EP0332634A1 - Procede et appareil d'imagerie a resonance magnetique oblique a angles multiples - Google Patents

Procede et appareil d'imagerie a resonance magnetique oblique a angles multiples

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Publication number
EP0332634A1
EP0332634A1 EP87907719A EP87907719A EP0332634A1 EP 0332634 A1 EP0332634 A1 EP 0332634A1 EP 87907719 A EP87907719 A EP 87907719A EP 87907719 A EP87907719 A EP 87907719A EP 0332634 A1 EP0332634 A1 EP 0332634A1
Authority
EP
European Patent Office
Prior art keywords
nmr
plane
gradient
sin
magnetic field
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP87907719A
Other languages
German (de)
English (en)
Other versions
EP0332634A4 (en
Inventor
David Smith
David Hertz
Robert Wolf
Robert H. Olsen
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Fonar Corp
Original Assignee
Fonar Corp
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Filing date
Publication date
Application filed by Fonar Corp filed Critical Fonar Corp
Publication of EP0332634A1 publication Critical patent/EP0332634A1/fr
Publication of EP0332634A4 publication Critical patent/EP0332634A4/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/483—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy
    • G01R33/4833—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
    • G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils

Definitions

  • 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.
  • 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
  • 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.
  • the planes 1-7 within a given scan are parallel, and have a constant distance D therebetween.
  • 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.
  • the present invention entails a method and apparatus for obtaining in the course of a single scan
  • 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
  • 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 NM R excitation and magnetic field gradient pulses.
  • E ach 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 positioni ng 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.
  • Figure 1 is a schematic diagram utilized to explain the oblique multi-slice technique of the prior art.
  • Figure 2 is a schematic diagram of a repetition sequence entailing various waveforms applied in accordance with conventional NMR imaging techniques.
  • Figure 3 is a schematic diagram of a generic gradient waveform utilized in a preferred embodiment of the present invention.
  • Figure 4 is a diagram of orthogonal slice selector and readout gradients which are rotated by an angle "a.” in accordance with the present invention.
  • Figure 5 is a schematic diagram of a display system and cursors thereon utilized with a preferred embodiment of the present invention.
  • Figure 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 Figure 5.
  • Figure 7 is a block diagram of an apparatus in accordance with the present invention.
  • Figure 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.
  • Figure 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.
  • Figure 10 is a schematic diagram illustrating a medical application of a preferred embodiment of the present invention.
  • 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.
  • present day NMR imaging apparatus 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the time of the produced spin-echo NMR signal after the 90 degree RF excitation pulse is designated as TE (for time of echo) .
  • TE for time of echo
  • 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.
  • 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.
  • 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.
  • 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.
  • additional selected planes can be sequenced utilizing different frequencies in a consecutive manner.
  • 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° RF pulse, a 180° 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 re _ by the sequence time interval TS.
  • 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 Figure 2.
  • the overall repetition interval T re _ (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.
  • imaging data for the 15 different planes or slices of an object are obtained during each repetition.
  • the vertical axis in Figure 9 has also been labeled with frequencies - ⁇ - f 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 Figure 2.
  • the P operation for each slice or plane within the overall repetition interval T re _ occurs during a corresponding one of the 15 sequence time intervals, TS, with no two P operations occurring during the same interval TS.
  • a 90" RF excitation pulse, and a 180° rephasing pulse are applied at a first frequency f ⁇ 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.
