NMR Spectroscopy and Imaging Coil

The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with the contemporaneous imaging or spectroscopic analysis of hydrogen and phosphorous nuclei and will be described with particular reference thereto. However, it is to be appreciated that the invention is also applicable to other multiple nuclei imaging and spectroscopy applications.

Heretofore, others have recognized that it would be advantageous to acquire proton magnetic resonance images and/or spectra concurrently with phosphorus (P31) images and/or spectra. Conventionally, the radio frequency transmission and reception coils of the magnetic resonance scanner are tuned to the resonance frequency of the nuclei to be imaged. The resonance frequency is proportional to the product of the gyromagnetic ratio of the nuclei and the strength of the magnetic field. In conventional magnetic resonance imaging equipment, the strength of the magnetic field is fixed. Because the gyromagnetic ratios of hydrogen and phosphorous are very different, they have distinctly different resonance frequencies. In a one and a half tesla magnetic field, for example, the resonance or Larmor frequency of phosphorous is about 26 MHz as opposed to 64 MHz for hydrogen.

One prior art approach was to conduct the resonance experiment with a first coil tuned to the resonance frequency of hydrogen. The hydrogen tuned resonance coil was physically removed and replaced with a phosphorous tuned resonance coil. Not only was the change of coils slow and labor-intensive, but the resultant images or spectroscopic data were difficult to align. With human patients, the subject tended to move during the coil replacement process creating further problems in determining a correspondence between phosphorous and hydrogen images.

Another approach is to use the same coil for both the hydrogen and phosphorous images. Rather than changing coils, the coil is retuned between the collection of hydrogen and phosphorous data. This is again a relatively slow and labor-intensive process during which human patients are apt to move.

U.S. Pat. No. 4,799,016 issued January 1989 to Rezvani illustrates a coil arrangement in which two concentric coils are used orthogonal to each other. By placing the coils orthogonal to each other, each coil is invisible to the other during their independent operation. One of the drawbacks of the Rezvani design is that it is not amenable to quadrature applications. Redesigning each coil to operate in a quadrature mode, i.e. a mode in which the coil is sensitive to signals from two orthogonal directions, would cause the coils to interfere with each other.

Another approach is illustrated in the article "Design and Evaluation of Double Tuned (.sup.1 H,.sup.31 P) Birdcage Head Coil for Use in Clinical Imaging and Spectroscopy at 1.5T", G. Isaac, et al. SMRM Book of Abstracts, page 180 (1989). This abstract discloses a double tuned birdcage coil that works in a low pass configuration at the resonance frequencies of both hydrogen and phosphorus. Operating in the low pass configuration for phosphorus is disadvantageous. The high RF transmit voltages required create such high voltages across the capacitors that arcing would result when kilowatt level pulses are used. Another disadvantage of the Isaac coil is that the two resonance frequencies cannot be tuned separately. Accordingly, set-up and calibration is extremely complex.

Another double tuned birdcage coil is illustrated in "A Double Resonance Birdcage Coil for Imaging of Fluorine and Protons", Joseph, et al. SMRM Book of Abstracts, page 263 (1988). However, the Joseph design is only suitable when the two resonance frequencies are close to one another, e.g. within 10%, as with fluorine and hydrogen.

In accordance with the present invention, a new and improved double tuned resonance coil is provided that overcomes the above-referenced problems and others.

In accordance with one aspect of the present invention, a double tuned birdcage type coil is provided which operates as a high pass coil at the frequency with the higher transmit power and as a low pass coil for the other frequency.

In accordance with another aspect of the present invention, a double frequency birdcage type coil is provided which includes separate means for adjusting each of two resonance frequencies.

In accordance with yet another aspect of the present invention, a double tuned birdcage type coil is provided which has a third operating mode. Specifically, means are provided for deresonating the coil in order to decouple the coil from both frequencies when a separate receive coil is used.

In accordance with yet another aspect of the present invention, a double tuned birdcage coil is provided which operates in a quadrature mode for both frequencies.

In accordance with yet another aspect of the present invention, a coupling arrangement is provided for double tuned birdcage coils. The coupling arrangement provides for the direct coupling in one mode of operation and for flux linkage inductive coupling in the other.

In accordance with yet another aspect of the present invention, the coupling circuity includes a tank circuit to prevent coupling in one mode to interfere with operation and symmetry of the coil in the other mode.

