Showing papers in "Magnetic Resonance in Medicine in 2007"
TL;DR: Practical incoherent undersampling schemes are developed and analyzed by means of their aliasing interference and demonstrate improved spatial resolution and accelerated acquisition for multislice fast spin‐echo brain imaging and 3D contrast enhanced angiography.
Abstract: The sparsity which is implicit in MR images is exploited to significantly undersample k -space. Some MR images such as angiograms are already sparse in the pixel representation; other, more complicated images have a sparse representation in some transform domain–for example, in terms of spatial finite-differences or their wavelet coefficients. According to the recently developed mathematical theory of compressedsensing, images with a sparse representation can be recovered from randomly undersampled k -space data, provided an appropriate nonlinear recovery scheme is used. Intuitively, artifacts due to random undersampling add as noise-like interference. In the sparse transform domain the significant coefficients stand out above the interference. A nonlinear thresholding scheme can recover the sparse coefficients, effectively recovering the image itself. In this article, practical incoherent undersampling schemes are developed and analyzed by means of their aliasing interference. Incoherence is introduced by pseudo-random variable-density undersampling of phase-encodes. The reconstruction is performed by minimizing the 1 norm of a transformed image, subject to data
TL;DR: The AFI method overcomes the limitation of previous methods that required long relaxation delays between sequence repetitions and is useful for time‐efficient whole‐body B1 mapping and correction of T1 maps obtained using a variable FA technique in the presence of nonuniform RF excitation.
Abstract: A new method has been developed for fast image-based measurements of the transmitted radiofrequency (RF) field. The method employs an actual flip-angle imaging (AFI) pulse sequence that consists of two identical RF pulses followed by two delays of different duration (TR(1) < TR(2)). After each pulse, a gradient-echo (GRE) signal is acquired. It has been shown theoretically and experimentally that if delays TR(1) and TR(2) are sufficiently short and the transverse magnetization is completely spoiled, the ratio r = S(2)/S(1) of signal intensities S(1) and S(2), acquired at the beginning of the time intervals TR(1) and TR(2), depends on the flip angle (FA) of applied pulses as r = (1 + n * cos(FA))/(n + cos(FA)), where n = TR(2)/TR(1). The method allows fast 3D implementation and provides accurate B(1) measurements that are highly insensitive to T(1). The unique feature of the AFI method is that it uses a pulsed steady-state signal acquisition. This overcomes the limitation of previous methods that required long relaxation delays between sequence repetitions. The method has been shown to be useful for time-efficient whole-body B(1) mapping and correction of T(1) maps obtained using a variable FA technique in the presence of nonuniform RF excitation.
TL;DR: A regularized, fast, and robust analytical solution for the Q‐ball imaging (QBI) reconstruction of the orientation distribution function (ODF) together with its detailed validation and a discussion on its benefits over the state‐of‐the‐art.
Abstract: We propose a regularized, fast, and robust analytical solution for the Q-ball imaging (QBI) reconstruction of the orientation distribution function (ODF) together with its detailed validation and a discussion on its benefits over the state-of-the-art. Our analytical solution is achieved by modeling the raw high angular resolution diffusion imaging signal with a spherical harmonic basis that incorporates a regularization term based on the Laplace– Beltrami operator defined on the unit sphere. This leads to an elegant mathematical simplification of the Funk–Radon transform which approximates the ODF. We prove a new corollary of the Funk–Hecke theorem to obtain this simplification. Then, we show that the Laplace–Beltrami regularization is theoretically and practically better than Tikhonov regularization. At the cost of slightly reducing angular resolution, the Laplace–Beltrami regularization reduces ODF estimation errors and improves fiber detection while reducing angular error in the ODF maxima detected. Finally, a careful quantitative validation is performed against ground truth from synthetic data and against real data from a biological phantom and a human brain dataset. We show that our technique is also able to recover known fiber crossings in the human brain and provides the practical advantage of being up to 15 times faster than original numerical QBI method. Magn Reson Med 58:497–510, 2007. © 2007 Wiley-Liss, Inc.
TL;DR: An iterative reconstruction method for undersampled radial MRI which is based on a nonlinear optimization, allows for the incorporation of prior knowledge with use of penalty functions, and deals with data from multiple coils is developed.
Abstract: The reconstruction of artifact-free images from radially encoded MRI acquisitions poses a difficult task for undersampled data sets, that is for a much lower number of spokes in k-space than data samples per spoke. Here, we developed an iterative reconstruction method for undersampled radial MRI which (i) is based on a nonlinear optimization, (ii) allows for the incorporation of prior knowledge with use of penalty functions, and (iii) deals with data from multiple coils. The procedure arises as a twostep mechanism which first estimates the coil profiles and then renders a final image that complies with the actual observations. Prior knowledge is introduced by penalizing edges in coil profiles and by a total variation constraint for the final image. The latter condition leads to an effective suppression of undersampling (streaking) artifacts and further adds a certain degree of denoising. Apart from simulations, experimental results for a radial spin-echo MRI sequence are presented for phantoms and human brain in vivo at 2.9 T using 24, 48, and 96 spokes with 256 data samples. In comparison to conventional reconstructions (regridding) the proposed method yielded visually improved image quality in all cases. Magn Reson Med 57:1086–1098, 2007. © 2007 Wiley-Liss, Inc.
