Single-shot fast spin-echo diffusion tensor imaging of the lumbar spine at 1.5 and 3 T

Diffusion tensor imaging (DTI) of the lumbar spine could improve diagnostic specificity. The purpose of this work was to determine the feasibility of and to validate DTI with single-shot fast spin-echo (SSFSE) for lumbar intervertebral discs at 1.5 and 3 T. Six normal volunteers were scanned with DTI-SSFSE using an eight- and a three-b-value protocol at 1.5 and 3 T, respectively. Apparent diffusion coefficient (ADC) values were computed and validated based on those obtained at 1.5 T from corresponding diffusion tensor scans using line scan diffusion imaging (LSDI), a technique that has been previously validated for use in the spine. Pearson correlation coefficients for LSDI and DTI-SSFSE ADC values were .88 and .89 for 1.5 and 3 T, respectively, with good quantitative agreement according to the Bland-Altman method. Results indicate that DTI-SSFSE is a candidate as a clinical sequence for obtaining diffusion tensor images of the lumbar intervertebral discs with scan times shorter than 4 min.     

Single-shot spatiotemporal terahertz field imaging

Using an optically chirped pulse in an electro-optic detection system, we image the spatiotemporal distribution of a terahertz (THz) pulse. We demonstrate single-shot THz field imaging of photoconductive emitters.     

Improved contrast at 1.5 tesla using half-Fourier imaging: application to spin-echo and angiographic imaging

Half-Fourier imaging is useful for reducing imaging time by requiring less than the usual number of phase-encoding steps. This increase in speed can be traded off for longer repeat times, TR, for improved contrast-to-noise in the same imaging time or to collect short asymmetric echoes. Consequently, it is shown to be especially useful for long TR spin-echo imaging where at 1.5 T a repeat time of 4 sec is recommended for a double-echo TE = 30/90 sequence or 3 sec for a double-echo TE = 15/90 sequence. Short TR FLASH imaging also benefits from a longer TR since there is more time to spoil the signal. In both cases, there is the advantage when a multislice acquisition mode is used that more slices (and hence, a larger volume) can be taken. Another application is to apply half-Fourier imaging in the read direction to avoid spin dephasing and motion artifacts. This is particularly useful in angiographic imaging where smaller pixel sizes and shorter echo times both reduce pixel dephasing. Again, even though taking less than the usual number of data points leads to a reduction in S/N, the improved signal and resolution for blood vessels can more than compensate this loss.     

Single-shot line scan imaging using stimulated echoes

A new high-speed MRI method is described for single-shot line scan imaging (LSI) based on stimulated echoes (STE). To allow for multislice imaging, the technique comprises a series of slice-selective preparation pulses (each corresponding to the first RF pulse of a STE sequence), a slab-selective refocusing pulse (second RF pulse), and multiple line-selective read pulses (third RF pulses). An alternative version employs packages of two slice-selective pulses followed by multiple line-selective read pulses. Experimental applications deal with human brain imaging on a clinical MRI system at 2.0 T. The technique offers user-selectable trade-offs between volume coverage (1-15 sections) and in-plane spatial resolution (1-5 mm linear pixel dimension) within total acquisition times of less than 500 ms. Although LSI yields a lower signal-to-noise ratio than Fourier imaging, single-shot LSI with STEs is free from resonance offset effects (e.g., magnetic field inhomogeneities and susceptibility differences) that are typical for echo-planar imaging. Moreover, the technique exhibits considerable robustness against motion and provides access to arbitrary fields-of-view, i.e., localized imaging of inner volumes without aliasing artifacts due to phase wrapping.     

Single-shot fast spin-echo diffusion tensor imaging of the brain and spine with head and phased array coils at 1.5 T and 3.0 T

In this study, we investigated the use of a single-shot fast spin-echo-based sequence to perform diffusion tensor imaging (DTI) with improved anatomic fidelity through the entire brain and the cervical spine. Traditionally, diffusion tensor images have been acquired by single-shot echo-planar imaging (EPI) methods in which large distortions result from magnetic susceptibility effects, especially near air-tissue interfaces. These distortions can be problematic, especially in anterior and inferior portions of the brain, and they also can severely limit applications in the spine. At higher magnetic fields these magnetic susceptibility artifacts are increased. The single-shot fast spin-echo (SSFSE) method used in this study utilizes radiofrequency rephasing in the transverse plane and thus provides diffusion images with negligible distortion even at 3 Tesla. In addition, the SSFSE sequence does not require multiple fast-receivers, which are not available on many magnetic resonance (MR) systems. Phased array coils were used to increase the signal-to-noise ratio of the images, offering a major inherent advantage in diffusion tensor imaging of the spine and brain. The mean diffusion measurements obtained with the SSFSE acquisition were not statistically different (p > 0.05) from EPI-based acquisitions. Compared to routine T(2)-weighted MR images, the DTI-EPI sequence showed up to 20% in elongation of the brain in the anterior-posterior direction on a sagittal image due to magnetic susceptibility distortions, whereas in the DTI-SSFSE, the image distortions were negligible. The diffusion tensor SSFSE method was also able to assess diffusion abnormalities in a brain stem hemorrhage, unaffected by the spatial distortions that limited conventional EPI acquisition.     

