Receiver coil for nuclear magnetic resonance imaging apparatus

Abstract: A receiver coil (10) for a nuclear magnetic resonance imaging apparatus, capable of fulfilling the condition for a satisfactory signal reception completely for the imaging of a region of interest that cannot be placed symmetrically with respect to the central axis of the receiver coil. The receiver coil (10) includes coil elements (10a, 10b) or defining an opening section into which a region of interest is to be inserted, and for receiving a nuclear magnetic resonance signal from the region of interest; and a coil support member for supporting the coil elements in a configuration in which a sensitivity central axis (LS10) of a region of uniform signal reception sensitivity for the coil elements is located away from a central axis (LG10) of the opening section. The coil elements are preferably supported in a configuration corresponding to a shape of the region of interest.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a receiver coil for receiving and collecting nuclear magnetic resonance signals in a nuclear magnetic resonance imaging apparatus. Description of the Background Art As a receiver coil for a nuclear magnetic resonance imaging apparatus, various types of the receiver coil have been developed for different imaging methods and different imaging object shapes. In particular, there are many examples of a receiver coil developed for imaging the spine at a neck portion, a chest portion, or a stomach portion of a patient. Examples of such a conventional receiver coil are shown in Figs. 1(A) to 6. An example of a receiver coil shown in Figs. 1(A) and 1(B) is called a crossing elliptic coil, and this receiver coil 1 comprises a pair of elliptic coil elements 1a and 1b which are wound around a cylindrical core 1c and are arranged such that they are crossing each other and electrically connected at cross sections P₁ and P₁′, where vertical positions of the cross sections P₁ and P₁′ coincide with that of the center of the cylindrical core 1c, as shown in Fig. 1(B). In this configuration, the sensitivity central axis L S1 of the region of uniform signal reception sensitivity and the central axis L G1 of the cylindrical core 1c coincide with each other. As shown in Figs. 2(A) and 2(B), the receiver coil 1 of the above example can be modified into a receiver coil 1′ which comprises a pair of elliptic coil elements 1a′ and 1b′ which are wound around a cylindrical core 1c′ and are arranged such that they are crossing each other and electrically connected at cross sections P₂ and P₂′, where vertical positions of the cross sections P₂ and P₂′ coincide with that of the center of the cylindrical core 1c′, as shown in Fig. 2(B), and where the angle formed between the elliptic coil elements 1a′ and 1b′ is made narrower than that between the coil elements 1a and 1b in the previous example. In this configuration, the sensitivity central axis L S1 ′ of the region of uniform signal reception sensitivity and the central axis L G1 ′ of the cylindrical core 1c also coincide with each other, as in the previous example. With this configuration, a closer fitting to a patient's body surface becomes possible. Another example of a conventional receiver coil is shown in Fig. 3, which is called a saddle coil, and this receiver coil 2 comprises a saddle shaped coil element 2a wound around a cylindrical core 2b. In this configuration, the sensitivity central axis L S2 of the region of uniform signal reception sensitivity and the central axis L G2 of the cylindrical core 2b also coincide with each other, as in the previous examples. Another example of a conventional receiver coil is shown in Fig. 4, which is another example of a saddle coil, and this receiver coil 3 comprises a pair of semi-saddle shaped coil elements 3a wound around a cylindrical core 3b. In this configuration, the sensitivity central axis L S3 of the region of uniform signal reception sensitivity and the central axis L G3 of the cylindrical core 3b also coincide with each other, as in the previous examples. Another example of a conventional receiver coil is shown in Fig. 5, which is called a Helmholtz coil, and this receiver coil 4 comprises a pair of circular coil elements 4a and 4b arranged with a prescribed separation formed therebetween. In this configuration, the sensitivity central axis L S4 of the region of uniform signal reception sensitivity and the central axis L G4 passing between the circular coil elements 4a and 4b also coincide with each other. Another example of a conventional receiver coil is shown in Fig. 6, which is called a modified Helmholtz coil, and this receiver coil 5 comprises a pair of warped circular coil elements 5a and 5b arranged with a prescribed separation formed therebetween. In this configuration, the sensitivity central axis L S5 of the region of uniform signal reception sensitivity and the central axis L G5 passing between the circular coil elements 5a and 5b coincide with each other, as in the last example. In all of these examples, the coil elements are supported by a core member formed by a non-magnetic resinous supporting material which