Free induction decay based magnetic resonance imaging methods
A pure phase encode measurement method where the FID signal is encoded by a brief pure phase encode magnetic field gradient pulse. Data collection occurs once the gradient is turned off. Multiple free induction decay points acquired are identically encoded such that a full k space data set is acquired for each FID point. Fourier transformation of these data sets generates one, two or three dimensional images with consistent fields of view. The image series which results may be fit to a T 2 * decay function and the T 2 * magnetic resonance (“MR”) lifetime mapped. Fluid content (proton density) images may also be generated by this simple fitting procedure.
1. A method of MRI comprising:
providing a sample space,
providing a sample in the sample space, wherein the sample is porous media wherein the porous media comprises a solid matrix with pores containing a fluid of interest,
applying a static magnetic field to the sample space,
applying an RF pulse to the sample space,
applying a phase-encode magnetic field gradient pulse to the static field thereby providing spatial encoding,
measuring a FID signal of the sample space after the RF and magnetic field gradient pulses have ceased,
repeating the steps of: applying an RF pulse, applying a phase-encode magnetic field gradient pulse and measuring a FID signal of the sample, wherein the step of applying a phase-encode magnetic field gradient pulse, when repeated, further comprises incrementally increasing or decreasing the phase-encode magnetic field gradient pulse relative to the last applied phase-encode magnetic field gradient pulse, and
deriving an image of the sample from the FID signal measurements comprising applying a Fourier transform to generate a plurality of images, one image at each dwell time on the FID,
wherein the plurality of images are used to generate an image of proton density comprising fitting a specified image pixel as a function of dwell time to yield a local value of a time constant describing the local FID decay.
2. The method of claim 1 , wherein each RF pulse is a 90 degree pulse or a low flip angle RF pulse.
3. The method of claim 2 , further comprising measuring the FID signals at a series of times separated by a dwell time.
4. The method of claim 3 , wherein the repeated steps are repeated a sufficient number of times in order to derive an image of interest of the sample.
5. The method of claim 1 , further comprising providing simultaneously two orthogonal magnetic field gradient pulses and wherein the image derived from the sample is a 2-D image.
6. The method of claim 1 , further comprising providing simultaneously three orthogonal magnetic field gradient pulses and wherein the plurality of images derived from the sample is a 3-D image.
7. The method of claim 1 , wherein the magnetic field gradient pulses have durations which are as short as possible such that the plurality of images have consistent FOVs.
8. The method of claim 7 , wherein the durations of the magnetic field gradient pulses is less than about 250 μs.
9. The method of claim 7 , wherein the durations of the magnetic field gradient pulses is between about 100 μs and about 250 μs.
10. The method of claim 1 , wherein the time constant is T 2 *.
11. The method of claim 1 , wherein the zero time intercept is proportional to fluid content in a porous media.
12. The method of claim 1 , wherein the time constant corresponds to a bi-exponential decay, an exponential decay, or a non-exponential decay.