First set of gated x-ray imaging diagnostics for the Laser Megajoule facility.

The Laser Megajoule (LMJ) facility located at CEA/CESTA started to operate in the early 2014 with two quadruplets (20 kJ at 351 nm) focused on target for the first experimental campaign. We present here the first set of gated x-ray imaging (GXI) diagnostics implemented on LMJ since mid-2014. This set consists of two imaging diagnostics with spatial, temporal, and broadband spectral resolution. These diagnostics will give basic measurements, during the entire life of the facility, such as position, structure, and balance of beams, but they will also be used to characterize gas filled target implosion symmetry and timing, to study x-ray radiography and hydrodynamic instabilities. The design requires a vulnerability approach, because components will operate in a harsh environment induced by neutron fluxes, gamma rays, debris, and shrapnel. Grazing incidence x-ray microscopes are fielded as far as possible away from the target to minimize potential damage and signal noise due to these sources. These imaging diagnostics incorporate microscopes with large source-to-optic distance and large size gated microchannel plate detectors. Microscopes include optics with grazing incidence mirrors, pinholes, and refractive lenses. Spatial, temporal, and spectral performances have been measured on x-ray tubes and UV lasers at CEA-DIF and at Physikalisch-Technische Bundesanstalt BESSY II synchrotron prior to be set on LMJ. GXI-1 and GXI-2 designs, metrology, and first experiments on LMJ are presented here.

Diffusion tensor imaging and tractography in Brown-Sequard syndrome.

STUDY DESIGN: Case report. OBJECTIVE: To demonstrate the utility of diffusion tensor imaging and tractography in two patients with Brown-Sequard syndrome after penetrating cervical cord injury. SETTING: Milwaukee, WI, USA. METHODS: Two patients, who presented with features of Brown-Sequard syndrome after sustaining stab wounds to the neck, underwent DTI and tractography of the cervical cord within a week of the injury. DTI metrics were measured within the left and right hemicord around the level of injury. Diffusion tensor tractography was performed to visualize the site of injury and injured fiber tracts. RESULTS: Axial fractional anisotropy maps at the site of injury showed unilateral damage to the cord structure, and FA was significantly reduced within the injured hemicord in both patients. Tractography allowed for visualization of the injured fiber tracts around the level of injury. Both DTI metrics and tractography showed an asymmetry that corresponded to the neurological deficits exhibited by the patients. CONCLUSION: This report illustrates the utility of DTI and DTT in delineating regions of cord injury in two patients with traumatic Brown-Sequard syndrome. Our results indicate that DTI provides clinically relevant information that supplements conventional MR imaging for patients with acute SCI.

Assessment of temporal state-dependent interactions between auditory fMRI responses to desired and undesired acoustic sources.

A confounding factor in auditory functional magnetic resonance imaging (fMRI) experiments is the presence of the acoustic noise inherently associated with the echo planar imaging acquisition technique. Previous studies have demonstrated that this noise can induce unwanted neuronal responses that can mask stimulus-induced responses. Similarly, activation accumulated over multiple stimuli has been demonstrated to elevate the baseline, thus reducing the dynamic range available for subsequent responses. To best evaluate responses to auditory stimuli, it is necessary to account for the presence of all recent acoustic stimulation, beginning with an understanding of the attenuating effects brought about by interaction between and among induced unwanted neuronal responses, and responses to desired auditory stimuli. This study focuses on the characterization of the duration of this temporal memory and qualitative assessment of the associated response attenuation. Two experimental parameters--inter-stimulus interval (ISI) and repetition time (TR)--were varied during an fMRI experiment in which participants were asked to passively attend to an auditory stimulus. Results present evidence of a state-dependent interaction between induced responses. As expected, attenuating effects of these interactions become less significant as TR and ISI increase and in contrast to previous work, persist up to 18s after a stimulus presentation.

Diffusion tensor MR imaging in chronic spinal cord injury.

