Design and verification of 5-channel 1.5T knee joint receiving coil based on wearable technology

Abstract

BACKGROUND:

Over the past 20 years, magnetic resonance receiving coil technology has developed rapidly. The traditional, commercial knee joint coil has a fixed mechanical structure. To meet the imaging needs of most patients, it is necessary to ensure that the mechanical geometry of the coil is as large as possible. Therefore, different quality images can be obtained by filling coefficients under loads of knees of different sizes. Lufkin et al. [1] have demonstrated that the signal-to-noise ratio (SNR) of coil imaging is directly proportional to its filling coefficient, which is $S/N\approx\sqrt{Q_{L}*\eta}$ . Thus, the pursuit of an optimal coil filling coefficient is an important way to improve the coil imaging quality.

OBJECTIVE:

This study combines wearable concepts and coil development techniques and applies flexible and elastic materials to coil designs.

METHODS:

We used an elastic material instead of the traditional fixed mechanical structure to develop a 1.5T 5-channel knee joint receiving coil that can be attached to knee joints of different sizes within a certain range, allowing the coil to achieve a maximum filling coefficient under the loads of knees of different sizes.

RESULTS:

Compared to commercial 8-channel knee coils, the phantom test and clinical knee joint imaging demonstrated that the SNR of the developed coil increased by four times in the shallow layer and two times in the deep layer, under different load conditions.

CONCLUSION:

This high SNR performance demonstrates potential for the realization of high resolution and fast imaging sequences in knee imaging.

Keywords

Magnetic resonance wearable coil SNR filling coefficient

1. Introduction

Since Roemer et al. [2] proposed the coil overlap decoupling and low input impedance preamplifier decoupling strategy in 2010, the phased array technology has been widely used in multichannel coil development to achieve large field of view (FOV), high signal-to-noise ratio (SNR), and accelerated imaging. Because high SNR of a coil is the basic requirement for using higher resolution and faster imaging sequences, different groups are pursuing better imaging performance of the coils by increasing the number of coil channels. In 2006, Wiggins et al. [3] increased the number of head-receiving coil channels to 32. Compared to the 8-channel head-receiving coil, the cortex and center SNR were increased by 3.5 times and 1.4 times, respectively. In 2012, Keil et al. [4] increased the number of channels to 64, which increased the SNR of cortex by 1.3 times compared to that of the 32-channel head-receiving coil, and the SNR of the center was similar to that in 32 channels. At the same time, the coil was further improved in terms of the acceleration performance by increasing the four-fold acceleration of the cortex and SNR of the center by 1.4 times and 1.2 times, respectively, compared to that of 32 channels. To determine whether high-density, multi-coil MR receiver arrays can achieve high-acceleration parallel imaging in practice, Hardy et al. [5] increased the number of channels to 128. However, this continuous increase in the number of channels decreases the size of the single turn and degrades the quality of the coil. In addition, more channels imply that a large number of electronic devices need to be laid out in a fixed area, and hence, this increase in the performance makes the coil more cumbersome, and less portable and comfortable. Therefore, it is not possible to increase the coil SNR by increasing the number of channels without limitations. Lufkin proposed that the coil SNR is proportional to its fill factor, which means that unlike the idea of increasing the number of coil channels, the strategy of increasing the coil fill factor can improve the coil SNR. Many research groups have conducted various explorations on increasing the coil filling factor using new materials and methods. The Institute of Biomedical Engineering at the University of Zurich in Switzerland uses an adjustable mechanical structure to adapt to the dimensional changes of the measured part [6]. The structure of this coil is complex and bulky, and does not involve decoupling between coils, resulting in the compromise of imaging quality. The Stanford University’s Department of Radiology and Electrical Engineering [7, 8] deploys coils on soft materials, but the coil array cannot be stretched and deformed. Its strong flexibility makes it suitable for several applications, and the imaging effect is good, but the bulky system line still remains the biggest obstacle to the application. Both the Freiburg University and the University of California at Berkeley printed coil inductance and “distributed capacitance” by performing an in-depth theoretical analysis and the technical accumulation of the relationship among different printed materials [9, 10, 11]. The inkjet printing and screen printing techniques are used to make coils that can fit the tested tissue. However, the technical method is costly (the material uses either silver paste or other high-cost good conductors such as “ink”, and the printing equipment is also expensive) [12, 13]. Moreover, the coil made of the current printing material has poor flexibility and is easy to break, such that the performance of the coil is deteriorated, and low frequency of use also limits its application scenarios. Currently, materials that meet the requirements of printing technology and high flexibility remain the bottleneck in the implementation of such magnetic resonance RF coils.

