The following relates to the magnetic resonance arts, medical imaging arts, and related arts.
Magnetic resonance (MR) imaging can be performed using sensitivity encoding (SENSE) or other parallel imaging techniques. In some parallel imaging techniques, multiple radio frequency (RF) transmit coils are used, or a single RF transmit coil may be driven using independent drive channels. As an example of the latter arrangement, a birdcage coil having “I” and “Q” drive ports may be driven using independent radio frequency power inputs to the I and Q channels. In such multiple RF transmit channel configurations, each transmit channel generally has an independent drive amplitude and phase, so that for N RF transmit channels there are 2N drive parameters.
To calibrate the RF transmit power, one or more power optimization acquisitions are performed using a multi-channel transmit configuration. The power optimization acquisitions are used to scale the RF transmit power to a desired level. A power optimization acquisition typically employs a 1D projection, which can be acquired relatively quickly and provides an average RF transmit field power level measure for use in the RF transmit power optimization.
In some cases, the RF transmit channels of a multi-channel transmit configuration are trimmed to provide a more uniform RF transmit field. In a usual approach, a B1 map is acquired and optimized respective to the B1 transmit field uniformity. This process is known as RF transmit field shimming.
Existing multi-channel RF transmit preparation techniques provide limited accuracy respective to RF transmit power. Because the 1D projection provides an average RF transmit power measure, it may fail to accurately measure the RF transmit power at a location of interest, such as over the volume of a heart, brain, or other organ that is the imaging target. This problem is enhanced at high magnetic fields due to shorter RF wavelength and enhanced spatial non-uniformity. Patient loading effects are also larger at high magnetic field due to more pronounced electrical properties of biological tissue.
The following provides new and improved apparatuses and methods which overcome the above-referenced problems and others.
In accordance with one disclosed aspect, a magnetic resonance method comprises: acquiring B1 maps for a plurality of radio frequency transmit channels of a magnetic resonance scanner; and computing optimized amplitude and phase parameters for the plurality of radio frequency transmit channels using the acquired B1 maps such that operating the plurality of radio frequency transmit channels together in a multi-channel transmit mode using the optimized amplitude and phase parameters generates a radio frequency transmit field that is both (i) shimmed respective to radio frequency transmit field uniformity and (ii) optimized respective to a radio frequency transmit power metric; wherein the computing is performed by a digital processor.
In accordance with another disclosed aspect, a magnetic resonance system is disclosed, comprising: a magnetic resonance scanner including a plurality of radio frequency transmit channels; and a processor configured to perform a method as set forth in the immediately preceding paragraph in cooperation with the magnetic resonance scanner.
In accordance with another disclosed aspect, a storage medium stores instructions executable by a digital processor to perform a method comprising: optimizing relative amplitude parameters and phase parameters for a plurality of radio frequency transmit channels using B1 maps corresponding to the plurality of radio frequency transmit channels such that operating the plurality of radio frequency transmit channels together in a multi-channel transmit mode using the optimized relative amplitude parameters and optimized phase parameters generates a radio frequency transmit field that is shimmed respective to radio frequency transmit field uniformity; and scaling the relative amplitude parameters using the B1 maps to generate optimized amplitude parameters such that operating the plurality of radio frequency transmit channels together in a multi-channel transmit mode using the optimized amplitude parameters and optimized phase parameters generates a radio frequency transmit field that is optimized respective to a radio frequency transmit power metric.
In accordance with another disclosed aspect, a magnetic resonance method comprises: loading a subject into a magnetic resonance scanner; with the subject loaded into the magnetic resonance scanner, acquiring B1 maps for a plurality of radio frequency transmit channels of the magnetic resonance scanner; shimming the plurality of radio frequency transmit channels and setting a radio frequency transmit power for the shimmed plurality of radio frequency transmit channels using the acquired B1 maps to generate optimized amplitude and phase parameters for the plurality of radio frequency transmit channels; acquiring magnetic resonance imaging data of the subject loaded into the magnetic resonance scanner including exciting magnetic resonance by operating the plurality of radio frequency transmit channels using the optimized amplitude and phase parameters; generating a reconstructed image from the acquired magnetic resonance imaging data; and displaying the reconstructed image.
