Patent Description:
Magnetic resonance (MR) devices employ transmit and receive radio frequency (RF) electronics that operate at high dynamic range. For example, MR excitation typically employs a sharply peaked RF pulse over a narrow frequency band generally centered at the MR frequency and thus has high amplitude at the pulse peak but much lower amplitude elsewhere. A typical received MR signal is an echo in which all of the spins add coherently at a moment in time near the center of k-space leading to a high signal, and are incoherent in some other areas of k-space leading to much lower signal. To handle such a large dynamic range, MR imaging or spectroscopy devices typically employ specialized RF modulators in the transmit RF chain, and specialized receivers, typically with relatively high speed ADC's and large numbers of bits in order to maintain acceptable SNR without clipping the signals.

These costly and specialized hardware requirements are made more onerous by the widespread use in MR imaging of local MR receiver coils or coil arrays. These have architectural advantages and can improve sensitivity, for example by locating a local coil or coil array on or closely proximate to the surface of the anatomy to be imaged, or using a head coil that encloses the head undergoing imaging, or so forth. But, the requirements for high dynamic range and low noise in these receive chains result in relatively high power and cost of these devices.

<CIT> describes a system and method of increasing the sampling rate for MR data acquisition. By implementing ensemble sampling techniques, the present system provides higher data sampling rates that are useful for several MR data acquisition applications including Echo Planar Imaging, Functional Magnetic Resonance Imaging, and Sensitivity Encoding Imaging (SENSE) techniques. By multiplying an MR signal by a series of pure sinusoids having the same frequency but shifted by an incremental phase, the MR signal may be separated into a number of channels which can be sampled at lower rates by analog-to-digital converters. The output from the converters may then be reconstructed using one of a number of interpolation techniques to create a single digital channel with increased bandwidth. The single channel with increased bandwidth may then be used to acquire MR data with an improved sampling rate.

From <CIT> a controller for an RF amplifier, in particular for a RF amplifier of an MR tomography apparatus, is known which has an IQ control element for adjusting the magnitude and phase an RF signal that is be fed to the RF amplifier. The IQ control element has a signal splitter that splits the RF signal into two partial signals having a <NUM>° phase offset in an I path and a Q path, each having a multiplier for multiplying the partial signal by an I factor in the I path and a Q factor in the Q path. A summing unit recombines the partial signals. A detector determines the actual phase difference and actual amplification between the RF signal fed to the IQ control element and the RF signal amplified by the RF amplifier. An IQ controller determines the I factor and the Q factor from the actual difference and a desired phase difference and the actual amplifier and a desired amplification. The IQ controller has an operating point at which the I factor and the Q factor are the same magnitude if the actual and desired phase differences and the actual and desired amplifications are the same.

The following discloses new and improved systems and methods.

The invention is defined in claims <NUM> and <NUM>.

Preferred features of the invention are defined in the dependent claims.

In one disclosed aspect, a radio frequency (RF) device for receiving a magnetic resonance (MR) signal in an MR imaging or spectroscopy subject is provided as defined in claim <NUM>.

The RF device comprises an MR receive coil configured to receive an MR signal in an MR frequency band, an analog dispersive delay line connected to disperse the MR signal received by the MR receive coil to generate a dispersed MR signal, and an A/D converter connected to generate a digitized dispersed MR signal from the dispersed MR signal.

In another disclosed aspect, an RF device for transmitting an RF excitation pulse for exciting a magnetic resonance (MR) signal in an MR imaging or spectroscopy subject is provided as defined in claim <NUM>. The RF device comprises a digital signal processing chain configured to generate a digital signal in an MR frequency band, a D/A converter configured to convert the digital signal to an analog signal, and an analog dispersive delay line connected to increase dynamic range of the analog signal to generate an RF excitation pulse. The RF device may further comprise an MR transmit coil tuned to transmit an MR excitation signal in response to receiving the RF excitation pulse generated by the analog dispersive delay line.

One advantage resides in providing a radio frequency (RF) transmit chain for a magnetic resonance (MR) imaging and/or spectroscopy device with reduced dynamic range requirements.

Another advantage resides in providing for retrofitting an existing RF transmit chain of such an MR device to provide the foregoing advantage with limited hardware modification.

