A method for measuring tissue texture using NMR spectroscopy with a VOI length in the analysis direction defined by the receiver bandwidth

Abstract

A selective sampling method for assessing the texture of a specimen using magnetic resonance (MR) involves exciting and refocusing the specimen to provide a sample rod within the specimen. An encoding gradient pulse is applied to induce phase wrapping, creating a spatial encoding for a specific k value and orientation. A low, non-zero magnitude gradient (140), acting as a time-dependent phase encoding, is then applied to generate a time-varying trajectory through 3D k-space of the k value encoding (146), while simultaneously recording multiple consecutive samples (138) of the signal at a range of k values ​​near the specific k value. A receiver bandwidth is set during data sampling to define the length of a VOI within the rod. The samples at the range of k values ​​recorded within the time span during which the non-zero magnitude gradient is applied are then post-processed to characterize the texture features of the tissue within the VOI.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims priority to U.S. Provisional Patent Application No. 62769666, filed Nov. 20, 2018, entitled A METHOD TO MEASURE TISSUE TEXTURE USING NMR SPECTROSCOPY WITH VOI LENGTH IN AN ANALYSIS DIRECTION DEFINED BY RECEIVER BANDWIDTH, having the same assignee as this application, the disclosure of which is incorporated herein by reference.

[0002] The method claimed herein relates to the field of evaluation of fine texture in biological systems and in the inspection of materials and structures in industrial and engineering research. More specifically, the embodiments disclosed herein perform selective excitation of a first slice and a second slice - selective refocusing sequence for defining a rod within a specimen of interest, and then apply a gradient along an analysis direction and sweep a small k - value range having a receiver bandwidth set narrow to define the length of a volume of interest (VOI).

Summary of the Invention

Means for Solving the Problems

[0003] The present disclosure provides a method of selective sampling for evaluating the texture of a specimen using magnetic resonance (MR). A first RF pulse is transmitted with a first gradient selected for first slice selection in the specimen. A second RF pulse is transmitted by applying a second gradient selected for slice-selective refocusing in a region defined by the intersection of a first slice and a second slice that delineate a rod within the specimen. An encoding gradient pulse is applied to induce phase wrapping to create spatial encoding for a particular k-value and orientation. Thereafter, a gradient of low non-zero magnitude that acts as a time-dependent phase encoding is applied to generate a time-varying trajectory through the 3D k-space of the k-value encoding, while simultaneously recording a plurality of consecutive samples of the NMR RF signal at a series of k-values over a vicinity of a particular k-value defined by the height and pulse width of the gradient of non-zero magnitude, and setting a receiver bandwidth narrow enough to define the length of the VOI within the rod during data sampling. Thereafter, the samples at a series of k-values recorded within the time span while the gradient of non-zero magnitude is applied are post-processed to characterize the texture features of the tissue within the VOI.

Brief Description of the Drawings

[0004]

Figure 1

Figure 2

Figure 3

Best Mode for Carrying Out the Invention

[0005] Referring to the drawings, FIG. 1 shows an exemplary timing diagram of a pulse sequence for data acquisition using the method claimed herein. The RF pulses included in trace 102 are employed to excite a selected volume of the tissue under investigation. The first RF pulse 104 is transmitted simultaneously with the gradient pulse 108 at the first magnetic field gradient represented by trace 106. The first RF pulse 104 excites a single slice or slab of the specimen, and the positioning of that slice or slab is determined according to the direction and magnitude of the first gradient and the frequency included in the RF pulse. The pulse 110, which is a negative gradient pulse, refocuses the excitation within a predetermined thickness of the slice or slab.

[0006] The second RF pulse 112 is transmitted simultaneously with the gradient pulse 116 at the second gradient represented by trace 114, exciting a slice-selective refocusing of the spins, and this second tissue slice intersects the first slice or slab described above (this second RF pulse 112 tilts the net magnetic vector antiparallel to B0, thus causing spin inversion and subsequent refocusing, and thus resulting in a signal echo at a time point after the 180-degree RF pulse corresponding to the time between the 90° RF pulse and the 180° RF pulse). For the example shown, the first, higher-value gradient pulse 118 is a crusher or “spoiler” gradient designed to induce a large phase wrap across the specimen volume at the start of the gradient pulse 116. A similar gradient pulse 122 at the end of pulse 116 unwraps this phase wrap since it arrives after the 180-degree RF inversion pulse 112. In this way, excitations that do not exist prior to the 180 Degree RF degree RF pulse, such as excitations from defects in the pulse itself, do not have this pre-encoding and are thus not refocused by the second crusher and thus do not contribute to the signal. Note that crushers 118 and 122 are shown in FIG. 1 only on one axis. However, the crushers can be on any combination of axes.