TL;DR: The field‐dependent increases in 1H2O T1 values more than offset the well‐known decrease in typical MRI contrast reagent (CR) relaxivity, and simulations predict that this leads to lower CR concentration detection thresholds with increased magnetic field.
Abstract: Brain water proton (1H2O) longitudinal relaxation time constants (T1) were obtained from three healthy individuals at magnetic field strengths (B0) of 0.2 Tesla (T), 1.0T, 1.5T, 4.0T, and 7.0T. A 5-mm midventricular axial slice was sampled using a modified Look-Locker technique with 1.5 mm in-plane resolution, and 32 time points post-adiabatic inversion. The results confirmed that for most brain tissues, T1 values increased by more than a factor of 3 between 0.2T and 7T, and over this range were well fitted by T1 (s)=0.583(B0)0.382, T1(s)=0.857(B0)0.376, and T1(s)=1.35(B0)0.340 for white matter (WM), internal GM, and blood 1H2O, respectively. The ventricular cerebrospinal fluid (CSF) 1H2O T1 value did not change with B0, and its average value (standard deviation (SD)) across subjects and magnetic fields was 4.3 (+/-0.2) s. The tissue 1/T1 values at each field were well correlated with the macromolecular mass fraction, and to a lesser extent tissue iron content. The field-dependent increases in 1H2O T1 values more than offset the well-known decrease in typical MRI contrast reagent (CR) relaxivity, and simulations predict that this leads to lower CR concentration detection thresholds with increased magnetic field.
TL;DR: In this paper, pseudocontinuous arterial spin labeling (pCASL) was proposed for noninvasively measuring arterial flow by magnetically tagging the protons in arterial blood.
Abstract: Arterial spin labeling (ASL) is capable of noninvasively measuring blood flow by magnetically tagging the protons in arterial blood, which has been conventionally achieved using instantaneous (PASL) or continuous (CASL) RF pulses. As an intermediate method, pseudocontinuous ASL (pCASL) utilizes a train of discrete RF pulses to mimic continuous tagging that is often unavailable on imagers due to the requirement of continuous RF transmit capabilities. In the present study, we implemented two versions of pCASL (balanced and unbalanced gradient waveforms in tag and control scans) for both transmit/receive coils and array receivers. Experimental data show a 50% +/- 4% increase of signal-to-noise ratio (SNR) compared with PASL and a higher tagging efficiency than amplitude-modulated (AM) CASL (80% vs. 68%). Computer simulations predict an optimal tagging efficiency of 85% for flow velocities from 10 to 60 cm/s. It is theoretically and experimentally demonstrated that the tagging efficiency of pCASL is dependent upon the resonance offset and flip angle of the RF pulse train. We conclude that pCASL has the potential of combining the merits of PASL, including less hardware demand and higher tagging efficiency, and CASL, which includes a longer tagging bolus and thus higher SNR. These improvements provide a better balance between tagging efficiency and SNR.
TL;DR: Two approaches to reduce the effects of T1 are demonstrated: small flip angle (flip angle) and dual flip angle methods and two methods to reduce noise bias are shown to effectively minimize deviation of the measured fat‐fraction from its true value.
Abstract: Quantification of hepatic steatosis is a significant unmet need for the diagnosis and treatment of patients with nonalcoholic fatty liver disease (NAFLD). MRI is capable of separating water and fat signals in order to quantify fatty infiltration of the liver (hepatic steatosis). Unfortunately, fat signal has confounding T1 effects and the nonzero mean noise in low signal-to-noise ratio (SNR) magnitude images can lead to incorrect estimation of the true lipid percentage. In this study, the effects of bias from T1 effects and image noise were investigated. An oil/water phantom with volume fat-fractions ranging linearly from 0% to 100% was designed and validated using a spoiled gradient echo (SPGR) sequence in combination with a chemical-shift based fat-water separation method known as iterative decomposition of water and fat with echo asymmetry and least squares estimation (IDEAL). We demonstrated two approaches to reduce the effects of T1: small flip angle (flip angle) and dual flip angle methods. Both methods were shown to effectively minimize deviation of the measured fat-fraction from its true value. We also demonstrated two methods to reduce noise bias: magnitude discrimination and phase-constrained reconstruction. Both methods were shown to reduce this noise bias effectively from 15% to less than 1%. Magn Reson Med 58:354–364, 2007. © 2007 Wiley-Liss, Inc.
TL;DR: The results suggest that directional diffusivities may reveal abnormalities that are not obvious with SMI‐31 and LFB staining, depending on the type of injury.
