P. C. M. Van Zijl

SENSE-DTI at 3 T

While holding vast potential, diffusion tensor imaging (DTI) with single‐excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal‐to‐noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in‐plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. A further significant increase in SNR efficiency was accomplished by SENSE acquisition, exploiting enhanced encoding speed for echo time reduction. Based on the resulting image data, high‐resolution tensor mapping is demonstrated. Magn Reson Med 51:230–236, 2004.

Comparison of the dependence of blood R2 and R2* on oxygen saturation at 1.5 and 4.7 Tesla.

Gradient‐echo (GRE) blood oxygen level‐dependent (BOLD) effects have both intra‐ and extravascular contributions. To better understand the intravascular contribution in quantitative terms, the spin‐echo (SE) and GRE transverse relaxation rates, R2 and R  2* , of isolated blood were measured as a function of oxygenation in a perfusion system. Over the normal oxygenation saturation range of blood between veins, capillaries, and arteries, the difference between these rates, R′2 = R  2* − R2, ranged from 1.5 to 2.1 Hz at 1.5 T and from 26 to 36 Hz at 4.7 T. The blood data were used to calculate the expected intravascular BOLD effects for physiological oxygenation changes that are typical during visual activation. This modeling showed that intravascular ΔR  2* is caused mainly by R2 relaxation changes, namely 85% and 78% at 1.5T and 4.7T, respectively. The simulations also show that at longer TEs (>70 ms), the intravascular contribution to the percentual BOLD change is smaller at high field than at low field, especially for GRE experiments. At shorter TE values, the opposite is the case. For pure parenchyma, the intravascular BOLD signal changes originate predominantly from venules for all TEs at low field and for short TEs at high field. At longer TEs at high field, the capillary contribution dominates. The possible influence of partial volume contributions with large vessels was also simulated, showing large (two‐ to threefold) increases in the total intravascular BOLD effect for both GRE and SE. Magn Reson Med 49:47–60, 2003.

Diffusion tensor imaging of periventricular leukomalacia shows affected sensory cortex white matter pathways

Abstract—The authors used diffusion-tensor imaging to examine central white matter pathways in two children with spastic quadriplegic cerebral palsy. Corticospinal tracts projecting from cortex to brainstem resembled controls. In contrast, posterior regions of the corpus callosum, internal capsule, and corona radiata were markedly reduced, primarily in white matter fibers connected to sensory cortex. These findings suggest that the motor impairment in periventricular leukomalacia may, in part, reflect disruption of sensory connections outside classic pyramidal motor pathways.

Independent component analysis of fMRI data in the complex domain

In BOLD fMRI a series of MR images is acquired and examined for task‐related amplitude changes. These functional changes are small, so it is important to maximize detection efficiency. Virtually all fMRI processing strategies utilize magnitude information and ignore the phase, resulting in an unnecessary loss of efficiency. As the optimum way to model the phase information is not clear, a flexible modeling technique is useful. To analyze complex data sets, independent component analysis (ICA), a data‐driven approach, is proposed. In ICA, the data are modeled as spatially independent components multiplied by their respective time‐courses. There are thus three possible approaches: 1) the time‐courses can be complex‐valued, 2) the images can be complex‐valued, or 3) both the time‐courses and the images can be complex‐valued. These analytic approaches are applied to data from a visual stimulation paradigm, and results from three complex analysis models are presented and compared with magnitude‐only results. Using the criterion of the number of contiguous activated voxels at a given threshold, an average of 12–23% more voxels are detected by complex‐valued ICA estimation at a threshold of |Z| > 2.5. Additionally, preliminary results from the complex models reveal a phase modulation similar to the magnitude time‐course in some voxels, and oppositely modulated in other voxels. Magn Reson Med 48:180–192, 2002.

Diffusion Tensor Imaging in Children with Periventricular Leukomalacia: Variability of Injuries to White Matter Tracts

BACKGROUND AND PURPOSE: Conventional MR imaging shows evidence of brain injury and/or maldevelopment in 70%–90% of children with cerebral palsy (CP), though its capability to identify specific white matter tract injury is limited. The great variability of white matter lesions in CP already demonstrated by postmortem studies is thought to be one of the reasons why response to treatment is so variable. Our hypothesis is that diffusion tensor imaging (DTI) is a suitable technique to provide in vivo characterization of specific white matter tract lesions in children with CP associated with periventricular leukomalacia (PVL). MATERIALS AND METHODS: In this study, 24 children with CP associated with PVL and 35 healthy controls were evaluated with DTI. Criteria for identification of 26 white matter tracts on the basis of 2D DTI color-coded maps were established, and a qualitative scoring system, based on visual inspection of the tracts in comparison with age-matched controls, was used to grade the severity of abnormalities. An ordinal grading system (0 = normal, 1 = abnormal, 2 = severely abnormal or absent) was used to score each white matter tract. RESULTS: There was marked variability in white matter injury pattern in patients with PVL, with the most frequent injury to the retrolenticular part of the internal capsule, posterior thalamic radiation, superior corona radiata, and commissural fibers. CONCLUSION: DTI is a suitable technique for in vivo assessment of specific white matter lesions in patients with PVL and, thus, a potentially valuable diagnostic tool. The tract-specific evaluation revealed a family of tracts that are highly susceptible in PVL, important information that can potentially be used to tailor treatment options in the future.

