Quiet and distortion-free, whole brain BOLD fMRI using T2 -prepared RUFIS.

PURPOSE: To develop and evaluate a novel MR method that addresses some of the most eminent technical challenges of current BOLD-based fMRI in terms of 1) acoustic noise and 2) geometric distortions and signal dropouts. METHODS: A BOLD-sensitive fMRI pulse sequence was designed that first generates T2-weighted magnetization (using a T2 preparation module) and subsequently undergoes three-dimensional (3D) radial encoding using a rotating ultrafast imaging sequence (RUFIS). The method was tested on healthy volunteers at 3T with motor, visual, and auditory tasks, and compared relative to standard gradient and spin echo planar imaging (EPI) methods. RESULTS: In combination with parallel imaging the method achieves efficient and robust 3D whole brain coverage (3 mm isotropic resolution in 2.65 s scan time). Compared with standard EPI-based fMRI, the method demonstrated 1) T2-weighted imaging clean of geometrical distortions and signal dropout, 2) an acoustic noise reduction of ∼40 dB(A), and 3) a consistent BOLD response that is less sensitive (∼1.3% BOLD change) but spatially more specific. CONCLUSION: T2-prepared RUFIS provides quiet and distortion-free whole brain BOLD fMRI with minimal demands on the gradient performance. In particular, auditory fMRI and/or studies involving brain regions near air-tissue interfaces are expected to greatly benefit from the proposed method, especially if performed at ultrahigh field strengths.

Zero TE MR bone imaging in the head.

PURPOSE: To investigate proton density (PD)-weighted zero TE (ZT) imaging for morphological depiction and segmentation of cranial bone structures. METHODS: A rotating ultra-fast imaging sequence (RUFIS) type ZT pulse sequence was developed and optimized for 1) efficient capture of short T2 bone signals and 2) flat PD response for soft-tissues. An inverse logarithmic image scaling (i.e., -log(image)) was used to highlight bone and differentiate it from surrounding soft-tissue and air. Furthermore, a histogram-based bias-correction method was developed for subsequent threshold-based air, soft-tissue, and bone segmentation. RESULTS: PD-weighted ZT imaging in combination with an inverse logarithmic scaling was found to provide excellent depiction of cranial bone structures. In combination with bias correction, also excellent segmentation results were achieved. A two-dimensional histogram analysis demonstrates a strong, approximately linear correlation between inverse log-scaled ZT and low-dose CT for Hounsfield units (HU) between -300 HU and 1,500 HU (corresponding to soft-tissue and bone). CONCLUSIONS: PD-weighted ZT imaging provides robust and efficient depiction of bone structures in the head, with an excellent contrast between air, soft-tissue, and bone. Besides structural bone imaging, the presented method is expected to be of relevance for attenuation correction in positron emission tomography (PET)/MR and MR-based radiation therapy planning.

Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods.

Current spokes pulse design methods can be grouped into methods based either on sparse approximation or on iterative local (gradient descent-based) optimization of the transverse-plane spatial frequency locations visited by the spokes. These two classes of methods have complementary strengths and weaknesses: sparse approximation-based methods perform an efficient search over a large swath of candidate spatial frequency locations but most are incompatible with off-resonance compensation, multifrequency designs, and target phase relaxation, while local methods can accommodate off-resonance and target phase relaxation but are sensitive to initialization and suboptimal local cost function minima. This article introduces a method that interleaves local iterations, which optimize the radiofrequency pulses, target phase patterns, and spatial frequency locations, with a greedy method to choose new locations. Simulations and experiments at 3 and 7 T show that the method consistently produces single- and multifrequency spokes pulses with lower flip angle inhomogeneity compared to current methods.

Bedside detection of intracranial midline shift using portable magnetic resonance imaging.

