Adiabatic radiofrequency pulse forms in biomedical nuclear magnetic resonance

B 1 + inhomogeneity mitigation for diffusion weighted MRI at 7T using TR-FOCI pulses

PURPOSE

The purpose of this study is to improve the image quality of diffusion-weighted images obtained with a single RF transmit channel 7 T MRI setup using time-resampled frequency-offset corrected inversion (TR-FOCI) pulses to refocus the spins in a twice-refocused spin-echo readout scheme.

METHODS

We replaced the conventional Shinnar-Le Roux-pulses in the twice refocused diffusion sequence with TR-FOCI pulses. The slice profiles were evaluated in simulation and experimentally in phantoms. The image quality was evaluated in vivo comparing the Shinnar-Le Roux and TR-FOCI implementation using a b value of 0 and of 1000 s/mm2.

RESULTS

The b0 and diffusion-weighted images acquired using the modified sequence improved the image quality across the whole brain. A region of interest-based analysis showed an SNR increase of 113% and 66% for the nondiffusion-weighted (b0) and the diffusion-weighted (b = 1000 s/mm2) images in the temporal lobes, respectively. Investigation of all slices showed that the adiabatic pulses mitigatedB 1 + $$ {B}_1^{+} $$ inhomogeneity globally using a conventional single-channel transmission setup.

CONCLUSION

The TR-FOCI pulse can be used in a twice-refocused spin-echo diffusion pulse sequence to mitigate the impact ofB 1 + $$ {B}_1^{+} $$ inhomogeneity on the signal intensity across the brain at 7 T. However, further work is needed to address SAR limitations.

Rapid 3D absolute B1 + mapping using a sandwiched train presaturated TurboFLASH sequence at 7 T for the brain and heart

PURPOSE

To shorten the acquisition time of magnetization-prepared absolute transmit field (B₁ ⁺ ) mapping known as presaturation TurboFLASH, or satTFL, to enable single breath-hold whole-heart 3D B₁ ⁺ mapping.

METHODS

SatTFL is modified to remove the delay between the reference and prepared images (typically 5 T₁ ), with matching transmit configurations for excitation and preparation RF pulses. The new method, called Sandwich, is evaluated as a 3D sequence, measuring whole-brain and gated whole-heart B₁ ⁺ maps in a single breath-hold. We evaluate the sensitivity to B₁ ⁺ and T₁ using numerical Bloch, extended phase graph, and Monte Carlo simulations. Phantom and in vivo images were acquired in both the brain and heart using an 8-channel transmit 7 Tesla MRI system to support the simulations. A segmented satTFL with a short readout train was used as a reference.

RESULTS

The method significantly reduces acquisition times of 3D measurements from 360 s to 20 s, in the brain, while simultaneously reducing bias in the measured B₁ ⁺ due to T₁ and magnetization history. The mean coefficient of variation was reduced by 81% for T₁ s of 0.5-3 s compared to conventional satTFL. In vivo, the reproducibility coefficient for flip angles in the range 0-130° was 4.5° for satTFL and 4.7° for our scheme, significantly smaller than for a short TR satTFL sequence, which was 12°. The 3D sequence measured B₁ ⁺ maps of the whole thorax in 26 heartbeats.

CONCLUSION

Our adaptations enable faster B₁ ⁺ mapping, with minimal T₁ sensitivity and lower sensitivity to magnetization history, enabling single breath-hold whole-heart absolute B₁ ⁺ mapping.

Interleaved binomial kT-Points for water-selective imaging at 7T

PURPOSE

We present a time-efficient water-selective, parallel transmit RF excitation pulse design for ultra-high field applications.

METHODS

The proposed pulse design method achieves flip angle homogenization at ultra-high fields by employing spatially nonselective k T $$ {\mathrm{k}}_T $$ -points pulses. In order to introduce water-selection, the concept of binomial pulses is applied. Due to the composite nature of k T $$ {\mathrm{k}}_T $$ -points, the pulse can be split into multiple binomial subpulse blocks shorter than half the precession period of fat, that are played out successively. Additional fat precession turns, that would otherwise impair the spectral response, can thus be avoided. Bloch simulations of the proposed interleaved binomial k T $$ {\mathrm{k}}_T $$ -points pulses were carried out and compared in terms of duration, homogeneity, fat suppression and pulse energy. For validation, in vivo MP-RAGE and 3D-EPI data were acquired.

