown, including two lasers and a beam splitter. Most etalons are made with two gold layers [8], with a thickness of 30 nm. To generate the ultrasound a PMDS layer is used.

Figure 2.5: Schematic view of an optical transducer. [8]

The downside of this method is that the receiving and sending elements cannot be the same, as can be seen in Figure 2.5.

Summary

Table 2.1 summarizes the different types of transducers with pros and cons. None of the trans-

ducers are for sale commercially and no sale prices are available for this reason.

Table 2.1: Overview of current MRI transducers

Transducer Benefits Downsides

Marius Good quality imaging.

Imaging area can be changed.

Not locatable/traceable inside MRI. Additional casing for McRobot.

Optical transducer No electronic needed. Might not work for lower frequencies. Research still very new.

Tang transducer Adapted from already ex- isting technology. Locat- able in MRI.

Image quality not good.

Cu-Epoxy transducer Custom-made material that can be altered to purposes.

Image quality better with different MRI settings.

Custom-made materials

are more difficult to

replicate.

2.2 Materials

An ultrasound transducer consists of several components that each have their own function. To make sure that the transducer is MRI compatible, each part will have to be examined separately to make sure it does not contain ferromagnetic materials. Dia- and paramagnetic materials are not a direct problem inside the MRI, as their effect on the magnetic field is limited. This means these materials can be used, but will have to be tested to make sure their effect is limited and does not harm the patient or the image quality and hence is fully MRI compatible, as defined before.

A basic transducer is made of the following components [3]:

• The ultrasound generating element is a piezoelectric element in most transducers and is responsible for generating ultrasound waves. In recent years research has shown the possibility of using fully optical transducers to generate ultrasound.

• The matching layer is required to transfer the ultrasound waves correctly from the gen- eration element to the imaging object.

• A backing material is needed to prevent excessive vibration of the ultrasound generating element. It causes the element to generate shorter pulse lengths, thus improving axial resolution. Current backing materials often contain metals to increase the density of the material, which decreases the MRI compatibility.

• The housing is needed to contain the whole structure of backing and matching layers and the piezoelectric element.

• An acoustic lens is used to focus the ultrasound beam.

The housing and acoustic lens will not be considered here, as this thesis focusses on the basics of a transducer, which means the piezoelectric element, the backing layer and the matching layer. To make it MRI compatible, shielding materials will also be considered.

Besides the standard components, MRI markers will also be covered. MRI markers are needed for locating the ultrasound transducer inside the MRI, which is essential for fusing the MRI and ultrasound images

In the following section possible materials for the aforementioned components will be covered in depth.

2.2.1 Piezoelectric materials

Choosing a piezoelectric material depends on the purpose of the final transducer. There are several material properties that have to be considered before making a final choice.

First there is the coupling coefficient, which represents the efficiency of converting mechanical energy into electrical energy and vice versa. Depending on the geometry of the piezoelectric el- ement and the usage, different coupling coefficients are important. For a piezoelectric element of the thickness expander group, the thickness coupling coefficient [18] is most important and a value in the range of 0.45-0.55 is needed for optimal performance. Besides a high thickness mode coupling coefficient, a low planar coupling coefficient is optimal, as it limits the energies in the plane of the material.

Another characteristic to look at is the acoustic impedance (Z). The acoustic impedance is de- fined as the ratio of the density ( ρ) and the acoustic velocity (V) of a material as can be seen in equation 2.1.

Z = ρ

V (2.1)

For ultrasound applications an acoustic impedance close to that of tissue is preferred, which

is 1.5 Mrayls [18]. The lower the material’s impedance, the fewer matching layers are needed

to facilitate the movement of the ultrasound waves from the ultrasound generating element to the tissue.

There are several types of piezoelectric elements, with each their benefits and downsides. A summary can be seen in 2.2.

Table 2.2: Overview of piezoelectric material types [18] [19]

Material type Benefits Downsides

Piezoceramics(PZT) Low planar coupling co- efficient and high thick- ness mode coupling co- efficient

Lead based ceramics have often high acous- tic impedance (>30 MRayls).

Piezopolymers Very low acoustic

impedance. Good

receiving.

