Variable-rate Selective Excitation Research Articles

Open-source myocardial T1 mapping with simultaneous multi-slice acceleration: Combining an auto-calibrated blipped-bSSFP readout with VERSE-MB pulses.

Enabling fast and accessible myocardial T1 mapping is crucial for extending its clinical application. We introduce Open-MOLLI-SMS combining simultaneous multi-slice (SMS) with auto-calibration and variable-rate selective excitation (VERSE)-multiband pulses to obtain all slices in a fast single-shot T1 mapping sequence. Open-MOLLI-SMS was developed by integrating SMS with the open-source method Open-MOLLI previously implemented in Pulseq. Three methods were integrated for Open-MOLLI-SMS: (1) auto-calibration blip patterns to ensure consistency between the data and coil information; (2) a blipped-balanced SSFP (bSSFP) readout to induce controlled aliasing in parallel imaging shifts without disturbing the bSSFP frequency response; and (3) a VERSE-multiband pulse for minimizing the achievable TR and the specific absortion rate (SAR) impact of SMS. Two (SMS2) or three (SMS3) slices were excited simultaneously and encoded with an in-plane acceleration factor of 2. Experiments were performed in the International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology phantom and five healthy volunteers. Phantom results show accurate T1 estimates for reference values between 400 to 2200 ms. Artifacts were visible for Open-MOLLI-SMS3 but not replicated in vivo. In vivo Open-MOLLI-SMS (T1 SMS2 = 993 ± 10 ms; T1 SMS3 = 1031 ± 17 ms) provided similar values to mean T1 single-band Open-MOLLI estimates (T1 Open-MOLLI = 1005 ± 47 ms). Open-MOLLI-SMS2 provided the closest estimates to the reference. This proof-of-principle implementation study demonstrates the feasibility of speeding up T1 -mapping acquisitions and increasing coverage by combining auto-calibration strategies with a blipped-bSFFP readout and VERSE multiband RF excitation pulses. The proposed methodology was built on the Open-MOLLI mapping sequence, which provides a fast means for prototyping and enables open-source sharing of the method.

Evaluating of the Quality of Hepatic Diffusion Weighted Imaging Using Multiband Imaging With Variable-Rate Selective Excitation.

To assess the image quality of diffusion-weighted imaging (DWI) using multiband (MB) imaging with variable-rate selective excitation (VERSE) and compare it to conventional DWI. We retrospectively evaluated hepatic DWI images of patients (n = 76) according to either the conventional method (SENSE, acceleration factor = 2) (n = 38) or fast scanning method (MB imaging with VERSE, acceleration factor = 2 × 2) (n = 38). We also conducted a volunteer study (n = 15) for those scanning methods. During quantitative analysis, the signal-to-noise ratio (SNR), apparent diffusion coefficient values, and contrast in the liver, spleen, and spinal cord were compared between the 2 groups. During qualitative analysis, all images were independently and blindly evaluated by 2 board-certified radiologists. The image contrast, noise, artifacts, and sharpness were assessed, and the performance of classification was measured using receiver operating characteristic curve analysis. In the retrospective study, the SNRs of the hepatic parenchyma and spinal cord between the 2 protocols were significantly different (liver, 8.9 [interquartile range {IQR}, 7.6-12.2] vs 13.0 [IQR, 10.0-16.7]; P < 0.001 and spinal cord, 6.0 [IQR, 4.7-9.4] vs 4.3 [IQR, 3.8-6.8]; P < 0.02). No significant differences between the 2 protocols in the other retrospective analyses were noted. In the receiver operating characteristic curve analysis, area under the curve was 0.49 (95% confidence intervals, 0.40-0.58). Multiband VERSE reduced scan time and SNR of hepatic DWI; however, subjective image quality parameters were not significantly impacted.