  • another sequence of a 90" RF excitation pulse, a 180" 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.
  • 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.
  • consecutive sets of pulses and reading out of signals, at different frequencies can be accomplished in one repetition time interval T re _.
  • 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.
  • 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 re _.
  • the slice selector magnetic field gradient is disposed along one of the three orthogonal axes, which may arbitrarily be designated as the Z axis.
  • the readout and the phase-encoding magnetic field gradients may arbitrarily be designated as disposed along the Y and X axes.
  • a generic wave form 10 is depicted from which the slice selector gradient wave form and the readout gradient wave form may be generated.
  • the wave form 10 rises from an initial value of -.5 to a maximum value of .5, after which it descends to its mini um of value of -1.5, thereafter rises once again to its maximum of .5, and then dips, finally, to once again --5.
  • the ideal slice selector gradient wave form depicted in Figure 2 is simply the wave .form 10 shifted upwardly by .5. That is, the amplitude of the slice selector gradient wave form at any time is merely that of the wave form 10 plus .5. Accordingly, the ideal slice selector gradient wave form possesses two flat peaks of amplitude 1 with a valley therebetween dipping to -1 and for the remainder of the time is 0. Similarly, the readout gradient wave, form is generated from the wave form 10 by taking the negative of the wave form 10 and adding .5 to it. That is, the amplitude of the ideal readout gradient wave form at any particular time is simply the negative of the value of the wave form 10 at that time to which .5 has been added.
  • each of the rotated axes, considered as vectors may be described as a linear combination of the Y and Z axes, considered as vectors.
  • Each of the magnetic field gradients may be considered a vector pointing in the direction of the gradient. If the slice selector gradient in the Z direction is designated Z ss , and a slice selector gradient in the Y direction is designated Y SS and if a slice selector gradient in the Z ⁇ direction is designated z ⁇ gs' then in accordance with the above equations, 1SS is equal to COS(a)Z ss + SIN(a)Y ss .
  • a readout gradient in the Z direction is designated Z R0
  • a readout gradient in the Y direction is designated Y R0
  • a readout gradient in the Y ⁇ ⁇ direction is designated ⁇ iRO '
  • Y 1R0 i s e al to -SIN(a)Z R0 + COS(a)Y RQ may be generated from a linear combination of the slice selector gradients in the Z and Y directions.
  • a readout gradient in the - ⁇ direction may be generated from a linear combination of the readout gradients in the Z and Y directions.
  • Conventional NMR imaging apparatus comprise X, Y and Z coils for generating magnetic field gradients in, respectively, the X, Y and Z directions.
  • the slice selector wave form, SS(t) is applied to the Z coil.
  • the slice selector wave form, SS(t) is applied to the Y coil.
  • a readout gradient in the Z direction is created by applying the readout wave form, RO(t), to the Z coil, and a readout gradient in the Y direction is created by applying the readout wave form RO(t) , to the Y coil.
  • the wave form applied to the Z coil is G(t)[COS(a) + SIN(a)] + A[COS(a) - SIN(a)], and
  • the waveform applied to the Y coil is G(t) [SIN(a) - COS(a) ] + A[SIN(a) + COS(a)].
  • the slice selector waveform SS(t) is multiplied by a constant, denoted C ss , such that the resulting waveform provides a predetermined slice selector gradient, denoted G gauss per inch.
  • the readout waveform is multiplied by a constant, denoted C ro , such that the readout gradient possesses a predetermined value, which preferably is also G gauss per inch. Accordingly, to provide a slice selector gradient in the Z 1 direction, and a readout gradient in the Y ⁇ direction having predetermined values, the above waveform applied to the Z coil is multiplied by a constant, and the above waveform applied to the Y coil s multiplied by a constant.
  • the waveform applied to the Z coil is multiplied by C ss
  • the waveform applied to the Y coil is multiplied by C ro .
  • the waveform applied to the Z coil is [G(t)[C0S(a) + SIN(a) ] + A[ (COS(a)-SIN(a) ]] C ss
  • the waveform applied to the Y coil is [G(t) [SIN(a)-COS(a) ] + A[SIN(a) + COS(a)]] C rQ .
  • a display system 70 utilized in conjunction with a preferred embodiment of the present invention is depicted.
  • image data from an object in an NMR scanner taken in a plane 92 parallel to the YZ plane, and having a particular X coordinate, X Q is displayed on 5 the system 70.
  • the image data in the plane 92 is obtained by initially applying a slice selector gradient along the X axis. Subsequently, the slice selector, the read out, and the phase encoding gradients are in, respectively, the Z, Y and X 10 directions.
