Simultaneous quantitative mapping of conductivity and susceptibility using a double-echo ultrashort echo time sequence: Example using a hematoma evolution study.

PURPOSE: The primary purpose of this study is to propose a method for the simultaneous quantitative three-dimensional (3D) mapping of conductivity and susceptibility using double-echo ultrashort echo time (UTE) imaging. The secondary purpose is to investigate the changes of these properties over time during in vitro hematoma evolution in blood samples. METHODS: The first and second set of echo data for a UTE sequence were used to perform quantitative conductivity mapping (QCM) and quantitative susceptibility mapping (QSM), respectively. A simulation study was conducted to determine the echo time (TE) range that was acceptable for QCM. Subsequently, a NaCl phantom experiment and in vivo 3D QCM and QSM demonstrations were performed. The changes in electromagnetic (EM) properties over time were studied using in vitro blood coagulation experiments with venous blood from healthy volunteers. RESULTS: Quantitative and qualitative analyses showed small differences in the QCM for TE values up to 300 µs. The estimated conductivity and susceptibility values monotonically increased during the first few hours of the hematoma evolution experiments. However, although the susceptibility values continued to increase, the conductivity values were steady after 24 h. CONCLUSION: The proposed method can be useful for determining EM property changes (including those during hemorrhage) and providing additional information about the state of the blood. Magn Reson Med 76:214-221, 2016. © 2015 Wiley Periodicals, Inc.

Initial study on in vivo conductivity mapping of breast cancer using MRI.

PURPOSE: To develop and apply a method to measure in vivo electrical conductivity values using magnetic resonance imaging (MRI) in subjects with breast cancer. MATERIALS AND METHODS: A recently developed technique named MREPT (MR electrical properties tomography) together with a novel coil combination process was used to quantify the conductivity values. The overall technique was validated using a phantom study. In addition, 90 subjects were imaged (50 subjects with previously biopsy-confirmed breast tumor and 40 normal subjects), which was approved by our institutional review board (IRB). A routine clinical protocol, specifically a T2 -weighted FSE (fast spin echo) imaging data, was used for reconstruction of conductivity. RESULTS: By employing the coil combination, the relative error in the conductivity map was reduced from ~70% to 10%. The average conductivity values in breast cancers regions (0.89 ± 0.33S/m) was higher compared to parenchymal tissue (0.43 S/m, P < 0.0001) and fat (0.07 S/m, P < 0.00005) regions. Malignant cases (0.89 S/m, n = 30) showed increased conductivity compared to benign cases (0.56 S/m, n = 5) (P < 0.05). In addition, invasive cancers (0.96 S/m) showed higher mean conductivity compared to in situ cancers (0.57 S/m) (P < 0.0005). CONCLUSION: This study shows that conductivity mapping of breast cancers is feasible using a noninvasive in vivo MREPT technique combined with a coil combination process. The method may provide a tool in the MR diagnosis of breast cancer.

Simultaneous imaging of dual-frequency electrical conductivity using a combination of MREIT and MREPT.

PURPOSE: To propose a single magnetic resonance scan conductivity imaging technique providing dual-frequency characteristics of tissue conductivity. METHODS: Using a modified spin-echo pulse sequence, the magnetic flux density induced by externally injected currents and the B1+ phase map with injected current effects removed were acquired simultaneously. The low-frequency conductivity was reconstructed from the measured magnetic flux density by the projected current density method, while the high-frequency conductivity was reconstructed using the B1+ maps. Three different conductivity phantoms were used to demonstrate low- and high-frequency conductivity characteristics. RESULTS: A conductivity spectrum at two frequencies was successfully acquired with the proposed scheme. Magnetic resonance electrical impedance tomography is advantageous for seeing an anomaly itself wrapped with a thin insulating membrane. In addition, if the membrane is porous, the membrane property can be quantitatively visualized with magnetic resonance electrical impedance tomography. Magnetic resonance electrical properties tomography does not detect such membranes, which enable it to probe things inside an insulating membrane. CONCLUSION: Considering these pros and cons and also the fact that the conductivity of biological tissue changes with frequency, a dual-frequency conductivity imaging incorporating both magnetic resonance electrical impedance tomography and magnetic resonance electrical properties tomography in future animal and human experiments is suggested.

Artifact reduction from metallic dental materials in T1-weighted spin-echo imaging at 3.0 tesla.

