Compact magnetic resonance imaging low-temperature probe system

Abstract

The compact type magnetic resonance imaging low-temperature probe system comprises a vacuum shell, the vacuum shell is cylindrical, one end of the vacuum shell is provided with a concave shell arc plate part, the interior of the vacuum shell is hollow, the other end of the vacuum shell is open, and a hollow area in the vacuum shell is a low-temperature cavity; a normal-temperature pickup coil and a normal-temperature pumping coil are arranged on the outer surface of the vacuum shell, a cold conducting finger is arranged in the low-temperature cavity, a parameter amplification module and a low-temperature wireless coil unit are arranged on the cold conducting finger, and the parameter amplification module is connected with the low-temperature wireless coil unit. The low-temperature wireless coil unit is inductively coupled with the normal-temperature pickup coil and the normal-temperature pumping coil respectively; the open end of the vacuum shell is connected with a shell of the small pulse tube refrigerator, and the cold conduction finger is connected with a cold end heat exchanger of the small pulse tube refrigerator. By arranging the parametric amplification module and the normal-temperature pumping coil, the use of an active amplifier in the probe is avoided, so that the calorific value of the probe is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of magnetic resonance imaging systems, in particular to a compact magnetic resonance imaging cryogenic probe system suitable for high-sensitivity nuclear magnetic resonance signal detection. BACKGROUND

[0002] Magnetic resonance imaging (MRI) is a non-invasive imaging technique widely used in medical and scientific research fields. The quality of MRI imaging largely depends on the signal-to-noise ratio (SNR) of the system. In multi-nuclear MRI of non-proton nuclei such as sodium (Na), xenon (Xe) and phosphorus (P), it is challenging to obtain high SNR images due to the low nuclear magnetic sensitivity. 23 129 31

[0003] To improve imaging sensitivity, it is common practice to place the radio frequency receiving coil and preamplifier in a cryogenic environment (usually below 30 K) for operation, thereby effectively reducing thermal noise and improving SNR.

[0004] However, existing cryogenic probe systems rely on large Gifford-McMahon (GM) refrigerators, making them bulky and complex in structure, often requiring an external compressor and connecting pipeline, which is heavy and complex to deploy, not conducive to compact integration and portable applications of the system, and not suitable for space-limited experimental environments or small MRI systems.

[0005] Small pulse tube refrigerators have the advantages of small size, low vibration and high integration, providing a feasible solution for the development of compact cryogenic probe systems. However, their limited refrigeration power poses higher requirements for thermal design and structural arrangement. Therefore, there is an urgent need for a compact cryogenic probe system with efficient thermal management and easy deployment to adapt to more clinical and research application scenarios. SUMMARY

[0006] The present application aims to solve the problems of existing magnetic resonance imaging cryogenic probe systems, such as large size and inconvenient deployment, and provides a compact magnetic resonance imaging cryogenic probe system.

[0007] The above-mentioned purpose of the present application is achieved by the following technical means:

[0008] ​​​The utility model provides a compact magnetic resonance imaging cryogenic probe system, including vacuum shell, vacuum shell is cylindrical and is provided with the concave shell arc plate part in one end, the inside of vacuum shell is hollow and the other end is open, the inside hollow area of vacuum shell is cryogenic chamber, is provided with the cold -conducting finger in the cryogenic chamber, is provided with parametric amplification module and low temperature wireless coil unit on the cold -conducting finger, parametric amplification module is connected with low temperature wireless coil unit, and low temperature wireless coil unit respectively with normal temperature pickup coil and normal temperature pump coil inductive coupling, the open end of vacuum shell is connected with the shell of small pulse tube refrigerator, and the cold -conducting finger is connected with the cold end heat exchanger of small pulse tube refrigerator.

[0009] The low temperature wireless coil unit includes a first wireless low temperature coil and a second wireless low temperature coil, and the parametric amplification module includes a first capacitor, a first inductor, a variable capacitor, a second capacitor, and a second inductor.

[0010] One end of the first wireless low temperature coil and the first capacitor in parallel is connected with one end of the first inductor, the other end of the first wireless low temperature coil and the first capacitor in parallel is connected with one end of the second wireless low temperature coil, the other end of the first inductor is connected with one end of the variable capacitor, one end of the second capacitor and the second inductor in parallel is connected with the other end of the variable capacitor, and the other end of the second capacitor and the second inductor in parallel is connected with the other end of the second wireless low temperature coil.

