Patent Description:
<CIT> describes selective sampling for assessing structural spatial frequencies with specific contrast mechanisms.

An invention is set out in claim <NUM>. The invention provides a method for selective sampling to assess tissue texture in a specimen using magnetic resonance (MR). A first RF pulse is transmitted with a first gradient chosen for first slice selection in the specimen. A second RF pulse is transmitted with application of a second gradient chosen for slice selective refocusing in a region defined by an intersection of the first slice and a second slice defining a rod within the specimen. An encoding gradient pulse is applied to induce phase wrap to create a spatial encode for a specific k-value and orientation. A low non-zero magnitude gradient is then applied acting as a time dependent phase encode to produce a time varying trajectory through 3D k-space of k-value encodes while simultaneously recording multiple sequential samples of the NMR RF signal at a sequence of k-values across a neighborhood proximate the specific k-value defined by height and pulse width of the non-zero magnitude gradient and setting receiver bandwidth narrowly enough to delineate a length of a VOI within the rod during the data sampling. The samples are then post processed at the sequence of k-values, recorded within a time span while the non-zero magnitude gradient is applied, to characterize the textural features of tissue in the VOI.

Referring to the drawings, <FIG> shows an example timing diagram for a pulse sequence for data acquisition using the method claimed herein. RF pulses included in trace <NUM> are employed to excite selected volumes of the tissue under investigation. A first RF pulse, <NUM>, is transmitted coincidentally with a gradient pulse <NUM> on a first magnetic field gradient, represented in trace <NUM>. This excites a single slice, or slab, of a specimen, the positioning of which is dependent on the orientation and magnitude of the first gradient, and the frequencies contained in the RF pulse. The negative gradient pulse, pulse <NUM>, rephases the excitation within the defined thickness of the slice or slab.

A second RF pulse <NUM> is transmitted coincidentally with gradient pulse <NUM>, on a second gradient, represented in trace <NUM>, exciting a slice-selective refocus of spins, this second tissue slice intersecting with the first slice or slab described above. (As this second RF pulse <NUM> tips the net magnetic vector to antiparallel to B<NUM>, it results in inversion of spins and subsequent refocusing, thus leading to a signal echo at a time after the <NUM> degree RF pulse equivalent to the time between the <NUM>° and <NUM>° RF pulses. ) For the example shown, an initial higher value gradient pulse, <NUM>, at the start of gradient pulse <NUM> is a crusher, or "spoiler" gradient, designed to induce a large phase wrap across the specimen volume. A similar gradient pulse, <NUM>, at the trailing end of pulse <NUM>, as it comes after the <NUM> degree RF inversion pulse <NUM>, unwinds this phase wrap. In this way, any excitation that is not present prior to the <NUM> degree RF pulse, such as excitations from imperfections in the <NUM> pulse itself, will not have this pre-encode so will not be refocused by the second crusher, hence will not contribute to the signal. Note that the crushers <NUM> and <NUM> are shown in <FIG> as on only one axis. However, the crushers could be on any combination of axes.

The second RF pulse, in combination with the applied second gradient, provides slice selective refocusing of the signal in a region defined by the intersection of the first slice <NUM> and the second slice <NUM> set by this second gradient thereby defining lateral dimensions of a rod or core <NUM> through the specimen as represented in <FIG>. Those skilled in the art will recognize that there are a number of ways to generate an internally excited rod using time varying gradient pulses and RF excitations. Parameter selection for the various methods can be done with SNR optimization in mind. As described herein the rod or core <NUM> generated by a slice selective excitation and mutually- orthogonal slice selective refocusing pulses is represented in <FIG>.

An encoding gradient pulse <NUM>, on trace <NUM>, sets an initial phase wrap, hence k-value encode, along the direction of gradient pulse <NUM>. In general, the k-value encode can be oriented in any direction, by vector combination of the machine gradients but for ease of visualization is represented as on the second gradient. The negative encoding gradient pulse <NUM> winds up phase such that, in the signal echo signal acquisition starts at selected k-value, which may then be subsequently incremented or incremented or varied in orientation. While shown separated from second gradient pulse <NUM>, the encoding gradient pulse <NUM> may be combined with pulse <NUM>.

A low non-zero magnitude phase encode gradient <NUM> acting as a time dependent phase encode is applied and data samples <NUM> are taken from an initial k-value <NUM> for time varying k-values, seen in trace segment <NUM>. The encode <NUM> for the initial k-value and the subsequent time varying k-values <NUM> do not need to be aligned with the axis of the excited rod. However, alignment of the phase encode gradient is aligned with the delineated direction of the VOI and, by definition aligned with an analysis direction, as described subsequently.

A receive gate <NUM> is opened to receive the RF signal, which is shown in <FIG> as pulse <NUM> on signal trace <NUM>. The RF signal in trace <NUM> is a representation showing only the signal present in the receive gate window without showing the actual details of the RF signal outside the window. Sampling occurs as represented by trace <NUM> beginning with the initial k-value, <NUM>, seen on trace <NUM>. Note that, at the scale of the drawing, the sampling rate is high enough that the individual triggers of the analog to digital converter (A/D) have merged together in trace <NUM>. The receiver bandwidth is set to delineate a length <NUM> of a VOI <NUM> within the rod or core <NUM> (seen in <FIG>) during the data sampling. The initial phase wrap may be selected to provide an initial k-value with a magnitude corresponding to a low SNR region. The encoding gradient <NUM> may be employed to wind up to the lowest or highest k-value in a targeted texture and the non-zero magnitude gradient pulse is imposed in the necessary direction (increasing or decreasing k) to reach the other limit in k-space to define the texture. In an exemplary implementation a <NUM> read time (i.e., gradient on time) would be employed with a wavelength range <NUM> to <NUM>, Bzero = 1T, H1 nucleus (<NUM>/T). A VOI analysis length <NUM> could be obtained with a gradient of <NUM> mT/m and bandwidth of ~<NUM>. In practice the data would be acquired in a first VOI with a first significantly larger bandwidth with later data analysis and selection of multiple subset VOIs by post hoc selection of the bandwidth within the data, as described subsequently.