  • a center 90 of the display system 70 corresponds to a center 91 of the NMR magnet.
  • An object 81 appears in the plane 92 displayed on the system 70.
  • a cursor 78 having a center 79 designated with a dot is positionable on the system 70 via 15 controls 76 and 72. If image data from the object 81 is required in a plane orthogonal to the Y-Z plane at an angle "c" from the Y axis, then the cursor 78 is positioned such that the center dot 79 of the cursor 78 coincides with a desired point.82 on the object 81 0 and the cursor 78 is rotated to the desired angle " "c” .
  • the 0 display system 70 is coupled to a computer 71 to which is output the coordinate and angle information for the object point 82.
  • the computer 71 receives, from the system 70, the X coordinate, X Q , of the plane 92; the Y and Z coordinates of the object O point 82 relative to the center 90 of the system. 70, and the angle "c" of the desired plane, orthogonal to the Y-Z plane, passing through the object point 82. From these coordinates the location of the point 82 relative to the center 91 of the NMR magnet can be calculated.
  • the slice selector, readout and phase encoding gradients have been calibrated such that the magnetic field strength changes in their respective directions in a predetermined amount per inch. Preferably, this change is uniform for each of the gradients and is G gauss per inch.
  • a particular RF frequency required to excite nuclei in a plane having that particular strength is a particular RF frequency required to excite nuclei in a plane having that particular strength.
  • the position of the point under consideration on the system 70 can be translated into corresponding frequencies associated with the gradient planes passing through the point. For example, if the point under consideration has a Z coordinate of 6 inches, then, if there is a slice selector gradient along the Z direction of G gauss per inch, a plane orthogonal to the Z axis at 6 inches passing through the point will have a magnetic field strength of 6G + B Q , where B Q is the strength of the magnetic field at the origin.
  • the frequency of the RF pulse required to excite nuclei in this plane perpendicular to the slice selector gradient at 6 inches can be determined.
  • a plane 100 extending into the paper through the object 81 is disposed at an angle "c" from the readout gradient direction Y.
  • the plane 100 is perpendicular to the rotated slice selector gradient Z . That is, to generate a plane through the object 81 at the angle *c* having a constant magnetic field strength, the slice selector gradient Z is rotated by the angle *c" to form a rotated slice selector gradient Z 1 .
  • the length of a line 101 which extends from the origin perpendicularly to the plane 100, is calculated. Since the magnetic field changes by G gauss per inch, the change in the strength from the origin to the plane 100 may be calculated based upon the length of the line 101. To calculate the strength of the field in the plane 100 this calculated change is simply added to the strength of the field at the origin. In this fashion, one may derive the strength of the magnetic field in the plane perpendicular to the rotated slice selector axis, that passes through the object 81; and accordingly, infer the frequency of the RF pulse that will excite the nuclei in this plane.
  • the strength of the magnetic field in the plane 105 which is perpendicular to the rotated readout gradient Y ⁇ and which passes through the object 81 may be calculated from the length of the line 104, which is derivable from the angle of rotation " " .
  • the coordinate and angle data output by the display system 70 permit the computer 71 to calculate the strength of the magnetic field in the planes perpendicular to the rotated slice selector gradient, and the rotated readout gradient, which pass through the object under consideration. From the strength of the magnetic field in the plane perpendicular to the rotated slice selector gradient, the frequency of the RF pulse required to excite the nuclei in that plane may be derived.
  • the computer 71 can calculate the frequency required to demodulate the NMR signal during the read out period. Accordingly, the data from the system 70 provides all the necessary information for spatially encoding NMR signals output by the object corresponding to rotated slice selector and readout gradients.
  • the computer 71 performs all the necessary calculations for transforming the data output by the syste 70 into corresponding coordinates and f requencies required for the imaging process.
  • the display system 70 of Figure 5 is coupled to the computer 71 to provide information regarding the angle *c* of the position cursor 78.
