US20060006865A1

US20060006865A1 - Method and apparatus for magnetic resonance imaging and spectroscopy using microstrip transmission line coils

Info

Publication number: US20060006865A1
Application number: US11/224,436
Authority: US
Inventors: Xiaoliang Zhang; Kamil Ugurbil; Wei Chen
Original assignee: University of Minnesota
Current assignee: University of Minnesota
Priority date: 2000-10-09
Filing date: 2005-09-12
Publication date: 2006-01-12
Also published as: AU2001296738A1; US7023209B2; JP2005270674A; US20020079996A1; WO2002031522A1; US20060277749A1; JP2004511278A; EP1344076A1

Abstract

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.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

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.

GOVERNMENT RIGHTS

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.

TECHNICAL FIELD OF 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.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a method according to one example embodiment of the invention.
FIGS. 2-14 illustrate various example embodiments of the apparatus of the invention.

DETAILED DESCRIPTION OF THE INVENTION

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.

Method Embodiments

According to a first method embodiment of the invention, as illustrated in FIG. 1, a target is positioned within the field of a main magnetic field of a magnet resonance imaging (MRI) system, at least one coil is positioned proximate the target wherein the coil is constructed using at least one microstrip transmission line, and the main magnet and the MTL coil are used to obtain MRI images from the target. According to one use of the microstrip transmission line (MTL) coil, it is operated as a receiver (pickup coil) or a transmitter (excitation coil) or both during an imaging procedure. As used herein the term “MTL coil” generally refers to any coil formed using a microstrip transmission line.
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.