Neuroimaging is crucial for assessing mass effect in brain-injured patients. Transport to an imaging suite, however, is challenging for critically ill patients. We evaluated the use of a low magnetic field, portable MRI (pMRI) for assessing midline shift (MLS). In this observational study, 0.064 T pMRI exams were performed on stroke patients admitted to the neuroscience intensive care unit at Yale New Haven Hospital. Dichotomous (present or absent) and continuous MLS measurements were obtained on pMRI exams and locally available and accessible standard-of-care imaging exams (CT or MRI). We evaluated the agreement between pMRI and standard-of-care measurements. Additionally, we assessed the relationship between pMRI-based MLS and functional outcome (modified Rankin Scale). A total of 102 patients were included in the final study (48 ischemic stroke; 54 intracranial hemorrhage). There was significant concordance between pMRI and standard-of-care measurements (dichotomous, κ = 0.87; continuous, ICC = 0.94). Low-field pMRI identified MLS with a sensitivity of 0.93 and specificity of 0.96. Moreover, pMRI MLS assessments predicted poor clinical outcome at discharge (dichotomous: adjusted OR 7.98, 95% CI 2.07-40.04, p = 0.005; continuous: adjusted OR 1.59, 95% CI 1.11-2.49, p = 0.021). Low-field pMRI may serve as a valuable bedside tool for detecting mass effect.

Fast radiofrequency flip angle calibration by Bloch-Siegert shift.

In a recent work, we presented a novel method for B 1+ field mapping based on the Bloch-Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off-resonance radiofrequency pulses were added to a slice-selective spin echo sequence. The off-resonance pulses induce a Bloch-Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B(1) (2) . The signal is further spatially localized by a readout gradient, and the signal-weighted average B(1) field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch-Siegert-based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers.

B1 mapping by Bloch-Siegert shift.

A novel method for amplitude of radiofrequency field (B1+) mapping based on the Bloch-Siegert shift is presented. Unlike conventionally applied double-angle or other signal magnitude-based methods, it encodes the B(1) information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch-Siegert frequency shift is caused by irradiating with an off-resonance radiofrequency pulse following conventional spin excitation. When applying the off-resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of B1(2). Adding gradient image encoding following the off-resonance pulse allows one to acquire spatially resolved B(1) maps. The frequency shift from the Bloch-Siegert effect gives a phase shift in the image that is proportional to B(1)(2). The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off-resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch-Siegert B(1) mapping with 25 sec/slice is demonstrated to be quantitatively comparable to a 21-min double-angle map. As such, this method enables robust, high-resolution B(1)(+) mapping in a clinically acceptable time frame.

Regional temperature changes in the brain during somatosensory stimulation.

Time-dependent variations in the brain temperature (Tt) are likely to be caused by fluctuations of cerebral blood flow (CBF) and cerebral metabolic rate of oxidative consumption (CMRO2) both of which are seemingly coupled to alterations in neuronal activity. We combined magnetic resonance, optical imaging, temperature sensing, and electrophysiologic methods in alpha-chloralose anesthetized rats to obtain multimodal measurements during forepaw stimulation. Localized changes in neuronal activity were colocalized with regional increases in Tt (by approximately 0.2%), CBF (by approximately 95%), and CMRO2 (by approximately 73%). The time-to-peak for Tt (42+/-11 secs) was significantly longer than those for CBF and CMRO2 (5+/-2 and 18+/-4 secs, respectively) with a 2-min stimulation. Net heat in the region of interest (ROI) was modeled as being dependent on the sum of heats attributed to changes in CMRO2 (Qm) and CBF (Qf) as well as conductive heat loss from the ROI to neighboring regions (Qc) and to the environment (Qe). Although tissue cooling because of Qf and Qc can occur and are enhanced during activation, the net increase in Tt corresponded to a large rise in Qm, whereas effects of Qe can be ignored. The results show that Tt increases slowly (by approximately 0.1 degrees C) during physiologic stimulation in alpha-chloralose anesthetized rats. Because the potential cooling effect of CBF depends on the temperature of blood entering the brain, Tt is mainly affected by CMRO2 during functional challenges. Implications of these findings for functional studies in awake humans and temperature regulation are discussed.

