Slice Gradient Research Articles

Segmented K-space blipped-controlled aliasing in parallel imaging for high spatiotemporal resolution EPI.

A segmented k-space blipped-controlled aliasing in parallel imaging (skipped-CAIPI) sampling strategy for EPI is proposed, which allows for a flexible choice of EPI factor and phase encode bandwidth independent of the controlled aliasing in parallel imaging (CAIPI) sampling pattern. With previously proposed approaches, exactly two EPI trajectories were possible given a specific CAIPI pattern, either with slice gradient blips (blipped-CAIPI) or following a shot-selective CAIPI approach (higher resolution). Recently, interleaved multi-shot segmentation along shot-selective CAIPI trajectories has been applied for high-resolution anatomical imaging. For more flexibility and a broader range of applications, we propose segmentation along any blipped-CAIPI trajectory. Thus, all EPI factors and phase encode bandwidths available with traditional segmented EPI can be combined with controlled aliasing. Temporal SNR maps of moderate-to-high-resolution time series acquisitions at varying undersampling factors demonstrate beneficial sampling alternatives to blipped-CAIPI or shot-selective CAIPI. Rapid high-resolution scans furthermore demonstrate SNR-efficient and motion-robust structural imaging with almost arbitrary EPI factor and minimal noise penalty. Skipped-CAIPI sampling increases protocol flexibility for high spatiotemporal resolution EPI. In terms of SNR and efficiency, high-resolution functional or structural scans benefit vastly from a free choice of the CAIPI pattern. Even at moderate resolutions, the independence of sampling pattern, TE, and image matrix size is valuable for optimized functional protocol design. Although demonstrated with 3D-EPI, skipped-CAIPI is also applicable with simultaneous multislice EPI.

Gradient-controlled local Larmor adjustment (GC-LOLA) for simultaneous multislice bSSFP imaging with improved banding behavior.

Simultaneous multislice (SMS) accelerated balanced SSFP (bSSFP) imaging can be impaired by off-resonance effects, due to slice-specific alterations in the frequency response. In this work, we introduce gradient-controlled local Larmor adjustment as a means to restore the frequency response and to stabilize SMS-accelerated bSSFP imaging with respect to banding artifacts. Providing each simultaneously excited slice with an individual RF phase cycle in SMS-accelerated bSSFP imaging results in the sequence's frequency response being shifted slice-specifically along the off-resonance axis. The net available pass-band for imaging is effectively reduced, increasing the measurement's susceptibility toward B0 inhomogeneities. To overcome these issues, gradient-controlled local Larmor adjustment modifies the Larmor frequency locally and aligns the slice-specific frequency responses on resonance by (1) unbalancing the slice gradient by a small constant amount and (2) modifying the RF phase cycles homogeneously across all slices. The concept is investigated using simulations and phantom experiments and applied to SMS-accelerated bSSFP cine cardiovascular MR at 3 T. Phantom and in vivo measurements demonstrate the successful removal of banding artifacts and restoration of the bSSFP frequency response using gradient-controlled local Larmor adjustment. For large slice thicknesses and small slice distances, banding artifacts become slightly widened. Gradient-controlled local Larmor adjustment successfully restores the frequency response in SMS-accelerated bSSFP imaging without increasing the sequence's susceptibility toward eddy current effects. The concept facilitates combinations of the different SMS encoding concepts and provides a powerful way to actively control off-resonance effects in slice-accelerated bSSFP imaging.

Radiofrequency pulse design with numerical optimization in the Fourier domain.

A radiofrequency (RF) pulse design technique is presented that uses iterative constrained minimization to determine Fourier domain coefficients for an optimal time domain RF pulse. The design of new RF pulses is especially beneficial for field strengths of 7.0T and above, where challenges pertaining to specific absorption rate (SAR) are exacerbated. A pair of 90° and 180° spin-echo pulses was designed to lower SAR without the need for a variable slice gradient. The optimized pulses were deployed to a 7.0T human scanner to demonstrate a reduction in SAR while retaining signal-to-noise (SNR) ratio.

