Spread spectrum MRI

A method for acquiring data from voxels in a contour of tissue includes positioning the tissue in a magnetic field. For the present method, a z-gradient is imposed on the contour tissue to spread the spectrum of all of the voxels over a same range of Larmor frequencies. Additionally, voxels in the contour tissue are selectively encoded with different x-gradients and y-gradients to distinguish the various voxels from each other. In the presence of the z-gradient, nuclei of the encoded voxel are tilted and then refocused at a rate proportional to the z-gradient. Due to this refocusing, spin echo signals are generated which are useful for acquiring data from the tissue. The intravoxel z-gradient used for the present invention is the same for all voxels and is greater than either the x-gradient or the y-gradient which are used for encoding. The z-gradient may be substantially constant. Importantly, the z-gradient is sufficiently extensive to spread the spectrum of the spin echo signals for suppression of exogenous noise and make the signals immune to static field perturbations.

FIELD OF THE INVENTION 
The present invention pertains generally to methods for acquiring data that 
is useful for magnetic resonance imaging (MRI). More particularly, the 
present invention pertains to methods and techniques for acquiring image 
data for magnetic resonance imaging when the magnetic field is 
characterized by a z-gradient. The present invention is particularly, but 
not exclusively, useful for employing spread spectrum techniques in 
magnetic resonance imaging to suppress exogenous noise, to make MRI 
signals immune to static field perturbations and to overcome perceived 
inefficiencies of nonhomogeneous magnetic fields. 
BACKGROUND OF THE INVENTION 
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 volume of tissue 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, which are characteristic of the irradiated 
tissue, are then recorded from the tilted nuclear magnetic moments. By 
using imaging techniques well known in the art, the signal contributions 
of individual volume elements (voxels) in the tissue are distinguished 
from each other. These voxels are ultimately displayed on a computer 
monitor or film for use by the physician or researcher. 
For any system which relies on nuclear magnetic resonance techniques for 
acquiring data, one very important design consideration is the system's 
static magnetic field. For purposes of discussing the static magnetic 
field, consider an orthogonal x-y-z coordinate system. 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 x, y 
and z components of the field and by spatial derivatives of the field 
strength. Specifically, the x, y and z components of the field magnetic 
vector, B.sub.0, are designated B.sub.x, B.sub.y and B.sub.z. The magnetic 
field can then be further characterized by the gradients which are the 
rate of change (first derivatives) of the field strength in the x, y and z 
directions. The field gradients are designated G.sub.x, G.sub.y and 
G.sub.z. It is to be appreciated that higher order derivatives may, and 
most likely are, present. For purposes of discussing the present 
invention, however, only the field gradients G.sub.x, G.sub.y and G.sub.z 
need be considered. 
In a very general sense, a homogeneous magnetic field exists in a small 
neighborhood of a point where all of the field gradients, i.e. G.sub.x, 
G.sub.y and G.sub.z, are zero. 
To by-pass the difficulties encountered with the manufacture of homogeneous 
MRI systems, recent efforts have been made to effectively use the more 
commonplace and more cost effectively established nonhomogeneous magnetic 
field. For example, U.S. Pat. No. 5,304,930 which is assigned to the 
assignee of the present invention, and which issued to Crowley et al. for 
an invention entitled "Remotely Positioned MRl System" (hereinafter the 
'930 patent) discloses a device and method for magnetic resonance imaging 
with a nonhomogeneous magnetic field. As clearly disclosed in the '930 
patent, a nonhomogeneous field is a field that has a non-zero gradient 
G.sub.z. Regardless whether the magnetic field is homogeneous or 
nonhomogeneous, in order to perform an MRI procedure it is necessary to 
distinguish various voxels within the tissue to be imaged. To do this, the 
tissue is typically encoded with spatial patterns. 
One widely recognized encoding procedure for imparting spatial patterns in 
the tissue volume to be imaged involves the application of gradient 
magnetic fields. These so-called gradient magnetic fields consist of an 
additional range of field values, denoted by .DELTA. B.sub.o, that are 
superimposed on the static field. At this point it should be noted that 
the x and y spatial variations of .DELTA. B.sub.o are determined by the 
respective x and y gradients, G.sub.x and G.sub.y, of the superimposed 
field values. Through the Larmor constant, a range of Larmor frequencies 
determined by the expression 
EQU .DELTA.f=.lambda..DELTA.B.sub.o 
exists during the application of the gradient, either G.sub.x or G.sub.y or 
both. The effect of this range of Larmor frequencies is a spatial pattern 
of phase accumulation in the magnetic moments across 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 (G.sub.x 
or G.sub.y) spatially distinguish one voxel from another in the respective 
x and y directions. With a suitable number of gradient encodings, which 
are each followed by a measurement of spin echo signals, data is obtained 
that can be reconstructed into an image of the array of voxels. 
