Spatially resolved magnetic resonance spin-spin relaxation distribution measurement methods

A slice-selective CPMG pulse sequence with a DANTE-Z selective scheme for measuring spatially-resolved T2 distributions.

FIELD

The present invention relates to magnetic resonance imaging in general and spatially resolved magnetic resonance spin-spin relaxation distribution measurements in particular.

BACKGROUND

A spatially resolved measurement of T2relaxation (T2mapping) is one of the most basic magnetic resonance imaging (“MRI”) core analysis measurements employed to determine a wide variety of fluid/matrix properties. MRI employing T2distribution measurements, including T2distribution mapping, is an appealing technique for chemical and petroleum engineering, including core analysis, due to its ability to probe the occupancy of pores by water and oil phases [1]. It is suitable, in principle, for studies of a variety of miscible and immiscible processes, including enhanced oil recovery, and for characterizing porous rocks with regard to mass transfer between flowing and stagnant fluids. Recently, it has been recognized to be a promising technique for spatially resolved analysis of the irreducible water saturation of porous rocks [2]. T2distribution measurements, including T2distribution mapping, are also widely adopted in clinical applications as well.

It is desirable that a T2mapping scheme provide as wide an interval of measurable T2as possible for a comprehensive analysis of relaxation data. Ideally, spatially resolved T2measurements are expected to give as realistic T2distributions as regular bulk CPMG measurements. High sensitivity signal-to-noise ratio (“SNR”) measurements are also important since low field magnets are traditionally used for core rock analysis.

Two pulse sequences for one-dimensional (“1-D”) T2mapping which employ phase encode magnetic resonance imaging techniques, namely CPMG-prepared SPRITE and spin-echo single-point imaging (“SE-SPI”) are described in [3]. The CPMG-prepared SPRITE sequence has no hardware restrictions on the echo timing other than those for a regular CPMG experiment but is relatively slow, as the measurement time is proportional to T2dimension, and has a worse SNR due to a small radio frequency (“r.f.”) pulse flip angle. The spin-echo SPI provides much faster measurements and with good SNR, but has a restriction on the first echo acquisition time. The latter makes it difficult to measure short T2(<1 ms) characteristic, e.g., for water in clay-containing rocks and cement-based materials.

Prior art MRI based methods are unable to reliably measure short lifetime signal components in a T2distribution.

SUMMARY OF THE DISCLOSURE

In one implementation, the present disclosure is directed to a method of measuring a signal component in a T2distribution including using a slice selection method to provide a local T2measurement through magnetization selected from a specific location through a DANTE-Z method (a known selective excitation method). In a further implementation, the signal component includes more than one signal component and which are short lived signal components.

In another implementation, the present disclosure is directed to a method for generating a magnetic resonance pulse sequence for the investigation of a sample by magnetic resonance, including generating a selective scan; and generating a non-selective scan.

In another implementation, the present disclosure is directed to an MRI based method for measuring spatially-resolved T2distributions including providing a sample to be imaged; using a slice selection method, providing a local T2measurement through magnetization selected from a specific location in the sample through a selective excitation method.

DETAILED DESCRIPTION

Certain embodiments of the invention include a slice-selective CPMG pulse sequence with a DANTE-Z selective scheme for measuring spatially-resolved T2distributions. This is also referred to by the inventors as DANTE-Z-CPMG.

Certain embodiments of the invention include a method of measuring spatially-resolved T2which is based on a slice selective CPMG sequence. Only T2at a particular spatial position is measured at a time. There are a number of applications where only monitoring of T2at particular locations is required. For example, if a sample has well-defined large-scale heterogeneities, only T2relaxation someplace in those heterogeneous regions may be of interest. Another case is when one monitors T2change during a long repetitive experiment (e.g., a core flood experiment) and, to avoid processing a large amount of data, analyzes T2relaxation only from few chosen locations.

