Patent Application
20120153950
The present application relates to the magnetic resonance arts. It finds particular application in conjunction with radio-frequency (RF) shimming of parallel transmit systems. It is to be appreciated, however, that the present application will also find application in conjunction with other types of magnetic resonance imaging, spectroscopy, and other diagnostic techniques which use radio frequency coils.
Magnetic resonance imaging (MRI) and spectroscopy (MRS) systems are often used for the examination and treatment of patients. By such a system, the nuclear spins of the body tissue to be examined are aligned by a static main magnetic field B0 and are excited by transverse magnetic fields B1 oscillating in the radiofrequency band. In imaging, relaxation signals are exposed to gradient magnetic fields to localize the resultant resonance. The relaxation signals are received in order to form in a known manner a single or multi-dimensional image. In spectroscopy, information about the composition of the tissue is carried in the frequency component of the resonance signals.
Two types of MR systems that are in common use include “open” MR systems (vertical system) and “bore-type” systems. In the former, the patient is introduced into an examination zone which is situated between two magnetic poles connected by a C-shaped unit. The patient is accessible during the examination or treatment from practically all sides. The latter comprises a cylindrical examination space (axial system) into which a patient is introduced.
An RF coil system provides the transmission of RF signals and the reception of resonance signals. In addition to the RF coil system which is permanently built into the imaging apparatus, special purpose coils can be flexibly arranged around or in a specific region to be examined Special purpose coils are designed to optimize signal-to-noise ratio (SNR), particularly in situations where homogeneous excitation and high sensitivity detection is required. Furthermore, special sequences of RF signals, higher field strengths, high flip angles or real-time sequences can be realized and generated by multi-channel antenna arrangements, and multi-dimensional excitations can be accelerated.
MR imaging and spectroscopy benefit from improved signal-to-noise (SNR) ratios and contrast-to-noise ratios (CNR) at higher static magnetic field strengths, for example greater than 3 Tesla (T), because a larger number of the protons align along the main magnetic field and thus increase longitudinal magnetization and increase precession rates. Nonetheless, wave propagation effects diminish SNR and CNR at main field strengths of about 3 T and above. One such factor in this reduction is B1 field inhomogeneities which cause non-uniform SNR and CNR across the imaging volume. Conductive loading of patient tissue coupled with dielectric resonances created by objects longer than the transmit wavelength results in the B1 field inhomogeneities.
Effective methods have been developed to mitigate B1 field inhomogeneities such as adiabatic pulses, novel coil designs, and image processing techniques. However, adiabatic pulses suffer from high SAR absorption, coil designs cannot account for the subject's shape and size, and image processing techniques merely normalize pixel intensities which do not improve SNR or CNR.
Parallel RF transmission systems have the potential of compensating for B1 field inhomogeneities through RF shimming RF shimming can be performed in two different ways. Basic RF shimming adjusts the global amplitude and phase of the currents in each independent transmit element, aiming at a constant B1 in the region of interest. Basic RF shimming applies standard slice selective RF pulses, typically with a sinc shape, corresponding to a one-dimensional (through-plane) trajectory in the excitation k-space. By adjusting the global amplitude and phase of the currents in each transmit element, one can achieve a relatively constant B1 amplitude in the region of interest in many situations. For 3D volume imaging, 3D RF shimming is facilitated using different frequencies for the deferent transmit elements. The elements of a transmit array are driven with different frequencies to excite different slabs in the excitation volume via the underlying gradient. Amplitudes and phases can be optimized for each slab individually to achieve optimal homogeneity. The advantage of basic RF shimming is that it can be easily combined with nearly every MR sequence, since basic RF shimming does not require any change of sequence timing or sequence gradients. On the other hand, basic RF shimming is of limited flexibility, i.e., not all B1 signal inhomogeneities can be compensated, particularly when using only two RF transmit channels.
Tailored RF shimming can be performed via multi-dimensional RF pulses designed to achieve a spatially constant excitation pattern. Typically, a two-dimensional, in-plane trajectory in the excitation k-space is used, which allows the excitation of an arbitrary spatial magnetization pattern. Moreover, additional dimensions might be taken into account, like through-plane or spectral dimension. Multi-dimensional RF pulses do not require parallel transmission; however, parallel transmission allows the acceleration of multi-dimensional RF pulses with Transmit SENSE or alternative techniques. Assuming a sufficient pulse length, nearly all B1 signal inhomogeneities can be compensated. Although tailored RF shimming has a very high RF shimming potential, it has a big impact on sequence timing and sequence gradients. Even with acceleration techniques, multi-dimensional RF pulses are typically much longer than standard 1D sinc pulses.
The present application provides a new and improved radio-frequency shimming apparatus and method which overcomes the above-referenced problems and others.
In accordance with one aspect, a radio-frequency (RF) shimming apparatus is comprised of a spatial sensitivity unit which determines a transmit spatial sensitivity distribution of at least one RF coil. A selection unit selects an excitation pattern with an excitation k-space trajectory. An optimization unit curves the excitation k-space trajectory of the selected excitation pattern according to the generated spatial sensitivity distribution, and supplies the curved excitation k-space trajectory to at least one transmitter which causes the at least one RF transmit coil to transmit the selected excitation pattern with the curved excitation k-space trajectory.