Due to the fact the present invention contemplates an MRI operation with a 
z-gradient (G.sub.z), several consequences which involve data acquisition 
and the suppression of exogenous noise are pertinent. First, the data 
acquisition techniques in the presence of a z-gradient are quite different 
from those used for conventional MR1 in a homogeneous magnetic field. This 
data acquisition aspect has been fully considered and disclosed in U.S. 
Pat. No. 5,304,930, which has been cited above and which is incorporated 
herein by reference. Second, the suppression of exogenous noise is 
accomplished by imposing a z-gradient, G.sub.z, which is greater than 
either the x or y encoding gradients (G.sub.x and G.sub.y). An additional 
benefit from this relationship between the gradients is the fact that the 
system is less sensitive to static field perturbations. 
Spread spectrum techniques are widely used in the communications industry 
to avoid the corruption of transmitted signals by interfering noise 
sources. The method is particularly effective in the presence of a 
discrete set of noise sources that occupy narrow frequency bands. 
Conventional radio or television signals fall into this category. 
The basic idea in spread spectrum techniques is to send and receive signals 
that occupy a range of frequencies that is significantly wider than that 
of the individual interfering noise sources. In this manner, the effects 
of the individual noise sources are minimized. 
The use of such techniques is not conventionally taught in the art since, 
as mentioned above, most magnetic resonance equipment can be shielded from 
the effect of external noise by enclosing the system in an r.f. shielded 
room (Faraday cage). However, the present invention recognizes there are 
benefits to systems that are not enclosed in shielded rooms, especially 
for cost of operation and smaller portable systems. 
In light of the above, it is an object of the present invention to provide 
methods for acquiring data from voxeis in a contour of tissue which is 
accomplished using a z-gradient, G.sub.z. It is another object of the 
present invention to provide methods for acquiring data from voxels in a 
contour of tissue which uses an extensive z-gradient, G.sub.z, to suppress 
exogenous noise making the data less sensitive to static field 
perturbations. Still another object of the present invention is to provide 
methods for MRI which are relatively easy to accomplish and comparatively 
cost effective. 
SUMMARY OF THE INVENTION 
A method in accordance with the present invention involves positioning a 
tissue sample to be imaged in a magnetic field. Specifically, a 
nonhomogeneous magnetic field will have a permanent z-gradient (G.sub.z) 
which may be intentionally imposed, but is most typically an inherent 
characteristic of the magnet system that generates the magnetic field. 
Once the tissue sample has been positioned in the magnetic field and the 
z-gradient, G.sub.z, 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 subsequenty impressed with an x-gradient (G.sub.x) and a y-gradient 
(G.sub.y). The effect of impressing the x and y gradients (G.sub.x, 
G.sub.y) on the tissue sample is to encode a portion of the tissue as a 
contour of voxels. Due to this encode, each of the voxels will have its 
own slightly different field strengths in the x and y directions. It is 
this spatial information which distinguishes one voxel from another voxel 
in the contour. On the other hand, as indicated above, all of the voxels 
in the contour are subjected to the same z-gradient. Thus, rather than 
distinguishing the voxels from each other, the effect of the z-gradient is 
to spread the spectrum of each of the encoded voxels. Importantly, the 
z-gradient (G.sub.z) is typically orders of magnitude (i.e. ten to one 
hundred times) larger than either the x-gradient (G.sub.x) or the 
y-gradient (G.sub.y). 
As is well known in the art, tilted nuclei generate spin echo signals which 
contain informational data that can be received and processed to image the 
tissue. For the present invention, the x, y and z gradients are 
appropriately activated, and the nuclei in the tilted and encoded voxels 
are refocused to form spin echoes which are received for data acquisition 
purposes. Specifically, as disclosed in great detail in the '930 patent, 
the data acquisition process envisioned by the present invention involves 
a series of refocusing pulses which are applied at a rate that is 
proportional to the z-gradient. 
Several aspects of the present invention are particularly noteworthy. 
Firstly, the z-gradient does not impart encoding patterns. Nevertheless, 
the z-gradient causes all of the voxels' spins to oscillate at a same 
range of Larmor frequencies. Further, the extensive z-gradient allows use 
of spread spectrum techniques for the suppression of exogenous noise and 
desensitization to static field perturbations.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIG. 1, a magnet system in accordance with the 
present invention is shown and generally designated 10. As shown, the 
magnet system includes a North pole face 12 and a South pole face 14 which 
are both mounted on a base 16. As mounted on the base 16, North pole face 
12 and the South pole face 14 generate a magnetic field which is 
represented by the magnetic field lines 18. On the preferred embodiment, 
the magnetic field is nonhomogeneous and, with reference to the orthogonal 
x-y-z coordinate system, the magnetic field has an inherent permanent 
z-gradient, G.sub.z. For purposes of the present invention, G.sub.z will 
be above approximately 0.2 gauss per millimeter and, most likely, will be 
somewhere around 3 gauss per millimeter. 