In accordance with yet another aspect of the present invention, a PIN diode combiner/divider circuit is provided which reduces the number of cables attached to the coil.

One advantage of the present invention is that the birdcage coil is simultaneously tuned to two frequencies.

Another advantage of the present invention is that the coil operates in quadrature, i.e. circularly polarized in one or both modes.

Another advantage of the present invention is that it is readily decoupled from both frequencies during a receive period.

Another advantage of the present invention is that is simplifies fine tuning. The coil does not need to be retuned for each patient due to its distributed capacitance design. Moreover, both frequencies of the coil can be tuned independently without interfering with each other.

Another advantage of the present invention is that it can receive high power input pulses for manipulating the resonance of nuclei with small gyromagnetic ratios.

Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.

The invention may take form in various components and arrangements of components and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention

FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus in accordance with the present invention;

FIG. 2 is an enlarged, more detailed view of the birdcage RF coil of the magnetic resonance imager of FIG. 1;

FIG. 3 is an electrical schematic diagram of the coil of FIG. 2;

FIG. 4 is an equivalent circuit of the circuit of FIG. 3 when the coil operates in the high pass mode;

FIG. 5 is an equivalent circuit to the circuit of FIG. 3 when the coil operates in the low pass mode;

FIG. 6 is an equivalent circuit to the coil of FIG. 3 illustrating the quadrature coupling;

FIG. 7 is an equivalent circuit of the coil of FIG. 2 including appropriate leads and components for decoupling the coil;

FIG. 8 is a block diagram of the combiner/divider of FIG. 1.

Primary magnetic field coils 10 under the control of a primary magnetic field control means 12 create a static, uniform magnetic field longitudinally through an examination region 14. Gradient magnetic field coils 16 under the control of a gradient magnetic field control 18 selectively cause magnetic field gradients along and across the primary magnetic field.

A radio frequency transmitter means 20 selectively transmits pulses of radio frequency energy at selectable frequencies. In the preferred embodiment, the radio frequency transmitter means includes a transmitter means 20a for transmitting RF pulses at the resonance frequency of a first nucleus and a second frequency transmitter means 20b for transmitting RF pulses at the resonance frequency of a second nucleus. In the preferred embodiment, the RF frequencies are 26 MHz and 64 MHz--the resonance frequencies of phosphorous and hydrogen, respectively, in a 1.5 T magnetic field. The RF pulses are conveyed to corresponding upper and lower RF frequency quad combiner/divider circuits 22a, 22b. The quad divider circuits circularly polarize the RF pulses by providing two 90.degree. phase shifted components, e.g. sine and cosine components which are applied 90.degree. apart to a double tuned or dual frequency radio frequency birdcage coil 24.

When the birdcage coil 24 receives 26 MHz signals, it is tuned such that it operates in a high pass mode in which it is resonant at 26 MHz. When the birdcage coil receives 64 MHz signals, it operates in a low pass mode in which it is resonant at 64 MHz. In the receive mode, the quad combiner/dividers combine the quadrature components of the selected 26 or 64 MHz signal. The recombined and phase shifted signals are supplied to a receiving means 30 including a 26 MHz receiver 30a and a 64 MHz receiver 30b. The receivers include mixers of the appropriate frequencies to separate the carrier and resonance signals and supply appropriate data lines to a reconstruction means 32 for reconstructing images or spectra from the data line, such as a two dimensional inverse Fourier transform reconstruction means for reconstructing images. Reconstructed image or spectra representations are stored in a data memory means 34. Preferably, the data memory means includes a first nuclei or phosphorous memory means 34a and a second nuclei or hydrogen memory means 34b for storing corresponding phosphorous and hydrogen images or spectra. A display monitor 36 displays the corresponding phosphorous and hydrogen images and/or spectra side by side, superimposed, superimposed in different colors, or the like, as the operator may select.

The gradient control means 18, the radio frequency transmitter 20, and the quad combiner/divider means 22 are controlled by a sequence control means 40 to implement spin echo, gradient echo, and other conventional imaging sequences and to select which nucleus is to be imaged. In one preferred embodiment, phosphorous and hydrogen data are collected alternately to generate hydrogen and phosphorous images or spectra. Alternately, a surface coil 42 may be placed adjacent a region of interest of the subject for receiving the generated magnetic resonance signals. When the surface coil is utilized, the sequence control means 40 causes the birdcage coil 24 to enter a third, de-tuned mode in which no resonant frequency exists.

