RF receiver system with adjustable impedance matching

Abstract

According to the invention a radio frequency receiver system for an MRI apparatus is provided, comprising a receive coil, an amplifier, an analog-to-digital converter, a matching network and a noise calculation unit; the receive coil is attached to a patient, exhibiting a total effective coil impedance comprising a coil impedance and a patient impedance when attached to the patient; the amplifier exhibits a lowest noise impedance, connected to the receive coil to amplify a signal and output an amplified output signal; the analog-to-digital converter is connected to the amplifier to convert the amplified output signal into a digital signal for further processing; the matching network is interconnected between the receive coil and the amplifier, comprising an adjustable impedance matching system to match the total effective coil impedance to the lowest noise impedance; the noise calculation unit is connected to the analog-to-digital converter to receive a digital output signal of the analog-to-digital converter, adapted to calculate a noise of the output signal of the analog-to-digital converter, adapted to adjust the adjustable impedance of the matching network. This provides the possibility to individually calibrate the matching network for each patient before a scanning process.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging (MRI), and more particularly to the field of radio frequency receiver systems for magnetic resonance imaging (MRI) apparatus. Background Technology

[0002] Every MRI setup includes a receiver system for receiving and converting MRI signals. Typically, a receiver system comprises a radio frequency (RF) receiver coil, a matching network, a low-noise amplifier (LNA), and an analog-to-digital converter (ADC). Precession of net magnetization induces a current in the RF receiver coil via electromagnetic induction. The LNA amplifies the received signal so that it can subsequently be digitized by the ADC and processed to form an image. Therefore, achieving a minimum noise figure (NF) is of paramount importance. A minimum NF is achieved if the source impedance is transformed and matched to the optimal noise impedance of the LNA. The RF receiver coil is a section of wire with inductance and coil resistance. If a patient is loaded onto the RF receiver coil, the total resistance of the RF receiver coil becomes the sum of the RF receiver coil resistance and the patient's resistance. However, the conventional matching network of an RF receiver system is designed to match only for a specific impedance. Therefore, for such a system, image quality cannot be maintained at a constant level due to the NF, and thus the signal-to-noise ratio (SNR) varies from patient to patient.

[0003] The paper by B. Sporrer et al., “A fully integrated dual-channel on-coil CMOS receiver for array coils” (IEEE Trans. Biomed. Circ. and Syst. 11(2017) 1245-1255), describes an on-board coil CMOS receiver array with both capacitors and inductors integrated with matching circuitry for amplifiers and (implicit) noise calculation. Summary of the Invention

[0004] Therefore, the object of the present invention is to provide a patient-specific tuned RF receiver system for matching networks.

[0005] According to the invention, this objective is achieved through the subject matter of the independent claims. The matching network includes a capacitive portion having at least one capacitor or several capacitors. A matched amplifier is integrated with the capacitive portion of the matching network and a noise calculation unit. Preferred embodiments of the invention are described in the dependent claims.

[0006] Therefore, according to the present invention, a radio frequency receiver system for an MRI apparatus is provided, wherein the radio frequency receiver system includes a receiving coil, an amplifier, an analog-to-digital converter, a matching network, and a noise calculation unit, the receiving coil being arranged in a suitable position to receive magnetic resonance signals, for example, attached to or mounted relative to a patient to be examined by the MRI apparatus, and in this state where the receiving coil is attached to the patient, the receiving coil exhibits a total effective coil impedance, the total effective coil impedance including the coil impedance of the coil itself and the patient impedance due to the patient to whom the coil is attached, the amplifier exhibits a minimum noise impedance, and the amplifier... A receiver coil is connected to the amplifier to amplify the signal received from the receiver coil and output an amplified output signal. The analog-to-digital converter (ADC) is connected to the amplifier to convert the amplified output signal from the amplifier into a digital signal for further processing. A matching network is interconnected between the receiver coil and the amplifier, and the matching network includes a matching system with adjustable impedance to match the total effective coil impedance with the lowest noise impedance. A noise calculation unit is connected to the ADC to receive the digital output signal of the ADC, and the noise calculation unit is adapted to calculate the noise of the ADC's output signal and to adjust the adjustable impedance of the matching network.