RESULTS

Simulation results show that interleaved binomial k T $$ {\mathrm{k}}_T $$ -points pulses achieve shorter total pulse durations, improved flip angle homogeneity and more robust fat suppression compared to available methods. Interleaved binomial k T $$ {\mathrm{k}}_T $$ -points can be customized by changing the number of k T $$ {\mathrm{k}}_T $$ -points, the subpulse duration and the order of the binomial pulse. Using shorter subpulses, the number of k T $$ {\mathrm{k}}_T $$ -points can be increased and hence better homogeneity is achieved, while still maintaining short total pulse durations. Flip angle homogenization and fat suppression of interleaved binomial k T $$ {\mathrm{k}}_T $$ -points pulses is demonstrated in vivo at 7T, confirming Bloch simulation results.

CONCLUSION

In this work, we present a time efficient and robust parallel transmission technique for nonselective water excitation with simultaneous flip angle homogenization at ultra-high field.

Parallel transmit pulse design for saturation homogeneity (PUSH) for magnetization transfer imaging at 7T

PURPOSE

This work proposes a novel RF pulse design for parallel transmit (pTx) systems to obtain uniform saturation of semisolid magnetization for magnetization transfer (MT) contrast in the presence of transmit field B1+ inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied B1+ field saturating but not rotating its magnetization; thus, standard pTx pulse design methods do not apply.

THEORY AND METHODS

Pulse design for saturation homogeneity (PUSH) optimizes pTx RF pulses by considering uniformity of root-mean squared B1+ , B1rms , which relates to the rate of semisolid saturation. Here we considered designs consisting of a small number of spatially non-selective sub-pulses optimized over either a single 2D plane or 3D. Simulations and in vivo experiments on a 7T Terra system with an 8-TX Nova head coil in five subjects were carried out to study the homogenization of B1rms and of the MT contrast by acquiring MT ratio maps.

RESULTS

Simulations and in vivo experiments showed up to six and two times more uniform B1rms compared to circular polarized (CP) mode for 2D and 3D optimizations, respectively. This translated into 4 and 1.25 times more uniform MT contrast, consistently for all subjects, where two sub-pulses were enough for the implementation and coil used.

CONCLUSION

The proposed PUSH method obtains more uniform and higher MT contrast than CP mode within the same specific absorption rate (SAR) budget.

Terminology and concepts for the characterization of in vivo MR spectroscopy methods and MR spectra: Background and experts' consensus recommendations

With a 40-year history of use for in vivo studies, the terminology used to describe the methodology and results of magnetic resonance spectroscopy (MRS) has grown substantially and is not consistent in many aspects. Given the platform offered by this special issue on advanced MRS methodology, the authors decided to describe many of the implicated terms, to pinpoint differences in their meanings and to suggest specific uses or definitions. This work covers terms used to describe all aspects of MRS, starting from the description of the MR signal and its theoretical basis to acquisition methods, processing and to quantification procedures, as well as terms involved in describing results, for example, those used with regard to aspects of quality, reproducibility or indications of error. The descriptions of the meanings of such terms emerge from the descriptions of the basic concepts involved in MRS methods and examinations. This paper also includes specific suggestions for future use of terms where multiple conventions have emerged or coexisted in the past.