The DANTE-Z [4; 5] method is based on a selective inverting a DANTE sequence [6], a series of short pulses which act as an effective 180° pulse upon a band of resonance frequencies. Since the magnetization selected by this pulse train lies along z axis, it is affected only by T1relaxation while waiting for eddy currents to settle after a slice gradient, which is advantageous for short T2samples with T1>T2(the latter inequality is typical for water in rocks [7]). For the same reason, it does not introduce any phase problems when combined with another pulse sequence, which may take place for the selective excitation with a global flip angle of 90° when two transverse components of magnetization are created. Besides, it is easy to program and can be implemented on “routine spectrometers, without the need for sophisticated hardware”[8].

Certain embodiments of the invention include supplementing the slice-selective CPMG T2mapping method with an MRI sequence employing the DANTE-Z selective excitation scheme, in order to visualize the slice of interest. In certain embodiments, a one-time running of such an MRI sequence prior to the slice-selective CPMG helps one to adjust the width and the location of the slice.

The DANTE-Z Selective Excitation Scheme

The DANTE-Z scheme consists of two successive scans: during the first scan, the magnetization is inverted from z to −z within a selected band of resonance frequencies, and during the second scan, it is all maintained along z. Subtracting the signals coming from those scans leaves only selected frequency components in the resulting signal, no matter what happens to the magnetization outside the frequency band after applying DANTE-Z (seeFIG. 1). The pulse sequence realizing this scheme is known in the art from the article [10] and can be written as:
[θ−θ±x−τ]n−αx−Acq±(1)
where 2ηθ=180° and a denotes a pulse that brings the z magnetization to the transverse plane for acquisition or precession purposes. The sequence (1) differs from a basic DANTE pulse train in that the second θ-pulse has been found to cancel all negative sidebands at (2 k+1)/2τ Hz, so that the first (positive) sideband appears only at 1/τ Hz from the central frequency. This diminishes the problem of multiple selective excitation.

DANTE-Z has been shown to yield a good selectivity profile with reduced side lobes as compared to a conventional DANTE sequence with a 90° flip angle [10]. In order to remove the side lobes completely and to improve the profile toward a quasi-rectangular shape, the amplitude of θ pulses is modulated by a sinc function [9; 11]. The resulting profile is shown inFIG. 2. The sequence (1) differs from in that the second θ pulse is applied immediately after the first one. The pairwise application of θ pulses cancels all negative sidebands at (2 k+1)/2τ Hz, which appear in the basic DANTE-Z sequence [4], so that the first (positive) sideband appears only at 1/τ Hz from the central frequency [10]. This helps one to avoid more effectively a multiple selective excitation (see below).

FIG. 3 (a)shows a DANTE-Z-CPMG sequence according to an embodiment of the invention. The sequence comprises a combination of DANTE-Z given by sequence (1) with a regular CPMG pulse train. Here and in all following sequences, DANTE-Z with n=40 and τ=40 μs is used, and the CPMG 180° pulse period is 0.4 ms. There is a delay for eddy currents between the switching off of the slice gradient and applying the CPMG train, which is 300 μs for a the gradient coil used. Including this delay, the total duration of the DANTE-Z slice selection block approaches 2 ms. The slice thickness is varied by the gradient strength, while the position of the slice is controlled by the carrier frequency of the 0 pulses.

The same DANTE-Z block is used in an auxiliary MRI sequence, which is intended for visualization and adjustment of the slice of interest. In one embodiment of the invention, SPRITE, a purely phase encoding MRI technique, namely its double half k-space (DHK) variant [12], is chosen for this purpose The intrinsic idea of DANTE-Z, to subtract the signals acquired with the alternate storage of the magnetization along ±z axis, well suits SPRITE, where the subtraction is needed when one wants to remove the steady-state component of the SPRITE signal and thereby to make the image intensity be proportional to a prepared magnetization [13]. Thus, no modification is required of the acquisition scheme of SPRITE with prepared magnetization [3; 13]. A diagram of a DANTE-Z/DHK SPRITE sequence according to an embodiment of the present invention is shown inFIG. 3 (b). To improve SNR, four FID points are acquired after every a pulse, following by the chirp-z transform of the acquired k-space data for the image reconstruction [14].

Test Measurements

Sequences according to the invention were tested on a set of five 2 ml cylindrical vials of water doped with GdCl3, stacked together in a raw of 6 cm long. T2of water in the vials varied as 19, 48, 88, 143 and 190 ms. The profile of such a composite sample is shown inFIG. 4(a), as obtained with a regular (non-selective) DHK SPRITE, andFIG. 4(b)shows bulk CPMG measurements on this sample.

