US 20060006865 A1
Apparatus and method for MRI imaging using a coil constructed of microstrip transmission line (MTL coil) are disclosed. In one method, a target is positioned to be imaged within the field of a main magnetic field of a magnet resonance imaging (MRI) system, a MTL coil is positioned proximate the target, and a MRI image is obtained using the main magnet and the MTL coil. In another embodiment, the MRI coil is used for spectroscopy. MRI imaging and spectroscopy coils are formed using microstrip transmission line. These MTL coils have the advantageous property of good performance while occupying a relatively small space, thus allowing MTL coils to be used inside restricted areas more easily than some other prior art coils. In addition, the MTL coils are relatively simple to construct of inexpensive components and thus relatively inexpensive compared to other designs. Further, the MTL coils of the present invention can be readily formed in a wide variety of coil configurations, and used in a wide variety of ways. Further, while the MTL coils of the present invention work well at high field strengths and frequencies, they also work at low frequencies and in low field strengths as well.
1. A method, comprising:
positioning a target to be imaged within the field of a main magnetic field of a magnet resonance imaging (MRI) system;
positioning at least one coil proximate the target wherein the coil is constructed using at least one microstrip transmission line;
imaging the target using the main magnet and the coil with MRI.
This application is a continuation application of U.S. patent application Ser. No. 09/974,184, filed Oct. 9, 2001, which is a continuation of provisional application Ser. No. 60/239,185, filed, Oct. 9, 2000, and entitled “Microstrip Resonator RF Surface and Volume Coils and Methods for NMR Imaging and Spectroscopy at High Fields.” The entire contents of U.S. application Ser. No. 60/239,185 are hereby incorporated herein by reference.
This invention was partially supported by NIH grants NS38070 (W.C.), NS39043 (W.C.), P41 RR08079 (a National Research Resource grant from NIH), Keck Foundation, National Foundation for Functional Brain Imaging and the US Department of Energy. The Government may have certain rights in the invention.
This invention pertains generally to magnetic resonance imaging (MRI) and more specifically to surface and volume coils for MRI imaging and spectroscopy procedures.
Surface and volume coils are used in MRI imaging or spectroscopy procedures in order to obtain more accurate or detailed images of tissue under investigation. Preferably, a MRI coil performs accurate imaging or spectroscopy across a wide range of resonant frequencies, is easy to use, and is affordable. Further, the operating volume inside the main magnet of many MRI systems is relatively small, often just large enough for a patient's head or body. As a result, there is typically little space available for a coil in addition to the patient. Accordingly, it is advantageous if a surface or volume coil itself occupies as little space as possible.
In high fields (3 Tesla and beyond), due to the high Larmour frequencies required, radiation losses of RF coils become significant which decreases a coil's quality factor or Q factor, and a low Q factor can result in low signal-to-noise ratio (SNR) in MRI procedures. One existing solution to reducing radiation losses is adding a RF shielding around the coil(s). The RF shielding, however, usually makes the physical size of RF coil much larger, which as noted above is not desired in the MR studies, especially in the case of high field operations.
According to certain example embodiments of the invention there are provided a MRI coil formed of microstrip transmission line. According to various embodiments of the invention, MRI coils according the present invention are easy to manufacture with relatively low cost components, and compact in design. In addition, the coil's distributed element design provides for operation at relatively high quality factors and frequencies and in high field (4 Tesla or more) environments. Further, microstrip coils according to the present invention exhibit relatively low radiation losses and require no RF shielding. As a result of not requiring RF shielding, the coils may be of compact size while having high operating frequencies for high field MR studies, thus saving space in the MRI machine. Further, the methods and apparatus of the present invention are not just good for high frequency MR studies, but also good for low frequency cases.
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only be the appended claims.
According to a first method embodiment of the invention, as illustrated in
The microstrip transmission line, according to one example design, is formed of a strip conductor, a ground plane and a dielectric material that may be air, a vacuum, low loss dielectric sheets such as Teflon or Duroid, or liquid Helium or liquid Nitrogen. Further, the strip conductor or ground plane are, in one embodiment, formed in whole or in part from a non-magnetic conductive material such as copper or silver. According to another example embodiment of the invention, the ground planes for multiple strip conductors are arranged in one single piece foil so as to reduce radiation loss.
In another example embodiment, the MTL coil is a volume MTL coil having a plurality of microstrip transmission lines. In still another example embodiment, the volume MTL coil is detuned using PIN diodes. In yet another example embodiment, the MTL coil includes bisected ground planes and the PIN diodes are positioned in the gap of the bisected ground planes.
According to still other example embodiments of the methods of the invention, a MTL coil is tuned by varying capacitive termination of the MTL coil wherein, for example but not by way of limitation, the MTL coil is tuned by varying capacitive termination on each end of the MTL coil.