  • a second cursor 92 having a center dot 93 is selectively positioned through a second portion of the object 81 at a required angle "d*.
  • Additional cursors may be utilized to take additional slices through selected portions of the object 81 at desired angles, which may be the same or different, as required.
  • slices may be taken through the same portion of the object 81 at different angles, through different portions at the same angle, through different portions at different angles, or any combination of the above. That is, in a single scan, slices may be taken through any desired portions of the object at any desired angles.
  • the position of the portion of the object through which a slice is taken is determined by the movement of a cursor such that its center coincides with the portion of the object 81 through which the slice is taken.
  • the center of the image corresponding to the slice will be the portion of the object 81 which coincides with the center of the cursor.
  • This information in conjunction with the X coordinate of the plane being viewed on the system 70 is sufficient to determine the strength of the magnetic field in the planes which pass through the object and are perpendicular to the rotated slice selector and readout gradients, and the phase encoding gradient.
  • the coordinate and angle data afforded by the system 70 regarding the portion of the object 81 through which a slice is taken provides all the necessary parameters for the corresponding NMR imaging process.
  • the computer 71 calculates all the necessary transformations from the data provided by the system 70 to the coordinates and corresponding frequencies of the NMR apparatus. In this fashion, utilizing a plurality of cursors on the system 70, any number of slices of corresponding portions of the object 81 may be taken at various angles within one scan. One simply positions the cursors in sequence at various selected portions of the object 81 at corresponding desired angles. Preferably, the cursors do not cross within the region of interest; since, image information may be impaired in the intersecting region.
  • the expression for the waveform applied to the Z coil above when expanded, contains a constant coefficient of the generic waveform G(t) , and a constant term.
  • the coefficient of G(t) is designated a multiplier and equals [COS(a) + SIN (a)] C ss .
  • the constant term is designated an offset and equals A[COS(a)-SIN(a) ] C ss .
  • the waveform applied to the Y coil contains a multiplier term, which is the coefficient of the generic waveform G(t) , and contains a constant term, designated an offset.
  • the multiplier for the Y coil waveform equals [SIN(a)-COS(a) ] C ro
  • the offset term equals A[SIN(a)+COS(a) ] C rQ .
  • the computer 71 calculates the multiplier and offset terms for the waveform applied to the Z coil, and calculates the multiplier and offset terms for the waveform applied to the Y coil. These values, associated with this slice, are output by the computer 71 to a RAM 28 where they are stored. This is done, in sequence, for each of the slices taken through the object 81 via the corresponding cursors on the system 70.
  • the computer 71 calculates for each slice an RF excitation frequency for the plane orthogonal to the direction of the slice selector gradient rotated by the angle of the slice, which passes through the - selected portion of the object, and a frequency required to demodulate the corresponding NMR signal during the read out period. These two frequencies are output by the computer 71 to a frequency synthesizer controller 73 where they are stored in sequence.
  • a generic gradient waveform generator 20 contains the generic waveform G(t) stored in digital
  • the generator 20 also stores the phase encoding waveform, as illustrated in Figure 2, in digital form. Preferably, the generator 20 stores these particular waveforms; but, may store others that suffice for purposes of the present invention.
  • a pulse programmer 24 controls the waveform generator 20, and a slice pointer 31.
  • the computer 71 controls a level pointer.35. In response to a pulse from the programmer 24, the generator 20 outputs, from a terminal 21, a word of stored waveform data corresponding to a particular axis. Simultaneously, a signal is output from a terminal 22 representing the axis corresponding to the output waveform data.
  • a word of the digitally stored phase encoding waveform is output; for the Y axis, a word of the digitally stored generic waveform G(t) is output, and for the Z axis, a word of the digitally stored generic waveform G(t) is also output.
  • the order of the axes is predetermined, but not necessarily cyclical. That is, the outputs of the terminal 22 are not necessarily X, Y, Z, X, Y, Z, etc. or some fixed permutation thereof; each axis need not appear every three outputs.
