Composite pulses for time reversal in NMR imaging

Claim 3

The method of Claim 1, wherein the pulse sequence applying step comprises the steps of: applying a first pulse for producing a first rotation of the nuclear spins about a first axis in said transverse planar region; then applying a second pulse, following the first pulse, for producing a second rotation of the nuclear spins about a second axis in the transverse planar region that is substantially orthogonal to the first axis; and thereafter applying a third pulse, following the second pulse, for producing a third rotation of the nuclear spins about the first axis.

Claim 4

The method of Claim 2, including the steps of: forming the first and third pulses to produce substantially a 90° rotation about the first axis; and.forming the second pulse to. produce substantially a 180° rotation about the second axis.

Claim 5

The method of Claim 3, further comprising the steps of: providing each of the first, second and third pulses at the same amplitude; and providing the second pulse with a duration that is twice the duration of each of the first and third pulses.

Claim 6

The method of Claim 2, further comprising the step of applying the pulses of the pulse sequence along a selected one of the first and second axes.

Claim 7

The method of Claim 5, further comprising the step of providing each of the pulses as a gated RF pulse.

Claim 8

The method of Claim 6, further comprising the steps of: applying the second pulse immediately following the end of the first pulse; applying the third pulse immediately following the end of the second pulse; and providing a 90° phase difference between each of the first and second pulses and between each of the second and third pulses.

Claim 9

The method of Claim 7, further comprising the step of deriving said pulses from an RF source that produces in-phase and quadrature-phase RF outputs, the first pulse being derived from one of said outputs and the second pulse being derived from the other of said outputs.

Claim 10

The method of Claim 1, wherein step (c) includes the steps of: Fourier-transforming the spin-echo NMR signals to produce complex signals having real and imaginary components; and step (d) includes the step of deriving the magnitude of the complex signals from both the real and imaginary components of the complex signals.

Claim 11

The method of Claim 1, further comprising the step of applying a plurality of successive composite pulses to the sample to successively time-reverse the nuclear spins and generate at least one of a sequence of multiple spin-echo signals responsive to each composite pulse.

Claim 12

The method of Claim 1, further comprising the step of providing the composite pulse as a selective 180° pulse for rotating by 180° only the nuclear spins in a selected thin planar slice of the sample.

Claim 13

The method of Claim 11, wherein the selective 180° pulse is provided by the steps of: selecting the RF carrier frequency to excite the nuclear spins in the thin planar slice; selecting a sequence of in-phase and quadrature-phase components of the RF carrier; and amplitude-modulating the selected-phase-sequenced RF carrier with a modulation envelope selected to confine the frequency spectrum of the pulse to a narrow band of frequencies about the selected RF carrier frequency.

Claim 14

A composite RF pulse generator for NMR imaging apparatus, wherein an imaging sequence is applied t6 a sample and NMR signals are detected to form an NMR image of a selected transverse planar region of the sample, said generator for time-reversing nuclear spins in the selected transverse planar region and comprising: RF source means for generating an RF carrier having a selected frequency substantially at the NMR frequency of the nuclear spins in the selected region; means connected to the RF source for producing in-phase and quadrature-phase components of the RF carrier; means for combining the in-phase and quadrature-phase components of said RF carrier into at least one composite RF pulse which, when applied to said sample, each sequentially produce a first rotation of the nuclear spins in the selected region about a first transverse axis, a second rotation of the nuclear spins about a second transverse axis substantially orthogonal to the first axis, and a third rotation of the spins about the first axis so as to time-reverse the nuclear spins; and means for applying the composite RF pulse to the sample.

Claim 15

A composite RF pulse generator for NMR imaging apparatus, wherein an imaging sequence is applied t6 a sample and NMR signals are detected to form an NMR image of a selected transverse planar region of the sample, said generator for time-reversing nuclear spins in the selected transverse planar region and comprising: RF source means for generating an RF carrier having a selected frequency substantially at the NMR frequency of the nuclear spins in the selected region; means connected to the RF source for producing in-phase and quadrature-phase components of the RF carrier; means for combining the in-phase and quadrature-phase components of said RF carrier into at least one composite RF pulse which, when applied to said sample, each sequentially produce a first rotation of the nuclear spins in the selected region about a first transverse axis, a second rotation of the nuclear spins about a second transverse axis substantially orthogonal to the first axis, and a third rotation of the spins about the first axis so as to time-reverse the nuclear spins; and means for applying the composite RF pulse to the sample.

Claim 16

The apparatus of Claim 13, wherein said applying means comprises means for applying the composite RF pulse to the sample along one of said first and second transverse axes.

Claim 17

The apparatus of Claim 13, wherein the first and third rotations are substantially 90°, and the second rotation is substantially 180°.

Claim 18

The apparatus of Claim 15, wherein the composite RF pulse comprises a first in-phase pulse, a second quadrature-phase pulse immediately following the first pulse, and a third in-phase pulse immediately following the second pulse.

Claim 19

The apparatus of Claim 16, further comprising means for controlling the durations of the respective first, second and third pulses to produce said respective rotations therewith.

Claim 20

The apparatus of Claim 13, further comprising means for applying a selected modulation envelope to the composite RF pulse to confine the frequency spectrum of the composite RF pulse to a selected narrow band of frequencies substantially about said NMR frequency.

Claim 21

The apparatus of Claim 13, further comprising: means for processing the NMR signals received from said sample to produce complex signals having real and imaginary components; and means for forming the NMR image from the magnitude of said complex signals.

Claim 22

The apparatus of Claim 19, wherein said received NMR signals.comprise spin-echo signals, and further comprising: means for generating a successive sequence of composite RF pulses; and means for applying said sequence of composite pulses to the sample as part of the imaging sequence to produce multiple spin-echo received signals. GHK:nbK