MR TOMOGRAPHY WITH ELECTRICAL PROPERTIES

Abstract

The invention relates to a method for MR imaging of an object (10) arranged in an examination volume of an MR system (1). An object of the invention is to enable improved differentiation between abnormal / malignant tissue and tissue with a high water content in MR-EPT. The method of the invention comprises the following steps: generating MR signals by exposing the object (10) to an imaging sequence comprising RF pulses and switched magnetic field gradients; acquiring the MR signals; reconstructing a size-MR image and a phase map from the MR signals for a given field of view within the object (10), wherein the phase map is related to the spatial RF field distribution induced by the RF pulses within the field of view.Deriving an electrical conductivity map from the phase map, wherein the electrical conductivity map represents a measurement of the distribution of electrical conductivity within the field of view; deriving a further electrical conductivity map from the size MR image, wherein the further electrical conductivity map represents an estimate of the electrical conductivity resulting from the distribution of water within the field of view; and identifying discrepancies between the two electrical conductivity maps. Furthermore, the invention relates to an MR system for performing this method and a computer program to be executed on an MR system.

Description

FIELD OF THE INVENTION

[0001] The invention relates to the field of magnetic resonance imaging (MR imaging). It relates to a method for MR imaging of an object. The invention also relates to an MR system and a computer program to be executed on an MR system. BACKGROUND OF THE INVENTION

[0002] MR imaging techniques, which use the interaction between magnetic fields and nuclear spins to produce two-dimensional or three-dimensional images, are widely used today, particularly in the field of medical diagnostics, as they are superior to other imaging techniques for imaging soft tissue in many ways, do not require ionizing radiation, and are generally non-invasive.

[0003] Electrical properties (EPs), also known as dielectric properties, including electrical conductivity, are fundamental properties of materials. Conductivity characterizes the ability to transmit electrical current within a medium. In biological tissues, electrical conductivity is determined by the tissue's basic biophysical properties, such as water content, ion concentration, molecular composition, fraction of intracellular space, cell membrane permeability, cell structure, etc. Electrical conductivity has the potential to be used as a biomarker for tissue health in clinical applications. Previous studies have shown that various diseases cause local changes in EPs compared to healthy, nearby tissue.Among the various types of cancer, breast cancer tissue has been shown to exhibit the highest number of EPs compared to normal breast tissue. Benign breast tissue exhibits significantly different conductivity than malignant breast carcinoma.

[0004] Magnetic resonance imaging (MR-EPT) is a technique that uses MR imaging to investigate the electrical conductivity of tissue (see, e.g., Katscher et al., NMR Biomed., 2017, 30). Electrical conductivity can be obtained using a simple and common imaging sequence (e.g., a so-called balanced fast-field echo (bFFE) sequence). The excitation of the magnetic resonance is related to the spatial RF (magnetic) field distribution. The RF field distribution can be directly measured using conventional B1 mapping techniques. The unknown electrical conductivity can be calculated from an acquired phase map by post-processing the RF field distribution within the framework of Maxwell's equations. The phase map is related to the spatial RF field distribution induced in the subject by the RF pulses.

[0005] For a reliable diagnosis, it is important to consider that high electrical conductivity does not necessarily indicate abnormal tissue, and conversely, a severely abnormal tissue condition is not necessarily associated with high electrical conductivity. Electrical conductivity is primarily determined by the water content of the tissue being examined, not by the degree of tissue abnormality. Therefore, with conventional MR-EPT techniques, there is the problem that regions of high electrical conductivity can be intuitively misinterpreted as pathological, while in reality the region simply has a high water content and is completely healthy. Misdiagnosis can result. SUMMARY OF THE INVENTION

[0006] From the foregoing, it is readily apparent that there is a need for an improved MR-EPT imaging method. Therefore, it is an object of the invention to enable improved discrimination between abnormal / malignant tissue and tissue that only has a high water content in MR-EPT.

[0007] According to the invention, a method for MR imaging of an object placed in an examination volume of an MR system is disclosed. The method comprises the following steps: - Generating MR signals by subjecting the subject to an imaging sequence comprising RF pulses and switched magnetic field gradients; - Acquiring the MR signals; - reconstructing a magnitude MR image and a phase map from the MR signals for a given field of view within the object, wherein the phase map is related to the spatial RF field distribution induced by the RF pulses within the field of view; - deriving an electrical conductivity map from the phase map, the electrical conductivity map being a measurement of the distribution of electrical conductivity within the field of view; - deriving a further electrical conductivity map from the size MR image, wherein the further electrical conductivity map represents an estimate of the electrical conductivity resulting from the distribution of water within the field of view; and - Identify discrepancies between the two electrical conductivity maps.