Simultaneous parallel inclined readout image technique

Sensitivity-encoded phase undersampling has been combined with simultaneous slice excitation to produce a parallel MRI method with a high volumetric acquisition acceleration factor without the need for auxiliary stepped field coils. Dual-slice excitation was produced by modulating both spin and gradient echo sequences at +/-6 kHz. Frequency aliasing of simultaneously excited slices was prevented by using an additional gradient applied along the slice axis during data acquisition. Data were acquired using a four-channel receiver array and x4 sensitivity encoding on a 1.5 T MR system. The simultaneous parallel inclined readout image technique has been successfully demonstrated in both phantoms and volunteers. A multiplicative image acquisition acceleration factor of up to x8 was achieved. Image SNR and resolution was dependent on the ratio of the readout gradient to the additional slice gradient. A ratio of approximately 2:1 produced acceptable image quality. Use of RF pulses with additional excitation bands should enable the technique to be extended to volumetric acquisition acceleration factors in the range of x16-24 without the SNR limitations of pure partially parallel phase reduction methods.

REPRESENTATION OF IMAGINED and EXECUTED SEQUENTIAL FINGER MOVEMENTS OF ADULTS POST STROKE and HEALTHY CONTROLS.

Purpose/Hypothesis: The purpose of this study was to describe the effects of brain injury on imagery ability and representation by examining executed and imagined complex finger movement sequences with each hand. We hypothesized that individuals post-stroke would exhibit similar patterns of brain activation for executed and imagined movements to healthy controls (HC) when using their ipsilesional hand but not their contra-lesional hand. Number of Subjects: Five individuals post stroke (at least one year post-stroke, right CVA with preserved motor function in the contralesional hand) and five age matched HC. Materials/Methods: Subjects were tested in two sessions with one week interval. In the first session imagery ability was examined using the Motor Imagery Questionnaire (MIQ) and chronometric break tests. Subjects practiced executed and imagined paced sequential finger movement sequences with each hand. Audio taped instructions were provided for practice of the tasks at home. During the second session prior to brain scanning accuracy of finger movements was confirmed. Data were acquired with a 3T Allegra MR scanner (Siemens) using an echo-planar imaging (EPI) gradient system. Functional studies (fMRI) were acquired using a 32 slice gradient echo EPI (TR=4000, TE=30, 64x64 matrix, FOV=22 cm); and a blocked paradigm, each consisting of 4 epochs of task performance alternating with visual attention. The four trials of hand movements were: executed ipsi-lesional, imagined ipsi-lesional, exe-cuted contra-lesional, imagined contra-lesional. MRI data were analyzed using SPM 99, with a corrected height (p=.05) threshold. Group analyses were performed to determine the volume of activation in pre-determined regions of interest. Results: MIQ scores did not differ significantly between groups. HC's pattern of activation during executed tasks by either hand included the contralateral primary sensory (1, 2, 3) motor (4) parietal (5, 7) and premotor (PMC) and supplementary motor areas (SMA) (6). Individuals post stroke pattern of brain activation when using their ipsilesional hand was similar to that of HC. For the stroke affected hand executed movements were bilaterally represented while imagined movements were lateralized to the brains' primary lesioned side. For all tasks performed by the individuals post-stroke there was greater activation (50–500%) of area six on the con-tralesional side of the brain. Conclusions: The pattern of activation observed for tasks by the stroke affected hand suggest that it was easier for individuals post-stroke to imagine rather than execute the movements. The increased activity of the pre-motor and supplementary motor areas of the contra-lesional side during all tasks for the stroke affected subjects suggests that there may be a compensatory shift in the tasks of planning and sequencing both imagined and executed sequential finger movements. Clinical Relevance: Mental imagery by individuals post-stroke may serve as a stimulus for plasticity of stroke affected side of the brain and possibly be a good tool for rehabilitation of motor function.

