Abstract:
The invention relates to a multi-channel (e.g. quadrature) MRI transmit system in which RF power amplifiers having different power capabilities are used in different transmit channels. This results in reduced system costs, due to the avoidance of an unused excess of RF power capability when the power demand for obtaining a homogeneous B1-field (RF shimming) is asymmetric and the asymmetry is qualitatively the same for different imaging applications. The multi-channel transmit unit may also comprise a commutator which enables to selectively connect each RF power amplifier to each drive port of transmit coil arrangement (e.g. a birdcage coil).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of magnetic resonance (MR) imaging. It concerns a method of MR imaging of at least a portion of a body of a patient placed in an examination volume of a MR device. The invention also relates to a MR device and to multi-channel transmit unit for a MR device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive. 
         [0003]    According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field of the RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession about the z-axis. This motion of the magnetzation describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle) 90°. The RF pulse is radiated toward the body of the patient via a RF coil arrangement of the MR device. The RF coil arrangement typically surrounds the examination volume in which the body of the patient is placed. 
         [0004]    After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T 1  (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T 2  (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within the examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils. 
         [0005]    In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation. 
         [0006]    It is generally desirable to have a relatively uniform homogeneity of the generated RF field (B 1  field) for excitation of magnetic resonance throughout a cross section of the imaged portion of the patient&#39;s body. However, as the MR frequency increases with increasing main magnetic field strength, this becomes more difficult due to conductive losses and wavelength effects within the body of the patient. Multi-channel transmit MR imaging has been accepted as a standard method of operating volume RF coils to achieve a relatively uniform B 1  field. Compared with a single channel mode of operation, a two-channel transmit technique results in a significantly increased B 1  homogeneity. 
         [0007]    In known multi-channel transmit systems the RF power signal is typically supplied to the RF coil arrangement via RF drive ports being connected to individual resonator elements of the RF coil arrangement. The RF coil arrangement may be a so-called birdcage resonator comprising a plurality of rungs arranged in parallel to a longitudinal axis of the main magnetic field, wherein the birdcage resonator surrounds the imaged body portion. In this case, the RF drive ports are connected to two or more rungs of the birdcage resonator. Two-channel transmit MR systems typically use two independent RF transmit chains and amplifiers for applying the RF power signals to the RF drive ports of the coil arrangement. The RF power applied to the different RF drive ports can be controlled individually in order to optimize the homogeneity of the RF field (so-called RF shimming). 
         [0008]    It turns out that different imaging tasks that utilize RF shimming have different RF power demands from each of the two channels. Dual channel shimming can therefore lead to a significantly asymmetric power demand at the two RF drive ports of the RF coil arrangement. The extent of asymmetry depends upon the anatomy in which the RF field homogeneity needs to be optimized and also upon the extent to which shimming is required. Typically, the RF power amplifiers within a multi-channel transmit system have equal power capabilities. However, the asymmetry occurring in the power demand results in a situation in which one of the RF power amplifiers does most of the work while the other RF power amplifiers idle. 
         [0009]    Moreover, a drawback of known multi-channel transmit MR systems is that the asymmetry in the RF power demand changes according to, for example, a changed patient anatomy and/or a changed imaging task. For this reason it is necessary in conventional multi-channel systems to employ RF power amplifiers in all channels having sufficient excess power capabilities in order to be able to fulfill the requirements in any possible application. This results disadvantageously in increased system costs. 
       SUMMARY OF THE INVENTION 
       [0010]    From the foregoing it is readily appreciated that there is a need for an improved MR imaging technique. It is consequently an object of the invention to provide a MR method and a MR device that uses the available power capabilities of the RF power amplifiers efficiently for all possible applications. Unused power capabilities in the transmit chain of a MR apparatus should be avoided to the largest possible extent. 
         [0011]    In accordance with the invention, a method of MR imaging of at least a portion of a body of a patient placed in an examination volume of a MR device is disclosed. The method of the invention comprises the following steps: 
         [0012]    subjecting the portion of the body to an imaging sequence comprising RF pulses and switched magnetic field gradients, the RF pulses being transmitted toward the portion of the body via a RF coil arrangement surrounding the examination volume, wherein RF signals are supplied to the RF coil arrangement via two or more transmit channels in an asymmetric fashion relative to a horizontal and/or vertical axis of the cross section of the portion of the body; 
         [0013]    acquiring MR signals from the portion of the body ( 110 ); and 
         [0014]    reconstructing a MR image from the acquired MR signals. 
