MR Electrical Characteristics Tomography

Abstract

The invention relates to a method for MR imaging of an object placed in an examination volume of an MR system. The object of the invention is to enable an improved discrimination between abnormal / malignant tissue and tissue with a high water content in MREPT. The method comprises the steps of generating MR signals by subjecting the object to an imaging sequence including 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 in the object, deriving a conductivity map from the phase map, the phase map relating to the spatial RF magnetic field distribution induced by the RF pulses in the field of view, the conductivity map representing a measurement of the distribution of conductivity in the field of view, deriving a further conductivity map from the magnitude MR image, the conductivity map further representing an estimate of the conductivity due to the distribution of water in the field of view, and identifying deviations between the two conductivity maps.

Description

[Technical field]

[0001] The present invention relates to the field of magnetic resonance (MR) imaging. The present invention relates to a method for MR imaging of an object. The present invention also relates to an MR system and to a computer program executed on an MR system. [Background technology]

[0002] MR imaging, which utilizes the interaction between a magnetic field and atomic nuclear spins to form two- or three-dimensional images, is widely used today, especially in the field of medical diagnostics, for imaging of soft tissues, since it is superior in many ways to other imaging methods, does not require ionizing radiation, and is usually non-invasive.

[0003] Electrical properties (EP), also known as dielectric properties including electrical conductivity, are fundamental properties of materials. Conductivity characterizes the ability to transmit electric current within a medium. In biological tissues, EP is determined by the fundamental biophysical properties of the tissue, such as water content, ionic concentration, molecular composition, intracellular space fraction, cell membrane permeability, cell structure, etc. Electrical conductivity can potentially be used as a biomarker to indicate tissue health in clinical applications. Previous studies have shown that various diseases cause localized changes in EP compared to healthy neighboring tissues. Among various cancers, breast cancer tissue has been shown to have the most different EP compared to normal breast tissue. Benign breast tissue shows significantly different conductivity compared to malignant breast cancer.

[0004] Magnetic resonance electrical property tomography (MREPT) is a technique that uses MR images to investigate the electrical conductivity of tissues (see, for example, Katscher et al., NMR Biomed., 2017, 30). The electrical conductivity can be obtained from simple and common imaging sequences (for example, the so-called balanced fast field echo sequence bFFE). The excitation of magnetic resonance is related to the spatial RF (magnetic field) distribution. The radio frequency field distribution can be measured directly using conventional B1 mapping techniques. The unknown electrical conductivity can be calculated by post-processing of the RF field distribution in the framework of Maxwell's equations from the acquired phase map, which is related to the spatial RF field distribution induced by the RF pulse in the object.

[0005] With regard to a reliable diagnosis, it is important to consider that high conductivity does not necessarily indicate abnormal tissue, and conversely, a highly abnormal state of tissue is not necessarily associated with high conductivity. Conductivity is primarily determined by the water content of the tissue being examined, not the degree of tissue abnormality. The problem is therefore that with conventional MREPT techniques areas of high conductivity may be intuitively misinterpreted as pathology, whereas areas may in fact only have a high water content and be perfectly healthy. As a result, misdiagnosis may occur. Summary of the Invention [Problem to be solved by the invention]

[0006] From the above, it is easily understood that there is a need for an improved MREPT imaging method. Therefore, the object of the present invention is to enable improved discrimination between tissues having only high water content and abnormal / malignant tissues during MREPT. [Means for solving the problem]

[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 comprising: generating MR signals by exposing the object to an imaging sequence having 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, the phase map relating to a spatial RF magnetic field distribution induced by an RF pulse within the field of view; deriving a conductivity map from the phase map, the conductivity map representing a measure of the distribution of conductivity within the field of view; - deriving a further conductivity map from said magnitude MR image, said further conductivity map representing an estimate of the conductivity resulting from the distribution of water within said field of view; identifying a deviation between the two conductivity maps; has.

