Abstract:
It is proposed herein to improve the specifications of a low-noise amplifier (LNA) by integrating it in a chip. In order to cover a range of operating frequencies using a single chip, the integrated-circuit amplifier proposed herein comprises an input port configured to receive a magnetic resonance (MR) signal from a radio-frequency (RF) coil, one or more LNAs configured to amplify the received MR signal, and an output port configured to output the amplified MR signal from the one or more LNAs. The operating frequency of the RF coil depends on the field strength, The matching circuit, if present, needs to be tuned to operate at the operating frequency of the RF coil, and depends on the component values in the loop, thus on loop size. In contrast, the proposed integrated-circuit amplifier is capable of directly connecting to RF coils with different loop sizes, without the need for a matching circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of low-noise amplifiers (LNA) for radio-frequency (RF) applications, and particularly to LNA devices for RF coils in a magnetic resonance (MR) system. 
       BACKGROUND OF THE INVENTION 
       [0002]    The European patent application EP0344293B1 discusses a nuclear magnetic resonance signal acquisition apparatus that includes a coil, a matching means and a preamplifier. The coil has a source impedance (designated R s  in the aforementioned European patent application), while the preamplifier has an optimum source impedance R opt  and an input impedance R in , with R in  being less than R opt . The matching means connects the coil to the preamplifier such that the source impedance of the coil R s  is transformed by the matching means to be approximately equal to the optimum source impedance R opt  at the point of connection between the matching means and the preamplifier. At the same time, the low input impedance of the preamplifier is transformed by the matching means to be a value higher than the optimum source impedance R opt  at the point of connection between the matching means and the coil. 
       SUMMARY OF THE INVENTION 
       [0003]    A matching means as described above could have fairly large dimensions which could add to the size of the integrated-circuit amplifier device. It could also add unwanted noise to the signal received by the coil itself. In addition, in the case of coils with multiple transmitting or receiving loops, e.g., phased-array coils, the matching means of one loop might couple with the B 1  transmit fields of other loops, and could also result in an increased number of parasitic loops. 
         [0004]    It is therefore proposed herein to improve the specifications of the LNA by integrating it in a chip. In order to cover a range of operating frequencies using a single chip, the integrated-circuit amplifier proposed herein comprises an input port configured to receive an MR signal from an RF coil, one or more LNAs configured to amplify the received MR signal, and an output port configured to output the amplified MR signal from the one or more LNAs. The operating frequency of the RF coil is dependent on the field strength of the main magnetic field. The matching circuit, as mentioned in the prior art, needs to be tuned to operate at the operating frequency of the RF coil. The component values in the matching circuit depend on the loop size. In contrast, the proposed integrated-circuit amplifier is capable of directly connecting to RF coils with different loop sizes, without the need for a matching circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    These and other aspects will be described in detail hereinafter, by way of example, on the basis of the following embodiments, with reference to the accompanying drawings, wherein: 
           [0006]      FIG. 1  shows an embodiment of the integrated-circuit amplifier disclosed herein, wherein multiple LNAs are provided on a single chip with individual inputs and outputs for each LNA; 
           [0007]      FIG. 2  shows a second embodiment of the integrated-circuit amplifier with multiple LNAs that are selectable based on a control signal; 
           [0008]      FIG. 3  shows a third embodiment having multiple input ports and a single output port; 
           [0009]      FIG. 4  shows a fourth embodiment wherein the LNA is formed from a field-effect transistor device, and wherein an additional bias voltage is applied; 
           [0010]      FIG. 5  shows a fifth embodiment in which a feedback loop is added to the LNA; 
           [0011]      FIG. 6  shows a sixth embodiment wherein a particular degenerating inductor is selected by a switch; and 
           [0012]      FIG. 7  shows a magnetic resonance system utilizing an integrated-circuit amplifier as disclosed herein. 
       
    
    
       [0013]    Corresponding reference numerals when used in the various figures represent corresponding elements in the figures. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0014]    Phased-array coils, sometimes also referred to as “Synergy” coils, employ multiple receive loops simultaneously to receive MR signals from a subject under examination. Due to the proximity of the coil loops to one another, noise currents in one loop could couple to other loops. If each of the loops were connected to a high impedance, the flow of potentially problematic noise currents could be blocked. 
         [0015]    Since the MR signals received by the loops are very weak, the signals need to be amplified in order to obtain reasonable image quality. LNAs are often used for amplifying the MR signals. However, currently known LNA topologies generate low noise only within a specific, small range of input impedances (optimal impedance Z opt ); in other words, outside of this range of input impedances, the noise levels generated by the LNA are high. 