[0011] The variable capacitor is a zero-bias varactor diode.

[0012] The first wireless low temperature coil and the first capacitor constitute a first resonant circuit, the first resonant circuit is tuned to the Larmor frequency v1 of the target to be measured, the second inductor and the second capacitor constitute a second resonant circuit, the second resonant circuit is tuned to the pump frequency v3, the first inductor and the variable capacitor constitute a third resonant circuit, the third resonant circuit is tuned to the difference frequency v2, and v3=v1+v2.

[0013] The normal temperature pickup coil is a single-frequency resonant coil, the normal temperature pickup coil is tuned to the Larmor frequency v1 of the target to be measured, and is connected with the magnetic resonance imaging system; the normal temperature pump coil is tuned to the pump frequency v3.

[0014] The cold -conducting finger includes a cylindrical part, the end face of one end of the cylindrical part is a concave end face, the low temperature wireless coil unit is arranged on the concave end face, and the other end of the cylindrical part is connected with the cold end heat exchanger.

[0015] The optical fiber temperature sensor is arranged on the cylindrical part of the cold -conducting finger.

[0016] The cold -conducting finger, the low temperature wireless coil unit and the optical fiber temperature sensor are all provided with a gap between the vacuum shell.

[0017] The material of the low-temperature wireless coil unit is yttrium barium copper oxide or bismuth strontium calcium copper oxide.

[0018] The small-sized pulse tube refrigerator is connected with a compressor, and a magnetic shielding layer is arranged outside the compressor.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] 1. The low-temperature wireless coil unit is connected with a parametric amplification module, the parametric amplification module and the normal-temperature pumping coil constitute a parametric amplifier, the use of an active preamplifier is avoided, the heat source in the low-temperature chamber is reduced, the requirement for the refrigeration power of the refrigerator is lowered, the use of a large-sized refrigeration platform is avoided, and thus the system volume and weight are reduced, the system is convenient to integrate in a narrow space, and the operation and maintenance costs are lowered.

[0021] 2. The material of the low-temperature wireless coil unit is a high-temperature superconducting material, which is beneficial to relax the minimum refrigeration temperature requirement, and makes it possible to apply a small-sized low-power refrigerator to a low-temperature probe.

[0022] 3. The present application has high miniaturization and integration degree, is convenient to deploy, and is suitable for various application scenarios such as small animal MRI, portable MRI and low-field MRI. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a structural schematic diagram of the present application;

[0024] Figure 2 is a connection circuit diagram of the low-temperature wireless coil unit and the parametric amplification module;

[0025] Figure 3 is a structural schematic diagram of the cold guide;

[0026] Figure 4 is a structural schematic diagram of the vacuum shell;

[0027] Figure 5 is a structural schematic diagram of the cooperation installation of the vacuum shell, the cold guide and the small-sized pulse tube refrigerator;

[0028] Among them, 1 is a screw, 2 is a coaxial cable, 3 is a normal-temperature pumping coil, 4 is a cold guide, 5 is a low-temperature wireless coil unit, 6 is a vacuum shell, 7 is a normal-temperature pickup coil, 8 is a temperature sensor, 9 is an optical fiber, 10 is a small-sized pulse tube refrigerator, 11 is a cold end heat exchanger, 12 is a gas pipe, 13 is a coupling flange, 14 is a compressor, 201 is a first wireless low-temperature coil, 202 is a first capacitor, 203 is a first inductor, 204 is a second wireless low-temperature coil, 205 is a second capacitor, 206 is a second inductor, and 207 is a variable capacitor. DETAILED DESCRIPTION

[0029] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to embodiments. The embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0030] Example 1:

[0031] like Figure 1 As shown, a compact magnetic resonance imaging cryogenic probe system includes a vacuum housing 6, a cooling finger 4, a cryogenic wireless coil unit 5, a room temperature pickup coil 7, a room temperature pump coil 3, and a small pulse tube refrigerator 10.

