Wireless Radio Frequency Coil for Magnetic Resonance
Image of Knee Joint
Tomohiro Sahara, Hiroshi Tsutsui, Shigehiro Hashimoto,
Shuichi Mochizuki, Kenichi Yamasaki, Hideo Kondo, Teruo Miyazaki
Department of Biomedical Engineering, Osaka Institute of Technology,
Osaka, 535-8585, Japanhashimoto@bme.oit.ac.jp
http://www.oit.ac.jp/bme/andMasayasu Motozuka, Kazuo Okuyama
Division of Radiological Technology, Osaka City University Hospital,
Osaka, JapanandYuichi Inoue
Department of Radiology, Osaka City University Medical School,Osaka, Japan

ABSTRACTAn inductively coupled wireless radio frequency(RF) coil has been developed to improve thequality in the magnetic resonance imaging.
The wireless coils of a Helmholtz type and abirdcage type were manufactured, and theresonance frequency was tuned at 64 MHz.
Several magnetic resonance images were taken
on a phantom, on a rabbit’s head and on ahuman knee joint with the RF coils. The signal
to noise ratio (SNR) has been compared at theacquired images. The experimental results
show that the SNR is improved with applicationof the inductive coupling between the RF coils.
Keywords:
Biomeasurement,
MagneticResonance Imaging, Radio Frequency Coil,
Signal to Noise Ratio, Inductive Coupling

1. INTORDUCTION
Magnetic Resonance Imaging (MRI) is acomputed tomography, which being used anuclear magnetic resonance phenomenon [1].
It is useful tool for diagnostic medical imaging,since it is a non-invasive tool to study bothanatomy and function of human body.
The radio frequency coils, known as RF (radiofrequency) resonators or RF probes, are the key
components in a magnetic resonance scanner [2].
They pick up the radio frequency waves whichbeing radiated inside the human body in thestrong static magnetic field. The RF coils havebeen studied to improve their capabilities bymany researchers. Arrayed divided coils weredesigned to decrease noise in the Magnetic
Resonance Image, for example, because noise
increases with the size of the coil [3]. In theprevious study, an Archimedean spiral coil for
MRI was designed and the electric characterwas simulated [4, 5].
In the RF coil, an electric noise increases withits dimension, which decreases the signal tonoise ratio (SNR). To maintain the quality ofmagnetic resonance images, several factors
have been investigated in many trials.
The RF coils are classified into two types: asurface type and a volume type. The surface
coil has an advantage to increase local
resolution, where the volume coil has anadvantage to get the uniform signal field. The
volume coil has a cylindrical shape to generate a
uniform magnetic field and is called birdcage
coil. The uniformity is disturbed, however,when an asymmetrical subject like a knee isinserted.
The inductive coupling in radio
frequency probe is one of the factors to improve
the image [6-10].
In the present study, a wireless coil is inserted
in the birdcage coil to improve the SNR in MRI.
An inductively coupled wireless RF coil with thebirdcage coil has been developed in whole-bodymagnetic resonance (MR) scanners in thepresent study.

primary

secondaryFig. 2: Equivalent circuit of dual coils.

Secondary probe

Primary probe
Fig. 3: Animal test.

2. METHODS
Inductive Coupling Dual Coils System
An inductive coupling dual coils system wasdesigned (Figs. 1 & 2) [11].
The system
includes two coils: a birdcage coil and a wirelesscoil of a cylindrical type or of a Helmholtz type.

Secondary probe

Primary probe

Fig. 1: Inductively coupling RF probe

Animal Test
The RF probe of the cylinder with 280mm
diameter and with 280 mm height was applied
for the primary coil, which is conventionallyused for human head. To generate a uniform
magnetic field at resonance, a wireless probe of
the secondary coil was manufactured in acylindrical type, which is the same kind of shapeas the primary coil. The coil consists of eightelements along the lateral surface of the plastic
cylinder. The secondary coil has dimensions of
120 mm diameter and 100 mm height to fit
rabbit’s head (Figs. 3 & 4). The secondary coil
was located at the centre of the primary coil.
Magnetic resonance images were taken on a
rabbit’s head under anesthesia in RF probe withand without the wireless secondary probe,
respectively.
T1-weighted and T2-weighted
images at spin-echo sequence (TR/TE=4000ms/96 ms) were compared between two types of
RF coil arrangements. Resolution of the image

birdcage alone, longitudinal and vertical. Fig.
8 shows two positions in the phantom study:
longitudinal and vertical.

was 0.39 mm×0.39 mm×3.0 mm voxel.
Phantom Study
The frequency of resonance was adjusted to 64
MHz, which corresponds to 1.5 teslas of a MR
system (Siemens Magnetom Vision) (Fig. 6).
Fig. 6 shows MR system with the volume coil,which was applied to the phantom.
Tominimize the shift of resonance frequency with
the inserted wireless coil, the reactancecomponent is neutralized with capacitance ofhigh voltage proof.

