Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1998-12-08
2001-03-06
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S307000
Reexamination Certificate
active
06198289
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention disclosed and claimed herein is generally directed to a system for driving an MR gradient coil, so that the coil is operated in two or more different modes to produce pulses of selectively different gradient amplitudes and slew-rates during a single MR pulse sequence. More particularly, the invention pertains to a system wherein the gradient coil is operated in a given mode to produce gradient pulses having any gradient amplitude and any slew-rate up to a maximum gradient amplitude and maximum slew-rate, respectively, which correspond to the given mode. A preferred application of the gradient system of the invention is diffussion-weighted echo-planar imaging.
Persons of skill in the art have now recognized that an MR imaging sequence which combines diffusion-weighted (DW) imaging and echo-planar imaging (EPI) can serve as an effective clinical tool for the early diagnosis of acute stroke. As is well known, a DW imaging sequence is sensitive to particle motion resulting from diffusion, and comprises two successive gradient pulses of comparatively long time duration. Diffusion sensitivity is characterized by a parameter referred to as b-value which depends quadratically on DW gradient pulse amplitude. Because of the long time duration of the DW pulses, slew-rate, which is a measure of the increment of gradient amplitude in a unit time, is not of primary importance. However, it has been determined that use of a maximum gradient amplitude for the DW gradient waveform results in a shorter echo time (TE), for a given b-value, and therefore provides a higher signal-to-noise ratio (SNR). In one wholebody MR scanner, for example, TE can be reduced from about 99 msec to 72 msec when the amplitude of the DW gradient waveform is increased from 22 mT/m to 40 mT/m, for a b-value of 1100 sec/mm
2
. Such reduction in TE results in a considerable increase in SNR for brain tissue. Moreover, reduced TE decreases repetition time, and therefore increases volume coverage for the same imaging time. Alternatively, higher gradient amplitude allows a higher b-value with greater attendant diffusion sensitivity for a given TE. Using a higher gradient amplitude also makes DW trace imaging more feasible.
In EPI, a series of bipolar trapezoidal gradient pulses is used for data acquisition. It is very desirable to provide a high slew-rate, for successive pulses of the series, to reduce the spacing therebetween. In the readout of a combined DW-EPI sequence, a reduced spacing results in smaller off-resonance effects, such as image distortion and blurring. Moreover, it is anticipated that a substantial increase in slew-rate, over the currently used slew-rate of 120 T/m/sec, could provide these benefits in single shot EPI, which is the imaging method of choice for DW imaging.
From the above, it would appear that significant increases in both gradient amplitude and slew-rate would be very desirable in a combined DW-EPI sequence. However, simultaneously increasing both parameters could cause peripheral nerve stimulation and be in violation of FDA regulations. As is well known by those of skill in the art, the Reilly curve defines the limits of gradient amplitude and slew rate which are likely to result in nerve stimulation. If the values selected for the amplitude and slew-rate of a particular gradient pulse collectively exceed limits established by the Reilly curve, undesirable peripheral nerve stimulation could occur. Also, systems currently available in the prior art to provide two slew-rates generally require two different coils for each gradient axis. It would be very desirable, both for simplicity and to reduce the cost of gradient amplifiers, to provide a system which could operate a single gradient coil to produce pulses of different combinations of maximum gradient amplitudes and slew-rates.
SUMMARY OF THE INVENTION
The present invention is directed to a new gradient system, which is disposed to operate a single gradient coil in multiple modes. The system is capable of operating the coil to provide gradient pulses having multiple combinations of maximum gradient amplitudes and maximum slew rates. That is, each mode of operation has a corresponding maximum gradient amplitude and maximum slew-rate. When operated in a given mode, the gradient system can produce pulses of any gradient amplitude, up to the corresponding maximum amplitude, and of any slew-rate, up to the corresponding maximum slew-rate. Accordingly, the gradient system can provide optimal gradient performance for both diffusion-weighted imaging and echo-planar imaging. At the same time, performance parameters are employed which ensure that limits established by the Reilly curve, including a selected safety factor, are not exceeded. In one useful embodiment, the invention comprises a dual slew-rate gradient system, which is capable of operating the same gradient coil to selectively produce pulses of two different maximum slew-rates, and of two different maximum amplitudes.
Generally, the invention can be embodied as a method for operating an MR gradient coil during a specified imaging sequence. Such method comprises the steps of driving the coil in a first mode, during a first sub-sequence, to produce a number of first gradient pulses in which the gradient amplitudes are equal to or lower than a first maximum gradient amplitude and the slew-rates are equal to or lower than a first maximum slew-rate, and driving the coil in a second mode, during a second sub-sequence, to produce a number of second gradient pulses, in which the gradient amplitudes and slew-rates are equal to or lower than a second maximum gradient amplitude and second maximum slew-rate. In general, the operating modes can be in any order. In a preferred embodiment, the coil is operated during the first sub-sequence to produce a set of diffusion-weighted imaging pulses, and is operated during the second sub-sequence to produce a train of EPI pulses.
In a further embodiment, the invention is directed to a gradient amplifier or other apparatus for driving a single MR gradient coil in multiple modes of operation. The amplifier is operable in a first mode disposed to drive the coil to produce a number of first gradient pulses, each having an amplitude and slew-rate which do not exceed a first maximum gradient amplitude and a first maximum slew-rate, respectively, and operable in a second mode to drive the coil to produce a number of second gradient pulses, each having an amplitude and a slew-rate which do not exceed a second maximum gradient amplitude and a second maximum slew-rate, respectively. The second maximum gradient amplitude is selectively less than the first maximum gradient amplitude, and the second maximum slew-rate is selectively greater than the first maximum slew-rate. The apparatus further comprises a switch disposed to switch the gradient amplifier from one of the modes to the other, during a single brief time period. In a preferred embodiment, the amplifier is operable in the first mode to drive the gradient coil to produce a set of diffusion-weighted imaging pulses, and is operable in the second mode to dive the coil to produce a train of echo planar imaging pulses.
REFERENCES:
patent: 5492123 (1996-02-01), Edelman
patent: 5663647 (1997-09-01), Wirth et al.
Du Yiping
King Kevin F.
Cabou Christian G.
General Electric Company
Oda Christine
Price Phyllis Y.
Shrivastav Brij B.
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