Single-shot time-frequency imaging spectroscopy using an echelon mirror

We demonstrate single-shot time-frequency imaging spectroscopy with an echelon mirror for measuring ultrashort laser pulses as well as ultrafast responses of materials using the same optical setup. The echelon mirror produces a spatially encoded time delay for the probe pulse whereby both the probe and pump pulses are focused on samples with small spot size. Using the optical Kerr gate apparatus, we successfully mapped the time-frequency images of ultrashort laser pulses and subsequently evaluated the chirp characteristics with the phase-retrieval procedure on a single-shot basis. By simply replacing the Kerr medium with samples, we could also visualize the phonon-polariton oscillations in ferroelectric LiNbO3.     

Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging

We describe a multi-layer (“sandwich”) configuration detector consisting of two x-ray imaging flat-panel detectors for single-shot (single-kV) dual-energy imaging. An intermediate copper filter is used to increase spectral separation between the two detectors to improve contrast at the expense of image noise. Monte Carlo and cascaded-systems analyses of the signal and noise performance are described that quantify performance characteristics. Image quality of dual-energy images obtained from a prototype sandwich-detector system is evaluated using a figure of merit (FOM), defined as the squared contrast-to-noise ratio normalized by x-ray exposure for a mouse phantom for preclinical imaging. Demonstration dual-energy bone and soft-tissues images of a postmortem mouse are obtained using the prototype system. While the FOM with the single-shot detector is lower than that achieved using a conventional dual-shot (dual-kV) method, the single-shot approach may be preferable when imaging speed or insensitivity to motion artifacts is a primary concern.     

A method for in vivo assessment of reversible rat pancreatic ischemia using 31P NMR spectroscopy at 2.0 tesla

A surgical method is described which allows in vivo assessment of reversible rat pancreatic ischemia using ³¹P NMR spectroscopy at 2.0 T. Phosphorous-31 NMR spectra acquired during the ischemic period show the expected increase in inorganic phosphate with a concomitant decrease in ATP levels and pH as compared to controls. Upon reperfusion, inorganic phosphate and ATP returned to control levels while pH recovered to a more alkaline value. This method provides a means of studying in vivo changes in high energy metabolite associated with acute pancreatitis (AP) and maintains the secretory ability of the gland so that different forms of AP, such as those arising from pancreatic juice edema, can be studied.     

Simple RF design for human functional and morphological cardiac imaging at 7 tesla

Morphological and functional cardiac MRI can potentially benefit greatly from the recent advent of commercial high-field (7 tesla and above) MRI systems. However, conventional hardware configurations at lower field using a body-coil for homogeneous transmission are not available at these field strengths. Sophisticated multiple-transmit-channel systems have been shown to be able to image the human heart at 7 tesla but such systems are currently not widely available. In this paper, we empirically optimize the design of a simple quadrature coil for cardiac imaging at 7 tesla. The size, geometry, and position have been chosen to produce a B₁ field with no tissue-induced signal voids within the heart. Standard navigator echoes for gating were adapted for operation at the heart/lung interface, directly along the head–foot direction. Using this setup, conventional and high-resolution cine functional imaging have been successfully performed, as has morphological imaging of the right coronary artery.     

Single-shot extreme ultraviolet laser imaging of nanostructures with wavelength resolution

We have demonstrated near-wavelength resolution microscopy in the extreme ultraviolet. Images of 50 nm diameter nanotubes were obtained with a single ~1 ns duration pulse from a desktop-size 46.9 nm laser. We measured the modulation transfer function of the microscope for three different numerical aperture zone plate objectives, demonstrating that 54 nm half-period structures can be resolved. The combination of near-wavelength spatial resolution and high temporal resolution opens myriad opportunities in imaging, such as the ability to directly investigate dynamics of nanoscale structures.     