Low thickness mode coupling coefficient and small dielectric con- stants, thus low transmit ability.

Piezoceramic/polymer Tailored properties, higher thickness mode coefficient and lower densities. Higher poly- mer fraction gives lower acoustic impedance.

Diminished dielectric constant means more difficult system cou- pling. More costly and difficult to manufacture than the others.

Currently PZTs are the most used piezoelectric material used in medical transducers, either in pure form or in a composite with a polymer. This is because of their favourable coupling coefficients, their physical strength and the low cost of manufacturing [20].

PZT-5H and PZT-5A [18] are the most commonly used PZT materials. PZT-5A is recommended for instrument applications because of its high sensitivity, temperature stability and time sta- bility. The main difference between 5H and 5A is the dielectric constant, which is much higher for 5A(5A ≈ 1700, 5H ≈ 3400) [19]. 5H on the other hand has a better temperature stability.

While this sounds very clear cut, in reality companies have made many small variations on the basic PZT materials.

Both 5H and 5A can be used in piezoceramic/polymer materials, as can most other piezoelec-

tric materials. Most commonly is the 1-3 connectivity materials, for which some characteris-

tics can be seen in table 2.3 Smart Materals [21] manufactures 1-3 composites with PZT-5H and

PZT-5A materials with the methods of arrange & fill and dice & fill. Due to simpler production

method arrange & fill is the cheaper method.

Table 2.3: 1-3 random fill piezo composite specifications. [21]

Frequency range 40kHz ... 10MHz Available fill factors

® 800µm fiber 80%*

® 250µm fiber 65%*

® 105µm fiber 35%*

Acoustic impedance ∼14 ... 24 MRayl Coupling coefficient ∼0.65

Maximum dimensions 60mm x 60mm

Max. temperatures 100

C (130

C HT version) Matrix material Epoxy / Polyuretane

Available electrodes 1.5 µm CuSn ≤ 5MHz ≤ 800nm Au

The choice of the final type of piezoelectric element depends on several factors. Composites cost more due to a more complex production method, but the upside of this is that the com- posite can be tailored to the specifications of the user. While PZT materials cannot be tailored to specifications as easily they are much cheaper to produce.

2.2.2 Matching layers

Matching layers in transducers are needed to negate the ringing problem. The ringing problem, which is an undesired oscillation of a signal, is caused by internal reflection of ultrasound at the backing layer and matching layer boundary. This reflection is caused by differences between the acoustic impedance of two materials.

Piezoelectric elements have an acoustic impedance of around 30 MRayls for ceramics, 3-4.5 MRayls for polymers and everything in between for composites [22]. As tissue has an average acoustic impedance of 1.5 MRayls, matching layers are needed to make sure there is sufficient transmission between the different layers. The ratio of reflection from medium 1 to medium 2 can be calculated with equation 2.2 with Z being the acoustic impedance of the respective medium [23].

R =

µ Z

− Z

Z

+ Z

¶

(2.2) This is optimised for single matching layers when [23]:

Z

= q

Z

• Z

(2.3)

Equation 2.3 is only correct when the matching layer has a thickness of 1/4 λ. While this equa- tion is sufficient for use in medical transducers, it assumes that all materials are fluids and does not consider the resonance of the piezoelectric elements and is thus not precise [22].

Optimized matching layers must not only have a sufficient acoustic impedance, but also a low attenuation coefficient to decrease the loss of ultrasound. The attenuation of a material is the loss of ultrasound energy in said material [2]. An attenuation of 10 dB/mm or lower is in general considered good, but this of course depends on the situation. It might be beneficial to choose a material with a better matching impedance even when it has a higher attenuation, but this has to be evaluated on a case to case basis.

As most polymers have a acoustic impedance of 1-5 MRayls and other materials already have an impedance of at least 14 MRayls (glass) [22], composites are often used as matching layers to get a desired acoustic impedance.