Another advantage resides in providing an RF receive chain for an MR imaging and/or spectroscopy device with reduced dynamic range requirements.

Another advantage resides in providing for retrofitting an existing RF receive chain of such an MR device to provide the foregoing advantage with limited hardware modification.

Another advantage resides in providing for one or more of the foregoing advantages implemented by way of a lossless signal decompression and/or compression methodology.

In drawings presenting log or service call data, certain identifying information has been redacted by use of superimposed redaction boxes.

With reference to <FIG>, a magnetic resonance (MR) imaging and/or spectroscopy device <NUM> includes a gantry or housing <NUM> that contains internal components (not shown) typically including a static magnet (resistive or superconducting with appropriate cryogenic containment) producing a static magnetic field (typically denoted B<NUM>) and sets of magnetic field gradient windings for superimposing magnetic field gradients on the B<NUM> field along various spatial directions (e.g. transverse x- and y-gradient windings and longitudinal z-gradient windings). The MR device <NUM> typically includes other accessory and/or auxiliary hardware, e.g. magnetic field gradient pulse generators, monitoring sensors, et cetera also not shown. A medical patient or other imaging subject is loaded into an examination region (illustrative bore) <NUM> via a patient couch or other subject support <NUM> for MR imaging or for MR spectroscopy or MR spectroscopic imaging.

As further depicted diagrammatically in <FIG>, the MR device <NUM> further includes radio frequency (RF) devices for exciting and receiving MR signals, such as an illustrative MR transmit chain <NUM> driving an MR transmit coil <NUM>, and an MR receive chain <NUM> processing MR signals received by an MR receive coil <NUM>. The MR excitation and received MR signals are in an MR frequency band, which is generally centered at the MR frequency given by <MAT> where B<NUM> is the static magnetic field (e.g. as non-limiting examples some commercial MR imaging devices employ |B<NUM>| of <NUM> Tesla or <NUM> Tesla depending on the employed static magnet) and γ is the gyrometric ratio which depends on the excited nuclear spins, e.g. for MR imaging using <NUM>H spins <MAT> MHz/Tesla so at a magnetic field of <NUM> Tesla fMR = <NUM>. The MR frequency band will have some bandwidth approximately centered at fMR, typically on the order of <NUM>-<NUM> although larger (e.g. <NUM>) or smaller MR bandwidth may be employed, and is in general dependent on the value of |B<NUM>| and design parameters of the MR imaging device, as well as on the parameters of the MR imaging and/or spectroscopy pulse sequence employed in a particular imaging and/or spectroscopy session, e.g. may be larger for multi-nuclei imaging in which MR signals from spins other than <NUM>H are acquired.

The MR transmit coil <NUM> and the MR receive coil <NUM> are each illustrated in <FIG> as a single loop coil. More generally, however, the term "MR coil" and similar phraseology as used herein is to be understood as encompassing other coil types and coil arrays known for use in exciting or receiving MR signals. For example, the MR transmit and receive coils <NUM>, <NUM> could be a whole-body birdcage coil, a head coil or coil array, a limb coil or coil array, a surface coil array, or so forth. The MR coils <NUM>, <NUM> may employ any suitable RF circuit technology, e.g. may employ strip line or micro-strip conductors, transmission line configurations, solid conductors, or so forth, with lumped and/or distributed capacitive and/or inductive tuning elements, and/or so forth. Furthermore, in some embodiments the same physical coil (that is, an MR transceiver coil or coil array) may serve as both the MR transmit and receive coils <NUM>, <NUM> using suitable switching circuitry.

The MR transmit chain <NUM> is first considered. In a conventional design, the transmit chain employs digital circuitry to generate a digital RF pulse at baseband which is converted to the analog domain and modulated (i.e. frequency-shifted) using an analog or digital mixer or the like to the MR frequency band, amplified and applied to the MR transmit coil. In this conventional approach, the digital RF pulse has a large dynamic range (in terms of amplitude) and accordingly the components of the MR transmit chain must be of sufficiently high speed and high (analog) sensitivity or high (digital) resolution to process the RF pulse without clipping or undue distortion. The MR transmit chain <NUM> of <FIG> avoids this problem as follows.

