Low R.F. dosage magnetic resonance imaging of high velocity flows
First Claim
1. A method of magnetic resonance imaging comprising:
- (a) applying a resonance excitation pulse;
(b) applying a slice selection gradient pulse sequence;
(c) applying a read gradient pulse sequence;
(d) applying a phase encode gradient;
(e) at least one of the slice selection and read gradient pulse sequences including complementary pulses of a like product of duration and amplitude between the resonance excitation pulse and a resonance field echo and of opposite polarity such that the effective first moment in time is substantially zero to cause the resonance field echo;
(f) monitoring the field echo.
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Accused Products
Abstract
A main magnetic field coil (10) and control (12) cause a generally uniform main magnetic field through an image region. A resonance excitation control (22) causes an R.F. coil (20) to generate excitation pulses (100). A slice gradient control (32) and a read gradient control (34) cause a gradient coil (30) to generate complementary slice selection gradient profiles (112, 114) and complementary read gradient profiles (122, 124) in such a manner that the effective first moment in time is substantially zero. By time shifting a pulse in one or both of the slice selection and read gradient sequences (FIGS. 3 and 4), resonating nuclei in the selected slice can be phase encoded. A transform algorithm (40) transforms field echo signals (102) received by the R.F. coil into image representations. A first memory (54) receives real and imaginary portions of the image representations when the read and slice selection gradients are not shifted and a second memory (56) receives the image representations when one or both of the read and slice selection gradients are time shifted. A phase difference map (70) is calculated (60) from the arctangent of phase difference values derived from the first and second images. The intensity of each pixel of the phase difference map varies with phase shift, hence velocity.
32 Citations
19 Claims
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1. A method of magnetic resonance imaging comprising:
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(a) applying a resonance excitation pulse; (b) applying a slice selection gradient pulse sequence; (c) applying a read gradient pulse sequence; (d) applying a phase encode gradient; (e) at least one of the slice selection and read gradient pulse sequences including complementary pulses of a like product of duration and amplitude between the resonance excitation pulse and a resonance field echo and of opposite polarity such that the effective first moment in time is substantially zero to cause the resonance field echo; (f) monitoring the field echo. - View Dependent Claims (2, 3, 4, 5)
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6. A method of magnetic resonance imaging of moving substances, the method comprising:
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(a) applying a resonance excitation pulse; (b) applying a plurality of slice selection gradient pulses of opposite polarity; (c) applying a read gradient pulse sequence including a plurality of read gradient pulses of opposite polarity, one of the read gradient pulses being applied during collection of resonance echo signals from a resonance echo; (d) applying a phase encode gradient between application of the resonance excitation pulse and collection of the resonance echo signals; and
,(e) repeating steps (a) through (d) with at least one of the slice selection gradient pulses and read gradient pulses altered such that altered gradient echo resonance signals are collected. - View Dependent Claims (7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. A method of magnetic resonance examination of moving substances, the method comprising;
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(a) acquiring a first image having real and imaginary components with first gradient pulse sequences; (b) acquiring a second image having real and imaginary components with second gradient pulse sequences in which at least one pulse is altered relative to the first gradient pulse sequence; (c) multiplying the real components of the first and second images, multiplying the imaginary components of the first and second images, and adding the multiplication products; (d) multiplying the real component of the first image with the imaginary component of the second image, multiplying the imaginary component of the first image with the real component of the second image, and subtractively combining the products; (e) taking an arctangent of the sum of step (c) and the difference of step (d) to produce a phase map whose components vary in accordance with phase difference.
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19. A magnetic resonance apparatus for examining moving substances, the apparatus comprising:
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a main magnetic field control means for generating a generally uniform magnetic field through an image region;
at least one coil for transmitting and receiving radio frequency signals, the coil being disposed adjacent the image region;a resonance excitation control means for selectively causing the coil to transmit a resonance excitation pulse; a receiving means for receiving a radio frequency resonance signal received by from the coil; a gradient coil means for selectively causing magnetic field gradients across the main magnetic field; a slice selection gradient means for selectively causing the gradient coil means to generate gradient pulse sequences for selecting a slice through the image region to be imaged; a read gradient control means for selectively causing the gradient coil means to generate a read gradient pulse sequences across the slice; a phase encoder means for selectively causing the gradient coil means to phase encode resonating nuclei in the selected slice; a transform means for selectively transforming the resonance signal from the receiving means into an image representation which has real and imaginary values each having a phase component for each of a plurality of pixels; a first memory means for storing the real and imaginary values of a first image representation which is produced when the slice selection gradient control means and the read gradient control means apply slice selection and read gradient pulse sequences in such a manner that an effective first moment in time in the selected slice is substantially zero; a second memory means for storing the real and imaginary values of a second image representation which is produced when at least one of the slice selection gradient control means and the read gradient control means flow encodes resonating nuclei in the selected slice by shifting at least one pulse of the respective slice selection and read gradient pulse sequences; a phase difference determining means for determining a phase map whose intensity values vary with phase difference, the phase difference determining means being operatively connected with the first and second memory means for receiving phase components of the first and second image real and imaginary values and determining a phase map whose pixel valves vary with phase difference between phase components of corresponding pixels.
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Specification