Frequency swept excitation for magnetic resonance
A method of magnetic resonance is provided that uses a frequency swept excitation wherein the acquired signal is a time domain signal is provided. In one embodiment, the method comprises, applying a sweeping frequency excitation and acquiring a time domain signal. The sweeping frequency excitation has a duration and is configured to sequentially excite isochromats having different resonant frequencies. Acquisition of the time domain signal is done during the duration of the sweeping frequency excitation. The time domain signal is based on evolution of the isochromats.
1. A method comprising:
using a magnetic resonance scanner to apply an excitation sequence to a specimen, the excitation sequence including a series of repeated pulses, each pulse having a pulse duration less than a repetition time for the repeated pulses, and each pulse having a plurality of alternating excitation segments and quiescent segments and each pulse modulated in frequency or modulated in phase;
acquiring a response signal simultaneous with application of the excitation sequence; and
generating an image using the response signal.
2. The method of claim 1 wherein radio frequency power of the excitation sequence is off during a quiescent segment.
3. The method of claim 1 wherein the frequency of the excitation sequence is swept linearly.
4. The method of claim 1 wherein the excitation sequence includes a hyperbolic secant pulse.
5. The method of claim 1 wherein the excitation sequence includes a truncated hyperbolic secant pulse.
6. The method of claim 1 further including applying a magnetic field gradient during the excitation sequence.
7. The method of claim 6 wherein the excitation sequence includes a swept frequency configured to excite the specimen in a time sequential manner along a direction of the magnetic field gradient.
8. The method of claim 6 further including modulating an amplitude of the magnetic field gradient.
9. The method of claim 8 wherein modulating the amplitude of the magnetic field gradient is at a selected rate to maintain a steady state.
10. The method of claim 6 wherein a difference between the pulse duration and the repetition time exceeds a time for setting the magnetic field gradient.
11. The method of claim 1 wherein the excitation sequence is configured to coincide with a motion corresponding to the specimen.
12. The method of claim 1 wherein generating the image includes forming a plurality of projections, each projection corresponding to the response signal.
13. A method comprising:
using a magnetic resonance scanner to provide an excitation signal to a specimen, the excitation signal including a plurality of pulses, each pulse having modulated excitation segments separated by quiescent segments, the excitation segments modulated in phase or modulated in frequency;
acquiring a response signal corresponding to relaxation of isochromats of the specimen; and
generating an output based on the response signal.
14. The method of claim 13 wherein each pulse has a duration and the acquiring occurs simultaneous with the duration.
15. The method of claim 13 further including applying a magnetic gradient simultaneous with the excitation signal.
16. The method of claim 15 wherein applying the magnetic gradient includes applying a modulated gradient.
17. The method of claim 13 wherein the excitation signal includes a first frequency swept signal having a first direction and includes a second frequency swept signal having a second direction.
18. The method of claim 17 wherein the first direction is opposite the second direction.
19. The method of claim 13 wherein the excitation signal includes a carrier frequency, and further including varying the carrier frequency to introduce jitter.
20. The method of claim 13 wherein the excitation signal includes a first sequence and a second sequence, and further including providing a first gradient during the first sequence and providing a second gradient during the second sequence, wherein at least one of the first gradient and the second gradient is configured to provide a gradient-echo, a spin-echo, or a simulated echo.