With continuing reference to <FIG>, the illustrative MR transmit chain <NUM> includes a digital signal generator <NUM> which generates a digital signal at baseband representing the desired RF excitation pulse. In illustrative <FIG>, this digital signal is a digital chirp signal <NUM>, in which the signal frequency is ramped up linearly over a time T from an initial frequency f1B to a final frequency f2B both within the baseband. In the illustrative example the chirp signal <NUM> is monotonically increasing with f2B>f1B; however, in a variant embodiment the chirp signal may be monotonically decreasing. The chirp signal <NUM> advantageously has a low dynamic range (in terms of amplitude) compared with a sharply peaked pulse signal, and the dynamic range can be adjusted by adjusting the time T and the start and end frequencies f1B and f2B. The digital signal <NUM> generated by the digital signal generator <NUM> is modulated to the MR frequency band by operation of a digital RF modulator <NUM>, and then converted to the analog domain by a digital-to-analog (D/A) converter <NUM>. In an alternative embodiment, the D/A converter <NUM> may be applied first followed by an analog RF modulator. In either case, the result is an analog signal at the MR frequency band. An RF amplifier <NUM> is applied to amplify the analog signal at the MR frequency band to the desired signal power. However, this signal is still a chirp signal (in the illustrative embodiment, or more generally a signal with a lower dynamic range, in terms of amplitude, than a narrowly peaked signal of the type usually preferred for performing MR excitation).

As disclosed herein, the analog signal at the MR frequency band is suitably increased in dynamic range by an analog dispersive delay line <NUM> to generate the desired RF excitation signal with a narrow and large peak (and hence a larger dynamic range). To accomplish this, the analog dispersive delay line <NUM> imposes a frequency-dependent signal delay <NUM> that is monotonically increasing or (as in the illustrative example) or monotonically decreasing (in an alternative embodiment) over the MR frequency band. More particularly, the frequency-dependent signal delay <NUM> is chosen to cancel the linear signal frequency ramp of the chirp signal <NUM>, so that all signal frequencies align in time, thereby producing a higher dynamic range RF excitation pulse with a large peak. As the illustrative chirp signal <NUM> is monotonically increasing in frequency from f1B to f2B over the time interval T, the frequency-dependent signal delay <NUM> monotonically decreases from a longest delay at a frequency f1MR (which is equal to the baseline frequency f1B modulated into the MR frequency band by the RF modulator <NUM>) to a shortest (or optionally zero) delay at a frequency f2MR (which is equal to the baseline frequency f2B modulated into the MR frequency band by the RF modulator <NUM>). The difference between the longest time delay for f1MR and the shortest (or zero) delay for f2MR is equal to the time interval T to provide maximum dynamic range adjustment of the chirp signal <NUM>. (By analogy, if the generated chirp signal were monotonically decreasing then the frequency-dependent signal delay imposed by the analog dispersive delay line would appropriately be monotonically increasing). The resulting RF excitation pulse is applied to the MR transmit coil <NUM> which radiates the MR excitation pulse into the examination region <NUM>.

The analog dispersive delay line <NUM> may be constructed using any known technology that can generate the desired frequency-dependent signal delay <NUM>. For example, the analog dispersive delay line <NUM> may be a surface acoustical wave (SAW) device that leverages frequency-dependent acoustic delays, or may be a reflection mode delay line leveraging frequency-dependent reflection phase shifts.

With continuing reference to <FIG>, the MR receive chain <NUM> is now considered. The MR receive coil <NUM> receives an MR signal which typically has a high dynamic range due to a large signal peak in the vicinity of the center of k-space and much lower signal strength in peripheral areas of k-space. Alternatively, viewed in the time domain the signal strength is highest at initial RF excitation of the MR signal and at any subsequent spin echo or gradient echo peaks, and is much lower at other points in the imaging sequence. The received MR signal is also typically weak, and accordingly a pre-amplifier <NUM> may be initially applied to boost the MR signal strength prior to the MR signal being digitized by an analog-to-digital (A/D) converter <NUM> and demodulated by a digital RF demodulator <NUM> that shifts the MR signal from the MR frequency band to baseband. In an alternative embodiment, the receive chain may include an analog RF demodulator followed by an A/D converter to digitize the MR signal after demodulation to baseband.