One advantage resides in providing more accurate radio frequency transmit power optimization.
Another advantage resides in reduction in MR acquisition time.
Further advantages will be apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.
With reference to
A plurality of radio frequency (RF) transmit channels 20 are provided, as shown in
The plurality of radio frequency transmit channels 20 can be variously embodied. For example, in some embodiments the plurality of radio frequency transmit channels 20 is embodied as a single birdcage-type volumetric radio frequency coil having I and Q ports that are independently driven, such that the number of RF transmit channels N=2 for such embodiments. In other embodiments, the plurality of radio frequency transmit channels 20 is embodied as a set of N independent coil elements, such as N independent surface coils, or N decoupled rods or rungs of a degenerate whole-body RF coil, or so forth. In these embodiments, the N independent coil elements may be variously configured, for example as separately housed coil elements, or coil elements that are electrically isolated but physically housed in a common housing (for example, a dedicated N-element coil array assembly), or so forth.
Additionally, one or more magnetic resonance receive coils are provided. In some embodiments one, some, or all of the RF transmit channels of the plurality of RF transmit channels 20 are configured as transmit/receive coils that are suitably switched to a receive mode to receive the magnetic resonance. In other embodiments, one or more magnetic resonance receive coils (not illustrated) that are separate from the plurality of RF transmit channels 20 are provided to perform the magnetic resonance receive operation.
With continuing reference to
In a typical imaging sequence the subject to be imaged is loaded into the imaging region of the bore 16 using the loading system 14, the RF transmit channels of the plurality of RF transmit channels 20 are energized in a multi-channel transmit mode to excite magnetic resonance in the subject, the magnetic field gradient coils are operated before, during, and/or after the magnetic resonance excitation in order to spatially limit and/or spatially encode or otherwise manipulate the magnetic resonance, and the magnetic resonance is received via the MR receive coils and stored in an acquired MR data storage 24. The acquired MR data are suitably reconstructed by an MR image reconstruction module 26 to generate one or more reconstructed MR images that are stored in a reconstructed MR images storage 28. The reconstruction module 26 employs a reconstruction algorithm that is operative with the spatial encoding employed during acquisition of the MR imaging data. For example, if the MR imaging data are acquired as k-space samples using Cartesian encoding, then a Fourier transform-based reconstruction algorithm may be suitably employed by the reconstruction module 26.
In this illustrative imaging sequence, the RF transmit channels of the plurality of RF transmit channels 20 are energized in a multi-channel transmit mode to excite magnetic resonance in the subject. In the multi-channel transmit mode each RF transmit channel is independently controlled in terms of RF excitation amplitude and phase. Thus, for N RF channels there are 2N independently adjustable parameters. It is desired to adjust these 2N parameters to provide a substantially (spatially) uniform B1 transmit field and to provide a B1 transmit field of a desired radio frequency transmit power. Adjusting the RF channels to provide a substantially uniform B1 transmit field is known as RF shimming. The adjustment of the RF channels to provide a desired radio frequency transmit power is typically done to provide a desired flip angle in the subject, such as a target 90° flip angle, or to limit the specific absorption rate (SAR) or another subject safety measure, or so forth. The uniformity of the B1 transmit field for a given set of 2N multi-channel transmit parameters can be substantially influenced by electrical and/or magnetic susceptibility properties of the subject undergoing imaging, so that the “optimal” transmit parameters are in general subject-specific. The influence of the subject on the B1 transmit field tends to increase as the static (B0) magnetic field increases.