PURPOSE: To investigate and propose a method of artifact reduction arising from metallic dental materials by applying a slice-encoding for metal artifact correction (SEMAC) technique on T1-weighted spin-echo (SE) imaging at 3 Tesla. MATERIALS AND METHODS: The view angle tilting (VAT) technique was adapted to conventional T1-weighted spin-echo (SE) sequence to correct the in-plane distortion, and the SEMAC technique was used for correcting the remaining through-plane distortions. Fourier transform based B0 field simulations were performed to estimate the amount of field perturbation and a scout imaging method was developed which guide in selecting the number of slice-encodings needed in SEMAC sequences. Phantoms of six different dental materials with various shapes and sizes that are used in practice (amalgam; titanium implant; gold and Ni-Cr crowns; Ni-Ti and stainless steel orthodontic wires) were imaged. In vivo images of two subjects were also acquired. The amounts of artifact reduction were quantified in phantom studies. RESULTS: Compared with conventional SE imaging in phantom studies, in-plane artifacts were reduced by up to 43% in the VAT SE images and 80% in the SEMAC images. Through-plane artifacts were reduced by up to 65% in SEMAC images. In vivo SEMAC images also showed reduced artifacts. CONCLUSION: The SEMAC technique can mitigate artifact caused by metallic dental materials for T1w-SE imaging.

3D imaging using magnetic resonance tomosynthesis (MRT) technique.

PURPOSE: To introduce an alternative approach to three-dimensional (3D) magnetic resonance (MR) imaging using a method that is similar to x-ray tomosynthesis. METHODS: Variable angle tilted-projection images are acquired using a multiple-oblique view (MOV) pulse sequence. Reconstruction is performed using three methods similar to that of x-ray tomosynthesis, which generate a set of tomographic images with multiple 2D projection images. The reconstruction algorithm is further modified to reformat to the practical imaging situations of MR. The procedure is therefore termed magnetic resonance tomosynthesis (MRT). To analyze the characteristics of MRT, simulations are performed. Phantom and in vivo experiments were done to suggest potential applications. RESULTS: Simulation results show anisotropic features that are structurally dependent in terms of resolution. Partial blurrings along slice direction were observed. In phantom and in vivo experiments, the reconstruction performance is particularly noticeable in the low SNR case where improved images with lower noise are obtained. Reformatted reconstruction using thinner slice thickness and∕or extended field-of-view can increase spatial resolution partially and alleviate slice profile imperfection. CONCLUSIONS: Results demonstrate that MRT can generate adequate 3D images using the MOV images. Various reconstruction methods in tomosynthesis were readily adapted, while allowing other tomosynthesis reconstruction algorithms to be incorporated. A reformatted reconstruction process was incorporated for applications relevant to MR imaging.

Fast B1+ mapping using three consecutive RF pulses and balanced gradients for improved bSSFP imaging.

PURPOSE: To develop a B1+ mapping during the transient phase of balanced steady state free precession (bSSFP) imaging which can be used for subsequent B1+ inhomogeneity compensation. METHODS: Two images with different flip angles (FA) are acquired using single-shot spiral technique during the transient phase of bSSFP with three consecutive RF pulses and balanced gradients. Under the assumptions that the transmit (B1+) field varies slowly in spatial domain and T1 and T2 relaxation effects are negligible during 2·TR, B1+ was estimated using the two magnitude images and bSSFP data was sequentially acquired. B1+ estimation error due to the assumptions and other factors such as FA and off-resonance were assessed using Bloch simulation. Phantom and in vivo experiments were performed with α-2α-3α scheme. RESULTS: The simulation results indicated that the proposed method was less sensitive to T1 relaxation and B1+ mapping FA (α) of approximately 60° produced minimum estimation error. The B1+-induced intensity variation was reduced with the proposed method in the phantom experiment. For both the phantom and in vivo experiments, the estimated B1+ map showed comparable to the conventional B1+ map using spin-echo DAM. CONCLUSION: B1+ map was estimated during the transient phase of bSSFP and subsequently compensated bSSFP images. There was no scan time increment and hence the technique can be used in a prescan manner for B1+ mapping or shimming.

Fast Spin Echo Imaging-Based Electric Property Tomography With K-Space Weighting via ${T}_{2}$ Relaxation (rEPT).