If we consider the transmission between PZT (Z ≈ 30MRa yl s) and tissue (Z ≈ 1.5MRa yl s) we get a reflection coefficient of (1.5 − 30)

/(30 + 1.5)

= 0.82. This shows that less than 20%

of the wave can pass the PZT-tissue barrier. The acoustic impedance of an optimal matching

layer between PZT and tissue is equal to p

1.5 ∗ 30 = 6.7MRa yl s. Matching between those two materials will thus require a composite.

An aluminium oxide (Al

O

) composite can be used as matching layer. Depending on the volume fraction of aluminium oxide the impedance varies between 3 and 6 MRayls. This may be improved even more by increasing the volume percentage beyond 35% or switching up the used polymer. The acoustic attenuation for this material lies between 12 and 16 dB/mm [8].

These measurements are all done at a frequency of 30 MHz, so the attenuation at lower fre- quencies will be less.

Another option is to use multiple matching layers, which enables other materials to be used, as more matching layers change the appropriate acoustic impedances of the matching layers.

A schematic of a multiple layer transducer can be seen in figure 2.6.

Figure 2.6: Transducer with two matching layers. Zp is the piezoelement, Zt is the tissue and Z1 and Z2 show the matching layers and Z is the width of both matching layers.

The downside of this is the production is more complicated, as it requires more layers to be attached. The equations for two matching layers can be seen in equations 2.4 and 2.5 [23] [24].

Z

= q

Z

∗ Z (2.4)

Z

= p

Z ∗ Z

(2.5)

With Z being equal to 2.6;

Z = q

Z

∗ Z

(2.6)

Table 2.4 shows the acoustic impedances required for 1 and 2 matching layers for a generic PZT material and a generic PZT composite. As can be seen two matching layers increase the chance of finding materials when matching PZT, as glass often has an impedance of around 14 MRayls and plastics have impedances of 3 MRayls.

Table 2.4: Needed acoustic impedance for different materials.

PZT (MRayls) (Z ≈ 30MRa yl s)

PZT composite (MRayls) (Z ≈ 16MRa yl s)

1 layer Z

= 6.7 Z

= 4.9 2 layers Z

= 14.2 Z

= 8.9

Z

= 3.2 Z

= 2.7

2.2.3 Backing materials

As mentioned earlier backing materials reduce the excessive vibrations of the piezoelectric ma- terial and minimize the energy returned to the piezoelectric material [25]. There are several options for the type of backing that can be used. One option is a high impedance backing, which has an impedance close to that of the piezoelectric element. Also called a heavy back- ing, it makes sure that as much energy as possible is transmitted to the backing. This creates a greater bandwidth, shorter pulses and a lower amplitude. The other type of backings are soft backings(2.5-3.5 MRayls), which have a very low acoustic impedance. Soft backings increase the sensitivity of the transducers, but also lengthen the pulse duration and lower the band- width. [25].

Depending on the preferred characteristics of the transducer, a backing with an impedance between that of a soft and heavy backing can be used. This way a transducer with sufficient bandwidth but also enough axial resolution can be made [26].

The ultrasound backing should have a high attenuation to absorb the ultrasound quickly [27].

This loss is caused by a scattering of ultrasound and the ultrasound is then absorbed and trans- formed into heat [28]. The thickness of the backing necessary is determined by the amount of reflected ultrasound from the body, the intensity of ultrasound and the attenuation of the material [26].

Polymer composites are mostly used as backing materials. Depending on what is needed, poly- mers can be combined with several materials to create composites with different characteris- tics. Wang et al. [27] uses the EPO-TEK 301 epoxy and fills it with tungsten. As can be seen in figure 2.7 the maximum attenuation is approximately 42 dB/mm at a volume fraction of 8%, while the acoustic impedance keeps rising with the volume fraction of tungsten.

Figure 2.7: Effect of different percentages of filling on acoustic impedance (l) and attenuation (r) at 30 Mhz. [27]

Martha et al. [28] backs up this research and also shows that increasing particle size increases

the attenuation, despite a constant percentage of volume. Martha et al. also did research on

PZT/epoxy composites and alumina/epoxy composites. Figure 2.8 shows us the attenuation of

different materials at frequencies ranging from 3 to 7 Mhz.

