Apparatus and method for multiple angle oblique magnetic resonance imaging

What is claimed is:

1. A method for obtaining in the course of a single scan NMR image data for a plurality of differently oriented selected planes in an object using nuclear magnetic resonance techniques, said method comprising the steps of:

(a) positioning an object in a static homogeneous magnetic field;

(b) determining first and second selected planes in said object for which NMR image data is to be obtained, said first selected plane being located at a first portion of said object and having a first orientation with respect to a predetermined direction and said second selected plane being located at a second portion of said object and having a second orientation with respect to said predetermined direction, said first and second orientations being different from one another;

(c) subjecting said object to a plurality of repetitions of a first repetition sequence composed of NMR excitation and magnetic gradient field pulses, each of said repetitions of said first repetition sequence including the steps of applying an excitation pulse and reading out of an NMR signal produced by said excitation pulse, said excitation pulse for said first repetition sequence being applied at a first predetermined frequency in the presence of a first predetermined slice selector magnetic field gradient having a gradient direction extending perpendicular to said first selected plane, said first predetermined frequency being chosen so that said application of said excitation pulse at said first predetermined frequency only excites selected nuclei in said first selected plane, and said plurality of repetitions of said first repetition sequence being carried out in a manner to encode spatial information into a first collection of said NMR signals, said first collection of NMR signals being representative of NMR image data for said first selected plane; and

(d) subjecting said object to a plurality of repetitions of a second repetition sequence composed of NMR excitation and magnetic field gradient pulses, each of said repetitions of said second repetition sequence including the steps of applying an excitation pulse and reading out of an NMR signal produced, by said excitation pulse, said excitation pulse for said second repetition sequence being applied at a second predetermined frequency in the presence of a second predetermined slice selector magnetic field gradient having a gradient direction extending perpendicular to said second selected plane, said second predetermined frequency being chosen so that said application of said excitation pulse at said second predetermined frequency only excites selected nuclei in said second selected plane, said second predetermined slice selector magnetic field gradient and said second predetermined frequency being different from said first predetermined slice selector magnetic field gradient and said first predetermined frequency, respectively, and said plurality of repetitions of said second repetition sequence being carried out in a manner to encode spatial information into a second collection of NMR signals, said second collection of NMR signals being representative of NMR image data for said second selected plane;

said plurality of repetitions of said first and second repetition sequences each being carried out during the course of a single scan of said object and each being continued substantially throughout said single scan, the repetition time interval for repeating each of said first and second repetition sequences being substantially the same and said steps of applying an excitation pulse and reading out of an NMR signal for each repetition of said second repetition sequence being performed at a different time during said repetition time interval than each of said steps of applying an excitation pulse and reading out of an NMR signal for said first repetition sequence.

2. The method of claim 1, wherein step (c) comprises generating said magnetic gradient field pulses of said first repetition sequence via a first and a second waveform, corresponding to said first orientation of said first selected plane, which produce said first predetermined slice selector magnetic field gradient, and a first predetermined read out magnetic field gradient having a direction orthogonal to that of said first predetermined slice selector magnetic field gradient; and

wherein step (d) comprises generating said magnetic field gradient pulses of said second repetition sequence via a third and a fourth waveform, corresponding to said second orientation of said second plane, which produce said second predetermined slice selector magnetic field gradient, and a second predetermined read out magnetic field gradient having a direction orthogonal to that of said second predetermined slice selector magnetic field gradient.