To more readily accommodate the typically high dynamic range of the MR signal received by the MR receive coil <NUM>, an analog dispersive delay line <NUM> is inserted into the MR receive chain after the MR coil <NUM> (and, in the illustrative embodiment, after the pre-amplifier <NUM>, and before the downstream A/D converter <NUM> and RF demodulator <NUM>. The analog dispersive delay line <NUM> imposes a frequency-dependent signal delay <NUM> that is monotonically increasing or (as in the illustrative example) or monotonically decreasing (in an alternative embodiment) over the MR frequency band. The frequency-dependent signal delay <NUM> is chosen to spread the peaked MR signal so as to produce an MR signal that has lower dynamic range compared with the received (and optionally pre-amplified) MR signal. The illustrative frequency-dependent signal delay <NUM> is monotonically increasing from a shortest delay at a lower f1MR to a longest delay at a higher frequency f2MR (that is, f2MR>f1MR where both f1MR and f2MRare in the MR frequency band), with the time difference between the longest delay at f2MR and the shorted delay at f1MR being denoted as time difference T.

As with the analog dispersive delay line <NUM> of the MR transmit chain <NUM>, the analog dispersive delay line <NUM> of the MR receive chain <NUM> may be constructed using any known technology that can generate the desired frequency-dependent signal delay <NUM>. For example, the analog dispersive delay line <NUM> may be a surface acoustical wave (SAW) device that leverages frequency-dependent acoustic delays, or may be a reflection mode delay line leveraging frequency-dependent reflection phase shifts.

The received MR signal after dispersion by the analog dispersive delay line <NUM> is processed by an image reconstruction processor <NUM> (for example, implemented on an illustrative server computer <NUM>, desktop computer <NUM>, or on some other computer or electronic digital processor) to generate a reconstructed MR image that is suitably displayed on a display <NUM> of a computer <NUM>, stored in a Picture Archiving and Communication System (PACS, not shown), and/or otherwise utilized. The image reconstruction processor <NUM> can utilize any suitable image reconstruction algorithm appropriate for the spatial encoding employed by the MR imaging pulse sequence executed by the MR imaging device <NUM> to generate the MR signal, e.g. the image reconstruction processor <NUM> may employ a Fourier image reconstruction, an iterative reconstruction algorithm, or so forth. Likewise, if MR spectroscopy is perform suitable Fourier or other processing of the MR signal is performed to generate MR spectrum data (optionally spatially encoded, e.g. an MR spectroscopy image).

Besides the advantageous reduction in dynamic range, the impact of the frequency-dependent signal delay <NUM> imposed on the MR signal by the analog dispersive delay line <NUM> is to shift only the phase of the signal at the different frequencies, but not the amplitudes. That is, the analog dispersive delay line <NUM> operates as a phase-only linear dispersion filter. As a consequence, if the image reconstruction processor <NUM> operates to generate an amplitude image that does not rely upon the phase information contained in the MR signal, then the frequency-dependent signal delay <NUM> introduced by the analog dispersive delay line <NUM> has no impact on the reconstructed image. Likewise, for MR spectroscopy if only amplitude information is leveraged then the frequency-dependent signal delay <NUM> introduced by the analog dispersive delay line <NUM> has no impact on the extracted MR spectral information. In such cases, insertion of the analog dispersive delay line <NUM> into the analog signal processing sub-chain of the MR receive chain <NUM> has no practical impact beyond the advantageous dynamic range reduction, and no further modification on either the analog or digital signal processing sub-chains of the MR receive chain <NUM> is needed.