         [0016]    In general, there will be a mismatch between the requirements on the impedance seen by the loop and the optimal impedance at the input of the LNA. This problem is generally solved in phased-array or synergy coils by adding a transformer network (also known as a matching network) between the receive loop and the LNA. The disadvantage of adding a matching network is that it might add additional noise and could also take up significant space, which prevents miniaturization of the complete preamplifier assembly. In addition, the matching circuits themselves might mutually couple to each other, due to the high quality, core-less inductors needed in their construction. 
         [0017]      FIG. 1  shows an embodiment in which the integrated-circuit amplifier  102  is formed on a single chip having several LNAs on-board. Four LNAs  106   1 ,  106   2 ,  106   3  and  106   4  are shown integrated on a single integrated-circuit amplifier chip  102 . Each LNA  106   1 ,  106   2 ,  106   3  and  106   4  has its own input port  104   1 ,  104   2 ,  104   3  and  104   4 , and its own output port  108   1 ,  108   2 ,  108   3  and  108   4 , respectively. 
         [0018]    As each LNA has its own separate input and output, it is possible to select the appropriate LNA by connecting a receive coil to the appropriate input port  104   1 ,  104   2 ,  104   3  or  104   4  of the integrated-circuit amplifier  102 . In the exemplary embodiment shown in  FIG. 1 , the four different LNAs  106   1 ,  106   2 ,  106   3  and  106   4 , have four different optimal input impedances, for example 20 Ohm, 54 Ohm, 9 Ohm and 22 Ohm, respectively. The operational frequency of the first two LNAs  106   1 ,  106   2  is 128 MHz, while that of the last two LNAs  106   3 ,  106   4  is 64 MHz. It is possible to have more LNAs on the same chip, operating at the above or other operational frequencies. It is also possible to have LNAs with other optimal input impedances as well. 
         [0019]    The above design of the integrated-circuit amplifier permits an end-user, for example an imaging technician at a healthcare facility, to connect RF coils having various loop sizes (and therefore, operating at different resonance frequencies) to the same integrated-circuit amplifier by simply connecting the coil to the appropriate input  104   1 ,  104   2 ,  104   3  or  104   4 , thereby selecting the appropriate LNA within the integrated-circuit amplifier chip  102 . Though this would be a useful item for clinical research sites, in the normal product these items are preassembled into a coil. The real customer value is improved image quality (as there is no noise added by the matching circuit), more compact coil design and less weight on the patient. For example, a cascode amplifier with a shunt inductor, manufactured using PHILIPS&#39; CMOS 250 nm technology, makes it feasible to directly couple the RF receive loop to the LNA, without a matching network. 
         [0020]      FIG. 2  shows an embodiment of the integrated-circuit amplifier disclosed herein, wherein the integrated-circuit amplifier  202  is configured as a single chip have multiple LNAs  206   1 ,  206   2 ,  206   3  and  206   4  on-board and with a single input  204  and single output  208 . The input stage of a particular LNA  206   1 ,  206   2 ,  206   3  or  206   4  is chosen by a low-loss multiplex or switching device  210 , for example a micro-electromechanical switch (MEMS). A similar switching device  212  could be used to connect the output stage of an LNA  206   1 ,  206   2 ,  206   3  or  206   4  to the output port  208  of the integrated-circuit amplifier  202 , if different output stages are beneficial. The MEMS or other switching or selector devices  210 ,  212  are operated based on a control signal CTRL. It is also conceivable that different control signals are used to trigger the operation of the switching devices  210 ,  212  at the input and output ports  204 ,  208 , respectively. 
         [0021]    As in the embodiment discussed above, the four different LNAs  206   1 ,  206   2 ,  206   3  and  206   4 , have four different optimal input impedances, for example 20 Ohm, 54 Ohm, 9 Ohm and 22 Ohm, respectively. The operational frequency of the first two LNAs  206   1 ,  206   2  is 128 MHz, while that of the last two LNAs  206   3 ,  206   4  is 64 MHz. As in the above-mentioned embodiment, it is possible to have fewer or more than four LNAs on the same chip, operating at the above or other operational frequencies. It is also possible to have LNAs with other optimal input impedances as well. 
