MAGNETIC RESONANCE IMAGING APPARATUS

Abstract: Gradients are caused across a magnetic field in an image region by gradient field coils (22). A transmitter (30) causes an RF antenna (32) to generate excitation pulses to excite selected dipoles from the image region to resonance. A first receiving coil (50) and a second receiving coil (52) are disposed along the gradient to receive magnetic resonance signals from first and second fields of view (102,104) within the image region. First and second phase sensitive detectors (70,72) demodulate the magnetic resonance signals at different frequencies. Analog-to-digital converters (80,82,84,86) digitize magnetic resonance signal components from the phase sensitive detectors to provide digital data for reconstruction by Fourier transform circuits (90,92). By interconnecting each of the receiving coils with one of the phase sensitive detectors, two of the analog-to-digital converters, and one of the Fourier transform circuits, two images each corresponding to one of the fields of view can be generated concurrently. Alternately, a single receiving coil may provide a single magnetic resonance signal which is split among the phase sensitive detectors and the analog-to-digital converters. In this manner parallel and processing channels are provided to reduce the noise by the square root of two.

Description: This invention relates to magnetic resonance imaging apparatus. Conventionally, magnetic resonance imagers have included a single receiving channel for receiving magnetic resonance signals for conversion into an image representation. Commonly, a single coil having a configuration particularly adapted to the region of the patient to be imaged is disposed within the bore of the magnetic resonance imager and interconnected with the receiver. In a quadrature detection system, the receiver includes a signal splitter which divides the magnetic resonance signal into sine and cosine components. The sine and cosine components are each demodulated to eliminate the carrier frequency and filtered to remove stray frequency components. One analog-to-digital converter digitizes the filtered sine component and a second converter digitizes the filter cosine component. A two dimensional Fourier transform computer subroutine transforms the digitized sine and cosine resonance signal data into phase maps or image representations. Using surface coils, imaging a plurality of body parts has heretofore required a corresponding plurality of scans. After each scan, the coil is moved adjacent a second body part and a second scan is conducted to generate the data for the image of the second body part. Alternatively, the adjacent body parts could be imaged in one scan by increasing the size of the coil and the field of view. However, such an increase in the field of view increases the noise and lowers the resolution. It is an object of the present invention to provide a new and improved magnetic resonance imaging technique and apparatus which overcomes the above referenced problems. According to a first aspect of the present invention there is provided a magnetic resonance imaging apparatus comprising means for generating a main magnetic field through an image region; means for causing a gradient magnetic field across the image region; means for exciting magnetic resonance of dipoles in the image region; means for receiving and demodulating magnetic resonance signals from resonating dipoles in the image region; and image reconstruction means for reconstructing an image representation from the received demodulated magnetic resonance signals, characterised in that said receiving and demodulating means receives and demodulates magnetic resonance signals for each of at least two different demodulation frequencies and said image reconstruction means comprises means for constructing a respective image representation from the demodulated signals for each said demodulation frequency. According to a second aspect of the present invention there is provided a method of magnetic resonance imaging comprising the steps of: generating a main magnetic field through an image region; causing a gradient magnetic field across the image region; exciting magnetic resonance of dipoles in the image region; receiving magnetic resonance signals from resonating dipoles in the image region; demodulating the received magnetic resonance signals; and using the demodulated magnetic resonance signals to construct an image representation characterised in that in the demodulating step demodulated signals for at least two different demodulating frequencies are produced; and in the image reconstructing step an image representation is constructed from the demodulated signals for each said demodulation frequency. Two embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic illustration of a magnetic resonance image apparatus in accordance with the present invention; Figure 2 illustrates dual receiver coil placement for concurrently imaging two regions of the spine; Figure 3 illustrates those components of the apparatus of Figure 1 which are utilized for concurrently deriving images from a plurality of surface coils, and Figure 4 is an alternative embodiment illustrating components for implementing a two coil, concurrent image technique. With reference to Figure 1, a magnetic resonance imaging apparatus includes a plurality of main magnetic field coils 10. A main magnetic field control means 12 controls the coils to cause the creation of a substantially uniform magnetic field longitudinally through an image region. A gradient field control means 20 causes a plurality of gradient coils 22 to generate magnetic field gradients across the image region. In particular, the gradient field control means causes slice select, read, and phase encode gradient pulses to be applied to the gradient coils to cause corresponding magnetic field gradients across the image region. A transmitter