         [0015]    The gist of the invention is the application of RF energy to the RF coil arrangement of the used MR apparatus in a geometrically suitable asymmetric fashion relative to the horizontal and/or vertical axis of the imaged body, which is appropriate for most applications. The power demand for obtaining a homogeneous B 1  field is asymmetric according to the invention, exactly like in the prior art. It turns out, however, that the asymmetry is qualitatively the same for the different imaging applications, which is due to the special asymmetric RF feeding of the RF coil arrangement. Consequently, the power demand in one transmit channel is always smaller than the power demand in another transmit channel. Hence, it is possible to make provision for RF power amplifiers having different power capabilities in the different transmit channels. An unused excess of RF power capability, which is always present in known multi-channel transmit systems for the reasons described above, can be avoided by the invention. This results in significantly reduced system costs. 
         [0016]    The RF coil arrangement used in accordance with the invention may be a birdcage resonator comprising a plurality of rungs arranged in parallel to a longitudinal axis of the body of the imaged patient, wherein the birdcage resonator comprises two or more RF drive ports connected to different rungs. Due to the asymmetric positioning of the rungs to which RF power is supplied, the power demand in one channel will always be smaller than the power demand in the other channel. Hence, a RF power amplifier having a smaller power capability can be connected to one of the RF drive ports, while a RF power amplifier having a larger power capability can be connected to the other RF drive port. The total installed RF power can be reduced significantly in this way without constraining the general usability of the system for different imaging applications. 
         [0017]    The invention does not only relate to a MR method but also to a MR device. According to the invention the MR device includes: 
         [0018]    a main magnet for generating a uniform, steady magnetic field within an examination volume along a main field axis, 
         [0019]    a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, 
         [0020]    at least one RF coil arrangement for generating RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, which RF coil arrangement surrounds the examination volume and comprises two or more RF drive ports connected to resonator elements of the RF coil arrangement, which resonator elements are positioned asymmetrically relative to a horizontal and/or vertical axis of the cross section of the RF coil arrangement and/or of the body of the patient, 
         [0021]    a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, 
         [0022]    a reconstruction unit, and 
         [0023]    a visualization unit. 
         [0024]    According to yet another aspect of the invention, a multi-channel transmit unit for supplying RF power signals to a RF coil arrangement of a MR device is disclosed, wherein the transmit unit comprises two or more transmit channels, each transmit channel including a RF power amplifier, wherein the RF power amplifiers of the different transmit channels have unequal power capabilities. Such a multi-channel transmit unit can be used in combination with a MR device, in which RF power signals Are supplied to the RF coil arrangement in a geometrically asymmetric fashion relative to the symmetry of the imaged body portion of the patient. Because of the unequal power capabilities of the RF power amplifiers, an unused excess of installed RF power is avoided and overall systems costs are reduced. It turns out that the invention enables to realize a multi-channel transmit unit, in which the power capability of a RF power amplifier of one transmit channel is at least double the power capability of a RF power amplifier of another transmit channel. In practice the total installed RF power can be reduced by up to 35% as compared to conventional systems, in which the RF coil arrangement is symmetrically driven. Moreover, the multi-channel transmit unit of the invention can be used in a very flexible manner in all cases in which an asymmetric RF power demand occurs. To this end the multi-channel transmit unit of the invention may advantageously comprise a commutator which enables to selectively connect each RF power amplifier to each RF drive port of the RF coil arrangement. The commutator may be controllable by the control unit of the respective MR device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings: 
           [0026]      FIG. 1  shows a MR device according to the invention; 
           [0027]      FIG. 2  shows a cross section of a RF coil arrangement with RF drive ports positioned in a conventional manner; 
           [0028]      FIG. 3  shows a cross section of a RF coil arrangement with RF drive ports positioned according to the invention; 
           [0029]      FIG. 4  shows a cross section of a RF coil arrangement with RF drive ports positioned according to the invention; 
           [0030]      FIG. 5  shows diagrams illustrating the power level of the RF signals supplied via the RF drive ports in the RF coil arrangements of  FIGS. 2 and 3 ; 
           [0031]      FIG. 6  schematically shows a multi-channel transmit unit according to the invention. 