[0008] In other words, the invention proposes using the estimated electrical conductivity map associated with water content to "normalize" the conductivity maps actually measured by MR-EPT. The resulting normalized tissue conductivity can be directly interpreted as the degree of tissue abnormality, indicated by deviations between the two electrical conductivity maps. The implementation of the invention consists in reconstructing the "measured" electrical conductivity from the phase image and estimating the electrical conductivity associated with tissue water content from the magnitude MR image (which indicates, among other things, the proton density and thus the water content). This enables a direct voxel-by-voxel calculation of a normalized electrical conductivity from a single MR image, e.g., by subtracting the two electrical conductivity maps.Tissue abnormalities can be identified by non-zero (positive or negative) voxel values ​​in the thus normalized electrical conductivity map.

[0009] In one embodiment, the imaging sequence is a steady-state gradient-echo sequence, preferably a balanced fast-field echo (bFFE) sequence. Generally, steady-state imaging sequences are based on a gradient-echo sequence with a short repetition time TR. Steady-state sequences include transverse coherences from overlapping spin echoes of several orders of magnitude and stimulated echoes. This is typically achieved by refocusing the phase-encoding gradient at each repetition interval to keep the phase integral (or gradient moment) constant. Fully balanced steady-state imaging sequences achieve zero phase by refocusing all imaging gradients. The phase map is suitable for MR-EPT if it refers only to B1 and is not affected by the inhomogeneity of the main magnetic field B0.Conventionally, any undesirable influence of B0 inhomogeneity is avoided by using spin-echo-based imaging sequences. However, the influence of B0 inhomogeneity on the phase map is negligible in steady-state acquisitions, as the phase remains virtually unchanged at off-resonances within the passband of the imaging sequence (frequency range corresponding to 1 / TR). The B1-related phase map can be derived directly from the real and imaginary parts of the respective voxel value for any position in a reconstructed complex MR image.

[0010] In another embodiment, deriving the further electrical conductivity map involves rescaling the size image. The invention takes advantage of the fact that the relationship between the voxel values ​​of the size MR image and the electrical conductivity at the respective position can be parameterized by a simple empirical rescaling function. The rescaling can then be based on a calibration in which an empirical rescaling function is adapted to a measured electrical conductivity map for a different field of view, i.e., in healthy tissue, where no deviations are expected between the electrical conductivity measured by MR-EPT and the electrical conductivity resulting from the water distribution.

[0011] In another embodiment, the method further comprises the step of acquiring a B1 magnitude map representing the distribution of the magnitude of the RF field induced by the RF pulses within the field of view, wherein the electrical conductivity map is derived from the phase map and the B1 magnitude map. For the accurate calculation of the electrical conductivity according to Maxwell's equations, both the B1 magnitude and the phase are required. Therefore, it may be advantageous not to rely solely on the phase map but to also perform an additional B1 mapping step to also have the distribution of the B1 magnitude available. However, a sufficiently good approximation of the conductivity is achieved in most cases if only the phase map is used and the B1 magnitude is assumed to be constant. This approximation can be used in practice because it eliminates the time-consuming task of measuring the B1 magnitude map.

[0012] The method according to the invention does not necessarily have to be performed in direct combination with the acquisition of the MR signals. It can also be performed independently outside of an MR system as a pure post-processing step of the acquired and reconstructed image data. In this regard, the invention relates, as a further aspect, to a computer-implemented method comprising the following steps: - Providing a magnitude MR image and a phase map for a given field of view; - deriving an electrical conductivity map from the phase map, the electrical conductivity map being a measurement of the distribution of electrical conductivity within the field of view; - deriving a further electrical conductivity map from the size image, the further electrical conductivity map being an estimate of the electrical conductivity resulting from the distribution of water within the field of view; and - Identify discrepancies between the two electrical conductivity maps.

[0013] The methods of the invention described so far can be carried out using an MR system that includes at least one main magnetic coil for generating a uniform, static magnetic field in an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions in the examination volume, at least one RF coil for generating RF pulses in the examination volume and / or for receiving MR signals from an object positioned in the examination volume, a control unit for controlling the temporal sequence of RF pulses and switched magnetic field gradients, and a reconstruction unit. The method of the invention can be implemented, for example, by appropriately programming the reconstruction unit and / or the control unit of the MR system.

