In the MRI apparatus, imaging of a subject to be examined is performed by placing the subject in a uniform static magnetic field generated by magnets, irradiating electromagnetic fields to excite nuclear spins inside the subject body, and then by receiving nuclear magnetic resonance signals that are the electromagnetic field generated by the nuclear spins. The irradiation of electromagnetic waves and the reception of nuclear magnetic resonance signals are conducted by the RF coil which transmits or receives electromagnetic waves of radio frequency (RF). And the transmitting coils, the receiving coils or the transmitting-receiving coils in various shapes suitably used for the MRI apparatus have been developed.
The signals to be measured usually by the MRI apparatus are the signals from the hydrogen nucleus contained in water and fat. The sensitivity of the MRI apparatus has been improved by the employment of the higher magnetic field in the MRI apparatus in recent years. As a result, it has become possible to obtain signals from the nucleus of other than hydrogen (signals from non-hydrogen nuclides), including those of phosphor, carbon, fluorine and sodium. It is expected that the imaging of signals from phosphor and carbon may enable the acquisition of information associated with the metabolism of muscles and proteins.
In general, MRI signals from non-hydrogen nuclides are significantly weaker or 10 to 100 times weaker, than those from hydrogen nucleus, so that it is difficult to obtain detailed images. Therefore, the MRI signals with low image resolution from non-hydrogen nuclide are usually displayed by superimposing them onto the morphological image produced from the MRI signals from hydrogen. In such case, it is desirable to image the MRI signals from hydrogen and of non-hydrogen nuclides at the same time or in succession.
Since the coil used for the transmission and reception of MRI signals has high frequency selectivity, the coil for obtaining MRI signals from hydrogen and the coil for obtaining signals from non-hydrogen nuclide were usually installed separately. But an attempt has been made recently to use a single coil to be tuned with two frequencies.
For example, the patent reference 1 discloses how to be tuned with two or more frequencies in a multiple patch resonator system by using an RF coil in which multiple number of conductive wires, rung, are disposed in the axial direction between dual cylinders on concentric circles. The reference 2 proposes how to be tuned with two or more frequencies by installing inductor/condenser resonant circuit in parallel with each rung of bird cage type coil. Further, the reference 3 proposes the method to be tuned with two or more frequencies by providing plural sets of so called endring of bird cage type.    [Patent document 1] U.S. Pat. No. 5,557,247    [Patent document 2] U.S. Pat. No. 4,916,418    [Patent document 3] U.S. Pat. No. 6,100,694