With reference to FIGS. 2 and 3, the RF coil is a birdcage type coil with four fold symmetry. The coil includes 8 main or longitudinal conductors 50a-h, although other multiples of four are also contemplated. Each conductor includes a tank circuit 52a-h which is resonant at a frequency between the resonant frequencies of the two nuclei to be examined, i.e. between 26 and 64 MHz for phosphorous and hydrogen. Each tank circuit includes a parallel connected capacitance 54a-h and inductance 56a-h. Additional capacitors 58a-h interconnect the main conductors 50 in a loop at one end and capacitors 60a-h interconnect the main conductors in a loop adjacent to their other end. A fine tuning ring 62 is mounted adjacent each of conductors containing capacitors 60a-h. By adjusting the physical displacement between the fine tuning ring and the capacitors 60a-h, the capacitance of capacitors 60a-h are selectively adjusted or fine tuned. The tank circuits 52a-h and the capacitors 54a-h and 60a-h are selected to have values such that the coil resonates at the two selected resonance frequencies, e.g. 26 and 64 MHz.

Small variations in the capacitance of the capacitors 54a-h are primarily responsible for the fine tuning of the 64 MHz resonance frequency; whereas, adjustment of the capacitances 60a-h is primarily responsible for fine tuning the 26 MHz resonance frequency. In this manner, the two resonance frequencies are independently fine tunable.

With continued reference to FIGS. 2 and 3, and further reference to FIG. 4, in the 26 MHz mode, the tank circuits have an inductive reactance. The inductance which each tank circuit has can be determined by mathematical calculation or by replacing each tank circuit with an adjustable inductance which is adjusted until the coil becomes resonant at the selected resonance frequency, e.g. 26 MHz. In the coil of the preferred embodiment, the reactance was found to be about 200 nH. With reference to FIG. 5, the inductors are removed and replaced with capacitors which are adjusted until the coil is resonant at 64 MHz. In the preferred embodiment, capacitors of 15 pF cause the desired 64 MHz resonance. Tank circuits are now inserted having a capacitance and an inductance which are calculated mathematically such that the tuning circuit has an inductive reactance equivalent to 200 nH at 26 MHz and a capacitive reactance equivalent to 15 pF at 64 MHz. Capacitances of 55 pF and inductors of 154 nH meet this criteria. One of the capacitors in the tank circuits, the capacitors 54 in the preferred embodiment, are adjustable to enable the coil to be fine tuned. Note that changing the variation in the capacitors 54 of the tank circuit does not effect the 26 MHz tuning. Conversely, it should be noted that adjusting the capacitance of capacitors 60 does not effect the 64 MHz tuning.

Normally, a higher RF transmit power is required to resonate phosphorus nuclei. At 26 MHz, the birdcage coil resonates in a high pass mode in which sixteen capacitors 58a-h and 60a-h divide the total applied voltage. Because the applied voltage is divided sixteen ways, the relative voltage across any one capacitor is relatively low, preventing arcing.

With reference again to FIG. 2 and further reference to FIG. 6, coupling means are provided for coupling the RF coil with the quad combiner/divider circuits 22 in both modes. The two couplings are effected in such a manner that an impedance of about 50 Ohms is seen at the input and such that coupling in one mode does not disturb the other mode. Interference between the coupling of the modes could cause a drop in the orthogonality of the quadrature mode or cause de-tuning. In the 64 MHz mode, an inductive coupling means is provided. The inductive coupling means includes a pair of identical coupling loops 70a, 70b connected 90.degree. offset around the RF coil. The coupling loops are partially resonated by tank circuits 72a, 72b that include parallel connected capacitors 74a, 74b and inductors 76a, 76b. The tank circuit is designed to have a capacitive reactance at 64 MHz, but to have a high impedance at 26 MHz such that substantially no 26 MHz RF current flows into the loop. In this manner, interference of the 26 and 64 MHz modes is prevented. The high impedance at 26 MHz is achieved by adjusting the tank circuits such that they are resonant at 26 MHz. A pair of lumped-element baluns 78a, 78b provide for balanced operation. If the coil is to be operated in a linear rather than a quadrature mode at 64 MHz, one of the 64 MHz coupling loops is simply eliminated. The invention is intended to include variations such that linear or quad operation at one or both frequencies and any combination thereof is also included.

