Magnetic resonance imaging apparatus and signal processing method
In an MRI apparatus, a dynamic range of an MR signal is extended without increasing the time for imaging nor manufacturing cost. The MRI apparatus is provided with a receiver 200 configured to perform the A/D conversion on two-system nuclear magnetic resonance signals according to a direct sampling method. The receiver 200 comprises a distributor 201 configured to divide each of the nuclear magnetic resonance signals into two-system signals, an attenuator 202 configured to attenuate an overflowed signal outputted from the distributor, and a switch 203 configured to switch between the signals having the same gain to output the switched signal. Also provided is a digital processing means having ADCs of the same number as that of the nuclear magnetic resonance signals to perform the AD conversion respectively for the two types of signals, restore the attenuated signals, and recombine the signals. After switching, the signals outputted with different gains are combined to increase the number of bits of digital data with respect to all the sampling points, thereby extending the dynamic range.
1. A magnetic resonance imaging apparatus comprising
a magnetic field generator configured to generate nuclear magnetic resonance in a subject,
a receiver having a receive coil configured to receive a nuclear magnetic resonance signal emitted from the subject, and
an arithmetic unit configured to perform a process including image reconstruction using the nuclear magnetic resonance signals received by the receiver, wherein
the receiver comprises
a distributor configured to divide the nuclear magnetic resonance signal into at least two-system signals; a first-system signal and a second-system signal,
a gain adjuster configured to perform gain adjustment on at least one of the first-system signal and the second-system signal,
a first AD converter configured to perform analog-to-digital conversion on the first-system signal and a second AD converter configured to perform analog-to-digital conversion on the second-system signal, the first-system signal and the second-system signal having been adjusted by the gain adjuster to have different gains, and
a digital processor configured to restore the gain of the digitized signal after adjusted by the gain adjuster, to an original gain, and to combine the digitized two-system signals, wherein
the digital processor further comprises a control unit configured to control inputs to the first AD converter and to the second AD converter and the outputs from the first AD converter and from the second AD converter.
2. The magnetic resonance imaging apparatus according to claim 1 , wherein
the number of the nuclear magnetic resonance signals inputted to the receiver is more than one, and
the control unit performs control with respect to each of the nuclear magnetic resonance signals in such a manner that the inputs to the first AD converter and to the second AD converter are performed with the same timing, and the outputs from the first AD converter and from the second AD converter are performed with the same timing.
3. The magnetic resonance imaging apparatus according to claim 1 , wherein
the first-system signal is directly inputted to the first AD converter, and the second-system signal is subjected to gain adjustment by the gain adjuster, and then inputted to the second AD converter.
4. The magnetic resonance imaging apparatus according to claim 1 , wherein
the receiver receives multiple nuclear magnetic resonance signals, and as the distributor, multiple distributors are provided respectively in association with the multiple nuclear magnetic resonance signals, and
switches are provided respectively, between an output from each distributor and the first AD converter and between the other output from the distributor and the second AD converter, and by switching in the switches, the first-system signals are switched therebetween and the second-system signals are switched therebetween with respect to each of the nuclear magnetic resonance signals, and the switched signals are subjected to analog-to-digital conversion.
5. The magnetic resonance imaging apparatus according to claim 2 , wherein
the multiple nuclear magnetic resonance signals are complex signals.
6. The magnetic resonance imaging apparatus according to claim 2 , wherein
the receive coil is a multi-channel receive coil having multiple small receive coils, and the multiple nuclear magnetic resonance signals are received respectively by the multiple small receive coils.
7. The magnetic resonance imaging apparatus according to claim 1 , wherein
the gain adjuster is an attenuator.
8. The magnetic resonance imaging apparatus according to claim 1 , wherein
the digital processor further comprises a gain calculator configured to calculate an amount of the gain adjustment performed by the gain adjuster.
9. The magnetic resonance imaging apparatus according to claim 8 , wherein
the gain calculator calculates the amount of the gain adjustment, using the nuclear magnetic resonance signals acquired during a prescan or during imaging.
10. A signal processing method that processes a nuclear magnetic resonance signal measured by a magnetic resonance imaging apparatus, comprising
dividing the nuclear magnetic resonance signal into two-system signals; a first-system signal and a second-system signal, performing analog-to-digital conversion on the first-system signal as it is or after gain adjustment, performing analog-to-digital conversion on the second-system signal after the gain adjustment to have a gain different from that of the first-system signal, and after restoring the adjusted gain to an original gain, combining the digitized first-system signal with the digitized second-system signal, wherein
the signal processing method controls switching the analog-to-digital conversion in response to the number of nuclear magnetic resonance signals to be processed, along with synchronizing the timing of the analog-to-digital conversion of the first-system signal with the timing of the analog-to-digital conversion of the second-system signal, with respect to one nuclear magnetic resonance signal.