Methods for optimal gradient design and fast generic waveform switching
A computer-implemented method for sequencing magnetic resonance imaging waveforms uses a multistage sequencing hardware. A method comprises creating, with the aid of a computer processor, an active memory region that includes waveforms and schedules being played, and creating one or more buffer memory regions that contain waveforms and schedules not currently being played. Next, the waveforms and schedules in the one or more buffer memory regions may be updated while waveforms may be played in the active memory region. Upon completion of the waveform playback in the active memory region, the active and buffer memory regions may be swapped so that the former buffer memory region becomes the active memory region, and the former active memory region becomes the buffer memory region. The method may be repeated as needed until the imaging process is completed or otherwise halted.
1. A method of generating a multi-dimensional gradient waveform for use in a magnetic resonance imaging (MRI) device coupled to a computer system, the method comprising:
(a) providing a set of design constraints for the multi-dimensional gradient waveform along each dimension of the multi-dimensional gradient waveform, wherein the set of design constraints comprise a gradient start amplitude and a gradient end amplitude;
(b) providing (i) a set of hardware limitations for maximum gradients achievable by the MRI device, the set of hardware limitations comprising a maximum gradient slew-rate on each physical axis of the MRI device and a maximum gradient amplitude on each physical axis, and (ii) a set of regulatory limitations for maximum gradient fields that may be safely applied to a scan subject, the set of regulatory limitations comprising a maximum safe rate of change of a magnetic field of the MRI device for the scan subject;
(c) applying, with the aid of a computer processor, one or more geometric transformations to the set of design constraints, the set of hardware limitations, and the set of regulatory limitations, thereby placing the design constraints, and also the hardware limitations, and the regulatory limitations, in a geometrically transformed coordinate space;
(d) generating, with the aid of the computer processor, the multi-dimensional gradient waveform by calculating a gradient waveform for each gradient dimension (1) that simultaneously satisfies all of the design constraints, the hardware limitations, and the regulatory limitations, in the geometrically transformed coordinate space and (11) where each dimension of the gradient waveform meets an equal length condition of having the same time length as all other dimensions of the gradient waveform; and
(e) generating a magnetic field gradient pulse, with the MRI device, according to criteria of the multi-dimensional gradient waveform of step (d).
2. The method of claim 1 , wherein the multi-dimensional gradient waveform generated encompasses two or three gradient axes, respectively corresponding to a plane or volume in Cartesian space.
3. The method of claim 1 , wherein the set of design constraints further comprises one or more of a gradient area, a gradient first moment, or a higher order gradient moment.
4. The method of claim 1 , wherein one or more of the maximum gradient amplitude or maximum gradient slew-rate limit is calculated as a peak or as a root-mean-square limit.
5. The method of claim 1 , wherein the set of regulatory limitations limit induced tissue currents in the scan subject and minimize intolerable peripheral nerve stimulations in the scan subject.
6. The method of claim 1 , wherein the set of regulatory limitations comprises an integrated maximum rate of change of a magnetic field (dB/dt) for the waveform on each physical axis or as a vector sum.
7. The method of claim 1 , wherein the set of regulatory limitations comprises a maximum root-mean-square gradient slew rate.
8. The method of claim 1 , wherein the one or more geometric transformations is a spatial rotation, with the spatial rotation comprising one or more of a planar rotation, a three-dimensional rotation, or a vector rotation.
9. The method of claim 8 , wherein step (e) further comprises performing an inverse transformation of the multi-dimensional gradient waveform back into a native coordinate system of the MM device.
10. The method of claim 1 , wherein the one or more geometric transformations is rotative, proportional, or an increase or decrease in magnitude.
11. A method of generating a multi-dimensional gradient waveform for use in a magnetic resonance imaging (MRI) device coupled to a computer system, the method comprising:
(a) providing a set of design constraints for the multi-dimensional gradient waveform along each dimension of the multi-dimensional gradient waveform, wherein the set of design constraints comprise a gradient start amplitude and a gradient end amplitude;
(b) providing a set of hardware limitations for maximum gradients achievable by the MRI device, the set of hardware limitations comprising a maximum gradient slew-rate on each physical axis of the MRI device and a maximum gradient amplitude on each physical axis, wherein the set of hardware limitations constrain maximum gradient fields below a set of regulatory limitations for maximum gradient fields that may be safely applied to a scan subject, the set of regulatory limitations comprising a maximum safe rate of change of a magnetic field of the MRI device for the scan subject;
(c) applying, with the aid of a computer processor, one or more geometric transformations to the set of design constraints and the set of hardware limitations, thereby placing the design constraints, and also the hardware limitations, in a geometrically transformed coordinate space;
(d) generating, with the aid of the computer processor, the multi-dimensional gradient waveform by calculating a gradient waveform for each gradient dimension (1) that simultaneously satisfies all of the design constraints and the set of hardware limitations, in the geometrically transformed coordinate space and (ii) where each dimension of the gradient waveform meets an equal length condition of having the same time length as all other dimensions of the gradient waveform; and
(e) generating a magnetic field gradient pulse, with the MRI device, according to criteria of the multi-dimensional gradient waveform of step (d).