3 4 5 6 7 0

20 40 60 80

Frequency (MHz)

Attenuation (dB /mm )

Figure 2.8: Attenuation of different composites at frequencies from 3 to 7 MHz in Spurr epoxy. From top to bottom: tungsten, PZT, alumina and pure Spurr epoxy. [28]

As can be seen in figure 2.8 tungsten composites are more attenuating than alumina and PZT composites and a greater the particle size increases the attenuation, while impedance is in- creased with a higher volume fraction [28]. The bandwidth of the transducer also increases with higher impedance. Abas et Al. [29] showed that a backing layer of tungsten powder (100µm)and a slow setting epoxy at a weight ratio of 4:1 can increase the bandwidth for a 5 MHz transducer to 12.77%. Button et Al. [30] reached a bandwidth of 23.74% with an epoxy-tungsten composite and a bandwidth of 33.49% with an epoxy-alumina-tungsten composite. While exact ratios and attenuation values are not given, this shows the possibility of higher bandwidth backings with tungsten and alumina.

Even though tungsten looks like the best option, oxide fillers like aluminium oxide can be re- quired for array transducers, as tungsten composites can be conductive [27].

When soft backings are needed a pure polymer can be used. For this Ecogel 1365-80 epoxy with an attenuation of 120 dB/cm or Dispersion-236 rubber with an attenuation of 139 dB/cm can be used. The polymers have an impedance of around 2.3 MRayls [28].

A short summary of possible materials can be seen in table 2.5.

Table 2.5: Summary of possible backing materials

Material Impedance

(MRayls)

Attenuation

(dB/cm) Comments

Heavy backings

Tungsten-epoxy

(volume 10% - 40.1%) 4.9-14.6 39-42

Depending on volume%

used, can be conductive.

Not usefull for arrays if conductive.

Tungsten-alumina-epoxy 14.2-14.4 14.2-14.4

Attenuation likely to be between that of tungsten-epoxy and aluminium oxide-epoxy.

Provides a bandwidth between 18.35% and 34.45% at about 2 MHz.

Intermediate

backing Aluminium oxide-epoxy 3.3-7.0 3.5-40

Not conductive, but a low attenuation and impedance depending on the volume percent- age.

Soft backing Ecogel 1365-80 epoxy 2.5 120

Increases the sensitivity, but can only be used for very specific imaging due to the low bandwidth.

Dispersion-236 rubber 2.1 139

2.2.4 Shielding materials

As said before the ultrasound transducer will have to be shielded, so that the MRI is not in- fluenced by its RF waves and the ultrasound is not affected by the RF waves generated by the magnetic field of the MRI. To shield the MRI against RF waves conductive materials can be used, which was shown in chapter 1. The same principle of shielding works to shield the ultrasound used against the RF waves generated by the MRI. The main problem to consider are eddy currents that can be caused by the changing magnetic field of the MRI. Heating can occur in metals as result of these currents, burning the patient if they come into contact with tissue [31]. Furthermore eddy currents inside the MRI create field shifts and gradients, which creates artefacts in the final image [32]. As the body is naturally conductive, small pieces of conductive material will create currents that can be absorbed by the human body [31] and as a result will not be a problem. The amount and size of RF fields that can be absorbed depends on the body weight and size, so this is specific for each patient.

An interesting solution is the use of a non-conductive polymer with a conductive filling as is done by Gerold et al [17]. The composite of copper powder and epoxy provides high fre- quency shielding, while avoiding macroscopic eddy currents. The material acts as a conductor at high frequencies, while being a insulator at low frequencies. A 45% volume of copper powder created a composite with enough conductivity for shielding, but which was not DC conductive, so that only a AC current could pass through. Because the material is not DC conductive, eddy current loops will not occur as easily.

Morari et al. [33] showed the possibility of using silicone rubber filled with ferrite and graphite

powder. SFU20, a composite with 20 volume% graphite and ferrite powder, and SFU15, con-

taining 15 volume% ferrite, show good shielding abilities in the 0.06-3 THz range. As both

materials contain ferrite, it will have to be tested for ferromagnetic properties to ensure MRI compatibility.

