Offset gradient fields in nuclear magnetic resonance imaging systems

Claim 2

In a nuclear magnetic resonance imaging system, including a magnet for developing a main magnetic field, apparatus for producing a magnetic field gradient comprising: means for producing a gradient magnetic field within a region of said main magnetic field, said gradient magnetic field being distributed about a reference point of gradient field intensity within said region; and bias field means for producing a substantially uniform offset in the magnetic field intensity within said region.

Claim 3

The nuclear magnetic resonance imaging system of Claim 1, wherein the magnetic field intensity at points within said region is the sum of components of said main magnetic field, said gradient magnetic field, and a bias field produced by said bias field means.

Claim 4

The nuclear magnetic resonance imaging system of Claim 1, wherein said main magnetic field is oriented in a given'direction; wherein said gradient magnetic field contributes to said main magnetic field in differing amounts of intensity at points along a given axis within said region; and wherein said bias field means contributes to said main magnetic field in substantially uniform amounts of intensity at said points along said given axis.

Claim 5

The nuclear magnetic resonance imaging system of Claim 3, wherein said reference point of gradient field intensity is a null point on said given axis at which said gradient magnetic field makes substantially no contribution to said main magnetic field, said contributions at points on respective sides of said null point being of respective opposite senses.

Claim 6

The nuclear magnetic resonance imaging system of Claim 4, wherein said bias field means is capable of producing a non-zero contribution to the field intensity at said null point.

Claim 7

The nuclear magnetic resonance imaging system of Claim 1, wherein said gradient magnetic field producing means includes a gradient field coil; and wherein said bias field means includes a bias field coil.

Claim 8

The nuclear magnetic resonance imaging system of Claim 6, wherein said main magnetic field is oriented in the direction of a given axis, and wherein said bias field coil is wound in a generally spiral form about said given axis, said region being located within the generally cylindrical volume within said bias field coil.

Claim 9

The nuclear magnetic resonance imaging system of Claim 7, wherein said bias field coil is wound in a Helmholz coil configuration.

Claim 10

In a nuclear magnetic resonance imaging system, including a magnet for developing a main magnetic field, magnetic field gradient apparatus comprising: means for producing a first gradient magnetic field within a region of said main magnetic field, said first gradient magnetic field being distributed about a first reference point of gradient field intensity, which first reference point is located at a first point on an axis within said region; and means for producing a second gradient magnetic field within said region of said main magnetic field, said second gradient magnetic field being distributed about a second reference point of gradient field intensity, which second reference point is located at a second point different than said first point on said axis.

Claim 11

In a nuclear magnetic resonance imaging system, including a magnet for developing a main magnetic field, magnetic field gradient apparatus comprising: means for producing a first gradient magnetic field within a region of said main magnetic field, said first gradient magnetic field being distributed about a first reference point of gradient field intensity, which first reference point is located at a first point on an axis within said region; and means for producing a second gradient magnetic field within said region of said main magnetic field, said second gradient magnetic field being distributed about a second reference point of gradient field intensity, which second reference point is located at a second point different than said first point on said axis.

Claim 12

The nuclear magnetic resonance imaging system of Claim 9, wherein said first and second gradient magnetic fields both exhibit field gradients varying in substantially the same direction, and wherein said first and second gradient magnetic fields are superimposed within said region.

Claim 13

In a nuclear magnetic resonance imaging system, including a magnet for developing a main magnetic field, magnetic field gradient apparatus comprising: means for producing a first gradient magnetic field within a region of said main magnetic field, said first gradient magnetic field being distributed about a first reference point of gradient field intensity, which first reference point is located on an axis within said region; and means for producing a second gradient magnetic field within said region of said main magnetic field, said second gradient magnetic field being distributed about a second reference point of gradient field intensity, which second reference point is located on said axis, wherein said first and second gradient magnetic fields form a composite gradient magnetic field exhibiting a reference point of composite gradient field intensity located on said axis.

Claim 14

In a nuclear magnetic resonance imaging system, including a magnet for developing a main magnetic field, magnetic field gradient apparatus comprising: means for producing a first gradient magnetic field within a region of said main magnetic field, said first gradient magnetic field being distributed about a first reference point of gradient field intensity, which first reference point is located on an axis within said region; and means for producing a second gradient magnetic field within said region of said main magnetic field, said second gradient magnetic field being distributed about a second reference point of gradient field intensity, which second reference point is located on said axis, wherein said first and second gradient magnetic fields form a composite gradient magnetic field exhibiting a reference point of composite gradient field intensity located on said axis.

Claim 15

The nuclear magnetic resonance imaging system of Claim 11, wherein the magnetic field intensity at points within said region is the sum of components of said main magnetic field said first gradient magnetic field, and said second gradient magnetic field.

Claim 16

The nuclear magnetic resonance imaging system of Claim 12, wherein components of said first gradient magnetic field exhibit a variation in magnetic field intensity along a given direction, and wherein components of said second gradient magnetic field exhibit a variation in magnetic field intensity along said given direction.

Claim 17

The nuclear magnetic resonance imaging system of Claim 11, wherein the location of said reference point of composite gradient field intensity is a function of the relative intensities and distribution of said first and second gradient magnetic fields.

Claim 18

The nuclear magnetic resonance imaging system of Claim 11, wherein said main magnetic field is oriented in a given direction; wherein said first reference point is a null point at which said first gradient magnetic field makes substantially no contribution to said main magnetic field in said given direction; and wherein said second reference point is a null point at which said second gradient magnetic field makes substantially no contribution to said main magnetic field in said given direction.

Claim 19

The nuclear magnetic resonance imaging system of Claim 15, wherein said reference point of composite gradient field intensity is a composite null point at which the sum of components of said first and second gradient magnetic field makes substantially no contribution to said main magnetic field in said given direction.

Claim 20

The nuclear magnetic resonance imaging system of Claim 11, wherein said first gradient magnetic field producing means includes a first reverse Helmholz coil configuration, and wherein said second gradient magnetic field producing means includes a second reverse Helmholz coil configuration.

Claim 21

The nuclear magnetic resonance imaging system of Claim 11, wherein said first gradient magnetic field producing means includes a first Golay coil configuration, and wherein said second gradient magnetic field producing means includes a second Golay coil configuration.

Claim 22

The nuclear magnetic resonance imaging system of Claim 18, wherein each Golay coil configuration includes a pair of coils which are asymmetric with respect to each other and are positioned around a central axis.

Claim 23

The nuclear magnetic resonance imaging system of Claim 11, wherein said gradient magnetic field producing means comprise spirally wound coils surrounding a cylindrical volume, said axis being the longitudinal axis of said cylindrical volume.