[0032] like Figure 4 As shown, the vacuum housing 6 is cylindrical and has a concave arc plate at one end. The vacuum housing 6 is hollow inside and open at the other end. The hollow area inside the vacuum housing 6 is a low-temperature chamber, and the area enclosed by the arc plate is a normal temperature detection area. The open end of the vacuum housing 6 is used to install the cooling finger 4.

[0033] like Figure 3 As shown, due to the limited cooling capacity of the small pulse tube cryostat 10, the number of circuits in the low-temperature chamber should be minimized to reduce the heat generated by the circuitry. The low-temperature wireless coil unit 5 and the cooling finger 4 are located within the low-temperature chamber. The cooling finger 4 includes a cylindrical portion with a concave end face at one end and a cooling finger connection portion at the other end. The concave end face matches the shape of the outer shell's arc plate. The low-temperature wireless coil unit 5 is positioned on the concave end face. Heat transfer between the cooling finger 4 and the low-temperature wireless coil unit 5 carries away the heat generated by the low-temperature wireless coil unit 5, allowing it to operate at a low temperature and improving the signal-to-noise ratio of magnetic resonance imaging. Gaps are provided between the cooling finger 4 and the low-temperature wireless coil unit 5 and the vacuum shell 6 to reduce heat transfer between the vacuum shell 6 and the cooling finger 4, and between the vacuum shell 6 and the low-temperature wireless coil unit 5, thereby reducing the risk of frostbite on the object being tested in the ambient temperature detection area.

[0034] Understandably, the smaller the distance between the low-temperature wireless coil unit 5 and the target under test, the higher the signal-to-noise ratio of the coil. In order to make the coil closer to the target under test, the vacuum housing 6 is made as thin as possible while ensuring a high vacuum, so that the distance between the low-temperature wireless coil unit 5 and the target under test outside the vacuum housing 6 is as small as possible.

[0035] The low-temperature wireless coil unit 5 is inductively coupled to the ambient temperature pickup coil 7 and ambient temperature pump coil 3 outside the vacuum housing 6. Using this type of wireless coil effectively avoids the radio frequency heating problem caused by wired connection cables and eliminates the thermal isolation challenges posed by wired connections passing through the vacuum housing.

[0036] To achieve preamplification, an active preamplifier is typically used. Active preamplifiers rely on a power supply and utilize active components such as transistors and operational amplifiers. These components generate static and dynamic power consumption during operation, leading to heat accumulation. To reduce cooling consumption, the parametric amplification module is housed in a cryogenic chamber. A radio frequency pump signal provided by a room-temperature pump coil 3 via magnetic induction wirelessly powers the parametric amplification module, enabling signal amplification without the need for an additional DC power supply line, further reducing heat generation. The radio frequency signal received by the cryogenic wireless coil unit 5 can be directly preamplified by the parametric amplification module, thereby reducing the impact of noise in the subsequent radio frequency transmission path on the overall signal-to-noise ratio and avoiding the heat generated by the DC bias circuit in existing active preamplifiers.

[0037] In some embodiments, due to the limited cooling capacity of the small refrigerator, the wires in the low-temperature wireless coil unit 5 are made of high-temperature superconducting materials, such as yttrium barium copper oxide (YBCO) or bismuth strontium calcium copper oxide (BSCCO), which can achieve superconductivity at temperatures above 77K. Compared with conventional copper conductors, thermal noise is further reduced, coil sensitivity is further improved, and imaging SNR (signal-to-noise ratio) is enhanced.

[0038] like Figure 2 As shown, Figure 2 The circuit diagram showing the connection between the low-temperature wireless coil unit 5 and the parametric amplifier module is displayed. The overall circuit of the low-temperature wireless coil unit 5 and the parametric amplifier module is a three-frequency resonant circuit structure. Wireless power supply to the parametric amplifier module is achieved using the room-temperature pump coil 3.

[0039] The low-temperature wireless coil unit 5 includes a first low-temperature wireless coil 201 and a second low-temperature wireless coil 204. The parametric amplification module includes a first capacitor 202, a first inductor 203, a variable capacitor 207, a second capacitor 205, and a second inductor 206. In this embodiment, the variable capacitor 207 is a zero-bias varactor diode.