Besides composites a pure metal like aluminium pr gold can also be used as a shielding ma- terial. The problem with using pure metals is that it is DC conductive and thus creates eddy currents faster and heats up more. As aluminium and gold have a good combination of low resistivity and low conductivity, they do not heat up as fast as other metals. The application of aluminium as a shielding material inside MRI is shown by Tang et al [16]. The downside is that the effectiveness of the shielding depends on the settings of the MRI.

Table 2.6: Possible shielding materials for ultrasound

Material Composition Comments

polymer- metal composite

SFUG20/15

20%/15% graphite/fer- rite powder in silicone rubber

Contains ferrite and might thus have ferromagnetic properties.

0-3 copper- epoxy

45 volume% copper in epoxy

Showed good shielding in 1.5 T MRI and was not DC conduc- tive with good mixing.

Metal Aluminium

Good balance between resistiv- ity and conductivity, cheap to use.

Gold

Similar to aluminium in resis- tivity and conductivity. Costly to use for shielding.

When a shielding material is used, it has to be grounded to the shielding of the MRI room to be effective.

2.2.5 MRI Markers

Markers can be used to allow the ultrasound image to be linked to the MRI image. Fiducial

markers for MRI are already widely available on the market. External markers are in general

made of a reservoir filled with a fluid. In the case of Favazza et Al. markers with different

concentrations of coppers sulfate solution were tested. Best results were found with the 1 g/L

copper sulfate solution, which was used for the final prototype [34]. While these markers are

not commercially available, Beekley Medical [35] offers a range of MRI markers for external

use. Beekley uses markers filled with a gadolinium liquid [36], which is a commonly used MRI

contrast agent for internal use [37]. Another option is to use small containers with oil or resin

markers, but these are not officially tested and approved and thus require more extensive test-

ing. Several markers can be combined to make the transducer locatable inside the MR image.

3 Requirements

Several design requirements are set up following the Moscow Method for the final transducer.

Table 3.1 shows the MoSCoW requirements sorted. Later in the chapter some requirements will be explained further.

Table 3.1: Requirements according to the MoSCoW-method.

Must 1 MRI-Compatible

The transducer has to be able to sit in the MRI while switched on and pose no hazards to the patient or have an negative effect on the MRI image. The MRI may not interfere with ultrasound measurements.

2 Comply with FDA/CE regulations

The transducer must satisfy sensitivity, reliability, accuracy, precision, sterilizability (or disposability), cost-effectiveness and safety requirements for surgical procedures ac- cording to FDA/CE regulations.

3 Compactness

The transducer has to be as compact as possible and has to fit within an MRI machine.

4 Locatable

The ultrasound transducer must be locatable inside MRI images.

Should 6 Multimodality compatible

The transducer should be compatible with CT/PET.

7 Cost

The overall cost of the transducer and design should be as low as possible.

8 Compability McRobot

Should be integratable in the McRobot version 3.2.

Could 9 Performance

The ultrasound transducer could provide adequate image qualities.

10 Combining elastography/MRI/ultrasound

Using a mechanism to apply force to see how the tissue reacts to the force by looking at the MRI and ultrasound images.

MRI-compatibility & multi-modality compatible

As the ultrasound transducer will be used inside an MRI, it has to be MRI compatible. This means that it is at least MRI-conditional and preferable MRI-safe, as defined in chapter 1. MRI- compatibility also includes that the materials may not affect the final MRI image in such a way that the image quality decreases significantly. To facilitate this, ferromagnetic materials can- not be used and conductive parts should be avoided as much as possible, because conductive materials may heat up and will in turn hurt the patient. Another problem is changes in the magnetic field, which are caused by metallic parts, as they will affect the final MRI image.

As a could multi-modality compatibility is also considered, which incorporates PET and CT

compatibility. Due to the strong regulations on MRI safety, materials that are safe in the MRI

are also safe in PET and CT scanners, but this does not cover imaging quality. As both imaging

modalities detect radiation to determine the imaged area, the quality is affected when there are

more materials present in the imaging area. To limit the image degradation quality, the density

of all materials used has to be as low as possible, so the attenuation is low for x-rayx and gamma

rays.

