WO2008064231A1

WO2008064231A1 - Magnet for magnetic resonance imaging

Info

Publication number: WO2008064231A1
Application number: PCT/US2007/085251
Authority: WO
Inventors: Michael Garwood
Original assignee: Steady State Imaging
Priority date: 2006-11-21
Filing date: 2007-11-20
Publication date: 2008-05-29

Abstract

A relatively inexpensive magnet for use in magnetic resonance imaging is provided. In one embodiment, the magnet is relatively inhomogenous. The magnet may be used, for example, with a method of magnetic resonance that uses a frequency swept excitation to sequentially excite isochromats having different resonant frequencies and acquisition of the time domain signal is done during the duration of the sweeping frequency excitation.

Description

FREQUENCY SWEPT EXCITATION FOR MAGNETIC RESONANCE

CROSS REFERENCE TO RELATED APPLICATION^) This application claims priority to U.S. Patent Application No. 60/866,777, filed November 21, 2006, the contents of which is incorporated in it's entirety herein by reference.

FIELD A magnet for magnetic resonance is provided. More specifically, a relatively inexpensive magnet such as a relatively inhomogeneous magnet is provided.

BACKGROUND Magnetic resonance techniques are applied in nuclear magnetic resonance

(NMR) experiments and magnetic resonance imaging (MRI). hi magnetic resonance techniques, the response of nuclear spins in a magnetic field is observed following excitation by a radio frequency (RF) field. There are three basic types of the RF excitation: sequential, simultaneous and random. Currently available techniques for both NMR and MRI using these types of RF excitation have limitations.

Several different NMR techniques have been developed including: continuous wave (CW), pulsed, stochastic, and rapid scan correlation spectroscopy. MRI has technical requirements beyond those of NMR. Because the objects of interest (such as the brain or breast) is usually much larger than a test tube, the static and RF fields used in MRI are more inhomogeneous than those used in high resolution NMR. For applications on living systems, the need to avoid tissue heating places limits on the intensity and duty cycle of the RF irradiation that can be safely used, also known as the specific absorption rate (SAR) limitations. Similarly, when used in vivo, the subject must remain generally stationary during the MRI. Thus, slow techniques are difficult in clinical applications. Several MRI techniques have been developed including CW MRI, pulsed FT MRI, including, Ultra-short Echo Time (UTE) MRI, and stochastic MRI. Additionally, rapid scan techniques have been considered for imaging using electron paramagnetic resonance (EPR). There are three major field sources in MRI: the magnet, the gradient coil, and the RF coil. In theory, all these sources should deliver perfectly linear fields in the imaging region. However, due to engineering feasibility, cost factors, and physical constraints, these fields generally are non-linear. The non-linearity of the fields causes artifacts within an image. Imaging in inhomogeneous magnetic fields in MRI and generating accurate images can be challenging.

MRI works by exposing the subject to a strong magnetic field and measuring the responsive magnetic signals output by the subject. Some elements, including hydrogen atoms inside water and body fat, respond to a strong magnetic field by lining up with it. The magnetic field needs to be very strong to cause the elements to respond. Once the responding atoms (e.g., hydrogen atoms) have lined up, they create their own magnetic signal. Because hydrogen atoms in different tissues have slightly different signals, the MRI scanner measures those differences, detecting contrast in an image. For this second step measuring the difference between, for example, a hydrogen atom inside a tumor and one inside muscle the magnetic field generally must be extremely precise. The field generally does not vary by more than one ten-thousandth of a percent.

Inhomogeneity of the static magnetic field, produced by the MRI scanner as well as by object susceptibility, is largely unavoidable in MRI. The large value of gyromagnetic coefficient causes a significant frequency shift even for few parts per million field inhomogeneity, which in turn causes distortions in both geometry and intensity of the MR images. Generally, manufacturers try to make the magnetic field as homogeneous as possible, especially at the core of the scanner. Even with very little inhomogeneity, there may be problems such as undesired changes in the intensity or brightness of pixels, which may cause problems in determining different tissues and reduce the maximum achievable image resolution. The only magnets currently available that are both very strong and homogeneous are superconducting magnets. Such magnets are expensive and are the biggest single cost in an MRI scanner.

It would be desirable to provide a relatively inexpensive magnet for use in MRI or spectroscopy.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 illustrates a first view of a magnet in accordance with one embodiment.

Figure 2 illustrates a second view of a magnet in accordance with one embodiment.

Figure 3 illustrates a third view of a magnet in accordance with one embodiment.

Figure 4 illustrates a frequency swept excitation method suitable for use with a magnet as provided herein.