Ultra-high resolution blood volume fMRI and BOLD fMRI in humans at 9.4 T: Capabilities and challenges

Functional mapping of cerebral blood volume (CBV) changes has the potential to reveal brain activity with high localization specificity at the level of cortical layers and columns. Non-invasive CBV imaging using Vascular Space Occupancy (VASO) at ultra-high magnetic field strengths promises high spatial specificity but poses unique challenges in human applications. As such, 9.4 T B1+ and B0 inhomogeneities limit efficient blood tagging, while the specific absorption rate (SAR) constraints limit the application of VASO-specific RF pulses. Moreover, short T2* values at 9.4 T require short readout duration, and long T1 values at 9.4 T can cause blood-inflow contaminations. In this study, we investigated the applicability of layer-dependent CBV-fMRI at 9.4 T in humans. We addressed the aforementioned challenges by combining multiple technical advancements: temporally alternating pTx B1+ shimming parameters, advanced adiabatic RF-pulses, 3D-EPI signal readout, optimized GRAPPA acquisition and reconstruction, and stability-optimized RF channel combination. We found that a combination of suitable advanced methodology alleviates the challenges and potential artifacts, and that VASO fMRI provides reliable measures of CBV change across cortical layers in humans at 9.4 T. The localization specificity of CBV-fMRI, combined with the high sensitivity of 9.4 T, makes this method an important tool for future studies investigating cortical micro-circuitry in humans.

MASE-sLASER, a short-TE, matched chemical shift displacement error sequence for single-voxel spectroscopy at ultrahigh field

B₁ inhomogeneity and chemical shift displacement error (CSDE) increase with the main magnetic field strength and are therefore deleterious for magnetic resonance spectroscopy (MRS) at ultrahigh field. A solution is to use adiabatic pulses which operate over a broad range of B₁ and thus are insensitive to B₁ inhomogeneity. Moreover, adiabatic pulses usually have a relatively higher bandwidth, which makes CSDE low to negligible. The use of exclusively adiabatic pulses for single-voxel spectroscopy (SVS) typically brings the disadvantage of a long echo time (TE), but the advantage of a low and matched CSDE. Herein, we took advantage of short-duration, low-power, matched-phase adiabatic spin echo (MASE) pulses to implement a matched CSDE semi-localized by adiabatic selective refocusing (sLASER) sequence capable of attaining short TEs, while CSDE is matched and still comparatively low. We also demonstrate here the feasibility of the direct measurement of the γ-aminobutyric acid (GABA) resonance at 2.28 ppm well separated from the neighboring glutamate resonance at 7 T using the implemented MASE-sLASER sequence at TEs of 68 and 136 ms. The shorter duration of MASE pulses also made it possible to implement a Mescher-Garwood-semi-localized by adiabatic selective refocusing (MEGA-sLASER) (with MASE) sequence with TE = 68 ms for editing GABA at 7 T, the results for which are also shown.

Fast and reproducible in vivo T1 mapping of the human cervical spinal cord

PURPOSE

To develop a fast and robust method for measuring T₁ in the whole cervical spinal cord in vivo, and to assess its reproducibility.

METHODS

A spatially nonselective adiabatic inversion pulse is combined with zonally oblique-magnified multislice echo-planar imaging to produce a reduced field-of-view inversion-recovery echo-planar imaging protocol. Multi- inversion time data are obtained by cycling slice order throughout sequence repetitions. Measurement of T₁ is performed using 12 inversion times for a total protocol duration of 7 min. Reproducibility of regional T₁ estimates is assessed in a scan-rescan experiment on five heathy subjects.

RESULTS

Regional mean (standard deviation) T₁ was: 1108.5 (±77.2) ms for left lateral column, 1110.1 (±83.2) ms for right lateral column, 1150.4 (±102.6) ms for dorsal column, and 1136.4 (±90.8) ms for gray matter. Regional T₁ estimates showed good correlation between sessions (Pearson correlation coefficient = 0.89 (P value < 0.01); mean difference = 2 ms, 95% confidence interval ± 20 ms); and high reproducibility (intersession coefficient of variation approximately 1% in all the regions considered, intraclass correlation coefficient = 0.88 (P value < 0.01, confidence interval 0.71-0.95)).

CONCLUSIONS

T₁ estimates in the cervical spinal cord are reproducible using inversion-recovery zonally oblique-magnified multislice echo-planar imaging. The short acquisition time and large coverage of this method paves the way for accurate T₁ mapping for various spinal cord pathologies. Magn Reson Med 79:2142-2148, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Acceleration of ASL-based time-resolved MR angiography by acquisition of control and labeled images in the same shot (ACTRESS)

PURPOSE

Noncontrast 4D-MR-angiography (MRA) using arterial spin labeling (ASL) is beneficial because high spatial and temporal resolution can be achieved. However, ASL requires acquisition of labeled and control images for each phase. The purpose of this study is to present a new accelerated 4D-MRA approach that requires only a single control acquisition, achieving similar image quality in approximately half the scan time.