Applying the slice gradient of 0.78 G/cm and changing the carrier frequency of 0 pulses from −10 to 10 kHz with respect to the central frequency, individual vials could be selected with good accuracy (FIG. 4(c)). The DANTE-Z-CPMG measurements undertaken at those individual vials' positions give one-exponential decays with T2that agree very well with the reference values (FIG. 4(d)). The back extrapolated amplitudes of the decays are all about one-fifth of that of the bulk (non-selective) CPMG decay. These data demonstrate the accuracy of the technique at measuring T2as well as its ability to preserve the quantitative information about the spin density at chosen positions. Note that no sideband appears even if the slice is moved close the edges of the 8-cm field of view.

The next experiment was carried out on three sandstone samples—Berea, sandstone #15 and Wallace, all saturated with water, to test the goodness of the DANTE-Z-CPMG sequence at measuring T2distributions.FIG. 5(a)shows the whole profile of the Berea sample and the slices chosen for measuring T2distributions, and the distributions are shown inFIG. 5(b), in the respective colors. The distributions by DANTE-Z-CPMG virtually coincide with the bulk T2measurements obtained in a regular CPMG experiment (FIG. 5(b), dash line). This is a noticeable improvement compared to SE-SPI T2mapping technique presented earlier [3] (FIG. 5(b), dot line). The next sample, water-saturated sandstone #15, is more challenging than Berea for its two-humped T2distribution with a pronounced peak at T20.5 ms (FIG. 6(a), dash line). Nevertheless, the peak is well reproducible in DANTE-Z-CPMG measurements (FIG. 6(a)). Again, this considerably exceeds the limit of SE-SPI for short T2(FIG. 6(a), dot line). The bulk T2distribution in the third sample, Wallace sandstone, exhibits a very similar to sandstone #15 peak at T20.4 ms (FIG. 6(b), dash line). However, this peak is now irreproducible in DANTE-Z-CPMG measurements (FIG. 6(b), line2). Such different performance of DANTE-Z-CPMG for sandstone #15 and Wallace can be explained by the difference in their T1distributions. In Wallace, the latter stretches a wide interval of 1 ms to 2 s, while in sandstone #15 it is considerably narrower, ranging only from 0.5 ms to 0.4 s. A narrow T1distribution means that components with different relaxivity are attenuated more similarly during the selective pulses and following storage period, thus better preserving the relative intensities in the CPMG signal.

For the last test, DANTE-Z-CPMG was applied to water-filled Berea and Bentheimer sandstone core samples that had been partly de-saturated by 3.5-hrs centrifuging at 2000 r.p.m. Profiles of the samples in their initial and de-saturated states are shown inFIGS. 7(a, c). The goal of these experiments was to generate within a porous rock sample a variable water saturation and to measure associated variable T2distributions. This type of measurements have been conducted with the SE-SPI technique [2]. The previous tests show, however, that using DANTE-Z-CPMG instead of SE-SPI permits a more accurate evaluation of the T2distribution, particularly for the short T2components.FIGS. 7(bandd) show T2distributions measured by DANTE-Z-CPMG at different positions along partly de-saturated Berea and Bentheimer core samples, respectively. The obtained distributions can be further analyzed in the terms of average T2values, which have been found to correlate with SW[2], to estimate the irreducible water saturation.

The tests above show that DANTE-Z-CPMG can probe T2components down to 0.2 ms. This presents a noticeable advantage over SE-SPI, the previous T2mapping technique we introduced for porous solids (seeFIGS. 5(b) and 6(a)). The ability of DANTE-Z-CPMG to measure that short T2is, however, not general but depends on T1relaxation of the sample. Thus, the comparison of sandstone #15 and Wallace, having similar peaks at T2˜0.4-0.5 ms, demonstrates a worse performance of DANTE-Z-CPMG in the latter case. This can be explained by different T1distributions in those samples (see above). The spread of T1's may manifest itself during both the lengthy selective inversion and the following storage period, when the different T1components experience different attenuation. In this case, a narrow T1distribution is favorable for the DANTE-Z-CPMG performance. The storage period is necessary for eddy currents to settle down after the slice gradient and amounts for the hardware used in the experiments to 0.3 ms. The same delay is applied in SE-SPI after the encoding gradient inserted between 90° and first 180° pulses [3]. Together with the encoding period, it shifts the first echo acquisition time in SE-SPI to 1.2-1.6 ms after excitation, causing an irrevocable loss of T2components shorter than 1-2 ms (seeFIG. 6(a)). In DANTE-Z-CPMG, the acquisition delay is considerably shorter and it affects T2distribution only via T1mechanism of relaxation, as mentioned above, thus providing better-fitting T2distributions than in SE-SPI for short T2components.