means 30 causes radio frequency transmission coils 32 to generate magnetic resonance excitation, inversion, and manipulation pulses in the image region. A carrier frequency oscillator 34 sets a carrier frequency for the transmitter. A sequence timing and control means 40 controls at least the gradient field control means 20 and the transmitter 30 such that the gradient pulse sequences and magnetic resonance excitation and manipulation pulses are applied with the appropriate timing. The timing and control means also control a plurality of switch means 42 for selectively interconnecting the below discussed components in different arrangements. A pair of receiving coils 50, 52 are disposed in the image region to receive magnetic resonance signals from an examined patient or subject. Preferably, the receiving coils are surface coils which conform to preselected regions of the patient's anatomy. For example, one coil may conform to the patient's back adjacent the lumbar region of the spine and the other might conform to a region of the patient's back adjacent the thoracic region of the spine. Alternatively, the receiving coils might be configured to conform to complementary body parts such as the kidneys, inner ear, knees, or the like. Preferably, the coils are coplanar to reduce or eliminate cross talk or interference between the received signals. Optionally, additional coils may be used. A separate receiver is connected with each of the receiving coils. More specifically, a first preamplifier 60 is connected with the first receiving coil and a second preamplifier 62 is connected with the first receiving coil and a second preamplifier 62 is connected with the second receiving coil. A detector means includes a pair of phase sensitive detectors 70 and 72 that are selectively interconnectable with the preamplifiers by the switching means 42. A frequency shift means 74 provides an offset to the carrier frequency generated by the oscillator 34. The offset is selected in accordance with the strength of the magnetic field gradient that lies along the direction of the receiving coils. The carrier or central frequency of one of the coils is offset from the central or carrier frequency of the signals received by the other coil by the frequency offset. The switching means 42 selectively switches the carrier frequency and the offset from the offset means 74 with the phase sensitive detectors 70, 72. The phase sensitive detectors each provide sine and cosine components of the received magnetic resonance signals. A plurality of analog-to-digital converters 80, 82, 84 and 86 are selectively interconnectable with the phase sensitive detectors by the switching means 42. An image reconstruction means, such as a first two dimensional transform means 90 and a second two dimensional Fourier transform means 92, is selectively interconnected with the analog-to-digital converters by the switching means 42. First and second image memories 94, 96 selectively store the reconstructed image representation as received from the Fourier transform means through the switching means 42. A display means 98 converts the image representation into a man readable display. With reference to Figures 2 and 3, the surface coils 50, 52 are disposed adjacent the thoracic and lumbar regions of a patient's spine or other selected body parts, which are disposed in the image region along a z-axis. A read gradient 100 is applied along the z direction to set up a first preselected field of view 102, e.g. a 15 centimeter field of view. The resonance signals emanating from the first field of view region of the patient have a center frequency f₀ and a bandwidth ± ▵f. The first preamplifier 60 amplifies the resonance signals received by the first coil 50. The first phase sensitive detector 79 demodulates the resonance frequency about the center frequency f₀ and divides it into first and second components that have a preselected phase relationship, preferably sine and cosine components that have a 90° phase difference. The sine and cosine components are individually digitized by analog-to-digital converters 80, 82 to provide digital inputs for the first two dimensional Fourier transform means 90. The resultant image of the first field of view is stored in the first image memory 94 for display on the display means or the like. The second coil 52 is centered in a second field of view 104 to receive magnetic resonance signals emanating from that region. The magnetic field gradient is selected such that the resonance signal from the second field of view has a center frequency f₀ + 2▵f and a bandwidth ±▵f. The signal from the second coil 52 is amplified by the second preamplifier 62 and demodulated by the second phase sensitive detector 72. The second phase sensitive detector demodulates the resonance signals from the second coil at a frequency which differs from the center or demodulation frequency of the first phase sensitive detector by 2▵f. The 2▵f bandwidth is selected such that the first and second fields of view do not overlap under the implemented imaging conditions. The 2▵f is equal to or greater than the average of the bandwidths spanned by the fields of view. It might be noted that if the second receiving coil were connected with the first phase sensitive detector without adjustments to the read gradient or other imaging conditions, then all received frequencies from the second field of view would be outside of the first field of view and would be filtered out by bandwidth limiting filters of the first phase sensitive detector. Analogously, if the first coil were connected with the second phase sensitive detector, the resonance signal components received from the first field of view would be filtered by the filters of the second phase sensitive detector. The sine and cosine components from the second phase sensitive detector are digitized by analog-to-digital converters 84, 86. The second two dimensional