[0007] As further explained above, the RF receiver coil is a section of wire with inductance and coil resistance, which changes to a total effective coil impedance when a patient is loaded. This total effective coil impedance is a combination of the RF receiver coil resistance and the patient's resistance. According to the invention, it has been recognized that patient resistance varies over a very wide range (i.e., typically from 0.1Ω to 30Ω). The invention addresses this fact by avoiding a fixed impedance in the matching network, which is only intended to provide a "typical" average matching impedance. Instead, the invention provides the possibility of adapting the matching network for each individual patient prior to the scanning procedure.

[0008] For a wide range of source impedances, the impedance seen by the amplifier can be adapted to an optimal point. Therefore, “matching the total effective coil impedance to the lowest noise impedance” means that the matching system transforms the total effective coil impedance so that the combination of the effective coil impedance and the adjustable impedance of the matching network approximates the amplifier’s lowest noise impedance. In this respect, the term “lowest noise impedance” refers to the impedance with the lowest noise figure provided by the amplifier. Therefore, the present invention aims to match the individual patient impedance to the impedance with the lowest noise figure provided by the amplifier.

[0009] The noise calculation unit receives the digital signal from the ADC and then generates digital bits to tune the matching network to the optimal minimum noise impedance. The term "optimal" means that, in this case, the noise floor level is below the target noise figure, which is preferably 1 dB. For this purpose, the noise floor is calculated from the ADC bitstream. The (digital) noise floor represents the ADC output when zero input signal is present. This can be done by simply calculating the standard deviation of the noise.

[0010] The functionality of this invention is independent of LNA configuration.

[0011] This invention can also be a technical solution for automatically tuned, potentially ultra-flexible, and adaptive RF coils that may be the future of coils.

[0012] According to a preferred embodiment of the invention, the amplifier in the radio frequency receiver system is a low-noise amplifier. A low-noise amplifier (LNA) is an electronic amplifier capable of amplifying very low-power signals without significantly reducing their signal-to-noise ratio. An amplifier will increase the power of both the signal and noise present at its input, but it will also introduce some additional noise. An LNA is designed to minimize this additional noise. A preferred LNA of this application can provide a power gain of 100 (20 dB) while reducing the signal-to-noise ratio by less than 2 times (3 dB noise figure (NF)). For an LNA, an NF of less than 0.5 dB is most preferred.

[0013] According to a preferred embodiment of the invention, the matching system includes an adjustable capacitor bank, preferably having a target range and resolution. It is possible to tune the matching network via the capacitor bank by deriving the noise figure (NF) of the ADC data output and to configure the matching network to optimal impedance prior to the scanning process.

[0014] Typically, different types of capacitor banks can be used in this invention. According to a preferred embodiment of the invention, the capacitor bank is a monolithically integrated capacitor bank. A capacitor bank is a group of several identical capacitors interconnected in parallel or series.

[0015] According to a preferred embodiment of the invention, the matching system, the amplifier, and the analog-to-digital converter are integrated on a single die. The matching capacitor, amplifier, and ADC of the matching system are mounted on the same die, also referred to as "on-chip." The most important components for calibrating the matching network for each individual patient are integrated on the same chip and can be easily installed.

[0016] Furthermore, according to a preferred embodiment of the invention, the matching network further includes at least one capacitor, preferably two capacitors, which are separate from the monolithic die and referred to as "off-chip". The capacitors are arranged such that the first capacitance is the sum of the capacitances of the on-chip capacitor and at least one off-chip capacitor. The second capacitance is the sum of the capacitances of the second on-chip capacitor and the second off-chip capacitor. If the ratio of the first capacitance to the second capacitance is varied such that the sum of the first and second capacitances remains constant, the optimal noise impedance travels along the resistance axis as a function of the first capacitance divided by the second capacitance. This enables the provision of even better matching impedances for each source impedance with only a slight increase in cost and complexity.

[0017] Typically, different types of capacitors can be used in this invention. According to a preferred embodiment of the invention, the at least one capacitor, separate from the monolithic mold, comprises a fixed capacitor.

[0018] Furthermore, according to the present invention, a method for operating a radio frequency receiver system for an MRI apparatus is provided. The method includes the steps of: attaching a receiving coil to a patient to be examined by the MRI apparatus, wherein the receiving coil exhibits a total effective coil impedance, the total effective coil impedance including the coil impedance itself and the patient impedance due to the patient to whom the coil is attached; receiving a signal from the receiving coil by an amplifier exhibiting a minimum noise impedance; amplifying the signal received from the receiving coil by the amplifier; outputting the amplified output signal from the amplifier to an analog-to-digital converter; converting the amplified output signal from the amplifier into a digital signal for further processing in the analog-to-digital converter; and matching the total effective coil impedance to the minimum noise impedance by a matching network interconnected between the receiving coil and the amplifier, the matching network comprising a matching system with adjustable impedance. In this context, "matching" means "approximate," i.e., aiming to make the difference between impedances small, preferably less than a predefined threshold. Preferably, the receiving coil includes multiple receiving channels having their own amplifiers and matching networks, and the method is performed for all of these receiving channels.

