Abstract:
A pair of quadrature radio frequency coils ( 32, 34 ) disposed adjacent an imaging region ( 10 ) are typically loaded differently due to factors such as subject geometry, subject mass, and a relative distance from the subject. A tip angle adjustment circuit ( 50 ) monitors a combined tip angle adjacent a mid-plane of the examination region, such as by analyzing delivered and reflected power to each of the coils. An adjustment circuit ( 54 ) adjusts relative RF power or amplitude to produce a selected, combined tip angle in the examination region.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with open MRI systems with a main magnetic field greater than 0.5 T and will be described with particular reference thereto. It will be appreciated, however, that the present invention is useful in conjunction with other systems containing more than one radio frequency coil, such as bore type MRI systems, spectroscopy systems, low field systems, and the like, and is not limited to the aforementioned application. 
     In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic resonance systems, the main magnetic field is typically vertical, perpendicular to the subject between upper and lower poles. A series of radio frequency (RF) pulses are applied to two RF coils, one adjacent each pole, to excite and manipulate magnetic resonance. Gradient magnetic fields are conventionally produced by gradient coils mounted between the RF coils and the poles to encode spatial position and other information in the excited resonance. The magnetic resonance signals are detected with the two RF coils or localized coils and processed to generate two or three dimensional image representations of a portion of the subject in the examination region. 
     Typically, the patient is placed in the examination region on his/her back close to the bottom pole assembly and further from the upper pole assembly to maximize openness in front of the patient. This causes uneven loading of the upper and lower RF coils during transmit, resulting in each coil having different RF power coupling to the patient. That is, the coils contribute unevenly to the imaging process. Patient geometry also contributes to different loading of the two RF coils. The anterior and posterior shapes of the body are not the same and are different at different positions along the length of the patient. The loading and RF power coupling experienced by the two coil assemblies due to this geometry variation are different. 
     With relatively low main fields of existing open systems, this phenomenon has not been a significant problem. However, as main fields increase, the uneven loading of the RF coils can become problematic in that the output images have non-uniform intensity such that they become unusable for diagnostic imaging. 
     In the higher fields of bore systems, a birdcage RF coil design is typically used, with the RF quadrature drive for the coil located 45 degrees to either side of the down position so that it is least affected by vertical patient location and shape. Generally though, the head to foot central axis of the patient is very close to the main axis of the bore, i.e. loading is relatively symmetric. 
     Conventionally, calibrated RF pulses are used to excite and manipulate the MR signal. That is, the excitation or tip angles, 180° inversion angles, other spin system manipulation as well as which nuclei are resonated are precisely achieved with carefully calibrated RF pulses. In order to reorient the magnetization, the amplitude and phase of the RF envelope as a function of time is precisely controlled. The patient mass influences the loading of the RF coils which affects the reflected RF power and the forward RF power coupled into the coil. For each patient anatomy, the RF field is separately amplitude calibrated to achieve the proper tip angle. For a patient disposed symmetrically, this typically consists of exciting a transverse slab with a nominal 90° pulse and adjusting the RF amplitude until a 90° magnetization tip or flip angle is achieved. Depending on patient girth, the loading of the RF coils can differ greatly from patient to patient. 
     The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a magnetic resonance apparatus includes a vertical field main magnet system which generates a main temporally constant field through an examination region. An upper radio frequency coil and a lower radio frequency coil excite magnetic resonance in selected nuclei located in the examination region. An RF transmitter drives upper and lower RF coils that generate radio frequency magnetic fields that excite and manipulate magnetic resonance. The same coils may be used to receive resonance signals as well. An RF tip angle adjustment circuit is used to separately calibrate two RF coils to achieve the desired spin tip angles that are produced when driving the combined coils. 
     In accordance with a more limited aspect of the present invention, the tip angle analyzer contains a means by which it detects the tip angle caused by the RF coils. In order to adjust the tip angle for the desired effect, the RF tip angle analyzer changes the RF power directed to the transmitter coils. 