maintains the geometrical configuration of the coil elements. Now, in these conventional receiver coils, the region of uniform signal reception sensitivity is a space formed in the opening section of the coil elements. The condition for a satisfactory signal reception is to make this region as large as the region of interest to be imaged, and to place this region of interest at the center of the region of uniform signal reception sensitivity. As an example, the operation of the receiver coil 1 of Figs. 1 (A) and 1 (B) will now be described with references to Figs. 7 and 8. This receiver coil 1 is one for imaging head and neck portions, and has the diameter of the opening section approximately equal to 260 mm, such that it can be used for both imaging the head portion and imaging the neck portion. As shown in Fig. 7, in case of imaging the head portion H, the head portion H is inserted inside the opening section of the receiver coil 1 completely. In this case, the region of uniform signal reception sensitivity coincides with the size of the head portion H, and the center of the head portion H is located on the sensitivity central axis L S1 of the receiver coil 1, so that the condition for the satisfactory signal reception is fulfilled, i.e., the nuclear magnetic signals can be received with a high S/N ratio. However, as shown in Fig. 8, in case of imaging the neck portion N, when the neck portion N is inserted inside the opening section of the receiver coil 1 completely, the central axis Na of the neck portion N is located below the sensitivity central axis L S1 of the receiver coil 1, so that the condition for the satisfactory signal reception cannot be fulfilled completely, i.e., the nuclear magnetic signals can be received only with a low S/N ratio. Thus, in a conventional receiver coil, because the coil elements are wound such that the sensitivity central axis and the central axis of the opening section coincide with each other, the condition for a satisfactory signal reception cannot be fulfilled completely for imaging a region of interest that cannot be placed symmetrically with respect to the central axis of the receiver coil such as the neck portion. In particular, the neck portion usually ends up being placed off the sensitivity central axis of a conventional receiver coil, so that the quality of imaging the neck portion has usually been poorer. In the document M. Mehdizadeh et al.: "An Open Quadrature Coil for NMR Imaging Sections of the Torso"; Society for Magnetic Resonance Imaging, 7 th meeting, February 1989, Oak Brook, USA, page 61, there is disclosed an open quadrature coil which constructed on a semi-cylindrical former with 45 cm in diameter and consists of a saddle-type coil and a rectangular coil with a curved plane. There is no statement whether these coils are completely separated from each other or at what locations the saddle-type coil and the rectangular coil are located. Further, there is stated, that within the area centered 10 cm above the bottom surface the S/N ratio is up to twice that of a 60 cm quadrature body coil at .5T. This means, that the S/N ratio can be at most twice that of the quadrature body coil at a location somewhere in this area with the S/N being not uniform in this area. The location of the central axis of a uniform signal reception sensitivity region is not defined. Similarly, the central axis of the opening section, defined by the open quadrature coil, is not well defined. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a receiver coil for a nuclear magnetic resonance imaging apparatus, capable of fulfilling the condition for a satisfactory signal reception completely for imaging a region of interest that cannot be replaced symmetrically with respect to the central axis of the receiver coil, such as the neck portion. According to the present invention there is provided a receiver coil for a nuclear magnetic resonance imaging apparatus comprising coil element means adapted to define an opening section into which a region of interest is to be inserted, and to receive a nuclear magnetic resonance signal from the region of interest; and coil support means adapted to support the coil element means characterized in that said coil element means are defining the central axis of said opening section and that said coil support means are supporting the coil element means in a configuration in which the central axis of the region of uniform signal reception sensitivity for the coil element means is located away from the central axis of the opening section. Further advantageous features of the inventive receiver coil are characterized by the amended/appended subclaims. Other features and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(A) and 1(B) are a perspective view and a side view, respectively, of one example of a conventional receiver coil. Figs. 2(A) and 2(B) are a perspective view and a side view, respectively, of another example of a conventional receiver coil. Fig. 3 is a perspective view of another example of a conventional receiver coil. Fig. 4 is a perspective view of another example of a conventional receiver coil. Fig. 5 is a