3. The method of claim 2 using an NMR imaging apparatus having first means for generating a first magnetic field gradient having a first direction, and second means for generating a second magnetic field gradient having a direction orthogonal to said first direction, a direction of said first predetermined slice selector gradient being said first direction rotated by a first angle a, and a direction of said second predetermined slice selector gradient being said first direction rotated by a second angle b, wherein the step of generating said magnetic gradient field pulses of said first repetition sequence comprises applying said first waveform to said first means, said first waveform being of a form [G(t)[COS(a)+SIN(a)]+A[(COS(a)-SIN(a)]]C.sub.ss, where G(t) is a predetermined gradient waveform, and A, and C.sub.ss are predetermined constants; and

applying said second waveform to said second means, said second waveform being of a form [G(t)[SIN(a)-COS(a)]+A[SIN(a)+COS(a)]]C.sub.ro, where C.sub.ro is a predetermined constant; and

wherein the step of generating said magnetic field gradient pulses of said second repetition sequence comprises applying said third waveform to said first means, said third waveform being of a form [G(t)[COS(b)+SIN(b)]+A[(COS(b)-SIN(b)]]C.sub.ss ; and

applying said fourth waveform to said second means, said fourth waveform being of a form [G(t)[SIN(b)-COS(b)]+A[SIN(b)+COS(b)]]C.sub.ro.

4. A method for conducting an examination of an object along two different selected planes using nuclear magnetic resonance techniques, said method comprising the steps of:

(a) positioning an object in an NMR imaging apparatus which includes means for generating a magnetic field, means for exciting selected nuclei to generate NMR signals and for reading of such NMR signals to provide a collection of NMR signals from selected regions of an object placed in said NMR imaging apparatus, means for applying gradient magnetic fields, means for obtaining NMR imaging data from said collection of NMR signals and means for producing an image from said NMR imaging data;

(b) operating said NMR imaging apparatus to obtain an NMR scout image for a portion of said object of said examination;

(c) while said object remains positioned in said NMR imaging apparatus, using said scout image to select a first plane and a second plane of said object for which NMR image data is to be obtained, said first plane and said second plane each being transverse to said scout plane, and said first plane having a first orientation relative to said scout plane and said second plane having a second orientation relative to said scout plane, said first orientation being different from said second orientation;

(d) conducting a plurality of NMR sampling operations to obtain NMR imaging data from said first selected plane of said object, said step of conducting said sampling operations for said first plane being commenced at a first time during the course of a single scan and being continued substantially throughout said single scan so as to obtain NMR imaging data for said first-selected plane of said object;

(e) conducting a plurality of NMR sampling operations to obtain NMR imaging data from said second selected plane of said object which is different from said first selected plane, said step of conducting said sampling operations for said second selected plane being commenced at a second time during the course of said single scan which is later than said first time, but prior to completion of said step of conducting said sampling operations for said first selected plane, and said step of conducting said NMR sampling operations for said second selected plane being continued substantially throughout said single scan to obtain NMR imaging data for said second selected plane;

each of said plurality of NMR sampling operations including an NMR excitation operation and an NMR reading operation, said NMR excitation operations for each of said selected planes being carried out in a manner so as to excite selected nuclei in said each of said selected planes, and said NMR reading operations for each of said selected planes being carried out in a manner to encode spatial information into said obtained NMR imaging data, each of said NMR excitation and NMR reading operations being performed at a different time during the course of said single scan than each other of said excitation and reading operations.

5. The method of claim 4, wherein step (d) comprises applying a first and a second waveform, corresponding to said first orientation of said first plane, to said means for applying gradient magnetic fields, to produce a first predetermined slice selector magnetic field gradient having a direction orthogonal to said first plane; and wherein

step (e) comprises applying a third and a fourth waveform, corresponding to said second orientation of said second plane, to said means for applying gradient magnetic fields, to produce a second predetermined slice selector magnetic field gradient having a direction orthogonal to said second plane.