Clinical evaluation of zero-echo-time MR imaging for the segmentation of the skull.

UNLABELLED: MR-based attenuation correction is instrumental for integrated PET/MR imaging. It is generally achieved by segmenting MR images into a set of tissue classes with known attenuation properties (e.g., air, lung, bone, fat, soft tissue). Bone identification with MR imaging is, however, quite challenging, because of the low proton density and fast decay time of bone tissue. The clinical evaluation of a novel, recently published method for zero-echo-time (ZTE)-based MR bone depiction and segmentation in the head is presented here. METHODS: A new paradigm for MR imaging bone segmentation, based on proton density-weighted ZTE imaging, was disclosed earlier in 2014. In this study, we reviewed the bone maps obtained with this method on 15 clinical datasets acquired with a PET/CT/MR trimodality setup. The CT scans acquired for PET attenuation-correction purposes were used as reference for the evaluation. Quantitative measurements based on the Jaccard distance between ZTE and CT bone masks and qualitative scoring of anatomic accuracy by an experienced radiologist and nuclear medicine physician were performed. RESULTS: The average Jaccard distance between ZTE and CT bone masks evaluated over the entire head was 52% ± 6% (range, 38%-63%). When only the cranium was considered, the distance was 39% ± 4% (range, 32%-49%). These results surpass previously reported attempts with dual-echo ultrashort echo time, for which the Jaccard distance was in the 47%-79% range (parietal and nasal regions, respectively). Anatomically, the calvaria is consistently well segmented, with frequent but isolated voxel misclassifications. Air cavity walls and bone/fluid interfaces with high anatomic detail, such as the inner ear, remain a challenge. CONCLUSION: This is the first, to our knowledge, clinical evaluation of skull bone identification based on a ZTE sequence. The results suggest that proton density-weighted ZTE imaging is an efficient means of obtaining high-resolution maps of bone tissue with sufficient anatomic accuracy for, for example, PET attenuation correction.

Adiabatic refocusing pulses for volume selection in magnetic resonance spectroscopic imaging.

Because of their excellent slice profiles and high immunity to RF inhomogeneity, adiabatic full passage (AFP) pulses are ideal for use in spatial localization. The nonlinear, position-dependent phase of a single AFP pulse generated during refocusing of transverse magnetization traditionally is eliminated by using identical pairs of AFP pulses, at the expense of increased RF power deposition and increased echo time (TE). Here it is shown that one can achieve significant phase refocusing by executing single AFP pulses along non-equivalent spatial axes. When used for volume selection in MR spectroscopic imaging (MRSI) the remaining nonlinear phase becomes inconsequential when the phase across a spectroscopic volume is small. Selection of rectangular and octagonal volumes is demonstrated with half the number of AFP pulses used in the traditional approach. It is shown that octagonal volume selection in the human brain provides excellent suppression of extracranial lipids, and thus allows multislice (1)H MRSI at 4 Tesla to be performed within the guidelines for RF power deposition.

Dynamically shimmed multivoxel 1H magnetic resonance spectroscopy and multislice magnetic resonance spectroscopic imaging of the human brain.

In vivo multivoxel Magnetic Resonance Spectroscopy (MRS) and multislice Magnetic Resonance Spectroscopic Imaging (MRSI) are extremely susceptible to poor homogeneity of the static magnetic field. Existing room-temperature (RT) shim technology can adequately optimize the B(0) homogeneity of local volumes, such as single voxels. However, the widespread global homogeneity required for in vivo spectral acquisitions from multiple volumes in the human brain cannot be attained with a single RT shim setting. Dynamic shim updating (DSU) allows for use of local RT shim B(0) homogeneity compensation capabilities in a global fashion. Here, by updating first- and second-order shims on a voxel- and slice-specific basis using a pre-emphasized DSU system, we present multivoxel MRS and multislice MRSI of the human brain. These results demonstrate that DSU can increase multivoxel MRS acquisition capabilities and significantly improve the quality of multislice MRSI data.