perspective view of another example of a conventional receiver coil. Fig. 6 is a perspective view of another example of a conventional receiver coil. Fig. 7 is a side view of the receiver coil of Figs. 1(A) and 1(B), for explaining its operation in imaging a head portion. Fig. 8 is a side view of the receiver coil of Figs. 1(A) and 1(B), for explaining its operation in imaging a neck portion. Figs. 9(A) and 9(B) are a perspective view and a side view, respectively, of a first embodiment of a receiver coil according to the present invention. Fig. 10 is a perspective view of a second embodiment of a receiver coil according to the present invention. Fig. 11 is a perspective view of a third embodiment of a receiver coil according to the present invention. Fig. 12 is a perspective view of a fourth embodiment of a receiver coil according to the present invention. Fig. 13 is a vertical cross sectional view of the receiver coil of Fig. 12, for explaining its operation in imaging the spine. Figs. 14(A) and 14(B) are a side view and a front view, respectively, of a fifth embodiment of a receiver coil according to the present invention. Fig. 15 is a side view of the receiver coil of Figs. 14(A) and 14(B), for explaining its operation in imaging the neck portion. Fig. 16 is another side view of the receiver coil of Figs. 14(A) and 14(B), for explaining its operation in imaging the neck portion. Fig. 17 is a top plan view of the receiver coil of Figs. 14(A) and 14(B), for explaining its operation of opening a coil support unit. Fig. 18 is an enlarged view of the pivotal crossing section of the receiver coil of Figs. 14(A) and 14(B). Figs. 19(A) and 19(B) are a side view and a top view, respectively, of the upper attaching member in the pivotal crossing section of Fig. 18. Figs. 20(A) and 20(B) are a top view and a side view, respectively, of the lower attaching member in the pivotal crossing section of Fig. 18. Fig. 21 is a side view of a sixth embodiment of a receiver coil according to the present invention. Fig. 22 is a side view of a seventh embodiment of a receiver coil according to the present invention. Fig. 23 is a side view of a eighth embodiment of a receiver coil according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figs. 9(A) and 9(B), a first embodiment of a receiver coil according to the present invention will be described. This receiver coil 10 comprises a pair of elliptic coil elements 10a and 10b which are wound around a cylindrical core 10c and are arranged such that they are crossing each other and electrically connected at cross sections P 1⊘ , where the vertical positions of the cross sections P 1⊘ are located below that of the central axis L G1⊘ of the cylindrical core 10c, as shown in Fig. 9(B). In this configuration, the sensitivity central axis L S1⊘ of the region of uniform signal reception sensitivity is located below the central axis L G1⊘ of the cylindrical core 10c. Thus, by placing a region of interest to be imaged inside the opening section 10d of the cylindrical core 10c, asymmetrically with respect to the central axis L G1⊘ of the cylindrical core 10c, a condition for a satisfactory signal reception can be fulfilled completely for imaging a region of interest that cannot be placed symmetrically with respect to the central axis L G1⊘ of the cylindrical core 10c. Referring now to Fig. 10, a second embodiment of a receiver coil according to the present invention will be described. This receiver coil 11 comprises a pair of saddle shaped coil elements 11a and 11b which are arranged such that they are facing each other, where the coil element 11a has curved portions along a circle of radius R1 while the coil element 11b has curved portions along a circle of radius R2 smaller than R1. In this configuration, the sensitivity central axis L S11 of the region of uniform signal reception sensitivity is located below the central axis L G11 , passing through a middle of the coil elements 11a and 11b. Thus, by placing a region of interest to be imaged inside the opening section of the receiver coil 11 asymmetrically with respect to the central axis L G11 , a condition for a satisfactory signal reception can be fulfilled completely for imaging a region of interest that cannot be placed symmetrically with respect to the central axis L G11 . Referring now to Fig. 11, a third embodiment of a receiver coil according to the present invention will be described. This receiver coil 12 comprises a saddle shaped coil element 12a having curved portions along a circle of radius R3 and a rectangular coil element 12b which are arranged such that they are facing each other. In this configuration, the sensitivity central axis L S12 of the region of uniform signal reception sensitivity is located below the central axis L G12 passing through the middle of the coil elements 11a and 11b. Thus, by placing a region of interest to be imaged inside the opening section of the receiver coil 12 asymmetrically with respect to the central axis L G12 , a condition for a satisfactory signal reception can be fulfilled completely for imaging a region of interest that cannot be placed symmetrically with respect to the