With continuing reference to
The processing modules 22, 26, 30 are suitably embodied by a digital processor 40, which in the illustrative embodiment of
It is also to be understood that the various processing modules 22, 26, 30 can be embodied by a storage medium storing instructions that are executable by the illustrated processor 40 of the computer 42 or by another processor in order to perform the operations disclosed herein, including the operations performed by the module 30 including the computing of optimized amplitude and phase parameters for the plurality of radio frequency transmit channels 20 using acquired B1 maps to both (i) shim the multi-channel RF transmit field and (ii) optimize radio frequency transmit power. The storage medium storing such instructions may, for example, be a hard disk drive or other magnetic storage medium, or an optical disk or other optical storage medium, or a random access memory (RAM), read-only memory (ROM), flash memory or other electronic storage medium, or so forth.
With reference to
The illustrative example of
In a suitable approach for the B1 mapping operation 68, a two- or three-dimensional B1 map of a slice or volume of interest (preferably inside or coincident with the loaded imaging subject) is acquired. The B1 mapping may suitably employ RF pulses of a pre-determined target B1 amplitude (e.g., amplitude scale 1.0) and the RF power (e.g., power Pcalib). The power level Pcalib can be a fixed and typically low power level, and is optionally derived from a traditional RF drive scale determination. The B1 map should map the complex B1 values (that is, the B1 values including phase information) and represent the actual B1 values or relative B1 values that are relative to a target or nominal B1 value. The B1 map for a given RF transmit channel represents the actual transmit sensitivity of that RF transmit channel.
With continuing reference to
With reference to
In an operation 88, this B1 map that would be obtained in multi-channel transmit mode using the plurality of RF transmit channels 20 operated with the initial parameters selected in the operation 82 is analyzed respective to spatial uniformity. The operation 88 suitably employs a figure of merit comprising a measure of RF transmit field uniformity. In some embodiments, the coefficient of variance is used as the figure of merit measuring RF transmit field uniformity; however, other uniformity figures of merit can be employed. If the operation 88 finds that the uniformity is unsatisfactory (for example, the computed variance figure of merit is larger than an acceptable maximum variance threshold) then in an operation 90 the amplitudes (or amplitude scales) and phases are adjusted in an attempt to improve the figure of merit. The operation 90 can employ any suitable iterative adjustment algorithm, such computing the partial derivatives of the variance respective to the various amplitude and phase parameters and employing a gradient-descent improvement step. Processing then flows back to operation 84 to generate an adjusted B1 map that would be obtained in multi-channel transmit mode using the plurality of RF transmit channels 20 operated with the amplitude and phase parameters as adjusted by the adjustment operation 90, and a new figure of merit is computed in the operation 86 which is compared with the maximum variance threshold or other satisfactory uniformity criterion in the operation 88, and so forth iteratively until at the operation 88 it is determined that the iteratively adjusted parameters are now yielding a multi-channel transmit mode B1 map of satisfactory spatial uniformity. This final map is suitably considered as a shimmed B1 map 92.
The iterative shimming process implemented by the operations 82, 84, 86, 88, 90 is an illustrative example, and other shimming processes may be employed. In general, any fitting method may be used which determines the optimum relative amplitude and phase parameters by which to combine the individual B1 maps 72 for minimum coefficient of variance (or as measured by another uniformity optimization criterion). A brute force approach is also contemplated, which involves sequentially iterating phase and amplitude coefficients while testing the uniformity of the combined B1 map.
The shimmed B1 map 92 is representative of the shimmed B1 field that would exist inside the imaging subject upon application by the plurality of RF transmit channels 20 of the shimmed multi-channel RF excitation. The amplitudes optimized by the shimming operations 82, 84, 86, 88, 90 are optimized relative amplitudes, because it is the values of the optimized amplitudes relative to one another that determines the B1 transmit field uniformity in multi-channel transmit mode. Accordingly, the optimized relative amplitudes output by the shimming operations 82, 84, 86, 88, 90 do not (in general) provide any particular RF transmit power level. However, an advantageous property of the shimmed B1 map 92 is that the values can be directly related to the individual channel powers and phases for achieving a desired B1 amplitude (or, equivalently, for achieving a desired RF transmit power level).
