Patrick Poulichet
email: p.poulichet@esiee.fr

Latifa Fakri-Bouchet
email: latifa.fakri-bouchet@creatis.univ-lyon1.fr

Christophe Delabie

Lionel Rousseau

Abdenasser Fakri

Anne Exertier

Optimisation and realisation of a portable NMR apparatus and Micro Antenna for NMR

Keywords: NMR, single side, portable, magnet, micro coil. I

This paper is focused on two designs and realizations. The first one concerns a prototype of a portable NMR (nuclear magnetic resonance) apparatus. The second one concerns NMR micro antenna realization.

For the first part, our goal is the NMR magnetic field homogeneity and the signal-to-noise ratio (SNR) improvement. Since de the volume of the sample to analyse is around 1 cm 3 , the design is optimized to obtain a good SNR. Particularly, the magnet is chosen to obtain a high magnetic field with limited inhomogeneities. The receiver antenna is designed and optimized to have high feeling factor and then more sensitivity. A mixer and a low-pass filter are used in order to limit the bandwidth and reduce the thermal noise. The FID is digitized and addressed to a FPGA which averages successive acquisitions in order to increase the SNR. The final acquisition is processed for determining the FID spectrum.

In the second part, a new concept of micro coil is presented in order to measure the small volumes and small concentrations samples by NMR spectroscopy at 4.7 T (200 MHz proton frequency resonance).

This micro sensor would offer the possibility of new investigation techniques based on micro coils' implantation used for in vivo study of local cerebral metabolites of animals models.

INTRODUCTION

The NMR signal measurement requires a static magnetic field B0 and a pulsed frequency varying B1 magnetic field. The studied sample is submitted to these magnetic fields. Then, the B1 magnetic field is applied at a Larmor frequency (1T correspond to 42.57 MHz), is applied, the signal FID (Free Induction Decay) generated by magnetization motion is acquired using a detection coil.

In mobile NMR, B0 is generated with an arrangement of magnets in order to deliver a homogeneous magnetic field. For measurement of the relaxation (T2 or T1) or a spectrum, inhomogeneities must be as low as possible particularly if the objective of the measurement is spectroscopy (B0/B0  10 ppm).

Reference [START_REF] Demas | Compact Magnets for Magnetic Resonance[END_REF] is a review of the different ways of the array magnets setup in order to obtain the best B0 homogeneity. Reference [START_REF] Danieli | Mobile sensor for high resolution NMR spectroscopy and imaging[END_REF] reports the construction of a mobile tomography device by exploiting the concept of movable permanent magnets in the shim unit of a Halbach array. The cross section of a banana placed inside the magnets is presented. Reference [START_REF] Hugon | Design of arbitrarily homogeneous permanent magnet systems for NMR and MRI: Theory and experimental developments of a simple portable magnet[END_REF] proposes a complete procedure for permanent magnet design, fabrication, characterization and shimming. 1H NMR spectrum of a 3 mm 3 sample of water doped with CuSO4 in the shimmed magnet is presented. The half-height full width (HHFW) is about 12 ppm. Reference [START_REF] Changa | Single-sided mobile NMR with a Halbach magnet[END_REF] presents a single-sided mobile NMR apparatus with a small Halbach magnet. It is lightweight, compact and exhibits good sensitivity.

Reference [START_REF] Takeda | OPENCORE NMR: Open-source core modules for implementing an integrated FPGA-based NMR spectrometer[END_REF] the spectrometer design that uses an FPGA. The system is composed of an FPGA chip and several peripheral boards for USB communication, direct-digital synthesis (DDS), RF transmission, signal acquisition, etc. In the FPGA have been implemented a number of digital modules including three pulse programmers, the digital part of DDS, a digital quadrature demodulator, dual digital lowpass filters, and a PC interface have been implemented.

The feasibility to use a new generation of microcoils was proposed in a recent study [6]. It demonstrated potential opportunities in terms of increased signal-to-noise ratio (SNR), spatial resolution, and limits of detection (LOD) [7] compared to the surface-coil [START_REF] Kadjo | BIOGRAPHIE Patrick Poulichet received his degree of electrical engineer at CNAM-Paris in[END_REF]. Their use for localized spectroscopic studies of NMR observable cerebral metabolites into 2mm 3 region of interest (ROI), aims to push limits of in vivo detection.