Altered saturation pulse with a high flip angle for venous saturation on 7\xa0T time-of-flight magnetic resonance angiography

This study aimed to apply minimum-time variable-rate selective excitation (MinVER) to a presaturation pulse (PSP) with a high flip angle on 7 T time-of-flight magnetic resonance angiography (7T TOF-MRA), to attain a superior vessel-to-tissue contrast (VTCR), short acquisition time, and minor off-resonance effect. An altered PSP modified by using the 90° flip angle (FA)-MinVER was implemented in the 7 T TOF-MRA, and its performance was evaluated with a signal profile and vessel-tissue contrast ratios and compared to the conventional PSP and 45 FA-TOF. The 90 FA-MinVER showed a similar signal profile to that of the conventional PSP and improved the vessel-tissue contrast ratios (0.313 ± 0.80) compared to all conventional types (45 FA-TOF: 0.088 ± 0.84, 90 FA-TOF: 0.203 ± 0.72). Moreover, this noteworthy approach achieved substantially reduced total acquisition time (5 min and 55 s) with a short repeat-to-time (28 ms), indicating that at the 7 T TOF-MRA, the 90 FA-MinVER could be applied by default to suppress the venous signals regardless of individual human status and the specific absorption ratio constraint and with rapid imaging. Ultimately, its application could also help to observe subtle microvascular changes in the early stages and serve as key biomarkers in various vascular diseases.

Hybrid -shimming and gradient adaptions for improved pseudo-continuous arterial spin labeling at 7 Tesla.

To improve pseudo-continuous arterial spin labeling (pcASL) at 7T by exploiting a hybrid homogeneity- and efficiency-optimized -shim with adapted gradient strength as well as background suppression. The following three experiments were performed at 7T, each employing five volunteers: (1) A hybrid (ie, homogeneity-efficiency optimized) -shim was introduced and evaluated for variable-rate selective excitation pcASL labeling. Therefore, -maps in the V3 segment and time-of-flight images were acquired to identify the feeding arteries. For validation, a gradient-echo sequence was applied in circular polarized (CP) mode and with the hybrid -shim. Additionally, the gray matter (temporal) signal-to-noise ratio (tSNR) in pcASL perfusion images was evaluated. (2) Bloch simulations for the pcASL labeling were conducted and validated experimentally, with a focus on the slice-selective gradients. (3) Background suppression was added to the -shimmed, gradient-adapted 7T sequence and this was then compared to a matched sequence at 3T. The -shim improved the signal within the labeling plane (23.6%) and the SNR/tSNR increased (+11%/+11%) compared to its value in CP mode; however, the increase was not significant. In accordance with the simulations, the adapted gradients increased the tSNR (35%) and SNR (45%) significantly. Background suppression further improved the perfusion images at 7T, and this protocol performed as well as a resolution-matched protocol at 3T. The combination of the proposed hybrid -phase-shim with the adapted slice-selective gradients and background suppression shows great potential for improved pcASL labeling under suboptimal conditions at 7T.

Minimum TR radiofrequency-pulse design for rapid gradient echo sequences.

A framework to design radiofrequency (RF) pulses specifically to minimize the TR of gradient echo sequences is presented, subject to hardware and physiological constraints. Single-band and multiband (MB) RF pulses can be reduced in duration using variable-rate selective excitation (VERSE) VERSE for a range of flip angles; however, minimum-duration pulses do not guarantee minimum TR because these can lead to a high specific absorption rate (SAR). The optimal RF pulse is found by meeting spatial encoding, peripheral nerve stimulation (PNS) and SAR constraints. A TR reduction for a range of designs is achieved and an application of this in an MB cardiac balanced steady-state free-precession (bSSFP) experiment is presented. Gradient imperfections and their imaging effects are also considered. Sequence TR with low-time bandwidth product (TBP) pulses, as used in bSSFP, was reduced up to 14%, and the TR when using high TBP pulses, as used in slab-selective imaging, was reduced by up to 72%. A breath-hold cardiac exam was reduced by 46% using both MB and the TR-optimal framework. The importance of RF-based correction of gradient imperfections is demonstrated. PNS was not a practical limitation. The TR-optimal framework designs RF pulses for a range of pulse parameters, specifically to minimize sequence TR.

Improving PCASL at ultra-high field using a VERSE-guided parallel transmission strategy.

