Fernando E. Boada

Fast-kz three-dimensional tailored radiofrequency pulse for reduced B1 inhomogeneity

This article presents a small‐flip‐angle, three‐dimensional tailored RF pulse that excites thin slices with an adjustable quadratic in‐plane spatial variation. The quadratic spatial variation helps to compensate for the loss in image uniformity using a volume coil at 3 T due to the wavelike properties of the RF field. The pulse is based on a novel “fast‐kz” design that uses a series of slice‐select subpulses along kz and phase encoding “blips” along kx–ky. The method is demonstrated by acquiring a series of 5‐mm‐thick T2‐weighted images of the human brain at 3 T using pulses 4.8 ms in length with a 45 ° flip angle. Magn Reson Med, 2006.

Three-dimensional tailored RF pulses for the reduction of susceptibility artifacts in T*2-weighted functional MRI

A three‐dimensional tailored RF pulse method for reducing intravoxel dephasing artifacts in T *2‐weighted functional MRI is presented. A stack of spirals k‐space trajectory is employed to excite a disk of magnetization for small tip angles. Smaller disks with a linear through‐plane phase are inserted into the disk to locally refocus regions which are normally dephased due to susceptibility variations. Numerical simulations and imaging experiments which use the tailored RF pulses are presented. Limitations of the method and improvements are also discussed. Magn Reson Med 44:525–531, 2000.

High-definition fiber tractography of the human brain: neuroanatomical validation and neurosurgical applications.

BACKGROUND High-definition fiber tracking (HDFT) is a novel combination of processing, reconstruction, and tractography methods that can track white matter fibers from cortex, through complex fiber crossings, to cortical and subcortical targets with subvoxel resolution. OBJECTIVE To perform neuroanatomical validation of HDFT and to investigate its neurosurgical applications. METHODS Six neurologically healthy adults and 36 patients with brain lesions were studied. Diffusion spectrum imaging data were reconstructed with a Generalized Q-Ball Imaging approach. Fiber dissection studies were performed in 20 human brains, and selected dissection results were compared with tractography. RESULTS HDFT provides accurate replication of known neuroanatomical features such as the gyral and sulcal folding patterns, the characteristic shape of the claustrum, the segmentation of the thalamic nuclei, the decussation of the superior cerebellar peduncle, the multiple fiber crossing at the centrum semiovale, the complex angulation of the optic radiations, the terminal arborization of the arcuate tract, and the cortical segmentation of the dorsal Broca area. From a clinical perspective, we show that HDFT provides accurate structural connectivity studies in patients with intracerebral lesions, allowing qualitative and quantitative white matter damage assessment, aiding in understanding lesional patterns of white matter structural injury, and facilitating innovative neurosurgical applications. High-grade gliomas produce significant disruption of fibers, and low-grade gliomas cause fiber displacement. Cavernomas cause both displacement and disruption of fibers. CONCLUSION Our HDFT approach provides an accurate reconstruction of white matter fiber tracts with unprecedented detail in both the normal and pathological human brain. Further studies to validate the clinical findings are needed.

Reduction of transmitter B1 inhomogeneity with transmit SENSE slice‐select pulses

Parallel transmitter techniques are a promising approach for reducing transmitter B1 inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B1 inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eight‐channel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast‐kz design and produced 5‐mm‐thick slices at a flip angle of 30° with only a 4.3‐ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse. Magn Reson Med 57:842–847, 2007.

Three-dimensional triple-quantum-filtered 23Na imaging of in vivo human brain

A scheme for the generation of three‐dimensional, triple‐quantum–filtered (TQ) sodium images from normal human brain is presented. In this approach, a three‐pulse, six‐step, coherence transfer filter was used in conjunction with a fast twisted projection imaging sequence to generate spatial maps of the TQ signal across the entire brain. It is demonstrated, theoretically as well as experimentally, that the use of the three‐pulse coherence filter leads to TQ sodium images in which the dependence of the image intensity on the spatial variation of the flip angle is less pronounced than it is in the “standard,” four‐pulse, TQ filter. Correction for the variation of the TQ signal intensity across the field of view because of radio‐frequency (RF) inhomogeneity is straightforward with this approach. This imaging scheme allows the generation of RF inhomogeneity–corrected, TQ, sodium images from human brain at moderate field strength (3.0 T) in times acceptable for routine clinical examinations (20 minutes). Magn Reson Med 42:1146–1154, 1999.

Loss of Cell Ion Homeostasis and Cell Viability in the Brain: What Sodium MRI Can Tell Us

This chapter demonstrates the use of sodium magnetic resonance imaging (MRI) as a noninvasive, in vivo means to assess metabolic changes that ensue from loss of cell ion homeostasis due to cell death in the brain. The chapter is organized in two sections. In the first section, the constraints imposed on the imaging methods by the nuclear magnetic resonance (NMR) properties of the sodium ion are discussed and strategies for avoiding their potential limitations are addressed. The second section illustrates the use of sodium MRI for monitoring focal brain ischemia in permanent and temporary primate models of endovascular middle cerebral artery occlusion.