  • the exact order of the axes is predetermined; but, may be adapted to the needs of the particular situation.
  • the two outputs of the generator 20 are fed to an arithmetic unit 25.
  • the output of the terminal 22 of the generator 20 is conveyed to the multiplier and offset parameter RAM 28.
  • the slice pointer 31 in response to a pulse from the pulse programmer 24, outputs a signal to the RAM 28 and to the frequency synthesizer controller 73 indicating which of the slices, in the sequence of slices of the object 81, is to be taken.
  • the level pointer 35 indicates which of the repetitions of the . repetition sequence is to transpire, and conveys this information to the RAM 28 which also stores amplitude values for each of the repetitions.
  • An output terminal 41 of the arithmetic unit 25 is coupled to digital-to-analog converters 40, 42, and 45. An output terminal.
  • the 32 of the arithmetic unit 25 is also coupled to each of the digital-to-analog converters 40, 42, and 45.
  • the terminal 41 provides waveform data for a particular axis, and the terminal 32 indicates the axis corresponding to that data.
  • the digital-to-analog converters 40, 42, and 45 are coupled to, respectively, power supplies 47, 50 and 51, which are coupled, respectively, to an X coil 53, a Y coil 57, and Z coil 60, in the NMR apparatus.
  • the generic gradient waveform generator 20 in response to a pulse from the programmer 24, outputs from the terminal 21, a word of waveform data corresponding to a particular axis.
  • the waveform data and the axis signal are conveyed to the arithmetic unit 25, and the axis signal is conveyed to the multiplier and offset parameter RAM 28.
  • the slice pointer 31 provides a signal to the RAM 28 indicating the particular slice from the sequence of slices on the system 70 from which images are to be generated.
  • the RAM 28 In response to the axis indicating signal from the generator 20, and the slice indicating signal from the slice pointer 31, the RAM 28 outputs the above- described multiplier and offset terms for the corresponding axis and slice. Specifically, for an input from the slice pointer 31 indicating the second slice, and an input from the generator 20 indicating the Z axis, the RAM 28 outputs the multiplier [COS(d) + SIN(d)] C ss , and outputs the offset A[COS(d)-SIN(d) ] C ss .
  • the arithmetic unit 25 multiplies the digital data representing the generic waveform G(t) by the multiplier term and to this expression adds the offset term.
  • the digital-to-analog converter 40 corresponding to the Z coil is accessed by the axis indicating signal output by the terminal 32, and the digital data output by the terminal 41 is directed to the converter 40 where it is converted to analog.
  • the output of the converter 40 is applied to the Z coil 53 after being increased in amplitude by the power supply 47.
  • a similar operation is performed with respect to the Y axis to provide the corresponding waveform segment to the Y coil.
  • the arithmetic unit 25 seirves only to multiply the digital data representing the generic phase encoding waveform by a constant output by the RAM 28 in response to the repetition indicated by the level pointer 35. The resulting quantity is output from the terminal 41 of the arithmetic unit 25 to the X coil.
  • the Z coil, the Y coil, and the X coil receive corresponding waveforms in a predetermined sequence.
  • the waveforms applied to the Z coil and the Y coil serve to rotate the slice selector and the read out gradients in, respectively, the Z and Y axes by the angle corresponding to the slice indicated by the slice pointer 31.
  • the conventional NMR technique for eliciting signals representing image data is applied.
  • the process for generating images from the various rotated planes through the corresponding portions of the object 81 is similar to the conventional multi-slice technique, except that the waveforms are no longer necessarily the conventional ones indicated in Figure 2; but, the waveforms applied to the Z coil and Y coil are functions of the angle of the slice which is being taken.
  • the slices or planes taken in accordance With the present invention are excited one after another during different portions of an interval between repetitions by packing an integral number of slice excitations between successive excitations in one particular plane or slice. That is, when selective RF pulses are applied in the presence of a magnetic field gradient, only a limited region of the object is excited due to satisfaction of the resonance conditions.
  • the repetition sequence for any particular slice involves excitation followed by reading of the produced signal and then followed by a recovery interval before applying the excitation pulse in a subsequent repetition
  • the nuclei in different 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.