         [0022]      FIG. 3  shows a single chip integrated-circuit amplifier  302  with a single output port  308  and multiple input ports  304   1 ,  304   2 ,  304   3 ,  304   4 . Each input port connects to a different first stage amplifier  306   1 ,  306   2 ,  306   3 ,  306   4  such that each input (or combination of inputs) has a different optimal impedance. The various first stage amplifiers  306   1 ,  306   2 ,  306   3 ,  306   4  connect to a second stage amplifier  310 . Though only one second stage amplifier  310  is shown in this embodiment, it is possible to have multiple second stage amplifiers as well. 
         [0023]    Both the first and second-stage amplifiers shown in this embodiment utilize field-effect transistors (FETs), though other implementations are also possible, such as one utilizing bipolar-junction transistors (BJTs). The advantage of using multiple input stages with different optimal impedances is that RF coils having different loop sizes can be connected directly to the same integrated-circuit amplifier  302 , by connecting the coil to the appropriate external pins. The inputs of the unused FETs are grounded externally and do not play a role in the circuit anymore. This implementation using FETs has the additional advantage that it requires less space and has fewer external pins than some of the other embodiments. It may be noted that the unused FETs may be left unbiased in order to attain improved power dissipation in the circuit. 
         [0024]      FIG. 4  shows a single integrated-circuit amplifier chip  402 , with a single input port  404  and a single output port  408 . A first-stage FET amplifier  406  has its source grounded internally via a degenerating inductor  414 , while its drain is connected to the source of a second-stage amplifier  410 . This “grounded-gate” configuration results in a low input impedance of the second-stage amplifier. Another way of achieving low input impedance is to use grounded-base bi-polar transistor amplifiers. The second-stage amplifier  410  converts an input current into an amplified output voltage at the output port  408 . A control signal CTRL supplied to the gate of the first stage FET amplifier  406  determines the input impedance of the integrated-circuit amplifier  402 . The input impedance may be controlled via adjustments in biasing (external controls) or with active and externally controlled feedback. In this embodiment the bias voltage supplied to the gate of the first-stage FET amplifier  406  is made adjustable via an external control signal CTRL, which results in the ability to control the input impedance of the LNA circuit over a limited range. This in turn, makes it possible to connect RF coils of various loop sizes (within a limited range) to the LNA. By incorporating multiple such LNAs in a single chip, the range of RF coils (i.e., loop sizes) that may be connected to the integrated-circuit amplifier may be increased. 
         [0025]      FIG. 5  shows an integrated-circuit amplifier  502  having a single input port  504  and a single output port  508 . The source of the first-stage FET amplifier  506  is grounded internally via a degenerating inductor  514 , while its drain is connected to the source of a second-stage FET amplifier  510 . A feedback signal is added to the first-stage FET amplifier  506  by connecting it source to the drain of the second-stage FET amplifier  508  via a feedback amplifier  512 . The feedback response curve may be controlled externally via the control signal CTRL, and may be used to control the optimal frequency. 
         [0026]      FIG. 6  shows an integrated-circuit amplifier  602  with a single input  604  and a single output  608 . The drain of a first-stage FET amplifier  606  is connected to the source of a second-stage FET amplifier  610 , while the source of the first-stage FET amplifier  606  is connected to one of a number of degenerating inductors L 1 , L 2 , L 3 , L 4 , via a switching mechanism  608 . This embodiment differs from the embodiment of  FIG. 4  in that the inductor connected to the source of the first-stage FET is selectable. The switching mechanism may be a low-noise switch like a MEMS. The switching mechanism is controlled by a control signal CTRL which may be supplied externally. 
         [0027]    Some of the advantages of integrating an LNA on a chip include less power consumption, which may be important especially for wireless coils, and less requirements for board-space, which may be especially important for the RF coils used in massive parallel imaging schemes. Several chip technologies exist that might be used for integrating the LNA as disclosed herein, like metal-oxide semiconductor (MOS) using either N-channel or P-channel) FETs, complementary metal-oxide semiconductor (CMOS) using either N-channel or P-channel FETs, bipolar junction transistors (BJT) using bipolar devices, etc. 
         [0028]    Though the embodiments of  FIGS. 3 ,  4 ,  5  and  6  have been depicted as utilizing FETs, it is possible to replace the FETs with BJTs to arrive at alternate configurations having the same functionality. In such cases, the configurations utilizing BJTs may be directly derived from the FET configurations by replacing the words “drain, gate and source” for the FET with the words “collector, base and emitter”, respectively for the BJT. 