[0019] According to a preferred embodiment of the present invention, the method further includes the following steps: receiving the digital output signal of the analog-to-digital converter by a noise calculation unit, adjusting the adjustable impedance, and then calculating the noise of the output signal of the analog-to-digital converter at each impedance point. The noise calculation unit then calculates the optimal minimum noise impedance. "Optimal" means that, in this case, the noise floor level is below a predefined threshold level.

[0020] According to a preferred embodiment of the present invention, the method further includes the following method step: adjusting the adjustable impedance by changing the ratio between a first adjustable impedance of the first capacitor and a second adjustable impedance of the second capacitor. The capacitance of the first matching capacitor and the capacitance of the second matching capacitor are tuned in such a way that the sum of the capacitances of the first matching capacitor and the second matching capacitor is constant to achieve optimal impedance.

[0021] This adjustment can be performed in different ways. According to a preferred embodiment of the invention, the adjustment of the adjustable impedance is automatically controlled. The adjustment is digitally controlled, so that the user does not need any specific knowledge about the implementation method.

[0022] Furthermore, according to the present invention, a non-transient computer-readable medium is provided, comprising instructions stored thereon, which, when executed on a processor, are used by the radio frequency receiver system of an MRI apparatus to perform the method described above. Attached Figure Description

[0023] These and other aspects of the invention will become apparent and explained with reference to the embodiments described below. However, such embodiments do not necessarily represent the full scope of the invention, and therefore reference is made to the claims and this document for interpreting the scope of the invention.

[0024] In the attached diagram:

[0025] Figure 1 An RF receiver system according to a preferred embodiment of the present invention is schematically depicted;

[0026] Figure 2 A second RF receiver system according to a preferred embodiment of the present invention is schematically depicted; and

[0027] Figure 3 A scheme of method according to a preferred embodiment of the present invention is illustrated schematically.

[0028] List of reference numerals

[0029] Radio Frequency Receiver System 1

[0030] Receiving coil 2

[0031] Coil inductance 3

[0032] Patient impedance 4

[0033] coil impedance 5

[0034] Amplifier, LNA 6

[0035] Analog-to-digital converter 7

[0036] Matching Network 8

[0037] Matching System 10

[0038] Single-piece mold 11

[0039] Capacitor 12, separate from the monolithic mold

[0040] The first capacitor 12A is separate from the monolithic mold.

[0041] The second capacitor 12B is separate from the monolithic mold.

[0042] Fixed capacitor 14

[0043] First fixed capacitor 14A

[0044] Second fixed capacitor 14B

[0045] Noise Calculation Unit 15

[0046] Adjustable impedance 16

[0047] First adjustable impedance 16A

[0048] Second adjustable impedance 16B

[0049] Capacitor 17

[0050] First capacitor 17A

[0051] Second capacitor 17B

[0052] Attach the receiving coil to patient S1

[0053] The amplifier receives signal S2 from the receiving coil.

[0054] Amplify the signal S3 received from the receiving coil

[0055] The amplified output signal S4 is obtained from the amplifier output.

[0056] Convert the amplified output signal S5 from the amplifier.