Magnetization transfer MRI demonstrates spinal cord abnormalities in adrenomyeloneuropathy

Background: In adrenomyeloneuropathy (AMN) conventional MRI detects only spinal cord atrophy in the late stages. Objective: To apply a magnetization transfer-weighted (MTw) imaging to patients with AMN and AMN-like syndrome in order to visualize and quantitatively assess the pathology of white matter tracts in the cervical spinal cord. Methods: MTw studies were conducted in nine men with AMN, eight symptomatic heterozygous women, and 10 age- and sex-matched controls and compared to the Expanded Disability Status Scale (EDSS) and quantitative tests of vibratory sense and postural sway. MTw data sets were obtained at the level of C1 to C3 using a three-dimensional gradient echo acquisition technique, these images were then standardized between subjects by using the in-slice CSF signal as a normalization reference, allowing a quantitative assessment of the MTw signal. Results: In contrast to conventional MRI, MTw images showed signal hyperintensities in the lateral and dorsal columns of all patients. The MT signal quantified in the dorsal column showed significant differences between patients with AMN, X-linked adrenoleukodystrophy heterozygotes, and controls. MT hyperintensity in the dorsal column correlated with EDSS, vibratory sense, and postural sway. Conclusion: Magnetization transfer-weighted imaging is a sensitive modality for the visual and quantitative assessment of spinal cord pathology in adrenomyeloneuropathy, and is a potential tool for evaluation of new therapies.

Diffusion Tensor Imaging of the Optic Nerve in Multiple Sclerosis: Association with Retinal Damage and Visual Disability

BACKGROUND AND PURPOSE: There is a well-known relationship between MS and damage to the optic nerve, but advanced, quantitative MR imaging methods have not been applied to large cohorts. Our objective was to determine whether a short imaging protocol (<10 minutes), implemented with standard hardware, could detect abnormal water diffusion in the optic nerves of patients with MS. MATERIALS AND METHODS: We examined water diffusion in human optic nerves via DTI in the largest MS cohort reported to date (104 individuals, including 38 optic nerves previously affected by optic neuritis). We also assessed whether such abnormalities are associated with loss of visual acuity (both high and low contrast) and damage to the retinal nerve fiber layer (assessed via optical coherence tomography). RESULTS: The most abnormal diffusion was found in the optic nerves of patients with SPMS, especially in optic nerves previously affected by optic neuritis (19% drop in FA). DTI abnormalities correlated with both retinal nerve fiber layer thinning (correlation coefficient, 0.41) and loss of visual acuity, particularly at high contrast and in nerves previously affected by optic neuritis (correlation coefficient, 0.54). However, diffusion abnormalities were overall less pronounced than retinal nerve fiber layer thinning. CONCLUSIONS: DTI is sensitive to optic nerve damage in patients with MS, but a short imaging sequence added to standard clinical protocols may not be the most reliable indicator of optic nerve damage.

Anisotropy of the diamagnetic susceptibility of benzene. Determination by high-field deuterium NMR

Abstract A doublet due to quadrupole splitting cannot be detected in the deuterium NMR spectrum of perdeuterobenzene at 141 kG, whether neat, in dilute solution, or in the gas phase. However, the proton-decoupled spectrum of monodeuterobenzene at 296 K consists of a doublet with spacing of 0.50 Hz at 1 mol% concentration in acetone- d 6 , decreasing to 0.42 Hz at 80 mol% concentration. The observed splitting provides a value for the anisotropy of the diamagnetic susceptibility of -1.11 :K 10 −28 emu/molecule at I% concentration and -0.93 × 10 −28 emu/molecule at 80% concentration, where transient dimers may be formed. The difficulty with perdeuterobenzene is traced to the effect of the scalar spin-spin coupling between deuterons.

Determination of cerebral glucose transport and metabolic kinetics by dynamic MR spectroscopy

A new in vivo nuclear magnetic resonance (NMR) spectroscopy method is introduced that dynamically measures cerebral utilization of magnetically labeled [1-13C]glucose from the change in total brain glucose signals on infusion. Kinetic equations are derived using a four-compartment model incorporating glucose transport and phosphorylation. Brain extract data show that the glucose 6-phosphate concentration is negligible relative to glucose, simplifying the kinetics to three compartments and allowing direct determination of the glucose-utilization half-life time [ t ½ = ln2/( k 2 + k 3)] from the time dependence of the NMR signal. Results on isofluorane ( n = 5)- and halothane ( n = 7)- anesthetized cats give a hyperglycemic t ½ = 5.10 ± 0.11 min-1 (SE). Using Michaelis-Menten kinetics and an assumed half-saturation constant Kt = 5 ± 1 mM, we determined a maximal transport rate T max = 0.83 ± 0.19 μmol ⋅ g-1 ⋅ min-1, a cerebral metabolic rate of glucose CMRGlc = 0.22 ± 0.03 μmol ⋅ g-1 ⋅ min-1, and a normoglycemic cerebral influx rate CIRGlc = 0.37 ± 0.05 μmol ⋅ g-1 ⋅ min-1. Possible extension of this approach to positron emission tomography and proton NMR is discussed.

The Role of Imaging in Acute Brain Injury

There is increasing awareness of the preponderance and complexity of neurological dysfunction in the intensive care unit (ICIJ). The critical care physician has a central role in the evaluation and treatment not only of patients admitted with complex traumatic and non-traumatic brain injuries, but also of patients who develop secondary neurological dysfunction as a result of a systemic insult such as cardiac arrest, liver failure, or sepsis.