6. The method of claim 5, wherein said means for applying gradient magnetic fields comprises first means for generating a first magnetic field gradient having a first direction, and second means for generating a second magnetic field gradient having a direction orthogonal to said first direction, a direction of said first predetermined slice selector gradient being said first direction rotated by a first angle a, and a direction of said second predetermined slice selector gradient being said first direction rotated by a second angle b;

wherein the step of applying said first and second waveforms comprises applying said first waveform to said first means, said first waveform being of a form [G(t)[COS(a)+SIN(a)]+A[(COS(a)-SIN(a)]]C.sub.ss, where G(t) is a predetermined waveform, and A and C.sub.ss are predetermined constants;

and applying said second waveform to said second means, said second waveform being of a form [G(t)[SIN(a)-COS(a)]+A[(SIN(a)+COS(a)]]C.sub.ro, where C.sub.ro is a predetermined constant; and wherein

the step of applying said third and said fourth waveforms comprises applying said third waveform to said first means, said third waveform being of a form [G(t)[COS(b)+SIN(b)]+A[(COS(b)-SIN(b)]]C.sub.ss, and applying said fourth waveform to said second means, said fourth waveform being of a form [G(t)[SIN(b)-COS(b)]+A[(SIN(b) +COS(b)]]C.sub.ro.

7. An apparatus for obtaining, in a course of a single scan, NMR image data for a plurality of selected planes, in an object, dispose at different angles relative to a predetermined direction, comprising:

means for providing generic gradient waveforms; and

means, coupled to said providing means, for generating gradient waveforms in said single scan that produce slice selector magnetic field gradients having, respectively, directions which are orthogonal to, respectively, said plurality of selected planes.

8. The apparatus of claim 7, wherein said generating means comprises:

means, coupled to said providing means, for providing multiplier and offset parameters corresponding to said different angles of said selected planes; and

means, coupled to generic gradient waveform providing means and to said multiplier and offset parameter providing means, for combining at least one of said generic gradient waveforms with said multiplier and offset parameters to produce said gradient waveforms.

9. The apparatus of claim 8, wherein said multiplier and offset parameter providing means includes means for providing multiplier parameters [COS(a)+SIN(a)]C.sub.ss and [SIN(a)-COS(a)]C.sub.ro for values of "a" corresponding to said different angles, where C.sub.ro and C.sub.ss are each predetermined constants, and for providing offset parameters A[(COS(a)-SIN(a)]]C.sub.ss and A[(SIN(a)+COS(a)]]C.sub.ro for values of "a" corresponding to said different angles, where A is a predetermined constant;

wherein said generic gradient waveform providing means includes means for providing a generic gradient waveform G(t); and wherein

said combining means includes means for generating waveforms having a form [G(t)[COS(a)+SIN(a)]A[(COS(a)-SIN(a)]]C.sub.ss and [G(t)[SIN(a)-COS(a)]+A[(SIN(a)+COS(a)]]C.sub.ro for values of "a" corresponding to said different angles.

10. An apparatus adapted to be coupled to a generator providing a generic gradient waveform G(t), for obtaining, in a course of a single scan, NMR image data for a plurality of selected planes, in an object, disposed at different angles relative to a predetermined direction comprising:

slice pointer means for providing signals representing, respectively, said plurality of selected planes; and

means coupled to said slice pointer means, for generating gradient waveforms in said single scan that produce slice selector magnetic field gradients having, respectively, directions which are orthogonal to, respectively, said plurality of selected planes, wherein said generating means comprises:

means, coupled to said slice pointer means, and adapted to be coupled to said generator, for providing multiplier and offset parameters corresponding to said different angles of said selected planes; and

means, coupled to said multiplier and offset parameter providing means, and adapted to be coupled to said generator, for combining said waveform G(t) with said multiplier and offset parameters to produce said gradient waveforms.

11. The apparatus of claim 10 wherein said multiplier and offset parameter providing means includes means for providing multiplier parameters [COS(a)+SIN(a)]C.sub.ss and [SIN(a)-COS(a)]C.sub.ro for values of "a" corresponding to said different angles, where C.sub.ro and C.sub.ss are predetermined constants, and for providing offset parameters A[COS(a)-SIN(a)]C.sub.ss and A[SIN(a)+COS(a)]C.sub.ro for values of "a" corresponding to said different angles, where A is a predetermined constant; and wherein

said combining means includes means for generating waveforms having a form [G(t)[COS(a)+SIN(a)]+A[COS(a)-SIN(a)]]C.sub.ss and a form [G(t)[SIN(a)-COS(a)]+A[SIN(a)+COS(a)]]C.sub.ro, for values of "a" corresponding to said different angles.

