Probe for Magnetic Resonance Inspection Apparatus

This invention relates to a probe for a magnetic resonance inspection apparatus.

A magnetic resonance inspection apparatus constituted by utilizing magnetic resonance imaging irradiates a radio frequency magnetic field so as to generate nuclear magnetic resonance at an arbitrary position inside a subject, and includes a probe for receiving the magnetic resonance signal generated inside the living body. In magnetic resonance imaging, efficiency of the probe and uniformity of the radio frequency magnetic field must be improved to increase measured image quality and to shorten the imaging time. A quadrature detection probe is known as one of the means for improving performance of the probe, as disclosed in "Journal of Magnetic Resonance", Vol. 69 (187), pp. 236-242. This probe will be referred to as the "QD probe" in this specification. The probe of this kind is also described in JP-A-61-230052.

FIG. 1 is a perspective view of this QD probe and FIG. 2 shows its wiring structure. As is obvious from these drawings, this QD probe comprises four I-shaped first conductors 100, 200, 300, 400, ring-like guard rings 11, 12, capacitors or capacitance elements 31-38, 41, 42, and power supply circuits 500, 600. The first conductors 100, 300 and the guard rings 11, 12 constitute a coil, that is, an induction element, and this coil has a parallel relation with predetermined capacitance elements. This induction element and the predetermined capacitance elements constitute one probe. On the other hand, the first conductors 200, 400 and the guard rings 11, 12 form another coil. This coil detects a signal whose phase deviates by 90.degree. from the signal detected by the former coil. This coil, too, has a parallel relation with the predetermined capacitance elements and constitutes another probe.

When a subject such as a human head is inspected, a radio frequency magnetic field is applied generally to induce a maximum current in the probe and improve the signal-to-noise ratio (S/N). Thus,

where .omega. is an angular velocity of the Larmor's precession, .gamma. is a gyromagnetic ratio and Ho is the intensity of a static magnetic field.

The intensity Ho is determined on the basis of this relation so that .omega. becomes the resonance frequency of the probe. For example, Ho is set to 21.1 MHz.

The studies conducted by the inventors of the present invention reveal that the resonance characteristics of the probe described above are such as shown in FIG. 3. In the diagram, an impedance .vertline.Z.vertline. can be determined from the reactance component due to the coil and the capacitance elements and from the resistance component of a subject and the conductor itself in one probe. A first peak 74 represents resonance of the probe. A second peak 73 appears immediately adjacent to the first peak. Although the reasons why this second peak appears have not yet been clarified, it is assumed that another probe, that is, the probe whose phase deviates by 90.degree. from that of the probe which measures the impedance, affects the impedance due to the structure of the QD probe shown in FIG. 1. The first peak 74 is affected by the second peak 73 and the waveform of the first peak 74 becomes asymmetric with 21.1 MHz being the center.

When the first peak 74 undergoes deformation, the magnitude of the induced current which is distributed at frequencies near 21.1 MHz and detected on the basis of a gradient field produces a detection signal which is distorted due to deformation of the impedance.

It is therefore a first object of the present invention to make the first peak corresponding to the resonance frequency symmetric on both right and left sides and to correct frequency dependence of a detection signal to prevent the detection signal from distortion depending on the frequency.

To accomplish this object, the separates the second peak 73 from the first peak 74 by reducing the frequency range in which the second peak 73 appears. This makes the first peak 74 symmetric on both right and left sides and improves frequency dependence of the detection signal.

According to the studies conducted by the present inventors, however, the Q value of the probe is reduced if the second peak 74 is shifted to a lower frequency range. When the Q value is reduced, the S/N ratio is undesirably affected.

It is therefore a second object of the present invention to improve frequency dependence of the detection signal while maintaining the Q value.

In order to accomplish this object, the present invention shifts the second peak to a higher frequency range than the first peak.

It is still another object of the present invention to improve an electric balance of a probe.

To accomplish this object, the present invention disposes capacitance elements at least between both ends of a first conductor in its axial direction and guard rings, and secures electric symmetry of the probe in the axial direction.

When unbalanced power is supplied to the probe, the ratio of the capacitance of the power supply side of the probe to the capacitance of the opposite side the probe is adjusted. This minimizes the sensitivity differences between the power supply side and the opposite side of the probe.

FIG. 1 is a perspective view showing a probe of the prior art;

FIG. 2 is a schematic view showing a wiring structure of the probe of the prior art;

FIG. 3 is a waveform diagram showing the resonance characteristics of the probe of the prior art;

FIG. 4 is a perspective view showing a probe in accordance with an embodiment of the present invention;

FIG. 5 is a schematic view showing a wiring structure of the probe shown in FIG. 4;

FIGS. 6 and 7 are perspective views each showing the probe in accordance with another embodiment of the present invention;

FIG. 8 is a waveform diagram showing the resonance characteristics of the probe of the present invention;

FIG. 9 is a graph showing the relation between the ratio of a combined capacitance on the opposite side to a power supply side to the combined capacitance on the power supply side and a sensitivity;

FIG. 10 is a photo showing an image taken in the embodiment of the invention;

FIG. 11 is a photo showing an image taken in a comparative example;

FIG. 12 is a photo showing an image taken in the other embodiment of the present invention; and

FIG. 13 is a photo showing an image taken in a comparative example.

