Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-12-31
2003-09-02
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06614225
ABSTRACT:
FIELD
This patent specification is in the field of magnetic resonance imaging (MRI) using pulse sequences such as sequences making use of slice selective radiofrequency (RF) pulses.
BACKGROUND
Pulse sequences can be repeated with different RF pulse frequency to give resonance at different positions on a magnetic gradient to accomplish multi-slice coverage. Mansfield has described use of slice selective excitation. To increase signal recovery by T
1
for higher SNR in images, Ernst has proposed that multiple slice planes can be obtained within the same repetition time (TR) of the sequence. In spin echo (SE) imaging, Crooks has proposed applying, within the same TR, frequency offsets to RF pulses (90° and 180°) to accomplish multi-slice imaging, and this method is commonly used currently for many other types of pulse sequences. Some are known under the names gradient echo (GR), FLASH, EPI, SE-EPI, RARE, TSE, FSE, GRASE, BURST, and stimulated echo imaging.
The efficient interleaving of slices within the same TR of a SE sequence can, if time permits, be applied to interleaving of slices within the same TE, as proposed by Bishop in T
2
-weighted SE imaging. See Bishop, J. E. and Plewes, D. B. (Department of Medical Biophysics, University of Toronto), A new Multi-slice Technique Based on TE-Interleaving. The large time delays between RF pulses and SE signal can by utilized to record additional slices by interleaving multiple complete sequences of the 90° and 180° RF pulses, applied with different frequency offsets to independently excite and independently refocus signals from different slices within a shared TE time interval.
A very different method called BURST uses a set of low flip angle RF pulses applied on the same slice. This generates signals from fragments of the total magnetization. These fragmented or BURST signals are refocused with a constant unipolar gradient. The BURST technique creates an image extremely fast; however, the image is intrinsically lower in SNR than echo train sequences such as EPI or RARE. Unlike BURST, which refocuses fractions of the slice magnetization in each signal, the EPI and RARE sequences refocus the slice's total magnetization into multiple signals.
SUMMARY
An MRI process in accordance with the disclosure herein gives each of a number of excited slices a respective phase history different from those of the other slices. The process then simultaneously refocuses these slices, and acquires the MRI signals from them at respective different times related to the respective different phase histories of the respective slices. These MRI signals can be used as known to reconstruct respective images of the slices. This process can be called Simultaneous Image Refocusing (SIR).
For example, two consecutive 90° RF excitation pulses at respective frequencies create magnetization in two slices s
1
and s
2
of a body, such as by using the known MRI practice of employing a main magnetic field and a slice selection gradient field acting on a body. A first dephasing pulse, on a readout axis Gr, is applied after the first but before the second 90° RF pulse, thereby encoding the magnetization of the first slice (s
1
) but not of slice s
2
since the second slice magnetization has not yet been created. A second dephasing pulse, on the same axis Gr, is applied after both slices have been magnetized by their respective 90° FR pulses and, therefore, encodes the magnetization of both slices equally. This gives the two slices respective different phase histories relative to the Gr direction. If the subsequent readout pulses on the Or gradient axis are of unit area, and the first and second dephasing pulses on the Gr axis have areas of 0.50 and 0.25, respectively, the magnetization pathways they produce in the two slices will refocus echoes (MRI signals) within opposite halves of each read period, alternating with each EPI (echo planar imaging) gradient switching, due to the respective different phase histories of the two slices relative to the read gradient direction.
As one example, with equal signal bandwidth in conventional EPI (single slice) and SIR EPI (two slices), the echo trains are 30 ms and 45 ms, respectively, for 64 readout periods with linear phase encoding on a 1.5 T MRI scanner.
A significant advantage of the SIR pulse sequence disclosed herein is that it reduces the number of gradient switchings that cause physiologic neurostimulation and thereby impose performance limits on MRI data acquisition. When applied to EPI sequences, SIR with simultaneous refocusing of two slices improves the threshold to physiologic stimulation (pain) by a factor of approximately two. As is known, when conventional EPI is performed with very high performance gradients (high maximum (dB/dx) at greater net slew rate (dB/dx/dt) in the sequence) the neurostimulation of the body rather than gradient performance limits the data acquisition rate and number of slices imaged.
In pulse sequences such as RARE, and to some degree GRASE, typically 180° RF pulses are used to refocus the signals, and RF dependent heating of the body ultimately limits the imaging speed and limits the number of slices acquired during an image sequence. In RARE there typically is a one-to-one relationship between the number of signals and 180° RF pulses. An advantage of the SIR technique applied to RARE sequences, when for example two slices are acquired in a read period, is to reduce to half the number of 180° degree RF pulses per slice. This greatly reduces RF heating of the body and therefore permits faster image acquisition using a shorter TR or a larger number of slices in the same time as RARE, or tradeoffs of shorter TR or more slices in the same time period.
While for simplicity most examples discussed herein involve two simultaneously refocused slices, it should be clear that when more than two slices are encoded simultaneously in SIR, there is a further reduction of RF heating per slice. It should also be clear that the techniques described herein as applied to slices can be applied to volumes having a greater thickness than that of a typical MRI slice, and that the multiple simultaneously refocused slices need not be parallel to each other, and need not be perpendicular to a longitudinal axis of the MRI magnet. Each slice in SIR can be at its own angle in space, defined for example by the known technique of using a respective combination of concurrent gradients along different axis.
After SIR EPI data acquisition of two slices, “time reversal” of data on alternate polarity read gradients separates data from the two slices onto two halves of the frequency axis of k-space. This acquired k-space is separated into two k-spaces, by directly dividing it in half on the frequency axis. Another method of separating the k-spaces is by a deconvolution method which separates the high spatial frequency k-space data overlapping in time. In fact, there is an extension of the two highest resolution regions of k-space of two slices, or leakage of low amplitude, highest spatial frequency k-space data between the two slices k-space.
Phase alternation of one of the two 90° RF excitation pulses can be used in two acquisitions of the same sequence to create a polarity change in one of the two simultaneously acquired slices' k-space. Taking different linear combinations (i.e. adding or subtracting) the two phase alternating data sets nulls data from one or the other slice. The resulting highest spatial frequency data can therefore be utilized in a larger k-space data matrix with advantages of higher spatial resolution, higher SNR/unit volume, and more complete separation of signal leakage between two simultaneously acquired slices. Another similar approach to obtaining different combinations and dependent manipulation of signals is to change the order of slice excitation with the 90° pulses in multiple average acquisitions (i.e., multiple acquisitions of the entire imaging sequence). This produces different linear combinations of k-space overlaps, allowing for separation of k-spaces of different slices.
Applie
Cooper & Dunhamm, LLP.
Feinberg David
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