BACKGROUND AND PURPOSE: Diffusion tensor MR imaging is emerging as an important tool for displaying anatomic changes in the brain after injury or disease but has been less widely applied to disorders of the spinal cord. The aim of this study was to characterize the diffusion properties of the entire human spinal cord in vivo during the chronic stages of spinal cord injury (SCI). These data provide insight into the structural changes that occur as a result of long-term recovery from spinal trauma. MATERIALS AND METHODS: Thirteen neurologically intact subjects and 10 subjects with chronic SCI (>4 years postinjury) were enrolled in this study. A single-shot twice-refocused spin-echo diffusion-weighted echo-planar imaging pulse sequence was used to obtain axial images throughout the entire spinal cord (C1-L1) in <60 minutes. RESULTS: Despite heterogeneity in SCI lesion severity and location, diffusion characteristics of the chronic lesion were significantly elevated compared with those of uninjured controls. Fractional anisotropy was significantly lower at the chronic lesion and appeared dependent on the completeness of the injury. Conversely, mean diffusivity measurements in the upper cervical spinal cord in subjects with SCI were significantly lower than those in controls. These trends suggest that the entire neuraxis may be affected by long-term recovery from spinal trauma. CONCLUSION: These results suggest that diffusion tensor imaging may be useful in the assessment of SCI recovery.

Diffusion tensor MR imaging of the neurologically intact human spinal cord.

BACKGROUND AND PURPOSE: The aim of this study was to characterize the diffusion properties of the entire human spinal cord in vivo. These data are essential for comparisons to pathologic conditions as well as for comparisons of different pulse sequence design parameters aimed to reduce scan time and more accurately determine diffusion coefficients. MATERIALS AND METHODS: A total of 13 neurologically intact subjects were enrolled in this study. A single-shot, twice-refocused, spin-echo, diffusion-weighted, echo-planar imaging (EPI) pulse sequence was used to obtain axial images throughout the entire spinal cord (C1-L1) in 45 minutes. RESULTS: Diffusion images indicated slight geometric distortions; however, gray and white matter contrast was observed. All measurements varied across the length of the cord. Whole cord diffusion coefficients averaged 0.5-1.3 x 10(-3) mm(2)/s depending on orientation, mean diffusivity (MD) averaged 0.83 +/- 0.06 x 10(-3) mm(2)/s, fractional anisotropy (FA) averaged 0.49 +/- 0.05, and volume ratio (VR) averaged 0.73 +/- 0.05. CONCLUSION: This study provided normative diffusion values for the entire spinal cord for use in comparisons with pathologic conditions as well as improvements in pulse sequence design.

Presurgical and intraoperative mapping of the motor system in congenital truncation of the precentral gyrus.

A 43-year-old man presented with a grade II astrocytoma in the left postcentral gyrus and superior parietal lobule. Preoperative functional MR imaging and diffusion tensor imaging mapped distal upper-extremity primary motor cortex and white matter, respectively, adjacent to the tumor, within a congenitally truncated precentral gyrus. Because of the congenital anomaly, this region of primary motor cortex was inaccessible to direct visualization or intraoperative electrocortical stimulation. The integration of preoperative and intraoperative mapping data facilitated resection of the tumor while avoiding a postoperative motor deficit.

Characterizing the attenuation and/or saturation effect of the acoustic scanner noise in auditory event-related functional magnetic resonance imaging.

The analysis of event-related functional magnetic resonance imaging when presenting auditory stimuli and/or investigating auditory cortex and related areas is hindered by inherent acoustic scanner noise (ASN), which can alter the properties of the acquired time-series data. Therefore, it is necessary to account for ASN in the analysis, and one step towards this goal is to characterize the attenuation and/or saturation effect of the hemodynamic response due to ASN. Towards this end, this study examined how the effect of ASN is dependent on repetition time (TR) and the inter-stimulus interval (ISI), two experimental parameters that affect the acoustic signal-to-noise ratio of the experimental paradigm. Results indicate that a decrease in TR (e.g., 6 s to 1.5 s) results in an increase in saturation and an attenuation of the estimated hemodynamic response peak with respect to the baseline signal level. There was no statistical difference in peak response between the two ISI values used, 12 s and 18 s.