     In accordance with a more limited aspect of the present invention, the magnetic resonance apparatus transmits and receives magnetic resonance manipulation signals in quadrature. 
     In accordance with another aspect of the present invention, a magnetic resonance apparatus is given. A magnet assembly generates a main magnetic field through and examination region. First and second RF coil assembles are disposed opposite each other, adjacent the imaging region. An RF transmitter provides RF pulses to the coil assemblies. An RF monitor measures the RF power delivered by the coil assemblies separately. An RF tip angle calculator measures the tip of induced resonance in order to produce more desired tip angles. 
     According to a more limited aspect of the present invention, RF tip angle calculator retrieves adjustment data from a memory look up table. 
     According to another aspect of the present invention, a method of magnetic resonance is given. Two RF transmit coils are disposed adjacent an examination region. Magnetic resonance is selectively excited and tip angles of the resonance are measured and adjusted. The tip angles are adjusted by adjusting an RF pulse amplitude and a radio frequency. 
     According to another aspect of the present invention, a method of magnetic resonance is given. Two RF transmit coils are disposed adjacent an examination region. Magnetic resonance is selectively excited and tip angles of the resonance are measured and adjusted to produce a tip angle of a desired amount. 
     According to another aspect of the present invention, a method of magnetic resonance is given. Two RF coils are disposed opposite each other, adjacent an examination region. Total power and reflected power for each of the coils is measured. Power delivered into the examination region is calculated. The relative power to each coil is adjusted, thereby adjusting the tip angle in a selected region near a midplane. 
     One advantage of the present invention is that it provides RF transmit coils for open machines with higher main fields. 
     Another advantage of the present invention is that it permits off center placement of the subject, vertically and longitudinally. 
     Another advantage of the present invention is that it allows for images with better signal uniformity. 
     Another advantage of the present invention is that it provides better RF transmit fields for MRI. 
     Yet another advantage resides in a higher signal-to-noise ratio. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging system in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a representation of a two transmitter embodiment; 
     FIG. 3 is a representation of a single transmitter embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, in an open MRI system, an imaging region  10  is defined between an upper pole assembly  12  and a lower pole assembly  14 . A pair of annular super-conducting magnets  16 ,  18  surround upper and lower pole pieces  20 ,  22  generating a temporally constant, main magnetic field B 0  through the imaging region  10 . It is to be appreciated that the open MRI apparatus may have a variety of pole pieces or, in some instances, no pole pieces at all. The magnets for generating the main magnetic field can be positioned at other locations. Optionally, a ferrous flux return path is provided between the pole assemblies remote from the imaging region  10 . 
     For imaging, magnetic field gradient coils  24 ,  26  are disposed on opposite sides of the imaging region  10  adjacent the pole pieces  20 ,  22 . In the preferred embodiment, the gradient coils are planar coil constructions which are connected by gradient amplifiers  28  to a gradient magnetic field controller  30 . The gradient magnetic field controller  30  causes current pulses which are applied to the gradient coils  24 ,  26  such that gradient magnetic fields are superimposed on the temporally constant and uniform field B 0  across the imaging region  10 . The gradients of the fields aligned with the main field are typically oriented along a longitudinal or y-axis, a vertical or z-axis and a transverse or x-axis. 
     For exciting magnetic resonance in selected nuclei, an upper radio frequency coil  32  and a lower radio frequency coil  34  are disposed between the gradient coils  24 ,  26  adjacent the imaging region  10 . The coils  32 , 34  generate RF frequency magnetic fields, typically denoted B 1 , within the imaging region. The coils  32 ,  34  can be connected to one or more RF transmitters  38 . RF screens  36  are disposed between the RF coils  32 ,  34  and the gradient coils  24 ,  26  to minimize the generation of RF eddy currents in the gradient coils  24 ,  26 . The RF coils  32 ,  34  transmit B 1  magnetic field pulses into the imaging region. The received signals are processed into in-phase and 90° out-of-phase quadrature signals. 
