Abstract:
A head coil for a magnetic resonance apparatus has a supporting body that carries a number of antenna elements. The supporting body has an end section that is shaped as a spherical cap. A butterfly antenna is mounted at the end section, and is annularly surrounded by at least one group antenna that overlaps the butterfly antenna.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a head coil provided as a radio-frequency antenna for a magnetic resonance apparatus, the head coil being configured to be arranged around the head of a patient to be examined with a magnetic resonance apparatus. 
     2. Description of the Prior Art 
     A head coil (also known as a head array antenna) for a magnetic resonance tomography apparatus is known from DE 195 05 062 A1, for example. An arrangement of sub-antennas is located on the generated surface of a support body of the known head coil, wherein the generated surface can represent an inner or outer surface of a hollow cylindrical segment. The known head coil furthermore has an end segment that can be fashioned as a spherical cap, such that the head array antenna approximates a helmet in its shape. Just as on the generated surface, butterfly antennas can be arranged on the end segment. Individual sub-antennas on the generated surface are formed by a loop antenna and a butterfly antenna. In contrast to this, an auxiliary antenna arranged on the end segment is formed as a circularly polarized antenna made up of two butterfly antennas arranged crossed relative to one another. The imaging region should be homogenized via a partial, mutual overlapping of the butterfly antennas. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a head coil that is suitable for magnetic resonance tomography that has both a good adaptation to the anatomy of the human head and advantageous electrical properties, in particular with regard to the achievable signal-to-noise ratio. 
     This object according to the invention is achieved according to the invention by a head coil having a support body carrying a number of antenna elements, the support body having an end segment fashioned as a spherical cap on which a butterfly antenna is arranged. The butterfly antenna is annularly surrounded by a plurality of loop antennas arranged overlapping with this. 
     As used herein, the shape of a spherical cap is not only a shape that is identical in the strictest geometric sense to a segment of a spherical surface, but also encompasses spherical cap-like geometries that are adapted at least approximately to the calvarium of a patient to be examined with a magnetic resonance apparatus. 
     In an embodiment that affords a particularly simple, patient-friendly handling, the support body of the head coil is composed of a lower shell and an upper shell. At the same time, an inductive geometric decoupling can be achieved in that an antenna element arranged on the upper shell overlaps with an antenna element arranged on the lower shell. 
     Independent of whether the support body is designed as one shell or multiple shells (in particular two shells), advantageous physical properties resulting in particular in a good signal-to-noise ratio can be achieved in that two respective adjacent loop antennas which as a whole annularly surround the butterfly antenna overlap. 
     Surprisingly, it has been shown that the arrangement of only a single butterfly antenna in the region of the end segment (i.e. in the region of the spherical cap of the head coil) is particularly advantageous. 
     According to a preferred embodiment, a cylindrical support segment connects to the end segment of the support body that is fashioned as a spherical cap. The cylindrical support segment of the head coil is thus facing towards the neck and the body of the patient. In a further embodiment, the support body has two eye openings, in particular in the region of the cylindrical support segment. The patient can thus look out from the support body through the eye openings. It is therefore possible to expose the patient to specific visual stimuli during a magnetic resonance examination and to detect activity of the brain stimulated by these stimuli. Each eye opening is advantageously completely surrounded by at least one antenna element. At the same time, in a preferred geometry design of the head coil a nose cutout is fashioned in the support body between the eye openings. The individual antenna elements in the region of the view of the patient can therefore be positioned near to the skull. 
     The butterfly antenna advantageously has the following geometric features, which can be realized individually or in different combinations: 
     Electrical conductors (in particular conductor traces) of the butterfly antenna describe two arc segments fashioned so as to be mirror-symmetrical to one another. These are preferably arranged (at least in one projection) on the circumference of one and the same circle. Conductor segments of lesser curvature (advantageously uncurved linear segments of the butterfly antenna) connect to the ends of the arc segments. In the projection view, the linear segments describe an intersection point that coincides with the center point of the circle described by the arc segments. An angle of at least 15° (preferably between 30° and 60° and most preferably 45°) is enclosed between the intersecting linear segments. 
     The advantage of the invention in particular lies in that a head coil provided for magnetic resonance tomography and acting as a radio-frequency antenna is adapted well to the anatomy of the human head so that the individual antenna elements are only slightly distanced from the head. An end piece of the head coil that covers the calvarium of the patient, in which end piece are integrated a central butterfly antenna as well as multiple loop antennas annularly surrounding said butterfly antenna overall, provides a significant contribution to a high spatial resolution and an advantageous signal-to-noise ratio, wherein every loop antenna forms overlaps both with two adjacent loop antennas in the circumferential direction and with the central butterfly antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a magnetic resonance tomography apparatus with a head coil employed as a local coil. 