           [0032]      FIG. 7  shows a further embodiment of a multi-channel transmit unit according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0033]    With reference to  FIG. 1 , a MR device  101  is shown. The device comprises superconducting or resistive main magnet coils  102  such that a substantially uniform, temporally constant main magnetic field is created along a z-axis through an examination volume. 
         [0034]    A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging. 
         [0035]    Most specifically, a gradient pulse amplifier  103  applies current pulses to selected ones of whole-body gradient coils  104 ,  105  and  106  along x, y and z-axes of the examination volume. A digital multi-channel RF frequency transmitter  107  transmits RF pulses or pulse packets via two RF drive ports  119 ,  120  to a whole-body volume RF coil  109  to transmit RF pulses into the examination volume. 
         [0036]    A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body  110  positioned in the examination volume. The MR signals are also picked up by the whole-body volume RF coil  109 . 
         [0037]    For generation of MR images of limited regions of the body  110  by means of parallel imaging, a set of local array RF coils  111 ,  112 ,  113  are placed contiguous to the region selected for imaging. The array coils  111 ,  112 ,  113  can be used to receive MR signals induced by body-coil RF transmissions. 
         [0038]    The resultant MR signals are picked up in the depicted embodiment by the array RF coils  111 ,  112 ,  113  and demodulated by a receiver  114  preferably including a preamplifier (not shown). The receiver  114  is connected to the RF coils  111 ,  112  and  113  via switch  108 . 
         [0039]    A host computer  115  controls the gradient pulse amplifier  103  and the transmitter  107  to generate any of a plurality of MR imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like. For the selected sequence, the receiver  114  receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system  116  performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system  116  is a separate computer which is specialized in acquisition of raw image data. 
         [0040]    Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor  117  which applies a Fourier transform and other appropriate reconstruction algorithms, such like SENSE or SMASH. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor  118  which provides a man-readable display of the resultant MR image. 
         [0041]    With continuing reference to  FIG. 1  and with further reference to  FIGS. 2 ,  3 , and  4 , the asymmetric RF driving of the RF coil arrangement  109  according to the invention is described for the case of a birdcage resonator as a RF coil arrangement  109 . 
         [0042]      FIGS. 2 ,  3 , and  4  show cross sections of birdcage resonators comprising a plurality of rungs  1  to  16  arranged in parallel to the main field axis z. The birdcage resonator surrounds the examination volume of the MR device  1 . As depicted in  FIGS. 2 and 3 , the body  110  of the patient is positioned within the birdcage resonator close to its centre.  FIGS. 2 ,  3 , and  4  further show a vertical symmetry axis  201  of the birdcage resonator which constitutes simultaneously a vertical symmetry axis of the body  110  of the patient. 
         [0043]      FIG. 2  shows the conventional approach, in which the RF drive ports  119  and  120  are connected to rungs  7  and  11 , i.e. in a symmetrical fashion relative to the vertical axis  201 . 
         [0044]      FIG. 3  shows an arrangement according to the invention, in which the two RF drive ports  119  and  120  are connected to rungs  6  and  10  which are positioned asymmetrically with respect to the vertical axis  201  of the cross section of the RF coil arrangement  109 . The angular distance between the RF drive ports  6  and  10  is 90 degrees. 
         [0045]      FIG. 4  shows a further arrangement according to the invention, wherein the RF drive ports  119  and  120  are connected to rungs  8  and  12  which are again positioned asymmetrically with respect to the vertical axis  201  of the cross section of the RF coil arrangement  109 . 
         [0046]    The diagrams in  FIG. 5  show the RF power (in kW) required to achieve optimum RF uniformity in different arrangements and applications. In each of the diagrams  501  to  506  the left column shows the RF power in the first transmit channel, while the middle column shows the RF power in the second channel. The right column shows the total RF power demand. 
         [0047]    Diagram  501  in  FIG. 5  relates to the case of body (torso) imaging using the configuration of the invention as shown in  FIG. 3 , in which the first transmit channel is connected to RF drive port  119  while the second transmit channel is connected to RF drive port  120 . It turns out that the RF power demand in the first transmit channel is much smaller than the RF power demand in the second channel for obtaining maximum RF homogeneity. In fact, the RF power in the first channel amounts to less than 4.0 kW while the RF power in the second channel amounts to more than 22.0 kW. The situation is similar in the case of diagram  502 . Diagram  502  shows the situation for breast imaging using the same configuration of the RF drive ports  119 ,  120  ( FIG. 3 ). Again, the RF power in the first transmit channel is much smaller than the RF power in the second transmit channel in order to achieve optimum RF uniformity. 