Fourier transform means 92 transforms the digitized components into a second image representation that is stored in the second image memory 96. In this manner, at the end of a single imaging sequence or scan, two complete sets of image data are collected with no loss in the signal-to-noise ratio or the resolution. Analogously, third or additional coils may be provided along with additional receivers, phase sensitive detectors, analog-to-digital converters, and image reconstruction means. With reference to Figure 4, two separate images can be derived from the first and second coils by time sharing the operation of a single receiver. The switching means 42 includes a first multiplexing means 110 which alternately connects each coil to the first phase sensitive detector 70. The signal from one coil is sampled on the first echo and the signal from the other coil is sampled during the second echo. The mulitplexer 110 alternately connects the demodulating frequencies from the oscillator 34 and the frequency shift means 74 with the phase sensitive detector. The components are digitized and Fourier transformed into image data. The switching means also includes a second mutliplexing means 112 that alternately channels the image data from the two dimensional Fourier transform means 90 into the first and second image memories 94, 96. In alternate views, the multiplexing means 110 reverses the order which coil is sampled during the first echo and which is sampled during the second echo. In this manner, two complete sets of image data are collected concurrently without adversely affecting the signal to noise ratio or the resolution. However, the images have a mixture of contrast attributable to T1 and T2 echo times due to the difference in the T1 and T2 weighting of the first and second echoes. It is to be appreciated that this imaging technique is not limited to imaging the spine. It is applicable to the imaging of other body parts, particularly where coplanar or otherwise non-interacting surface coils can be utilized. Where appropriate, the read gradient may be applied along the x-axis or the y-axis, i.e. transverse to the scans. It is further to be appreciated that the first and second phase sensitive detectors are not required to demodulate the received resonance signals on resonance. However, to avoid overlap of the images, the demodulation frequencies should be greater than the imaging bandwidth determined by the sampling time and the read gradient.

Claims: A magnetic resonance imaging apparatus comprising means (10, 12) for generating a main magnetic field through an image region; means (20, 22) for causing a gradient magnetic field across the image region; means (30, 32) for exciting magnetic resonance of dipoles in the image region; means (50, 52, 70, 72) for receiving and demodulating magnetic resonance signals from resonating dipoles in the image region; and image reconstruction means (90, 92) for reconstructing an image representation from the received demodulated magnetic resonance signals, characterised in that said receiving and demodulating means (50, 52, 70, 72) receives and demodulates magnetic resonance signals for each of at least two different demodulation frequencies (f₀, f₀ + 2▵f), and said image reconstruction means (90, 92) comprises means for constructing a respective image representation from the demodulated signals for each said demodulation frequency (f₀, f₀ + 2▵f). An apparatus according to Claim 1 wherein the demodulated signals are passed to said image reconstruction means (90, 92) via analog-to-digital converter means (80, 82, 84, 86) for digitising said demodulated signals. An apparatus according to Claim 2 wherein said converter means (80, 82, 84, 86) comprises a pair of analog-to-digital converters for the demodulated signals for each said demodulation frequency. An apparatus according to Claim 3 wherein said receiving and demodulating means (50, 52, 70, 72) produces, for each said demodulating frequency, first and second components having a preselected phase relationship which are respectively applied to said pair of converters for that demodulation frequency. An apparatus according to any preceding claim wherein said receiving and demodulating means (50, 52, 70, 72) includes a respective coil (50, 52) for receiving magnetic resonance signals for each said demodulating frequency (f₀, f₀ + 2▵f), said coils being disposed adjacent to the imaging region. An apparatus according to Claim 5 wherein said coils (50, 52) are coplanar. An apparatus according to any preceding claim wherein the image reconstruction means includes a respective two-dimensional Fourier transform means (90, 92) for the demodulated signals for each said demodulation frequency. An apparatus according to any preceding claim wherein said receiving and demodulating means (50, 70) comprises a single demodulating means (70) and multiplexing means (110) for applying the received magnetic resonance signals for each said demodulating frequency to said single demodulating means (70) sequentially. A method of magnetic resonance imaging comprising the steps of: generating a main magnetic field through an image region; causing a gradient magnetic field across the image region; exciting magnetic resonance of dipoles in the image region; receiving magnetic resonance signals from resonating dipoles in the image region; demodulating the received magnetic resonance signals; and using the demodulated magnetic resonance signals to construct an image representation characterised in that in the demodulating step demodulated signals for at least two different demodulating frequencies (f₀, f₀ + 2▵f) are produced; and in the image reconstructing step an image representation is constructed from the demodulated signals for each said demodulation frequency.

Inventors: HOLLAND, NEIL G.
Applicants: PICKER INTERNATIONAL, INC.
Application year: 1988
Main classification: A61B5
IPC codes: G01R33