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IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012 2877 Direct Detection of Magnetic Resonance Signals in Ultra-Low Field MRI Using Optically Pumped Atomic Magnetometer With Ferrite Shields: Magnetic Field Analysis and Simulation Studies Takenori Oida, Masahiro Tsuchida, and Tetsuo Kobayashi Department of Electrical Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan An ultra-low eld (ULF) magnetic resonance imaging (MRI) system with an optically pumped atomic magnetometer (OPAM) has re- cently been proposed. However, to measure MR signals with high sensitivity in ULF-MRI systems with OPAMs, the resonant frequencies of a sample and an OPAM must be matched. In this study, we propose a direct detection method of the MR signals using ferrite shields. In addition, the magnetic eld distribution analyses and MR signal intensity simulations were performed to improve the sensitivity of direct MR signal detection by using an OPAM with ferrite shields. The magnetic eld distribution analyses and MR signal intensity simulations indicated that the MR signals could be detected by the proposed method and that the ferrite shields with high relative per- meability improved the sensitivity of direct MR signal detection. Index Terms—Ferrite shield, nuclear magnetic resonance (NMR), optically pumped atomic magnetometer, ultra-low eld (ULF) mag- netic resonance imaging (MRI). I. INTRODUCTION A MAGNETIC resonance imaging (MRI) scanner utilizes a high magnetic eld to achieve high signal-to-noise ratio (SNR), which enables the visualization of the anatomy and func- tion of the human body. However, the conventional high mag- netic eld scanner has some limitations such as high cost, large chassis, and risk for patients with metal implants. To measure MR signals without these limitations, ultra-low eld (ULF) MRI systems are used that depend on superconducting quantum inter- ference devices (SQUIDs) that have attracted attention in recent years [1], [2]. Recently, MR signal detection, which is an op- tically pumped atomic magnetometer (OPAM) application and includes ULF-MRI, has been proposed by Savukov et al. [3]. In this system, the remote detection of MR signals using an OPAM with a ux transformer (FT) was proposed. In addition, the output coil of the FT is placed in the vicinity of a glass cell and its number of turns and radius are optimized to improve the SNR of the detector. However, the SNR of the detector was lim- ited by not only the sensitivity of the OPAM but also by that of the FT [4]. The OPAM is based on the magneto-optical effect of an optically pumped alkali metal vapor. In general, OPAM con- sists of an alkali metal vapor in a glass cell. A pump and probe laser are used for taking measurements in an OPAM. A magnetic eld is applied to the alkali metal as shown in Fig. 1 and has magnetic sensitivity in the region where the pump and probe laser beams cross each other. Since the gyromagnetic ratio of an alkali metal is different from that of a proton, which is targeted most frequently in MRI; the magnetic eld applied to an OPAM should be reduced to that to a measured object (biological sample). Manuscript received February 29, 2012; revised April 13, 2012; accepted May 07, 2012. Date of current version October 19, 2012. Corresponding au- thor: T. Oida (e-mail: [email protected]). Color versions of one or more of the gures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identier 10.1109/TMAG.2012.2199469 Fig. 1. Schematic of an optically pumped atomic magnetometer (OPAM). In this study, we propose a direct detection method of MR signals in a ULF-MRI system using an OPAM and ferrite mag- netic shields. Ferrites exhibit the following characteristics: they have high relative permeability, high electrical resistivity, and the magnetic noise caused by the ferrite is less, (approximately 0.75 ) [5]. Therefore, we adopted ferrite shields as magnetic shields in this study. In addition, magnetic eld dis- tribution analyses were performed to improve the sensitivity of MR signal detection by adjusting the magnetic elds that are applied to the sample and OPAM. II. MATERIALS AND METHODS A schematic of proposed MR signal detection methods in ULF-MRI using an OPAM and ferrite shields is shown in Fig. 2. The ULF-MRI consists of static magnetic eld coils, orthogonal triaxial gradient magnetic eld coils, and prepolarizing mag- netic eld coils. An MR signal detection system with the OPAM is considered, when the sample used is a proton and the alkali metal vapor used is potassium. In such a system, the resonant frequency of the OPAM is tuned to that of the sample and the ratio of the magnetic elds, which is applied to the sample and the OPAM, is adjusted to approximately 164. In the proposed method, the magnetic elds, which are generated by the static and gradient magnetic eld coils of the MRI system as shown in Fig. 2, are reduced by the ferrite shields. In addition, to produce symmetrical magnetic eld distortion, two magnetic shields are symmetrically placed with respect to the sample. 0018-9464/$31.00 © 2012 IEEE