12. Apparatus for obtaining NMR image data from a plurality of selected planes in an object comprising:

(a) means for applying magnetic fields to the object;

(b) means for applying radio frequency excitation pulses to the object;

(c) means for actuating and controlling said magnetic field applying means and said radio frequency applying means to:

(1) apply a first sequence including a first slice selector magnetic field gradient in a first direction concomitantly with a first RF excitation pulse at a first frequency to thereby excite nucleii only in a first plane perpendicular to said first direction, whereby a first NMR signal will be emitted only by nucleii in said first plane, said first sequence further including at least one encoding magnetic field gradient operative to encode spatial information into said first NMR signal;

(2) apply a second sequence including a second slice selector magnetic field gradient in a second direction different from said first direction concomitantly with a second RF excitation pulse at a second frequency different from said first frequency to thereby excite nucleii only in a second plane perpendicular to said second direction whereby a second NMR signal will be emitted only by nucleii in said second plane, said second sequence further including at least one encoding magnetic field gradient operative to encode spatial information into said second NMR signal; and

(3) a repeat said first and second sequences a plurality of times during the course of a single scan so as to excite nucleii in said first and second planes alternatively and produce said first and second NMR signals alternately while varying said at least one encoding gradient in each said sequence on each repetition; and

(d) means for receiving said first and second NMR signals and recovering NMR image data therefrom.

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FIELD OF THE INVENTION

The present invention relates generally to magnetic resonance imaging and, more particularly, to a method and apparatus capable of obtaining, in a single scan, NMR image data from selected planes, in an object, disposed at various angles, having varying distances between each other, and having image centers shifted relative to each other.

BACKGROUND OF THE INVENTION

Conventional prior art magnetic resonance imaging apparatus and techniques entail oblique imaging which permits image data to be taken from a plane through an object which is at an angle to one of the three orthogonal axes. Also, the prior art in this domain entails multi-slice imaging which permits image data, in one scan, to be taken in a plurality of parallel planes, through the object, which are orthogonal to one of the three orthogonal axes, which are uniformly spaced one from the other, and whose image centers are all aligned.

Further, present magnetic resonance imaging apparatus entail a combination of the above methods in an oblique multi-slice technique which was disclosed by FONAR Corporation in a technical exhibit at a conference of the Radiological Society of North America in November, 1984. Referring to FIG. 1, the oblique multi-slice technique permits images of an object 11, in one scan, to be obtained in planes, such as 1-7 extending into the paper, which are disposed at an angle P relative to one of the three primary orthogonal axes, arbitrarily designated Y. However, the planes 1-7 within a given scan are parallel, and have a constant distance D therebetween. Further, the positioning of the center of the image corresponding to the plane 1 determines the center of the image corresponding to each of the planes 2-7. That is, when the center of the image corresponding to the plane 1 is selected to be at a point 12 on the object 11, the centers of the images of the planes 2-7 are necessarily therefore at, respectively, points 13-18. The selection of the center of the image corresponding to the plane 1 at the point 12 determines the centers of the images corresponding to the other planes 2-7.

Accordingly, in the prior art, to generate an image from a first plane disposed at a first angle relative to one of the orthogonal axis, and to generate an image from a second plane disposed at a second angle, two scans are required. A full scan including the first plane disposed at the first angle must be taken, and then a second full scan including the second plane disposed at the second angle must be taken. Similarly, if the distance between planes is desired to be varied, then once again several scans are required, each having one of the desired distances between planes. Further, if the centers of the images corresponding to two or more planes are not desired to be aligned as in the prior art, then a separate scan is required to center the image of a particular plane. For example, to center the image corresponding to the plane 6 at the point 19 on the object 11, a second scan would be required; since, in the first scan the center of the image corresponding to the plane 6 coincides with the point 17.