4 Conceptual design

In this chapter the concept for an MRI compatible ultrasound transducer will be designed. For each component of the transducer the benefits and possible downsides of the alternatives will be analyzed, after which the best option will be chosen. Finally, at the end of this chapter the conceptual design will be presented.

Piezoelectric element

There are two viable options for piezoelectric elements, depending on the The first option is a piezoceramic/polymer composite, which would be the most optimal choice, as a compos- ite can be tailored to our needs and has a lower impedance than a pure ceramic. The lower impedance would make it easier to get a backing layer that would give the transducer a suffi- cient bandwidth. In the end the chosen piezoelectric element is Pz27 (Ferroperm Piezoceram- ics A/S). It is a modified PZT material for which the characteristics can be seen in appendix A. As it is a fully ceramic piezoelectric element, its acoustic impedance is rather high at 34.8 MRayls. The reason for choosing this material is its availability

The PZT material has a thickness of 0.5 mm and has a theoretical resonance frequency of 4.094 MHz in the thickness direction and has a capacitance of 2507 pF [38].

Matching layers

For the matching layer there is the choice between one and two layers. Here both options will be explored and materials will be suggest for the one-layer and two-layer system.

Using a single matching layer would mean that according to equation 2.3 an impedance of 7.22 MRayls is needed. This impedance can only be obtained by using a specially created compos- ite. Another option is to use double matching layers. Table 4.1 shows the required values and possible materials that can be used, including manufacturers for each of the materials.

Table 4.1: Needed acoustic impedances of matching layers with possible materials. [39] [40]

Impedance Material (MRayls) Attenuation (dB/cm) 1 layer Z = 7.22

Carbon - vitreous, Sigradur K (Si) (Z = 7.38)

Hysol - C9-4183/3561, 57.5 phe C5W (H, Li) (Z = 7.04)

2 layers

Z

= 15.86 Glass - flint (Z = 16.0 MRayls) 10

Z

= 3.29

Acrylic, Clear, Plexiglas G Safety Glazing

(Rohm) (Z = 3.26) 6.4

PVC, Grey, Rod Stock

(Port) (Z = 3.27) 11.2

Araldite - 0.52/956, 10 phe CW (Ciba, Li) (Z = 3.19)

A one-layer system has the benefit of easier construction. When glue is used to put together the

layers, the glue cannot be discarded in the system of matching layers. This would affect the total

transmission adversely. As the two-layer system requires more connections, the transmission

might be worse than the one-layer system, but more materials with the correct impedance

might be available.

The first choice is the two-layer system with glass and plexiglas. Plexiglas has a favourable at- tenuation of 6.4 dB/mm, while the exact attenuation of glass-flint is unknown. As most glasses have an attenuation of around 10 dB/mm, this is a good sign for the attenuation of flint.

A second choice for the concept would be a one-layer system with carbon as material.

While carbon is conductive, the thickness of the matching layer and the favourable acoustic impedance make this the first choice. As the attenuation is not yet known for this material, it is only considered favourable as long as the attenuation is about 10 dB/mm or lower. This is also immediately the reason why it is not the first choice for the concept and why finding materials is difficult, as the relevant properties like attenuation and acoustic impedance are not always known.

Backing layers

The next step is choosing the backing layer. As covered in section 2.2.3 the backing layer is of importance for determining the bandwidth of the transducer. The bandwidth is not only influenced by the backing layer, but also slightly by the matching layers and piezoelectric ma- terials [41].

In general, a bandwidth of at least 40% is preferred to obtain adequate images. [42]. This is of course fully dependent on the aim of the imaging. In this case a backing layer will be chosen that can increase the bandwidth to a desirable percentage. While a bandwidth of 40% cannot be reached with the backing layers that have been found, electrical impedance matching and the matching layer will also contribute to increasing the bandwidth in the design.

The favourable choice for a simple transducer would be a matching layer of tungsten-epoxy.

The reason for this choice is the high attenuation and the high impedance that can be reached with this composite. Conductivity can be controlled by careful mixing so that the tungsten is not lumped together, also lower amounts of tungsten powder should decrease the conductivity in case this is needed. A second option would be a backing of tungsten-alumina-epoxy. The addition of alumina can result in more control of the conductivity of the backing, in case this is a problem. Bandwidths of over 33% can be reached at a frequency of 2 MHz, so while this will be lower for 5 MHz, it is a good sign for higher frequencies. This material is not the first choice because there is nothing known about its mixing rations and attenuation.