[0057] Matching total effective coil impedance S6

[0058] Receive digital output signal S7

[0059] Calculate the minimum noise impedance S8

[0060] Adjustable impedance S9 Detailed Implementation

[0061] Figure 1An RF receiver system 1 according to a preferred embodiment of the present invention is schematically depicted. The RF receiver system 1 includes a receiving coil 2, wherein the receiving coil 2 includes a coil inductance 3, a patient impedance 4, and a coil impedance 5. The patient impedance 4 varies depending on the patient and is typically in the range of 0.1Ω to 30Ω. A second part of the RF receiver system 1 is a matching network 8. The matching network 8 includes a capacitor 12 separate from the monolithic die 11, a so-called "off-chip" capacitor, and a matching system 10, wherein the matching system 10 includes another capacitor 17 having an adjustable impedance 16. The capacitor 17 is integrated on the monolithic die and is also referred to as an "on-chip" capacitor. In addition to the "on-chip" capacitor 17, other components are integrated on the monolithic die 11. An amplifier 6 and an analog-to-digital converter 7 for a low-noise amplifier (LNA) with the lowest noise impedance are also integrated on the monolithic die 11. The LNA 6 and the ADC 7 are arranged such that the ADC 7 receives signals from the LNA 6. The noise calculation unit 15 is also integrated on the monolithic die 11 and is arranged in such a way that it receives digital signals from the ADC 7 and adjusts the adjustable impedance 16 of the "on-chip" capacitor 17 to make the noise floor level of the LNA 6 as low as possible. Thus, the capacitor 17 with adjustable impedance 16, the LNA 6, the ADC 7, and the noise calculation unit 15 are integrated on the same monolithic die 11.

[0062] Figure 2 A second RF receiver system 1 according to a preferred embodiment of the present invention is schematically depicted. The receiving coil 2 and... Figure 1 The receiving coil 2 is the same. The matching network 8 now includes first and second combined capacitors instead of an "on-chip" capacitor 17 and an "off-chip" capacitor 12. The first combined capacitor is the sum of a first capacitor 17A with a first adjustable impedance 16A and a first capacitor 12A with a fixed capacitance 14, which is separate from the monolithic die 11. The second combined capacitor is the sum of a second capacitor 17B with a second adjustable impedance 16B and a second capacitor 12B with a fixed capacitance 14, which is separate from the monolithic die 11. The first capacitor 17A and the second capacitor 17B with adjustable impedances 16A and 16B are integrated on the monolithic die 11, while the first capacitor 12A and the second capacitor 12B are separate from the monolithic die 11. The LNA 6, ADC 7, and noise calculation unit 15 are also integrated on the monolithic die 11. Instead of adjusting only one impedance 16, the first capacitor 17A and the second capacitor 12B are integrated on the monolithic die 11. Figure 2The preferred embodiment of the invention shown now allows for adjusting the ratio of the first total impedance 16A+14A to the second total impedance 16B+14B. The sum of the first total impedance 16A+14A and the second total impedance 16B+14B is constant, such that the optimal noise impedance travels along the resistance axis as a function of the first total impedance 16A+14A divided by the second total impedance 16B+14B. This enables providing an even better-matched optimal impedance for each source impedance at a slightly increased complexity cost.

[0063] Figure 3 A scheme of method according to a preferred embodiment of the present invention is illustrated schematically.

[0064] S1: First, attach the receiving coil 2 to the patient. The patient will be examined by the MRI device. The impedance of the receiving coil 2 is the sum of the coil impedance 5 and the patient impedance 4. The receiving coil 2 is attached to the patient impedance 4 for examination.

[0065] S2: Second, the signal from the receiving coil 2 is received by the amplifier 6 (e.g., an LNA 6 with the lowest noise impedance).

[0066] S3: Third, the received signal is amplified by amplifier 6.

[0067] S4: Fourth, the amplified signal is output from amplifier 6 to ADC 7.

[0068] S5: Fifth, the amplified output signal is converted from analog to digital by ADC 7.

[0069] S7: Sixth, the digital output signal of ADC 7 is received by the noise calculation unit 15.

[0070] S8: Seventh, the noise calculation unit 15 calculates the optimal minimum noise impedance by deriving the noise of the output signal of the ADC 7, so that the noise floor level is as low as possible.

[0071] According to an embodiment of the invention, if the matching network comprises only one adjustable impedance 16 or a combination of a first adjustable impedance 16A and a second adjustable impedance 16B, then the next step is any of the following:

[0072] S6: The matching network 8, having a matching system 10 with adjustable impedance 16, matches the total effective coil impedance (and thus the combination of patient impedance 4 and coil impedance 5) to the lowest noise impedance. "Matching" means that the adjustable impedance 16 is adjusted to correspond to the lowest noise impedance; or

[0073] S9: Match the total effective coil impedance by adjusting the ratio between the first adjustable impedance 16A of the first "on-chip" capacitor 17A and the second adjustable impedance 16B of the second "on-chip" capacitor 17B.