Passive tracking exploiting local signal conservation: the white marker phenomenon.

This article presents a novel approach to passive tracking of paramagnetic markers during endovascular interventions, exploiting positive contrast of the markers to their background, so-called "white marker tracking." The positive contrast results from dephasing of the background signal with a slice gradient, while near the marker the signal is conserved because a dipole field induced by the marker compensates the dephasing gradient. Theoretical investigation shows that a local gradient induced by the local dipole field will nearly always cancel the dephasing gradient somewhere, regardless of marker composition, gradient strength, orientation, and acquisition parameters. The actual appearance of the white marker is determined by the marker strength, echo-time, slice thickness, and gradient strength, as shown both theoretically and experimentally. The novel concept is demonstrated by tracking experiments in a flow phantom and in pig models and is shown to allow reliable and robust depiction of paramagnetic markers with positive contrast and significant suppression of the background signal.

Evaluation of carotid endarterectomy with sequential MR perfusion imaging: A preliminary 12-month follow up

AIM: The clinical benefit of carotid endarterectomy is partially determined by peri-operative mortality and morbidity. Post-operative abnormalities in cerebral perfusion may be a risk factor for cerebral haemorrhage, and may be estimated from Bolus Arrival Time (BAT) as demonstrated by MR perfusion imaging. We aimed to use MR perfusion imaging to determine the temporal extent of these changes. MATERIALS AND METHODS: A single slice gradient recalled echo sequence was employed in five patients who underwent carotid endarterectomy. Sequential studies were undertaken pre-operatively, 3–5 days post carotid endarterectomy, and additionally at 3, 6 and 12 months. RESULTS: Asymmetric BATs were demonstrated in 3/5 patients, changes occurring as late as 6 to 12 months after carotid endarterectomy. These changes were not associated with either clinical or conventional MR morphological complications. CONCLUSIONS: MR perfusion imaging is able to demonstrate changes in BAT characteristics for up to 12 months after carotid endarterectomy. The clinical significance and underlying cause of these changes, including any association with post carotid endarterectomy hyperaemia, remains unknown.

Elimination of artifacts in short echo time H MR spectroscopy of the frontal lobe.

Localized 1H spectra from the frontal part of the human brain are often distorted by "ghost" artifacts, especially with short echo time PRESS sequences. These artifacts are caused by insufficient dephasing of unwanted coherences involving magnetization in the sinuses and the mouth. Because the various unwanted coherences of a PRESS sequence are dephased differently, the order of the slice gradients has a pronounced effect on the spectral quality. For frontal voxels, the last slice gradient has to be applied in the axial direction. With this slice order, the major metabolites can be quantified with an interindividual variability of about 10%.

Diffusive Effect Induced by a Strong Magnetic Field Gradient in (1)H-NMR Micro-imaging of Plant Tissues.

A micro-imaging probe, the best planar resolution of which was 0.0125 mm × 0.0125 mm, was devised using an NMR spectrometer with a superconducting magnet (JEOL GSX-270 WB). A strong Z-axis magnetic field (slice) gradient was required to lessen slice thickness. Application of a strong slice gradient around the 180° refocus pulse of the spin-echo 2D-FT method induced extra-reduction of signal intensity beyond that expected. The problem which was considered to be due to a diffusive effect, was overcome by saving the slice gradient on application of the 180° refocus pulse. Using the probe, measuring conditions (combinations of parameters) for a young tomato fruit of several mm in diameter were examined. Reduction of spatial resolution (voxel) was limited by a decline of image quality and the minimum voxel suitable for observation of the sample was 0.05 mm × 0.05 mm planar resolution and 0.25 mm slice thickness corresponding to two or three plant cells vertically aligned. NMR micro-imaging can be used for studying cell conditions in plant materials of small sizes.

