Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-11-29
2001-10-09
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C600S421000
Reexamination Certificate
active
06300761
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an antenna array for magnetic resonance examinations of the type having array elements that are decoupled from each other and are independent, and which are arranged in two adjacent rows.
2. Description of the Prior Art
An antenna array of this type is described in an article by R. Jones with the title “Twelve Antenna Element Lower Extremity/Pelvic Array for MRI (A)”, published in the conference volume for the ISMRM 1998 on page 440, “Proceedings of the International Society for Magnetic Resonance in Medicine, 6
th
Scientific Meeting and Exhibition, May 1998, Sidney, Australia.” The antenna array is fashioned as a bilateral extremity array for the examination of the entire vessel system below the aorta bifurcation. In two rows, four saddle coil pairs are adjacently arranged with one coil per pair per row, which cover the normal length of the lower extremities of an adult. For decoupling, apart from a strictly symmetrical construction of the two rows, the saddle coil pairs of one row are arranged so as to be rotated 90° relative to the other row.
The article by K. Y. Koyima et al.: “Lower Extremities: MR Angiography with a Unilateral Telescopic Phased-Array Coil” published in Radiology, vol. 196, No. 3, page 871 through 875, 1995 describes a unilateral telescopic phased-array-antenna with six saddle coils, which are sensitive to a linearly polarized field vector. The upper saddle coils are fashioned larger than the lower ones in order to achieve a good filling factor.
Not only the bilateral antenna array but also the unilateral antenna array has array elements with a linear antenna characteristic; this means that the primary sensitivity direction of the array elements extends along one single axis.
U.S. Pat. No. 5,430,378 discloses an antenna array with circularly polarizing array elements. Each array element is formed by two ring coils that are oriented perpendicularly to one another, whereby one ring coil is mounted in a base plate and the other ring coil is mounted on a carrier that is perpendicularly and symmetrically arranged relative to the base plate. The antenna array is fashioned for the examination of the lower extremities, the sensitivity area of the two ring coils of the individual antenna elements covering both extremities. Coupling ensues by a strictly symmetrical arrangement of the ring coils and by a partial overlap of neighboring ring coils.
The antenna array described in U.S. Pat. No. 5,548,218 also has circularly polarizing array elements that are fashioned for MR examinations of the lower extremities. Each array elements is formed by a ring coil and a butterfly coil. The middle area of the array elements is installed in a patient bed, whereas the outer areas are flexibly fashioned. This allows the outer elements to be “unfolded” away in order to position the patient. After the positioning, the outer areas are put on the patient, so that the entire lower extremities can be imagined with a high filling factor of the antenna. Here, the sensitivity area of the circularly polarizing array elements also encompassed both extremities.
The antenna array described in the U.S. Pat. No. 5,594,337 also has array elements whose sensitivity area encompasses both extremities.
Two decoupling modes are described in PCT Application WO 89/05115. The first mode is to arrange neighboring antenna elements in a partially overlapping manner. The second one transforms the input impedance of a connected pre-amplifier in a high-resistance manner.
U.S. Pat. No. 5,708,361 describes a way to decouple neighboring antenna elements, which does not require a specific geometrical arrangement of the array elements to be decoupled. The conductors of the array elements to be decoupled have an interruption (gap), and are connected in parallel to one another. The interruptions are short-circuited with at least one capacitor, with the decoupling being effected by a selection of the capacitance.
The antenna arrays described above enable a continuous display of the blood vessels in the lower extremities without repositioning the patient relative to the array. In comparison to the imaging of the blood vessels with a whole body resonator, a higher signal quality is therewith achieved. It is still desirable, however, to further increase the signal quality and therewith the signal-to-noise ratio in order to be able to also image finer blood vessels.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an antenna array that has an improved signal-to-noise ratio for the examination of the lower extremities.
This object is achieved in an antenna array having array elements each formed by two antenna elements whose respective sensitivity axes reside perpendicular to one another. During the application, only one leg is situated in the sensitivity area of each array element, as a result of which noise produced the leg that is not being examined at that moment can no longer superimpose on the useful signal. In addition, each array element has a circularly polarized antenna characteristic, as a result of which the signal-to-noise ratio is further improved.
Coupling of the antenna elements of the first row, which neighbor the antenna elements of the second row, are suppressed by a decoupling circuit. The decoupling circuit is connected to the neighboring antenna elements to be decoupled. Therewith, by means of magnetic resonance imaging, a continuous display of the blood vessels in the lower extremities with a high signal-to-noise ratio is obtained without having to reposition the patient relative to the array.
In an embodiment, neighboring parts of the array elements of the two rows are fastened by a bar that is arranged between the rows. With the patient in a recumbent position, the bar is placed upon the patient bed between the legs. The antenna elements of the two rows that are opposite to one another thereby remain in a fixed spacial position relative to one another. The coupling of the two antenna elements that is determined by the geometrical arrangement is compensated by the decoupling circuit. Proceeding away from the bar, the individual array elements are curved, so that they are shaped in a U-shaped manner in a plane perpendicular to the direction of the magnetic basic field of the magnetic imaging apparatus and so that they cover the legs.
In a further embodiment, the parts of the array elements projecting from the bar in the transverse direction are flexibly fashioned. The two rows of array elements thus can be adapted to examination areas of different sizes.
Given examination of the lower extremities, a particularly high filling factor results in further embodiment, wherein a first group of the array elements is more strongly curved and surrounds a smaller examination space than a second group of the array elements. Therewith, the antenna array is well-adapted to the different dimensions of thigh and lower leg.
In another embodiment the array elements form a part of a cover or “hood”, that is open at the bottom, for a patient bed. Due to the construction that is open at the bottom, the antenna array can be combined with many support aids for pressure-free support of knees and heels. In contrast to known antenna arrays, the imaging of a moderately angled leg supported in the knee is also possible. Many times, patients with an advanced occlusive disease of the lower extremities can only stretch the legs fully with difficulty and need careful support in order to minimize the danger of pain and therewith motion artefacts during the magnetic resonance examination.
In further embodiment, a pre-amplifier that is arranged in the middle bar is connected to the respective antenna elements. Particularly short signal lines to the pre-amplifier result. This allows the pre-amplifier input impedance to be matched to the antenna elements in an almost ideally high-resistance manner.
REFERENCES:
patent: 5389880 (1995-02-01), Mori
patent: 5430378 (1995-07-01), Jones
patent: 5548218 (1996-08-01), Lu
patent
Hagen Juergen
Kreischer Ludwig
Reykowski Arne
Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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