II. MOBILE NMR

In our design, we used the mains parts describe in [START_REF] Takeda | OPENCORE NMR: Open-source core modules for implementing an integrated FPGA-based NMR spectrometer[END_REF] in order to realize portable NMR apparatus dedicated to portable NMR. The aim is to optimize the SNR. The magnet (alnico) uses two large piece of steel in order to reduce inhomogeneities of the magnetic field. Fig. 1 shows the functional schematic of the spectrometer connected to the PA (Power Amplifier) and connected to the LNA (Low Noise Amplifier). The duplexer use a circuit with two diodes for ensuring a good isolation when a signal is applied to the generation coil. The FID value is :

0 1 0 . . . . K B M V  
when K is an homogeneity factor, B1 the alternative magnetic field and V the volume of the sample. In order to increase the B1 magnetic value, a separate inductance from the detection coil was chosen. With a current of nearly 5 A in the generation inductance,  = 14 µV. By taking into account the bandwidth of the detection coil Δf = 500 kHz, the spectral density of noise across this inductance is calculated: en = 1,6.10 -10 V. With the noise factor of the preamplifier, the SNR is calculated: SNR  100. In this calculus, the noise is underestimated because only the thermal noise from the detection coil and the noise from the LNA are take into account.

B. Spectrometer

Fig. 2 shows the functional schematic and the realization of the spectrometer used to control FID generation, acquisition and processing. A Cyclone III FPGA is used to control the FID record, acquisition time, sequence repetition time used for NMR, FID storage and transfer to the PC using USB protocol. There are some daughter cards connected to the FPGA as shown in Fig. 2. The FPGA also ensures that the delivery signal parameters are correct.

Fig. 2. Functional schematic and realization of the spectrometer

To digitize the FID, an ADC 14 bits -65 MIPS is used. Then data are fed into the FPGA memory. After M samples of N points, the digital word is sent to the PC via USB port.

When the acquisition parameters are defined, the generated signal is sent by the DDS and an amplitude modulation is applied if necessary via the DAC. The amplitude and frequency modulation can be used to generate complex shapes of chirps in order to compensate the inhomogeneities of B0.

Fig. 3. Magnet and antenna connected to the duplexer

The magnet and the duplexer of the Fig. 3 were connected to the LNA and the PA.

Using two large pieces of steel, low inhomogeneities are expected. The sample of liquid of the Fig. 3 is placed inside the air gap of the magnet. MICROCOIL NMR NMR micro coil represented in Fig. 6 detects the FID and is connected to the PCB for tuning and matching circuit. It is connected to the PCB for tuning and matching. Then, the signal is fed to an external amplifier. To improve the SNR, the resistance of the micro coil has to be very low. Therefore, the thickness of the copper used to realize the coil has to be thick so usually it uses electroplating. We optimize the electroplating process: flow of the electrolyte, current density, additive in the electrolyte in order to obtain good aspect and good uniformity of the deposited layer. The process to deposite the thick resist is difficult to optimise because the thickness is around 50 µm. The resist have to be etched for 50 µm on a width of 20 µm. Thus, at the bottom of the hole, if there are some rests of the resist, the adherence of the cooper will be poor. It's this problem that we can see on the Fig. 7; there a lack of cooper inside the cercle. The pattern represents in the Fig. 8 is used to verify that the resistivity of the layer copper deposited is correct. The thickness is 30 µm, the width is 42 µm and the total length of the pattern is about 10.9 mm. Thus resistivity calculation gives

III. CONCLUSION

Two important parts necessary for NMR measurement are described in this paper. In the first part, a portable NMR apparatus is described. In the future, the array of magnets will be modified to reduce the inhomogeneities of B0. The running works concern also the achievement of PC supervisor linked to the Portable NMR divice.

In the second part, the realization of a micro coil is described.

The two parts are designed to constitute an original NMR portable system for the analysis of low volumes samples.

A special thanks to the folowing students that worked on one part of this projet: LIMA MIGLIORINI Fabricio, ZHANG Shu and ZHANG Hao.

Fig. 1 .

 1 Fig. 1. Functional schematic of the connexions between the spectrometer and the coils

Fig. 4

 4 Fig.4shows an acquired signal with a superposed noise and the same signal with noise removed. This last signal is

Fig. 4 .

 4 Fig. 4. Signal generated with and without the noise

Fig. 5 :

 5 Fig. 5 : Shape of the generated signal from the spectrometer

Fig. 6 .

 6 Fig. 6. Micro coil on the right and the PCB used to tune and match the signal

Fig. 7 :

 7 Fig. 7 : Photography of the microcoil NMR

Fig. 8 .

 8 Fig. 8. Shape used to determine the resistivity of the electroplate copper