Characterizing the amplitude and spatial extent of the cortical response in auditory cortex to acoustic scanner noise generated during echo-planar image acquisition in functional magnetic resonance imaging.

Acoustic scanner noise produced during event-related functional magnetic resonance imaging (ER-fMRI) studies can hinder auditory fMRI research analysis by altering the properties of the acquired time-series data. Given the desire to obtain the most accurate results possible using ER-fMRI experiments, this study seeks to characterize the amplitude and spatial extent of the auditory fMRI cortical response, in isolation, generated by the acoustic scanner noise associated with echo-planar acquisition. Results from this study indicate that the pure cortical response is non-trivial, is comparable to a standard hemodynamic response function, and should be accounted for in analysis using ER-fMRI models.

Magnetic resonance analysis of postsurgical temporal lobectomy.

BACKGROUND AND PURPOSE: The effect of temporal lobe transection area, volume of postoperative gliosis, and surgical technique on patients' seizure-free outcome is unknown. The authors studied the effects of these variables on patients' seizure-free outcome. METHODS: A retrospective review of magnetic resonance imaging examinations acquired 3 to 18 months after temporal lobe resection was carried out for 18 patients with intractable temporal lobe seizures and known postsurgical outcomes for more than 2 years. The total volume of radiologically probable gliosis evident on axial proton-density-weighted images was calculated for each patient using software on an independent console. The total area of temporal lobe surface transected by the scalpel was calculated as well, using sagittal T1-weighted images. The total volume of gliosis, the total area of transected temporal lobe, and the specific type of surgery (sparing vs no sparing of the superior temporal gyrus) were then correlated with the postsurgical outcome of the patients. An examiner with no prior knowledge of the patients' postsurgical outcomes carried out the above calculations and measurements. The patients' postoperative outcome was defined using Engel classifications, and patients were divided into 2 groups: group A with Engel class 1 (n = 9) and group B with Engel classes 2-4 (n = 9). RESULTS: The mean volumes of postoperative gliosis were not significantly different between group A (3592.3 mm3) and group B (4270 mm3). The mean area of transected temporal lobe was also similar between group A (1865.2 mm2) and group B (1930 mm2). With regard to surgical technique, there were 5 patients who had the superior temporal gyrus resected and 13 who did not. Eighty percent of patients with the superior temporal gyrus resected were Engel class 1 or 2, whereas only 20% were of Engel class 3 or 4. CONCLUSIONS: The authors found no clear association between postoperative outcome and residual temporal lobe gliosis, the surgical technique, or the total area of temporal lobe transected by the scalpel.

Use of jackknife resampling techniques to estimate the confidence intervals of fMRI parameters.

PURPOSE: The objective of this study was to determine the reliability and the confidence intervals of task-activated functional MRI (fMRI) parameters using a computer-intensive resampling technique. The jackknife, a commonly used method for resampling mathematical data, was used to calculate the confidence interval of fMRI parameters for a simple bilateral finger-tapping paradigm. METHOD: Four healthy test subjects (three men, one woman) were used to test the correlation coefficient and variability in the data. Each subject performed 4.5 cycles, each cycle having 20 s of bilateral finger tapping alternating with rest periods of equal time, producing 90 images. One additional scan of 10 cycles (200 images) was used to test the stability of the method itself. One thousand jackknifed resampled data sets of 85 elements each (from 90 original points) were generated, and the correlation coefficient was determined using an idealized "on/off" box-car reference waveform. RESULTS: Activation maps were generated that had the same confidence intervals at each pixel. These maps were more localized with less extraneous activated pixels than the maps generated with a fixed correlation coefficient threshold. There was no significant difference in the distribution of correlation coefficients between the 85, 90, and 95 element, jackknifed data sets; similar robustness was seen, as well. CONCLUSION: The jackknife resampling technique for data analysis produced reliable distributions and statistical parameters. The jackknife estimates were shown to be stable, even from a small initial sample size. This method may be used in lieu of test-retest analysis.