Abstract: Diffusion tensor imaging (DTI) has been widely applied to investigate injuries in the central nervous system (CNS) white matter (WM). However, the underlying pathological correlates of diffusion changes have not been adequately determined. In this study the coregistration of histological sections to MR images and a pixel-based receiver operating characteristic (ROC) analysis were used to compare the axial (lambda( parallel)) and radial (lambda( perpendicular)) diffusivities derived from DTI and histological markers of axon (phosphorylated neurofilament, SMI-31) and myelin (Luxol fast blue (LFB)) integrity, respectively, in two different patterns of injury to mouse spinal cord (SC) WM. In contusion SC injury (SCI), a decrease in lambda( parallel) matched the pattern of axonal damage with high accuracy, but lambda( perpendicular) did not match the pattern of demyelination detected by LFB. In a mouse model of multiple sclerosis (MS), lambda( perpendicular) and lambda( parallel) did not match the patterns of demyelination or axonal damage, respectively. However, a region of interest (ROI) analysis suggested that lambda( perpendicular)-detected demyelination paralleled that observed with LFB, and lambda( parallel) decreased in both regions of axonal damage and normal-appearing WM (NAWM) as visualized by SMI-31. The results suggest that directional diffusivities may reveal abnormalities that are not obvious with SMI-31 and LFB staining, depending on the type of injury.
TL;DR: The objective of this study was to develop a BOLD MR‐based method that allows quantitative evaluation of tissue hemodynamic parameters, such as the blood volume, deoxyhemoglobin concentration, and oxygen extraction fraction (OEF).
Abstract: Since Ogawa et al. (Proc Natl Acad Sci USA 1990;87:9868-9872) made the fundamental discovery of blood oxygenation level-dependent (BOLD) contrast in MRI, most efforts have been directed toward the study of dynamic BOLD (i.e., temporal changes in the MRI signal during changes in brain activity). However, very little progress has been made in elucidating the nature of BOLD contrast during the resting or baseline state of the brain, which is important for understanding normal human performance because it accounts for most of the enormous energy budget of the brain. It is also crucial for deciphering the consequences of baseline-state impairment by cerebral vascular diseases. The objective of this study was to develop a BOLD MR-based method that allows quantitative evaluation of tissue hemodynamic parameters, such as the blood volume, deoxyhemoglobin concentration, and oxygen extraction fraction (OEF). The proposed method, which we have termed quantitative BOLD (qBOLD), is based on an MR signal model that incorporates prior knowledge about brain tissue composition and considers signals from gray matter (GM), white matter (WM), cerebrospinal fluid (CSF), and blood. A 2D gradient-echo sampling of spin-echo (GESSE) pulse sequence is used for the acquisition of the MRI signal. The method is applied to estimate the hemodynamic parameters of the normal human brain in the baseline state.
TL;DR: T1 maps obtained with modified Look‐Locker inversion recovery (MOLLI) can be used to measure myocardial T1 and provide the potential to dichotomously identify areas of infarction, according to standardized T1 threshold values.
Abstract: T1 maps obtained with modified Look-Locker inversion recovery (MOLLI) can be used to measure myocardial T1. We aimed to evaluate the potential of MOLLI T1 mapping for the assessment of acute and chronic myocardial infarction (MI). A total of 24 patients with a first MI underwent MRI within 8 days and after 6 months. T1 mapping was performed at baseline and at selected intervals between 2–20 min following administration of gadopentetate dimeglumine (Gd-DTPA). Delayed-enhancement (DE) imaging served as the reference standard for delineation of the infarct zone. On T1 maps the myocardial T1 relaxation time was assessed in hyperenhanced areas, hypoenhanced infarct cores, and remote myocardium. The planimetric size of myocardial areas with standardized T1 threshold values was measured. Acute and chronic MI exhibited different T1 changes. Precontrast threshold T1 maps detected segmental abnormalities caused by acute MI with 96% sensitivity and 91% specificity. Agreement between measurements of infarct size from T1 mapping and DE imaging was higher in chronic than in acute infarcts. Precontrast T1 maps enable the detection of acute MI. Acute and chronic MI show different patterns of T1 changes. Standardized T1 thresholds provide the potential to dichotomously identify areas of infarction. Magn Reson Med 58: 34 – 40, 2007. © 2007 Wiley-Liss, Inc. In present imaging techniques for in vivo infarct sizing, such as cardiac magnetic resonance (CMR) and singlephoton emission computed tomography (SPECT), the viewing window (i.e., the range of gray/color values to be selected for viewing) is determined arbitrarily before reporting, and thus requires a subjective preassessment of the images. In CMR the delayed-enhancement (DE) method produces high contrast between infarcted and noninfarcted myocardium (bright dead). The observable high contrast is reliant upon the imaging strategy employed. A preparatory inversion-recovery pulse in a T1-weighted gradientecho sequence nulls signal from noninfarcted myocardium, while infarcted myocardium, which retains a higher concentration of a gadolinium (Gd)-based extracellular contrast agent, returns higher signal due to its reduced spin-lattice relaxation time (T1 time) (1). The contrast between infarcted and noninfarcted areas on the resulting images is therefore enhanced over and above the underlying physical differences in T1 relaxation properties between these areas. The apparent degree of enhancement is dependent on the effectiveness of the nulling process, which is a function of the inversion time (TI) selected for the preparation pulse. Incomplete nulling will cause reduced contrast and create significant variation in the signal of remote myocardium, which is used as the reference to determine the threshold between infarcted and noninfarcted myocardium (2). The choice of TI therefore ultimately influences the infarct size as measured with DE techniques. T1-mapping CMR techniques circumvent the influences of windowing and variations in signal enhancement by directly measuring the underlying T1 relaxation times of the different areas of the myocardium. At a given magnetic field strength, each tissue has a normal range for spinlattice T1 relaxation time (3,4), much like the normal ranges of tissue X-ray attenuation in computed tomography (CT) measured in Hounsfield units. Thus by encoding CMR images by actual T1 relaxation times, one could prescribe standardized cutoffs between “normal” and “affected” myocardium to eliminate observer-input in delineation of diseased myocardium. The modified Look-Locker inversion recovery (MOLLI) technique was recently proposed as a means of creating high-resolution single-slice maps of myocardial T1 relaxation times (5), and normal values for human myocardium have been established using this technique (6). We hypothesized that thresholds derived from these normal values, when applied to T1 maps of patients with myocardial infarction (MI), could accurately delineate areas of MI.