The echo can be refocused within the same excitation and again read with a phase encode gradient of the same magnitude as phase encode gradient <NUM> but in opposite direction to sweep back through the same range of k-values allowing implementation of phase cycling and achieving a higher SNR.

Using bandwidth to select the specimen length has the additional advantage that the band pass may be offset one way or another within the first bandwidth to access the sample in slightly different regions in the specimen along the same initially excited rod. As an example, the location of the width dimension <NUM> can also be set by the center frequency of the bandwidth chosen. This way multiple VOIs along the rod <NUM> defined by the first and second slice <NUM>, <NUM> may be selected. This can be done by post processing the received broadband data set.

As shown in <FIG>, an additional aspect of the disclosed method is iterating the disclosed method on an array of VOIs for a plurality of measured or derived values and plotting each VOI in the array as an intensity or color on a 2D or 3D grid matching the structure of the analyzed specimen to generate an image displaying the distribution of the measured textures. The method disclosed here is particularly well suited to efficiently gathering the multiplicity of VOI measurements. For each internally excited rod <NUM>, setting a center frequency and sufficiently large bandwidth to cover the portion of the specimen intended for analysis for a first VOI can then be followed by repetitive selection of varying center frequency and bandwidth for a plurality of subset VOIs and post processing (filtering) along the axis of the gradient (rod <NUM>) and, with each iteration, generating a 1D image of the measured (or derived) values. By interleaving in time domain, the excitation, signal acquisition, and recovery time for a rod with additional excitation, signal acquisition, and recovery for additional non intersecting rods, 1D images for multiple rods can also be efficiently acquired.

This additional aspect provides a sensitive method for locating boundaries of structures in the analyzed specimen for cases where the rod <NUM> intersects a boundary between differing texture types in the specimen. Setting the bandwidth in multiple narrow ranges within the first bandwidth during post processing has the effect of increasing the resolution for locating the position of the boundary.

Incorporating the excitation pulses and gradients as described with respect to <FIG>, a first RF pulse is transmitted with a first gradient chosen for first slice selection in a specimen, step <NUM>. A second RF pulse is transmitted with application of a second gradient chosen for slice selective refocusing in a region defined by an intersection of the first slice and a second slice defining a rod within the specimen, step <NUM>. An encoding gradient pulse is applied to induce phase wrap to create a spatial encode for a specific k-value and orientation, step <NUM>. By applying a low non-zero magnitude gradient acting as a time dependent phase encode a time varying trajectory through 3D k-space of k-value encodes is produced, step <NUM>. A first receiver bandwidth is set to delineate a length of a first VOI within the rod during the data sampling, step <NUM>, and multiple sequential samples of the NMR RF signal are simultaneously recorded at a sequence of k-values across a neighborhood proximate the specific k-value defined by height and pulse width of the non-zero magnitude gradient, step <NUM>. The samples of the sequence of k values, recorded within a time span while the non-zero magnitude gradient was applied, are then post processed. Resetting receiver bandpass for one or more additional bandwidths within the first receiver bandwidth provides one or more subsets of the VOI, step <NUM>. Alternatively, or in combination, one or more alternative center frequencies within the first receiver bandwidth are selected to provide one or more subsets of the VOI, step <NUM>. By interleaving in time domain, excitation, signal acquisition, and recovery time for the rod with additional excitation, signal acquisition, and recovery for additional non intersecting rods, 1D images for multiple non-intersecting rods are created, step <NUM>. By iterating for a selected set of pass bands within the first bandwidth to provide an array of subset VOIs for a plurality of measured or derived values, each subset VOI in the array may be plotted as an intensity or color on a 2D or 3D grid matching the structure of the specimen to allow generation of an image displaying the distribution of the measured textures, step <NUM>. For identifying boundaries in measured textures in the specimen, setting the bandwidth in multiple ranges with less bandwidth than the first bandwidth increases resolution for locating the position of the boundary, step <NUM>.

Claim 1:
A method for selective sampling to assess tissue texture in a specimen using magnetic resonance (MR)
comprising:
transmitting (<NUM>) a first RF pulse (<NUM>) with a first gradient chosen for first slice (<NUM>) selection in the specimen;
transmitting (<NUM>) a second RF pulse (<NUM>) with application of a second gradient chosen for slice selective refocusing in a region defined by an intersection of the first slice and a second slice (<NUM>) defining a rod (<NUM>) within the specimen;
applying (<NUM>) an encoding gradient pulse (<NUM>) to induce phase wrap to create a spatial encode for a specific k-value and orientation;
applying a low non-zero magnitude gradient (<NUM>) having a first magnitude acting as a time dependent phase encode to produce (<NUM>) a time varying trajectory through 3D k-space of k-value encodes, while
simultaneously recording multiple sequential samples of the NMR RF signal at a sequence of k-values across a neighborhood proximate the specific k-value defined by height and pulse width of the non-zero magnitude gradient in a single excitation;
setting (<NUM>) a first receiver bandwidth to delineate a length (<NUM>) of a volume of interest, VOI, (<NUM>) within the rod during the data sampling; and
post processing the samples at the sequence of k-values, recorded within a time span while the non-zero magnitude gradient is applied, to characterize the textural features of tissue(s) of the specimen in the VOI.