PurposeTo improve the labeling efficiency of pseudo‐continuous arterial spin labeling (PCASL) at 7T using parallel transmission (pTx).MethodsFive healthy subjects were scanned on an 8‐channel‐transmit 7T human MRI scanner. Time‐of‐flight (TOF) angiography was acquired to identify regions of interest (ROIs) around the 4 major feeding arteries to the brain, and B1+ and B0 maps were acquired in the labeling plane for tagging pulse design. Complex weights of the labeling pulses for each of the 8 transmit channels were calculated to produce a homogenous radiofrequency (RF) ‐shimmed labeling across the ROIs. Variable‐Rate Selective Excitation (VERSE) pulses were also implemented as a part of the labeling pulse train. Whole‐brain perfusion‐weighted images were acquired under conditions of RF shimming, VERSE with RF shimming, and standard circularly polarized (CP) mode. The same subjects were scanned on a 3T scanner for comparison.ResultsIn simulation, VERSE with RF shimming improved the flip‐angles across the ROIs in the labeling plane by 90% compared with CP mode. VERSE with RF shimming improved the temporal signal‐to‐noise ratio by 375% compared with CP mode, but did not outperform a matched 3T sequence with a matched flip‐angle.ConclusionWe have demonstrated improved PCASL tagging at 7T using VERSE with RF shimming on a commercial head coil under conservative SAR limits at 7T. However, improvements of 7T over 3T may require strategies with less conservative SAR restrictions.

Using variable-rate selective excitation (VERSE) radiofrequency pulses to reduce power deposition in pulsed arterial spin labeling sequence at 7 Tesla.

Arterial spin labeling (ASL) is an established noninvasive MRI technique used for cerebral blood flow measurement, which generally suffers from a low signal-to-noise ratio (SNR). The use of ultra-high fields to enhance sensitivity inevitably results in an increase in TR because of specific absorption rate (SAR) constraints, causing inadequate sampling of hemodynamic response in functional MRI, and adversely affecting concurrent measurement such as blood oxygen level dependent. To address this problem, variable-rate selective excitation (VERSE) radiofrequency (RF) pulses were used. The conventional (sinc) selective RF pulses of the Q2TIPS block in the PICORE pulsed ASL (PASL) sequence used for blood saturation were replaced by their equivalent VERSE RF waveforms. Nine healthy volunteers were scanned using the conventional and VERSE PASL sequences on a head-only 7T scanner operating in parallel transmit mode. VERSE PASL sequence provides perfusion images similar to the conventional version, with comparable perfusion SNR (conventional, 3.33 ± 0.48; VERSE, 3.26 ± 0.55) and temporal SNR (conventional, 1.02 ± 0.20; VERSE, 1.05 ± 0.12) for TR = 3.5 seconds and 70 measurements. With shorter acquisition time (TR = 2.5 seconds), VERSE PASL sequence still provides similar results to those acquired using the conventional PASL sequence with longer TR = 3.5 seconds. The use of VERSE RF pulses in the Q2TIPS block of a PASL sequence allowed for the reduction of RF power deposition and, consequently, an increase in the temporal resolution and/or perfusion SNR.

High resolution time-of-flight MR-angiography at 7 T exploiting VERSE saturation, compressed sensing and segmentation