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Page 1: Direct Detection of Magnetic Resonance Signals in Ultra-Low Field MRI Using Optically Pumped Atomic Magnetometer With Ferrite Shields: Magnetic Field Analysis and Simulation Studies

IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012 2877

Direct Detection of Magnetic Resonance Signals in Ultra-Low Field MRIUsing Optically Pumped Atomic Magnetometer With Ferrite Shields:

Magnetic Field Analysis and Simulation StudiesTakenori Oida, Masahiro Tsuchida, and Tetsuo Kobayashi

Department of Electrical Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

An ultra-low field (ULF) magnetic resonance imaging (MRI) system with an optically pumped atomic magnetometer (OPAM) has re-cently been proposed. However, to measureMR signals with high sensitivity in ULF-MRI systems with OPAMs, the resonant frequenciesof a sample and an OPAM must be matched. In this study, we propose a direct detection method of the MR signals using ferrite shields.In addition, the magnetic field distribution analyses and MR signal intensity simulations were performed to improve the sensitivity ofdirect MR signal detection by using an OPAM with ferrite shields. The magnetic field distribution analyses and MR signal intensitysimulations indicated that the MR signals could be detected by the proposed method and that the ferrite shields with high relative per-meability improved the sensitivity of direct MR signal detection.

Index Terms—Ferrite shield, nuclear magnetic resonance (NMR), optically pumped atomic magnetometer, ultra-low field (ULF) mag-netic resonance imaging (MRI).

I. INTRODUCTION

A MAGNETIC resonance imaging (MRI) scanner utilizes ahigh magnetic field to achieve high signal-to-noise ratio

(SNR), which enables the visualization of the anatomy and func-tion of the human body. However, the conventional high mag-netic field scanner has some limitations such as high cost, largechassis, and risk for patients with metal implants. To measureMR signals without these limitations, ultra-low field (ULF)MRIsystems are used that depend on superconducting quantum inter-ference devices (SQUIDs) that have attracted attention in recentyears [1], [2]. Recently, MR signal detection, which is an op-tically pumped atomic magnetometer (OPAM) application andincludes ULF-MRI, has been proposed by Savukov et al. [3].In this system, the remote detection of MR signals using anOPAM with a flux transformer (FT) was proposed. In addition,the output coil of the FT is placed in the vicinity of a glass celland its number of turns and radius are optimized to improve theSNR of the detector. However, the SNR of the detector was lim-ited by not only the sensitivity of the OPAM but also by that ofthe FT [4].The OPAM is based on the magneto-optical effect of an

optically pumped alkali metal vapor. In general, OPAM con-sists of an alkali metal vapor in a glass cell. A pump andprobe laser are used for taking measurements in an OPAM. Amagnetic field is applied to the alkali metal as shown in Fig. 1and has magnetic sensitivity in the region where the pump andprobe laser beams cross each other. Since the gyromagneticratio of an alkali metal is different from that of a proton, whichis targeted most frequently in MRI; the magnetic field appliedto an OPAM should be reduced to that to a measured object(biological sample).

Manuscript received February 29, 2012; revised April 13, 2012; acceptedMay 07, 2012. Date of current version October 19, 2012. Corresponding au-thor: T. Oida (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2012.2199469

Fig. 1. Schematic of an optically pumped atomic magnetometer (OPAM).