FIG. 1 also shows that the system 10 includes a transmitting antenna system 
20 and a receiving antenna system 22 which are both mounted on the base 
16. As will be appreciated by the skilled artisan, for the purposes of the 
present invention many types of antenna systems 20, 22 can be used either 
separately or in combination 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 for the present invention, the magnet system 10 is designed 
with unshielded pole faces 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 subjected to unpredictable exogenous 
noise and static field perturbation which are represented by the arrows 26 
in FIG. 1. 
FIG. 1 also shows a tissue sample 28 (e.g. a hand) that has been positioned 
in the magnetic field of magnet system 1 0. It is to be appreciated that 
the tissue sample 28 can be oriented as desired. In all cases, however, 
the portion of tissue which will be imaged lies in an x-y plane, a slice 
or contour, which is perpendicular to the z axis and which is, thus, also 
perpendicular to the z-gradient (G.sub.z). 
FIG. 2 shows a representative contour 30 from the tissue sample 28 which is 
to be imaged. As shown, for a point 31 in the x-y-z coordinate system, the 
contour 30 includes a plurality of voxels 32 of which the voxel 32a is 
representative. As also shown in FIG. 2, the contour 30 and the voxels 32 
therein are bonded by an upper surface 34 and a lower surface 36. 
Accordingly, the contour 30 has a thickness t. importantly, the surfaces 
34, 36 are each of constant field magnitude, B.sub.OU and B.sub.OL 
respectively, and, consequently, the surfaces have different Larmor 
frequencies, f.sub.OU and f.sub.OL respectively. As indicated by earlier 
disclosure, the range of Larmor frequencies (f.sub.OU -f.sub.OL) across 
the thickness t of the contour 30 is due to G.sub.z. As stated above, 
G.sub.z will be greater than either G.sub.x or G.sub.y and, in most cases, 
much greater. Also, as defined herein, G.sub.z is permanent. Thus, G.sub.z 
will be imposed on the tissue sample 28 as soon as the tissue sample 28 is 
positioned in the magnetic field. Further, G.sub.z is the same for all of 
the voxels 32 in contour 30 and, thus, the range of Larmor frequencies 
will be the same for all voxels 32. Therefore, as used for the present 
invention, G.sub.z is, an intravoxel gradient. G.sub.x and G.sub.y are, in 
contrast, intervoxel gradients which serve to encode and thereby spatially 
differentiate the voxels 32 from each other. 
A method for operation in accordance with the present invention involves 
positioning a tissue sample 28 in the magnetic field of a magnet system 
10. This magnetic field is characterized by a z-gradient, G.sub.z. G.sub.z 
is preferably substantially constant and sufficiently large to suppress 
exogenous noise with spread spectrum techniques. Specifically, FIG. 3 
shows how the intravoxel range of Larmor frequencies, which result from 
G.sub.z, are spread in spectrum. As indicated in FIG. 3, the spectral 
density 38 resulting from the intravoxel gradient (G.sub.z), is rather 
extensive. En contrast, spectral densities of interfering exogenous noise 
sources and static field perturbations, shown by the lines 40 a-c, are 
small relative to the spread in signal spectrum. 
Once the tissue sample 28 has been properly positioned, the antenna system 
20 is activated to tilt nuclei in the tissue sample 28. After the tilting 
of the nuclei, or before if desired, the tissue sample 28 is encoded with 
x and y gradients. The result is a contour 30 of encoded voxels 32. The 
contour 30 is then irradiated with refocusing pulses which cause the 
nuclei to generate receivable spin echo signals. All of the actions; 
tilting, encoding, and refocusing are controlled by the computer 24. The 
resultant spin echo signals are then received by the antenna system and 
passed to computer 24 |where they are processed as desired. 
It is to be appreciated that the techniques disclosed herein are given in 
the context of a nonhomogeneous magnetic field. These techniques, however, 
are also applicable to homogeneous fields whenever a G.sub.z is superposed 
on the homogeneous field. 
While the particular spread spectrum MRI, as herein shown and disclosed in 
detail, is fully capable of obtaining the objects and providing the 
advantages herein before stated, it is to be understood that it is merely 
illustrative of the presently preferred embodiments of the invention and 
that no limitations are intended to the details of construction or design 
herein shown other than as described in the appended claims.