Method for producing an off-center image using an EPI pulse sequence
AssigneeGeneral Electric Company
InventorsJoseph Kenneth Maier, Frederick Howard Epstein, Xiaohong Zhou, Lawrence Edward Ploetz, Paul E. Licato
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Abstract
An off-center NMR image is produced using an EPI pulse sequence. The offset along the readout gradient axis is achieved by shifting the frequency of an RF reference signal used to demodulate the received NMR echo signals. Phase errors produced by the NMR system and by the shifts in reference signal frequency are corrected in "real time" by also shifting the phase of the RF reference signal.
Claims8
Method for correcting phase errors produced by an NMR system during the acquisition of an NMR image data array using an echo-planar pulse sequence, the steps comprising:
- a) calculating a phase shift (.phi.) for each NMR echo signal in the echo-planar pulse sequence;
- b) performing the echo-planar pulse sequence by
- i) producing an RF excitation pulse using an RF reference signal;
- ii) producing a readout magnetic field gradient which alternates in value to produce a corresponding series of NMR echo signals;
- iii) receiving each of said NMR echo signals and demodulating each with an RF reference signal that has been shifted in phase in synchronism with the alternating readout magnetic field gradient by its corresponding calculated phase shift (.phi.);
- and
- iv) storing each demodulated NMR echo signal to form an NMR data set;
- and
- c) reconstructing an image from the NMR data set.
A method for producing an off-center NMR image using an echo-planar sequence, the steps comprising:
- a) determining a distance .DELTA.x which the image is to be offset from isocenter along a readout magnetic field gradient axis;
- b) calculating a frequency shift (.DELTA.f) using the distance .DELTA.x and the magnitude of the readout magnetic field gradient used in the echo-planar pulse sequence;
- c) performing a scan by executing the echo-planar sequence by
- i) producing an RF excitation pulse using an RF reference signal;
- ii) producing the readout magnetic field gradient which alternates in value to produce a corresponding series of NMR echo signals;
- iii) receiving each of said NMR echo signals and demodulating each with an RF reference signal that has been shifted in frequency by the calculated frequency shift (.DELTA.f);
- and
- iv) storing each demodulated NMR echo signal to form an NMR data set;
- and
- d) reconstructing the off-center NMR image from the NMR data set.
Dependencies are inferred from the claim text.