In this study, we propose a direct detection method of MRsignals in a ULF-MRI system using an OPAM and ferrite mag-netic shields. Ferrites exhibit the following characteristics: theyhave high relative permeability, high electrical resistivity, andthe magnetic noise caused by the ferrite is less, (approximately0.75 ) [5]. Therefore, we adopted ferrite shields asmagnetic shields in this study. In addition, magnetic field dis-tribution analyses were performed to improve the sensitivity ofMR signal detection by adjusting the magnetic fields that areapplied to the sample and OPAM.

II. MATERIALS AND METHODS

A schematic of proposed MR signal detection methods inULF-MRI using an OPAM and ferrite shields is shown in Fig. 2.The ULF-MRI consists of static magnetic field coils, orthogonaltriaxial gradient magnetic field coils, and prepolarizing mag-netic field coils. AnMR signal detection system with the OPAMis considered, when the sample used is a proton and the alkalimetal vapor used is potassium. In such a system, the resonantfrequency of the OPAM is tuned to that of the sample and theratio of the magnetic fields, which is applied to the sample andthe OPAM, is adjusted to approximately 164. In the proposedmethod, the magnetic fields, which are generated by the staticand gradient magnetic field coils of the MRI system as shown inFig. 2, are reduced by the ferrite shields. In addition, to producesymmetrical magnetic field distortion, two magnetic shields aresymmetrically placed with respect to the sample.

0018-9464/$31.00 © 2012 IEEE

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2878 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012

Fig. 2. Schematic of proposed ULF-MR signal detection method using OPAM and ferrite shields.

In the present study, magnetic field distribution analyses ofthe fields, which were applied by static magnetic field coils,were performed for achieving the magnetic fields ratio of ap-proximately 164. The static magnetic field coils and gradientmagnetic field coils were assumed to wind on the FRP cylinderwith 600 mm in inner diameter. The magnetic field of 600 Tin maximum was assumed to be applied by the static magneticfield coil. The static magnetic field was analyzed because thecenter frequency of the MR signals depends on the static mag-netic field. In these analyses, we assumed that the relative per-meability of Ni-Zn ferrite was 2200 and that of Mg-Zn ferritewas 7500. The prepolarizing magnetic field coil was assumed towind on the ring of square shape with 144 mm in each edge. Thehomogeneous region of the prepolarizing magnetic field appliedby this coil was assumed to be the spherical region with about50 mm in diameter. The ferrite shields were assumed to havea 28.28 mm inner diameter and were placed as far as possiblein the FRP cylinder to ensure the homogeneity of the magneticfield applied to the sample.In addition, to estimate the MR signal intensities measured

by the OPAM, MR signals generated from 1 cc of water witha 30 mT prepolarizing magnetic field were simulated. In thesesimulations, the magnetization of water was expressed as aminute loop current, in which the radius of the loop was 0.1 mmand the current was calculated as follows:

(1)

where is the area of the minute loop, is the magnetizationin the sample, and is the volume of the sample. When thesample is exposed to the prepolarizing magnetic field , themagnetization is generated in the unit volume of the sample.

is calculated by following equation [6]:

(2)

where is the spin density, is the gyromagnetic ratio, isthe Planck’s constant divided by , is the spin quantumnumber, is the Boltzmann constant, and is the tempera-ture of the sample. We used as the gyro-magnetic ratio of proton , 1/2 as proton spin quantum number, and as the proton density of water [7].

Fig. 3. Magnetic field distribution in plane using ferrite shields with(a) 40 mm and (b) 130 mm height and the relative permeability of 2200.

These magnetic field distribution analyses and MR signal inten-sity simulations were performed using a “JMAG Studio Version9.1” software package (JSOL Corporation) [8].

III. RESULT

The magnetic field distributions in the plane using ferriteshields with a height of 40 and 130 mm, which consist of fer-rite having a relative permeability of 2200 and inner diameterof 28.28 mm are shown in Fig. 3(a) and (b), respectively. Themagnetic field distributions shown in Fig. 3 indicated that themagnetic flux converged on the ferrites and the magnetic field

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OIDA et al.: DIRECT DETECTION OF MR SIGNALS IN ULF-MRI USING OPAMWITH FERRITE SHIELDS 2879

Fig. 4. Resulting thickness of ferrite shields with relative permeability of 2200achieving a magnetic fields ratio of about 164.