[0008] In other words, the invention proposes to use an estimated conductivity map related to water content to "normalize" the conductivity map actually measured by MREPT. The resulting normalized tissue conductivity can be directly interpreted as the degree of tissue abnormality indicated by the deviation of the two conductivity maps. An implementation of the invention is to reconstruct the "measured" conductivity from the phase image and estimate the conductivity related to the water content of the tissue from the magnitude MR image (i.e., showing the proton density and therefore the water content). This allows for a direct voxel-by-voxel calculation of the normalized conductivity from a single MR scan, for example, by subtracting the two conductivity maps. Tissue abnormalities can be identified by non-zero (positive or negative) voxel values ​​in the conductivity map thus normalized.

[0009] In one embodiment, the imaging sequence is a steady-state gradient echo sequence, preferably a balanced fast field echo (bFFE) sequence. In general, the steady-state imaging sequence is based on a gradient echo imaging sequence with a short repetition time TR. The steady-state sequence includes transverse coherence tomography from overlapping multi-order spin echoes and stimulated echoes. This is usually achieved by refocusing the phase encoding gradients at each repetition interval to keep the phase integral (or gradient moment) constant. A fully balanced steady-state imaging sequence achieves zero phase by refocusing all imaging gradients. The phase map is suitable for MREPT when it is related only to B1 and is not affected by the inhomogeneity of the main magnetic field B0. Conventionally, by using a spin echo-based imaging sequence, any undesirable effect of B0 inhomogeneity is avoided. However, the effect of B0 inhomogeneity on the phase map can be neglected in steady-state acquisition because the phase changes little for off-resonance within the passband (frequency domain corresponding to 1 / TR) of the imaging sequence. The B1-related phase map can be derived directly from the real and imaginary parts of each voxel value for each location in the reconstructed complex MR image.

[0010] In another embodiment, the derivation of the further conductivity map comprises rescaling of the magnitude image. Thus, the invention exploits that the relationship between the voxel values ​​of the magnitude MR image and the conductivity at the respective location can be parameterized using a simple empirical rescaling function. The rescaling can then be based on a calibration, which comprises fitting the empirical rescaling function to measured conductivity maps of different fields of view, i.e., healthy tissue, where no deviations between the conductivity measured by MREPT and the conductivity resulting from the distribution of water are expected.

[0011] In yet another embodiment, the method further comprises the step of obtaining a B1 magnitude map representing the distribution of the magnitude of the RF field induced by the RF pulse in the field of view, where the conductivity map is derived from the phase map and the B1 magnitude map. In order to rigorously calculate the conductivity according to Maxwell's equations, both the magnitude and phase of B1 are required. It may therefore be advantageous to not only rely on the phase map, but also to perform an additional B1 mapping step, so that the distribution of the B1 magnitude is available as well. However, a sufficiently good approximation of the conductivity is most often obtained by using only the phase map and considering the B1 magnitude to be constant. This approximation can be used in practice, since it saves the time-consuming measurement of the B1 magnitude map.

[0012] The method of the invention does not necessarily have to be performed in direct combination with the acquisition of MR signals: it can also be performed independently outside the MR system, as a purely post-processing step of the acquired and reconstructed image data. In this respect, the invention relates in a further aspect to a method for the detection of MR signals. Providing a magnitude MR image and a phase map for a given field of view; deriving a conductivity map from the phase map, the conductivity map representing a measure of the distribution of conductivity within the field of view; - deriving a further conductivity map from the magnitude image, the further conductivity map representing an estimate of conductivity resulting from a distribution of water within the field of view; identifying a deviation between the two conductivity maps; The present invention relates to a computer-implemented method comprising:

[0013] The inventive method as described above can be performed by an MR system comprising at least one main magnet coil for generating a homogeneous static magnetic field in an examination volume, a plurality 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 arranged in the examination volume, a control unit for controlling the time sequence of the RF pulses and the switched magnetic field gradients, and a reconstruction unit. The inventive method can for example be implemented by corresponding programs in the reconstruction unit and / or in the control unit of the MR system.

[0014] The method of the present invention can be advantageously implemented in most MR systems currently in clinical use. For this purpose, it is simply necessary to utilize a computer program that controls the MR system to carry out the above-mentioned method steps of the present invention. The computer program may be present on a data carrier or in a data network, so as to be downloaded for installation in the control unit of the MR system. The computer program may also be executable independently of the MR system, for example on a commercially available personal computer.