TL;DR: It is observed that malignant tumors obtain larger exponents of the power law than benign tumors indicating a more liquid‐like behavior and the combination of the Breast Imaging Reporting and Data System (BIRADS) categorization obtained via MRM with viscoelastic information leads to a substantial rise in specificity.
Abstract: The purpose of this analysis is to explore the potential diagnostic gain provided by the viscoelastic shear properties of breast lesions for the improvement of the specificity of contrast enhanced dynamic MR mammography (MRM). The assessment of viscoelastic properties is done via dynamic MR elastography (MRE) and it is demonstrated that the complex shear modulus of in vivo breast tissue follows within the frequency range of clinical MRE a power law behavior. Taking benefit of this frequency behavior, data are interpreted in the framework of the exact model for wave propagation satisfying the causality principle. This allows to obtain the exponent of the frequency power law from the complex shear modulus at one single frequency which is validated experimentally. Thereby, scan time is drastically reduced. It is observed that malignant tumors obtain larger exponents of the power law than benign tumors indicating a more liquid-like behavior. The combination of the Breast Imaging Reporting and Data System (BIRADS) categorization obtained via MRM with viscoelastic information leads to a substantial rise in specificity. Analysis of 39 malignant and 29 benign lesions shows a significant diagnostic gain with an increase of about 20% in specificity at 100% sensitivity.
TL;DR: This work extends the traditional 2D gray‐level co‐occurrence matrix (GLCM) method to investigate a volumetric texture analysis approach and apply it for the characterization of breast MR lesions and shows that the classification performance of volumed texture features is significantly better than that based on 2D analysis.
Abstract: Automated image analysis aims to extract relevant information from contrast-enhanced magnetic resonance images (CE-MRI) of the breast and improve the accuracy and consistency of image interpretation. In this work, we extend the traditional 2D gray-level co-occurrence matrix (GLCM) method to investigate a volumetric texture analysis approach and apply it for the characterization of breast MR lesions. Our database of breast MR images was obtained using a T1-weighted 3D spoiled gradient echo sequence and consists of 121 biopsy-proven lesions (77 malignant and 44 benign). A fuzzy c-means clustering (FCM) based method is employed to automatically segment 3D breast lesions on CE-MR images. For each 3D lesion, a nondirectional GLCM is then computed on the first postcontrast frame by summing 13 directional GLCMs. Texture features are extracted from the nondirectional GLCMs and the performance of each texture feature in the task of distinguishing between malignant and benign breast lesions is assessed by receiver operating characteristics (ROC) analysis. Our results show that the classification performance of volumetric texture features is significantly better than that based on 2D analysis. Our investigations of the effects of various of parameters on the diagnostic accuracy provided means for the optimal use of the approach.
TL;DR: The in vivo quantification and cell trafficking patterns were verified using 19F spectroscopy and fluorescence microscopy in excised pancreata and the labeling procedure did not affect T‐cell migration in vivo.
Abstract: This article describes an in vivo imaging method for visualizing and quantifying a specific cell population. Cells are labeled ex vivo with a perfluoropolyether nanoparticle tracer agent and then detected in vivo using (19)F MRI following cell transfer. (19)F MRI selectively visualizes only the labeled cells with no background, and a conventional (1)H image taken in the same imaging session provides anatomical context. Using the nonobese diabetic mouse, an established model of type 1 diabetes, (19)F MRI data were acquired showing the early homing behavior of diabetogenic T cells to the pancreas. A computational algorithm provided T cell counts in the pancreas. Approximately 2% of the transferred cells homed to the pancreas after 48 hr. The technique allows for both unambiguous detection of labeled cells and quantification directly from the in vivo images. The in vivo quantification and cell trafficking patterns were verified using (19)F spectroscopy and fluorescence microscopy in excised pancreata. The labeling procedure did not affect T-cell migration in vivo. This imaging platform is applicable to many cell types and disease models and can potentially be used for monitoring the trafficking of cellular therapeutics.