On the other hand, if the image reconstruction processor <NUM> leverages phase information contained in the MR signal, as is the case in some imaging techniques such as some diffusion weighted imaging (DWI) approaches, then the frequency-dependent signal delay <NUM> imposed by the analog dispersive delay line <NUM> could be problematic. In such cases, a digital dispersive delay line <NUM> is suitably inserted into the digital signal processing sub-chain, preferably downstream of any components such as the digital RF demodulator <NUM> that may benefit from the reduced dynamic range imparted by the analog dispersive delay line <NUM>. The digital dispersive delay line <NUM> is tuned to impose a monotonically decreasing (as in the illustrative embodiment) or monotonically increasing (in an alternative embodiment) frequency-dependent signal delay <NUM> that is effective to cancel the frequency-dependent signal delay <NUM> imposed by the analog dispersive delay line <NUM>. Thus, as the illustrative frequency-dependent signal delay <NUM> imposed by the analog dispersive delay line <NUM> is monotonically increasing in frequency from f1MR to f2MR with the difference in delays being T, the frequency-dependent signal delay <NUM> monotonically decreases from a longest delay at a frequency f1B (which is equal to the frequency f1MR in the MR frequency band demodulated to baseband by the RF demodulator <NUM>) to a shortest (or optionally zero) delay at a frequency f2B (which is equal to the frequency f2MR in the MR frequency band demodulated to baseband by the RF demodulator <NUM>). The difference between the longest time delay for f1B and the shortest (or zero) delay for f2B is equal to the same time different T as in the illustrative frequency-dependent signal delay <NUM> imposed by the analog dispersive delay line <NUM>, thus providing cancellation of the delay <NUM>. (By analogy, if the frequency-dependent signal delay imposed by the analog dispersive delay line were monotonically decreasing then the frequency-dependent signal delay imposed by the digital dispersive delay line would appropriately be monotonically increasing). As the frequency-dependent signal delay <NUM> imposed by the digital dispersive delay line <NUM> cancels the frequency-dependent signal delay <NUM> imposed by the analog dispersive delay line <NUM>, the phase information is corrected and the image reconstruction processor <NUM> (or spectral analysis in the case of MR spectroscopy) can effectively utilize the corrected phase information.

The analog dispersive delay line <NUM> may, in general, be located with the MR receive coil <NUM>, or with the receiver electronics. For example, in a digital MR receive coil design, the A/D converter <NUM> is disposed with the MR receive coil <NUM> and the pre-amplifier <NUM> on a single receive coil substrate <NUM>. In such digital MR receive coil embodiments, the analog dispersive delay line <NUM> is also disposed on the single receive coil substrate <NUM> so as to be interposed along the MR receive chain between the MR receive coil <NUM> and the on-board A/D converter <NUM>. Thus, in these embodiments the single receive coil substrate <NUM> commonly supports the MR receive coil <NUM>, the analog dispersive delay line <NUM>, and the A/D converter <NUM>.

On the other hand, in analog MR receive coil embodiments in which the analog MR signal received by the MR coil <NUM> is ported off the receive coil substrate, the A/D converter <NUM> is then located with the receive electronics (e.g., in an electronic component housing containing the A/D converter <NUM> and also housing the RF demodulator <NUM>). In this case, the analog dispersive delay line <NUM> may be located either on the same receive coil substrate that supports the MR receive coil, or may be located with the receive electronics upstream along the receive RF chain of the A/D converter.

In <FIG>, the notation f1B, f2B, f1MR, f2MR, T used to denote the various parameters for the chirp signal <NUM> and the analog dispersive delay line <NUM> of the illustrative MR transmit chain <NUM>, on the one hand, and the notation f1B, f2B, f1MR, f2MR, T used to denote the various parameters for the analog and digital dispersive delay lines <NUM>, <NUM> of the illustrative MR receive chain <NUM>, on the other hand, are the same. However, this is not necessarily the case, and it is contemplated for the parameters f1B, f2B, f1MR, f2MR, T in the transmit and receive RF chains to have different values.

<FIG> illustrates employing the analog dispersive delay line <NUM> in the MR transmit chain <NUM>, on the one hand, and also employing the analog dispersive delay line <NUM> (and the optional digital dispersive delay line <NUM>) in the MR receive chain <NUM>. However, it will be appreciated that these are operationally independent.

For example, a given implementation may employ the analog dispersive delay line <NUM> in the MR transmit chain <NUM>, but not employ the analog dispersive delay line <NUM> (and the optional digital dispersive delay line <NUM>) in the MR receive chain <NUM>.

Likewise, in another example, a given implementation may employ the analog dispersive delay line <NUM> (and the optional digital dispersive delay line <NUM>) in the MR receive chain <NUM>, but not employ the analog dispersive delay line <NUM> in the MR transmit chain <NUM>.

