MXPA98005582A - Image formation by magnetic resonance of spectrum extend - Google Patents

Abstract

The present invention relates to a method for acquiring voxel data in a tissue contour, which includes placing the tissue in a magnetic field. For the present method, a gradient z is imposed on the contour tissue, to disperse the spectrum of all voxels over the same Larmor frequency range. Additionally, voxels in the contour tissue are selectively encoded with different gradients X and different Y gradients to distinguish the various voxels from each other. In the present z-gradient, coded voxel cores are tilted and then re-focused at a rate proportional to the gradient z. Due to this refocus, spin echo signals are generated that are useful for acquiring tissue data. In the intravolve z gradient employed for the present invention, it is the same for all voxels and is greater than either the z gradient or the gradient and which are used for coding. The gradient z can be substantially constant. Importantly, the z-gradient is large enough to disperse the spectrum of the spin echo signals for suppression of exogenous interference and make the signals immune to static field disturbances.

Description

FORMATION DB IMAGE BY MAGNETIC RESONANCE OF ESPBCTKO K? TKNDIDO CAMPQ ßE? MVmION The present invention relates generally to methods for acquiring data that are useful for magnetic resonance imaging (MRI) imaging. More particularly, the present invention relates to methods and techniques for acquiring image data, for magnetic resonance imaging, when the magnetic field is characterized by a gradient z. The present invention is particular but not exclusively useful for employing extended spectrum techniques in magnetic resonance imaging, for suppressing exogenous interference to make MRI signals immune to static field perturbations and for overcoming perceived inefficiencies of inhomogeneous magnetic fields.