TL;DR: Based on 2D in vivo data obtained with a 32‐element phased‐array coil in the heart, it is shown that the number of channels can be compressed to as few as four with only 0.3% SNR loss in an ROI encompassing the heart.
Abstract: Arrays with large numbers of independent coil elements are becoming increasingly available as they provide increased signal-to-noise ratios (SNRs) and improved parallel imaging performance. Processing of data from a large set of independent receive channels is, however, associated with an increased memory and computational load in reconstruction. This work addresses this problem by introducing coil array compression. The method allows one to reduce the number of datasets from independent channels by combining all or partial sets in the time domain prior to image reconstruction. It is demonstrated that array compression can be very effective depending on the size of the region of interest (ROI). Based on 2D in vivo data obtained with a 32-element phased-array coil in the heart, it is shown that the number of channels can be compressed to as few as four with only 0.3% SNR loss in an ROI encompassing the heart. With twofold parallel imaging, only a 2% loss in SNR occurred using the same compression factor.
TL;DR: The problem of error propagation in the sequential procedure of sensitivity estimation followed by image reconstruction in existing methods, such as sensitivity encoding (SENSE) and simultaneous acquisition of spatial harmonics (SMASH), is considered and reformulates the image reconstruction problem as a joint estimation of the coil sensitivities and the desired image, which is solved by an iterative optimization algorithm.
Abstract: Parallel magnetic resonance imaging (pMRI) using multichannel receiver coils has emerged as an effective tool to reduce imaging time in various applications. However, the issue of accurate estimation of coil sensitivities has not been fully addressed, which limits the level of speed enhancement achievable with the technology. The self-calibrating (SC) technique for sensitivity extraction has been well accepted, especially for dynamic imaging, and complements the common calibration technique that uses a separate scan. However, the existing method to extract the sensitivity information from the SC data is not accurate enough when the number of data is small, and thus erroneous sensitivities affect the reconstruction quality when they are directly applied to the reconstruction equation. This paper considers this problem of error propagation in the sequential procedure of sensitivity estimation followed by image reconstruction in existing methods, such as sensitivity encoding (SENSE) and simultaneous acquisition of spatial harmonics (SMASH), and reformulates the image reconstruction problem as a joint estimation of the coil sensitivities and the desired image, which is solved by an iterative optimization algorithm. The proposed method was tested on various data sets. The results from a set of in vivo data are shown to demonstrate the effectiveness of the proposed method, especially when a rather large net acceleration factor is used.
TL;DR: Free‐breathing T2‐prepared SSFP provides T1‐weighted images of acute MI with fewer artifacts and better diagnostic accuracy than conventional dark‐blood TSE.
Abstract: T2-weighted MRI of edema in acute myocardial infarction (MI) provides a means of differentiating acute and chronic MI, and assessing the area at risk of infarction Conventional T2-weighted imaging of edema uses a turbo spin-echo (TSE) readout with dark-blood preparation Clinical applications of dark-blood TSE methods can be limited by artifacts such as posterior wall signal loss due to through-plane motion, and bright subendocardial artifacts due to stagnant blood Single-shot imaging with a T2-prepared SSFP readout provides an alternative to dark-blood TSE and may be conducted during free breathing We hypothesized that T2-prepared SSFP would be a more reliable method than dark-blood TSE for imaging of edema in patients with MI In patients with MI (22 acute and nine chronic MI cases), T2-weighted imaging with both methods was performed prior to contrast administration and delayed-enhancement imaging The T2-weighted images using TSE were nondiagnostic in three of 31 cases, while six additional cases rated as being of diagnostic quality yielded incorrect diagnoses In all 31 cases the T2-prepared SSFP images were rated as diagnostic quality, correctly differentiated acute or chronic MI, and correctly determined the coronary territory Free-breathing T2 prepared SSFP provides T2-weighted images of acute MI with fewer artifacts and better diagnostic accuracy than conventional dark-blood TSE
TL;DR: The first successful use of IRON to produce positive contrast in areas of metallic spheres and SPIO‐labeled stem cells in vitro and in vivo is presented.
Abstract: In proton magnetic resonance imaging (MRI) metallic substances lead to magnetic field distortions that often result in signal voids in the adjacent anatomic structures. Thus, metallic objects and superparamagnetic iron oxide (SPIO)-labeled cells appear as hypointense artifacts that obscure the underlying anatomy. The ability to illuminate these structures with positive contrast would enhance noninvasive MR tracking of cellular therapeutics. Therefore, an MRI methodology that selectively highlights areas of metallic objects has been developed. Inversion-recovery with ON-resonant water suppression (IRON) employs inversion of the magnetization in conjunction with a spectrally-selective on-resonant saturation prepulse. If imaging is performed after these prepulses, positive signal is obtained from off-resonant protons in close proximity to the metallic objects. The first successful use of IRON to produce positive contrast in areas of metallic spheres and SPIO-labeled stem cells in vitro and in vivo is presented.
