Proton Density Maps Research Articles

STrategically Acquired Gradient Echo (STAGE) imaging, part I: Creating enhanced T1 contrast and standardized susceptibility weighted imaging and quantitative susceptibility mapping

PurposeTo provide whole brain grey matter (GM) to white matter (WM) contrast enhanced T1W (T1WE) images, multi-echo quantitative susceptibility mapping (QSM), proton density (PD) weighted images, T1 maps, PD maps, susceptibility weighted imaging (SWI), and R2* maps with minimal misregistration in scanning times <5min. MethodsStrategically acquired gradient echo (STAGE) imaging includes two fully flow compensated double echo gradient echo acquisitions with a resolution of 0.67×1.33×2.0mm3 acquired in 5min for 64 slices. Ten subjects were recruited and scanned at 3 Tesla. The optimum pair of flip angles (6° and 24° with TR=25ms at 3T) were used for both T1 mapping with radio frequency (RF) transmit field correction and creating enhanced GM/WM contrast (the T1WE). The proposed T1WE image was created from a combination of the proton density weighted (6°, PDW) and T1W (24°) images and corrected for RF transmit field variations. Prior to the QSM calculation, a multi-echo phase unwrapping strategy was implemented using the unwrapped short echo to unwrap the longer echo to speed up computation. R2* maps were used to mask deep grey matter and veins during the iterative QSM calculation. A weighted-average sum of susceptibility maps was generated to increase the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR). ResultsThe proposed T1WE image has a significantly improved CNR both for WM to deep GM and WM to cortical GM compared to the acquired T1W image (the first echo of 24° scan) and the T1MPRAGE image. The weighted-average susceptibility maps have 80±26%, 55±22%, 108±33% SNR increases across the ten subjects compared to the single echo result of 17.5ms for the putamen, caudate nucleus, and globus pallidus, respectively. ConclusionsSTAGE imaging offers the potential to create a standardized brain imaging protocol providing four pieces of quantitative tissue property information and multiple types of qualitative information in just 5min.

Quantitative Synthetic MRI in Children: Normative Intracranial Tissue Segmentation Values during Development.

Synthetic MR imaging is a new technique to create absolute R1 relaxivity (1/T1), R2 relaxivity (1/T2), and proton-density maps using a single multiple-spin-echo saturation recovery sequence. These relaxivity maps allow rapid automated intracranial segmentation of tissue types. To assess its utility in children, we created a normative data base of intracranial volume and brain parenchymal, GM, WM, CSF, and myelin volumes in a pediatric population with normal brain MRI findings using synthetic MR imaging. All multiple-spin-echo saturation recovery sequences containing brain MR imaging examinations performed during 34 months were retrospectively reviewed. Abnormal examination findings were excluded following a detailed radiographic and clinical chart review. The remaining normal examination findings were then quantitatively analyzed with synthetic MR imaging. Intracranial, brain parenchymal, GM, WM, CSF, and myelin volumes were plotted versus age. Qualitative assessment of segmentation accuracy was performed. Selected abnormal examination findings were compared with these normative curves. One hundred twenty-two MRI examinations with normal findings were included of individuals ranging from 0.1 to 21.5 years of age (median, 11.8 years). Resulting normative data plots compared favorably with previously published data obtained using more onerous techniques. Differentiation from pathologic states was possible using quantitative values in select cases. A pediatric data base of normal intracranial tissue volumes using a single sequence and rapid software analysis has been compiled and correlates with previously published data. This provides a framework for clinical interpretation of quantitative synthetic MR images during development. Improved age-based segmentation algorithms in young children are needed.

