Radio Frequency Probe for Improvementof Signal to Noise Ratio in Magnetic Resonance Image
with Inductively Coupled Wireless CoilTomohiro SAHARA, Shigehiro HASHIMOTO, Kana NAKAMURA, Hiroshi TSUTSUI,Kenzo AKAZAWA, Shuichi MOCHIZUKI
Department of Biomedical Engineering, Osaka Institute of Technology, Osaka, 535-8585, Japanhashimoto@bme.oit.ac.jp http://www.oit.ac.jp/bme/
andYuichiro MATSUOKA
Department of Molecular Imaging, Foundation for Biomedical Research and Innovation, Osaka, JapanandTakashi NISHII
Department of Orthopaedic Surgery, Osaka University Medical School, Osaka, JapanandKagayaki KURODA
Department of Information Technology and Electronics, Tokai University, Hadano, JapanABSTRACT
An inductively coupled wireless coil for a bidirectional radio
frequency (RF) probe coil has been designed to improve thesignal to noise ratio (SNR) in the magnetic resonance image(MRI). A birdcage type of a primary coil and a Helmholtz typeof a wireless secondary coil was manufactured. The coils wereapplied to a phantom study with a 3 T MRI system. SNR wascalculated in the magnetic resonance images.
Theexperimental results show that the designed coils are effective toincrease SNR and to decrease the mean RF power. The low RFpower decreases a specific absorption rate to a subject at highfrequency of resonance.
Keywords: Magnetic Resonance Image, Radio Frequency Probe,
Signal to Noise Ratio, Inductively Coupling, Wireless Coil andRadio Frequency Power.

1. INTRODUCTION
A magnetic resonance image (MRI) system creates an image
with signals from an excited subject at high frequency. Its
detector is called a radio frequency (RF) probe, because thefrequency belongs to the range of radio frequency. The RFprobe governs the quality of the image [1]. There are twokinds of RF coils. One is a single coil, and the other is a crosscoil [2]. The former coil works for both excitation andreception. In the latter coil, excitation and reception are
separated into two coils respectively. In the clinical cross coil,a built-in whole body coil is used for excitation, so that a wholesubject has to be excited even in the case of local measurement[3]. The specific absorption rate (SAR) tends to increase in thecross coil method.
A wireless RF coil works with inductive coupling, and has been
applied in MRI systems [4]. Hout et al. proposed its clinicalapplication to extend the area of an image for the magnetic
resonance [5]. In the system, the intensity of the magnetic field
was rather low with the resonance frequency of 8 MHz, so thatSAR was also low and limitation of SAR had not to beconsidered. Graf et al. developed a wireless probe inductively
coupled with volume type of RF coil of a 1.5 T MRI system forthe head [6]. They applied the probe to small animals, andshowed that MRI can be taken with high signal to noise ratio(SNR). In their study, transmitting RF power and SAR werenot discussed. A phased array method has been widely used toimprove detection efficiency of a radio frequency coil in theMRI system. In the method, SAR tends to increase, because
the whole subject is excited even in the case of taking a local
image. In MRI with a strong magnetic field, SAR would easily
exceed the safety standard value, because SAR is proportional tosquare intensity of the magnetostatic field.
In the present study, an inductively coupled wireless coil for abidirectional RF probe coil for a 3 T MRI system has been
designed to improve SNR and SAR in MRI.

2. METHODS
RF probe
Signa3.0T (GE Healthcare) was used for the MRI system (Fig.
1). The RF probe consists of two coils: a primary coil and asecondary coil (Fig. 2). The bidirectional primary coil is abirdcage type, which has sixteen legs of 8 mm width each (Figs.
3 & 4). It has cylindrical shape with 200 mm diameter and 290mm height. The primary coil generates a linear drive type ofmagnetic field. The secondary coil is Helmholtz type with thedimension of 120 mm diameter and 120 mm height (Figs. 5 & 6).
The secondary coil is inserted into the primary coil, which has
an electric feeding point for transmission and reception ofsignals. For the MRI system of 3.0 T, the resonance frequency
is adjusted to 127.8 MHz with capacitance in the electric circuit.
The excitation efficiency increases, when the resonancefrequency of the secondary coil is same as that of the primarycoil. The frequency was measured with the radio frequency
network analyzer (Agilent Technologies, 8712ET).

Fig. 3: Dimension of primary coil.

Fig. 4: Primary coil with phantom ball.
Fig. 1: MRI system.
Signal to Noise Ratio
The SNR in inductively coupled coils is a function of a mutualinductance (m), a resonance frequency (f), and a resistance of a
primary coil (Rp) (Fig. 7). The SNR increases, when theproduct of the mutual inductance and the resonance frequency islarger than the resistance of the primary coil (Rp<<mf).
The methodology to calculate SNR from I (the mean intensity ofthe signal in the region of interest) and B (the standard deviationof the mean intensity in the background) in equation (1) is basedon the National Electric Manufactures Association (NEMA)
standards.
SNR = I / B(1)

The standard deviation in the background is calculated on theimage composed from subtraction signal of two images with thesame parameter.

Fig. 2: Two coils system.

SAR
A specific absorption rate into a subject (SAR, W/kg) iscalculated with the following equations.
SAR=0.5 Pabs/d(2)Fig. 7: Inductively coupled coils.

Fig. 5: Dimension of secondary coil.