[0040] One end of the first wireless cryogenic coil 201 and the first capacitor 202 connected in parallel is connected to one end of the first inductor 203. The other end of the first wireless cryogenic coil 201 and the first capacitor 202 connected in parallel is connected to one end of the second wireless cryogenic coil 204. The other end of the first inductor 203 is connected to one end of the variable capacitor 207. One end of the second capacitor 205 and the second inductor 206 connected in parallel is connected to the other end of the variable capacitor 207. The other end of the second capacitor 205 and the second inductor 206 connected in parallel is connected to the other end of the second wireless cryogenic coil 204.

[0041] The first wireless low-temperature coil 201 and the first capacitor 202 constitute a first resonant circuit, the first wireless low-temperature coil 201 is used for realizing signal transmission between the normal-temperature pickup coil 7, and the first resonant circuit is tuned to a Larmor frequency v1 of a target to be detected, and is used for receiving an RF (radio frequency) signal from the target to be detected;

[0042] The second inductance 206 and the second capacitor 205 constitute a second resonant circuit, the second resonant circuit is tuned to a pump frequency v3;

[0043] The first inductance 203 and the variable capacitor 207 constitute a third resonant circuit, the third resonant circuit is tuned to a difference frequency v2.

[0044] The second wireless low-temperature coil 204 is coupled with the normal-temperature pump coil 3, and is used for receiving a pump signal emitted by the normal-temperature pump coil 3 to provide energy for the parametric amplifier module.

[0045] The RF signal (the frequency is the Larmor frequency v1 of the target to be detected) and the pump signal (the frequency is the pump frequency v3) output by the normal-temperature pump coil are mixed when passing through the variable capacitor 207 to generate an amplified difference frequency signal (the frequency is the difference frequency v2), the difference frequency signal is mixed with the pump signal again, and secondary amplification output is generated at the first wireless low-temperature coil 201, so that the input RF signal is amplified. The Larmor frequency v1, the pump frequency v3 and the difference frequency v2 satisfy v3=v1+v2.

[0046] The normal-temperature pickup coil 7 is a single-frequency resonant coil, which is used for exciting and receiving an MR (magnetic resonance) signal, is arranged outside the vacuum shell 6, can be fixed on the outer surface of the vacuum shell 6, and is connected to an MRI (magnetic resonance imaging) system through a coaxial cable 2. As shown in the figure, the normal-temperature pickup coil 7 is coupled to the first wireless low-temperature coil 201 in the low-temperature wireless coil unit 5, and at this time, the normal-temperature pickup coil 7 is tuned to the Larmor frequency (v1). In the excitation stage, the first wireless low-temperature coil 201 generates a B1 field after receiving a radio frequency pulse signal from the MRI system, and excites the target to be detected; in the receiving stage, the first wireless low-temperature coil 201 senses the radio frequency signal of the target to be detected, and after local amplification by the parametric amplifier module, the signal is transmitted to the normal-temperature pickup coil 7 by the first wireless low-temperature coil 201, and the normal-temperature pickup coil 7 transmits the received amplified signal to the receiving channel of the MRI system through the coaxial cable 2 for digitization and image reconstruction. Figure 2 The normal-temperature pump coil 3 is an inductive coil tuned to the pump frequency v3, which is arranged on the outer surface of the vacuum shell 6 and is coupled with the second wireless low-temperature coil 204. The normal-temperature pump coil 3 emits a high-frequency pump signal, the pump signal drives the variable capacitor 207 to modulate, realizes mixing and amplification of the signal.

[0047]

[0048] ​The small-sized pulse tube refrigerator 10 generates cold energy at the cold end heat exchanger 11, which is provided with a cold end heat exchanger connecting part connected with the cold-finger connecting part, so that the cold energy is transmitted to the low-temperature wireless coil unit 5 through the cold finger 4, thereby realizing refrigeration of the low-temperature wireless coil unit 5, reducing the thermal noise of the low-temperature wireless coil unit 5 itself, improving the sensitivity of the low-temperature wireless coil unit 5, and realizing the improvement of the imaging quality.