METHODS

In a multi-phase Look-Locker sequence, the first phase was used as the control image and the labeling pulse was applied before the second phase. By acquiring the control and labeled images within a single Look-Locker cycle, 4D-MRA was generated in nearly half the scan time of conventional ASL. However, this approach potentially could be more sensitive to off-resonance and magnetization transfer (MT) effects. To counter this, careful optimizations of the labeling pulse were performed by Bloch simulations. In in-vivo studies arterial visualization was compared between the new and conventional ASL approaches.

RESULTS

Optimization of the labeling pulse successfully minimized off-resonance effects. Qualitative assessment showed that residual MT effects did not degrade visualization of the peripheral arteries.

CONCLUSION

This study demonstrated that the proposed approach achieved similar image quality as conventional ASL-MRA approaches in just over half the scan time. Magn Reson Med 79:224-233, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Elevated adiabatic T1ρ and T2ρ in articular cartilage are associated with cartilage and bone lesions in early osteoarthritis: A preliminary study

PURPOSE

To evaluate adiabatic T_1ρ and T_2ρ of articular cartilage in symptomatic osteoarthritis (OA) patients and asymptomatic volunteers, and to determine their association with magnetic resonance imaging (MRI)-based structural abnormalities in cartilage and bone.

MATERIALS AND METHODS

A total of 24 subjects (age range: 50-68 years; 12 female) were enrolled, including 12 early OA patients and 12 volunteers with normal joint function. Patients and volunteers underwent 3T MRI. T₂ , adiabatic T_1ρ , and T_2ρ relaxation times of knee articular cartilage were measured. Proton density (PD)- and T₁ -weighted MR image series were also obtained and separately evaluated for morphological changes using the MRI OA Knee Scoring (MOAKS) system. Comparisons using the Mann-Whitney nonparametric test were performed after dividing the study participants according to physical symptoms as determined by Western Ontario and McMaster Universities (WOMAC) score or presence of cartilage lesions, bone marrow lesions, or osteophytes.

RESULTS

Elevated adiabatic T_1ρ and T_2ρ relaxation times of articular cartilage were associated with cartilage loss (P = 0.024-0.047), physical symptoms (0.0068-0.035), and osteophytes (0.0039-0.027). Elevated adiabatic T_1ρ was also associated with bone marrow lesions (0.033).

CONCLUSION

Preliminary data suggest that elevated adiabatic T_1ρ and T_2ρ of cartilage are associated with morphological abnormalities of cartilage and bone, and thus may be applicable for in vivo OA research and diagnostics.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;46:678-689.

Slice profile and B1 corrections in 2D magnetic resonance fingerprinting

PURPOSE

The goal of this study is to characterize and improve the accuracy of 2D magnetic resonance fingerprinting (MRF) scans in the presence of slice profile (SP) and B₁ imperfections, which are two main factors that affect quantitative results in MRF.

METHODS

The SP and B₁ imperfections are characterized and corrected separately. The SP effect is corrected by simulating the radiofrequency pulse in the dictionary, and the B₁ is corrected by acquiring a B₁ map using the Bloch-Siegert method before each scan. The accuracy, precision, and repeatability of the proposed method are evaluated in phantom studies. The effects of both SP and B₁ imperfections are also illustrated and corrected in the in vivo studies.

RESULTS

The SP and B₁ corrections improve the accuracy of the T₁ and T₂ values, independent of the shape of the radiofrequency pulse. The T₁ and T₂ values obtained from different excitation patterns become more consistent after corrections, which leads to an improvement of the robustness of the MRF design.