Human Knee
Another coil of a Helmholtz type (Fig. 5) wasmanufactured in the dimension of 120 mm
diameter to be applied to the human knee,where the cylindrical type is hard to fit.
Magnetic resonance images were taken on ahuman knee of a volunteer.

The signal to noise ratio (SNR) of the image wascompared about the position of the coils: aSignal to Noise Ratio
The methodology to calculate SNR (R) from I(the mean intensity of the signal in the region ofinterest) and B (the standard deviation of themean intensity in the background) in equation(1) is based on the National Electric
Manufactures Association (NEMA) standards.

Fig. 4: Secondary probe in animal test.

Fig. 6: Magnetic resonance scanner.

Several magnetic resonance images were taken
on a cylindrical phantom of copper sulfate of 50
mm diameter and 150mm height (Fig. 7).

Main Static fields

Secondaryprobe

Secondaryprobe

Main Static fields
ParallelVerticalFig. 5: Two types of position between primary and secondary probes: parallel and vertical.

tLL

Fig. 7: Helmholtz coil (wireless coil)
L=120 mm, t=10 mm.
R=I/B

(1)

The standard deviation in the background is
calculated on the image composed fromsubtraction signal of two images with the sameparameter.

3. RESULTS

Fig. 8: Rabbit’s head; T2-weighted image,top; T1-weighted image, bottom; without
wireless coil, left; with wireless coil, rightFig. 8 shows the image of the sagittal section ofrabbit. The experimental results on rabbit’shead show that SNR with the secondary
wireless coil is higher than that without thewireless coil (Table 1).
Fig. 9 shows the proton density image of thelongitudinal section of the phantom column ateach position of coils in the phantom experiment.
Table 2 shows SNR calculated from each imageof phantom. The experimental results with thephantom show that SNR with the additional
wireless coil winded around the phantom is twotimes higher than that without the wireless coil(one coil).
Fig. 10 shows the proton density image of thehuman knee in each position of coils. Fig. 11
shows five regions to measure SNR in the imagearound the articular cartilage of the knee. Fig.

Fig. 9: Phantom study: parallel, left;
vertical, right: phantom, bottom right.
12 shows SNR calculated from the image ateach region in three positions of coils. The

Table 1: Signal to noise ratio in animal test

SN R

H e a d w ith w ir e le ss

H e a d a lo n e

Im p r o ve m e n t

T 1 w e ig h te d im a g e

1 3 .39 .1

1 .5

T 2 w e ig h te d im a g e

2 3 .81 1 .2

2 .1

Table 2: Signal to noise ratio in phantom test.

Phantom
SNR

Head alone
11.3

Pallarel
28.2

results in proton images of human knee showthat SNR in the vertical position is almost
double that with the birdcage alone in most ofregions.

4. DISCUSSION
The articular cartilage of the knee joint is easy
to be degenerated with aging. Taking themagnetic resonance image of the articular

60
Vertical50
Signal to noise ratio

The results show that the developed inductively
coupled coil improves the signal to noise ratio.

Vertical25.1

40
Birdcage alone
30
20Horizontal

10
0
12

345

Number of region (I1-I5 in Fig.6)

Fig. 12: Signal to noise ratio in eachregion (I1-I5 in Fig. 11)

(A)

(C)

(B)

Fig. 10: Proton density images ofknee. (A) birdcage alone, (B) wirelessvertical, (C) wireless horizontal.

I5I1
I4I2I3BFig. 11: Regions to measure SNR
in proton density image of knee

cartilage, however, is not easy, because the area
includes only small amount of proton.
Maximizing the signal to noise ratio of the radio
frequency probe is effective to improve thequality of the image.
Helmholtz coil is selected for the wireless coil to
get uniform magnetic field among the space
between two circles of the coil. The electric
current in the birdcage coil increases by
inductive coupling with the wireless coil, which
improves signal detection. The variation ofresonance frequency with inductive coupling
between coils should be considered, because it
affects SNR.
The additional inductively
coupled wireless radio frequency coil can besimply applied to conventional radio frequency
coils to improve the quality of magnetic
resonance images.
The volume ratio of the subject to the innerspace surrounded by the coil is called filling
factor.
Maximized filling factor increases
signal to noise ratio in magnetic resonanceimage with the volume type of the radio
frequency coil.
Experimental results onrabbit’s head show that a small cylindricalsecondary coil is effective to increase the filling
factor. A wireless coil of flexible material
should be easy to adjust the subject and improve
the filling factor.
Moderate inductive coupling between primaryand secondary coils generates resonance, andincrease electric current in the primary coil,which might improve detection efficiency.
5. CONCLUSION
The experimental results show that thedeveloped inductively coupled Wireless Radio
Frequency coils are effective to improve thesignal to noise ratio on magnetic resonanceimages and to be applied to the human knee
joint.

6. ACKNOWLEDGMENT
This work was supported in part by aGrant-in-Aid for Scientific Research from theJapanese Ministry of Education, Culture,Sports, Science and Technology.
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