In accordance with another aspect, a method for radio-frequency shimming is comprised of determining a transmission spatial sensitivity distribution of at least one RF transmit coil, and selecting an excitation pattern with an excitation k-space trajectory. The excitation k-space trajectory of the selected excitation pattern is curved according to the generated spatial sensitivity distribution. At least one transmitter is controlled to cause the at least one RF coil to transmit the selected excitation pattern with the curved excitation k-space trajectory.
One advantage resides in that homogeneity of a B1 excitation field is improved.
Another advantage resides in reduced specific absorption rate (SAR) hot spots.
Another advantage resides in improved signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR).
Another advantage resides in improved acquisition times.
Another advantage resides in enabling standard MR sequences notwithstanding improved RF shimming.
Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description.
The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.
With reference to
A radio-frequency (RF) coil assembly 18, such as a whole-body radio frequency coil, is disposed adjacent the examination region. The RF coil assembly generates radio frequency pulses for exciting magnetic resonance in aligned dipoles of the subject. The radio frequency coil assembly 18 also serves to detect magnetic resonance signals emanating from the imaging region. Optionally, local, surface or in vivo RF coils 18′ are provided in addition to or instead of the whole-body RF coil 18 for more sensitive, localized spatial encoding, excitation, and reception of magnetic resonance signals. The whole body coil can comprise of a single coil or a plurality of coil elements of an array as in a parallel transmit system. In parallel transmit systems, the k-space trajectory can be configured for a specific spatial sensitivity which ultimately shortens the overall pulse length. In one embodiment, the k-space trajectory determined by the gradient system, i.e. the gradient coil 16 and gradient controller 22, is the same for all transmit coils. In another embodiment, different B1 pulses are determined individually for each transmit element of the transmit coil (18,18′) array.
To acquire magnetic resonance data of a subject, the subject is placed inside the examination region 14, preferably at or near an isocenter of the main magnetic field. A scan controller 20 controls a gradient controller 22 which causes the gradient coils to apply the selected magnetic field gradient pulses across the imaging region, as may be appropriate to a selected magnetic resonance imaging or spectroscopy sequence. The scan controller 20 also controls at least one RF transmitter 24 which causes the RF coil assembly to generate magnetic resonance excitation and manipulation of B1 pulses. In a parallel system, the RF transmitter 24 includes a plurality of transmitters or a single transmitter with a plurality of transmit channels, each transmit channel operatively connected to a corresponding coil element of the array. To improve homogeneity of the B1 pulses in the examination region 14, a spatial sensitivity distribution of the transmit coils 18, 18′ are determined by a spatial sensitivity unit 30, e.g. by a short measurement prior to the actual imaging sequence to compensate for dielectric resonances occurring in patient tissue at high frequencies, i.e. Larmor frequency at static fields strengths of 3 T or greater.
After the spatial sensitivity distribution is determined, an excitation pattern with an excitation k-space trajectory is selected by a selection unit 32. The excitation k-space trajectory typically includes of a single spoke or a one-dimensional, slice-selective straight line in the through-plane direction kz as shown in
In a next step, an optimization unit 34 determines RF pulses for the individual transmit channels based on the selected excitation pattern, the corresponding excitation k-space trajectory, and the determined spatial sensitivity distribution. The RF pulses can be determined using known techniques such as Transmit SENSE or the like. The optimization unit 34 utilizes the determined RF pulses to optimize the through-plane spoke of the excitation k-space trajectory by curving the spoke in the in-plane direction(s) kx or ky. With reference to
kx=A sin(2πfkz/kmax+ψ) equation 1
where A is an amplitude, kmax is a maximum of a k-space range, f is a frequency of the sine function in the through-plane direction, kz is a running variable in k-space in the z-direction, and ψ is a phase of the sine function. The amplitude A, frequency f, and phase ψ of the curved excitation k-space trajectory in one embodiment are iteratively varied to find the optimal curvature. Alternatively, optimization algorithms such as simulated annealing, conjugate gradients, or the like can be employed to determine the optimal curvature. Alternatively, a look-up table can be employed to match several curved trajectories stored in a memory in the optimization unit 34 to the corresponding determined RF pulses.
With returning reference to
With reference to
With reference to
kx=A sin(2πfkz/kmax+ψ)cos(φtwistkz/kmax+φoff) equation 2
ky=A sin(2πfkz/kmax+ψ)sin(φtwistkz/kmax+φoff) equation 3
where additional parameters φtwist is a magnitude of the twist and φoff is a offset of the trajectory's twist. The result is a twist of the excitation k-space trajectory about the central axis kx=ky=0. It should also be appreciated that different parameterizations of curved trajectories are also contemplated. For example, alternatives to equation 1 are defined by:
kx=a
0(kz−a1)exp(−(kz−a2)2/a3) equation 4
kx=b
0(kz−b1)(kz−b2)(kz−b3) equation 5
where constants a0, a1, a2, a3 and b0, b1, b2, b3 are optimized individually.
The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed 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 |
|---|---|---|---|---|
| PCT/IB10/53550 | 8/5/2010 | WO | 00 | 2/15/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61240415 | Sep 2009 | US |