CONCLUSION

This study demonstrates that MRF is sensitive to both SP and B₁ effects, and that corrections can be made to improve the accuracy of MRF with only a 2-s increase in acquisition time. Magn Reson Med 78:1781-1789, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Parallel transmission for ultrahigh-field imaging

The development of MRI systems operating at or above 7 T has provided researchers with a new window into the human body, yielding improved imaging speed, resolution and signal-to-noise ratio. In order to fully realise the potential of ultrahigh-field MRI, a range of technical hurdles must be overcome. The non-uniformity of the transmit field is one of such issues, as it leads to non-uniform images with spatially varying contrast. Parallel transmission (i.e. the use of multiple independent transmission channels) provides previously unavailable degrees of freedom that allow full spatial and temporal control of the radiofrequency (RF) fields. This review discusses the many ways in which these degrees of freedom can be used, ranging from making more uniform transmit fields to the design of subject-tailored RF pulses for both uniform excitation and spatial selection, and also the control of the specific absorption rate. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Test-retest analysis of multiple 31 P magnetization exchange pathways using asymmetric adiabatic inversion

PURPOSE

A ³¹ P-MR inversion transfer (IT) method with a short adiabatic inversion pulse is proposed and its test-retest reliability was evaluated for two spectral fitting strategies.

METHODS

Assessment in a test-retest design (3 Tesla, vastus muscles, 12 healthy volunteers, 14 inversion times, 22 ms asymmetric adiabatic inversion pulse, adiabatic excitation); spectral fitting in Fitting Tool for Interrelated Arrays of Datasets (FitAID) and Java Magnetic Resonance User Interface (jMRUI); least squares solution of the Bloch-McConnell-Solomon matrix formalism including all 14 measured time-points with equal weighting.

RESULTS

The cohort averages of k[PCr→γ-ATP] (phosphocreatine, PCr; adenosine triphosphate, ATP) are 0.246 ± 0.050s^-1 versus 0.254 ± 0.050s^-1 , and k[Pi→γ-ATP] 0.086 ± 0.033s^-1 versus 0.066 ± 0.034s^-1 (average ± standard deviation, jMRUI versus FitAID). Coefficients of variation of the differences between test and retest are lowest (9.5%) for k[PCr→γ-ATP] fitted in FitAID, larger (15.2%) for the fit in jMRUI, and considerably larger for k[Pi→γ-ATP] fitted in FitAID (43.4%) or jMRUI (47.9%). The beginning of the IT effect can be observed with magnetizations above 92% for noninverted lines while inversion of the ATP resonances is better than -72%.

CONCLUSION

The performance of the asymmetric adiabatic pulse allows an accurate observation of IT effects even in the early phase; the least squares fit of the Bloch-McConnell-Solomon matrix formalism is robust; and the type of spectral fitting can influence the results significantly. Magn Reson Med 78:33-39, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Validation and optimization of adiabatic T1ρ and T2ρ for quantitative imaging of articular cartilage at 3 T

PURPOSE

The aim of the present work was to validate and optimize adiabatic T_1ρ and T_2ρ mapping for in vivo measurements of articular cartilage at 3 Tesla (T).

METHODS

Phantom and in vivo experiments were systematically performed on a 3T clinical system to evaluate the sequences using hyperbolic secant HS1 and HS4 pulses. R_1ρ and R_2ρ relaxation rates were studied as a function of agarose and chondroitin sulfate concentration and pulse duration. Optimal in vivo protocol was determined by imaging the articular cartilage of two volunteers and varying the sequence parameters, and successively applied in eight additional subjects. Reproducibility was assessed in phantoms and in vivo.

RESULTS

Relaxation rates depended on agarose and chondroitin sulfate concentration. The sequences were able to generate relaxation time maps with pulse lengths of 8 and 6 ms for HS1 and HS4, respectively. In vivo findings were in good agreement with the phantoms. The implemented adiabatic T_1ρ and T_2ρ sequences demonstrated regional variation in relaxation time maps of femorotibial cartilage. Reproducibility in phantoms and in vivo was good to excellent for both adiabatic T_1ρ and T_2ρ .

CONCLUSIONS

The findings indicate that sequences are suitable for quantitative in vivo assessment of articular cartilage at 3 T. Magn Reson Med 77:1265-1275, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Increasing the quantitative bandwidth of NMR measurements

The frequency range of quantitative NMR is increased to hundreds of kHz, yielding accurate integrals even for ¹⁹F.