Accordingly, the shimmed B1 map 92 is used to derive the RF power levels (that is, drive scales) by relating the known power levels used to acquire the individual channel B1 maps to the B1 field distribution and amplitude obtained following correction using the shim coefficients derived from the shimming analysis (operations 82, 84, 86, 88, 90). This ensures that the target B1 field is obtained accurately when driving the individual RF channels with the phase and amplitude coefficients determined to provide the most uniform excitation. Toward this end, an RF transmit power metric is computed for the shimmed B1 map 92 in an operation 94. The RF transmit power metric can be, for example: (i) average RF transmit power in a region of interest; (ii) average RF transmit power in a slice of interest; (iii) RF transmit power at a point in space of interest; or so forth. Because the complete shimmed B1 map 92 is available for processing by the operation 94, there is substantial flexibility in choosing an RF transmit power metric that is appropriate for the imaging task of interest. For example, if it is important to have a 90° flip angle at the center of the image, then the RF transmit power metric can be the RF transmit power at the center of the imaging volume. For imaging a slice, the choice of RF transmit power metric may be average RF transmit power over the slice.
The RF transmit power metric determined by the operation 94 is compared with a desired value for the RF transmit power metric to determine a power scaling factor in an operation 96, and the shimmed amplitudes for the RF transmit channels are scaled by the power scaling factor to arrive at the optimized amplitudes and phases 98 for achieving both RF shimming and desired RF transmit power. For example, if the RF transmit power metric determined by the operation 94 is denoted (in amplitude units) as B1meas and the desired value for the RF transmit power metric is denoted (again in amplitude units) as B1target, then the scaling factor is B1target/B1meas. The amplitudes are then suitably scaled by this scaling factor. In performing this adjustment, it should be noted that the choice of RF transmit power metric here is in amplitude units, and so the amplitudes being scaled by the scaling factor (B1target/B1meas) results in the corresponding RF transmit power being scaled by the factor (B1target/B1meas)2. The choice of RF transmit power metric can be either in amplitude units or in power units. Using a power units example, if the RF transmit power metric determined by the operation 94 is denoted (in power units) as P1meas and the desired value for the RF transmit power metric is denoted (in power units) as P1target, then the scaling factor for the amplitudes is (P1target/P1meas)1/2, and the corresponding RF transmit power is scaled by (P1target/P1meas).
In the embodiment of
In other embodiments, the shimming and the RF transmit power optimization can be performed concurrently, in a single process, again using the acquired B1 maps. For example, in one such embodiment the figure of merit employed in the decision block 88 is modified to be a figure of merit that combines (i) a measure of RF transmit field uniformity (such as the coefficient of variance) and (ii) a measure of RF transmit field power (such as the average B1 field over a slice or region of interest). In such an embodiment, for example, the figure of merit may be a weighted sum of (i) the coefficient of variance and (ii) a term (B1target-B1meas)2 which compares the measure of RF transmit field power (B1meas) with a target RF transmit field power (B1target). With this modified figure of merit, the iterative operations 82, 84, 86, 88, 90 can concurrently perform the shimming (by optimizing the coefficient of variance term) and the RF transmit power (by optimizing the term term (B1target-B1meas)2), with the weighting between the two terms selecting which aspect (field uniformity or RF transmit power optimization) dominates the optimization. In this embodiment, the operations 94, 96, 98 are suitably omitted since the modified figure of merit ensures that the optimization operations 82, 84, 86, 88, 90 optimize the RF transmit power metric.
In the B1 mapping approach of
This application has described one or more preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the application be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2010/053558 | 8/5/2010 | WO | 00 | 2/29/2012 |
Number | Date | Country | |
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61243196 | Sep 2009 | US |