Accordingly, to obtain images from planes which are not parallel to each other, or which possess varying distances between one another, or which have misaligned image centers, requires additional scans and time that is wasted in capturing nonessential information.

Thus, there is a need for an apparatus and method which permit, in a single scan, magnetic resonance images to be obtained from planes disposed at different angles, having varying distances therebetween, and having shifted image centers.

SUMMARY OF THE INVENTION

The present invention entails a method and apparatus for obtaining in the course of a single scan NMR image data for a plurality of differently oriented selected planes in an object using nuclear magnetic resonance techniques. A method in accordance with the present invention comprises positioning an object in a static homogeneous magnetic field; and determining first and second selected planes in the object for which NMR image data is to be obtained. The first selected plane is located at a first position of the object and has a first orientation with respect to a predetermined direction, and the second selected plane is located at a second portion of the object and has a second orientation with respect to the predetermined position The first and second orientations being different from one another.

The object is subjected to a plurality of repetitions of a first repetition sequence composed of NMR excitation and magnetic gradient field pulses. Each of the repetitions of the first repetition sequence includes the steps of applying an excitation pulse and reading out of an NMR signal produced by the excitation pulse. The excitation pulse for the first repetition sequence is applied at a first predetermined frequency in the presence of a first predetermined slice selector magnetic field gradient having a gradient direction extending perpendicular to the first selected plane. The first predetermined frequency is chosen so that application of the excitation pulse at the first predetermined frequency only excites selected nuclei in the first selected plane. The plurality of repetitions of the first repetition sequence is carried out in a manner to encode spatial information into a first collection of the NMR signals which are representative of NMR image data for the first selected plane.

The object is subjected to a plurality of repetitions of second repetition sequence composed of NMR excitation and magnetic field gradient pulses. Each of the repetitions of the second repetition sequence includes the steps of applying an excitation pulse and reading out of an NMR signal produced by the excitation pulse. The excitation pulse for the second repetition sequence is applied at a second predetermined frequency in the presence of a second predetermined slice selector magnetic field gradient having a gradient direction extending perpendicular to the second selected plane. A second predetermined frequency is chosen so that the application of the excitation pulse at the second predetermined frequency only excites selected nuclei in the second selected plane. The second predetermined slice selector magnetic field gradient and the second predetermined frequency are different from the first predetermined slice selector magnetic field gradient and the first predetermined frequency, respectively. The plurality of repetitions of the second repetition sequence are carried out in a manner to encode spatial information into a second collection of NMR signals which are representative of NMR image data for the second selected plane.

The plurality of repetitions of the first and second repetition sequence are each carried out during the course of a single scan of the object and each are continued substantially throughout the single scan. The repetition time interval for repeating each of the first and second repetition sequences is substantially the same. The steps of applying an excitation pulse and reading out of an NMR signal for each repetition of the second repetition sequence are performed at a different time during the repetition time interval than each of the steps of applying an excitation pulse and reading out of an NMR signal for the first repetition sequence.

The present invention also entails a method for conducting an examination of an object along two different selected planes using nuclear magnetic resonance techniques. This method comprises positioning an object in an NMR imaging apparatus. The apparatus includes means for generating a magnetic field, means for exciting selected nuclei to generate NMR signals and for reading of such NMR signals to provide a collection of NMR signals from selected regions of an object placed in the NMR imaging apparatus, and means for applying gradient magnetic fields The apparatus further includes means for obtaining NMR imaging data from the collection of NMR signals and means for producing an image from the NMR imaging data. The method further comprises operating the NMR imaging apparatus to obtain an NMR scout image for a portion of the object of the examination. While the object remains positioned in the NMR imaging apparatus, the scout image is used to select a first plane and a second plane of the object for which NMR image data is to be obtained. The first plane and the second plane are each transverse to the scout plane, and the first plane has a first orientation relative to the scout plane, and the second plane has a second orientation relative to the scout plane. The first orientation is different from the second orientation.