Two-dimensional J-resolved LASER and semi-LASER spectroscopy of human brain

PURPOSE

Two-dimensional J-resolved localized and semi-localized by adiabatic selective refocusing (LASER and semi-LASER) spectroscopy, named "J-resolved LASER" and "J-resolved semi-LASER", were introduced to suppress chemical shift artifacts, additional J-refocused artifactual peaks from spatially dependent J-coupling evolution, and sensitivity to radiofrequency (RF) field inhomogeneity.

METHODS

Three pairs of adiabatic pulses were employed for voxel localization in J-resolved LASER and two pairs in J-resolved semi-LASER. The first half of t1 period was inserted between the last pair of adiabatic pulses, which was proposed in this work to obtain two-dimensional adiabatic J-resolved spectra of human brain for the first time. Phantom and human experiments were performed to demonstrate their feasibility and advantages over conventional J-resolved spectroscopy (JPRESS).

RESULTS

Compared to JPRESS, J-resolved LASER or J-resolved semi-LASER exhibited significant suppression of chemical shift artifacts and additional J-refocused peaks from spatially dependent J-coupling evolution, and demonstrated insensitivity to the change of RF frequency offset over large bandwidth.

CONCLUSION

Experiments on phantoms and human brains verified the feasibility and strengths of two-dimensional adiabatic J-resolved spectroscopy at 3T. This technique is expected to advance the application of in vivo two-dimensional MR spectroscopy at 3T and higher field strengths for more reliable and accurate quantification of metabolites.

Large dynamic range relative B1+ mapping

Purpose

Parallel transmission (PTx) requires knowledge of the B1+ produced by each element. However, B1+ mapping can be challenging when transmit fields exhibit large dynamic range. This study presents a method to produce high quality relative B1+ maps when this is the case.

Theory and Methods

The proposed technique involves the acquisition of spoiled gradient echo (SPGR) images at multiple radiofrequency drive levels for each transmitter. The images are combined using knowledge of the SPGR signal equation using maximum likelihood estimation, yielding an image for each channel whose signal is proportional to the B1+ field strength. Relative B1+ maps are then obtained by taking image ratios. The method was tested using numerical simulations, phantom imaging, and through in vivo experiments.

Results

The numerical simulations demonstrated that the proposed method can reconstruct relative transmit sensitivities over a wide range of B1+ amplitudes and at several SNR levels. The method was validated at 3 Tesla (T) by comparing it with an alternative B1+ mapping method, and demonstrated in vivo at 7T.

Conclusion

Relative B1+ mapping in the presence of large dynamic range has been demonstrated through numerical simulations, phantom imaging at 3T and experimentally at 7T. The method will enable PTx to be applied in challenging imaging scenarios at ultrahigh field. Magn Reson Med 76:490–499, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Cartilage health in high tibial osteotomy using dGEMRIC: Relationships with joint kinematics

PURPOSE

The aims of this study are to determine how opening-wedge high tibial osteotomy (HTO) affects cartilage health in the tibiofemoral (TF) joint and patella, and to explore relationships between TF and patellofemoral (PF) joint kinematics and cartilage health in HTO.

METHODS

14 knees (13 subjects) with medial TF osteoarthritis (OA) were examined before HTO and 6 and 12 months after HTO using delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) to evaluate cartilage health at the TF joint and patella. They were also examined using a validated 3D MR knee kinematics measurement to obtain 11 rotations and translations at both TF and PF joints.

RESULTS

No statistically significant differences in overall TF or patellar dGEMRIC score were found at 6 or 12 months after HTO. However three subjects had large decreases (mean 105 ms) in TF dGEMRIC at 6 months that recovered at 12 months. Kinematics for these subjects were compared to subjects who did not have decreases in TF dGEMRIC at 6 months (n=5). Differences were observed between groups with HTO in anterior and proximal tibial translation (mean differences 3.05 mm and 1.35 mm), and patellar flexion (mean difference 3.65°). These changes were consistent between 6 and 12 months, despite recovery of TF dGEMRIC values.