[0014] The method of the invention can be advantageously carried out in most MR systems currently in clinical use. For this purpose, it is merely necessary to use a computer program to control the MR system in such a way that it performs the method steps of the invention explained above. The computer program can either be present on a data carrier or in a data network so that it can be downloaded for installation in the control unit of the MR system. The computer program can also be executed independently of an MR system, e.g., on a commercially available personal computer. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are for illustrative purposes only and are not intended to define the limits of the invention. In the drawings: Fig. 1 shows an MR system for carrying out the method of the invention; Fig. Figure 2 illustrates the method of the invention as a flow chart; Fig. Figure 3 shows a diagram illustrating the calibration for rescaling the size MR image according to the invention; and Fig. Figure 4 shows schematic conductivity maps illustrating the normalization approach of the invention. DETAILED DESCRIPTION OF EMBODIMENTS

[0016] With reference to Fig. 1 shows an MR system 1. The device comprises superconducting or resistive main magnetic coils 2, 2' such that a substantially uniform, temporally constant main magnetic field is generated along a z-axis through an examination volume.

[0017] A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

[0018] More specifically, a gradient pulse amplifier 3 applies current pulses to selected whole-body gradient coils 4, 5, and 6 along the x, y, and z axes of the examination volume. A digital RF transmitter 7 transmits RF pulses or pulse packets via a transmit / receive switch 8 to a whole-body volume RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence consists of a packet of short-duration RF pulse segments, each taken together and with any applied magnetic field gradients, achieves a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance, and to select a portion of a body 10 positioned within the examination volume. The MR signals are also recorded by the whole-body volume RF coil 9.

[0019] To generate MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 is positioned along with the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions.

[0020] The resulting MR signals are picked up by the whole-body RF coil 9 and / or by the array RF coils 11, 12, 13 and demodulated by a receiver 14, which preferably includes a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12, and 13 via transmit / receive switches 8.

[0021] A host computer 15 controls the gradient pulse amplifier 3 and the transmitter 7 to generate any of a variety of MR imaging sequences, e.g., a balanced fast-field echo (bFFE) imaging sequence or the like. For the selected sequence, the receiver 14 receives a single or multiple MR data lines in rapid succession after each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line into a digital format suitable for further processing. In modern MR systems, the data acquisition system 16 is a separate computer specialized for acquiring raw image data.

[0022] Finally, the raw digital image data is reconstructed into an image representation by a reconstruction processor 17, which applies a Fourier transform or other suitable reconstruction algorithms such as SENSE, SMASH, or GRAPPA. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image buffer, where it is accessible to convert slices, projections, or other parts of the image representation into a suitable format for visualization, for example, via a video monitor 18, which provides a human-readable display of the resulting MR image.

[0023] With further reference to Fig. 1 and with further reference to Fig. 2 to 4, an embodiment of the method of the invention is explained below.

[0024] In step 21 ( Fig. 2) The body 10 is subjected to a steady-state BFFE imaging sequence. The generated echo signals are acquired and reconstructed into a complex MR image of a specific field of view (2D or 3D). A magnitude MR image is derived from the complex MR image, and a B1-related phase map is generated for each position in the reconstructed MR image directly from the real and imaginary parts of the respective complex image value S c derived: φ(r)=atan2(Im(Sc),Re(Sc))

[0025] This phase information is used in step 22 according to the conventional phase-based EPT method to calculate an electrical conductivity map, which is a measurement of the distribution of electrical conductivity σ measwithin the field of view. In step 23, another field of view, i.e., a “healthy” area within the imaged anatomy, is identified in which no electrical conductivity abnormalities are expected. The other field of view can, for example, be another image section or a subregion within the given field of view for which the size image and the electrical conductivity maps have already been derived in steps 21 and 22. To estimate the electrical conductivity resulting from the distribution of water within the field of view, a rescaling function is determined in step 24. On an empirical basis, the relationship between the bFFE size MR image S m and the estimated electrical conductivity σ est of healthy tissue at the respective position, e.g., by σest=a1+bSm−c

[0026] Both σ es t as well as S mare related to the water content. An example of this relationship is shown in the diagram of Fig. 3 (scatter plot of electrical conductivity reconstructed from the bFFE phase versus the bFFE magnitude). The parameters a, b, and c depend on the system settings and must be adjusted individually for each MR imaging scan (where a adjusts the scaling of the conductivity, b adjusts the curvature of the function, and c adjusts the range of the bFFE magnitude MR image). To calibrate the rescaling function, a least-squares fit is performed to find the best match between the magnitude MR image and the measured electrical conductivity in the identified healthy area. The solid line in Fig. Figure 3 shows the fitting based on the above equation and illustrates the relationship between S m and σ meas. For the in vivo example shown, the optimal parameters were set to a = 1.14, b = 40.0, c = 211. Once the optimal parameters a, b and c have been found in this way, the size MR image is rescaled for the entire given field of view (including a suspicious, possibly abnormal tissue region) using the rescaling function in step 25 to obtain the estimated electrical conductivity σ est resulting from the distribution of water within the given field of view. Finally, in step 26, the estimated electrical conductivity σ est from the measured conductivity σ meas subtracted over the entire given field of view: σnorm=σmeas−σest=σmeas−a1+bSm−c which is the normalized conductivity σ norm results in σ norm can be negative or positive and shows deviations of the measured conductivity σ meas from the estimated conductivity σ estand thus indicates abnormal areas. In the (supposedly healthy) area used to adjust the constants a, b, c in step 24, the resulting σ norm therefore be approximately zero.