Shielding

To create an MRI compatible transducer, shielding is one of the most important components.

The difficulty with this is that little research has been done on shielding inside the MRI specifi- cally. One of the most promising possibilities is the use of a particle/polymer composite in the form of a 0-3 copper-epoxy composite with a volume percentage of 45% copper. Other combi- nations might also be possible, such as an aluminium-epoxy composite. As long as the creation of the material is done carefully the composite should not be DC conductive. An easier option might be aluminium shielding, but this only works in certain MRI environments and even then the shielding capabilities are limited.

Total setup

A first concept would consist of a Pz27 piezo disc with a diameter of 10 mm and a thickness of

0.5 mm, because of the availability of the material. A two-layer matching system will be used,

with glass - flint the first matching material and Plexiglas the second matching material. The

backing layer to be used is a tungsten-epoxy composite with a ratio of 4:1. The back side of

the piezo can be connected by a wire through the backing layer. While a conductive backing

layer can be used, the bigger size of the backing layer can cause problems in the MRI. Lastly,

the shielding can be done with a copper-epoxy composite. The thickness of the matching layer

should be

λ, while the thickness of the backing layer depends on the attenuation and the

intensity of the signal. When the ultrasound signal reaches the end of the backing layer, the intensity should be close to zero.

As this is a concept based on theoretical information and assumptions, it will have to be exten-

sively tested to make sure it fits the data found in literature.

5 Prototyping & Testing

Due to limited material availability the conceptual design and actual prototype will not be the same. The protoype is simplified and made with materials readily available, so that testing can occur. The testing is done to determine the effects of and ultrasound transducer being present inside the MRI.

The Prototype

The prototype is made of P z27, a material created by Ferroperm [43]. The piezodisc has a diameter of 10 mm and a thickness of 0.5 mm. The theoretical resonance of the piezo is 4.094 MHz, with a capacitance of 2507 pF at 1 kHz [38]. The piezo has screen-printed silver electrodes with a thickness of 8-10 microns [44].

The piezoelectric element is connected with a wire and a piece of copper-coated circuit board, with the wire soldered on the positive side of the piezoelectric element. The copper layer on the circuit board has a thickness of 30-40 microns and the type of solder used is 40/60 soldering tin with a ersin type core. A silver conductive glue connects the piezoelectric element to the circuit board, after which a wire is soldered to the circuit board. The wires used are made of copper and plated with silver and are covered in the isolating material Kynar 460. The whole setup is covered in a silicon conformal coating(Electrolube DCA200H) so that the outside is not conductive. The main reason for using these materials is their availability. The circuit board acts as a backing layer, while the silicon coating takes on the function of matching layer while also electrically isolating the components.

Figure 5.1: The copper prototype(left) and piezo prototype(right).

Figure 5.1 shows the finished prototype. The piezoelectric element is located slightly off center and some left over silver conductive glue can be seen to one side of the piezoelectric element.

This is due to movement of the piezoelectric element after gluing it on the circuit board and this could affect the performance of the prototype.

A second prototype has been made, which consists only of a piece of copper with wires at- tached, as can be seen in figure 5.1. This prototype was made to determine the effect of the copper circuit board. From now on this prototype will be called the copper prototype.

Testing

Before testing in the MRI is done, the prototype is measured and a phantom is made. The

resonance frequency of the actual prototype was measured to be 4.480 MHz, instead of the

theoretical 4.094 MHz.

A phantom is used, which is made of Pastileurre Soft(Bricoleurre) and Assouplissant Pastileurre(Bricoleurre). The phantom itself consists of two parts, as can be seen in figure A.1 in Appendix A. The outer part is made of 100% Pastileurre soft, while the inner part is made of a non-homogeneous mixture of 50% of 51.01% Assouplissant Pastileurre\48.99% Pastileurre Soft and 50% of 20.66% Assouplissant Pastileurre\77.34% Pastileurre soft.