In still other example embodiments of the method, the microstrip transmission line is arranged in a rectangular or circular configuration, or, in the alternative, in an S shape. In one advantageous embodiment, the MTL coil is constructed using at least two turns to improve the homogeneity of the magnetic field characteristics.
In still other example embodiments, one or more lumped elements are connected to the transmission line and operated so as to match the impedance of the line.
In yet still another embodiment, an MTL coil is operated in a resonant mode by bisection of the ground plane and tuning of the resonance by adjusting displacement of the ground planes. In another embodiment, at least two of the MTL coils are operated in a quadrature mode. In still another embodiment, a coil is arranged so as to operate as a ladder MTL coil. In yet another embodiment, at least two MTL coils are arranged and operated as a half volume MTL coil.
In still another example embodiment, an inverted imaging MTL coil is formed wherein the dielectric material is positioned in a plane on the side of the strip conductor plane in the direction of the field, and wherein coupling is capacitive.
In yet another example embodiment of the methods of the invention, the MTL coil is driven using a capacitive impedance matching network. In still another example embodiment of the methods of the invention, the dielectric constant Er is adjusted to change the resonant frequency of the MTL coil.
In yet still another example embodiment of the method, the coil dielectric substrate is flexible, and the MTL coil is formed and used in more than one configuration allowing a single coil to be adapted to multiple purposes. According to still another embodiment, the substrate is formed of thin layers of Teflon or other dielectric material allowing the substrate to be bent or twisted.
Referring first to
According to a first embodiment of the apparatus of the invention, as illustrated in
Further, the strip conductor or ground plane are, in one embodiment, formed in whole or in part from a non-conductive material such as copper or silver. As also illustrated, the strip conductor and ground plane, in this embodiment and others described below, is connected to a source of electrical excitation or RF detection circuitry, for example through a coax or other connector (not shown). According to still another example embodiment, because corners of the coil tend to radiate surface waves and thus have a potential to cause hot spots in images and degrade the Q value of coils, the corners may be chamfered to reduce the radiation loss and improve B1 distribution. According to another example embodiment 30 of the invention as illustrated in
In still other configurations, the coils may assume an “S” shape, as may be advantageously used for example in a volume coil design, or any other arbitrary shape. Further, as illustrated in
In another example embodiment as illustrated in
In still another example embodiment illustrated in
According to still other example embodiments of the apparatus of the invention illustrated in
In yet still another embodiment illustrated in
In another example embodiment shown in
According to still another embodiment, for the individual microstrip resonant element, the resonant frequency can be modified by choosing appropriate dielectric substrate with different relative dielectric constant. Therefore, doubly tuned frequency operation can be easily achieved by making two different resonant frequencies for the microstrip elements in the volume coil, alternatively. Namely, one set of microstrip resonant elements with even numbers can be set to one resonance frequency while another set of microstrip resonant elements with odd numbers set to a different resonance frequency. Multiple tuned RF coils also can be designed using the same approach. Each resonance can be quadraturely driven with an appropriate quadrature hybrid.
In still another example embodiment shown in
Still another example embodiment 120 of the invention is illustrated in
In yet still another example embodiment of the apparatus shown in
Referring now to
According to still yet another example embodiment, the MTL coil is formed as a dome-shaped coil which offers an increased filling factor and a great sensitivity and homogeneity in the top area of the human head. By applying the microstrip resonator volume coil technique, the dome-shaped coil can be constructed for higher field applications.
According to still another embodiment of the invention, the unbalanced circuit of the microstrip coil provides that there is no need to use the balun circuit commonly used in surface coils and balanced volume coils to stabilize the coil's resonance and diminish the so-called ‘cable resonance’.
Thus, there has been described above method and apparatus for forming MRI imaging and spectroscopy coils using microstrip transmission line. Due to its specific semi-open transmission line structure, substantial electromagnetic energy is stored in the dielectric material between the thin conductor and the ground plane, which results in a reduced radiation loss and a reduced perturbation of sample loading to the RF coil, compared to conventional surface coils. The MTL coils of the present invention are also characterized by a high Q factor, no RF shielding, small physical coil size, lower cost and easy fabrication. These MTL coils have the advantageous property of good performance while occupying a relatively small space, thus allowing MTL coils to be used inside restricted areas more easily than some other prior art coils. Further, the MTL coils of the present invention can be readily formed in a wide variety of coil configurations, and used in a wide variety of ways. Further, while the MTL coils of the present invention work well at high field strengths and frequencies, they also work at low frequencies and in low field strengths as well.
Further information concerning the design, operation and theory of MTL coils is found in Zhang, X. et al., “Microstrip RF Surface Coil Design for Extremely High-Field MRI and Spectroscopy”, Magn. Reson. Med. 2001 September; 46(3):443-50 and Zhang X. et al., “A Novel RF Volume Coil Design Using Microstrip Resonator for NMR Imaging and Spectroscopy”, submitted for publication. The entire contents of both of the aforementioned papers are incorporated herein by reference.