Fig. 8: Relationship between reflection power andfrequency.
SAR in the present study.
Phantom Study
A cupper sulfate aqueous solution packed in a plastic ball of 100
mm diameter was used for the phantom measurement (Fig, 4).
To check the contrast of magnetic resonance image, an orange of
120 mm diameter was also used for a phantom.

Fig. 6: Secondary coil.

Pall=Pabs+Prad(3)Where Pall is all of radio frequency power (W/m3), Pabs isabsorbed power into a subject (W/m3), Prad is radiated power(W/m3), and d is density of the tissue (kg/m3). SAR can beapproximately calculated with Pall in the present study, because
Prad is very small compared with Pabs in the frequency range
around 127.8 MHz (Eq. 4).
SAR=0.5 Pall/d(4)The mean radio frequency power were measured to estimate

While T1 and T2 weighted magnetic resonance images weretaken at the phantom with and without the wireless coil, thetransmitting gain and the mean radio frequency power (W) weremeasured. The maximum transmitting gain is 200, which
generates the maximum transmitting radio frequency power of 8kW.
SNR was calculated for the phantom study of the cupper sulfateaqueous solution in the ball.

3. RESULTS
Fig. 8 illustrates relationship between reflection power andFig. 9: T1 weighted images for cupper sulfate aqueous
solution packed in plastic ball: without wireless coil (left),
and with wireless coil (right).

Fig. 10: T2 weighted images for cupper sulfate aqueous
solution packed in plastic ball: without wireless coil (left),
and with wireless coil (right).

Table 1: Transmit gain and mean RF power at ball.
Transmit gain

Without wireless coilWith wireless coil

T1
120
20

T270
7RF power [W]
T1
30.05

T220.07

frequency in the electric power measurement. The secondary
coil induced the secondary resonance frequency of 122.4 MHz(f2), which is 5.4 MHz smaller than the primary resonancefrequency of 127.8 MHz (f1). The experimental result showsthat the primary resonance frequency does not shift with thesecondary wireless coil: f1 is equal to f0.
Table 1 shows the transmitting gain and the mean RF power,
while the magnetic resonance images were taken at the cupper

Fig. 11: T1 weighted images for orange: without wirelesscoil (left), and with wireless coil (right).

Fig. 12: T2 weighted images for orange: without wirelesscoil (left), and with wireless coil (right).

sulfate aqueous solution packed in the plastic ball. The results
show that both transmitting gain and the mean RF power
decrease considerably with the wireless secondary coil at T1 andT2.
Figs. 9 and 10 show the T1 and T2 weighted images for thecupper sulfate aqueous solution packed in the plastic ball,respectively. The left picture shows the image without thewireless coil, and the right picture shows one with the wirelesscoil in Figs. 9 and 10.
The calculated SNR is shown in Table 2. The results show thatSNR increases with the wireless coil both in T1 and in T2
weighted images.
Table 3 shows the transmitting gain and the mean radio
frequency (RF) power, while the magnetic resonance images
were taken at the orange. The results show that bothtransmitting gain and the mean RF power decrease considerablywith the wireless secondary coil at T1 and T2. Figs. 11 and 12

Table 2: SNR in phantom study.

Without wireless coilWith wireless coil

T1T2

127
151

6275

Fitting the secondary wireless coil increases the magnetic fillingfactor, which improves SNR. Morphological preciseness of thecoils would provide a uniform magnetic field, which improves
MRI in the present system with the secondary wireless coil.
The designed coil is simple and easily applied to a conventional
magnetic resonance image system.

Table 3: Transmit gain and mean RF power at orange.
Transmit gain
T1 T2
Without wireless coilWith wireless coil

97318117

RF power [W]
T1
T2
2.95
0.03

1.79
0.5

5. CONCLUSION
The phantom study shows that the designed inductively coupledwireless coil for a bidirectional radio frequency probe coil
improves the signal to noise ratio in the magnetic resonanceimage and decrease the specific absorption rate into a subject.

6. ACKNOWLEDGMENT
show the T1 and T2 weighted images for the orange,respectively. The left picture shows the image without thewireless coil, and the right picture shows one with the wirelesscoil in Figs. 11 and 12.

This work was supported in part by a Grant-in-Aid for Scientific
Research from the Japanese Ministry of Education, Culture,Sports, Science and Technology.

7. REFERENCES
4. DISCUSSION
There are several ways to improve resolution in MRI: one is toincrease the resonance frequency, and another is to increase
SNR. Increase the resonance frequency has an advantage toimprove resolution of MRI, but has disadvantage to decrease
SAR.
The signal to noise ratio (SNR) decreases as the dimension of
the RF coil, so that expansion of imaging area decreases
resolution of image [7]. Roemer proposed a NMR phasedarray, which consists of many small probes in array arrangement[8]. Their system employs the cross coil method, so that SAR
tends to increase with the whole body excitation. SAR islimited to smaller value at the cross coil than at the single coil inthe standard of the International Electrotechnical Commission[9]. In the previous study, transmission power and SAR havenot been examined on the inductively coupled RF coils.
Although secondary resonance frequency was induced with thesecondary wireless coil, primary resonance frequency wasmaintained at the same value in the present study. Mutualinductance is very small compared with self-inductance of the
primary coil, so that the resonance frequency does not shift.
The difference between first and second resonance frequencies
is 5 MHz, which is large enough to be distinguished in the MRIsystem. Simultaneous resonance both at primary and at
secondary coils decreases the mean electric power to take MRI,
which decreases SAR.

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