3D MR fingerprinting with accelerated stack-of-spirals and hybrid sliding-window and GRAPPA reconstruction

PurposeWhole-brain high-resolution quantitative imaging is extremely encoding intensive, and its rapid and robust acquisition remains a challenge. Here we present a 3D MR fingerprinting (MRF) acquisition with a hybrid sliding-window (SW) and GRAPPA reconstruction strategy to obtain high-resolution T1, T2 and proton density (PD) maps with whole brain coverage in a clinically feasible timeframe. Methods3D MRF data were acquired using a highly under-sampled stack-of-spirals trajectory with a steady-state precession (FISP) sequence. For data reconstruction, kx-ky under-sampling was mitigated using SW combination along the temporal axis. Non-uniform fast Fourier transform (NUFFT) was then applied to create Cartesian k-space data that are fully-sampled in the in-plane direction, and Cartesian GRAPPA was performed to resolve kz under-sampling to create an alias-free SW dataset. T1, T2 and PD maps were then obtained using dictionary matching. ResultsPhantom study demonstrated that the proposed 3D-MRF acquisition/reconstruction method is able to produce quantitative maps that are consistent with conventional quantification techniques. Retrospectively under-sampled in vivo acquisition revealed that SW + GRAPPA substantially improves quantification accuracy over the current state-of-the-art accelerated 3D MRF. Prospectively under-sampled in vivo study showed that whole brain T1, T2 and PD maps with 1 mm3 resolution could be obtained in 7.5 min. Conclusions3D MRF stack-of-spirals acquisition with hybrid SW + GRAPPA reconstruction may provide a feasible approach for rapid, high-resolution quantitative whole-brain imaging.

Fast 3D magnetic resonance fingerprinting for a whole-brain coverage.

The purpose of this study was to accelerate the acquisition and reconstruction time of 3D magnetic resonance fingerprinting scans. A 3D magnetic resonance fingerprinting scan was accelerated by using a single-shot spiral trajectory with an undersampling factor of 48 in the x-y plane, and an interleaved sampling pattern with an undersampling factor of 3 through plane. Further acceleration came from reducing the waiting time between neighboring partitions. The reconstruction time was accelerated by applying singular value decomposition compression in k-space. Finally, a 3D premeasured B1 map was used to correct for the B1 inhomogeneity. The T1 and T2 values of the International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology MRI phantom showed a good agreement with the standard values, with an average concordance correlation coefficient of 0.99, and coefficient of variation of 7% in the repeatability scans. The results from in vivo scans also showed high image quality in both transverse and coronal views. This study applied a fast acquisition scheme for a fully quantitative 3D magnetic resonance fingerprinting scan with a total acceleration factor of 144 as compared with the Nyquist rate, such that 3D T1 , T2 , and proton density maps can be acquired with whole-brain coverage at clinical resolution in less than 5 min. Magn Reson Med 79:2190-2197, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A new algebraic method for quantitative proton density mapping using multi-channel coil data.

A difficult problem in quantitative MRI is the accurate determination of the proton density, which is an important quantity in measuring brain tissue organization. Recent progress in estimating proton density in vivo has been based on using the inverse linear relationship between the longitudinal relaxation rate T1 and proton density. In this study, the same type of relationship is being used, however, in a more general framework by constructing 3D basis functions to model the receiver bias field. The novelty of this method is that the basis functions developed are suitable to cover an entire range of inverse linearities between T1 and proton density. The method is applied by parcellating the human brain into small cubes with size 30mm x 30mm x 30mm. In each cube the optimal set of basis functions is determined to model the receiver coil sensitivities using multi-channel (32 element) coil data. For validation, we use arbitrary data from a numerical phantom where the data satisfy the conventional MR signal equations. Using added noise of different magnitude and realizations, we show that the proton densities obtained have a bias close to zero and also low noise sensitivity. The obtained root-mean-square-error rate is less than 0.2% for the estimated proton density in a realistic 3D simulation. As an application, the method is used in a small cohort of MS patients, and proton density values for specific brain structures are determined.

Spatiotemporal characterization of hydration process of asymmetric polymeric wound dressings for decubitus ulcers.