Background3D Time-of-Flight (TOF) MR-angiography (MRA) substantially benefits from ultra-high magnetic field strengths (≥7 T) due to increased Signal-to-Noise ratio and improved contrast. However, high-resolution TOF-MRA usually requires long acquisition times. In addition, specific absorption rate constraints limit the choice of optimal pulse sequence parameters, especially if venous saturation is employed. PurposeTo implement and evaluate an arterial TOF-MRA for accelerated high-resolution angiography at ultra-high magnetic field strength. Field strengths/sequence7 T modified gradient-echo TOF sequence including venous saturation using Variable-Rate Selective Excitation (VERSE), Compressed Sensing (CS) and sparse application of saturation pulses, called segmentation, were included for acceleration. AssessmentTo analyze the acceleration techniques all volunteers were examined with the same protocols. CS with different sampling patterns and regularization factors as well as segmentation were applied for acceleration. For comparison, conventional acceleration techniques were applied (GRAPPA PAT 3 and Partial Fourier (6/8 in slice/phase encoding)). Images were co-registered and 40 mm thick transversal maximum intensity projections were created to calculate the relative number of vessels. To analyze the visibility of small vessels, the lenticulostriate arteries (LSA) were examined. This was done via multiscale vessel enhancement filtering in a VOI and quantification via Fiji ImageJ as well as qualitatively evaluation by two radiologists. Additionally, the venous/arterial vessel-to-background ratios (vVBR/aVBR) were calculated for chosen protocols. ResultsFor the acceleration of a high resolution TOF-MRA (0.31 mm isotropic), under-sampling of 9.6 showed aliasing artifacts, whereas 7.2 showed no aliasing. The regularization factor R had a strong impact on the image quality according to smoothing (R = 0.01 to R = 0.005) and noise (R = 0.0005 to R = 0.00005). With the alternating sampling patterns it was shown that the k-space center should not be under-sampled too much. Additionally segmentation could be verified to be feasible for stronger acceleration with sufficient venous suppression. ConclusionThe combination of several independent techniques (VERSE, CS with acceleration factor 7.2, R = 0.001, Poisson disc radius of 80%, 3 segments) enables the application of high-resolution (0.31 mm isotropic) TOF-MRA with venous saturation at 7 T in clinical time settings (TA ≈ 5 min) and within the SAR limits.

Time-optimized 4D phase contrast MRI with real-time convex optimization of gradient waveforms and fast excitation methods.

To shorten 4D flow acquisitions by shortening TRs with fast RF pulses and gradient waveforms. Real-time convex optimization is used to generate these gradients waveforms on the scanner. RF and slab-select waveforms were shortened with a minimum phase SLR excitation and the time-optimal variable-rate selective excitation method. Real-time convex optimization was used to shorten bipolar and spoiler gradients by finding the shortest gradient waveforms that satisfied constraints on scan parameters, gradient hardware, M0 , M1 , and peripheral nerve stimulation. Waveforms were calculated and TE and/or TR values were compared for a range of scan parameters and compared to a conventional 4D flow sequence. The method was tested in flow phantoms, and in the aorta and neurovasculature of volunteers (N = 10). Additionally, eddy current error was measured in a large phantom. TEs and TRs were shortened by 21-32% and 20-34%, respectively, compared to the conventional sequence over a range of scan parameters. Bland-Altman analysis of 2 flow phantom configurations showed flow rate bias of 0.3 mL/s and limits of agreement (LOA) of [-6.9, 7.5] mL/s for a cardiac phantom and a bias of -0.1 mL/s with LOA = [-0.4, 0.2] mL/s for a neuro phantom. Similar agreement was also seen for flow measurements in volunteers (bias = -1.0 and -0.1 mL/s, LOA = [-34.9, 33.0] and [-0.7, 0.6] mL/s). Measured eddy currents were 39% larger with the CVX + mpVERSE method. The real-time optimized 4D flow gradients and fast slab-selection excitation methods produced up to 34% faster TRs with excellent flow measurement agreement compared to a conventional 4D flow sequence.

Squeezed Trajectory Design for Peak RF and Integrated RF Power Reduction in Parallel Transmission MRI.

