Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1998-09-11
2001-04-10
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06215307
ABSTRACT:
BACKGROUND
The present invention relates to the field of magnetic resonance imaging. It finds particular application in conjunction with radio frequency (r.f.) coils for use therein.
Magnetic resonance imaging (MRI) is a widely used diagnostic imaging method. MRI equipment includes a magnet apparatus for generating a very strong, homogeneous static magnetic field within an examination region. A so-called “open” magnet arrangement includes a pair of pole pieces disposed on opposite sides of the examination region. MRI systems also include an r.f. coil arrangement for exciting and detecting magnetic resonance in the examination region. In order to excite magnetic resonance, transmit coils are used to generate a rotating r.f. field.
A number of sometimes conflicting considerations influence r.f. coil design. In order to maximize the size of the examination region, it is desirable that the coils be as thin as possible. At the same time, it is desirable that the coils be as efficient as possible, so that for a given input power, the coils produce a relatively large r.f. field. It is also necessary that the coils be tuned to the r.f. excitation frequency, for example the Larmor frequency of hydrogen atoms influenced by the static magnetic field. In order to generate an r.f. field which rotates in a plane perpendicular to the main magnetic field, two r.f. coils rotated 90 degrees with respect to each other have been provided, with the coils driven in quadrature.
R.f. coils for so-called open magnet systems have included planar butterfly coils. Each half of a butterfly coil includes a relatively small number of conductor turns (e.g., two) configured to produce the desired r.f. field within the examination region. The coils are connected by circuit traces to a “bank” of discrete tuning capacitors. This arrangement has various drawbacks. First, there tends to be significant capacitive coupling between the rotated coils. This, together with the high voltage present on the coils, has a deleterious effect on coil efficiency. In addition, the circuit bridges associated with the discrete tuning capacitors tend to be narrow, thereby increasing stored magnetic energy which does not contribute to the useful magnetic field. Further, the tuning capacitors themselves must be of relatively high quality and also tend to be bulky. While a larger number of coil turns is desirable, increasing the number of turns tended to decrease the self-resonant frequency of the coil so as to be below the Larmor frequency. Radial current components associated with the coils have also led to undesirable components in the r.f. field.
A pair of coils has been associated with the upper pole piece and a corresponding pair associated with the lower pole piece. The coils nearest to the pole pieces have been oriented in a first angular position, and the coils nearest to the imaging region have been oriented at a second angular position rotated about the z-axis offset ninety degrees from the first. The coils nearest the pole piece have been driven to produce a first component of the rotating r.f. field, while the coils nearest the imaging region have been driven to produce a ninety degree offset field component. However, the efficiency of the coil pair nearer the pole pieces (and hence the r.f. screen or shield) has been lower than that of the pair nearer the examination region. As a consequence, the r.f. field at the center of the imaging region has had a undesirable linear component resulting from the difference in amplitude of the zero and ninety degree components.
The method and apparatus disclosed herein address these drawbacks, and others.
SUMMARY
According to a first aspect of the present invention, a magnetic resonance imaging apparatus includes a magnet including first and second pole pieces disposed on opposite sides of an examination region, means for generating time-varying gradient magnetic fields within the examination region, means for exciting magnetic resonance in an object located in the examination region, means for detecting magnetic resonance in the object, and means for generating an image indicative of the magnetic resonance signals detected by means for detecting. The means for exciting includes a coil for generating an r.f. field in the examination region. The coil includes a substrate having first and second major surfaces, a first coil segment disposed on the first major surface and a second coil segment disposed on the second major surface and connected electrically in series with the first coil segment. Overlapping portions of the first and second coil segments form a capacitor which provides the electrical connection therebetween.
According to a more limited aspect, the apparatus includes a first plurality of coil segments disposed on the first major surface and a second plurality of coil segments disposed on the second major surface. Overlapping portions of corresponding coil segments disposed on the first and second major surfaces form a capacitor which provides a series electrical connection therebetween.
According to a still more limited aspect of the present invention, the first and second plurality of coil segments form a generally spiral winding.
According to another more limited aspect, the winding is characterized by six turns.
According to another yet more limited aspect, the apparatus includes means for canceling radial current components.
According to another limited aspect of the present invention, the resonant frequency of the coil is equal to the frequency of an excitation signal applied to the coil.
According to another more limited aspect of the present invention, the width of the overlapping portion of the coil segments is equal to the width of the non-overlapping portion.
According to yet another limited aspect of the present invention, the apparatus further includes means for adjusting the area of the overlap between the first and second coil segments.
According to still another limited aspect of the present invention, the means for exciting includes a first coil disposed between the first pole piece and the examination region, a second coil disposed between the first coil and the examination region, a third coil disposed between the second pole piece and the examination region, and a fourth coil disposed between the third coil and the examination region. The first and fourth coils generate a first r.f. field component, and the second and third coils generate an r.f field component ninety degrees offset from the first r.f. field component.
According to a yet another limited aspect of the present invention, the substrate comprises woven fiberglass and PTFE.
According to another aspect of the present invention, a magnetic resonance examination apparatus includes open magnet means for generating a magnetic field in an examination region, means for imposing a series of time varying gradients on the main magnetic field, and means for detecting magnetic resonance in the examination region. The r.f. coil includes a plurality of capacitors and a plurality of coil segments connected electrically in series. The electrical connection between successive coil segments is provided by a capacitor.
According to a more limited aspect of the invention, the coil includes an insulating substrate having first and second sides. Successive ones of the coil segments are disposed on alternating first and second sides of the substrate. According to a still more limited aspect, the capacitors are formed by overlapping portions of successive ones of the coil segments. According to a still more limited aspect, the apparatus includes means for adjusting the capacitance of the capacitors.
According to another limited aspect of the invention, the coil is formed by etching a desired conductor pattern on a circuit board.
According to another more limited aspect, a coil segment on the first side of the substrate overlaps a coil segment on the second side of the substrate at a location where current flowing through each coil segment has a radial component and the voltage on each coil segment is zero. The respe
Fetzner Tiffany A.
Fry John J.
Gurin Timothy B.
Oda Christine
Picker Nordstar Oy
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