Apparatus Embodiments

Referring first to FIGS. 2A and 2B, there is illustrated in diagrammatic form an example embodiment of a microstrip transmission line (MTL) 20 having a strip conductor 21 with a width W and ground plane 22, on either side of a dielectric substrate 23 having a height H and dielectric coefficient Er. Magnetic field lines H are shown surrounding strip conductor 21 and emanating outward in the Y direction orthogonal to ground plane 22 and along the length of strip conductor 21. As illustrated, the field is contained in whole or in part on one side of the strip conductor 21 by the ground plane 22, and extends outwardly beyond the plane of the strip conductor in a direction extending away from the ground plane.
According to a first embodiment of the apparatus of the invention, as illustrated in FIGS. 2C and 2D, there is provided a single turn MRI imaging or spectroscopy MTL coil 23 constructed using at least one microstrip transmission line. The microstrip transmission line coil, according to one example design, is formed of a strip conductor 24, a ground plane 25 and a substrate 26 made of a dielectric material that may be air, a vacuum, a single or multilayer low loss dielectric sheets such as Teflon or Duroid materials, or liquid Helium or Nitrogen. According to one example embodiment, such coil is 9 cm×9 cm, has a substrate 26 that is 5-7 mm, uses copper foil 36 microns in thickness (for example an adhesive-backed copper tape such as is available from 3M Corporation of St. Paul, Minn.) for the strip conductors and ground plane, and has a resonant frequency of 300 MHz. According to one example embodiment, the MRI signal intensity is proportional to H when H<5 mm and reaches a maximum when H 5 mm. These results indicate that the optimized H value is about 5-7 mm for the above embodiments of the microstrip MTL coils according to the present invention. Further, the dielectric material thickness H, or more accurately, the ratio W/H, is an important parameter that affects the B1 penetration in air. If H is too small, or W/H too large, most of electromagnetic fields will be compressed around the strip conductor. Although the B1 penetration will increase with the increase of dielectric material thickness H, or the decrease of the ratio W/H, a thickness of 5-7 mm is suggested in practice because the radiation loss can become significant when the substrate is much thicker. This optimized H makes it possible to build a very thin surface coil at extremely high fields, where the coil thickness can, in certain circumstances, be less than the conventional surface coil with RF shielding.
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 FIGS. 3A and 3B, a two turn coil is illustrated. As shown, a single ground plane 32 is shared by the strip conductors 34, wherein the ground planes are formed for example with a single sheet of foil so as to reduce radiation loss. FIGS. 3C and 3D illustrate additional embodiments 35 and 36 wherein embodiment 35 has one turn and embodiment 36 has two turns 102 to improve the homogeneity of the magnetic field characteristics.
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 FIGS. 2C and 3A, one or more elements 27 and 31, respectively, are connected to the transmission line so as to match the impedance of the line.
In another example embodiment as illustrated in FIGS. 4A and 4B, the MTL coil 42 is a half-volume coil having a plurality of microstrip transmission lines each having a ground plane 46 and strip conductor 48.
In still another example embodiment illustrated in FIGS. 5A and 5B, the MTL coil 50 includes a bisected ground plane 52. In this configuration, tuning of the resonance frequency is accomplished by adjusting displacement 54 of at least one of the ground planes. As illustrated in FIG. 6, in yet another embodiment 60, PIN diodes 64 are positioned in the gap 66 of the bisected ground planes 62, and used to detune the coil.
According to still other example embodiments of the apparatus of the invention illustrated in FIG. 7A, a MTL coil 70 is tuned by varying capacitive termination elements 72 on one end of the coil. FIG. 7B illustrates a hypothetical plot of magnetic field profile vs. capacitive termination value for a range of capacitances. As illustrated, increasing capacitive termination raises the magnetic field profile at the end of the coil at which the termination is applied. FIGS. 7C and 7D illustrate an example embodiment and field profile for tuning a MTL coil 74 by varying capacitive termination on each end 75 of the coil. Further, fine tuning can also be accomplished by slightly changing the length of the strip conductor.
In yet still another embodiment illustrated in FIG. 8A, at least two of the MTL coils 80 are arranged to be operated in a quadrature mode. The equivalent electrical circuit for MTL coil 80 is illustrated in FIG. 8B. In this example schematic, Z0 is the characteristic impedance of each microstrip element. In the impendence jX, jX1, jX2, jX15, X, X1, X2, X15 are positive real numbers. For the mode 1, the current on each microstrip resonant element is modulated by a cosine function cos(npie/8) where n=0, 1, 2, . . . , 15. L denotes the length of the volume coil 80.
In another example embodiment shown in FIG. 9, an MTL coil 90 is formed as arranged and operated as a ladder MTL coil. In yet another embodiment illustrated in schematic form in FIGS. 10A and 10B, a volume coil 100 is provided. Coil 100 includes ground planes 102 on the outside of a cylinder of dielectric material (for example Acrylic) having a diameter of 260 mm, a length of 210 mm, and a material thickness 104 of 6.35 mm. Strip conductors 106 are placed on the inside of the coil 100 running parallel to the axis. Coaxial connectors 108 are provided to connect the ground planes and strip conductors to a source of electrical excitation or RF detectors, as is conventionally done in use of a MRI volume or surface coil. According to one example embodiment of the apparatus, the high permittivity of the human head, the dielectric resonance effect results in higher signal intensity in the central region of the image. This higher intensity can be taken into account in the design of a large volume coil at high fields. In one example embodiment, in order to achieve a relatively uniform MR image in the human head, an inhomogeneous B1 distribution in the transaxial plane in free space is intentionally designed to compensate for the dielectric resonance effect in the human head.
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 FIG. 11, an inverted MTL coil 110 is illustrated, wherein is coupling is capacitive adjacent microstrip elements to provide lower resonant frequency operation.
Still another example embodiment 120 of the invention is illustrated in FIG. 12, wherein the strip conductors 122 have ‘T’ shaped ends 124 and coupling gap 126 between tips 128 of the ends are adjusted to change the current and E field at the end of the coil, and thus allow the operating frequency to be raised.
In yet still another example embodiment of the apparatus shown in FIG. 13, the MTL coil 130 substrate dielectric is formed of one or more relatively thin flexible layers 132 so that the coil may be bent or twisted or otherwise formed. Such layers may be formed of Teflon, for example. According to this embodiment, the coil 130 may be bent or formed into a first configuration, and thereafter formed into a second or third or more different configurations, wherein the coil may be used in more than one configuration and thus have a multipurpose nature.
Referring now to FIG. 14, there is illustrated a photograph of yet one more example embodiment of a volume coil 140 according to the present invention.
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.

Claims

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.