Readout Bandwidths Research Articles

Accelerating Brain Imaging Using a Silent Spatial Encoding Axis.

PurposeTo characterize the acceleration capabilities of a silent head insert gradient axis that operates at the inaudible frequency of 20 kHz and a maximum gradient amplitude of 40 mT/m without inducing peripheral nerve stimulation.MethodsThe silent gradient axis' acquisitions feature an oscillating gradient in the phase‐encoding direction that is played out on top of a cartesian readout, similarly as done in Wave‐CAIPI. The additional spatial encoding fills k‐space in readout lanes allowing for the acquisition of fewer phase‐encoding steps without increasing aliasing artifacts. Fully sampled 2D gradient echo datasets were acquired both with and without the silent readout. All scans were retrospectively undersampled (acceleration factors R = 1 to 12) to compare conventional SENSE acceleration and acceleration using the silent gradient. The silent gradient amplitude and the readout bandwidth were varied to investigate the effect on artifacts and g‐factor.ResultsThe silent readout reduced the g‐factor for all acceleration factors when compared to SENSE acceleration. Increasing the silent gradient amplitude from 31.5 mT/m to 40 mT/m at an acceleration factor of 10 yielded a reduction in the average g‐factor (gavg) from 1.3 ± 0.14 (gmax = 1.9) to 1.1 ± 0.09 (gmax = 1.6). Furthermore, reducing the number of cycles increased the readout bandwidth and the g‐factor that reached gavg = 1.5 ± 0.16 for a readout bandwidth of 651 Hz/pixel and an acceleration factor of R = 8.ConclusionA silent gradient axis enables high acceleration factors up to R = 10 while maintaining a g‐factor close to unity (gavg = 1.1 and gmax = 1.6) and can be acquired with clinically relevant readout bandwidths.

RARE two-point Dixon with dual bandwidths.

To investigate the impact of dual readout bandwidths (dBW) in a dual echo fat/water acquisition and describe a dBW-rapid acquisition relaxation enhanced, or turbo spin echo sequence where the concept is used to improve SNR by removing dead times between refocusing pulses and avoiding redundant Chemical-shift encoded. Cramér-Rao bounds and Monte Carlo simulations were used to investigate a two-point fat/water model where the difference in bandwidths is incorporated. In vivo images were acquired at 1.5 and 3 T with the dBW-rapid acquisition relaxation enhanced, or turbo spin echo sequence. Typical bandwidth ratios were 1:2. SNR was compared with a single bandwidth sequence under identical scan parameters at 3T. Monte Carlo simulations and Cramér-Rao analysis demonstrate that number of signal averages can be improved with dual bandwidths compared to conventional single bandwidth acquisitions. The dBW-rapid acquisition relaxation enhanced, or turbo spin echo sequence can acquire images with high readout resolutions with well-conditioned sampling. An SNR improvement of 52% was measured, in line with the theoretical gain of 54%. The proposed dBW-rapid acquisition relaxation enhanced, or turbo spin echo sequence is a highly SNR-efficient two-point rapid acquisition relaxation enhanced, or turbo spin echo sequence without dead times, and can acquire images at higher resolutions than current vendor-supplied alternatives.

Optimized MR Simulation Protocol for Framed Stereotactic Radiosurgery at 3T

Magnetic resonance imaging is often the primary imaging modality for framed stereotactic radiosurgery (SRS). However, eddy currents, chemical shift, gradient nonlinearities, and the magnetic susceptibility distribution of the patient can all contribute to geometric distortions that may affect the accuracy of SRS if not properly controlled. The goal of this work was to establish an optimized MR simulation protocol for framed SRS at 3T, and assess the severity of any residual geometric distortions. Imaging was performed on a 3T scanner using an SRS phantom (CIRS, model 603A). A titanium Leksell head frame was secured to the phantom. The fiducial box was then affixed to the head frame. The combined, phantom, head frame, and fiducial box assembly were then immobilized in treatment position using a holder of our design. The combination of two flexible phase-array coils and the integrated spine RF coil was used for signal reception. Magnetic field mapping was performed using a dual echo GRE sequence. Different shimming modes (tune up, standard, standard + local shim, standard + brain shim, advanced, advanced + local shim, advanced + brain shim) were applied prior to acquisition of each field map. In addition, 3D T1 FLASH images were acquired with different gradient slew rate modes (whisper, normal, fast) and readout bandwidths. Vendor-provided 3D gradient nonlinearity distortion correction was applied to all images prior to them being sent to a commercial deformable registration algorithm system for analysis. Large magnetic field variations were observed with tune up and standard shim modes. The advanced + brain shim mode resulted in smallest magnetic field variations over the brain. Shimming over the brain did not affect the geometric accuracy of the fiducial markers. The low slew rate whisper gradient mode reduced eddy current-induced distortion by 1.5mm compared to normal gradient mode. Reducing the readout bandwidth from 980 Hz/pixel to 440 Hz/pixel reduced eddy current-induced distortion further by 2mm and increased SNR, while minimizing the severity of chemical shift. Eddy currents induced by the gradients in the titanium frame can result in geometric distortions of fiducial that exceed 1mm in regions of the fiducial box within 2cm of the SRS frame. We established an optimized MR simulation protocol for framed SRS at 3T. Our recommendations include: 1) use of low slew rate gradient modes to reduce eddy current induced distortions, 2) reduce readout bandwidths to 440 Hz/pixel as a tradeoff between eddy current induced distortions and water-fat shift, 3) scan with the prescription volume centered at isocenter, 4) perform advanced high-order shimming with shim volume localized to brain, 5) confirm A-P phase encode direction to avoid flow artifacts, 6) apply 3D gradient nonlinearity distortion correction prior to using images for treatment planning, 7) avoid registering images in treatment planning system using fiducials within 2cm of the SRS frame.

