Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-07-12
2003-11-18
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06650118
ABSTRACT:
BACKGROUND
The invention relates to an RF coil system for an open MR apparatus, which system includes a plurality of resonator elements which are arranged at the area of a patient positioned in the examination volume of an MR apparatus, the resonator elements having at least one conductor element and at least one capacitor.
During MR imaging the nuclear magnetization in the examination volume is localized by means of temporally varying, spatially inhomogeneous magnetic fields (magnetic field gradients) which are superposed on a steady, as homogeneous as possible magnetic field. For the formation of images the magnetic resonance signal in the form of a voltage induced in the RF coil system of the MR apparatus is picked up under the influence of a suitable sequence of RF and gradient pulses in the time domain. The actual image reconstruction is then performed by Fourier transformation of the time signals. The sampling of the reciprocal k space is defined by the number, the distance in time, the duration and the strength of the gradient pulses used; this k space defines the volume to be imaged (FOV or field of view) as well as the image resolution. The number of phase encoding steps, and hence the duration of the imaging sequence, is defined by the requirements imposed on the image format and the image resolution. Contemporary MR apparatus aim to form images with an as high as possible quality and in an as short as possible period of time. Consequently, special requirements are imposed on the gradient system and the RF system of the MR apparatus.
In conventional MR apparatus the steady magnetic field, the RF fields and the magnetic field gradients are generated by essentially cylindrical coils which fully enclose the patient to be examined. In most cases cylindrical coil systems are also used for the detection of the MR signals. The RF system of customary MR apparatus includes a transmission and receiving coil such as, for example, an integrated body coil which can be used for volume imaging of the examination volume. The body coils used for the excitation as well as the detection of MR signals customarily are so-called birdcage resonators (birdcage coils). Such coils have a plurality of conductor rods which are arranged around the examination volume and extend parallel to the direction of the main field, said conductor rods being connected to one another via loop conductors at the extremities of the coil. The resonance behavior of the body coil is governed by capacitor elements which interconnect the conductor elements so as to form a network.
The steady magnetic field in a conventional MR system normally extends horizontally and parallel to the longitudinal axis of the cylindrical coil system. Granted, the cylindrical configuration enables high field strengths to be generated with a field distribution that can be suitably controlled. However, such a configuration has the drawback that the access to the examination volume and the patient present therein is strongly impeded. Because of the poor accessibility of the patient, the cylindrical geometry severely impedes or even inhibits interventional examinations. The narrow closed cylindrical tube of the MR apparatus, moreover, often gives rise to claustrophobic reactions of the patient.
For the above reasons so-called open MR apparatus have recently been developed; the examination volume therein is suitably accessible both for the patient and for a physician performing an examination. Some of the open MR apparatus utilize two disc-shaped magnet elements which are arranged at opposite sides of the examination volume and produce a vertically directed, steady magnetic field. The coils for generating the magnetic field gradients and the RF coils in such a system are customarily also constructed so as to be flat and disc-shaped, said coils being integrated in the pole faces of the main field magnet.
The flat construction of the RF coil system in open MR apparatus has the drawback that it is difficult to generate a suitably homogeneous RF field in the examination volume. Particularly strong inhomogeneities occur in the vicinity of the pole faces in which the conductor elements of the RF coils are integrated. The same holds for the sensitivity during the detection of the MR signals as this sensitivity disadvantageously also strongly dependent on the distance from the pole faces of the magnet in the open systems. A further problem resides in the fact, unlike in the conventional systems, no RF shielding is provided laterally around the examination volume in open MR apparatus. Therefore, the RF field strength decreases only slowly outside the examination volume and the spatial sensitivity profile during the detection also extends into areas far outside the examination volume. Consequently, outside the region of interest there are generated MR signals which, because of aliasing effects, become visible as undesirable artifacts in the selected FOV of the image formed.
Furthermore, flat RF coils in open MR apparatus are substantially more ineffective for the generating of RF fields than cylindrical coils. Therefore, open systems require a significantly higher transmission power in comparison with the conventional closed systems. Apart from the fact that stronger and hence more expensive power amplifiers are required, the level of the specific RF load whereto the patient to be examined is exposed is significantly higher than in customary MR apparatus. Because the conductor elements of the coil system at the area of the pole faces of the magnet are arranged very near to the body of the patient, the high RF field strengths may cause hazardous injuries (burnings) due to the absorption of the field energy in the body tissue. In order to avoid such risks, the transmission power must be limited in open MR systems. However, such limiting has an adverse effect on the imaging quality.
An alternative for the RF coils integrated in the pole faces of the main field magnet is the use of surface coils or systems with several surface coils, for example, as known from WO 99/27381. In conformity with the cited document, a plurality of surface coils, arranged notably at the area of the extremities of the patient to be examined, can operate in parallel in order to combine the respective detected MR signals so as to form an overall image. It is particularly advantageous that the limited spatial sensitivity range of the surface coils yields a larger signal-to-noise ratio. The combining of several surface coils for imaging is also known as SYNERGY.
However, the known arrangements of surface coils cannot be simply used in open MR apparatus. On the one hand, it should be noted that the orientation of the surface coils relative to the steady magnetic field has a decisive effect on the sensitivity upon detection. As has already been stated, the steady magnetic field is directed vertically in open MR systems. In this case surface coils which are arranged in a flat position on or underneath the patient have a minimum sensitivity only, because the nuclear magnetization which performs a precessional motion perpendicularly to the steady magnetic induces practically no voltage in such a coil system. However, surface coils which have each time a vertically oriented coil plane and are arranged to the side of the patient will be suitable. Unfortunately, however, it is a drawback that the sensitivity of surface coils decreases strongly in the direction from the coil plane to the volume to be examined. The high sensitivity inside the coil is not used.
Birdcage coils which are arranged around the patient are not suitable either for open MR apparatus. This is because these coils produce a circularly polarized RF field in a vertical plane. Because of the vertically oriented steady magnetic field, however, a horizontally oriented RF field which extends perpendicularly thereto is required.
SUMMARY
Considering the foregoing, the present invention has for its object to provide an RF coil system for open MR apparatus in which the RF field generated in the transmission mode and the spatial
Arana Louis
Lundin, Esq. Thomas M.
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