Magnetic resonance electrical property tomography (MREPT) is a technique used to extract the electrical properties of tissues (conductivity in particular) using a magnetic resonance imaging system. In this paper, we propose an improved data acquisition scheme for the electrical property tomography technique by utilizing T 2 modulation in fast spin echo (FSE) imaging. This technique was motivated by a numerical analysis of conductivity reconstruction in the frequency domain; results reveal the spatial frequency-dependent noise texture of conventional methods. A data-acquisition scheme using the FSE sequence was formulated to concentrate the signal within a specific frequency range where notable noise amplification is observed in the conventional method. Through numerical studies, the performance of the proposed acquisition was investigated. Furthermore, a compensation scheme was applied to reduce quantification errors due to tissue-specific T 2 modulation, which is inherent in FSE imaging. The technique was applied to phantom and in vivo experiments. Results showed improved conductivity contrasts in both experiments, as compared with conventional MREPT methods.

RF slice profile effects in magnetic resonance fingerprinting.

The radio frequency (RF) slice profile effects on T1 and T2 estimation in magnetic resonance fingerprinting (MRF) are investigated with respect to time-bandwidth product (TBW), flip angle (FA) level and field inhomogeneities. Signal evolutions are generated incorporating the non-ideal slice selective excitation process using Bloch simulation and matched to the original dictionary with and without the non-ideal slice profile taken into account. For validation, phantom and in vivo experiments are performed at 3T. Both simulations and experiments results show that T1 and T2 error from non-ideal slice profile increases with increasing FA level, off-resonance, and low TBW values. Therefore, RF slice profile effects should be compensated for accurate determination of the MR parameters.

MultiSlice CAIPIRINHA Using View Angle Tilting Technique (CAIPIVAT).

We aim to focus on improving the performance of slice parallel imaging while simultaneously correcting for spatial shift artifacts related to off-resonance. In multislice controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA), simultaneously excited slices are shifted along the phase-encoding direction by varying the radiofrequency phase for each slice, thereby obtaining virtually shifted coil sensitivity information. Meanwhile, the view angle tilting (VAT) technique provides additional shifts in the readout direction to further spread an image overlap while correcting for field inhomogeneity-induced spatial misregistration using a compensation gradient. By combining these features of CAIPIRINHA and VAT, named CAIPIVAT, the excited individual slices are shifted along both phase-encoding and readout directions. Consequently, the number of aliased voxels is reduced, and the virtual coil sensitivity information is more effectively used. Blurring due to the compensation gradient in VAT was alleviated by using a constrained least square filter. The advantages of CAIPIVAT are shown by signal-to-noise ratio simulation, phantom experiments, and in vivo experiments. Thus, CAIPIVAT can be useful for multislice parallel imaging while providing the correction of off-resonance-related spatial shift artifact.

Frequency-dependent conductivity contrast for tissue characterization using a dual-frequency range conductivity mapping magnetic resonance method.

Electrical conductivities of biological tissues show frequency-dependent behaviors, and these values at different frequencies may provide clinically useful diagnostic information. MR-based tissue property mapping techniques such as magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance electrical property tomography (MREPT) are widely used and provide unique conductivity contrast information over different frequency ranges. Recently, a new method for data acquisition and reconstruction for low- and high-frequency conductivity images from a single MR scan was proposed. In this study, we applied this simultaneous dual-frequency range conductivity mapping MR method to evaluate its utility in a designed phantom and two in vivo animal disease models. Magnetic flux density and B(1)(+) phase map for dual-frequency conductivity images were acquired using a modified spin-echo pulse sequence. Low-frequency conductivity was reconstructed from MREIT data by the projected current density method, while high-frequency conductivity was reconstructed from MREPT data by B(1)(+) mapping. Two different conductivity phantoms comprising varying ion concentrations separated by insulating films with or without holes were used to study the contrast mechanism of the frequency-dependent conductivities related to ion concentration and mobility. Canine brain abscess and ischemia were used as in vivo models to evaluate the capability of the proposed method to identify new electrical properties-based contrast at two different frequencies. The simultaneous dual-frequency range conductivity mapping MR method provides unique contrast information related to the concentration and mobility of ions inside tissues. This method has potential to monitor dynamic changes of the state of disease.

A modified multi-echo AFI for simultaneous B1(+) magnitude and phase mapping.

To simultaneously acquire the B1(+) magnitude and B1(+) phase, a modified multi-echo actual flip-angle imaging (AFI) sequence is proposed. A multi-echo gradient echo sequence was integrated into every even TR of AFI to measure both magnitude and phase of B1(+). In addition, to increase the signal-to-noise ratio of the B1(+) phase, a double-angle multi-echo AFI sequence, in which the flip-angle of the RF pulses is α at the odd TR and 2α at the even TR is proposed. Images were simulated to evaluate the performance of this method under various imaging and physical parameters. The performance was compared to the spin echo based B1(+) mapping method in phantom and in vivo studies. In the simulation, the estimation error decreased as TR1/T1 decreased and TR2/TR1 increased. For double-angle AFI, flip-angle ranges that could estimate B1(+) magnitude and phase better than the original AFI were identified. Using the proposed method, B1(+) phase estimation was similar to spin echo phase. In the phantom study, correlation coefficient between the estimated B1(+) phases using the spin echo and the proposed method was 0.9998. The results show that the B1(+) magnitude and B1(+) phase can be simultaneously acquired and accurately estimated using the proposed double-angle AFI method.

