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
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences

Definitions

• the present invention relates to an improved method for reducing the dead- periods in magnetic resonance imaging pulse sequences and to associated apparatus and. more specifically, it relates to a simplified means tor reducing dead-periods by employing a calculated waveform and. most specifically, is particularly advantageous in imaging oblique planes 2 Description of the Prior Art
• magnetic resonance imaging involves providing bursts of radio frequency energy to a specimen positioned m a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei
• the emitted signal may be detected in such a manner as to provide information as to the intensity of the response and the spatial origin ot the nuclei emitting the responsive magnetic signal.
• the imaging may be performed in a slice or plane, or multiple planes, or three-dimensional volume with information corresponding to the responsively emitted magnetic radiation being delivered to a computer which stores the information in the form of numbers corresponding to the intensity of the signal
• the computer establishes a pixel value as by employing Fourier Transformations which convert the signal amplitude as a function of time to signal amplitude as a function or frequency
• the signals may be stored in the computer and may be delivered with or without enhancement to a video screen display, such as a cathode-ray tube, tor example wherein the image created by the computer output w ill be presented through regions ot contrasting black and white which vary in intensity or color presentations which ⁇ arv in hue and intensitv
• a video screen display such as a cathode-ray tube
• the method of the present invention permits employing conventional magnetic resonance imaging equipment which has the computer controlling operation ot the gradient coils and amplifiers programmed, either by appropriate software or hardware embedded means.
• the duration of each dead-period is reduced.
• the term "dead-period" in a gradient-echo pulse sequence is the time interval between RF pulses and readout.
• a dead-period is a time interval in an MR pulse sequences where the waveform shape is not important, but its zeroeth. and/or higher moments are important.
• the nth moment of a gradient waveform in a dead-period is defined as
• t, and t c are the sta ⁇ ing and ending time of the waveform g.
• the sta ⁇ ing and ending values and zeroeth and/or higher moments of the slice, readout and phase encoding waveforms of a dead-period will be referred to as "the parameters of the dead-period "
• the zeroeth moment ill be the dead-period waveform area.
• the parameters of the dead-period are determined. These values are transformed into gradient amplifier coordinates and the performance characteristics of the amplifier are used to compute an optimal waveform trom the dead-period.
• the method involves tor each phase encoding step for the slice, phase encoding and readout directions, the sta ⁇ ing and ending gradient levels and the moments contained within the waveform are determined and are transformed into gradient amplifier coordinates The minimum dead-period is then employed with this calculated information to design a hardware optimized waveform for the desired imaging sequence.
• This approach is pa ⁇ icularly impo ⁇ ant in respect ot oblique scans.
• the apparatus of the present invention provides magnetic field generating means tor establishing a main magnetic field on the specimen, magnetic field gradient generating means for establishing gradients in said main magnetic field, and RF signal generating means tor emitting pulsed RF signals to at least a po ⁇ ion ot the magnetic field passing through the specimen
• Dead-period determining means to determine the minimum and maximum phase encoding steps by delivering for the slice, phase encoding and readout directions the scan plane pulse sta ⁇ ing and ending gradient levels and the zeroeth moment or zeroeth and higher moments contained within the dead-period waveform and transforming these values into gradient amplifier coordinates as by rotation after which the reduced or minimum dead-periods are calculated based upon performance characteristics of the amplifiers. While the preferred practice of the invention emplov s the minimum dead-period, reductions in the dead-period which do not reach as low as the minimum may be advantageously employed.
• the design waveform which is employable in oblique and non-oblique imaging, is obtained by design waveform creating means which for each phase encoding step for the slice, phase encoding and readout directions calculates the sta ⁇ ing and ending gradient levels and the gradient moments contained therein. These v alues are transformed to gradient amplifier coordinates by rotating the same A hardware optimized waveform is then determined employing the calculated minimum dead-periods.
• the apparatus also provides receiver means for receiving signals emitted trom the specimen responsive to the RF pulse being emitted and emitting responsive output signals which are delivered to computer means for establishing image information related thereto and visual display means tor displaying images from the image information received trom the computer means.
• ide larger capacity amplifiers and coils which would permit higher gradient levels and higher slew rates.