In the 26 MHz mode, a direct, inductive coupling is utilized. Two 26 MHz feed points 80a, 80b are connected by inductors 82a, 82b with two, 90.degree. displaced portions of the RF coil. A pair of 180.degree. baluns 84a, 84b extend from the feed points to partially lumped and partially distributed baluns 86a, 86b and are connected by inductors 82i c, 82d with 180.degree. opposite points on the birdcage coil 24. The inductors 82a-d are selected to provide a large impedance at 64 MHz. A typical value used for these inductors is 1.3 .mu.H. The partially lumped and partially distributed baluns provide phase shift of 180.degree.. In this manner, the 64 MHz mode is isolated from the 26 MHz mode.

With continuing reference to FIG. 2 and particular reference to FIG. 7, it is frequently desirable to use a surface coil to receive the spectroscopy signal while using the RF coil only as a transmitter. In this instance, the RF coil is de-tuned during the receive period to prevent the surface coil from being de-tuned. A DC signal is applied at de-tune feed point 90. Of course, to the DC current, inductors appear as short circuits and the capacitors appear as open circuits. The DC current flows through the inductors 82 and baluns 84, 86 of the 26 MHz feed circuit flowing through each of PIN diodes 92a-d to bias the diodes conductive. Note that the DC signal flows through one of parallel connected inductor elements of inductor 82a, through PIN diode 92a, back through the other parallel connected inductor element of inductor 82a, through the balun 84a, through one parallel connected portion of inductor 82c, through PIN diode 92b, and back through a second parallel connected inductive portion of inductor 82c, etc. The same principle carries the DC current through each of the PIN diodes to a ground 94. RF radio frequency chokes 96 are provided for preventing interference with the 26 or 64 MHz modes.

With reference to FIG. 8, the quadrature divider/combiner includes a .pi. network 100 which causes a 90.degree. phase shift. The .pi. network is connected with summing nodes 102a, 102b which are connected by like inductors 104a, 104b with feed points 106a, 106b to the birdcage coil coupling means. An RF signal applied to one of the nodes 102a is communicated to one feed point 106a with no phase shift and to the other feed point 106b 90.degree. lagging. When components of an RF signal are received at feed points 106a and 106b, one component is phase shifted by the .pi. network and summed with the unshifted component at one of the summing nodes. One can select which received component leads and which one lags by selecting which summing node to monitor.

A pair of feed paths 110a, 110b are connected with the summing nodes 102a, 102b at opposite ends of the .pi. network and with a common input/output point 112. The paths 110a, 110b include a pair of PIN diodes 114a, 114b. In the transmit mode, PIN diodes 114a and 114b are biased on by means of a DC bias current applied through a radio frequency choke 116. A tank circuit 118 tuned to the resonance frequency then provides a high impedance thereby connecting the input/output point 112 with one of the summing nodes 102b. In the receive mode, the DC bias current is removed and PIN diodes 114a and 114b are biased off becoming high impedance open circuits blocking RF current flow getting to summing node 102b. In this manner, the RF signal applied to one summing junction 102b in the transmit mode has a leading component transmitted directly to one feed point 106b and a lagging component transmitted to the other feed point 106a. In the receive mode, the RF component received at the feed point 106b is phase shifted 90.degree. by the .pi. circuit 100 to be the lagging component and is summed with the unphase shifted or leading component from the other feed point 106a at summing node 102a. Thus, which feed point carries the leading component and which carries the lagging component is reversed in the transmit and receive modes.

With reference to FIG. 7, when a surface coil is used, a negative DC bias is applied to the PIN diodes 92a, 92b, 92c, 92d and an isolation circuit 96 to ground to detune the birdcage resonance structure. The isolation circuit 96 includes a tank circuit that is resonant at the resonant frequency being applied and received such that transmitted and received signals, are not lost to ground. An analogous combiner/divider circuit is provided for the hydrogen mode, but with the capacitive and inductive values adjusted for resonance at 64 MHz.

The invention has been described with reference to the preferred embodiments. Of course, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such alterations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.