High peak RF amplitude and excessive specific absorption rate (SAR) are two critical concerns for hardware implementation and patient safety in scientific and clinical research for high field MRI using parallel transmissions (pTX). In this paper, we introduce a squeezing strategy to reduce peak RF amplitude and integrated RF power via direct reshaping of the k-space trajectory. In the existing peak RF / integrated RF power optimization methods gradient amplitude or slew rate is reduced, but the k-space trajectory remains unchanged. Unlike these traditional methods, we worked directly in the excitation k-space to reshape k-space traversal by a squeezing vector in order to achieve peak RF and total RF power optimization, using a particle swarm optimization algorithm. The squeezing strategy was applied to the conventional variable density spiral (CVDS) and the variable rate selective excitation (VERSE) trajectories, dubbed SVDS (squeezed variable density spiral) and SVERSE (squeezing trajectory with VERSE), respectively, for different excitation profiles of small or large tip angles. Pulse acceleration and off-resonance effects were evaluated for an 8-ch pTX via Bloch simulation. CVDS, VERSE, SVDS, and SVERSE pulses were implemented on a 3T scanner with a 2-ch pTX. Phantom and in vivo experiments were performed for reduced FOV (rFOV) imaging. The results show that SVDS pulses simultaneously reduce integrated RF power and peak RF by about 30% on average compared to CVDS pulses for a square pattern ( $80\times80$ mm2) with flip angles of 30°, 90°, and 180°. Compared with the VERSE method under the same peak RF constraints, the SVDS method reduces integrated RF power by an average of 20% for small tip excitations for profiles of slice, rectangular, square, and circle, and has slightly reduced excitation accuracy slightly (about 0.6%, from 6.8% to 7.4%). The SVERSE method shortens the duration of the VERSE pulse by 12.8% at large ti p angle (180°). Feasibility for rFOV imaging was demonstrated with phantom and in vivo experiments with squeezed pulses.

VERSE-guided parallel RF excitations using dynamic field correction.

In parallel RF pulse design, peak RF magnitudes and specific absorption rate levels are critical concerns in the hardware and safety limits. The variable rate selective excitation (VERSE) method is an efficient technique to limit the peak RF power by applying a local‐only RF and gradient waveform reshaping while retaining the on‐resonance profile. The accuracy of the excitation performed by the VERSEd RF and gradient waveforms strictly depends on the performance of the employed hardware. Any deviation from the nominal gradient fields as a result of frequency dependent system imperfections violates the VERSE condition similarly to off‐resonance effects, leading to significant excitation errors and the RF pulse not converging to the targeted peak RF power. Moreover, for iterative VERSE‐guided RF pulse design (i.e. reVERSE), the k‐space trajectory actually changes at every iteration, which is assumed to be constant. In this work, we show both theoretically and experimentally the effect of gradient system imperfections on iteratively VERSEd parallel RF excitations. In order to improve the excitation accuracy besides limiting the RF power below certain thresholds, we propose to integrate gradient field monitoring or gradient impulse response function (GIRF) estimations of the actual gradient fields into the RF pulse design problem. A third‐order dynamic field camera comprising a set of NMR field sensors and GIRFs was used to measure or estimate the actual gradient waveforms that are involved in the VERSE algorithm respectively. The deviating and variable k‐space is counteracted at each iteration of the VERSE‐guided iterative RF pulse design. The proposed approaches are demonstrated for accelerated multiple‐channel spatially selective RF pulses, and highly improved experimental performance was achieved at both 3 T and 7 T.

Advancing RF pulse design using an open-competition format: Report from the 2015 ISMRM challenge.

To advance the best solutions to two important RF pulse design problems with an open head-to-head competition. Two sub-challenges were formulated in which contestants competed to design the shortest simultaneous multislice (SMS) refocusing pulses and slice-selective parallel transmission (pTx) excitation pulses, subject to realistic hardware and safety constraints. Short refocusing pulses are needed for spin echo SMS imaging at high multiband factors, and short slice-selective pTx pulses are needed for multislice imaging in ultra-high field MRI. Each sub-challenge comprised two phases, in which the first phase posed problems with a low barrier of entry, and the second phase encouraged solutions that performed well in general. The Challenge ran from October 2015 to May 2016. The pTx Challenge winners developed a spokes pulse design method that combined variable-rate selective excitation with an efficient method to enforce SAR constraints, which achieved 10.6 times shorter pulse durations than conventional approaches. The SMS Challenge winners developed a time-optimal control multiband pulse design algorithm that achieved 5.1 times shorter pulse durations than conventional approaches. The Challenge led to rapid step improvements in solutions to significant problems in RF excitation for SMS imaging and ultra-high field MRI. Magn Reson Med 78:1352-1361, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Improved cerebral time-of-flight magnetic resonance angiography at 7 Tesla--feasibility study and preliminary results using optimized venous saturation pulses.