[0007] The second RF pulse, in combination with the applied second gradient, is set by this second gradient, thereby providing slice-selective refocusing of the signal in the region defined by the intersection of the first slice 202 and the second slice 204 that define the transverse dimension of the rod or core 206 through the specimen, as shown in FIG. 2. One of ordinary skill in the art will recognize that there are many ways to generate an internal excitation rod using time-varying gradient pulses and RF excitation. Parameter selection for the various methods can be done with SNR optimization in mind. As described herein, the rod or core 206 generated by slice-selective excitation and mutually orthogonal slice-selective refocusing pulses is represented in FIG. 2.

[0008] The encoding gradient pulse 126 on trace 114 sets an initial phase wrap, and thus a k-value encoding, along the direction of the gradient pulse 126. In general, the k-value encoding can be oriented in any direction by vector combination of the machine gradients, but is shown as being on the second gradient for ease of visualization. The negative encoding gradient pulse 126 winds up the phase in the signal so that echo signal acquisition begins at the selected k-value, and then the k-value can be incremented or Decrease otherwise varied or its direction changed. The encoding gradient pulse 126 is shown separated from the second gradient pulse 116, but can also be combined with pulse 122.

[0009] A low non-zero magnitude phase encoding gradient 140 that functions as a time-dependent phase encoding is applied, and the data sample 142 is obtained from an initial k-value 144 for the time-varying k-value seen in the trace segment 146. The encoding 140 for the initial k-value and the subsequent time-varying k-value 146 need not be aligned with the axis of the excited rod. However, the alignment of the phase encoding gradient is aligned with the defined direction of the VOI and, as described below, by definition with the analysis direction.

[0010] The receive gate 133 is opened to receive an RF signal, which is shown in FIG. 1 as a pulse 134 on signal trace 136. The RF signal on trace 136 is a representation showing only the signal present in the receive gate window and does not show the actual details of the RF signal outside the window. Sampling is performed as represented by trace 138 and begins with an initial k value 144 seen at trace 124. Note that at the scale of the drawing, the sampling rate is high enough that the individual triggers of the analog / digital converter (A / D) are merged into trace 138. The receiver bandwidth is set to define the length 208 of the VOI 210 within the rod or core 206 (seen in FIG. 2) during data sampling. The initial phase wrap can be selected to provide an initial k value having a magnitude corresponding to the low SNR region. An encoding gradient 126 can be employed to wind up to the lowest or highest k value in the target texture, and a gradient pulse of non-zero magnitude is imposed in the direction necessary to reach the other limit in k-space to define the texture (increase or decrease of k). In an exemplary implementation, a wavelength range of 0.050 to 0.100 mm, Bzero = 1 T, and a read time (i.e., time gradient) of 50 milliseconds at an H1 nucleus (4.258 MHz / T) are employed. A VOI analysis length of 5 mm can be obtained with a gradient of 23.5 mT / m and a bandwidth of approximately 500 Hz. In practice, the data is acquired in a first VOI having a first fairly wide bandwidth, as will be described later, by subsequent data analysis and post-selection of a plurality of subset VOIs of the bandwidth within the data.

[0011] The echoes can be refocused during the same excitation and read again with a phase encoding gradient of the same magnitude but in the opposite direction, sweeping back through the same k value range, enabling the implementation of phase cycling and achieving a higher SNR.

[0012] As a further advantage of using the bandwidth to select the specimen length, within the first bandwidth, the bandpass can be offset in either direction to access samples in slightly different regions along the same rod that was initially excited. As an example, the position of width dimension 208 can also be set by the center frequency of the selected bandwidth. In this way, a plurality of VOIs along rod 206 defined by the first and second slices 202, 204 can be selected. This can be done by post-processing the received broadband dataset.

[0013] As shown in Figure 3, a further aspect of the disclosed method is to repeat the disclosed method for an array of VOIs for a plurality of measured or derived values, and plot each VOI in the array as intensity or color on a 2D or 3D grid that matches the structure of the analyzed specimen, generating an image that displays the distribution of the measured texture. The method disclosed herein is particularly well-suited for efficiently collecting a large number of VOI measurements. For each internal excitation rod 206, after setting the center frequency and a sufficiently large bandwidth to cover the portion of the specimen intended for analysis of the first VOI, different center frequencies and bandwidths for a plurality of subset VOIs are repeatedly selected, post-processed (filtered) along the axis of the gradient (rod 206), and a 1D image of the measured (or derived) values can be generated for each iteration. In the time domain, the excitation, signal acquisition, and recovery times for one rod are interleaved with the further excitation, signal acquisition, and recovery Time for further non-intersecting rods, enabling efficient acquisition of 1D images for multiple rods.