With reference to <FIG>, in other variants, it is contemplated to employ a dispersive delay line with a non-linear, but still monotonic, frequency-dependent signal delay. <FIG> illustrates an alternative frequency-dependent signal delay <NUM>NL that is monotonically decreasing and is also non-linear, i.e. the delay as a function of frequency is not a straight line. This is an alternative embodiment to the illustrative linear monotonically increasing frequency-dependent signal delay <NUM> shown in <FIG> for the analog dispersive delay line <NUM>. If the delay-cancelling digital dispersive delay line <NUM> is also employed, <FIG> shows a suitable variant frequency-dependent signal delay <NUM>NL that is monotonically increasing and also nonlinear with a shape suitable to cancel the frequency-dependent delay introduced by the frequency-dependent signal delay <NUM>NL of <FIG>. Although not illustrated, in an analogous variant for the RF transmit chain <NUM> not encompassed by the claims, the digital signal generator <NUM> is contemplated to generate a digital signal at baseband with the shape shown in <FIG> (but at baseband), in which case the analog dispersive delay line <NUM> suitably imposes the frequency-dependent signal delay shown in <FIG> (but at the MR frequency band) so as to increase the dynamic range of the signal to produce a sharply peaked RF pulse.

In general, the digital components of the digital signal processing (sub-)chain <NUM>, <NUM> of the transmit chain <NUM> and of the digital signal processing (sub-)chain <NUM>, <NUM> of the receive chain <NUM> may comprises any type of hardwired or programmable digital component. As non-limiting illustrative examples, these digital components may comprise one or more of a microprocessor, a microcontroller, a field-programmable gate array (FPGA), a digital application-specific integrated circuit (ASIC), one or more discrete logic gate components, various combinations thereof, and/or so forth. The analog components of the analog signal processing (sub-)chain <NUM>, <NUM> of the transmit chain <NUM> and of the analog signal processing (sub-)chain <NUM>, <NUM> of the receive chain <NUM> may comprises any type of analog discrete component or integrated circuit (IC) or various combinations thereof, e.g. mixer IC chips may be used in implementing analog RF modulator or demodulator components, and as previously mentioned the analog dispersive delay lines may be implemented as SAW devices and/or reflection mode delay lines or so forth. The A/D and D/A converters may likewise be implemented as discrete and/or IC components, optionally employing a bank of A/D or D/A converters to handle the total number of bits (e.g. a bank of four eight-bit converters can provide <NUM>-bit conversion).

In general, the dispersive delay lines <NUM>, <NUM>, <NUM> used to adjust signal dynamic range to control peak amplitudes in the transmitted excitation RF signal or the received MR signal should be approximately matched to the frequency range of the MR acquisition. The smaller the bandwidth (or, more precisely, the smaller the value of T), the less dynamic range adjustment the filter will produce. It is contemplated to use multiple dispersive delay lines with appropriate switch-in/switch-out circuitry if communication from the system can inform the local circuits of the preferred state. The end result of effective use of this approach is a much lower dynamic range and a more predictable level for MR signal sampling.

Claim 1:
A radio frequency (RF) device for receiving a magnetic resonance (MR) signal in an MR imaging or spectroscopy subject, the RF device comprising:
- an MR receive coil (<NUM>) tuned to receive an MR signal in an MR frequency band;
- an analog signal processing chain (<NUM>, <NUM>) operatively connected with the MR receive coil and at least partly tuned to operate at the MR frequency band for receiving an analog signal at the MR frequency band from the MR receive coil, and including an analog dispersive delay line (<NUM>) tuned to impose a frequency-dependent signal delay (<NUM>) upon the analog signal at the MR frequency band received from the MR receive coil, the frequency-dependent signal delay being monotonically increasing or monotonically decreasing over the MR frequency band, the analog dispersive delay line being connected to disperse the MR signal received by the MR receive coil to generate a dispersed MR signal, and
- an A/D converter (<NUM>) connected to generate a digitized dispersed MR signal from the dispersed MR signal, and
- a digital signal processing chain (<NUM>, <NUM>) at least partly tuned to operate at baseband, wherein the A/D converter is connecting the digital signal processing chain and the analog signal processing chain.