EXPERIMENTAL

NMR measurements were carried out on an Oxford Instruments Maran DRX spectrometer equipped with a 0.35 T horizontal bore magnet (v=15 MHz), at room temperature. The r.f. probe used was a home-made 54-mm wide birdcage probe with a 90°-pulse duration of 19 μs. The acquisition parameters can be found in the text and figures captions. T2distributions were measured by using a Laplace inverse transform algorithm UPEN [15].

The invention can be implemented in a conventional MRI instrument apparatus as a programmed pulse sequence. For example,FIG. 8schematically shows an MRI measuring system which is suited for performing the inventive method. The system contains a main magnet M for generating the basic magnetic field which is substantially homogeneous and static in a volume under investigation V. Three sets of gradient coils GX, GY, and GZ are introduced into the bore of the main magnet M, which surround the volume under investigation V, and can superpose additional magnetic fields of controllable duration and strength with constant gradients on the basic field. Gradient amplifiers AX, AY, and AZ, which are driven by a sequence control unit SEQ for timely generation of gradient pulses, provide the gradient coils GX, GY, GZ with electric current for generating substantially linear gradient fields.

Several transmitting elements TA1to TAn are located in the gradient field system, the entirety of which is also called transmitting antenna means. They surround an object under investigation O and are fed by several independent RF power transmitters TX1. . . TXn. The RF pulses generated by these RF power transmitters TX1. . . TXn are determined by the sequence control unit SEQ and triggered at the correct time. The transmitting elements TA1to TAn irradiate RF pulses onto the object under investigation O located in the volume under investigation V (as described in more detail inFIG. 8), thereby exciting the nuclear spins. The resulting magnetic resonance signals are converted into electric voltage signals using one or more RF receiver elements RA1, . . . , RAm, which are then supplied to a corresponding number of receiver units RX1, . . . , RXm. The overall receiver elements RA1, . . . , RAm are also called receiver antenna means that consists of m receiver elements RA1, . . . , RAm. These are also located within the gradient coils GX, GY, GZ and surround the object under investigation O. In order to reduce the expense for equipment, the transmitting and receiver antenna means may also be designed and connected in such a fashion that one or more of the transmitting elements TA1to TAn are also used for receiving the magnetic resonance signals. In this case, which is not considered inFIG. 8, switching over between transmitting and receiving operation is effected by one or more electronic transmitting-receiver switch points that are controlled by the sequence control unit SEQ. This means that during the RF transmitting phases of the executed RF pulse sequence, this antenna(s) is/are connected to the corresponding RF power transmitter(s) and is/are separated from the allocated receiver channels, while for the receiver phases, the transmitters are separated and the receiver channel is connected. The received signals are amplified by the receiving units RX1to RXm shown inFIG. 1, and are converted into digital signals using conventional signal processing methods, and passed on to an electronic computer system COMP. In addition to the reconstruction of images and spectra and values derived from the received measured data, the controlling computer system COMP serves to operate the entire MRI measuring system and initiates performance of the pulse sequences through corresponding communication with the sequence control unit SEQ. The user-controlled or automatic execution of programs for adjusting the measuring system properties and/or for generating magnetic resonance images is also provided on this control computer system COMP, as well as the display of the reconstructed images, storage and management of measurement and image data and control programs. In order to perform these tasks, this computer system has at least one processor, one working memory, one computer keyboard KB, one display instrument PNTR, e.g. a computer mouse, one screen MON and one external digital storage unit DSK.

REFERENCES