A plurality of NMR sampling operations are conducted to obtain NMR imaging data from the first selected plane of the object The step of conducting the sampling operations for the first plane is commenced at a first time during the course of a single scan, and is continued substantially throughout the single scan so as to obtain NMR imaging data for the first selected plane of the object.

A plurality of NMR sampling operations are conducted to obtain NMR imaging data from the second selected plane of the object which is different from the first selected plane. The step of conducting the sampling operations for the second selected plane is commenced at a second time during the course of the single scan which is later than the first time, but prior to completion of the step of conducting the sampling operations for the first selected plane. The step of conducting the NMR sampling operations for the second selected plane is continued substantially through the single scan to obtain NMR imaging data for the second selected plane.

Each of the plurality of NMR sampling operations includes an NMR excitation operation and an NMR reading operation. The NMR excitation operations for each of the selected planes are carried out in a manner so as to excite selected nuclei in each of the selected planes, and the NMR reading operations for each of the selected planes are carried out in a manner to encode spatial information into the obtained NMR imaging data. Each of the NMR excitation and NMR reading operations is performed at a different time during the course of the single scan than each of the other excitation and reading operations.

An apparatus in accordance with the present invention for obtaining, in the course of a single scan, NMR image data for a plurality of selected planes, in an object, disposed at different angles, includes a generic gradient waveform generator, and a slice pointer for outputting signals representing, respectively, the selected planes. A level pointer outputs signals representing, respectively, the repetitions of a repetition sequence. A RAM, coupled to the waveform generator, and to the slice and level pointers, stores multiplier and offset parameters corresponding to the different angles of the selected planes An arithmetic unit, coupled to the waveform generator and the RAM, transforms generic gradient waveforms into waveforms that effect rotations of a slice selector and a read out gradient by the angles corresponding to the selected planes.

The present invention permits NMR image data to be acquired from planes at varying angles in a single scan. Unlike the prior art, one need not conduct an entire scan with a plurality of slices at one fixed angle, and only thereafter, in a second scan, alter the slices to a second desired angle. The slices in a given scan may vary in angle from one slice to the next in accordance with the needs of the occasion. Further, in a single scan, the distances between slices may vary, unlike the prior art; and, the center of the image corresponding to a slice may be shifted from one slice to the next.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram utilized to explain the oblique multi-slice technique of the prior art.

FIG. 2 is a schematic diagram of a repetition sequence entailing various waveforms applied in accordance with conventional NMR imaging techniques.

FIG. 3 is a schematic diagram of a generic gradient waveform utilized in a preferred embodiment of the present invention.

FIG. 4 is a diagram of orthogonal slice selector and readout gradients which are rotated by an angle "a" in accordance with the present invention.

FIG. 5 is a schematic diagram of a display system and cursors thereon utilized with a preferred embodiment of the present invention.

FIG. 6 is a diagram representing orthogonal magnetic field gradients which extend from a reference point in an NMR magnet, and a plane corresponding to an image displayed on the CRT screen of FIG. 5.

FIG. 7 is a block diagram of an apparatus in accordance with the present invention.

FIG. 8 is a diagram illustrating the determination of the magnetic field strength along a plane perpendicular to the direction of a rotated slice selector gradient, which passes through a selected portion of an object.

FIG. 9 is a schematic diagram depicting the timing of operations for fifteen slices during one repetition time interval of a multi-slice NMR imaging technique.

FIG. 10 is a schematic diagram illustrating a medical application of a preferred embodiment of the present invention.

Identical numerals in different figures refer to identical elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention entails a method and apparatus for obtaining NMR image data from a plurality of selected planes in an object which are nonparallel, in the course of a single scan.