  • the number of planes for which NMR images can be obtained is dependent upon the recovery time interval between successive excitation pulses in a single plane and the sequence interval required for exciting and reading out of an NMR signal in one plane plus the time for switching of the gradients.
  • the above-described apparatus of a preferred embodiment of the present invention calculates the slice selector, read out and phase encodings waveforms in real time, just before application to the corresponding magnet coils, and excitation of the corresponding slice. In this fashion, relatively little memory is required for storage - purposes; most quantities are merely calculated, as needed, then output.
  • the present invention permits a plurality of slices of an object to be taken at varying angles within the same scan. Accordingly, there is no longer the need to position all of the slices in a multi- slice scan at the same angle, and to therefore require a second scan for any second* required angle of a slice.
  • FIG. 10 a particular medical use of the apparatus of the present invention is depicted.
  • a patient is disposed in an NMR imaging apparatus, and a scout scan is taken of the patient's spine, which is displayed on a screen 130.
  • an operator positions, via controls 131 and 133, cursors 140-146 having, respectively, centers 150-156 to provide image data from corresponding planes through the spine 160, In this fashion, image data may be generated from planes whose angles correspond to the dispositions of the various vertebrae of the spine 160.
  • the operator may select slices of the spine 160 at any required angles within one single scan. Further, the distances between the required planes need not be uniform, and the centers of the images corresponding to the cursors 140-146 need not be aligned. That is, the centers of the images corresponding to the cursors 140-146 will coincide with, respectively, the center dots 150-156, despite the fact that the center dots 150-156 are not aligned. While the above-described embodiment of the present invention utilized image data on the system 70 from a plane having a particular X coordinate, this is not necessary. The present invention may be utilized to take image data from various planes disposed at corresponding angles with respect to any one particular axis.
  • the image on the system 70 may be a conventional television image of the object taken from outside the object, and the cursors positioned with respect to this image.
  • the embodiment described above utilized slice selector and readout gradients that were rotations of reference slice selector and. readout gradients that were fixed from slice-to-slice, this is not necessary for the present invention. That is, a first slice may correspond to slice selector and readout gradients that are rotations of a slice selector gradient along the Z axis and a readout gradient along the Y axis; but, a second slice may correspond to a rotation of a slice selector gradient disposed along the X axis and the readout gradient disposed along the Z axis.
  • the reference readout and slice selector gradients may be permuted among the three orthogonal axes.
  • all that is required are the X, Y and Z coordinates of the point through which a plane is desired, and the angle of the plane relative to an arbitrarily chosen read out gradient direction, or equivalently the angle which the plane is rotated about an arbitrarily chosen phase encoding gradient direction.
  • the system 70 of Figure 7 provides this information; but, any source of this data, including direct input, suffices for the purposes of the present invention.
  • the slice selector, read out and phase encoding gradient directions may be permuted among the orthogonal coordinates from slice to slice.
  • the present invention may be employed with the spin-echo NMR technique, the free induction decay (FID) technique, or with any NMR technique utilizing a slice-selector gradient.
  • the present invention also entails methods corresponding to the operations described above.
  • the present invention pertains to a method and apparatus for obtaining NMR image data from a plurality of selected planes in an object, in the course of a single scan.
  • a patient is disposed in an NMR imaging apparatus and a "scout" image of a portion of the patient's body is displayed on, for example, a screen.
  • an operator positions several cursors, each cursor having a center, at various positions and orientations on the screen.
  • image data is provided from planes which traverse the part of the patient's body that is displayed on the screen in the positions and orientations indicated by the cursors.
  • the angles of the various selected planes may be different and, additionally, the distances between the various selected planes may be different and the centers of the various images from the selected planes may be different.