         [0029]      FIG. 7  shows a possible embodiment of an MR system capable of utilizing the integrated-circuit amplifier as disclosed herein. The MR system comprises a set of main coils  701 , multiple gradient coils  702  connected to a gradient driver unit  706 , and RF coils  703  connected to an RF coil driver unit  707 . The function of the RF coils  703 , which may be integrated into the magnet in the form of a body coil, or may be separate surface coils, is further controlled by a transmit/receive (T/R) switch  713 . The multiple gradient coils  702  and the RF coils are powered by a power supply unit  712 . A transport system  704 , for example a patient table, is used to position a subject  705 , for example a patient, within the MR imaging system. A control unit  708  controls the RF coils  703  and the gradient coils  702 . The control unit  708 , though shown as a single unit, may be implemented as multiple units as well. The control unit  708  further controls the operation of a reconstruction unit  709 . The control unit  708  also controls a display unit  710 , for example a monitor screen or a projector, a data storage unit  715 , and a user input interface unit  711 , for example, a keyboard, a mouse, a trackball, etc. 
         [0030]    The main coils  701  generate a steady and uniform static magnetic field, for example, of field strength 1T, 1.5T or 3T; the embodiments disclosed herein may be used at other field strengths as well. The main coils  701  are arranged in such a way that they typically enclose a tunnel-shaped examination space, into which the subject  705  may be introduced. Another common configuration comprises opposing pole faces with an air gap in between them into which the subject  705  may be introduced by using the transport system  704 . To enable MR imaging, temporally variable magnetic field gradients superimposed on the static magnetic field are generated by the multiple gradient coils  702  in response to currents supplied by the gradient driver unit  706 . The power supply unit  712 , fitted with electronic gradient amplification circuits, supplies currents to the multiple gradient coils  702 , as a result of which gradient pulses (also called gradient pulse waveforms) are generated. The control unit  708  controls the characteristics of the currents, notably their strengths, durations and directions, flowing through the gradient coils to create the appropriate gradient waveforms. The RF coils  703  generate RF excitation pulses in the subject  705  and receive MR signals generated by the subject  705  in response to the RF excitation pulses. The RF coil driver unit  707  supplies current to the RF coil  703  to transmit the RF excitation pulse, and amplifies the MR signals received by the RF coil  703 . The transmitting and receiving functions of the RF coil  703  or set of RF coils are controlled by the control unit  708  via the T/R switch  713 . The T/R switch  713  is provided with electronic circuitry that switches the RF coil  703  between transmit and receive modes, and protects the RF coil  703  and other associated electronic circuitry against breakthrough or other overloads, etc. The characteristics of the transmitted RF excitation pulses, notably their strength and duration, are controlled by the control unit  708 . 
         [0031]    It is to be noted that though the transmitting and receiving coil are shown as one unit in this embodiment, it is also possible to have separate coils for transmission and reception, respectively. It is further possible to have multiple RF coils  703  for transmitting or receiving or both. The RF coils  703  may be integrated into the magnet in the form of a body coil, or may be separate surface coils. They may have different geometries, for example, a birdcage configuration or a simple loop configuration, etc. The control unit  708  is preferably in the form of a computer that includes a processor, for example a microprocessor. The control unit  708  controls, via the T/R switch  713 , the application of RF pulse excitations and the reception of MR signals comprising echoes, free induction decays, etc. User input interface devices  711  like a keyboard, mouse, touch-sensitive screen, trackball, etc., enable an operator to interact with the MR system. 
         [0032]    The MR signal received with the RF coils  703  contains the actual information concerning the local spin densities in a region of interest of the subject  705  being imaged. The received signals are reconstructed by the reconstruction unit  709 , and displayed on the display unit  710  as an MR image or an MR spectrum. It is alternatively possible to store the signal from the reconstruction unit  709  in a storage unit  715 , while awaiting further processing. The reconstruction unit  709  is constructed advantageously as a digital image-processing unit that is programmed to derive the MR signals received from the RF coils  703 . 
         [0033]    The various embodiments of the integrated-circuit amplifier disclosed herein are suitable for phased-array RF coils that employ multiple receive loops or elements. They are also suitable for other types of coils including one- or two-element coils, though in this case, the requirements regarding isolation of loops from one another are less strict. 
         [0034]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The disclosed embodiments can be implemented by means of hardware comprising several distinct elements, or by means of a combination of hardware and software. In the system claims enumerating several means, several of these means can be embodied by one and the same item of computer readable software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.