TL;DR: Intravascular blood oxygenation level dependent (BOLD) contributions during visual activation were estimated, and due to the large R 2* in venous blood, intravascular SE BOLD signal changes were larger than GRE effects at echo times above 30 ms.
Abstract: Knowledge of the transverse relaxation rates R2 and R of blood is relevant for quantitative assessment of functional MRI (fMRI) results, including calibration of blood oxygenation and measurement of tissue oxygen extraction fractions (OEFs). In a temperature controlled circulation system, these rates were measured for blood in vitro at 3T under conditions akin to the physiological state. Single spin echo (SE) and gradient echo (GRE) sequences were used to determine R2 and R, respectively. Both rates varied quadratically with deoxygenation, and changes in R were found to be due predominantly to changes in R2. These data were used to estimate intravascular blood oxygenation level dependent (BOLD) contributions during visual activation. Due to the large R in venous blood, intravascular SE BOLD signal changes were larger than GRE effects at echo times above 30 ms. When including extravascular effects to estimate the total BOLD effect, GRE BOLD dominated due to the large tissue volume fraction. Magn Reson Med 58:592–596, 2007. © 2007 Wiley-Liss, Inc.
TL;DR: A new signal‐to‐noise ratio (SNR) efficient method is introduced for the mapping of vascular territories based on pseudocontinuous arterial spin labeling (ASL), combined with a Hadamard or similar encoding scheme such that all vessels of interest are fully inverted or relaxed for nearly all of the encoding cycles, providing optimal SNR.
Abstract: A new signal-to-noise ratio (SNR) efficient method is introduced for the mapping of vascular territories based on pseudocontinuous arterial spin labeling (ASL). A pseudocontinuous tagging pulse train is modified using additional transverse gradient pulses and phase cycling to place some arteries in a tag condition, while others passing through the same tagging plane are in a control condition. This is combined with a Hadamard or similar encoding scheme such that all vessels of interest are fully inverted or relaxed for nearly all of the encoding cycles, providing optimal SNR. The relative tagging efficiency for each vessel is measured directly from the ASL data and is used in the decoding process to improve the separation of vascular territories. High SNR maps of left carotid, right carotid, and basilar territories are generated in 6 min of scan time.
TL;DR: A complete MRI‐compatible, high‐intensity focused US (HIFU) system is described together with adaptive methods that allow continuous MR thermometry and therapeutic US with real‐time tracking of a moving target, online motion correction of the thermometry maps, and regional temperature control based on the proportional, integral, and derivative method.
Abstract: Focused ultrasound is a unique and non-invasive technique for the local deposition of thermal energy deep inside the body. MRI guidance offers the additional benefits of excellent target visualization and continuous temperature mapping. However, treating a moving target poses severe problems because 1) motion related thermometry artifacts must be corrected, 2) the ultrasound focal point must be relocated according to the target displacement. In this paper, a complete MRI compatible High Intensity Focused Ultrasound (HIFU) system is described together with adaptive methods allowing continuous MR thermometry and therapeutic ultrasound with real-time tracking of a moving target, on-line motion correction of the thermometry maps, and regional temperature control based on the Proportional, Integral, and Derivative method. The hardware is based on a 256 element Phased-Array transducer with rapid, electronic, displacement of the focal point. The exact location of the target during ultrasound firing is anticipated using automatic analysis of periodic motions. The methods were tested with moving phantoms undergoing either rigid body or elastic periodical motions. The results show accurate tracking of the focal point. Focal and regional temperature control is demonstrated with a performance, similar to that obtained with stationary phantoms.
TL;DR: In patients with an identified muscular 23Na channelopathy (Paramyotonia congenita (PC)), induced muscle weakness led to a signal increase of ∼18% in the 23Na MR images, which was attributed to intracellular Na+ accumulation in this region.
Abstract: (23)Na MRI has the potential to noninvasively detect sodium (Na) content changes in vivo. The goal of this study was to implement (23)Na MRI in a clinical setting for neurooncological and muscular imaging. Due to the biexponential T(2) decay of the tissue Na signal with a short component, which ranges between 0.5-8 ms, the measurement of total Na content requires imaging techniques with echo times (TEs) below 0.5 ms. A 3D radial pulse sequence with a TE of 0.2 ms at a spatial resolution of 4 x 4 x 4 mm(3) was developed that allows the acquisition and presentation of Na images on the scanner. This sequence was evaluated in patients with low- and high-grade gliomas, and higher (23)Na MR signals corresponding to an increased Na content were found in the tumor regions. The contrast-to-noise ratio (CNR) between tumor and white matter increased from 0.8 +/- 0.2 to 1.3 +/- 0.3 with tumor grade. In patients with an identified muscular (23)Na channelopathy (Paramyotonia congenita (PC)), induced muscle weakness led to a signal increase of approximately 18% in the (23)Na MR images, which was attributed to intracellular Na(+) accumulation in this region.