Pressure ulcers belong to the most chalenging clinical problems. As hydration level of such wounds is important for optimal healing, preparation of new wound dressing (WD) materials for pressure ulcers requires thorough in vitro evaluation as prerequisite to final in vivo testing. The aims of the study were to: (a) develop a simple method of preparation of asymmetric polymeric membrane, (b) to propose a set of in vitro methods for membrane characterization during hydration. A polyvinyl alcohol asymmetric membrane with homogeneous skin layer and porous spongy layer was developed with nonadhesive properties and ability to absorb and retain the water. Complementary methods, including magnetic resonance imaging, allowed quantitative assessment of spatiotemporal aspects of membrane hydration, that is, global water uptake; swelling; local hydration in terms of proton density mapping; spatial distribution of T2 relaxation time; Young's modulus; piercing resistance. The proposed method of initial wound dressing evaluation seems to be promising to compare various WD formulations, to assess the time required to prepare WD membrane to be applied to the wound and to assess how long WD retains desired working properties. The developed asymmetric membrane seems to be a good candidate for further evaluation. It was found that: Young's modulus of hydrated membrane was comparable to those of human skin; asymmetrical structure was retained during the entire hydration period; each layer had its own distinct, hydration related, properties and their spatiotemporal evolution; relatively slow changes of membrane properties during the potential WD application time-span of several hours was observed. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 843-853, 2018.

Robust sliding-window reconstruction for Accelerating the acquisition of MR fingerprinting.

To develop a method for accelerated and robust MR fingerprinting (MRF) with improved image reconstruction and parameter matching processes. A sliding-window (SW) strategy was applied to MRF, in which signal and dictionary matching was conducted between fingerprints consisting of mixed-contrast image series reconstructed from consecutive data frames segmented by a sliding window, and a precalculated mixed-contrast dictionary. The effectiveness and performance of this new method, dubbed SW-MRF, was evaluated in both phantom and in vivo. Error quantifications were conducted on results obtained with various settings of SW reconstruction parameters. Compared with the original MRF strategy, the results of both phantom and in vivo experiments demonstrate that the proposed SW-MRF strategy either provided similar accuracy with reduced acquisition time, or improved accuracy with equal acquisition time. Parametric maps of T1 , T2 , and proton density of comparable quality could be achieved with a two-fold or more reduction in acquisition time. The effect of sliding-window width on dictionary sensitivity was also estimated. The novel SW-MRF recovers high quality image frames from highly undersampled MRF data, which enables more robust dictionary matching with reduced numbers of data frames. This time efficiency may facilitate MRF applications in time-critical clinical settings. Magn Reson Med 78:1579-1588, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

G-Ratio weighted imaging of the human spinal cord in vivo

The fiber g-ratio is defined as the ratio of the inner to the outer diameter of the myelin sheath. This ratio provides a measure of the myelin thickness that complements axon morphology (diameter and density) for assessment of demyelination in diseases such as multiple sclerosis. Previous work has shown that an aggregate g-ratio map can be computed using a formula that combines axon and myelin density measured with quantitative MRI.In this work, we computed g-ratio weighted maps in the cervical spinal cord of nine healthy subjects. We utilized the 300mT/m gradients from the CONNECTOM scanner to estimate the fraction of restricted water (fr) with high accuracy, using the CHARMED model. Myelin density was estimated using the lipid and macromolecular tissue volume (MTV) method, derived from normalized proton density (PD) mapping. The variability across spinal level, laterality and subject were assessed using a three-way ANOVA.The average g-ratio value obtained in the white matter was 0.76+/−0.03, consistent with previous histology work. Coefficients of variation of fr and MTV were respectively 4.3% and 13.7%. fr and myelin density were significantly different across spinal tracts (p=3×10−7 and 0.004 respectively) and were positively correlated in the white matter (r=0.42), suggesting shared microstructural information. The aggregate g-ratio did not show significant differences across tracts (p=0.6).This study suggests that fr and myelin density can be measured in vivo with high precision and that they can be combined to produce a g-ratio-weighted map robust to free water pool contamination from cerebrospinal fluid or veins. Potential applications include the study of early demyelination in multiple sclerosis, and the quantitative assessment of remyelination drugs.