DESCRIPTION

A relatively inexpensive magnet for use in magnetic resonance imaging or spectroscopy is provided, hi one embodiment, the magnet is relatively inhomogenous. Homogeneity (or uniformity) of the main magnetic field may be measured in ppm. hi a defined volume, it is the difference between maximum and minimum field strength and this multiplied by 1 million. The magnet provided herein may be used, for example, with a magnetic resonance imaging technique that uses a frequency swept excitation wherein the acquired signal is a time domain signal, such as the method disclosed in U.S. Patent Application Ser. No. 11/546,664, entitled Frequency Swept Excitation for Magnetic Resonance, filed October 11, 2006, and herein incorporated by reference.

Magnet types currently used for MRI are generally of superconducting, resistive, and permanent magnet designs ranging in strength from 0.08 to 4 T (T = 10,000 gauss). Magnets having higher strength, such as up to 12 T have also been designed. Most MR systems use superconducting magnets that provide fields of high strength and stability (or homogeneity). Most currently produced magnets are based on niobium-titanium (NbTi) alloys, which are reliable and use a liquid helium cryogenic system to keep the conductors at approximately 4.2 Kelvin (-268.8 Celcius). The RF coils used to excite nuclei may be quadrature coils which surround the head or body. Alternatively, the RF coils may be small (e.g. 6-10 cm) flat coils placed on the surface of the head or body. A relatively inexpensive and inhomogeneous magnet for use with MRI systems is provided. Figures 1-3 illustrate various views of a magnet in accordance with one embodiment. As shown, the magnet has a homogeneity of approximately 1000 ppm over 16cm x 30 cm x 90 cm. hi one embodiment, specifications of the magnet include: whole body access "CT" aspect ratio, 0.5 Tesla full field, magnet length of approximately 80 cm, and a cryocooler for zero boil-off (refrigerated technology) operation.

In alternative embodiments, magnets may be provided having varying inhomogeneity, varying strengths, different lengths, or different configurations. The material used to manufacture the magnet may vary. In various embodiments, the RF coils may be quadrature coils, flat coils, or other.

Inhomogeneous magnets as provided herein may be used with MRI methods such as the method of magnetic resonance disclosed in U.S. Patent App. Serial No. 11/546,664, entitled Frequency Swept Excitation for Magnetic Resonance, filed October 11, 2006, and herein incorporated by reference. That method uses a frequency swept excitation wherein the acquired signal is a time domain signal. The method may be referred to as a frequency swept excitation method and has applications in a plurality of fields including, but not limited to, human and animal medicine, dentistry, material science, security, and industry. Using the frequency swept excitation method, shown and discussed with reference to Figure 4, excitation of nuclear spins is achieved using a radio frequency sweep. The excitation may comprise a series of pulses, each pulse having an excitation segment and a quiescent segment wherein the frequency or phase is swept within each pulse. A signal is acquired as a time domain signal during the quiescent segment. The acquired signal is treated as a signal that varies as a function of time and is looked at in the time domain. After signal acquisition, the signal is processed, for example using a correlation method or a convolution, to correct the acquired signal by separating the spin system spectrum. The processed signal can then be used in imaging. The frequency swept excitation method has a near zero echo time (or TE approaching 0) because signal acquisition can begin within a few microseconds after excitation. Alternatively, the method may have a zero echo time where signal acquisition occurs during excitation. Because the frequency swept method has a zero or near zero "echo time," it is generally less sensitive to motion and flow artifacts than conventional magnetic resonance methods. Thus, the frequency swept method has a reduced signal loss due to either diffusion in the presence of a gradient or uncompensated motion as compared with other magnetic resonance methods. The method is insensitive to field inhomogeneity and thus can be used, for example, in imaging tissues near metal objects such as metal implants. Further, the method may be used in magnetic resonance imaging and spectroscopy using non-uniform (inhomogeneous) magnets (for example, having inhomogeneous main magnetic field or RF field).

As shown in Figure 4, nuclear spins in the object of interest are excited 12 using a frequency sweep comprising a series of frequency modulated pulses, also referred to as a series of frequency-swept pulses. In one embodiment, the frequency excitation is non-random but may not be continuous or linear. The frequency-swept pulses sequentially excite isochromats over each pulse, and as a result, the phase of the magnetization varies in a quadratic manner along the readout direction. The frequency of the RF irradiation used to excite signals may be modulated in time during each pulse. Alternatively, the phase of the RF irradiation used to excite signals is modulated in time during each pulse. The frequency-swept excitation distributes the signal energy in time. A time domain signal is acquired 14. Signal acquisition can be done nearly simultaneously with excitation. Specifically, signal acquisition may be done during quiescent segments of a pulse. In alternative embodiments, signal acquisition may be simultaneous with excitation. Although the invention has been described with reference to specific embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

CLAIMSWhat is claimed:

1. A magnet for use with a magnetic resonance imaging system: the magnet having a length of approximately 80 cm; the magnet having a homogeneity of approximately 100 ppm over 16 cm x 30 cm x 90 cm.