     The quadrature circuits  40  in the preferred embodiment have four ports, and operate in both transmit and receive modes. In transmit mode, the quadrature circuit  40  splits a signal received on a parallel port Q from one of the RF transmitters  38  and splits it into two components that are 90° out of phase with respect to each other. The circuit splits the signal, and sends the original out of an in-phase port I and phase-shifts the signal a quarter wave length and sends the out-of-phase component to an out-of-phase port O. The quadrature signals are then passed to the RF coils  32 ,  34 . 
     Prior to conducting an imaging sequence, a series of calibration pulses are applied for each patient to calibrate the macroscopic magnetization tip angle produced by the B 1  RF frequency pulses. A sequence controller  42  accesses a sequence memory  44  to withdraw one or more RF calibration pulse sequences, which are implemented by a RF pulse controller  46 . Typically, the sequence control causes the pulse controller and the RF transmitter to generate a stimulated echo pulse sequence or some other sequence sensitive to RF calibration. The resultant resonance is passed from the quadrature circuits to receivers  48  in the receive portions of the sequence by a transmit/receive (T/R) switch. An adjustment circuit  50  includes an RF patient calibration circuit  52 , more particularly a tip angle calculator. The RF patient calibration circuit  52  controls an amplitude adjustment circuit  54  to adjust an amplitude of the RF pulse envelopes until the desired tip angle is achieved for example, 90°. In the preferred embodiment, with the patient positioned between the two transmit coils, RF pulses are directed to either the top or the bottom coil. The RF pulse amplitude is then calibrated for each coil separately and then stored in the patient calibration memory  56 . Subsequently, half of the calibrated amplitude from memory  56  for the respective coil is applied for the combined transmit signal. Alternatively, the separate coil calibrations can be used to set the proper ratio of RF amplitude to the top and bottom coils then the combined transmit signal calibrated for the proper tip angle. 
     When the patient is moved into an imaging position in which the patient is not symmetrically disposed between the upper and lower RF coils, there is an imbalance in the loading on the two coils. In order to correct for the uneven loading, the amplitude of the RF envelope, which is sent to each of the upper and lower coils, is adjusted. 
     The relative position of the patient between the two RF coils can be determined in various ways. In one embodiment, patient loading is determined by reflected power. More specifically, when radio pulses are input into the Q port of the quadrature circuits  40 , an output signal appears on the fourth port {overscore (Q)}, of the quadrature hybrid. When the patient is centered, the reflected power at both coils is equal. When the patient is shifted toward one coil or the other, the reflected power also shifts. An RF monitor  60  measures the reflected power from each RF coil and provides an output indicative of the loading by the patient which addresses the calibration memory  58  to retrieve the appropriate gain factor for the adjustment circuit  54 . 
     In the embodiment of FIG. 2 the gain of two transmitters  38   u ,  38   l  is adjusted. In this embodiment, each transmitter has a separate amplitude driver dedicated to the purpose of adjusting its own signal. Thus, different RF amplitude pulses are transmitted to the two coils. In the embodiment of FIG. 3, a variable signal splitter  62  adjusts the relative amplitude of the RF pulses supplied to each coil. 
     In applications in which the radio frequency coils  32 ,  34  operate in both transmit and receive modes, magnetic resonance signals are picked up by the radio frequency coils  32 ,  34 . The resonance signals are demodulated by one or more receivers  48 , preferably digital receivers. In the illustrated embodiment, two receivers demodulate the quadrature signals from each quadrature combiner  40 . Alternately, The signals from each RF coil can be combined by the quadrature combiners before demodulation. The digitized signals are processed by a reconstruction processor  70  into volumetric or other image representations which are stored in a volumetric image memory  72  A video processor  74 , under operator control, withdraws selected image data from the volume memory and formats it into appropriate data for display on a human readable display  76 , such as a video monitor, active matrix monitor, liquid crystal display, or the like. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.