Abstract 466: MR prediction of tumor burden in Patient-Derived Mouse Xenografts model of glioblastoma using an adaptive model

Abstract Introduction: In human glioblastoma multiforme (GBM), infiltrating cells are found in remote locations, even in the hemisphere contralateral to the primary lesion. Magnetic Resonance Imaging (MRI) allows approximation of the extent of tumor cell infiltration. However, the actual extent of infiltration may be greater or less than the edema, and there is no standard MRI practice that allow exploring the infiltrating tumor burden. This pilot study investigates the feasibility of using a set of MR modalities for the development of an MRI estimate of infiltrating tumor burden in Patient-Derived Mouse Xenografts model of GBM using an adaptive model. Material and Methods: 8 mice implanted with GBM CSC HF2927 were studied. MRI studies were performed in a Direct Drive Varian 7 Tesla. The following image sets were acquired: high-res. T1-weighted; T2-weighted (TE/TR = (20, 40, 60, 80)/3000 ms); MT-weighted fast spin-echo. Magnevist (0.25 mmol/kg i.p) was injected about 5 minutes before the post-contrast T1-weighted image set was acquired. The animal was sacrificed immediately after MRI and stained for the presence of human GBM cells. We used the following MRI sequences to establish a basis set for training the AM: pre- and post-contrast (Magnevist, I.P.) T1-weighted, 4-echo T2, MT, 3-direction, 3 b-value diffusion-weighted. Maps of T2 and proton density (T2-PD) were produced by fitting the T2 data. The histology image was warped and co-registered to the T2-PD image using mouse brain anatomical landmarks. MR image sets were normalized to the white matter area of the brain, and each voxel profile extracted from the 9 image set was normalized to the summation of two normalized T2 images (echo 2 and 3), following which the normalized profile along with the co-registered histology was used for training and testing of an artificial neural network (ANN) with Multi-Layer perceptron architecture to predict the presence of local tumor burden in form of tumorous cell density. Results and Conclusions: The ANN was successfully trained and validated using K-Fold Cross Validation technique (KFCV) and the 9 MR modalities as its input set. Results imply that the chosen MRI feature set contains adequate information content for training the ANN. The predictive power of the ANN was ∼ 0.81. The correlation coefficient for the association between the predicted map and histology was ∼ 0.85. Given the success of training an ANN to predict infiltrating tumor burden, it may be possible to identify similar or different basis set of MRI images that can robustly predict both infiltrating and solid tumor burden in human GBM. Citation Format: Hassan Bagher-Ebadian, Ana deCarvalho, Azimeh NV Dehkordi, Tavarekere Nagaraja, Susan Irtenkauf, Swayamparva Panda, Meser M. Ali, Arbab S. Ali, Hamid Soltanian-Zadeh, Robert Knight, James R. Ewing. MR prediction of tumor burden in Patient-Derived Mouse Xenografts model of glioblastoma using an adaptive model. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 466.

Multimodal quantitative MRI assessment of cortical damage in relapsing-remitting multiple sclerosis.