Magnetic resonance imaging is a well-known and widely used method for obtaining medical images for both diagnostic and research purposes. In order to conduct a typical MRI procedure, a tissue volume is first placed in a static magnetic field. The tissue is then irradiated with radio frequency energy, to tilt the nuclear magnetic moments within the tissue. Spin echo signals that are characteristic of the irradiated tissue, are then recorded from the inclined nuclear magnetic moments. By using well-known imaging techniques in the art, the signal contributions of individual volume elements (voxel = volume elements) in the tissue are distinguished from each other. These voxels are finally displayed on a computer monitor or movie to be used by the doctor or researcher. For any system that relies on nuclear magnetic resonance techniques to acquire data, a very important design consideration is the static magnetic field of the system. For purposes of discussing the static magnetic field, consider a system of orthogonal x-y-z coordinates. With the origin of this coordinate system at any point in the magnetic field, the magnetic field at that particular point can be characterized by the respective components x, y, and z of the field and by the spatial derivatives of the field strength. Specifically, the components x, y, and z of the field magnetic vector, B0, are designated Bx, and B ,. The magnetic field can then be further characterized by the gradients that are the rate of change (first derivatives) of the intensity of change in the directions x, y and z. The field gradients are designated G », Gy and G-. It will be appreciated that higher-level derivatives can and are more likely to be present. For purposes of discussing the present invention, however, only the field gradients Gx, Gy and G. need to be considered. In a very general sense, a homogeneous magnetic field exists in a small neighborhood of a point, where all the field gradients, ie Gx, Gy and G, are zero. To derive the difficulties encountered with the fabrication of homogeneous MRI systems, recent efforts have been made to effectively utilize the most common non-homogeneous magnetic field and most cost effective. For example, U.S. Pat. No. 5,304,930 granted to the assignee of the present invention, and to be granted to Cro ley and collaborators for an invention with title "Remotely Positioned MRI system for Magnetic Resonance Imaging System" below will be referred to as the '930 patent, describes a device and method for resonance image formation with a non-homogeneous magnetic field. As clearly described in the '930 patent, an inhomogeneous field is a field having a non-zero gradient G,. Regardless of whether the magnetic field is homogeneous or non-homogeneous, in order to perform an MRI procedure, it is necessary to distinguish various voxels within the tissue that will be imaged. To do this, the tissue is typically encoded with spatial patterns. A coding method widely recognized for imparting spatial patterns in the volume of tissue that is imaged, involves the application of gradient magnetic fields. These so-called gradient magnetic fields consist of an additional range of field values, denoted by AB0, which overlap in the static field. At this point, it will be noted that the spatial variations X and Y of AB0 are determined by respective gradients X and Y, Gx and Gy of the superimposed field values. Through the constant Larmor, a range of Larmor frequencies determined by the expression? F =? AB0 exists during the application of the gradient, either Gx or Gy or both. The effect of this Larmor frequency range is a spatial pattern of phase accumulation at magnetic moments through the tissue of interest. For the purposes of the present invention, the key point is that the range of Larmor frequencies associated with a gradient field (Gx or Gy) spatially distinguishes one voxel from another in the respective directions x and y. With a convenient number of gradient coding, which is followed by a measurement of espln echo signals, data that can be reconstructed in an image of the set of voxels is obtained. Because the present invention contemplates an MRI operation with a gradient z (G, several consequences involving data acquisition and the suppression of exogenous interference, are pertinent.) First, data acquisition techniques in the presence of a z gradient are quite different from those used for conventional MRI in a homogeneous magnetic field This aspect of data acquisition has been fully considered and described in U.S. Patent No. 5,304,930, cited above, and which is incorporated herein by reference. Suppression of exogenous interference is achieved by imposing a gradient z (G, which is greater than either of the x or y coding gradients.) An additional benefit of this relationship between the gradients is the fact that the system is less sensitive to static field disturbances.Extended spectrum techniques are widely employed in the Communications Industry to avoid r Corruption of signals transmitted by sources of interference. The method is particularly effective in the presence of a described set of interference sources that occupy narrow frequency bands. Conventional radio or television signals fall into this category. The basic idea in extended spectrum techniques is to send and receive signals that occupy a range of frequencies that are significantly wider than those of the individual interference sources. In this way, the effects of individual interference sources are minimized. The use of these techniques is not conventionally illustrated in the art since, as mentioned above, most MRI equipment can be protected against the effect of external interference by circumscribing the system in an RF shielded room (Faraday cage). However, the present invention recognizes that there are benefits to systems that are not circumscribed in armored rooms, especially for operating cost and smaller portable systems. In light of the foregoing, an object of the present invention is to provide methods for acquiring voxel data in a tissue contour that is achieved using a gradient z, G,. Another objective of the present invention is to provide methods for voxel data of a tissue contour using a large z gradient, Gz, to suppress exogenous interference which makes the data less sensitive to static field disturbances. Yet another objective of the present invention is to provide methods for MRI that are relatively easy to achieve and comparatively cost effective. Compendium of the Invention A method according to the present invention involves placing a tissue sample that will be imaged in a magnetic field. Specifically, a non-homogeneous magnetic field will have a permanent z-gradient (Gt) that can be imposed intentionally, but more typically it is an inherent characteristic of the magneto system that generates the magnetic field. Once the tissue sample has been located in the magnetic field and the gradient z, G. is imposed, a slice or contour of the tissue sample is excited by the application of RF energy. The transmission of this RF energy corresponds to the range of Larmor frequencies in the contour, which will be the same for all voxels in the contour. The tissue sample is then printed subsequently with a gradient x (Gx) and a gradient y (Gy). The effect of printing the means of x and y (Gx Gy) on the tissue sample is to encode a position of the tissue as a voxel contour. Because of this coding, each of the voxels will have their own field strengths slightly different, in the x and y directions. It is this spatial information that distinguishes a voxel from another voxel in the contour. On the other hand, as indicated above, all the voxels in the contour are subjected to the same gradient z. In this way, instead of distinguishing the voxels from each other, the effect of the gradient z is to disperse the spectrum of each of the coded voxels. Importantly, the gradient z (G.) is typically orders of magnitude (10 to 100 times) larger than either the gradient x (Gx) or the gradient y (Gy). As is well known in the art, tilted cores generate spin echo signals that contain information data that can be received and processed to image the fabric. For the present invention, the gradients X, Y and X are appropriately activated and the cores in the inclined and coded voxels are refocused to form spin echoes that are received for data acquisition purposes. Specifically, as described in greater detail in the '930 patent, the data acquisition process envisioned by the present invention involves a series of refocus pulses that are applied at a rate that is proportional to the gradient z.

Several aspects of the present invention are particularly noteworthy. First, the z gradient does not impart coding patterns. However, the z gradient causes all voxel spins to oscillate in the same Larmor frequency range. In addition, the extended z-gradient allows the use of extended-spectrum techniques for the suppression of exogenous interference and sensitization to static field disturbances.

The novel features of this invention as well as the invention itself, both in terms of its structure and operation, will be better understood from the accompanying drawings taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts and wherein: Figure 1 is a schematic drawing of the components of a magnet system that is usable for the present invention; Figure 2 is an idealized visualization of a voxel contour that is imaged by the methods of the present invention; and Figure 3 is a comparison of the spectral densities of the static field gradient (G and exogenous interference.