Single-shot laser-induced fluorescence imaging of formaldehyde with XeF excimer excitation

Single-shot formaldehyde laser-induced fluorescence (LIF) imaging measurements in a technical scale turbulent flame have been obtained using XeF excimer laser excitation in the ?¹A₂-˜X¹A₁ transition at 353.2 nm. Measurements have been carried out in a 150 kW natural gas swirl burner where formaldehyde distribution fields have the potential, in combination with OH concentration fields, to visualize the heat release distribution and therefore give an optimal visualization of flame-front positions. The extended areas where formaldehyde was detected in the swirl flame indicates the presence of low temperature chemistry in preheated gas pockets before ignition. Received: 31 January 2000 / Revised version: 2 March 2000 / Published online: 5 April 2000     

Single-shot measurement of free-electron laser polarization at SDUV-FEL

In this paper, a division-of-amplitude photopolarimeter(DOAP) for measuring the polarization state of a free-electron laser(FEL) pulse is described. The incident FEL beam is divided into four separate beams, and four Stokes parameters can be measured in a single-shot. In the crossed-planar undulators experiment at the Shanghai deep ultraviolet FEL test facility, this DOAP instrument constructed in house responsed accurately and timely while the polarization-state of fully coherent FEL pulses were switched, which is helpful for confirming the crossed-planar undulators technique for short-wavelength FELs.     

Towards intrinsic R2* imaging in the prostate at 3 and 7 tesla

PurposeHypoxia is an important marker for resistance to therapy. In this study, we quantify the macroscopic effects of R2* mapping in prostate cancer patients incorporating susceptibility matching and field strengths effects.Materials and methods91 patients were scanned without endorectal coil (ERC) at 3 T. Only when rectal gas was absent, data was included for analysis. Another group of 10 patients was scanned using a susceptibility matched ERC. To assess the residual contamination of R2 and macroscopic field non-uniformities, a group of 10 patients underwent ultra-high resolution 7 T MRI.ResultsOf the patients scanned at 3 T 60% presented rectal gas and were excluded, due to susceptibility artifacts. At 3 T the tumor was significantly different (P < 0.01) from the healthy surrounding tissue in R2* values at intrapatient level. Using the measured median R2* value of 24.9 s^− 1 at 3 T and 43.2 s^− 1 at 7 T of the peripheral zone, the minimum contribution of macroscopic susceptibility effects is 15% at 3 T.ConclusionR2* imaging might be a promising tool for hypoxia imaging, particularly when minimizing macroscopic susceptibility effects contaminating intrinsic R2* of tissue, such as rectal gas. At 3 T macroscopic effects still contribute 15% in the R2* value, compared to ultra-high resolution R2* mapping at 7 T.     

Liver imaging at 1.5 tesla: pulse sequence optimization based on improved measurement of tissue relaxation times

In order to predict the most sensitive MR imaging sequence for detecting liver metastases at 1.5 T, in vivo measurements of T1 and T2 relaxation times and proton density were obtained using multipoint techniques. Based on these measurements, two-dimensional contrast contour plots were constructed demonstrating signal intensity contrast between hepatic lesions and surrounding liver parenchyma for different pulse sequences and pulse timing parameters. The data predict that inversion recovery spin echo (IRSE) imaging should yield the greatest contrast between liver metastases and liver parenchyma at 1.5 T, followed by short tau inversion recovery (STIR) and spin-echo (SE) pulse sequences. T2-weighted SE images provided greater liver/lesion contrast than T1-weighted SE pulse sequences. Calculated T1, T2, and proton density values of the spleen were similar to those of hepatic metastatic lesions, indicating that the signal intensity of the spleen may be used as an internal standard to predict the signal intensity of hepatic metastases on T1- and T2-weighted images at 1.5 T.     

Effects of echo time variation on perfusion assessment using dynamic susceptibility contrast MR imaging at 3 tesla

The implications of changing the echo time of a gradient-echo echo planar imaging sequence applied to dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) for perfusion imaging at 3T were investigated. Four echo times in the range of 21 to 45 ms were examined in a total of 17 patients who received a dose of 0.1 mmol/kg bodyweight Gadobutrol (Gadovist, 1.0 mmol/ml). As the primary optimization parameter, the concentration-to-noise ratio (SNRc) was selected as it takes effects of variations in baseline as well as in signal drop into account. In an analysis of gray matter, white matter and arterial regions of interest, SNRc showed the highest values for the shortest applied echo time in all cases. Maps of regional cerebral blood volume (rCBV) and blood flow (rCBF) were calculated using deconvolution based on singular value decomposition. The quality of rCBF and rCBV images was judged to be good or excellent in all cases, independent of the echo time. Calculated gray matter/white matter ratios of rCBF and rCBV displayed no significant dependence on the applied echo time. Considering the better SNRc and arterial signal saturation aspects, we found that the shortest investigated echo time was the superior one. We thus suggest that short echo times should be applied, taking technical limitations and clinical demands into consideration.     