         [0048]    The situation changes significantly when the conventional configuration as shown in  FIG. 2  is used, in which the RF drive ports  119  and  120  are arranged symmetrically relative to the vertical axis  201 . Diagram  503  shows the case of body (torso) imaging. In this case, the RF power in the first transmit channel is again smaller than the RF power in the second transmit channel. Diagram  504  relates to the case of breast imaging using the configuration shown in  FIG. 2 . Here the situation is inverted. The RF power in the first transmit channel is larger than the RF power in the second transmit channel. 
         [0049]    Diagrams  505  and  506  again refer to the cases of body (torso) imaging and breast imaging respectively. The RF drive ports are connected to rungs  8  and  12  ( FIG. 4 ). As can be seen in the diagrams  505  and  506 , the RF power demand is again asymmetric but also qualitatively different for the different imaging applications. 
         [0050]    As can be seen from diagrams  501  and  502  in  FIG. 5 , RF shimming for body (torso) imaging and breast imaging always yields an asymmetric RF power distribution over the two transmit channels, wherein the small and large RF power demands occur in the same channels. This can be exploited according to the invention by using a transmit unit  107  for supplying the RF energy to the RF drive ports  119  and  120 , wherein the transmit channels of the transmit unit  107  include RF power amplifiers having unequal power capabilities. For the case shown in  FIG. 3 , the power capability of the RF power amplifier in the first transmit channel can be less than half of the power capability of the RF power amplifier in the second transmit channel. The total installed RF power can be reduced by at least 25%. In practice, the reduction factor may be up to 35% as compared to the conventional situation illustrated in  FIG. 2 . Hence, the invention helps to reduce the total power requirements and consequently the associated system costs. 
         [0051]      FIG. 6  schematically shows the two-channel RF transmit unit  107  of the invention. The transmit unit  107  is used for supplying RF power to RF coil arrangement  109 . To this end output ports  601  and  602  are connected to the RF drive ports  119  and  120  of the RF coil arrangement  109 . The transmit unit  107  comprises two transmit channels, wherein each transmit channel includes a RF power amplifier  603 ,  604 . The RF power amplifiers  603 ,  604  have unequal power capabilities. The power capabilities of the two RF power amplifiers  603 ,  604  can be selected according to the diagrams shown in  FIG. 5 , depending on the respective arrangement of the RF drive ports  119 ,  120 . The RF power amplifiers  603 ,  604  are connected to the output ports  601 ,  602  via a commutator  605 , by which each RF power amplifier  603 ,  604  is selectively connectible to one of the output ports  601 ,  602  of the multi-channel transmit unit  107 . This allows to employ the RF transmit unit  107  depicted in  FIG. 6  even in the arrangement shown in  FIG. 4 . According to diagrams  505  and  506  in  FIG. 5 , the RF power distribution is asymmetric for body (torso) and breast imaging respectively. However, the small and large RF power demands occur in different channels for body (torso) and breast imaging. The commutator  605  enables to use the RF transmit unit  107  for both body (torso) and breast imaging in the arrangement shown in  FIG. 4 . 
         [0052]    Another embodiment is shown in  FIG. 7 , in which the RF transmit  107  comprises for output ports  601 ,  602 ,  606 ,  607 . The commutator  605  enables to switchably connect each RF power amplifier  603 ,  604  arbitrarily to one of the output ports  601 ,  602 ,  606 ,  607 . Output ports  601 ,  602  may be connected to rungs  6  and  10  while output ports  606  and  607  are connected to rungs  8  and  12 . In this case the arrangement shown in  FIG. 3  can be used for body (torso) imaging (see diagram  501  in  FIG. 5 ), while the arrangement shown in  FIG. 4  can be used for breast imaging (see diagram  506  in  FIG. 5 ). This advantageously enables to use RF power amplifiers  603 ,  604  having most significantly different power capabilities. The power capability of amplifier  603  should be 22 kW, while a power capability of only 4 kW is sufficient for amplifier  604 . 
         [0053]    In the above description body (torso) and breast imaging are mentioned as exemplary applications. It has to be noted, however, that the method and apparatus of the invention can be used with advantage as well for head imaging, cardiac imaging, liver imaging, imaging of the extremities and other imaging tasks.