Noninvasive quantification of total sodium concentrations in acute reperfused myocardial infarction using 23Na MRI

The transport of sodium and potassium between the intra‐ and extracellular pools and the maintenance of the transmembrane concentration gradients are important to cell function and integrity. The early disruption of the sodium pump in myocardial infarction in response to the exhaustion of energy reserves following ischemia and reperfusion results in increased intracellular (and thus total) sodium levels. In this study a method for noninvasively quantifying myocardial sodium levels directly from sodium (23Na) MRI is presented. It was used to measure total myocardial sodium on a clinical 1.5T system in six normal dogs and five dogs with experimentally‐induced myocardial infarction (MI). The technique was validated by comparing total sodium content measured by 23Na MRI with that measured by atomic absorption spectrophotometry (AAS) in biopsied tissue. Total sodium measured by 23Na MRI was significantly elevated in regions of infarction (81.3 ± 14.3 mmol/kg wet wt, mean ± SD) compared to noninfarcted myocardial tissue from both infarcted dogs (36.2 ± 1.1, P < 0.001) and from normal controls (34.4 ± 2.8, P < 0.0001). Myocardial tissue sodium content as measured by 23Na MRI did not vary regionally in the lateral, anterior, or inferior regions in normal hearts (ANOVA, P = NS). Sodium content measured by 23Na MRI agreed with the mean AAS estimates of 31.3 ± 5.6 mmol/kg wet wt (P = NS) in normal hearts, and did not differ significantly from AAS measurements in MI (P = NS). Thus, local tissue sodium levels can be accurately quantified noninvasively using 23Na MRI in normal and acutely reperfused MI. The detection of regional myocardial sodium elevations may help differentiate viable from nonviable, infarcted tissue. Magn Reson Med 46:1144–1151, 2001.

Multicomponent T2* mapping of knee cartilage: Technical feasibility ex vivo

Disorganization of collagen fibers is a sign of early‐stage cartilage degeneration in osteoarthritic knees. Water molecules trapped within well‐organized collagen fibrils would be sensitive to collagen alterations. Multicomponent effective transverse relaxation (T2*) mapping with ultrashort echo time acquisitions is here proposed to probe short T2 relaxations in those trapped water molecules. Six human tibial plateau explants were scanned on a 3T MRI scanner using a home‐developed ultrashort echo time sequence with echo times optimized via Monte Carlo simulations. Time constants and component intensities of T2* decays were calculated at individual pixels, using the nonnegative least squares algorithm. Four T2*‐decay types were found: 99% of cartilage pixels having mono‐, bi‐, or nonexponential decay, and 1% showing triexponential decay. Short T2* was mainly in 1‐6 ms, while long T2* was ∼22 ms. A map of decay types presented spatial distribution of these T2* decays. These results showed the technical feasibility of multicomponent T2* mapping on human knee cartilage explants. Magn Reson Med, 2010.

Acquisition-weighted stack of spirals for fast high-resolution three-dimensional ultra-short echo time MR imaging.

Ultra‐short echo time (UTE) MRI requires both short excitation (∼0.5 ms) and short acquisition delay (<0.2 ms) to minimize T2‐induced signal decay. These requirements currently lead to low acquisition efficiency when high resolution (<1 mm) is pursued. A novel pulse sequence, acquisition‐weighted stack of spirals (AWSOS), is proposed here to acquire high‐resolution three‐dimensional (3D) UTE images with short scan time (∼72 s). The AWSOS sequence uses variable‐duration slice encoding to minimize T2 decay, separates slice thickness from in‐plane resolution to reduce the number of slice encodings, and uses spiral trajectories to accelerate in‐plane data collections. T2‐ and off‐resonance induced slice widening and image blurring were calculated from 1.5 to 7 Tesla (T) through point spread function. Computer simulations were performed to optimize spiral interleaves and readout times. Phantom scans and in vivo experiments on human heads were implemented on a clinical 1.5T scanner (Gmax = 40 mT/m, Smax = 150 T/m/s). Accounting for the limits on B1 maximum, specific absorption rate (SAR), and the lowered amplitude of slab‐select gradient, a sinc radiofrequency (RF) pulse of 0.8ms duration and 1.5 cycles was found to produce a flat slab profile. High in‐plane resolution (0.86 mm) images were obtained for the human head using echo time (TE) = 0.608 ms and total shots = 720 (30 slice‐encodings × 24 spirals). Compared with long‐TE (10 ms) images, the ultrashort‐TE AWSOS images provided clear visualization of short‐T2 tissues such as the nose cartilage, the eye optic nerve, and the brain meninges and parenchyma. Magn Reson Med 60:135–145, 2008.

Stroke Onset Time Using Sodium MRI in Rat Focal Cerebral Ischemia

Background and Purpose— Thrombolytic therapy with intravenous tPA must be administered within 3 hours after stroke onset. However, stroke onset time cannot be established in 20% to 45% of potential patients. We propose that the rate of increase of the brain concentration of sodium ([Na+]br) after stroke, monitored using sodium MRI in a rat model of cortical ischemia, is linear in each individual animal, can locate the ischemic region, and can be used to estimate onset time. Methods— After induction of focal cortical ischemia in rats under isoflurane anesthesia, [Na+]br time course maps were acquired continuously on a 3 T whole body scanner from 2 to 7 hours after occlusion followed by T2-weighted proton images. Microtubule-associated protein-2 immunostained brain sections were used to verify the location of the infarct. Results— The ischemic region identified with microtubule-associated protein-2 corresponded to the region of maximum [Na+]br increase (P<0.001; n=5), and all of the animals demonstrated high linearity. [Na+]br increased at a mean rate of 25±4.7%/h in ischemic tissue (P=0.013) but not in normal cortex (1.0±1.1%/h; P=0.42). The mean onset time error was 1±4 minutes (n=4). Conclusions— These results of sodium MRI show that the region of maximum [Na+]br increase corresponds to the ischemic region. Although [Na+]br increases at a different rate in each animal, the increase is linear, and, therefore, onset time can be estimated. These findings suggest that this method can be used as a ticking clock to estimate time elapsed after vascular occlusion.