[0049] The cold finger 4 is responsible for transporting the cold energy generated by the small-sized pulse tube refrigerator 10 to the low-temperature wireless coil unit 5 and taking away the heat generated by the low-temperature wireless coil unit 5. To ensure efficient transmission of cold energy to the low-temperature wireless coil unit 5, the cold finger 4 is generally made of a solid material with high thermal conductivity and low dielectric loss, such as sapphire, Al2O3, aluminum nitride, etc. The cold finger 4 and the cold end heat exchanger 11 are fixed by screws 1 or other methods, and at the same time, the contact surfaces of the cold finger 4 and the cold end heat exchanger 11 are uniformly coated with cold-finger glue to fill the gap between the rigid contact surfaces, so that they can perform sufficient and efficient heat exchange.

[0050] The length of the cold finger 4 is as short as possible without affecting the installation, so as to minimize the heat radiation area of the cold finger 4, thereby reducing the demand for refrigeration capacity of the small-sized pulse tube refrigerator 10.

[0051] In some embodiments, an optical fiber temperature sensor 8 is fixed on the cylindrical part of the cold finger 4, and a gap is provided between the optical fiber temperature sensor 8 and the vacuum shell 6. The optical fiber temperature sensor 8 can obtain the surface temperature of the cold finger 4, and send the signal to the computer through the optical fiber 9 for monitoring the real-time temperature of the low-temperature wireless coil unit 5.

[0052] It can be understood that the smaller the distance between the first wireless low-temperature coil 201 in the low-temperature wireless coil unit 5 and the target to be measured, the higher the signal-to-noise ratio of the coil. In order to make the coil closer to the target to be measured, the thickness of the vacuum shell 6 is as thin as possible while ensuring high vacuum, and the concave end surface is as close as possible to the shell arc plate part, so that the distance between the low-temperature wireless coil unit 5 and the target to be measured outside the vacuum shell 6 is as small as possible.

[0053] As shown in Figure 5 The cold end heat exchanger 11, the cold finger 4 and the low-temperature wireless coil unit 5 are placed in the vacuum shell 6, which can effectively save heat and reduce heat transfer between the low-temperature area and the outside, so as to ensure that the inside of the vacuum shell 6 maintains a low temperature below 77K. In implementation, the outer surface of the connection between the cold end heat exchanger 11 and the cold finger 4 should be wrapped with multiple layers of thermal insulation film to reduce heat radiation loss.

[0054] The vacuum shell 6 and the shell of the small pulse tube refrigerator 10 are coupled through coupling flanges 13, are tightly fixed by using screws 1, and sealing rings are arranged between the coupling flanges 13 arranged on the vacuum shell 6 and the coupling flanges 13 arranged on the shell of the small pulse tube refrigerator 10, so as to ensure the air tightness in the vacuum shell 6. Meanwhile, the vacuum shell 6 is provided with a vacuum extraction interface, so as to ensure that the inside of the vacuum shell 6 maintains a high-vacuum environment, reduce the heat transfer between the cold-finger 4 and the vacuum shell 6, and reduce the loss of cold energy.

[0055] The small pulse tube refrigerator 10 has the characteristics of small volume, low vibration and high reliability, is made of non-magnetic material, can be placed in a magnetic resonance magnet, the cold end heat exchanger 11 and the cold-finger 4 are coupled, the refrigeration capacity of the small pulse tube refrigerator 10 is transmitted to the low-temperature wireless coil unit 5 through the cold-finger 4, and the low-temperature wireless coil unit 5 is kept at a temperature lower than 77K.

[0056] The small pulse tube refrigerator 10 is connected with the compressor 14 through the air pipe 12 (the small pulse tube refrigerator 10 is prior art, and the specific structure is not described in detail), compressed air generated by the compressor 14 enters the small pulse tube refrigerator 10 through the air pipe 12, and the small pulse tube refrigerator 10 is provided with acoustic power driving, and the air pipe 12 can be customized according to actual needs.

[0057] In some embodiments, the compressor 14 can be placed close to an MRI main magnet, and the compressor 14 is magnetically shielded by arranging a magnetic shielding layer outside the compressor 14, so as to avoid the influence of the compressor 14 on imaging, and the whole system can be more compact.

[0058] The small pulse tube refrigerator 10 can be replaced by a Stirling type pulse tube refrigerator.

[0059] It should be noted that the embodiments described in the present application are only examples for illustrating the spirit of the present application. Those skilled in the art of the present application can make various modifications or supplements to the described embodiments or use similar ways to replace, but will not deviate from the spirit of the present application or exceed the scope defined by the appended claims.