         FIG. 2  shows the head coil in accordance with the invention, of the magnetic resonance tomography apparatus of  FIG. 1 , in cross-section. 
         FIG. 3  is a plan view of the head coil of  FIG. 2 . 
         FIG. 4  illustrates the antenna arrangement of the head coil of  FIGS. 2 and 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows in a rough schematic a magnetic resonance tomography apparatus  1 ; reference with regard to its basic function is made by way of example to DE 102 11 567 C1. A patient  3  positioned on a patient bed  2  is examined with the magnetic resonance tomography apparatus  1 . 
     Electromagnets arranged in housing parts  4 ,  5  of the magnetic resonance tomography apparatus  1 , namely a basic magnet provided to generate a homogeneous basic magnetic field and a gradient magnet system, are not shown in  FIG. 1 . 
     In contrast to this, a head coil  6  acting as a high channel coil is recognizable in longitudinal section in  FIG. 1 , in which head coil  6  the head of the patient  3  is located. Details of the head coil  6  are described in the following using  FIGS. 2 through 4 . 
     A coordinate system with axes x, y, z is drawn in  FIG. 2 , which shows a cross section through the head coil  6 . As can be seen from  FIG. 2 , the head coil  6  is composed of two half-shells  7 ,  8 , namely of a lower shell  7  arranged on the patient bed  2  and an upper shell  8  connected with said lower shell  7  such that it can be detached. Lower shell  7  and upper shell  8  together form a support body  9  of the head coil  6 , wherein the lower shell  7  overlaps with the upper shell  8 . 
     Antenna elements  10  which likewise overlap in the region of the overlap between the lower shell  7  and the upper shell  8  are located both on said lower shell  7  and said upper shell  8 . In order to clarify the overlap between the antenna elements  10 , a representation that is in particular distorted in the x-direction is selected in  FIG. 2 . An inductive overlap is formed that electrically decouples the antenna elements  10  situated next to one another on the different half-shells  7 ,  8  in a manner that enables the acquisition of image data with a high resolution. The fact that no electrical connections exist between the lower shell  7  and the upper shell  8  to decouple the antenna elements  10  arranged on the different half-shells  7 ,  8  is particularly advantageous. 
     The section shown in  FIG. 2  shows an essentially cylindrical segment of the support body  9  that is designated as a support segment  11 . The term “support segment” does not imply any attribute as to the static function of this segment of the support body  9  in comparison to other parts of the support body  9 . As is recognizable in  FIGS. 1 and 3 , an end segment  12  of the support body  9  that is fashioned as a spherical cap attaches to the support segment  11  facing towards the body of the patient  3 . Like the support segment  11 , the end segment  12  is formed partially from the lower shell  7  and partially from the upper shell  8 . 
     Only a few antenna elements  10  are shown in  FIG. 3 , primarily in the region of the support segment  11 . Located in this segment are two eye openings  13  and a nose cutout  14  in the upper shell  8 . The eye openings  13  on the one hand have the purpose of enabling the patient to look out from the head coil  6 ; on the other hand, thanks to the eye openings  13  it is also possible to expose the patient to targeted optical stimuli, as is principally known from DE 101 18 472 C1, for example. The eye openings  13  according to the exemplary embodiment shown in  FIG. 3  are respectively completely surrounded by antenna elements  10 . The nose cutout  14  ensures that these antenna elements  10  can be arranged particularly close to the face of the patient  3 . 
     Details of the end segment  12  can be recognized in  FIG. 4 , which shows a projection in the z-direction. A central antenna element of the essentially spherically curved end segment  12  (which is well adapted to the shape of the calvarium of the patient  3 ) is a butterfly antenna  15  that is overall annularly surrounded by (in the exemplary embodiment six) loop antennas  16 . Contours of the half-shells  7 ,  8  are not shown in  FIG. 4  for clarity. 
     Each of the annular loop antennas  16  overlaps both with the two at least approximately similar loop antennas  16  that are adjacent in the circumferential direction and with the butterfly antenna  15 . 
     The butterfly antenna  15  has the basic shape of a circle and has two arc segments  17 ,  18  that are fashioned so as to be mirror-symmetrical to one another, which arc segments  17 ,  18  are arranged on the circumference of one and the same circle. Linear segments  19 ,  20  adjoin the ends of the arc segments  17 ,  18 , which linear segments  19 ,  20  form an X in the shown projection and enclose an angle α of less than 45° between them. The antenna geometry according to  FIG. 4  enables information to be acquired with a good signal-to-noise ratio (signal-to-noise), even in a region of the end segment  12  which is arranged essentially orthogonal to the z-axis. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.