Referring to FIG. 2, in order to construct images of an object, present day NMR imaging apparatus generally utilize magnetic field gradients for selecting a particular slice or plane of the object to be imaged and for encoding spatial information in signals provided by the object. For instance, one conventional technique involves subjecting an object to a continuous static homogeneous field extending along a first direction and to sets of sequences of orthogonal magnetic field gradients which each generate a magnetic field component in the same direction as the static field but whose strengths vary along the direction of the gradients. In accordance with this known technique, nuclear spins in a selected plane are excited by a selective RF pulse in the presence of one of the magnetic field gradients, the frequency of the selective RF pulse corresponding to the Larmour frequency for only the selected plane of the object as determined by the magnetic field gradient imposed on the static magnetic field. Conveniently, the applied magnetic field gradient is designated the slice selector gradient. The selected plane will thus extend in a direction perpendicular to the gradient direction of the slice selector magnetic field gradient. This gradient is generated by applying a wave form designated SS(t) to a coil disposed along one of the three orthogonal axes. The excited selected spins are then subjected to the other magnetic field gradients, which can be designated the readout and phase-encoding magnetic field gradients, utilizing a plurality of repetitions in which the amplitude of the phase-encoding gradient is varied for each repetition and in which the readout gradient is applied during the reading out of the generated NMR signals The readout magnetic field gradient is generated by a wave form designated RO(t) applied to a coil disposed along a second of the three orthogonal axes. The phase-encoding magnetic field gradient is generated by applying a wave form designated PE(t) to a coil disposed along the third of the three orthogonal axes. The received NMR signals are then transformed utilizing conventional two-dimensional Fourier transform techniques. The readout magnetic field and phase-encoding magnetic field gradients serve to encode spatial information into the collection of NMR signals so that two-dimensional images of the NMR signals in the selected plane can be constructed. As will be appreciated, during the scanning sequence, the various magnetic field gradients are repeatedly switched on and off at the desired intervals. Such a two-dimensional Fourier transform imaging technique and the pulse sequence for such a technique is described in the book entitled Nuclear Magnetic Resonance Imaging in Medicine, published in 1981 by Igaku-Shoin, Ltd., Tokyo, and is sometimes known as spin-warp imaging.

Furthermore, many NMR imaging schemes today rely on the collection of spin-echo NMR signals rather than free induction decay (FID) signals FID NMR signals are achieved by application of a 90 degree RF excitation pulse and then reading out of the produced signal The present invention may be utilized with NMR imaging techniques which employ either spin-echo NMR signals, or free induction decay NMR signals.

Referring to FIG. 2, in utilizing the spin echo signals, a 90 degree RF excitation pulse is followed by the application of a 180 degree rephasing RF pulse at a predetermined time interval after the 90 degree pulse. This produces a spin-echo signal at a corresponding time interval after the application of the 180 degree RF pulse. In NMR parlance, the tie of the produced spin-echo NMR signal after the 90 degree RF excitation pulse is designated as TE (for time of echo). Thus the 180 degree RF pulse is applied at a time interval of TE divided by 2 after the 90 degree RF pulse.

The technique of multi-slice imaging has been developed for obtaining NMR images from a multiple number of parallel planes of the object by exciting the nuclei in the planes and reading out NMR signals therefrom during a single scan. More particularly, in multi-slice imaging, the slices or planes in the imaging volume are excited one after another during different portions of the interval between repetitions by packing an integral number of slice excitations between successive excitations in one particular plane or slice. For example, when selective RF pulses are applied in the presence of a magnetic field gradient, only a limited region of the objected is excited due to satisfaction of the resonance conditions. Accordingly, different frequencies will excite different parts of the object. As the repetition sequence for any particular slice involves an excitation followed by reading of the new signal and then followed by a recovery interval before applying the excitation pulse in a subsequent repetition, the nuclei in differing regions or planes can be excited during the recovery interval for one particular plane, thus efficiently utilizing the recovery time interval to selectively excite nuclei and read out NMR signals in other planes. Generally, the number of planes for which NMR images can be obtained is dependent on the recovery time interval between successive excitation pulses in a single plane and the sequence interval required for exciting and reading out of a NMR signal in one plane plus the time for switching of the gradients. For example, in connection with a spin-echo imaging sequence, the slice interval will correspond to the time necessary to apply a 90 degree excitation RF pulse, to apply a 180 degree rephasing RF pulse, to observe the echo produced thereby, and to raise and lower the appropriate gradients. During the portion of a repetition sequence following the sequence time interval, additional selected planes can be sequenced utilizing different frequencies in a consecutive manner.

