Multiple-echo angiography with enhanced signal-to-noise ratio

Claim 1

A method for acquiring at least one nuclear magnetic resonance (NMR) angiographic image of flowing material in at least a selected portion of a sample, comprising the steps of: (a) immersing the sample in a main static magnetic field; (b) nutating, in the initial part of each of a first sequence and a second sequence of a sequential pair of imaging sequences for each of a multiplicity S of regions of said selected sample portion, the spins of all nuclei of a selected species; (c) applying a pair of alternating-polarity flow-encoding signal pulses in a first magnetic field gradient impressed upon the sample, in a first direction selected to establish one axis of the NMR angiographic projection image, to cause a resulting NMR response echo signal from the spin of a moving nucleus to differ from the NMR response each signal resulting from the spin of a substantially stationary nucleus; each of the flow-encoding pulses in the first sequence of each pair having a polarity opposite to the polarity of the like-positioned flow-encoding pulse in the second sequence of each pair; (d) acquiring, responsive to a readout magnetic field gradient impressed upon the sample in a second direction, substantially orthogonal to the first direction, a set of data from the NMR response echo signal evoked, from the same sample portion, in each different one of a plurality N of acquisition time intervals of the first and second sequences, and including the substeps of: generating a dephasing pulse in the readout gradient prior to only the first of the N acquisition time intervals in each sequence; scaling the amplitude of the single dephasing pulse to be a fixed multiple of, but of opposite polarity to, the amplitude of the first readout gradient; and then multiplying the amplitude of each j-th one of the N readout gradients by a factor (-1) .sup.(j'1), to alternate the polarity of all readout gradients in that sequence; (e) subtracting the data in each j-th one, where l≦j≦N, of the NMR response signal data that is acquired in a selected one of first and second sequences, from the data in the like-numbered j-th data set of the remaining one of the first and second sequences, to generate a j-th one of a plurality N of difference data sets from which response data obtained from stationary nuclei has been substantially removed; (f) summing all difference data sets to obtain a set of data with an increased signal-to-noise ratio (SNR); and (g) generating, responsive to the increased-SNR data set, at least one angiographic projection image, each lying in a plane having a preselected relationship with respect to the first and second directions.

Claim 2

A method for acquiring at least one nuclear magnetic resonance (NMR) angiographic image of flowing material in at least a selected portion of a sample, comprising the steps of: (a) immersing the sample in a main static magnetic field; (b) nutating, in the initial part of each of a first sequence and a second sequence of a sequential pair of imaging sequences for each of a multiplicity S of regions of said selected sample portion, the spins of all nuclei of a selected species; (c) applying a pair of alternating-polarity flow-encoding signal pulses in a first magnetic field gradient impressed upon the sample, in a first direction selected to establish one axis of the NMR angiographic projection image, to cause a resulting NMR response echo signal from the spin of a moving nucleus to differ from the NMR response each signal resulting from the spin of a substantially stationary nucleus; each of the flow-encoding pulses in the first sequence of each pair having a polarity opposite to the polarity of the like-positioned flow-encoding pulse in the second sequence of each pair; (d) acquiring, responsive to a readout magnetic field gradient impressed upon the sample in a second direction, substantially orthogonal to the first direction, a set of data from the NMR response echo signal evoked, from the same sample portion, in each different one of a plurality N of acquisition time intervals of the first and second sequences, and including the substeps of: generating a dephasing pulse in the readout gradient prior to only the first of the N acquisition time intervals in each sequence; scaling the amplitude of the single dephasing pulse to be a fixed multiple of, but of opposite polarity to, the amplitude of the first readout gradient; and then multiplying the amplitude of each j-th one of the N readout gradients by a factor (-1) .sup.(j'1), to alternate the polarity of all readout gradients in that sequence; (e) subtracting the data in each j-th one, where l≦j≦N, of the NMR response signal data that is acquired in a selected one of first and second sequences, from the data in the like-numbered j-th data set of the remaining one of the first and second sequences, to generate a j-th one of a plurality N of difference data sets from which response data obtained from stationary nuclei has been substantially removed; (f) summing all difference data sets to obtain a set of data with an increased signal-to-noise ratio (SNR); and (g) generating, responsive to the increased-SNR data set, at least one angiographic projection image, each lying in a plane having a preselected relationship with respect to the first and second directions.