PurposeConventional saturation pulses cannot be used for 7 Tesla ultra-high-resolution time-of-flight magnetic resonance angiography (TOF MRA) due to specific absorption rate (SAR) limitations. We overcome these limitations by utilizing low flip angle, variable rate selective excitation (VERSE) algorithm saturation pulses.Material and MethodsTwenty-five neurosurgical patients (male n = 8, female n = 17; average age 49.64 years; range 26–70 years) with different intracranial vascular pathologies were enrolled in this trial. All patients were examined with a 7 Tesla (Magnetom 7 T, Siemens) whole body scanner system utilizing a dedicated 32-channel head coil. For venous saturation pulses a 35° flip angle was applied. Two neuroradiologists evaluated the delineation of arterial vessels in the Circle of Willis, delineation of vascular pathologies, presence of artifacts, vessel-tissue contrast and overall image quality of TOF MRA scans in consensus on a five-point scale. Normalized signal intensities in the confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter were measured and vessel-tissue contrasts were calculated.ResultsRatings for the majority of patients ranged between good and excellent for most of the evaluated features. Venous saturation was sufficient for all cases with minor artifacts in arteriovenous malformations and arteriovenous fistulas. Quantitative signal intensity measurements showed high vessel-tissue contrast for confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter.ConclusionThe use of novel low flip angle VERSE algorithm pulses for saturation of venous vessels can overcome SAR limitations in 7 Tesla ultra-high-resolution TOF MRA. Our protocol is suitable for clinical application with excellent image quality for delineation of various intracranial vascular pathologies.

Hadamard slice encoding for reduced‐FOV diffusion‐weighted imaging

To improve the clinical utility of diffusion-weighted imaging (DWI) by extending the slice coverage of a high-resolution reduced field-of-view technique. Challenges in achieving high spatial resolution restrict the use of DWI in assessment of small structures such as the spinal cord. A reduced field-of-view method with 2D echo-planar radiofrequency (RF) excitation was recently proposed for high-resolution DWI. Here, a Hadamard slice-encoding scheme is proposed to double the slice coverage by exploiting the periodicity of the 2D echo-planar RF excitation profile. A 2D echo-planar RF pulse and matching multiband refocusing RF pulses were designed using the Shinnar-Le Roux algorithm to reduce band interference, and variable-rate selective excitation to shorten the pulse durations. Hadamard-encoded images were resolved through a phase-preserving image reconstruction. The performance of the method was evaluated via simulations, phantom experiments, and in vivo high-resolution axial DWI of spinal cord. The proposed scheme successfully extends the slice coverage, while preserving the sharp excitation profile and the reliable fat suppression of the original method. For in vivo axial DWI of the spinal cord, an in-plane resolution of 0.7 × 0.7 mm(2) was achieved with 16 slices. The proposed Hadamard slice-encoding scheme doubles the slice coverage of the 2D echo-planar RF reduced field-of-view method without any scan-time penalty.

Systematic Regional Variations of GABA, Glutamine, and Glutamate Concentrations Follow Receptor Fingerprints of Human Cingulate Cortex

Magnetic resonance spectroscopy (MRS) of glutamatergic or GABAergic measures in anterior cingulate cortex (ACC) was found altered in psychiatric disorders and predictive of interindividual variations of functional responses in healthy populations. Several ACC subregions have been parcellated into receptor-architectonically different portions with heterogeneous fingerprints for excitatory and inhibitory receptors. Similarly, these subregions overlap with functionally distinct regions showing opposed signal changes toward stimulation or resting conditions. We therefore investigated whether receptor-architectonical and functional segregation of the cingulate cortex in humans was also reflected in its local concentrations of glutamate (Glu), glutamine (Gln), and GABA. To accomplish a multiregion estimation of all three metabolites in one robust and reliable session, we used an optimized 7T-stimulated echo-acquisition mode method with variable-rate selective excitation pulses. Our results demonstrated that, ensuring high data retest reliability, four cingulate subregions discerning e.g., pregenual ACC (pgACC) from anterior mid-cingulate cortex showed different metabolite concentrations and ratios reflective of regionally specific inhibition/excitation balance. These findings could be controlled for potential influences of local gray matter variations or MRS voxel-placement deviations. Pregenual ACC was found to have significantly higher GABA and Glu concentrations than other regions. This pattern was not paralleled by Gln concentrations, which for both absolute and relative values showed a rostrocaudal gradient with highest values in pgACC. Increased excitatory Glu and inhibitory GABA in pgACC were shown to follow a regional segregation agreeing with recently shown receptor-architectonic GABAB receptor distribution in ACC, whereas Gln distribution followed a pattern of AMPA receptors.