[0074] Although the invention has been illustrated and described in detail in the accompanying drawings and the foregoing description, these illustrations and descriptions should be considered illustrative or exemplary, not restrictive; the invention is not limited to the disclosed embodiments. Those skilled in the art, through studying the drawings, disclosure, and claims, will be able to understand and implement other variations of the disclosed embodiments in practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality. Although specific measures are recited in dissimilar dependent claims, this does not indicate that combinations of these measures cannot be advantageously used. No reference numerals in the claims should be construed as limiting the scope. Furthermore, for clarity, not all elements in the drawings are provided with reference numerals.

Claims

1. A radio frequency receiver system (1) for an MRI apparatus, comprising: A receiving coil (2) is used to be attached to a patient to be examined by the MRI device, and in the state where the receiving coil is attached to the patient, it exhibits a total effective coil impedance, which includes the coil impedance (5) of the coil itself and the patient impedance (4) due to the patient to whom the coil is attached. An amplifier (6) exhibits the lowest noise impedance and is connected to the receiving coil (2) to amplify the signal received from the receiving coil (2) and output an amplified output signal. An analog-to-digital converter (7) is connected to the amplifier (6) to convert the amplified output signal from the amplifier (6) into a digital signal for further processing. A matching network (8) interconnected between the receiving coil (2) and the amplifier (6), and comprising a matching system (10) with adjustable impedance (16) for matching the total effective coil impedance to the lowest noise impedance, the matching network (8) further comprising a capacitor portion having at least one capacitor (12), and A noise calculation unit (15), connected to the analog-to-digital converter (7) for receiving the digital output signal of the analog-to-digital converter (7), and adapted to calculate the noise of the output signal of the analog-to-digital converter (7), calculate the target impedance, and adjust the adjustable impedance of the matching network (8) to the target impedance such that the noise floor level is lower than the target noise, wherein... The amplifier is integrated with the capacitor portion of the matching network and the noise calculation unit.

2. The radio frequency receiver system (1) according to claim 1, wherein, The amplifier (6) is a low-noise amplifier.

3. The radio frequency receiver system (1) according to any one of claims 1-2, wherein, The capacitive portion of the matching network includes an adjustable capacitor bank.

4. The radio frequency receiver system (1) according to claim 3, wherein, The capacitor bank is a monolithically integrated capacitor bank.

5. The radio frequency receiver system (1) according to any one of claims 1-2, wherein, The capacitor portion of the matching network, the amplifier (6), and the analog-to-digital converter (7) are integrated on a single die (11).

6. The radio frequency receiver system (1) according to claim 5, wherein, The matching network (8) also includes at least one inductor.

7. The radio frequency receiver system (1) according to claim 6, wherein, The matching network (8) also includes a capacitor separate from the monolithic mold (11).

8. The radio frequency receiver system (1) according to claim 7, wherein, The capacitor (12), which is separate from the monolithic mold, has a fixed capacitance (14).

9. A method for operating the radio frequency receiver system (1) for an MRI apparatus according to claim 1, in, The receiving coil (2) exhibits a total effective coil impedance, which includes the coil impedance (5) of the coil itself and the patient impedance (4) due to the patient to be examined by the MRI device and the coil (2) is positioned relative to the patient. The method includes the following steps: The signal is received from the receiving coil (2) by an amplifier (6) exhibiting impedance. The signal received from the receiving coil (2) is amplified by the amplifier (6). The amplified output signal is output from the amplifier (6) to the analog-to-digital converter (7). In the analog-to-digital converter (7), the amplified output signal from the amplifier (6) is converted into a digital signal for further processing. The total effective coil impedance is matched to the lowest noise impedance by a matching network (8) interconnected between the receiving coil (2) and the amplifier (6), and the matching network includes a matching system (10) with adjustable impedance (16). The digital signal from the analog-to-digital converter (7) is received by the noise calculation unit (15). Calculate the noise of the digital signal from the analog-to-digital converter (7). Calculate the target impedance, and The adjustable impedance of the matching network (8) is adjusted to the target impedance such that the noise floor level is lower than the target noise.

10. The method according to claim 9, wherein, The method further includes the following method steps: The adjustable impedance (16) is adjusted by changing the ratio between the first adjustable impedance (16A) of the first capacitor (17A) and the second adjustable impedance (16B) of the second capacitor (17B).

11. The method according to any one of claims 9-10, wherein, The adjustment of the adjustable impedance (16) is automatically controlled.

12. A non-transient computer-readable medium comprising instructions stored thereon, which, when executed on a processor, are used by a radio frequency receiver system of an MRI apparatus to perform the method according to any one of claims 9 to 11.

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