• Figure 1 is a schematic illustration of a magnetic resonance imaging system.
• Figure 2 is an illustration of a gradient-echo pulse sequence optimized for imaging non-oblique planes of the prior an.
• Figure 3 is a plot of the waveforms corresponding to Figure 2 as applied to gradient amplifiers/coordinates.
• Figure 4 is a plot of scan gradient values employed in the present invention and Figure 5 is a representation of the waveforms of Figure 4 after transrormation to gradient amplifier coordinates.
• Figure 6 illustrates hardware optimized trapezoids tor the gradient-ech pulse sequence showing amplifier currents and
• Figure 7 illustrates a corresponding gradient-echo pulse sequence which has been optimized
• FIGS 8 through 10 are logic diagrams showing a preferred practice o the present invention
• Figures 11 and 12 are contour plots of TR variation as a function ot plan orientation for. respectively, the practice of the present invention and prior an scan plan optimization
• Figure 13 is a plot of scan gradient coordinate values employing gradien moments and Figure 14 is a plot ot the waveforms of Figure 13 after conversion t gradient amplifier coordinates
• Figure 15 is an array ot eight types ot hardware optimized trapezoida pulses employable in the determination of the minimum dead-period
• Figure 16 is an array of waveforms of hardware optimized trapezoida pulses employable in determining waveforms for the amplifiers to be played du ⁇ ng th minimum dead-period
• Figure 17 is a plot of an optimized flow-compensated gradient-echo puls sequence employing hardware optimized trapezoidal pulses in the amplifier coordinat system
• Figure 18 is a flo -compensated gradient-echo pulse sequence optimize tor a non-oblique plane
• the term "specimen ' refers to any object placed in th mam magnetic field for imaging and shall expressly include, but not be limited t members of the animal kingdom, including humans, biological tissue samples, and tes specimens removed from such members of the animal kingdom It shall included inanimate objects which may be imaged by magnetic resonance or which contain wate or sources of other sensitive nuclei
• Figure 1 shows a schematic representation of the general concept o magnetic resonance imaging
• An RF source 2 provides a pulse radio trequencv energ to the specimen which, in the form shown, is a patient 4 in the main magnetic field whic is created by magnetic field generator 6
• the specimen is generallv aligned w ith th main magnetic field and the RF pulses are imposed perpendicular thereto.
• the angle of impingement of the RF vector representing the spatial gradient in the magnetic field will be angularly offset from either the x. ⁇ . or z directions. This arrangement results in excitation of the nuclei within the area or volume to be imaged and causes responsive emission of magnetic energy which is picked up bv receiver 8.
• the receiver 8 may be a coil which has a voltage induced in it as a result of such responsive emissions of magnetic energy.
• separate coils or identical coils may be employed as the RF source 2 and the receiver 8.
• the signal emerging from receiver 8 passes through analog-to-digital (A/D) convener 10 and enters computer 12.
• A/D analog-to-digital
• the Fourier Transformations of signals conve ⁇ the plot of amplitude versus time to a map of the distribution of frequencies by plotting amplitude versus frequency.
• the Fourier Transformations are performed in order to establish the intensity values and locations of specific pixels. These values may be stored, enhanced or otherwise processed and emerge to be displayed on a suitable screen, such as a cathode-ray rube 16. for example.
• the gradient- echo pulse sequence includes the RF pulse sequence 24. the slice pulse 26. the phase encoding pulse sequence 28 and the readout pulse sequence 30. It will be appreciated by looking at the legends at the uppermost po ⁇ ion of Figure 2 that the regions during which an RF pulse is being generated are labeled with the expression "RF" and the period for readout of data is indicated by the legend "READ. " The intermediate dead-periods are labeled, respectively. Dl and D2.
• the present invention targets reduction in the D l and D2 time periods which are the TE periods and the total elapsed time in the cycle of data acquisition which is the TR period.
• the present invention permits the use of existing equipment at a pa ⁇ icular facility with a customized designed waveform which facilitates reduction in the elapsed time periods Dl. D2. in which the waveform shape is not impo ⁇ ant for image formation, but the zeroeth. first and higher moments of the waveform are.