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  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)

Abstract

Un appareil permettant d'obtenir un seul balayage des données d'images à résonance magnétique nucléaire concernant une pluralité de plans sélectionnés d'un objet, inclinés avec différents angles, utilise un indicateur de tranche (31 de la Fig. 7) pour transmettre un signal indicateur d'un plan particulier à une mémoire à accès sélectif (28 de la Fig. 7) conservant des paramètres de multiplication et de décalage et fournissant des termes de multiplication et de décalage correspondant à l'angle du plan en question. Un générateur (20 de la Fig. 7) fournit une forme d'onde à gradient générique à une unité arithmétique (25 de la Fig. 7) qui combine cette forme d'onde avec les paramètres de multiplication et de décalage conservés par la mémoire à accès sélectif afin de créer des formes d'ondes qui, lorsqu'elles sont appliquées aux bobines de gradient (53, 57 et 60 de la Fig. 7) d'un appareil de résonance magnétique nucléaire, mettent en rotation le sélecteur de tranches et les gradients de lecture, ce qui permet d'obtenir des données d'images sur des plans inclinés avec différents angles. L'invention concerne également un procédé mis en oeuvre avec cet appareil.
EP19870907719 1986-11-14 1987-11-13 Apparatus and method for multiple angle oblique magnetic resonance imaging Withdrawn EP0332634A4 (en)

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US93133386A 1986-11-14 1986-11-14
US931333 1986-11-14

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GB8819705D0 (en) * 1988-08-19 1988-09-21 Royal Marsden Hospital Improvements in nmr spectroscopy localisation

Citations (2)

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GB2037996A (en) * 1978-11-16 1980-07-16 Emi Ltd Improvements in or relating to imaging systems
EP0216523A2 (fr) * 1985-08-27 1987-04-01 Resonex, Inc. Procédé pour l'imagerie par résonance magnétique nucléaire utilisant des gradients non-orthogonaux

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JPS5848839A (ja) * 1981-09-18 1983-03-22 Hitachi Ltd 核磁気共鳴を用いた検査装置
JPS5985651A (ja) * 1982-11-08 1984-05-17 株式会社東芝 診断用核磁気共鳴装置
US4673880A (en) * 1985-08-16 1987-06-16 Technicare Corporation Phase sensitive detection in multislice magnetic resonance imaging systems

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Publication number Priority date Publication date Assignee Title
GB2037996A (en) * 1978-11-16 1980-07-16 Emi Ltd Improvements in or relating to imaging systems
EP0216523A2 (fr) * 1985-08-27 1987-04-01 Resonex, Inc. Procédé pour l'imagerie par résonance magnétique nucléaire utilisant des gradients non-orthogonaux

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Title
AMERICAN JOURNAL OF RADIOLOGY, vol. 147, August 1986, pages 363-366; M.A. REICHER et al.: "Multiple-angle, variable-interval, nonorthogonal MRI" *
IEEE TRANSACTIONS ON NUCLEAR SCIENCE, vol. NS-31, no. 4, August 1984, pages 990-993, New York, US; A.A. MAUDSLEY et all.: "Electronics and instrumentation for NMR imaging" *
MEDICAL PHYSICS, vol. 13, no. 5, September/October 1986, pages 648-657, New York, US; M.A. BERNSTEIN et al.: "Pulse sequence generated oblique magnetic resonance imaging: Applications to cardiac imaging" *
See also references of WO8803655A1 *

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EP0332634A4 (en) 1991-03-20
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