CONCLUSIONS

We did not find significant differences in TF or patellar dGEMRIC before and after HTO with all subjects, however there were differences in kinematics between subjects who had a decrease in TF dGEMRIC at 6 months and those who did not. This suggests a link between joint kinematics and cartilage health in HTO.

CLINICAL RELEVANCE

The effect of opening-wedge high tibial osteotomy on cartilage GAG concentration may be linked to specific changes in knee kinematics following surgery.

Comparison of (31)P saturation and inversion magnetization transfer in human liver and skeletal muscle using a clinical MR system and surface coils

(31)P MRS magnetization transfer ((31)P-MT) experiments allow the estimation of exchange rates of biochemical reactions, such as the creatine kinase equilibrium and adenosine triphosphate (ATP) synthesis. Although various (31)P-MT methods have been successfully used on isolated organs or animals, their application on humans in clinical scanners poses specific challenges. This study compared two major (31)P-MT methods on a clinical MR system using heteronuclear surface coils. Although saturation transfer (ST) is the most commonly used (31)P-MT method, sequences such as inversion transfer (IT) with short pulses might be better suited for the specific hardware and software limitations of a clinical scanner. In addition, small NMR-undetectable metabolite pools can transfer MT to NMR-visible pools during long saturation pulses, which is prevented with short pulses. (31)P-MT sequences were adapted for limited pulse length, for heteronuclear transmit-receive surface coils with inhomogeneous B1 , for the need for volume selection and for the inherently low signal-to-noise ratio (SNR) on a clinical 3-T MR system. The ST and IT sequences were applied to skeletal muscle and liver in 10 healthy volunteers. Monte-Carlo simulations were used to evaluate the behavior of the IT measurements with increasing imperfections. In skeletal muscle of the thigh, ATP synthesis resulted in forward reaction constants (k) of 0.074 ± 0.022 s(-1) (ST) and 0.137 ± 0.042 s(-1) (IT), whereas the creatine kinase reaction yielded 0.459 ± 0.089 s(-1) (IT). In the liver, ATP synthesis resulted in k = 0.267 ± 0.106 s(-1) (ST), whereas the IT experiment yielded no consistent results. ST results were close to literature values; however, the IT results were either much larger than the corresponding ST values and/or were widely scattered. To summarize, ST and IT experiments can both be implemented on a clinical body scanner with heteronuclear transmit-receive surface coils; however, ST results are much more robust against experimental imperfections than the current implementation of IT.

A form-fitted three channel (31) P, two channel (1) H transceiver coil array for calf muscle studies at 7 T

PURPOSE

To enhance sensitivity and coverage for calf muscle studies, a novel, form-fitted, three-channel phosphorus-31 ((31) P), two-channel proton ((1) H) transceiver coil array for 7 T MR imaging and spectroscopy is presented.

METHODS

Electromagnetic simulations employing individually generated voxel models were performed to design a coil array for studying nonpathological muscle metabolism. Static phase combinations of the coil elements' transmit fields were optimized based on homogeneity and efficiency for several voxel models. The best-performing design was built and tested both on phantoms and in vivo.

RESULTS

Simulations revealed that a shared conductor array for (31) P provides more robust interelement decoupling and better homogeneity than an overlap array in this configuration. A static B1 (+) shim setting that suited various calf anatomies was identified and implemented. Simulations showed that the (31) P array provides signal-to-noise ratio (SNR) benefits over a single loop and a birdcage coil of equal radius by factors of 3.2 and 2.6 in the gastrocnemius and by 2.5 and 2.0 in the soleus muscle.

CONCLUSION

The performance of the coil in terms of B1 (+) and achievable SNR allows for spatially localized dynamic (31) P spectroscopy studies in the human calf. The associated higher specificity with respect to nonlocalized measurements permits distinguishing the functional responses of different muscles.