Claim 7

A method for acquiring at least one nuclear magnetic resonance (NMR) angiographic image of flowing material in at least a selected portion of a sample, comprising the steps of: (a) immersing the sample in a main static magnetic field; (b) nutating, in the initial part of each of a first sequence and a second sequence of a sequential pair of imaging sequences for each of a multiplicity S of regions of said selected sample portion, the spins of all nuclei of a selected species; (c) applying a pair of alternating-polarity flow-encoding signal pulses in a first magnetic field gradient impressed upon the sample in a first direction selected to establish one axis of the NMR angiographic projection image, to cause a resulting NMR response echo signal from the spin of a moving nucleaus to differ from the NMR response echo signal resulting from the spin of a substantially stationary nucelus; each of the flow-encoding pulses in the first sequence of each pair having a polarity opposite to the polarity of the like-positioned flow-encoding pulse in the second sequence of each pair; (d) acquiring, responsive to a readout magnetic field gradient impressed upon the sample in a second direction, substantially orthogonal to the first direction, and with said second direction being varied to be different in each j-th one, where l≦j≦N of a plurality N of response data acquisition time intervals of a sequence, with the second direction being the same in each like-numbered j-th acquisition interval of both sequences of all sequence pairs, a set of data from the NMR response echo signal evoked, from the sample portion, in each different one of the plurality N of acquisition time intervals of each of the first and second sequences, and including the steps of: acquiring in each sequence, and for at least one value of M greater than 1 and less than N, a plurality M of response data sets, each from a different one of M echoes in the same direction in that sequence, in each of at least one different preselected second direction, and with data sets being acquired from the same set of M echoes in each sequence; and summing the data values from all M response data sets in each sequence to obtain a single summed data set for that preselected second direction in that sequence; (e) subtracting the summed data set from one of the first and second sequences from the summed data set from the other sequence of the sequence pair, to generate a difference data set for that pair from which response data obtained from stationary nuclei has been substantially removed; and (f) generating for each difference data set a different angiographic image, with each image lying in a plane having a preselected relationship with respect to the first direction and to the different second direction assigned to that different image.

Claim 8

A method for acquiring at least one nuclear magnetic resonance (NMR) angiographic image of flowing material in at least a selected portion of a sample, comprising the steps of: (a) immersing the sample in a main static magnetic field; (b) nutating, in the initial part of each of a first sequence and a second sequence of a sequential pair of imaging sequences for each of a multiplicity S of regions of said selected sample portion, the spins of all nuclei of a selected species; (c) applying a pair of alternating-polarity flow-encoding signal pulses in a first magnetic field gradient impressed upon the sample in a first direction selected to establish one axis of the NMR angiographic projection image, to cause a resulting NMR response echo signal from the spin of a moving nucleaus to differ from the NMR response echo signal resulting from the spin of a substantially stationary nucelus; each of the flow-encoding pulses in the first sequence of each pair having a polarity opposite to the polarity of the like-positioned flow-encoding pulse in the second sequence of each pair; (d) acquiring, responsive to a readout magnetic field gradient impressed upon the sample in a second direction, substantially orthogonal to the first direction, and with said second direction being varied to be different in each j-th one, where l≦j≦N of a plurality N of response data acquisition time intervals of a sequence, with the second direction being the same in each like-numbered j-th acquisition interval of both sequences of all sequence pairs, a set of data from the NMR response echo signal evoked, from the sample portion, in each different one of the plurality N of acquisition time intervals of each of the first and second sequences, and including the steps of: acquiring in each sequence, and for at least one value of M greater than 1 and less than N, a plurality M of response data sets, each from a different one of M echoes in the same direction in that sequence, in each of at least one different preselected second direction, and with data sets being acquired from the same set of M echoes in each sequence; and summing the data values from all M response data sets in each sequence to obtain a single summed data set for that preselected second direction in that sequence; (e) subtracting the summed data set from one of the first and second sequences from the summed data set from the other sequence of the sequence pair, to generate a difference data set for that pair from which response data obtained from stationary nuclei has been substantially removed; and (f) generating for each difference data set a different angiographic image, with each image lying in a plane having a preselected relationship with respect to the first direction and to the different second direction assigned to that different image.