5271399 Three dimensional fourier transform, fast spin echo, black blood magnetic resonance angtography

Methods for producing an angiograph of a body portion are disclosed as including the steps of subjecting the body portion to electromagnetic energy so as to cause a response related to the magnetization of the body portion. Fast spin echo magnetic resonance measurements are performed on a three dimensional section of the body portion to generate three dimensional intensity information. The three dimensional intensity information is thereafter processed to construct an angiograph. In a preferred embodiment magnetic resonance data is produced by generating a slice gradient, generating a z axis slab selective 90° pulse, generating a z axis slab selective 180° pulse, generating a first phase encoding gradient pulse for a particular K y coordinate, generating a second phase encoding gradient pulse for a particular K z coordinate, and generating an x frequency encoding gradient and reading echo signal data from the body portion resulting from the 180° pulse. The method is repeated from the step of generating the 180° pulse, wherein the particular K z coordinate is incremented for each repetition. The method is thereafter repeated from the step of generating a 90° pulse, wherein the particular K y component is incremented after the K z coordinate has been incremented a prescribed number of times. Echo signal data read from the body portion is processed to produce an image.

Technical note: simultaneous multislice imaging with an additional slice gradient in the readout stage: an idea for multislice FLASH.

Conventional two-dimensional Fourier transform spin-echo multislice imaging can be performed in a time efficient manner by taking advantage of the relatively long repetition time (TR) compared with the sequence time (i.e. the time from excitation to end of signal acquisition), to observe other slices. Thus, given a sequence time of 1/n of TR, n slices can be observed with no time penalty. For some applications, however, e.g. cardiac imaging, it would be desirable to collect data for all slices of interest simultaneously and not sequentially. One method of simultaneous multislice imaging has previously been proposed (Muller, 1988). This method uses a multifrequency rf pulse to excite all the slices of interest simultaneously. Separation of the slices is then achieved by addition or subtraction of the time domain signals by altering the phase of the slice selection pulses. For n slices, the basic imaging sequence has to be repeated n times. We present here an alternative method to acquire multislice images simultaneously using an additional slice gradient at the readout stage to separate the slices. As a modification of the spin warp imaging method (Edelstein et al, 1988), the 2-D FT spin-echo (SE) imaging pulse sequence is widely used in magnetic resonance (MR) imaging. Three gradients are used in the sequence, one for frequency encoding, one for phase encoding, and one for slice selection. Usually the three gradients are orthogonal to each other and usually denoted as G x, G y and G z for the read, phase and slice gradients respectively.

Technical variables influencing the detection of acute deep vein thrombosis by magnetic resonance imaging

To establish which technical variables influence the detection of deep vein thrombosis by magnetic resonance imaging, 2 dogs, 5 normal volunteers and 17 patients were studied using a 1.5 T whole-body system. A sequential slice gradient echo acquisition (TR 25, TE 13, 0 = 30°, 2 NEX, flow compensation rephasing gradients) in the axial plane was found to be optimal for detecting venous thrombosis. Thus, when using appropriate technique, MRI may identify deep venous thrombosis accurately. It may also allow the diagnosis of conditions which may simulate venous thrombosis clinically since the most common of these, ruptured Baker's cyst, cellulitis, muscle tear, hematoma and external venous compression are all readily identified by MRI.

Low-power NMR volume selection by slicing z magnetization.

A novel method for reducing the rf power requirements of selective pulses employed for z magnetization slice inversion in localized NMR spectroscopy is presented. Following slice preparation by the use of frequency-incremented sinc pulses, the slice gradient is reduced, which narrows the frequency width of the slice. This allows the use of relatively low-power selective inversion pulses, an important consideration for in vivo applications.

RAPID--a new method for fast imaging using a single slice of Z-magnetization.

A single slice of z-magnetization is used to generate high-speed images. The slice selection step is achieved using two sin-sinc pulses. This procedure eliminates the need for continual slice gradient reversal, a requirement for the FLASH technique. The RAPID imaging method has less inherent T1 and/or T2 dependence than current fast imaging methods and is more sensitive.