Magnetic resonance ultrashort echo time spin-echo imaging of the deepest layers of articular cartilage

Magnetic resonance imaging (MRI) has emerged as an invasive radiologic technique to assess and characterize cartilage lesions in the setting of injury and degenerative joint disease. However, most of the currently available clinical and research MRI techniques, including proton-density weighted fast spin echo (FSE) [1], T2-weighted FSE [2], T2 mapping [3], and steady state free precession imaging [4] have focused on the superficial layers of cartilage. There is a growing interest in the deepest layers of articular cartilage, including the calcified and deep radial layers located just superficial to the subchondral bone [5,6]. There has been an emphasis on the role of lesions in the deep radial and calcified layers of cartilage in the pathogenesis of osteoarthritis [6,7]. The lack of imaging evaluation of the deep layers of cartilage stems largely from the fact that it is technically difficult to image. Conventional MRI pulse sequences with echo times (TE) of 1 ms or greater provide little or no detectable signal from many tissues and tissue components that have very short T2 relaxation times, such as calcified cartilage, menisci, tendons, ligaments and some forms of soft tissue calcification [8]. By using half-sinc radiofrequency (RF) pulses, radial mapping of k-space, rapid transmit/receive switching, and variable rate selective excitation, nominal TEs as short as 8 μs have been achieved with ultrashort TE (UTE) imaging [9,10]. Therefore, the UTE pulse sequences make it possible to directly image tissues with very short T2 and their adjacent tissues [11]. Although short T2 tissues are detectable with UTE sequences, positive visualization of deep layers of cartilage is limited by the presence of high signals from long T2 water and fat. The dual-echo gradient-echo UTE acquisition has been used previously to suppress long T2 signals in knee cartilage two-dimensional (2D) imaging [8]. With this approach, the second echo is subtracted from the first one, which is equivalent to band pass filtering. This selectively suppresses the signal from the longer T2 components, and typically provides high contrast in the short T2 range. However, the 2D UTE free induction decay (FID) acquisition is sensitive to eddy currents, gradient anisotropy and timing errors [12]. The latter echoes in this acquisition are sensitive to off-resonance effects. Therefore, we developed a dual-echo spin echo UTE acquisition approach. This letter describes the development and implementation of dual-echo spin-echo UTE sequences, and testing its relative efficacy in terms of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in imaging deep radial and calcified layers of articular cartilage.

Two-dimensional radial sodium heart MRI using variable-rate selective excitation and retrospective electrocardiogram gating with golden angle increments

Two-dimensional projection reconstruction methods provide advantages over three-dimensional techniques because of higher flexibility regarding the resolution and shorter scan time needed. To optimize a two-dimensional radial sequence with respect to signal-to-noise ratio, variable-rate selective excitation and retrospective electrocardiogram gating is investigated. The minimal radiofrequency pulse duration is simulated in dependence of the flip angle and coil parameters using sinc waveforms with two different variable-rate selective excitation approaches and a Fermi pulse. Retrospectively electrocardiogram-gated imaging with Golden Angle incremented projections was implemented to allow for continuous data acquisition enabling the possibility of dynamic electrocardiogram-gated heart imaging. Especially for abdominal coils with high transmitter voltages required, variable-rate selective excitation strongly reduces the radiofrequency pulse duration and echo time resulting in a signal-to-noise ratio gain up to 15.5% (if the fast relaxation component of sodium is in the order of the radiofrequency pulse duration) compared with standard sinc-shaped radiofrequency pulses. Retrospective electrocardiogram gating shows higher flexibility with regard to the trigger delay enabling the trade-off between heart motion artifacts and signal-to-noise ratio. A two-dimensional radial sequence is optimized for sodium heart imaging regarding signal-to-noise ratio. Different sodium contrasts of the human heart are shown, which can give additional information on heart diseases.