[0014] This further aspect provides a sensitive method for locating the boundaries of the structure within the specimen being analyzed when rod 206 intersects the boundaries between different texture types within the specimen. Setting the bandwidth within a plurality of narrow ranges within the first bandwidth during post-processing has the effect of increasing the resolution for locating the position of the boundary.

[0015] As described with respect to FIG. 1, a first RF pulse is transmitted with a first gradient selected for first slice selection in the specimen, incorporating an excitation pulse and a gradient (step 301). A second RF pulse is transmitted by application of a second gradient selected for slice-selective refocusing in a region defined by the intersection of the first slice and the second slice that delineate a rod within the specimen (step 302). An encoding gradient pulse is applied to induce phase wrapping to create spatial encoding for a particular k-value and orientation (step 303). By applying a gradient of low non-zero magnitude that acts as a time-dependent phase encoding, a time-varying trajectory through the 3D k-space of k-value encoding is generated (step 304). During data sampling, a first receiver bandwidth is set to define the length of a first VOI within the rod (step 305), and a plurality of consecutive samples of the NMR RF signal are simultaneously recorded at a series of k-values over a neighborhood of a particular k-value defined by the height and pulse width of a gradient of non-zero magnitude (step 305). Thereafter, the samples at the series of k-values recorded during the time span while the gradient of non-zero magnitude was applied are post-processed. By resetting the receiver bandwidth with respect to one or more additional bandwidths within the first receiver bandwidth, one or more subsets of the VOI are provided (step 306). 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 307). In the time domain, the excitation, signal acquisition, and recovery times for the rod are separated from the excitation, signal acquisition, and recovery for additional non-intersecting rods. TimeBy interleaving, a 1D image of a plurality of non-intersecting rods is created (step 308). To provide an array of VOIs for a plurality of measured or derived values, each VOI within the array can be plotted as intensity or color on a 2D or 3D grid that matches the structure of the specimen by repeating a selected set of passbands within a first bandwidth, enabling the generation of an image that displays the distribution of the measured texture (step 309). Setting the bandwidth within a plurality of ranges having a bandwidth smaller than the first bandwidth serves to increase the resolution for identifying the boundaries of the measured texture within the specimen (step 310).

[0016] Although various embodiments of the invention have been described in detail in accordance with the requirements of the patent law, those skilled in the art will understand the modifications and substitutions to the specific embodiments disclosed herein. Such modifications are also within the scope and spirit of the invention as defined in the following claims.

Claims

1. A method of selective sampling for evaluating the texture of tissue using magnetic resonance (MR), comprising: Transmitting a first RF pulse with a first gradient selected for first slice selection in a specimen; Transmitting a second RF pulse by applying a second gradient selected for slice-selective refocusing within a region defined by the intersection of the first slice and a second slice that defines a rod within the specimen; Applying an encoding gradient pulse to induce phase wrapping to create spatial encoding for a particular k-value and orientation; Applying a low non-zero magnitude gradient having a first magnitude that acts as a time-dependent phase encoding to generate a time-varying trajectory through the 3D k-space of k-value encoding; Simultaneously recording a plurality of series of samples of NMR RF signals at a series of k-values over a neighborhood of the particular k-value defined by the height and pulse width of the non-zero magnitude gradient in a single excitation; Setting a first receiver bandwidth to define the length of a volume of interest (VOI) within the rod during said data sampling; Post-processing the samples at the series of k-values recorded during a time span while the non-zero magnitude gradient is applied to characterize features of the texture of the specimen within the VOI. A method comprising the above steps.

2. The step of refocusing; Applying a low non-zero magnitude gradient having a magnitude equal to the first magnitude but in the opposite direction; Simultaneously recording a plurality of series of samples of the NMR RF signals with inversion of the series of k-values, sweeping back through the same k-value range to enable implementation of phase cycling. The method according to claim 1, further comprising the above steps.

3. The method according to claim 1, further comprising the step of selecting a center frequency of the first receiver bandwidth for positioning the VOI along the rod. **Claim 4** The method according to claim 3, wherein the post-processing step comprises resetting a receiver bandpass for one or more further bandwidths within the first receiver bandwidth and providing one or more subsets of the VOI. **Claim 5** The method according to claim 3, wherein the post-processing step comprises selecting one or more alternative center frequencies within the first receiver bandwidth and providing one or more subsets of the VOI. **Claim 6** The method according to claim 3, further comprising the step of interleaving, in the time domain, the excitation, signal acquisition, and recovery times for the rod with the excitation, signal acquisition, and recovery times for a further non-intersecting rod to create a 1D image for a plurality of rods.

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