Since the recovery time before reapplying an excitation pulse in connection with NMR images is generally long in comparison to the time needed to apply the excitation pulse, the rephasing RF pulse and the reading of the signals (together with the time needed to switch the appropriate gradients on and off), it is apparent that NMR signals can be generated and read out for a number of planes within the overall repetition time interval. For example, it is convenient to let TS represent a sequence time interval needed for a single slice to apply a 90.degree. RF pulse, a 180.degree. rephasing RF pulse, to read out the spin-echo signal and the time needed to raise and lower the appropriate gradients. A number of slices or planes for which imaging data can be obtained is thus equal to the largest integral number obtained from dividing the repetition time T.sub.rep by the sequence time interval TS.

Referring to FIG. 9, there is shown a schematic diagram of the scheme for exciting selected nuclei in each of 15 planes, and for collecting the NMR imaging data from the corresponding plane in a multi-slice technique for one repetition interval, utilizing the repetition sequence discussed hereinabove with reference to FIG. 2. In FIG. 9, the overall repetition interval T.sub.rep (along the horizontal axis) has been divided into 15 equal time slice intervals, TS. The vertical axis represents a number of slices of planes for which imaging data is to be collected, and has been labeled 1-15 to represent 15 different planes. Thus, imaging data for the 15 different planes or slices of an object are obtained during each repetition. Also, since the number of planes correspond to the number of different frequencies for the RF pulses, the vertical axis in FIG. 9 has also been labeled with frequencies f.sub.1 -f.sub.15. The term "P" is utilized to represent the operations for exciting selected nuclei and reading out of the generated NMR signal (together with the operations for switching on and off the appropriate gradient coils), corresponding to the intervals 1-4 depicted in FIG. 2.

In connection with the multi-slice imaging sequence, the P operation for each slice or plane within the overall repetition interval T.sub.rep occurs during a corresponding one of the 15 sequence time intervals, TS, with no two P operations occurring during the same interval TS.

Thus, in conventional multi-slice imaging, during a first sequence interval TS of the repetition sequence, a 90.degree. RF excitation pulse, and a 180.degree. rephasing pulse are applied at a first frequency f.sub.1 and the produced spin-echo signal then read out. This NMR signal will be representative of an NMR signal for nuclei in the first plane. Thereafter, during the recovery interval for the excited nuclei in the first plane, another sequence of a 90.degree. RF excitation pulse, a 180.degree. rephasing pulse and the reading out of the spin-echo NMR signal, with appropriate switching of the gradients, is carried out during the second sequence interval TS. This latter signal is representative of NMR signals from nuclei in the second plane. Thereafter, subsequent sequences of excitation, rephasing and reading out of NMR signals are carried out for the other planes in subsequent sequence intervals TS, when the excited nuclei in the first two planes are relaxing during their respective recovery intervals.

Accordingly, in a multi-slice NMR imaging method, consecutive sets of pulses and reading out of signals, at different frequencies, can be accomplished in one repetition time interval T.sub.rep. In particular, the various slices or planes in the object being imaged are excited one after another, and the appropriate sequence interval, with the overall repetition rate for one slice being utilized to pack an integral number of slice intervals between successive excitations of the same slice. Each of the RF excitation and rephasing pulses is applied at a different frequency so as to excite a different slice or plane of the object. A single frequency only repeats itself once for each plane during the repetition time interval T.sub.rep.

The slice selector magnetic field gradient is disposed along one of the three orthogonal axes, whi