• Such reduction of time is pa ⁇ icularly critical in respect of oblique imaging and imaging of objects which are in motion such as cardiac imaging, for example.
• Figure 3 there are shown the RF pulses 24' and waveforms of Figure 2 as convened to gradient amplifier coordinate systems for application to t he gradient amplifier. For purposes of Figures 2 and 3.
• the imaging plane has been ob t ained by respective rotations of 30°, 30° and 45° rotations about the X. Y" . Z" axis.
• An example of a suitable 30°. 30°. 45° rotation matrix is shown in Equation 2.
• Figure 3 is illustrative of the problems created as the darkened regions ot the respective gradient waveforms, as shown in pulse sequences 26 * . 28' and 30' . are regions wherein the waveform exceeds the maximum allowable gradient level and slew rate.
• effo ⁇ s to employ the pulses of Figure 2 applied to the gradient amplifiers when used in imaging oblique planes are inadequate because the waveform shown in Figure 2 are no longer applied to the gradient amplifier.
• the present invention provides a system for calculating reduced and preferably the minimum duration dead-periods for scanning a given plane, including oblique planes. This is accomplished by focusing on the requirements of the gradient pulses for the pa ⁇ icular scan plane during the dead-period based upon the starting gradient level, the ending gradient level and the moments contained within the waveform in the dead-period of the slice, phase encoding and readout directions. These values are transformed from the scan plane coordinates to amplifier coordinates with an appropriate ro t a t ion by an oblique rotation matrix. Within the amplifier coordinate system, the hardware optimized pulse for each gradient amplifier for a given sta ⁇ ing amplitude, ending amplitude, and moments with a maximu ⁇ . slew rate waveform.
• the dead-period Dl has waveforms for slice direction 40, phase encoding direction 42 and readout direction 44
• the sta ⁇ ing gradient level s s , s p . s r . respectively, and the ending gradient levels e,, e p , e r , as well as the zeroeth moment, i.e.. area included within the waveform between the respective sta ⁇ ing gradient levels and ending gradient levels may be determined.
• the present invention is useful as to all magnetic resonance imaging wherein the shape of the waveform within the dead- period is not of importance. This will include essentially all magnetic resonance imaging except spectroscopy not employing gradients.
• the invention uses of the three parameters, i.e. , the sta ⁇ ing gradient value, the ending gradient value and the moments, the waveform shape is not a required parameter. With respect to the moments, the zeroeth moment will be employed alone for some pulse sequences and with higher moments tor other pulse sequences.
• the present invention employs a 3x3 o ⁇ hogonal rotation matrix R that translates from the slice encoding waveform, the readout encoding waveform and the phase encoding waveform to the corresponding physical gradient amplifier coordinates This transformation can be accomplished by multiplication of the parameter vectors w ith the rotation matrix. R, in accordance with Equation 3.
• R is a matrix determined by the oblique angle being employed.
• lower case "s" and “e.” respectively, represent the starting an ending gradient values and lower case “a” represents- the zeroeth or higher moment
• "p” and “s” represent, respectively, the readout, phase encoding and slic directions and lower case "x. y. z” represent the amplifier current directions. In th way. for each amplifier current direction, the parameters of sta ⁇ ing and ending gradie values and moments can be determined.
• the waveform of desired reduction or the sho ⁇ est duration that satisfi the three parameters for the amplifier current directions to produce the reduced sho ⁇ est waveform satisfying these parameters is referred to as the "hardware optimize waveform. " As used herein, the term shall be deemed to refer to trapezoidal waveform triangular waveforms with maximum slew rate ramps or other waveforms designed t utilize specific hardware optimally.
• the equation for the moment covered by t hardware optimized waveform with arbitrary sta ⁇ ing and ending levels is a first second order equation.
• the resultant amplifier coordinate waveforms are shown i Figure 5 wherein for the "x.y.z” directions, respectively, s, and e, are the sta ⁇ ing an ending gradient values and "a,” is the zeroeth moment and for the " y” direction, the and e, beginning and ending gradient values with zeroeth moment a and for the "z direction. s 2 and e z with the zeroeth moment being _ ⁇ .