Specific absorption rate reduction using nonlinear gradient fields

The specific absorption rate is used as one of the main safety parameters in magnetic resonance imaging. The performance of imaging sequences is frequently hampered by the limitations imposed on the specific absorption rate that increase in severity at higher field strengths. The most well-known approach to reducing the specific absorption rate is presumably the variable rate selective excitation technique, which modifies the gradient waveforms in time. In this article, an alternative approach is introduced that uses gradient fields with nonlinear variations in space to reduce the specific absorption rate. The effect of such gradient fields on the relationship between the desired excitation profile and the corresponding radiofrequency pulse is shown. The feasibility of the method is demonstrated using three examples of radiofrequency pulse design. Finally, the proposed method is compared with and combined with the variable rate selective excitation technique.

Time-of-Flight Magnetic Resonance Angiography at 7 T Using Venous Saturation Pulses With Reduced Flip Angles

The visibility of the vasculature in time-of-flight (TOF) magnetic resonance angiography (MRA) highly profits from increased magnetic field strengths. However, the application of additional saturation pulses for suppression of the venous system is often not possible at 7 T; to remain within the regulatory specific absorption rate (SAR) limits, the repetition time (TR) needs to be prolonged, preventing the acquisition of high-resolution MRA data sets within clinically acceptable acquisition times. In this work, saturation pulses were modified regarding flip angle and duration to meet SAR constraints and minimize total measurement time. To ameliorate SAR restrictions, the variable-rate selective excitation (VERSE) algorithm was used for both excitation and saturation radio frequency pulses. In this way, saturation pulses (executed every TR) become applicable in high-resolution TOF MRA protocols but still lengthen total measurement time notably. In this work, saturation pulses were further modified in terms of flip angle and duration to meet SAR constraints and minimize total measurement time. In the considered parameter range for excitation flip angle α of 15° to 35° and TR of 20 ms to 35 ms, sufficient saturation flip angles (αSAT) were 30° to 50°. This work shows that by lowering the flip angle αSAT, saturation pulses can be applied in high-resolution clinical TOF protocols using a TR as short as 20 ms. An αSAT of α + 15° is sufficient for suppression of the venous system in TOF MRA protocols in the parameter range normally used at 7 T. Instead of the standard 90° saturation pulse, only half the flip angle (or even less) is necessary, substantially ameliorating SAR constraints and enabling acquisition of high resolution in acceptable imaging time.

Contrast enhancement in TOF cerebral angiography at 7 T using saturation and MT pulses under SAR constraints: Impact of VERSE and sparse pulses

Cerebral three-dimensional time of flight (TOF) angiography significantly benefits from ultrahigh fields, mainly due to higher signal-to-noise ratio and to longer T(1) relaxation time of static brain tissues; however, specific absorption rate (SAR) significantly increases with B(0). Thus, additional radiofrequency pulses commonly used at lower field strengths to improve TOF contrast such as saturation of venous signal and improved background suppression by magnetization transfer typically cannot be used at higher fields. In this work, we aimed at reducing SAR for each radiofrequency pulse category in a TOF sequence. We use the variable-rate selective excitation principle for the slab selective TOF excitation as well as the venous saturation radiofrequency pulses. In addition, magnetization transfer pulses are implemented by sparsely applying the pulses only during acquisition of the central k-space lines to limit their SAR contribution. Image quality, angiographic contrast, and SAR reduction were investigated as a function of variable-rate selective excitation parameters and of the total number of magnetization transfer pulses applied. Based on these results, a TOF protocol was generated that increases the angiographic contrast by more than 50% and reduces subcutaneous fat signal while keeping the resulting SAR within regulatory limits.