To investigate magnetization transfer ratio (MTR), T1 relaxation time, and proton density (PD) as indicators of gray matter damage in relapsing-remitting multiple sclerosis (RRMS), reflecting different aspects of microstructural damage and as imaging correlates of clinical disability. We aimed to determine which of these parameters may optimally quantify cortical damage, and serve as an imaging surrogate of clinical disability. In this study, cortical values of MTR, a surrogate for demyelination in MS, of PD, reflecting replacement of neural tissue by water, and of T1 , indicating a complex array of microstructural changes, were assessed in a group of RRMS patients in comparison to healthy controls (HC). 22 RRMS patients with varying disease duration (4.0 ± 6.54 years) and 10 HC received quantitative 3T magnetic resonance imaging (MRI) with MTR, T1 , and PD mapping. We tested for differences in cortical measurements between patients and HC. Additionally, correlation with disability as quantified by the Expanded Disability Status Scale was investigated. Cortical parameter values were significantly altered in the RRMS group, with increased values of T1 (P = 0.008) and PD (P = 0.028) and reduced values of MTR (P = 0.043). Only cortical T1 was correlated with clinical disability measurements (P = 0.001, r = 0.65). Receiver operating characteristic analysis demonstrated the best discriminatory power for T1 (area under the curve 0.79, PD: 0.75, MTR 0.73). Out of the parameters studied, cortical T1 is best suited to detect cortical damage as an imaging surrogate of clinical disability in RRMS. J. Magn. Reson. Imaging 2016;44:1600-1607.

An improved anatomical MRI technique with suppression of fixative fluid artifacts for the investigation of human postmortem brain phantoms.

Phantoms are often used to assess MR system stability in multicenter studies. Postmortem brain phantoms best replicate human brain anatomy, allowing for a combined assessment of the MR system and software chain for data analysis. However, a wash-out of fixative fluid affecting T1 values and thus T1-weighted sequences such as magnetization-prepared 180 degrees radiofrequency pulses and rapid gradient-echo (MP-RAGE) has been reported for brain phantoms, hampering their immediate use. The purpose of this study was the creation of anatomical data that provide the characteristics of conventional data while avoiding this artifact. Two brain phantoms were scanned at several time points, acquiring conventional MP-RAGE data and quantitative T1 and proton density (PD) maps. Assuming a suitable cutoff value T1cut , synthetic MP-RAGE data were created from these maps, being T1-weighted for T1 > T1cut to reduce fluid signal in the sulci, but PD-weighted for T1 < T1cut for artifact suppression. A time-dependent artifact was observed in the T1 but not in the PD maps. The temporal stability of the synthetic data was greatly improved as compared to the conventional data. The proposed method enables anatomical imaging of postmortem brain phantoms, avoiding artifacts induced by the wash-out of fixative fluid, and thus achieving high signal stability shortly after fixation. Magn Reson Med 77:1115-1123, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Quantitative T1 and proton density mapping with direct calculation of radiofrequency coil transmit and receive profiles from two-point variable flip angle data.

Quantitative T1 mapping of brain tissue is frequently based on the variable flip angle (VFA) method, acquiring spoiled gradient echo (GE) datasets at different excitation angles. However, accurate T1 calculation requires a knowledge of the sensitivity profile B1 of the radiofrequency (RF) transmit coil. For an additional derivation of proton density (PD) maps, the receive coil sensitivity profile (RP) must also be known. Mapping of B1 and RP increases the experiment duration, which may be critical when investigating patients. In this work, a method is presented for the direct calculation of B1 and RP from VFA data. Thus, quantitative maps of T1 , PD, B1 and RP can be obtained from only two spoiled GE datasets. The method is based on: (1) the exploitation of the linear relationship between 1/PD and 1/T1 in brain tissue and (2) the assumption of smoothly varying B1 and RP, so that a large number of data points can be fitted across small volume elements where B1 and RP are approximately constant. The method is tested and optimized on healthy subjects. Copyright © 2016 John Wiley & Sons, Ltd.

Changes and variability of proton density and T1 relaxation times in early multiple sclerosis: MRI markers of neuronal damage in the cerebral cortex.