[0015] The accompanying drawings disclose preferred embodiments of the invention, but it is to be understood that the drawings are designed for illustrative purposes only and are not intended as a definition of the limits of the invention. [Brief description of the drawings]

[0016] [Figure 1] 1 shows an MR system for carrying out the method of the present invention. [Diagram 2] The method of the present invention is illustrated as a flow chart. [Diagram 3] FIG. 2 shows a diagram illustrating the calibration for rescaling of magnitude MR images according to the present invention. [Figure 4]1 shows a conductivity map illustrating the normalization approach of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] With reference to Figure 1, there is shown an MR system 1. The apparatus comprises superconducting or resistive main magnet coils 2, 2' such that a substantially uniform, temporally constant main magnetic field is generated along the z-axis throughout the examination volume.

[0018] The 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 magnetic resonance, saturate spins, etc. to perform MR imaging.

[0019] More specifically, gradient pulse amplifiers 3 apply current pulses to selected ones of 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 to the body-volume RF coil 9 via a transmit / receive switch 8, which transmits RF pulses into the examination volume. A typical MR imaging sequence consists of packets of short duration RF pulse segments taken together with optional applied magnetic field gradients to achieve selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate resonance, excite resonance, invert magnetization, refocus resonance, or manipulate resonance to select a portion of a body 10 located within the examination volume. The MR signals are also picked up by the body-volume RF coil 9.

[0020] To generate MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 are positioned adjacent to the region selected for imaging. The array coils 11, 12, 13 may be used to receive MR signals induced by the body coil RF transmissions.

[0021] The resulting MR signals are picked up by a whole-body volume RF coil 9 and / or 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, 13 via a transmit / receive switch 8.

[0022] A host computer 15 controls the gradient pulse amplifiers 3 and the transmitter 7 to generate any of a number of MR imaging sequences, such as a balanced fast field echo (bFFE) imaging sequence. For the selected sequence, the receiver 14 receives single or multiple MR data lines in rapid succession following 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 most modern MR systems, the data acquisition system 16 is a separate computer dedicated to acquiring raw image data.

[0023] Finally, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17, which applies a Fourier transform or other suitable reconstruction algorithm 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, etc. The image is then stored in an image memory, where it can be accessed to convert slices, projections, or other portions of the image representation into an appropriate format for visualization, for example, via a video monitor 18, which provides a human-readable display of the resulting MR image.

[0024] With continued reference to FIG. 1 and further reference to FIGS. 2-4, an embodiment of the method of the present invention will now be described.

[0025] In step 21 (FIG. 2), the body 10 undergoes a steady-state bFFE imaging sequence. The generated echo signals are acquired and reconstructed into a complex MR image of a given field of view (2D or 3D). From the complex MR image, a magnitude MR image is derived, and each complex image value Sc From the real and imaginary parts of

[0026] Φ(r)=atan2(Im (S c ), Re(S c )) It is derived directly from

[0027] This phase information is used in step 22 of the conventional phase-based EPT method to calculate the electrical conductivity σ meas The electrical conductivity map is used to calculate a conductivity map representing a measure of the distribution of water in the FOV. In step 23, different fields of view are identified, i.e. "healthy" regions in the imaged anatomy, where no electrical conductivity abnormalities are expected. The different fields of view may for example be different image slices or sub-regions in a given field of view, for which magnitude images and electrical conductivity maps have already been derived in steps 21 and 22. A rescaling function is determined in step 24 to estimate the electrical conductivity resulting from the distribution of water in the FOV. Empirically, the bFFE magnitude MR image, the estimated electrical conductivity of healthy tissue at each location σ est For example, the relationship between It can be parameterized by TIFF2025504223000002.tif10130.