central axis L G12 . Referring now to Fig. 12, a fourth embodiment of a receiver coil according to the present invention will be described. This receiver coil 13 comprises circular coil elements 13a and 13b which are arranged such that they are facing each other, where the coil element 13a has the shape of a circle of diameter D1 while the coil element 13b has the shape of a circle of diameter D2 smaller than D1. In this configuration, the sensitivity central axis L S13 of the region of uniform signal reception sensitivity is located below the central axis L G13 passing through the middle of the coil elements 13a and 13b. Thus, by placing a region of interest to be imaged inside the opening section of the receiver coil 13 asymmetrically with respect to the central axis L G13 , a condition for a satisfactory signal reception can be fulfilled completely for imaging a region of interest that cannot be placed symmetrically with respect to the central axis L G13 . Namely, as shown in Fig. 13, in imaging a spine S1, this spine S1 can be placed on the sensitivity central axis L S13 by placing a patient between the coil elements 13a and 13b, with the face up. In this manner, the nuclear magnetic resonance image of the spine S1 can be obtained with a high S/N ratio. Referring now to Figs. 14(A) and 14(B), a fifth embodiment of a receiver coil according to the present invention will be described. This receiver coil 20, which is a specialized one for imaging the neck portion, comprises a head rest unit 21 having a head rest surface 21a; a coil unit 22 having a pair of elliptic coil elements 22a and 22b, which are wound around a virtual cylindrical core 22c and are arranged such that they are crossing each other and electrically connected at cross sections P 2⊘ ; a coil support unit 23 for supporting the coil elements 22a and 22b such that they are crossing each other with the vertical position of the cross sections P 2⊘ located below that of the central axis L G2⊘ of the virtual cylindrical core 22c; and a receiver coil attachment unit 24 for detachably attaching the receiver coil 20 to a bed T. In this configuration, the sensitivity central axis L S2⊘ of the region of uniform signal reception sensitivity is located below the central axis L G2⊘ of the virtual cylindrical core 22c, as shown in Fig. 14(A). The coil unit 22 has a pivotal crossing section 22d and an openable crossing section 22e, such that the coil support unit 23 can be opened up in the direction of arrow C by lifting the openable crossing section 22e around the pivotal crossing section 22d. The coil support unit 23 is opened in this manner when the patient is to be placed on the head rest unit 21, and then closed when the patient is placed on the head rest unit 21. The virtual cylindrical core 22c has a gradually increasing diameter toward the end in the direction of arrow D, which is a side on which the head of the patient is to be placed, and the central axis L G2⊘ of this virtual cylindrical core 22c has an angle of ϑ formed with respect to the top surface Ta of the bed T. The coil support unit 23 has a hard portion made of a resinous material formed around an outer edge of the coil support unit 23 so as to provide strength to maintain the geometrical configuration of the coil elements 22a and 22b, and a soft portion made of a foaming material formed around an inner edge of the coil support unit 23 to which the patient may make contact, such that a comfortable touch can be provided to the patient. In this embodiment, the coil support unit 23 is formed in the shape of the coil elements 22a and 22b, so as to provide a feeling of openness to the patient. The operation of this receiver coil 20 will now be described with references to Figs. 15 and 16. First, the receiver coil 20 is attached to the bed T by means of the receiver coil attachment unit 24. Then, the coil support unit 23 is opened up by lifting the openable crossing section 22e of the coil unit 22, the head H of the patient is placed on the head rest unit 21, and the coil support unit 23 is closed. In case of imaging with a backward bending posture, as shown in Fig. 15, a neck portion supporter 25 is inserted below the neck portion N of the patient P, such that the spine S₂ at the neck portion N in the backward bending posture can be placed on the sensitivity central axis L S2⊘ of the region of uniform signal reception sensitivity. Thus, in this arrangement, the nuclear magnetic resonance signals from the spine S₂ at the neck portion N can be collected with a high S/N ratio. In case of imaging with a forward bending posture, as shown in Fig. 16, a head portion supporter 26 is inserted below the head portion H of the patient P, such that the spine S₂ at the neck portion N in the forward bending posture can be placed on the sensitivity central axis L S2⊘ of the region of uniform signal reception sensitivity. Thus, in this arrangement, the nuclear magnetic resonance signals from the spine S₂ at the neck portion N can be collected with a high S/N ratio. Now, in this embodiment, as shown in Fig. 17, the coil support unit 23 opens up in a direction II pointing between the head portion