Several tests are done to determine the effects of metal and current inside the MRI. The MRI used is the G-scan Brio(Esaote S.p.A.) with a field strength of 0.25 Tesla. Tests are done with the piezo connected to a frequency generator. The waveform generator is set to 5 Volts and a frequency of 4.480 MHz, which is the resonance frequency of the piezo.

The components that will be tested are put inside the phantom to make sure they can be imaged and their effect can be observed. The total set-up is then wrapped in plastic foil to make sure that the prototype does not move inside the phantom. Figure 5.2 shows a set-up with a copper prototype.

Figure 5.2: Phantom with the copper prototype inserted and wrapped in plastic foil.

To start testing, first a scout is done to determine the location of the phantom inside the MRI coil. When this is completed, at spin echo T1 is done for the final imaging.

The following tests are done with this set-up:

• Phantom

• Piezo in phantom

• Prototype in phantom with and without power.

• Copper prototype in phantom with and without power.

• Copper prototype with aluminium in phantom with and without power.

For the tests with shielding, the aluminium was connected to the ground of the frequency gen-

erator, which was in turn grounded to the electrical outlet it was plugged into. Testing of the

prototype with shielding was scrapped, due to limited time and disappointing results with the

copper prototype.

6 Results

The results of the tests can be seen in figure 6.1 to figure 6.3. As said before, all tests are done with a Spin Echo T1 with a total of 7 slices of 5 mm thickness. To be able to compare the results, roughly the same slice is shown for each of the tests.

Figure 6.1: Results of the MRI-scans. A: Phantom B: Phantom + piezo C: Phantom + Copper prototype D: Phantom + transducer prototype

Image 6.1A shows the base phantom, which can be used as base comparison for the other im- ages to determine the effects of certain materials inside the MRI. Here the difference between the inner and outer part of the phantom can be seen easily. As the parts were added at an angle and the MRI takes slices of 5 mm thickness, there is some shadowing on the images that shows the parts at different depths.

The effect of the piezoelectric element on the phantom is negligible in this slice. This is partly due to the fact that the piezoelectric element is very small at that spot. In deeper slices the piezoelectric element is more noticeable, as can be seen in figure 6.2.

Figure 6.2: MRI-scan of phantom and piezoelectric element, deeper slice.

The image is slightly affected by the presence of the piezoelectric element. As can be seen

in figure 6.2 the boundaries are fuzzy and not well defined, while the actual piezo has strict

boundaries. A local non-homogeneous magnetic field can be the cause of the fuzzy boundaries,

as described before in artefacts. No zipper artefacts or others artefacts can be seen in the image.

Image 6.1C and 6.1D in figure 6.1 show the same artefacts. Around the location of the prototype there is some fuzziness, likely caused by a change in the local magnetic field. There are no major differences between the two images in terms of image quality. There is a small shadow in image 6.1D, under the prototype, this is likely caused by the prototype at different depths of the phantom. In none of the images there is sign of other artefacts. Image 6.1D is cut short on the bottom, but this is caused by the FOV, which ends there.

Figure 6.3: Results of the MRI-scans with power on the prototype. The green square shows the loca- tion of the prototype, while the yellow square denotes roughly the inner square in the phantom. A:

Copper prototype with aluminium B: Copper prototype without aluminium. C: Copper prototype with aluminium and power. D: Copper prototype without aluminium and power.

Figure 6.3 shows the effect that aluminium and powering up the prototype has on the MRI im- age. Image 6.3A has a greater distortion directly around the location of the prototype in com- parison to image 6.3B, which is highly likely cause by the presence of aluminium. Image 6.3C and image 6.3D both show zipper artefacts, which are caused by the powered up prototype.

These artefacts cannot be found in image 6.3A and B, which are made with the same prototype,

but without power. Comparing image 6.3C and D also shows that the presence of aluminium

increases the amount of zipper artefacts. On image 6.3C, only the outline of the phantom can

be seen, but internal differences are indistinguishable. While 6.3D is also affected, the phantom

itself is clearer as the corners can still be seen and the inside is visible faintly.