• the calculation time for the global minimum dea period may be reduced by considering only the maximum and minimum phase encodi steps. As the required area for each amplifier is a linear function of the phase encodi step, either the maximum phase encoding step or the minimum phase coding step wi give the longest of the minimum dead-periods. In this manner, by selecting the longe of the minimum dead-periods, which will become the global minimum, each amplifi will have the same dead-period and the amplifier ' s performance limits ill not exceeded. After determining the global minimum for each dead-period, a hardware optimized waveform pulse that satisfies the three parameters, i.e.. the sta ⁇ ing and ending current values and the moment and fits into this minimum time is designed.
• While the preferred embodiment of the present invention involves seeking the minimum dead-periods, it will be appreciated that in some instances, one may obtain benefits of the invention by employing a reduced dead-period which may be greater than the absolute minimum. Such reduced dead-periods, when attained through the use of the methods or apparatus of the present invention, shall be deemed “minimuu " for purposes of the invention. In such an approach, the targeted or desired degree of dead-period reduction will be the "minimum" in the practice of the invention.
• the minimum possible TR with this pulse sequence depends on the angle ot the oblique plane and the specific imaging parameters, such as slice thickness.
• FOV field of view
• receiver bandwidth and number of phase encoding steps
• a sho ⁇ axis image of the heart with a double oblique view was acquired using segmented k-space cardiac tagging as disclosed in McVeigh "Cardiac Tagging with Breath Hold CINE MRI. " Magn
• the logic diagram shows a preferred practice of the present invention wherein the initial stage 70 involves calculating the slice and readout gradient levels along with the phase encoding area.
• this information is processed by calculating the minimum dead-period 72 after which the longest of the minimum dead-periods is employed to design the hardware optimized waveform 74 which is applied to the gradient amplifiers This process is employed tor dead-period 76 as indicated by the arrow returning to block 72.
• the maximum and minimum phase encoding step 82 is determined b> calculating the sta ⁇ ing and ending gradient levels and the required area 84 Focusing on the maximum and minimum phase encoding step saves time.
• the next determination is whether the first or higher order moments are impo ⁇ ant 86 These would be impo ⁇ ant. for example, in the event the system were employed for velocity-encoded or flow- compensated computations. While these two examples employ the zeroeth and first order moments, other sequence uses will use the zeroeth moment (area) and/ or second or higher moments, for example The required moments are calculated 88 and the results 84.
• the output of the response to 84 is delivered direc t lv t o the transformation to gradient amplifier coordinates po ⁇ ion 90.
• the minimum dead-period based on i t s hardware limitation 94. The process is repeated for the next phase encoding step 96 and is fu ⁇ her repeated for each direction 98 of gradient amplifier. The longest of the minimum dead- periods 100 is then determined. This will be employed in the design of the hardware optimized waveform as shown in Figure 10.
• phase encoding and readout direction 112. the sta ⁇ ing and ending gradient levels, along with the required moments 114 are determined. If the first or higher order moments are impo ⁇ ant 113. the moments are calculated 117 and delivered along with the output of 1 16 to a transformation block 118. Such moments are impo ⁇ ant. for example, in respect ot velocity-encoding or flow-compensation systems. If the moments are not of consequence, the output of 116 is delivered directly to 118. wherein the values are transformed to gradient amplifier coordinates through multiplication by the rotation matrix. For each gradient amplifier coordinate 120. the hardware optimized waveforms employing the calculated dead-periods design hardware optimized waveforms 122. For the next direction of gradient amplifier 124. the process is repeated. For each phase encoding step 126. the process is repeated.
• the minimum TR and TE for the imaging parameters were determined.
• the minimum TR and TE values were calculated for all possible oblique orientations and phase and frequency encoding directions with a resolution of 5 ° .
• the results are summarized in Table 1 with the results being expressed in milliseconds.
• TR and TE values were also calculated for the prior an scan plane optimization method which was described in the Bernstein et al. 1994 anicle cited hereinbefore.
• the hardware optimized waveform approach of the present invention in respect of minimum TR and TE established a reduction of up to 30 and 40 percent. respectively, in TR and TE as compared with the prior an scan plane optimization.