H and a shoulder portion S of the patient P, which is not perpendicular to the body axis 1 of the patient P. This structure is adopted because, in positioning the patient P on the head rest unit 21, if the coil support unit 23 were to open in a direction perpendicular to the body axis 1, the opened coil support unit 23 would interfere with the shoulder portion S of the patient P, such that not only the positioning of the patient P becomes time-consuming but also an unnecessary pressure is exerted on the coil support unit 23 which may cause a deformation of the coil unit 22. By making the coil support unit 23 to open in a direction II, this problem can be eliminated. In order to provide this opening of the coil support unit 23 in an oblique direction, the pivotal crossing section 22d of the coil unit 22 has a detail configuration shown in Fig. 18. Namely, at the pivotal crossing section 22d, the upper part 23a and the lower part 23b of the coil support unit 23 are held together by a hinge structure formed by an upper attaching member 15 provided on the upper part 23a and a lower attaching member 16 provided on the lower part 23b. The coil elements 22a and 22b in the upper part 23a and the lower part 23b are electrically connected through a connector plate 20 provided inside the pivotal crossing section 22d, which makes contact with connector plates 19 attached to their ends, such that a closed loop is formed. Inside the lower part 23b, there is also provided a circuit connector plate 17 which connects the closed loop with circuits of a nuclear magnetic resonance imaging apparatus. The upper attaching member 15 has a structure shown in Figs. 19(A) and 19(B) which includes a base 15a, a cut 15b and a hole 15d provided along an oblique face 15c, while the lower attaching member 16 has a structure shown in Figs. 20(A) and 20(B) which includes a base 16a, a protrusion 16c provided obliquely to the base 16 which is to be engaged into the cut 15b of the upper attaching member 15 and which has a hole 16b to be continuous with the hole 15d of the upper attaching member 15 when the protrusion 16c is engaged into the cut 15b. These holes 15d and 16b are provided along the direction II shown in Fig. 17. The protrusion 16c and the cut 15b are relatively fixed by means of pin 13a piercing through the holes 15d and 16b. Thus, when the upper part 23a is lifted up, this upper part 23a rotates around the pin 13a, into the direction II shown in Fig. 17. It is to be noted that this feature of oblique opening of the coil support unit 23 may be equally beneficial in imaging a portion other than the neck portion, such as the chest portion, the lower stomach portion, or any portion in the vicinity of severe wound. It is also noted that the pivotal crossing section 22d of the above embodiment can be modified to be also openable such that the upper part 23a of the coil support unit 23 becomes completely detachable and the lower part 23b alone with the upper part 23a detached can be used as a receiver coil. The use of the lower part 23b alone has the advantages of reducing the artefact, improving the Q value of the receiver coil, and making the receiver coil to be adaptable to a larger region of interest. Referring now to Fig. 21, a sixth embodiment of a receiver coil according to the present invention will be described. This receiver coil 30, which is a specialized one for imaging the neck portion, comprises a pair of elliptic coil elements 30a and 30b which are wound around a core 30c having a tapering end portion 30d which has an increasing diameter toward the head portion of the patient in correspondence with the shape of the patient, such that the coil elements 30a and 30b can be placed closer to the neck portion of the patient P. Referring now to Fig. 22, a seventh embodiment of a receiver coil according to the present invention will be described. This receiver coil 31, which is a specialized one for imaging the leg portion, comprises a solenoid coil element 31a which is wound around a core 31b having a tapering end portion 31c which has an increasing diameter toward the foot portion of the patient in correspondence with the shape of the patient, such that the coil element 31a can be placed closer to the leg portion of the patient P. Referring now to Fig. 23, an eighth embodiment of a receiver coil according to the present invention will be described. This receiver coil 32, which is a specialized one for imaging the forearm portion, comprises a saddle shaped coil element 32a which is wound around a core 32b having a tapering end portion 32c which has an increasing diameter toward the hand portion of the patient in correspondence with the shape of the patient, such that the coil element 32a can be placed closer to the forearm portion of the patient P. It is to be noted that, although the above embodiments have been described as those of a receiver coil, they can be modified in a usual manner to make them operate as a receiver and transmitter coil. Besides those already mentioned, many modifications and variations of the above embodiments may be made without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.