[0027] This is in Fig. 4 is shown schematically using the example of a brain tumor. The given field of view is referred to as the 'tumor slice', while the other field of view, i.e., the healthy area used to calibrate the rescaling function, is referred to as the 'calibration slice'. The top row shows the EPTreconstructed electrical conductivity maps σ measfor the two layers (left: containing only healthy tissue, right: including tumor). Although the cerebral spinal fluid (CSF) shows the highest conductivity in the center of the given field of view, this is not the pathological area within the field of view. The bottom row shows the normalized electrical conductivity σ norm for the same two layers. Since the normalization is based on the 'healthy' calibration cut (left), the normalized conductivity σ norm This layer is zero. Normalization in the tumor layer shows zero in the healthy areas and non-zero in the tumor core, where conductivity is lower than normal, and in the tumor-surrounding edema, where conductivity is higher than normal. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] Katscher et al., NMR Biomed., 2017, 30

[0004]

Claims

[1] Method of MR imaging of an object (10) placed in an examination volume of an MR system (1), the method comprising the steps of: - generating MR signals by subjecting the object (10) to an imaging sequence comprising RF pulses and switched magnetic field gradients; - Acquiring the MR signals; - reconstructing a magnitude MR image and a phase map from the MR signals for a given field of view within the object (10), wherein the phase map is related to the spatial RF field distribution induced by the RF pulses within the field of view; - deriving an electrical conductivity map from the phase map, the electrical conductivity map being a measurement of the distribution of electrical conductivity within the field of view; - deriving a further electrical conductivity map from the size MR image, wherein the further electrical conductivity map represents an estimate of the electrical conductivity resulting from the distribution of water in the field of view; and - Identify discrepancies between the two electrical conductivity maps. [2] The method of claim 1, wherein the imaging sequence is a steady-state gradient echo sequence, preferably a balanced fast-field echo sequence. [3] The method of claim 1 or 2, wherein deriving the further electrical conductivity map includes rescaling the size image. [4] The method of claim 3, wherein the rescaling is based on a calibration in which an empirical rescaling function is fitted to a measured electrical conductivity map for a different field of view where no deviations are expected between the measured electrical conductivity and the electrical conductivity resulting from the water distribution. [5] The method of any one of claims 1 to 4, wherein identifying deviations of the two electrical conductivity maps includes subtracting the two electrical conductivity maps. [6] The method of any one of claims 1 to 5, further comprising the step of acquiring the B1 magnitude map of the RF field induced by the RF pulses, which represents the distribution of the magnitude of the RF field induced by the RF pulses within the field of view, wherein the electrical conductivity map is derived from the phase map and the B1 magnitude map. [7] A computer-implemented method, the method comprising the steps of: - providing a magnitude MR image and a phase map for a given field of view from MR signals generated by subjecting an object (10) to an imaging sequence comprising RF pulses and switched magnetic field gradients; - Deriving an electrical conductivity map from the phase map, the electrical conductivity map being a measurement of the distribution of electrical conductivity within the field of view, - deriving a further electrical conductivity map from the size image, the further electrical conductivity map being an estimate of the electrical conductivity resulting from the distribution of water within the field of view; and - Identify discrepancies between the two electrical conductivity maps. [8] MR system, including at least one main magnetic coil (2) for generating a uniform, static magnetic field in an examination volume, a number of gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions in the examination volume, at least one RF coil (9) for generating RF pulses in the examination volume and / or for receiving MR signals from an object (10) positioned in the examination volume, a control unit (15) for controlling the temporal sequence of RF pulses and switched magnetic field gradients and a reconstruction unit (17), wherein the MR system (1) is arranged to carry out the method according to one of claims 1 to 7. [9] Computer program comprising instructions which, when the program is executed by a computer, preferably by a reconstruction unit (17) of an MR system (1), cause the computer to carry out the method according to one of claims 1 to 7.

Images