Spin-lock magnetic resonance imaging of muscle in patients with autosomal recessive limb girdle muscular dystrophy.

Spin-lock imaging is a new magnetic resonance imaging (MRI) technique used to reflect the microstructural integrity of muscle. The purpose of this study was to characterize spin-lock contrast (SLC) of calf muscles in limb girdle muscular dystrophy (LGMD). The calf muscles of 5 patients with LGMD and 10 healthy volunteers were imaged with an off-resonance magnetic resonance (MR) spin-lock suppression pulse. Spin-lock suppression ratios were calculated for anterior tibialis, posterior tibialis, soleus, and gastrocnemius muscles. Clinical assessments of muscle strength were compared to the spin-lock suppression ratios in the LGMD group. Strong SLC was observed in healthy muscles, with mean (+/- SD) suppression ratios ranging from 51.2% (+/- 3.6%) to 56.3% (+/- 1.3%). In diseased muscle, spin-lock signal suppression was reduced by 8%-70%, demonstrating an inverse correlation between symptom duration and suppression ratios. Spin-lock contrast in the patients with LGMD, as a reflection of tissue integrity, was best preserved in posterior tibialis, anterior tibialis, soleus, and gastrocnemius muscles in descending order. Clinical assessments did a poorer job of differentiating than SLC did and were in poor agreement with spin-lock suppression ratios. Spin-lock MRI can quantify microstructural changes in LGMD and appears to provide information not obtainable from clinical evaluations. This suggests that this noninvasive technique may be useful in evaluating the extent, progression, and response to therapy of LGMD.

Magnetization transfer imaging of skeletal muscle in autosomal recessive limb girdle muscular dystrophy.

PURPOSE: The purpose of this work was to characterize magnetization transfer (MT) contrast of skeletal muscles in limb girdle muscular dystrophy (LGMD). METHOD: The calf muscles of five LGMD patients and 10 normal volunteers were imaged with an off-resonance MT suppression pulse applied to T1-weighted images. MT suppression ratios were calculated for anterior tibialis, posterior tibialis, soleus, and gastrocnemius muscles in the LGMD and control groups. The relationship between MT of individual muscles and the duration of LGMD symptoms was determined. RESULTS: Strong MT contrast was observed in normal calf muscles, with mean (+/-SD) suppression ratios ranging from 37.9% (+/-3.0) to 41.1% (+/-2.1). In diseased muscle, MT signal suppression ranged from 11 to 38%, demonstrating an inverse relationship between symptom duration and suppression ratios. MT contrast in the LGMD patients, as a reflection of muscle tissue integrity, was preserved in posterior or anterior tibialis, soleus, and gastrocnemius muscles, respectively. Suppression ratios were dramatically reduced in muscles with gross fatty infiltration but also were reduced in muscle tissues without visual evidence of fatty infiltration. CONCLUSION: MT imaging provides a quantitative measure of pathologic changes occurring within the skeletal muscles of patients with LGMD relative to normal and may be useful in evaluating disease extent, progression, and response to new therapies as they become available.

Depiction of intracranial vessels with MRA: utility of magnetization transfer saturation and gadolinium.