Claims: A receiver coil for a nuclear magnetic resonance imaging apparatus, comprising: coil element means (10a, 10b, 11a, 11b, 12a, 12b, 13a, 13b, 22a, 22b, 30a, 30b, 31a, 32a, 32b) adapted to define an opening section into which a region of interest is to be inserted, and for receiving a nuclear magnetic resonance signal from the region of interest; and coil support means (10c, 23, 30c, 31b, 31c, 32c) adapted to support the coil element means (10a, 10b, 11a, 11b, 12a, 12b, 13a, 13b, 22a, 22b, 30a, 30b, 31a, 32a, 32b) characterized in that said coil element means (10a, 10b, 11a, 11b, 12a, 12b, 13a, 13b, 22a, 22b, 30a, 30b, 31a, 32a, 32b) are defining the central axis of said opening section and that said coil support means (10c, 23, 30c, 31b, 31c, 32c) are supporting said coil element means (10a, 10b, 11a, 11b, 12a, 12b, 13a, 13b, 22a, 22b, 30a, 30b, 31a, 32a, 32b) in a configuration in which the central axis of the region of uniform signal reception sensitivity (L S10 , L S11 , L S12 , L S13 , L S20 ) for the coil element means is located away from the central axis (L G10 , L G11 , L G12 , L G13 , L G20 ) of the opening section. The receiver coil of claim 1, characterized in that the coil element means (11a, 11b, 12a, 12b, 13a, 13b) comprises a pair of coil elements having different sizes which are supported by the coil support means in a configuration to be facing each other. The receiver coil of claim 1, characterized in that the coil element means comprises a pair of elliptic coil elements (10a, 10b, 22a, 22b) which are wound to be crossing each other and to define the opening section in an approximately cylindrical shape, in a configuration in which vertical positions of cross sections (P₁₀, P₂₀) of the elliptic coil elements (10a, 10b, 22a, 22b) are located away from the vertical position of the central axis (L G10 , L G20 ) of the opening section. The receiver coil of claim 3, characterized in that the elliptic coil elements (22a, 22b) have a hinge structure (22d) at one of the cross sections, and an openable structure (22e) at another one of the cross sections, such that the coil support means (23) can be opened up. The receiver coil of claim 4, characterized in that the coil support means (23) is capable of opening up in a direction not perpendicular to the central axis of the region of interest. The receiver coil of claim 3, characterized in that the elliptic coil elements (22a, 22b) have openable structures (17, 19, 20) at the cross sections, such that the upper part (23a) of the coil support means (23) can be detached completely from the lower part (23b) of the coil support means (23). The receiver coil of claim 3, characterized in that the coil support means (23) is inclined by a prescribed angle with respect to the horizontal plane. The receiver coil of claim 3, characterized in that the coil support means (23) has an approximately similar shape as the elliptic coil elements (22a, 22b). The receiver coil of claim 3, characterized in that the opening section defined by the elliptic coil elements (22a, 22b) has a gradually increasing diameter in the direction of the central axis (L G20 ) of the opening section. The receiver coil of claim 1, characterized in that the coil support means (10c, 23, 30c, 31b, 31c, 32c) supports the coil element means (30a, 30b, 31a, 32a, 32b) in a configuration corresponding to the shape of the region of interest.

Inventors: MINEMURA TAKASHI, MINEMURA, TAKASHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G01R33
IPC codes: G01R33