Fig. 5. Resulting thickness of ferrite shields with relative permeability of 7500achieving a magnetic fields ratio of about 164.

Fig. 6. Resulting distances between the center of the sample and that of OPAMfor all ten combinations of relative permeability, height, and thickness (plottedin Figs. 4 and 5) of ferrite shields.

in the vicinity of the ferrite shields was distorted. On the con-trary, by comparing Fig. 3(a) and (b), it was observed that themagnetic field distortions in the vicinity of the sample and thosein the shields were changed according to the height of the fer-rite shields. From Fig. 3, it was found that the magnetic fieldwas more distorted in the vicinity of the sample by the ferriteshields that are 130 mm high compared to those that are 40 mmhigh.Figs. 4 and 5 show the resulting thickness for 40, 70, 100, 130,

and 160 mm high ferrite shields, respectively, with relative per-meabilities of 2200 and 7500. The ratio of the magnetic fields,which was applied to the sample and OPAM, was adjusted to ap-proximately 164. These results were sampled at a thickness of0.1 mm. In addition, the resulting distances between the centerof the sample and that of OPAM for all ten combinations ofpermeability, height, and thickness plotted in Figs. 4 and 5 areshown in Fig. 6. Figs. 4–6 indicated that the magnetic fields thatwere applied to the sample are about 164 times larger than thatapplied to the OPAM. The magnetic fields were achieved in var-ious combinations of height and thickness in the ferrite shields.Regardless of the height, the ferrite shields with relative perme-ability of 7500 could adjust the ratio of the magnetic fields with

Fig. 7. MR signal intensities for all ten combinations of relative permeability,height, and thickness (plotted in Figs. 4 and 5) of ferrite shields.

Fig. 8. Standard deviation of themagnetic field in the box region with 30mm ineach edge for all ten combinations of relative permeability, height, and thickness(plotted in Figs. 4 and 5) of ferrite shields.

thicknesses less than 3% of those with relative permeability of2200.Fig. 7 shows the MR signal intensities corresponding to all

ten combinations of permeability, height, and thickness plottedin Figs. 4 and 5. These results were simulated by placing theOPAM whose distance from the sample is shown in Fig. 6.From Fig. 7, it was found that the MR signals could be mea-sured directly with the ferrite shields except for those with rel-ative permeability of 2200 and whose heights are higher than70 mm, because the sensitivity limit of OPAM is expected tobe of [9]. On the contrary, MR signal inten-sities with ferrite shields whose heights are higher than 70 mmare smaller than the sensitivity limit of OPAM. Since MR signalintensities can be increased by increasing the prepolarized mag-netic field and/or volume of the sample, the MR signal with fer-rite shields whose heights are higher than 70 mm might also bedetectable. These results demonstrated that ferrite shields arefeasible for direct detection of MR signals in ULF-MRI usingOPAM.To evaluate the homogeneity of the magnetic field around

the sample position, we have estimated the homogeneity of themagnetic field in the box region with 30mm in each edge, whichhas diagonal dimension of about 52 mm. This box was assumedto be in the homogeneous region of the prepolarizing magneticfield. Fig. 8 shows the standard deviation (SD) of the magneticfield in the box region with 30 mm in each edge for all ten com-binations of relative permeability, height, and thickness (plottedin Figs. 4 and 5) of ferrite shields. The SDs were calculated fromthe magnetic fields at 125 locations, which were determined bydividing each edge of the box region equally among five. FromFig. 8, the homogeneity of the magnetic field changed in the rel-ative permeability, height and thickness of the shields. However,all the SDs around sample location was less than 1% against theaverages.