DESRIPTION, PE PREFERRED MODE With reference initially to Figure 1, a magnet system according to the present invention is illustrated and is generally designated 10. As shown, the magnet system includes a North pole face 12 and a South pole face 14 that both are mounted on a base 16. As mounted on the base 16, the North pole face 12 and the South pole face 14 generate a magnetic field which is represented by the magnetic field lines 18. In the preferred embodiment , the magnetic field is not homogeneous, and with reference to the orthogonal coordinate system x and z, the magnetic field has an inherent permanent Z gradient G, .. For purposes of the present invention, Gt will be about 0.2 Gauss per millimeter and more likely will be somewhere around 3 Gauss per millimeter. Figure 1 also shows that the system 10 includes a transmission antenna system 20 and a reception antenna system 22, both of which are mounted on the base 16. As will be appreciated by the person skilled in the art for the For purposes of the present invention, many types of antenna system 20, 22 may be employed either separately or in combination and in any manner well known in the art. In any case, the antenna systems 20, 22 are connected to a computer 24 which will control the transmission and reception of signals from the antenna systems 20, 22. As intended by the present invention, the magneto system 10 is designed with faces of non-shielded poles 12, 14, which are small enough to make the system 10 portable. Consequently, in addition to the permanent z-gradient, the magnetic field 18 will also be subject to non-predictable exogenous interference and static field perturbation, which are represented by the arrows 26 in Figure l. Figure 2 also illustrates a tissue sample 28 (e.g., a hand) that has been placed in the magnetic field of the magneto system 10. It will be appreciated that the tissue sample 28 can be oriented as desired. In all cases, however, the portion of tissue that will be formed in the image lies in a x-y plane, a slice or contour, which is perpendicular to the z-axis and thus also perpendicular to the gradient z (G.). Figure 2 shows a representative contour 30 of the tissue sample 28 to be imaged. As illustrated, for a point 31 the coordinate system x-y-z, the contour 30 includes a plurality of voxels 32 of which the voxel 32a is representative. As also illustrated in Figure 2, the contour 30 and the voxels 32 are limited by an upper surface 34 and a lower surface 36. Accordingly, the contour 30 has a thickness t. Importantly, the surfaces 34, 36 each are of constant field magnitude Bou and B01, respectively and as a consequence the surfaces have different frequencies Larmor, f0D and f0L respectively. As indicated by the previous description, the Larmor frequency range (f0u-fo ?,) through the thickness t of the contour 30 is due to G .. As stated above, Gt will be greater than either 6, or 6, and in most cases much greater. Also as defined here, G. is permanent. In this way, G. will be imposed on the tissue sample 28 as soon as the tissue sample 28 is placed in the magnetic field. Furthermore, Gt is the same for all voxels 32 in contour 30 and thus the Larmor frequency range will be the same for all voxels 32. Therefore, as used by the present invention G. is, a gradient intravoels. Gx and Gy in contrast are inter-pixel gradients that serve to encode and thus spatially differentiate the voxels 32 from each other. A method for operating in accordance with the present invention involves placing a tissue sample 28 in the magnetic field of a magneto system 10. This magnetic field is characterized by a gradient z, G ,. Gz preferably is substantially constant and large enough to suppress exogenous interference with spread spectrum techniques. Specifically, Figure 3 shows how the intravoxel range of Larmor frequencies resulting from G is dispersed in spectrum. As indicated in Figure 3, the spectral density 38 resulting from the intravoxel gradient (G.) is rather extensive. In contrast, spectral densities of exogenous interference sources and static field perturbations, illustrated by the lines 40a-c, are small with respect to the dispersion in the signal spectrum. Once the tissue sample 28 has been properly positioned, the antenna system 20 is activated to tilt the cores in the tissue sample 28. After the cores are tilted or earlier if desired, the tissue sample 28 is it encodes with the gradients X and Y. The result is a contour 30 of coded voxels 32. The contour 30 is then irradiated with refocusing pulses, which cause the cores to generate spin echo signals that can be received. All actions: tilt, coding and refocusing are controlled by the computer 24. The resulting spin echo signals are then received by the antenna system and passed to the computer 24 where they are processed as desired.

It will be appreciated that the techniques described herein are given in the context of a non-homogeneous magnetic field. These techniques, however, are also applicable to homogeneous fields whenever a Gt is superimposed on the homogeneous field. While the particular extended spectrum MRI as illustrated and described here in detail, is highly capable of obtaining the objects and providing the previously established advantages, it will be understood that it is merely illustrative of the preferred embodiments of the invention and that no limitations are intended to the details of construction or design shown herein, other than as described in the appended claims.