As shown in FIGS. 4 and 5, a probe in this embodiment is produced by adding capacitors 43-46 to the conventional probe shown in FIG. 1. The probe of this embodiment comprises first conductors 100, 200, 300, 400, guard rings 11, 12, capacitors or capacitance elements 31-38, 41, 42, and power supply circuits 500, 600.

The first conductor 100 comprises a vertical arm 1 of a belt-like conductor plate extending vertically in the drawings and wings 21, 25 expanding from the upper and lower ends of the vertical arm 1, and is substantially I-shaped when viewed from the front. Each of the other first conductors 200, 300, 400 comprises a vertical arm 2, 3, 4 and wings 22, 23, 24, 26, 27, 28, respectively. Each vertical arm 1, 2, 3, 4 of the first conductor is disposed in an axial direction of an imaginary cylinder surface in such a manner as to extend along this cylinder surface and moreover, equidistantly in the peripheral direction of the imaginary cylinder surface. The wings are disposed in the peripheral direction of the imaginary cylinder surface.

Each guard ring 11, 12 is a ring-like conductor plate. Ring 11 is disposed inside the wings 21, 22, 23, 24 ring 18 is disposed and inside the wings 25, 26, 27, 28. The enters of rings 11 and 12 are aligned with the center axis of the imaginary cylinder surface.

Each capacitance element comprises a first capacitor 31-38 disposed between the wings and a second capacitor 41-46 disposed between each first conductor and each guard ring.

The first conductors 100 and 300 and the guard rings 11 and 12, that oppose one another, respectively, constitute one induction element. This induction element is connected in parallel with a predetermined capacitance element and form one probe. Similarly, the first conductors 200 and 400 and the guard rings 11 and 12 form another induction element and the resulting induction element and another predetermined capacitor form another probe. These two probes detect those signals whose phases deviate by 90.degree. from each other. The resonance frequency of each probe is adjusted by regulating the capacitance element. In order to make fine adjustment of the resonance frequency, variable capacitors 81, 82, 91, 92 are preferably added to the capacitors 41, 42, 45, 46, respectively, as shown in FIGS. 6 and 7. A trimmer capacitor, for example, is used as the variable capacitor. One variable capacitance element can be used in place of the parallel circuit of the fixed capacitance element and the variable capacitance element used in the embodiment shown in the drawings.

A series circuit (power supply circuit) 500 composed of the capacitors 51 and 61 and another series circuit 600 composed of the capacitors 52 and 62 are interposed between the first conductor 100 and the guard ring 11 and between the first conductor 200 and the guard ring 11 constituting another probe, respectively. The combined capacitance of the capacitors 51 and 61 and that of the capacitors 52 and 62 are designed to be equal to the combined capacitances of the capacitors 41-46, respectively. In each circuit, a transmitter/receiver, not shown, is connected to both ends of the capacitors 61, 62, respectively, through the power supply lines 700, 800.

FIG. 8 shows the resonance characteristics of the probe having such a structure. As can be seen clearly from the diagram, the second peak 73 of the probe of this embodiment shifts to a higher frequency range than the first peak 74 and a large frequency difference exists between these peaks. The first peak 74 maintains its height (Q value) and symmetry.

FIG. 10 shows a tomogram of a phantom taken by the probe shown in FIG. 4. In the tomogram, line in the horizontal direction is the cursor that represents the position of measurement of the signal intensity. The profile of the signal at that position is represented by wave line in the tomogram. The probe of this embodiment can provide a uniform image not only from the image of the phantom but also from this signal profile.

FIG. 11 shows the image and signal profile when the phantom is tomographed using the probe shown in FIG. 1 under the same condition as described above. The signal profile deviates more greatly to the left side from the cursor. It can be understood from FIG. 11 that the image becomes non-uniform by use of the conventional probe.

In contrast with the probe shown in FIG. 1, the probe of this embodiment includes also the capacitance elements 43-46 between the wings 25-28 and the guard ring 12 of the inventive arrangement. Accordingly, electric symmetry of the probe in its axial direction is improved.

In the unbalanced power supply system shown in FIG. 4, power is supplied in parallel to the capacitors 61 and 62 (See Journal of Magnetic Resonance 36, 447-451 (1979).) The characteristic impedance of power supply cables 700, 800 at this time is 25 .OMEGA. when a ratrace circuit, is used and its characteristic impedance is 50 .OMEGA. at the time of normal power supply. Accordingly, electric asymmetry appears between the first conductors 100 and 300 and between 200 and 400 in each of the probes.

To solve this problem, the electrostatic capacitance of the capacitors 41-46, 51 and 61, and 52 and 62, is adjusted. The ratio of the combined capacitance of the capacitors 51, 61, 52, 62, 41 and 42 on the power supply side to the combined capacitance of the capacitors 43, 44, 45, and 46 on the opposite side to the power supply side is changed. FIG. 9 shows the difference of the sensitivity of the probe when the ratio of the combined capacitance C2 on the power supply side to the combined capacitance C1 on the opposite side is changed.

It is found from FIG. 9 that C1/C2 is preferably slightly greater than 1 and that the ratio is preferably from 1.05 to 1.11 and more preferably, 1.08.

FIG. 12 shows the image and signal profile when the phantom is tomographed with the probe having the ratio of 1.08. FIG. 13 shows the one when the ratio of the probe is 1.00. It can be understood from the Figs. that the former probe provides a more uniform image than the latter probe.