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2880 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012

IV. DISCUSSION

In this study, we discussed a direct detection method of theMR signals in ULF-MRI using an OPAM and ferrite magneticshields. Figs. 3–5 indicated that ferrite shields reduced the mag-netic field and were able to match the resonant frequencies ofthe sample and OPAM. Moreover, from Figs. 4–6, we foundthat various combinations of height and thickness in the ferriteshields could match the resonant frequencies of the sample andOPAM. From the results of the magnetic distribution analyses,the distortion of the magnetic field applied to the sample andthe magnetic field distribution in the shield were changed in thethickness and the height of the shield. In the setting of this study,since the distortion of the magnetic field was suppressed by re-ducing the thickness of the shield, it may seem that there is thelarge difference in the shielding capability than that of the per-meability. Fig. 7 shows that the ferrite shields also reduced theMR signals. It was found that the signal field was reduced inhundred order by the shield from the magnetic field distributionanalyses. This effect was considered to be reasonable comparedto the reduction of the static magnetic field. The MR signalsmeasured by the proposed detection method were changed byusing various combinations of relative permeability, height, andthickness of the ferrite shields. In addition, MR signals could bemeasured directly with ferrite shields that are 40 mm high whenthe Ni-Zn ferrite shields with relative permeability of 2200 and28.28 mm inner diameter were used. However, since MR sig-nals are proportional to the prepolarizing magnetic field andthe volume of the sample, as expressed by (2), the MR sig-nals expect to be detectable with various combinations of heightand thickness in the ferrite shields by increasing the value ofand .On the other hand, the magnetic field noise caused by the fer-

rite shields depends on the type and/or volume of the ferrites anddistance between the ferrites and the OPAM. Since the systemnoise may be limited by the ferrite shields, system noise shouldbe evaluated in the feature study. In addition, when the prepo-larizing field was used, the eddy current generated after the pre-polarization field was turned off may be a problem. However,since ferrites have the high electrical resistivity, the eddy cur-rent decays rapidly in the ferrites. In addition, when the MRsignal is detected with the OPAM, actively-shielded prepolar-izing coil should be utilized to decrease the eddy current of theother coils and the magnetic field noise in a similar manner tothe gradient coil of conventional MRI. In the case, the very littleeddy current is expected to be caused by the actively-shieldedprepolarizing coil.From Figs. 4–7, it is found that ferrite shields possessing a

high relative permeability should be selected to obtain a largeMR signal. Since the absorption of MR signals by the ferriteshields and the distance between the center of the sample andthat of OPAM were decided according to the height and thick-ness of ferrite shields, it was difficult to select the optimumheight and thickness of ferrite shields. The ferrite shields needto be placed as far as possible to ensure the homogeneity of themagnetic field applied to the sample. Low height and/or thinthickness of ferrite shields were appropriate to detect a largeMR signal.

V. CONCLUSION

In this study, we proposed a direct detection method ofMR signals in ULF-MRI using an OPAM and ferrite mag-netic shields. From the magnetic field distribution analysesand the MR signal intensity simulations, proposed systemwas expected to detect the MR signals directly in ULF-MRIwithout cooling system required by using SQUIDs. In addition,magnetic field distribution analyses and MR signal intensitysimulations were performed to improve the sensitivity of theMR signal detection by adjusting the magnetic fields applied tothe sample and OPAM. The magnetic field distribution analysesindicated that the ferrite shields, which had high permeabilityand high electric resistance, could be utilized to match theresonant frequencies. The resonant frequencies of the sampleand OPAM were matched by using the ferrite shields withvarious combinations of relative permeability, height, andthickness. In addition, the magnetic field caused by MR signalswas evaluated. These results indicated that regardless of thecombination of relative permeability, height, and thickness offerrite shields, the magnetic field caused by MR signals wasmeasured by the proposed method. Moreover, the ferrite shieldswith high relative permeability had the advantage to improvethe sensitivity of direct MR signal detection.

ACKNOWLEDGMENT

This work was supported in part by the InnovativeTechno-Hub for Integrated Medical Bio-imaging of the Projectfor Developing Innovation Systems and a Grant-in-Aid forChallenging Exploratory Research (22650116). Both of themare from the Ministry of Education, Culture, Sports, Science,and Technology (MEXT), Japan.

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