Claims

1. A compact cryogenic magnetic resonance imaging probe system, comprising a vacuum housing (6), characterized in that, The vacuum shell (6) is cylindrical and has a concave arc plate at one end. The vacuum shell (6) is hollow inside and open at the other end. The hollow area inside the vacuum shell (6) is a low-temperature chamber. A room temperature pickup coil (7) and a room temperature pump coil (3) are provided on the outer surface of the vacuum shell (6). A cooling finger (4) is provided inside the low-temperature chamber. A parametric amplification module and a low-temperature wireless coil unit (5) are provided on the cooling finger (4). The parametric amplification module is connected to the low-temperature wireless coil unit (5), and the low-temperature wireless coil unit (5) is inductively coupled to the room temperature pickup coil (7) and the room temperature pump coil (3) respectively. The open end of the vacuum shell (6) is connected to the shell of the small pulse tube refrigerator (10), and the cooling finger (4) is connected to the cold end heat exchanger (11) of the small pulse tube refrigerator (10).

2. The compact magnetic resonance imaging cryogenic probe system according to claim 1, characterized in that, The low-temperature wireless coil unit (5) includes a first low-temperature wireless coil (201) and a second low-temperature wireless coil (204). The parametric amplification module includes a first capacitor (202), a first inductor (203), a variable capacitor (207), a second capacitor (205), and a second inductor (206). One end of the first wireless cryogenic coil (201) and the first capacitor (202) connected in parallel is connected to one end of the first inductor (203). The other end of the first wireless cryogenic coil (201) and the first capacitor (202) connected in parallel is connected to one end of the second wireless cryogenic coil (204). The other end of the first inductor (203) is connected to one end of the variable capacitor (207). One end of the second capacitor (205) and the second inductor (206) connected in parallel is connected to the other end of the variable capacitor (207). The other end of the second capacitor (205) and the second inductor (206) connected in parallel is connected to the other end of the second wireless cryogenic coil (204).

3. The compact magnetic resonance imaging cryogenic probe system according to claim 2, characterized in that, The variable capacitor (207) is a zero-bias varactor diode.

4. A compact magnetic resonance imaging cryogenic probe system according to claim 3, characterized in that, The first wireless cryogenic coil (201) and the first capacitor (202) constitute a first resonant circuit, which is tuned to the Larmor frequency ν1 of the target under test. The second inductor (206) and the second capacitor (205) constitute a second resonant circuit, which is tuned to the pump frequency ν3. The first inductor (203) and the variable capacitor (207) constitute a third resonant circuit, which is tuned to the difference frequency ν2. ν3 = ν1 + ν2.

5. A compact magnetic resonance imaging cryogenic probe system according to claim 4, characterized in that, The ambient temperature pickup coil (7) is a single-frequency resonant coil. The ambient temperature pickup coil (7) is tuned to the Larmor frequency ν1 of the target to be measured and is connected to the magnetic resonance imaging system; the ambient temperature pump coil (3) is tuned to the pump frequency ν3.

6. A compact magnetic resonance imaging cryogenic probe system according to claim 1, characterized in that, The cooling finger (4) includes a cylindrical part, one end of which has a concave end face, and a low-temperature wireless coil unit (5) is provided on the concave end face. The other end of the cylindrical part is connected to the cold end heat exchanger (11).

7. A compact magnetic resonance imaging cryogenic probe system according to claim 2, characterized in that, A fiber optic temperature sensor (8) is provided on the cylindrical part of the cooling finger (4).

8. A compact magnetic resonance imaging cryogenic probe system according to claim 7, characterized in that, The cooling finger (4), the low-temperature wireless coil unit (5), and the fiber optic temperature sensor (8) are all provided with gaps between themselves and the vacuum housing (6).

9. A compact magnetic resonance imaging cryogenic probe system according to claim 2, characterized in that, The material of the low-temperature wireless coil unit (5) is yttrium barium copper oxide or bismuth strontium calcium copper oxide.

10. A compact magnetic resonance imaging cryogenic probe system according to claim 1, characterized in that, The small pulse tube refrigerator (10) is connected to the compressor (14), and the compressor (14) is provided with a magnetic shielding layer on the outside.

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