Proton density (PD) and T1 relaxation time are promising quantitative MRI (qMRI) markers of neuronal damage in multiple sclerosis (MS). However, it is unknown whether cortical differences of these parameters between patients and controls exist in the early stages of disease. This study investigates cortical T1 and PD in early MS stages, hypothesizing that these are altered and display a high spatial variability. Quantitative T1 and PD mapping was performed on 11 patients with clinically isolated syndrome (CIS)/early MS in remission and 11 healthy controls. The normal appearing cortical gray matter was extracted, lobar regions were identified, and mean values and standard deviations of both parameters were calculated within each region. Increased PD was detected in MS/CIS patients in the cerebral cortex as a whole and all subregions, indicating an increase of water content. Increase of PD variability reached significance in the whole cortex and in the frontal and parietal regions. Longer T1 relaxation times and increased variability were found in the cerebral cortex in all regions studied, indicating a change of microstructural tissue composition that is spatially heterogeneous. The data show spatially heterogeneous cortical involvement in early MS is reflected in T1 and PD qMRI. • Cortical involvement in early MS is reflected in T1/PD quantitative MRI. • The changes are spatially heterogeneous. • Cortical damage goes beyond increased water content.

Time-efficient, high-resolution, whole brain three-dimensional macromolecular proton fraction mapping.

Macromolecular proton fraction (MPF) mapping is a quantitative MRI method that reconstructs parametric maps of a relative amount of macromolecular protons causing the magnetization transfer (MT) effect and provides a biomarker of myelination in neural tissues. This study aimed to develop a high-resolution whole brain MPF mapping technique using a minimal number of source images for scan time reduction. The described technique was based on replacement of an actually acquired reference image without MT saturation by a synthetic one reconstructed from R1 and proton density maps, thus requiring only three source images. This approach enabled whole brain three-dimensional MPF mapping with isotropic 1.25 × 1.25 × 1.25 mm(3) voxel size and a scan time of 20 min. The synthetic reference method was validated against standard MPF mapping with acquired reference images based on data from eight healthy subjects. Mean MPF values in segmented white and gray matter appeared in close agreement with no significant bias and small within-subject coefficients of variation (<2%). High-resolution MPF maps demonstrated sharp white-gray matter contrast and clear visualization of anatomical details, including gray matter structures with high iron content. The proposed synthetic reference method improves resolution of MPF mapping and combines accurate MPF measurements with unique neuroanatomical contrast features.

SU‐E‐J‐233: Effect of Brachytherapy Seed Artifacts in T2 and Proton Density Maps in MR Images

Purpose:This study aims at investigating the influence of brachytherapy seeds on T2 and proton density (PD) maps generated from MR images. Proton density maps can be used to extract water content. Since dose absorbed in tissue surrounding low energy brachytherapy seeds are highly influenced by tissue composition, knowing the water content is a first step towards implementing a heterogeneity correction algorithm using MR images.Methods:An LDR brachytherapy (IsoAid Advantage Pd‐103) seed was placed in the middle of an agar‐based gel phantom and imaged using a 3T Philips MR scanner with a 168‐channel head coil. A multiple echo sequence with TE=20, 40, 60, 80, 100 (ms) with large repetition time (TR=6259ms) was used to extract T2 and PD maps.Results:Seed artifacts were considerably reduced on T2 maps compared to PD maps. The variation of PD around the mean was obtained as −97% to 125% (±1%) while for T2 it was recorded as −71% to 24% (±1%).Conclusion:PD maps which are required for heterogeneity corrections are susceptible to artifacts from seeds. Seed artifacts on T2 maps, however, are significantly reduced due to not being sensitive to B0 field variation.

TU-EF-BRA-01: NMR and Proton Density MRI of the 1D Patient

TU-EF-BRA-01: NMR and Proton Density MRI of the 1D Patient

TU-EF-BRA-03: Free Induction Decay (without the Decay) and Spin-Echo Imaging

TU-EF-BRA-03: Free Induction Decay (without the Decay) and Spin-Echo Imaging

TU‐EF‐BRA‐04: Into 2, 3, and 4 Dimensions

TU‐EF‐BRA‐04: Into 2, 3, and 4 Dimensions

TU-EF-BRA-02: Longitudinal Proton Spin Relaxation and T1-Imaging

TU-EF-BRA-02: Longitudinal Proton Spin Relaxation and T1-Imaging

TU-EF-BRA-00: MR Basics I

TU-EF-BRA-00: MR Basics I