TL;DR: The QRAPTEST method is presented as a means of quickly determining the longitudinal T1 and transverse T 2* relaxation time and proton density within a single sequence and provides a clinical example in which quantification of the whole brain using 1.5 mm3 voxels was achieved in less than 8 min.
Abstract: An imaging method called “quantification of relaxation times and proton density by twin-echo saturation-recovery turbo-field echo” (QRAPTEST) is presented as a means of quickly determining the long ...
TL;DR: A continuous‐wave MT pulse sequence with low irradiation power was used to study healthy human brains in vivo at 3 T and the asymmetry of the MT effects with respect to the water proton frequency was found to be a few percent from the water signal intensity.
Abstract: Magnetization transfer (MT) imaging provides a unique method of tissue characterization by capitalizing on the interaction between solid-like tissue components and bulk water. We used a continuous-wave (CW) MT pulse sequence with low irradiation power to study healthy human brains in vivo at 3 T and quantified the asymmetry of the MT effects with respect to the water proton frequency. This asymmetry was found to be a difference of approximately a few percent from the water signal intensity, depending on both the RF irradiation power and the frequency offset. The experimental results could be quantitatively described by a modified two-pool MT model extended with a shift contribution for the semisolid pool with respect to water. For white matter, this shift was fitted to be 2.34 +/- 0.17 ppm (N = 5) upfield from the water signal.
TL;DR: Elevated 13C lactate was observed in both primary and metastatic tumors, demonstrating the feasibility of studying cellular bioenergetics in vivo with DNP hyperpolarized 13C MRSI.
Abstract: The transgenic adenocarcinoma of mouse prostate (TRAMP) mouse is a well-studied murine model of prostate cancer with histopathology and disease progression that mimic the human disease. To investigate differences in cellular bioenergetics between normal prostate epithelial cells and prostate tumor cells, in vivo MR spectroscopic (MRS) studies with non-proton nuclei, such as 13C, in the TRAMP model would be extremely useful. The recent development of a method for retaining dynamic nuclear polarization (DNP) in solution permits high signal-to-noise ratio (SNR) 13C MRI or MRSI data to be obtained following injection of a hyperpolarized 13C agent. In this transgenic mouse study, this method was applied using a double spin-echo (DSE) pulse sequence with a small-tip-angle excitation RF pulse, hyperbolic-secant refocusing pulses, and a flyback echo-planar readout trajectory for fast (10–14 s) MRSI of 13C pyruvate (pyr) and its metabolic products at 0.135 cm3 nominal spatial resolution. Elevated 13C lactate (lac) was observed in both primary and metastatic tumors, demonstrating the feasibility of studying cellular bioenergetics in vivo with DNP hyperpolarized 13C MRSI. Magn Reson Med, 2007. © 2007 Wiley-Liss, Inc.
TL;DR: Dynamic spectra and spectroscopic images acquired in normal rats at 3T following the injection of 13C‐1‐pyruvate that was hyperpolarized by the dynamic nuclear polarization (DNP) method are presented for the first time.
Abstract: We present for the first time dynamic spectra and spectroscopic images acquired in normal rats at 3T following the injection of (13)C-1-pyruvate that was hyperpolarized by the dynamic nuclear polarization (DNP) method. Spectroscopic sampling was optimized for signal-to-noise ratio (SNR) and for spectral resolution of (13)C-1-pyruvate and its metabolic products (13)C-1-alanine, (13)C-1-lactate, and (13)C-bicarbonate. Dynamic spectra in rats were collected with a temporal resolution of 3 s from a 90-mm axial slab using a dual (1)H-(13)C quadrature birdcage coil to observe the combined effects of metabolism, flow, and T(1) relaxation. In separate experiments, spectroscopic imaging data were obtained during a 17-s acquisition of a 20-mm axial slice centered on the rat kidney region to provide information on the spatial distribution of the metabolites. Conversion of pyruvate to lactate, alanine, and bicarbonate occurred within a minute of injection. Alanine was observed primarily in skeletal muscle and liver, while pyruvate, lactate, and bicarbonate concentrations were relatively high in the vasculature and kidneys. In contrast to earlier work at 1.5 T, bicarbonate was routinely observed in skeletal muscle as well as the kidney and vasculature.
TL;DR: In vivo comparisons demonstrated that MSDE sequence can achieve better blood suppression and provide a more accurate depiction of the lumen boundaries by eliminating plaque mimicking artifacts in carotid artery imaging.