Zero-gradient-excitation ramped hybrid encoding (zGRF -RHE) sodium MRI.

Fast bi-exponential transverse signal decay compounds sodium image quality. This work aims at enhancing image characteristics using a special case of ramped hybrid encoding (RHE). Zero-gradient-excitation (zGRF )-RHE provides (1) gradient-free excitation for high flip angle, artifact-free excitation profiles and (2) gradient ramping during dead-time for the optimization of encoding time (tenc ) to reduce T2* signal decay influence during acquisition. Radial zGRF -RHE and standard radial UTE were investigated over a range of receiver bandwidths in simulations, phantom and in vivo brain experiments. Central k-space in zGRF -RHE was acquired through single point measurements at the minimum achievable TE. T2* blurring artifacts and image SNR and CNR were assessed. zGRF -RHE enabled 90° flip angle artifact-free excitation, whereas gradient pre-ramping provided greater tenc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (TRO ). Significant SNR improvements of up to 4.8% were observed for longer TRO . zGRF -RHE allows for artifact-free high flip angle excitation with time-efficient encoding improving on image characteristics. This hybrid encoding concept with gradient pre-ramping is trajectory independent and can be introduced in any center-out UTE trajectory design.

Development of diamond powder filled carbon fibre pipes

For the High Luminosity (HL) phase of the Large Hadron Collider (LHC) pixel detectors have to meet new and unprecedented challenges for electronics, read-out band widths and also mechanical support and cooling. Traditionally the mechanical support structures (staves) are built from carbon based materials, embedding a metallic cooling pipe through which the cooling liquid, typically CO2, is circulated. In this paper the development and use of a carbon pipe is discussed. Compared to the metallic pipes carbon pipes have potentially advantages like lightness, low radio activation and no CTE mismatch. Their disadvantage of lower transversal thermal conductivity can be mitigated by filling the fabric with diamond powder (DP). The paper reports about the production process of carbon pipes with DP filled epoxy and cyanate ester resin. It discusses that this process leads with reasonable efficiency to pipes that meet the requirements of pressure stability up to 200 bars and leak tightness of 10−6mbarLs-1 for use in HL-LHC staves. The remaining problems and an outlook on potential improvements and further work are presented.

Multicontrast multiecho FLASH MRI for targeting the subthalamic nucleus

The subthalamic nucleus (STN) is one of the most common stimulation targets for treating Parkinson's disease using deep brain stimulation (DBS). This procedure requires precise placement of the stimulating electrode. Common practice of DBS implantation utilizes microelectrode recording to locate the sites with the correct electrical response after an initial location estimate based on a universal human brain atlas that is linearly scaled to the patient's anatomy as seen on the preoperative images. However, this often results in prolonged surgical time and possible surgical complications since the small-sized STN is difficult to visualize on conventional magnetic resonance (MR) images and its intersubject variability is not sufficiently considered in the atlas customization. This paper proposes a multicontrast, multiecho MR imaging (MRI) method that directly delineates the STN and other basal ganglia structures through five co-registered image contrasts (T1-weighted navigation image, R2* map, susceptibility-weighted imaging (phase, magnitude and fusion image)) obtained within a clinically acceptable time. The image protocol was optimized through both simulation and in vivo experiments to obtain the best image quality. Taking advantage of the multiple echoes and high readout bandwidths, no interimage registration is required since all images are produced in one acquisition, and image distortion and chemical shift are reduced. This MRI protocol is expected to mitigate some of the shortcomings of the state-of-the-art DBS implantation methods.

Achieving sub electron noise in CCD systems by means of digital filtering techniques that lower 1/f pixel correlated noise

Scientific CCDs designed in thick high resistivity silicon (Si) are excellent detectors for astronomy, high energy and nuclear physics, and instrumentation. Many applications can benefit from CCDs ultra low noise readout systems. The present work shows how sub electron noise CCD images can be achieved using digital signal processing techniques. These techniques allow 0.4 electrons of noise at readout bandwidths of up to 10 Kpixels per second while keeping the full CCD spatial resolution and signal dynamic range.

Influence of selecting EPI readout-encoding bandwidths on arterial spin labeling perfusion MRI

The objective of this study was to investigate effects of varying readout bandwidths on the arterial spin labeling (ASL)-perfusion MRI measurements at a high magnetic field MRI system. Brain perfusion studies were performed on nine volunteers (four males, five females) using flow sensitive alternating inversion recovery (FAIR) ASL single-shot echo-planar imaging (EPI)-MRI. To investigate EPI bandwidth effects on the time-series perfusion-weighted imaging (PWI) data, two regions-of-interest (ROI) were placed outside the brain to determine the level of noise and another ROI inside the brain to determine the level of signal. Coefficients of variations (CoV) were calculated for the time-series PWI data. One-way analysis of variance (ANOVA) was used to investigate voxel-wise differences in the time-series PWI data between two different bandwidth values. At the level of ROI, there was no significant effect of changing EPI bandwidths on the time-series PWI data in any of the volunteers (P > 0.031). In contrast, CoV values over the dynamic PWI data varied with depending on selecting EPI bandwidths and voxel-based tests showed that N2 ghosting, modulated by EPI bandwidth, can appear in some brain regions, especially in areas that overlap with the spatial distribution of N2 ghosting artifacts. Although N2 ghosting can be reduced by adjusting the bandwidth of EPI on the time-series of PWI data, the effects cannot be entirely eliminated. In particular, N2 ghosting can bias CBF quantification if EPI control scans to determine the equilibrium-state signal are confounded by N2 ghosting. Therefore, careful tuning of the bandwidth of EPI is necessary to avoid artifacts in the ASL signal from N2-ghosting.