PURPOSE: The purpose of this work was to quantitate the individual and combined effects of magnetization transfer (MT) saturation and gadolinium (Gd) on the visualization of intracranial vessels with MR angiography (MRA). METHOD: Thirty-five subjects underwent two three-dimensional time-of-flight MRA sequences without and with MT and/or Gd. There were 14 MR angiograms without Gd or MT, 18 with MT only, 17 with Gd only, and 21 with both Gd and MT. On a projection image, a region of interest was drawn to delineate the arteries in the middle cerebral artery territory. The total area of blood vessels in the region of interest was calculated for each MR angiogram. Mean vessel areas for the four types of MRA were compared with analysis of variance. RESULTS: MRA with either MT or Gd alone showed significantly more vessel area than MRA without either (p < 0.05). MRA with MT alone and MRA with Gd alone were not different from each other (p = 0.29). The improvement in vessel area measured by using MT and Gd together was significantly more than expected from the cumulative improvement of adding each alone (p < 0.05). CONCLUSION: Combining MT and Gd synergistically improved the visualization of intracranial vessels on MRA.

Brain: gadolinium-enhanced fast fluid-attenuated inversion-recovery MR imaging.

PURPOSE: To determine the clinical utility of gadolinium-enhanced fluid-attenuated inversion-recovery (FLAIR) magnetic resonance (MR) imaging of the brain by comparing results with those at gadolinium-enhanced T1-weighted MR imaging with magnetization transfer (MT) saturation. MATERIALS AND METHODS: In 105 consecutive patients referred for gadolinium-enhanced brain imaging, FLAIR and T1-weighted MR imaging with MT saturation were performed before and after administration of gadopentetate dimeglumine (0.1 mmol per kilogram of body weight). Pre- and postcontrast images were evaluated to determine the presence of abnormal contrast enhancement and whether enhancement was more conspicuous with the FLAIR or T1-weighted sequences. RESULTS: Thirty-nine studies showed intracranial contrast enhancement. Postcontrast T1-weighted images with MT saturation showed superior enhancement in 14 studies, whereas postcontrast fast FLAIR images showed superior enhancement in 15 studies. Four cases demonstrated approximately equal contrast enhancement with both sequences. Six cases showed some areas of enhancement better with T1-weighted imaging with MT saturation and other areas better with postcontrast fast FLAIR imaging. Superficial enhancement was typically better seen with postcontrast fast FLAIR imaging. CONCLUSION: Fast FLAIR images have noticeable T1 contrast making gadolinium-induced enhancement visible. Gadolinium enhancement in lesions that are hyperintense on precontrast FLAIR images, such as intraparenchymal tumors, may be better seen on T1-weighted images than on postcontrast fast FLAIR images. However, postcontrast fast FLAIR images may be useful for detecting superficial abnormalities, such as meningeal disease, because they do not demonstrate contrast enhancement of vessels with slow flow as do T1-weighted images.

Blind source separation of multiple signal sources of fMRI data sets using independent component analysis.

PURPOSE: The objective of this study was to separate multiple signal components present in functional MRI (fMRI) data sets. Blind source separation techniques were applied to the analysis of fMRI data to determine multiple physiologically relevant independent signal sources. METHOD: Computer simulations were performed to test the reliability and robustness of the independent component analysis (ICA). Four subjects (3 males and 1 female between 14 and 29 years old) were scanned under various stimulus conditions: (1) rest while breathing room air, (2) bilateral finger tapping while breathing room air, and (3) hypercapnia during bilateral finger tapping. RESULTS: Simulations performed on synthetic data sets demonstrated that not only could the algorithm reliably detect the shapes of each of the source signals, but it also preserved their relative amplitudes. The algorithm also performed robustly in the presence of noise. With use of fMRI time series data sets from bilateral finger tapping during hypercapnia, distinct physiologically relevant independent sources were reliably estimated. One independent component corresponded to the hypercapnic cerebrovascular response, and another independent component corresponded to cortical activation from bilateral finger tapping. In three of the four subjects, the underlying fluctuations in signal related to baseline respiratory rate were identified in the third independent component. Principal component analysis (PCA) could not separate these two independent physiological components. CONCLUSION: With use of ICA, signals originating from independent sources could be separated from a linear mixture of observed data. Limitations of PCA were also demonstrated.