[0028] σ est and S. m Both are related to the water content. An example of this relationship is shown in the diagram of Figure 3 (scatter plot of the 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 (a: adjust the conductivity scaling, b: adjust the curvature of the function, c: adjust the range of the bFFE magnitude MR image). For calibration of the rescaling function, a least-squares fit is performed to find the best match between the magnitude MR image and the measured conductivity in the identified healthy regions. The solid line in Figure 3 represents the S m and σ meas, and the fit based on the above formula is shown. For the in vivo example shown, the adjusted optimal parameters were a = 1.14, b = 40.0, and c = 211. Once the optimal parameters a, b, and c are found in this way, the magnitude MR image for the entire given field of view (including the suspect, potentially abnormal tissue sites), rescaled using the rescaling function in step 25, is used to obtain the estimated conductivity σ resulting from the distribution of water within the given field of view. est Finally, in step 26, Estimated conductivity σ as in TIFF2025504223000003.tif16133 est is the conductivity σ measured over a given field of view meas The normalized conductivity σ is subtracted from norm We obtain σ norm can be negative or positive, and the estimated conductivity σ est The measured conductivity σ from meas , and thus indicates abnormal regions. In the (assumed to be healthy) regions used to adjust the constants a, b, c in step 24, the resulting σ norm should therefore be approximately 0.

[0029] This is shown diagrammatically in Fig. 4, which shows the example of a brain tumor. A given field of view is called a "slice through the tumor", while a different field of view, i.e. a healthy region used to calibrate the rescaling function, is called a "calibration slice". Top row shows the EPT-reconstructed conductivity maps σ for two slices (left: containing only healthy tissue, right: containing the tumor). meas The cerebrospinal fluid (CSF) shows the highest conductivity in the center of a given field of view, which is not a pathological region within the field of view. The bottom row shows the normalized conductivity σ for the same two slices. norm Since the normalization is based on the "healthy" calibration slice (left), the normalized conductivity of this slice σ normis 0. Normalization in the tumor slice shows zero in healthy regions where conductivity is lower than normal, non-zero in the tumor core, and zero in the edema surrounding the tumor where conductivity is higher than normal.

Claims

1. 1. A method for MR imaging of an object placed in an examination volume of an MR system, the method comprising: generating MR signals by exposing the object to an imaging sequence having 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, the phase map relating to a spatial RF magnetic field distribution induced by an RF pulse within the field of view; deriving a conductivity map from the phase map, the conductivity map representing a measure of the distribution of conductivity within the field of view; - deriving a further conductivity map from said magnitude MR image, said further conductivity map representing an estimate of the conductivity resulting from the distribution of water within said field of view; identifying a deviation between the two conductivity maps; The method comprising:

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 the deriving of the further conductivity map comprises rescaling the magnitude image.

4. 4. The method of claim 3, wherein the rescaling is based on a calibration, the calibration including fitting an empirical rescaling function to measured conductivity maps for different fields of view, and wherein deviations between the measured conductivity and conductivity resulting from water distribution are not estimated.

5. 5. The method of claim 1, wherein identifying the deviation between the two conductivity maps comprises subtracting the two conductivity maps.

6. B of the RF pulse, which represents the distribution of the magnitude of the RF magnetic field induced by the RF pulse within the field of view. 1 The method further comprises the step of acquiring an induced RF magnetic field according to a magnitude map, the conductivity map being a function of the phase map and the B 1 6. A method according to any preceding claim, derived from a magnitude map.

7. 1. A computer-implemented method, comprising: providing a magnitude MR image and a phase map for a given field of view from MR signals generated by exposing the object to an imaging sequence having RF pulses and switched magnetic field gradients; deriving a conductivity map from the phase map, the conductivity map representing a measure of the distribution of conductivity within the field of view; - deriving a further conductivity map from the magnitude image, the further conductivity map representing an estimate of conductivity resulting from a distribution of water within the field of view; identifying a deviation between the two conductivity maps; The method comprising:

8. 8. An MR system comprising at least one main magnet coil for generating a homogeneous static magnetic field in an examination volume, a plurality 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 arranged in the examination volume, a control unit for controlling the time sequence of the RF pulses and the switched magnetic field gradients, and a reconstruction unit, wherein the MR system is configured to perform the method according to any one of claims 1 to 7.

9. A computer program comprising instructions which, when said program is executed by a computer, preferably by a reconstruction unit of an MR system, cause said computer to carry out the method according to any one of claims 1 to 7.

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