To improve the SNR by increasing the coil filling factor while taking into account the comfort and portability of the coil, the methods adopted by each group pursue the close contact of the coil and measured tissue. We refer to these methods as the wearable technology. However, the shortcomings of these implementation methods and materials limit the application value of the coil. This study uses a wearable concept to maintain the performance of the coil when it is stretched and changed within a certain range. According to the traditional coil exploit technology, the coil is laid on an elastic material. The maximum filling factor and good imaging performance of the coil can be achieved under different loads while satisfying lightness and comfort. The coils were tested for phantoms of different sizes and clinical scan images were observed. The results were then compared to those of the 8-channel commercial knee joint receiving coils.

2. Methods

The array coil was exploited and tested using a 1.5T clinical scanner (Alltech EchoStar 1.5T). The coil was manufactured according to commercial standards and tested in accordance with the informed and permitted clinical requirements of the subject.

The components of the coil are laid out on an elastic material, and the experiments show that the material has no effect on imaging. In our design, the coil has a loop size radius of 4.5 cm, and the loop equivalent inductance uses a 24 AWG copper-core insulated wire. Three Dalipap-DLC10B series non-magnetic capacitors are connected to the wire to avoid the antenna effect of the coil wire, and an equivalent inductor can be used to form a resonant circuit for the reception of magnetic resonance signals. In addition, active and passive detuned circuits are placed in the loop to ensure that the circuit is detuned in the transmitting state to protect the receiving link from high voltage damage, and prevent the receiving loop from interfering with the B1 field. Active detuning is created by the diode on a parallel detuned loop that is turned on through the system power supply under the transmitting state. Passive detuning is a parallel detuned loop composed of two anti-parallel diodes. In the transmitting state, two parallel diodes are turned on to realize the resonant state of the circuit. The inductor in the active and passive parallel detuned loops is obtained by winding the insulated copper wire of radius 0.4 mm (inductor radius of 1.5 mm, 11 turns). By twisting the inductor, it is easy to freely adjust the inductance value so that the resonance can reach an optimal state. Five identical loops are produced using the above method.

Figure 1.

Layout process of 5-channel knee coil.

According to the overlapping decoupling and coil layout theory of Roemer et al. [2] and Wiggins [14], we created a paper model with five co-edge regular hexagons of side length 4.5 mm and combined these hexagons into a closed loop. Five loops that were previously debugged from the elastic material were fixed according to the circumscribed circular trajectories of five co-edge regular hexagons. The layout process is shown in Fig. 1. After the loop layout was completed, a low-impedance preamplifier was placed close to the top of each loop, powering the preamplifier and providing bias voltage to the active detuned circuit through the system line. The preamplifier input matching circuit was then adjusted to convert the input impedance to achieve preamplifier decoupling [7, 8] to eliminate coupling between adjacent channels. Next, the overlapping area of adjacent channels was fine adjusted, and the coupling of adjacent loops was measured in the range of $-$ 17 dB to $-$ 23 dB using a vector network analyzer (Agilent E5061A). To avoid the interference of the common mode current on the system line with the magnetic resonance signal, an LC balun [15] was added on the system line to suppress the common mode signal.

Figure 2.

5-channel knee coil.