• the TR and TE values as a function of slice orientation are shown in the contour plots of Figures 11 and 12.
• the plane orientation is given by azimuth and polar angles of the plane normal.
• the frequency in phase directions of the RF pulse width that gave the minimum TR were employed.
• the TR variation for the former is less than for the latter.
• the scan plane optimization as shown in Figure 12. causes an increase in TR as the slice becomes oblique. This behavior also was observed for TE with the pulses.
• the highest TE value of the present invention was observed for non-oblique planes, whereas the scan plane optimization method of the prior an had the lowest TE at the non- oblique planes.
• hardware optimized trapezoidal pulses give the same minimum TR and TE as that obtained with scan plane optimization.
• the moment parameter of the dead-period waveform may be the zeroeth moment (area), the zeroeth moment and higher moments depending upon the type of pulse sequences involved. For example, with flow- compensated and velocity -encoded gradient-echo pulse sequences and similar pulses. consideration is given to the first moments.
• the waveforms applied to the gradient amplifiers may total on a single gradient amplifier in excess of the slew rate and maximum amplitude specifications for the amplifier, such as was shown in Figures 2 and 3.
• the hardware optimized waveform pulses employed to create amplifier current waveforms in accordance with the present invention may be employed in connection with a wide variety of pulse sequences, such as for flow-compensated or velocity-encoded gradient-echo pulse sequences, for example As shown in Figure 13.
• the sta ⁇ ing and ending gradient levels in addition to the areas a r , a_, a v (zeroeth moment), the first moments of the gradient waveforms, which have been designated, respectively, m.. m.. m, are considered
• These parameters starting and ending gradients, zeroeth moments and first moments
• the convened zeroeth moments a,. a v . a. the first moments m 5 . m v , m.
• the shape of the dead-period waveform is not impo ⁇ ant to this determination.
• the amplifier current coordinates are shown with the moments being respectively, m,. m,. and m.
• the next step is to calculate the minimum possible dead-period from the waveform that has the highest demand on one of the amplifiers.
• the solution for the minimum dead-period is simplified by the fact that the waveforms use tor this pulse will be maximum slew rate ramps or maximum amplitude constants.
• eight hardware optimized trapezoidal waveform types such as that shown in Figure 15. will be considered with one of them being selected tor a given set of S-. s,. s z . e,.
• Equal duration constant waveforms are added to each plateau in the bipolar pulses of the hardware optimized trapezoidal waveforms obtained in the determination ot
• Th minimum achievable TE for the imaging conditions used for cardiac imaging was considered with a slice thickness of 5 cm FOV (field of view) of 280 mm: pa ⁇ ial ech panial excitation: a pixel band with a 250 Hz/pixel: RF pulse duration of 2.5 msec.
• FOV field of view
• pa ⁇ ial ech panial excitation a pixel band with a 250 Hz/pixel: RF pulse duration of 2.5 msec.
• the present invention offers a substantial reduction in TE by a no iterative method which does not require user interaction. Both anisotropic and isotropi gradient sets can be utilized optimally .
• the present invention provides a improved method and associated apparatus for enhancing or maintaining image qualit of magnetic resonance images while reducing the length of the uat acquisition perio All of this is accomplished in a manner which focuses three specific variabl within the dead-period and. in general, is independent ot the shape of the wavefor within the dead-period.
• the invention employs the sta ⁇ ing amplitude, the endin amplitude, and gradient moments of the dead-period to establish a reduced or minimum dead-period.
• the reduced or minimum dead-periods are transformed to gradient amplifier coordinates and are employed in combination with the sta ⁇ ing and ending amplitudes and the gradient moments to create a hardware optimized waveform, which may be a trapezoidal waveform.
• the hardware optimized waveform for both oblique and non-oblique imaging is provided.
• the invention is also employable with velocity-encoded or flow-compensated pulse sequences, for example. All of this is accomplished in a manner which facilitates employing the gradient amplifiers that exist at a pa ⁇ icular facility to maximum advantage.

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• 1994
  • 1994-11-25 US US08/346,130 patent/US5512825A/en not_active Expired - Lifetime
• 1995
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