Abstract: In this study, a turbo spin-echo (TSE) based motion-sensitized driven-equilibrium (MSDE) sequence was used as an alternative black-blood (BB) carotid MRI imaging scheme. The MSDE sequence was first optimized for more efficient residual blood signal suppression in the carotid bulb of healthy volunteers. Effective contrast-to-noise ratio (CNR(eff)) and residual signal-to-noise ratio (SNR) in the lumen measured from MSDE images were then compared to those measured from inflow saturation (IS) and double inversion-recovery (DIR) images. Statistically significant higher CNR(eff) and lower lumen SNR were obtained from MSDE images. To assess MSDE sequence in a clinical carotid protocol, 42 locations from six subjects with 50% to 79% carotid stenosis by duplex ultrasound were scanned with both MSDE and multislice DIR. The comparison showed that MSDE images present significantly higher CNR and lower lumen SNR compared to corresponding multislice DIR images. The vessel wall area and mean wall thickness measurements in MSDE images were slightly but significantly lower than those obtained with other blood suppression techniques. In conclusion, in vivo comparisons demonstrated that MSDE sequence can achieve better blood suppression and provide a more accurate depiction of the lumen boundaries by eliminating plaque mimicking artifacts in carotid artery (CA) imaging.
TL;DR: A spin‐echo single‐shot echo‐planar imaging (SS‐EPI) technique with a reduced field of view (FOV) in the phase‐encoding direction is presented that simultaneously reduces susceptibility effects and motion artifacts in diffusion‐weighted (DW) imaging of the spinal cord at a high field strength (3T).
Abstract: A spin-echo single-shot echo-planar imaging (SS-EPI) technique with a reduced field of view (FOV) in the phase-encoding direction is presented that simultaneously reduces susceptibility effects and motion artifacts in diffusion-weighted (DW) imaging (DWI) of the spinal cord at a high field strength (3T). To minimize aliasing, an outer volume suppression (OVS) sequence was implemented. Effective fat suppression was achieved with the use of a slice-selection gradient-reversal technique. The OVS was optimized by numerical simulations with respect to T(1) relaxation times and B(1) variations. The optimized sequence was evaluated in vitro and in vivo. In simulations the optimized OVS showed suppression to <0.25% and approximately 3% in an optimal and worst-case scenario, respectively. In vitro measurements showed a mean residual signal of <0.95% +/- 0.42 for all suppressed areas. In vivo acquisition with 0.9 x 1.05 mm(2) in-plane resolution resulted in artifact-free images. The short imaging time of this technique makes it promising for clinical studies.
TL;DR: A new method is presented that allows compensation of field fluctuations by modulating the B0 shims in real time to substantially improve the phase stability of EPI data and the image quality of multishot gradient‐echo (GRE) MRI at 7T.
Abstract: In MRI of human brain, the respiratory cycle can induce B0-field fluctuations through motion of the chest and fluctuations in local oxygen concentration. The associated NMR frequency changes can affect the MRI data in various ways and lead to temporal signal fluctuations, and image artifacts such as ghosting and blurring. Since the size of the effect scales with magnetic field strength, artifacts become particularly problematic at fields above 3.0T. Furthermore, the spatial dependence of the B0-field fluctuations complicates their correction. In this work, a new method is presented that allows compensation of field fluctuations by modulating the B0 shims in real time. In this method, a reference scan is acquired to measure the spatial distribution of the B0 effect related to chest motion. During the actual scan, this information is then used, together with chest motion data, to apply compensating B0 shims in real time. The method can be combined with any type of scan without modifications to the pulse sequence. Real-time B0 shimming is demonstrated to substantially improve the phase stability of EPI data and the image quality of multishot gradient-echo (GRE) MRI at 7T.
TL;DR: The feasibility of the technique for in vivo myocardial MRE by visualizing transverse wave propagation in the interventricular septum (IVS) is demonstrated and a twofold increase of the phase signal is predicted for liver‐like materials.
Abstract: In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency fv as the mechanical vibration. As a result, both the repetition time (TR) and echo time (TE) of such an MRE sequence are greater than the vibration period 1/fv. This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR
TL;DR: The dependence of CEST contrast on field distribution was solved and a correction algorithm was developed to compensate for field inhomogeneity‐induced CEST imaging artifacts, which showed nearly complete correction of Cest imaging measurement errors caused by moderate field inhmogeneity.
Abstract: Chemical exchange saturation transfer (CEST) imaging provides an indirect detection mechanism that allows quantification of certain labile groups unobservable using conventional MRI. Recently, amide proton transfer (APT) imaging, a variant form of CEST imaging, has been shown capable of detecting lactic acidosis during acute ischemia, providing information complementary to that of perfusion and diffusion MRI. However, CEST contrast is usually small, and therefore, it is important to optimize experimental conditions for reliable and quantitative CEST imaging. In particular, CEST imaging is sensitive to B0 and B1 field, while on the other hand; field inhomogeneities persist despite recent advances in magnet technologies, especially for in vivo imaging at high fields. Consequently, correction algorithms that can compensate for field inhomogeneity-induced measurement errors in CEST imaging might be very useful. In this study, the dependence of CEST contrast on field distribution was solved and a correction algorithm was developed to compensate for field inhomogeneity-induced CEST imaging artifacts. In addition, the proposed algorithm was verified with both numerical simulation and experimental measurements, and showed nearly complete correction of CEST imaging measurement errors caused by moderate field inhomogeneity. Magn Reson Med, 2007. © 2007 Wiley-Liss, Inc.