Finally, the active detuning, preamplifier decoupling, and loop sensitivity testing were performed on the coil using a test box that powered the pool amplifier and bias circuit. The test box was used to adjust the parameters, such as the center frequency, matching, coupling, and gain of the coil, setting the center frequency to 63.75 MHz ( $\pm$ 100 KHz), matching to $-$ 17 dB, coupling S21 between the adjacent coils at less than $-$ 17 dB, and the center frequency at the midpoint of the M-wave when measuring the coil gain and its sensitivity [16, 17].

After the coil bench test was completed, the coil was tested for imaging. To explore the performance of the coil under loads of different sizes in a certain range, three cylindrical water phantoms of circumferences 33.5 cm, 35 cm, and 37.5 cm were used in the water film test. In the clinical experiment of the human knee joint, the circumferences of the knee joints of three subjects were 33.5 cm, 36.5 cm, and 37.3 cm, respectively. To visually verify the designed coil performance, an 8-channel commercial knee coil was used for the imaging comparison test. The water film and clinical scan collected the signal and noise data of the region of interest, and the SNR results of the scanned image were obtained using the array calculation method that combines the optimal signal and noise data in each channel, as proposed by Kellman and Mcveigh [18]. The higher intensities of the red color of the pixel in the processed SNR graph indicates higher SNR values, whereas the higher intensities of the blue color indicates a lower SNR value of the pixel point.

3. Phantom verification result

First, the coil is subjected to a single-channel imaging test. Then the water (distilled water: 4400 g, NaCl: 19.294, NiCl ${}_{2}\cdot$ 6H ${}_{2}$ O: 5.698 g) film structure imaging test uses the FLASH sequence, with flip angle (FA) $=$ 60 ${}^{\circ}$ , repetition time (RT) $=$ 500 ms, echo time (ET) $=$ 5.6 ms, FOV $=$ 450 $\times$ 450, and thickness $=$ 5 mm, to obtain the data of sagittal, coronal, and cross-section images of water films of different sizes. In the experiment, the matching of coil can be maintained in the range of $-$ 15 dB to $-$ 25 dB under different water film load conditions, and the active and passive detuned loops provide better isolation than $-$ 35 dB for coils in the tuning and detuning states.

Figure 3.

Single channel imaging of a 5-channel knee coil.

Figure 4.

Normalized coupling matrix of 5-channel knee coil under three phantom loadings.

The images (from top to bottom) shown in Fig. 3 represent the coil single-channel imaging under loads of nos 1–3, respectively. The results show that the imaging area is concentrated and there is no crosstalk.

The images (from left to right) shown in Fig. 4 represent the coupled normalization matrices between the adjacent channels of the coil in water film under loading states of nos 1–3. The diagonal line represents the strongest coupling degree between each channel and itself, the value being 1. The blue colored blocks indicate that the coupling degree between the channels is small. It can be seen from three coupling matrices that the isolation of each channel of the coil is kept below 0.15.

Figure 5.

Three-phantom coronal imaging of 5- and 8-channel knee coils.

Figure 6.

SNR comparison of horizontal direction line of the fourth layer of coronal image.

Figure 5a and b show the SNR images of 5-channel and 8-channel knee coil coronal surfaces, respectively, under the water film loads of nos 1–3 from top to bottom. It can be seen that the overall SNR is relatively high, and gradually decreases from the shallow layer to the deep layer.

Figure 7.

Three-phantom transverse imaging of 5- and 8-channel knee coils.

Figure 8.

SNR comparison of horizontal direction line of the fourth layer of transverse image.

Figure 9.

Comparison of sagittal images of three subjects’ knee joints of two coils.

Figure 9.

continued.

Figure 6 shows the pixel contrast curves of 5-channel and 8-channel knee coils in the center horizontal direction of the image in the fourth layer of the coronal plane under the water film loads of three different sizes. The horizontal axis corresponds to the pixel position in the center horizontal direction, and the vertical axis represents the SNR values (repeated four times to obtain an average) at different positions. The larger the value of pixel position, the larger the SNR.