Computer-aided detection of metastatic brain tumors using magnetic resonance black-blood imaging.

OBJECTIVES: The objective of this study was to develop a computer-aided detection system for automated brain metastases detection using magnetic resonance black-blood imaging and compare its applicability with conventional magnetization-prepared rapid gradient echo (MP-RAGE) imaging. MATERIALS AND METHODS: Twenty-six patients with brain metastases were imaged with a contrast-enhanced, 3-dimensional, whole-brain magnetic resonance black-blood pulse sequence. Approval from the institutional review board and informed consent from the patients were obtained. Preprocessing steps included B1 inhomogeneity correction and brain extraction. The computer-aided detection system used 3-dimensional template matching, which measured normalized cross-correlation coefficient to generate possible metastases candidates. An artificial neural network was used for classification after various volume features were extracted. The same detection procedure was tested with contrast-enhanced MP-RAGE, which was also acquired from the same patients. RESULTS: The performance of the proposed detection method was measured by the area under the receiver operating characteristic curve (AUROC), sensitivity, and specificity values. In the black-blood case, detection process displayed an AUROC of 0.9355, a sensitivity value of 81.1%, and a specificity value of 98.2%. Magnetization-prepared rapid gradient echo data showed an AUROC of 0.6508, a sensitivity value of 30.2%, and a specificity value of 99.97%. CONCLUSIONS: The results demonstrate that accurate automated detection of metastatic brain tumors using contrast-enhanced black-blood imaging sequence is possible compared with using conventional contrast-enhanced MP-RAGE sequence.

Accelerated MR whole brain imaging with sheared voxel imaging using aliasing separation gradients.

PURPOSE: To accelerate data acquisition by undersampling phase encoding (PE) lines in MR imaging, an aliasing separation method by applying additional encoding gradients which result in voxel shape modification is proposed. METHODS: An imaging technique which achieves two directional accelerations by undersampling both PE lines in 3D imaging with aliasing separation gradients is proposed. A simple and fast reconstruction process using shift correction or gridding is followed. Phantom and in vivo experiments are performed to show the characteristics and acceleration capability of the proposed method. Further acceleration is achieved by combining with parallel acquisition techniques. RESULTS: The proposed technique shows anisotropic resolution pattern due to voxel shearing which increases proportionally with the acceleration factor. This characteristic is analyzed in both image domain and k-space and is illustrated by grid phantom imaging results. In in vivo experiment, 3D imaging results are presented with 6× acceleration using the proposed technique and ∼12× acceleration combining with parallel imaging technique. CONCLUSIONS: By applying separation gradients along both PE directions in 3D MR imaging, aliasing artifacts have been successfully separated and a high acceleration is achieved for whole brain 3D MR imaging with slight blurring due to voxel shearing.

Error analysis of nonconstant admittivity for MR-based electric property imaging.

Magnetic resonance electrical property tomography (MREPT) is a new imaging modality to visualize a distribution of admittivity γ = σ+iωε inside the human body where σ and ε denote electrical conductivity and permittivity, respectively. Using B1 maps acquired by an magnetic resonance imaging scanner, it produces cross-sectional images of σ and ε at the Larmor frequency. Since current MREPT methods rely on an assumption of a locally homogeneous admittivity, there occurs a reconstruction error where this assumption fails. Rigorously analyzing the reconstruction error in MREPT, we showed that the error is fundamental and may cause technical difficulties in interpreting MREPT images of a general inhomogeneous object. We performed numerical simulations and phantom experiments to quantitatively support the error analysis. We compared the MREPT image reconstruction problem with that of magnetic resonance electrical impedance tomography (MREIT) to highlight distinct features of both methods to probe the same object in terms of its high- and low-frequency conductivity distributions, respectively. MREPT images showed large errors along boundaries where admittivity values changed whereas MREIT images showed no such boundary effects. Noting that MREIT makes use of the term neglected in MREPT, a novel MREPT admittivity image reconstruction method is proposed to deal with the boundary effects, which requires further investigation on the complex directional derivative in the real Euclidian space [Formula: see text].