Claims (23)

1. CLAIMS 1. - A method for acquiring voxel data in a tissue contour, characterized in that it comprises the steps of: placing the tissue in a magnetic field; print a coding gradient on the fabric to encode the voxels; impose a z-gradient on the coded voxels to disperse the spectrum of all voxels over the same Larmor frequency range; the gradient z is greater than the x-y coding gradients; and acquire data from voxels over the same Larmor frequency range.
2. 2. - A method according to claim 1, characterized in that it also comprises the step of filtering the Larmor frequency range to attenuate exogenous interference.
3. 3. - A method according to claim 1, characterized in that the magnetic field is homogeneous.
4. 4. - A method according to claim 1, characterized in that the gradient z is inherent and maintains a value greater than zero during the acquisition stage.
5. 5. - A method according to claim 4, characterized in that the gradient z is substantially constant during the acquisition step.
6. 6. - A method according to claim 1, characterized in that the gradient z is greater than 0.2 Gauss per millimeter.
7. 7. - A method according to claim 1, characterized in that the contour has a surface and the gradient z is substantially perpendicular to the surface in the extended spectrum voxel.
8. 8. - A method according to claim 1, characterized in that the acquisition step further comprises the steps of: tilting cores in the extended spectrum voxel; refocus the inclined cores at a rate proportional to the z gradient to generate encoded spin echo signals; and receive the encoded spin echo signals.
9. 9. - A method according to claim 8, characterized in that it further comprises the step of converting the encoded spin echo signals into an image.
10. 10. - A method according to claim 8, characterized in that the reception stage includes averaging the encoded spin echo signals.
11. 11. - A method for acquiring data from a single voxel in a plurality of voxels of a tissue sample placed in a magnetic field, characterized in that it comprises the steps of: impressing at least one inter-pixel gradient in the tissue to encode the voxel; and impose an intravoxel gradient in the tissue to disperse the spectrum of all the voxels encoded over the same Larmor frequency range, the intravoxel gradient is greater than the inter-pixel gradient.
12. 12. - A method according to claim 11, characterized in that the intravoxel gradient is a gradient z and the intervoxel gradient includes a gradient x and a gradient y.
13. 13. - A method according to claim 12, characterized in that the gradient z is inherent and maintains a value greater than zero during the imposition step.
14. 14. - A method according to claim 12, characterized in that the gradient z is substantially constant during the imposition step and has a value greater than 0.2 Gauss per millimeter.
15. 15. - A method according to claim 11, characterized in that the contiguous voxels form a contour having a surface and the gradient z is substantially perpendicular to the surface of the coded voxel.
16. 16. - A method according to claim 15, characterized in that it further comprises the steps of tilting cores in the encoded voxel; refocus the inclined cores at a rate proportional to the gradient z, to generate encoded spin echo signals; and receive the encoded spin echo signals.
17. 17. - A method according to claim 16, characterized in that the reception stage includes averaging the encoded spin echoes and the method further comprises the step of converting the averaged spin echo signals into an image.
18. 18. - * A method for magnetic resonance processing, characterized in that it comprises the steps of: placing a sample of tissue in a magnetic field; use a first bandwidth in a tissue contour to distinguish there voxels; and using a second bandwidth in the tissue contour to disperse the voxel spectrum over the same Larmor frequency range for reception of spin echo signals, the second bandwidth is greater than the first bandwidth, and the second bandwidth is extensive to suppress exogenous interference.
19. 19. - A method according to claim 18, characterized in that it also comprises the steps of: tilting cores in the voxels; refocus the inclined cores at a rate proportional to the second bandwidth to generate encoded spin echo signals from the voxels; receive encoded spin echo signals; and converting the encoded spin echo signals into usable data.
20. 20. - A method according to claim 15, characterized in that the steps of using, tilting, refocusing, receiving and converting are repeated for a plurality of the voxels.
21. 21. A method according to claim 20, characterized in that the reception stage includes averaging the encoded spin echo signals and the conversion step includes using the data to create a tissue image.
22. 22. A method according to claim 19, characterized in that the first bandwidth results from a gradient x and a gradient y and the second bandwidth results from a gradient z.
23. 23. A method according to claim 22, characterized in that the gradient z is substantially constant and has a value greater than 0.2 Gauss per millimeter during the reception stage. The present invention relates to a method for acquiring voxel data in a tissue contour, which includes placing the tissue in a magnetic field. For the present method, a gradient z is imposed on the contour tissue, to disperse the spectrum of all voxels over the same Larmor frequency range. Additionally, voxels in the contour tissue are selectively encoded with different gradients X and different Y gradients to distinguish the various voxels from each other. In the present z-gradient, coded voxel cores are tilted and then re-focused at a rate proportional to the gradient z. Due to this refocus, spin echo signals are generated that are useful for acquiring tissue data. The intravole z gradient used for the present invention is the same for all voxels and is greater than either the z gradient or the gradient and which are used for coding. The gradient z can be substantially constant. Importantly, the gradient z is large enough to scatter the spectrum of the ester echo signals for suppression of exogenous interference and make the signals immune to static field perturbations. RS / frp / 27 / 1-046-10