Figure 7a and b show the SNR images of 5-channel and 8-channel knee coil cross-sectional surfaces, respectively, under the water film loads of nos 1–3. It can be seen that the overall SNR is relatively high and gradually decreases from the shallow layer to the deep layer.

Figure 8 shows the pixel contrast curves of 5-channel and 8-channel knee coils in the center horizontal direction of the image in the fourth layer of the coronal plane under the water film loads of three different sizes. The horizontal axis corresponds to the pixel position in the center horizontal direction, and the vertical axis represents the SNR values at different positions. The larger the value of pixel position, the larger the SNR.

By comparing Figs 5–8, it can be concluded that the designed 5-channel knee coil has higher SNR performance than the 8-channel commercial coil. The SNRs of the shallow and deep images are four times and two times that of the 8-channel knee coil, respectively. The water film imaging experiments show that the wearable knee joint receiving coil still exhibits better performance while maintaining the maximum filling coefficient.

4. In vivo verification results

The human knee joint clinical scan test uses the T2 FSE (fast spin echo) sequence, with TR $=$ 2700 ms, FA1 $=$ 90 ${}^{\circ}$ , FA2 $=$ 160 ${}^{\circ}$ , FOV $=$ 160 $\times$ 160 mm, phase resolution $=$ 0.75 mm, read resolution $=$ 0.6 mm, and thickness $=$ 4 mm. Subjects are required to wear sound-proof headphones lying in a supine position with advanced feet. The center of the coil and the positioning are at the lower edge of the patella. In this study, the coil test uses a flexible FR4 (flame retardant) material around the knee joint to form a relatively regulated cylindrical structure.

Figure 9 shows the middle three-layer SNR images of 5-channel elastic knee coil (left) and 8-channel commercial coil (right) in the sagittal plane of knee joint under the loads of three subjects: (a) no. 1, (b) no. 2, and (c) no. 3.

As shown in Fig. 9a–c, the 5-channel knee coil has a better imaging effect on the knee joint capsule and the connected ligament, and its SNR is 2–4 times that of the 8-channel knee coil. The larger FOV of the channel coil allows better imaging of the knee and flexor knee muscles, whereas the 5-channel knee coil limits the imaging of the knee-related muscle groups due to the smaller coil size.

5. Conclusions

In this paper, flexible and elastic materials are used to design coils by combining wearable systems with the coil development technology. The 1.5T 5-channel knee-joint receiving coil is fabricated using an elastic material instead of the traditional fixed mechanical structures. It can be attached to knee joints of different sizes in a certain range, so that the maximum filling coefficient of the coil can be achieved under knee loads of different sizes. In the water film test, the coil showed 4-fold and 2-fold SNR improvement in both shallow and deep layers under different sizes of loads compared to the in vivo knee imaging and the commercial 8-channel knee coil. Moreover, high SNR will facilitate the realization of high resolution and fast imaging sequence in knee joint imaging. Therefore, compared to the traditional commercial coils, the maximization of the filling coefficient of the wearable coils can greatly improve the SNR. The flexibility and miniaturization of the coil provide a better user experience; however, because of its single-row closed-loop structure, the FOV is relatively small, thereby limiting the clinical diagnosis. In the follow-up study, the design involving additional channels will be optimized.

Footnotes

Acknowledgments

This work was supported by the Leading Talent Training Project of Neijiang Normal University under Grant 2017 [Liu Yi-He], the Innovative Team Program of the Neijiang Normal University under Grant 17TD03, the Foundation of Ph. D. Scientific Research of the Neijiang Normal University under Grant RSC201704, the Sichuan Science and Technology Program under Grant 2019YJ0181, the Sichuan province academic and Technical Leader Training Funded projects under Grant 13XSJS002, the National Key Research and Development Program of China under Grants 2016YFC0100800 and 2016YFC0100802, and the Foundation of Ph. D. Scientific Research of Neijiang Normal University under Grants 2019 [Zhang Shuang] and [Wang Jiujiang].

Conflict of interest

None to report.

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