Single-shot terahertz pulse characterization via two-dimensional electro-optic imaging with dual echelons

A single-shot measurement of terahertz electromagnetic pulses is implemented using two-dimensional electro-optic imaging with dual echelon optics. The reported embodiment produces sequentially delayed multiprobe beamlets, routinely providing a time window of >10 ps with ~25 fs temporal step sizes. Because of its simplicity and robustness, the technique is ideally suited for real-time ultrashort relativistic electron bunch characterization.     

Laser cooling and magnetic trapping at several tesla

Laser cooling and magnetic trapping of (85)Rb atoms have been performed in extremely strong and tunable magnetic fields, extending these techniques to a new regime and setting the stage for a variety of cold atom and plasma experiments. Using a superconducting Ioffe-Pritchard trap and an optical molasses, 2.4 x 10(7) atoms were laser cooled to the Doppler limit and magnetically trapped at bias fields up to 2.9 T. At magnetic fields up to 6 T, 3 x 10(6) cold atoms were laser cooled in a pulsed loading scheme. These bias fields are well beyond an order of magnitude larger than those in previous experiments. Loading rates, molasses lifetimes, magnetic-trapping times, and temperatures were measured using photoionization and electron detection.     

Single-shot imaging of ground-state hydrogen atoms with a nonlinear laser spectroscopic technique

We report on a novel nonlinear laser spectroscopic technique for single-shot imaging of ground-state hydrogen atoms in harsh environments. H atoms from the ground state were first pumped to the 2s state via a two-photon excitation with a 243 nm laser beam, and the population of the 2s state was further probed through the 2s-4p transition with conventional polarization spectroscopy at 486 nm. A single Nd:YAG-pumped optical parametric oscillator laser system was enough to provide both laser beams. Single-shot visualization of native H atoms in an atmospheric pressure premixed H(2)/O(2) flame is demonstrated.     

Single-shot and lensless complex-amplitude imaging with incoherent light based on machine learning

We present a single-shot incoherent light imaging method for simultaneously observing both amplitude and phase without any imaging optics, based on machine learning. In the proposed method, an object with a complex-amplitude field is illuminated with incoherent light and is captured by an image sensor with or without a coded aperture. The complex-amplitude field of the object is reconstructed from a single captured image using a state-of-the-art deep convolutional neural network, which is trained with a large number of input and output pairs. In experimental demonstrations, the proposed method was verified with a handwritten character database, and the effect of a coded aperture printed on an overhead projector film in the reconstruction was examined. Our method has advantages over conventional wavefront sensing techniques using incoherent light, namely simplification of the optical hardware and improved measurement speed. This study shows the importance and practical impact of machine learning techniques in various fields of optical sensing.     

MR imaging and cervical fixation devices: Evaluation of ferromagnetism,heating, and artifacts at 1.5 tesla

The purpose of this study was to assess ferromagnetism, heating, and artifacts for cervical fixation devices exposed to a 1.5 T MR system. Cervical fixation devices (three halos, one tong and two halo vests) were evaluated for compatibility with MR procedures. Ferromagnetism was determined using a previously described technique. Heating was evaluated by measuring temperatures at various positions on the cervical fixation devices while applied to a volunteer subject before and during the use of various pulse sequences, including an magnetization transfer contrast (MTC) sequence. Artifacts associated with routine clinical MR imaging of the cervical spine were qualitatively evaluated with the cervical fixation devices applied to a volunteer subject. None of the devices displayed attraction to the magnetic field. The temperature changes were ±1.5°C in each instance. The MTC pulse sequence produced a sensation of “heating” the skull pins that may have been caused by vibration of the cervical fixation device. The MR images of the cervical spine were obtained without apparent artifacts using each routine, clinical pulse sequence. The lack of ferromagnetism, negligible heating, and capability of obtaining diagnostically acceptable studies of the cervical spine indicate that MR imaging performed at 1.5 T or less may be conducted safely in patients with each of the cervical fixation devices tested using conventional pulse sequences.