Phase-modulated LA-REDOR: a robust, accurate and efficient solid-state NMR technique for distance measurements between a spin-1/2 and a quadrupole spin

Distances between a spin-1/2 and a spin>1/2 can be efficiently measured by a variety of magic-angle spinning solid state NMR methods such as Rotational Echo Adiabatic Passage Double Resonance (REAPDOR), Low-Alpha/Low-Amplitude REDOR (LA-REDOR) and Rotational-Echo Saturation-Pulse Double-Resonance (R/S-RESPDOR). In this manuscript we show that the incorporation of a phase modulation into a long quadrupolar recoupling pulse, lasting 10 rotor periods that are sandwiched between rotor-synchronized pairs of dipolar recoupling π pulses, extends significantly the range of the values of the quadrupole moments that can be accessed by the experiment. We show by a combination of simulations and experiments that the new method, phase-modulated LA-REDOR, is very weakly dependent on the actual value of the radio-frequency field, and is highly robust with respect to off-resonance irradiation. The experimental results can be fitted by numerical simulations or using a universal formula corresponding to an equal-transition-probability model. Phase-modulated LA-REDOR (13)C{(11)B} and (15)N{(51)V} dipolar recoupling experiments confirm the accuracy and applicability of this new method.

Using the dGEMRIC technique to evaluate cartilage health in the presence of surgical hardware at 3T: comparison of inversion recovery and saturation recovery approaches

OBJECTIVE

To evaluate the effect of metal artifact reduction techniques on dGEMRIC T(1) calculation with surgical hardware present.

MATERIALS AND METHODS

We examined the effect of stainless-steel and titanium hardware on dGEMRIC T(1) maps. We tested two strategies to reduce metal artifact in dGEMRIC: (1) saturation recovery (SR) instead of inversion recovery (IR) and (2) applying the metal artifact reduction sequence (MARS), in a gadolinium-doped agarose gel phantom and in vivo with titanium hardware. T(1) maps were obtained using custom curve-fitting software and phantom ROIs were defined to compare conditions (metal, MARS, IR, SR).

RESULTS

A large area of artifact appeared in phantom IR images with metal when T(I) ≤ 700 ms. IR maps with metal had additional artifact both in vivo and in the phantom (shifted null points, increased mean T(1) (+151 % IR ROI(artifact)) and decreased mean inversion efficiency (f; 0.45 ROI(artifact), versus 2 for perfect inversion)) compared to the SR maps (ROI(artifact): +13 % T(1) SR, 0.95 versus 1 for perfect excitation), however, SR produced noisier T(1) maps than IR (phantom SNR: 118 SR, 212 IR). MARS subtly reduced the extent of artifact in the phantom (IR and SR).

CONCLUSIONS

dGEMRIC measurement in the presence of surgical hardware at 3T is possible with appropriately applied strategies. Measurements may work best in the presence of titanium and are severely limited with stainless steel. For regions near hardware where IR produces large artifacts making dGEMRIC analysis impossible, SR-MARS may allow dGEMRIC measurements. The position and size of the IR artifact is variable, and must be assessed for each implant/imaging set-up.

Composite slice-selective adiabatic excitation for prostate MRSI

Higher magnetic field strengths, such as 7 T, offer increased spectral resolution and higher signal-to-noise ratio. These properties can be very advantageous for MRSI. In particular, signals that generally overlap at lower fields, such as choline, polyamines and creatine, can be resolved at 7 T. However, higher magnetic field strengths suffer from strong radiofrequency (RF) field nonuniformities. These nonuniformities become even stronger when using surface transceivers, such as an endorectal coil for prostate imaging. In order to obtain uniform excitations for accurate MRSI measurements, adiabatic sequences are therefore recommended. Conventional adiabatic MRS sequences (i.e. localization by adiabatic selective refocusing, LASER) have relatively long TEs, especially when optimized to measure the strongly coupled spins of citrate in the prostate. The semi-LASER (sLASER) sequence has a significantly shorter TE, although it does not provide adiabatic excitation. Therefore, we propose an adiabatic sLASER sequence that either has a composite adiabatic slice-selective excitation (cLASER) or a non-slice-selective adiabatic excitation (nsLASER), allowing for shorter TEs, whilst maintaining the adiabatic spin excitation. Furthermore, the spatial properties of the composite adiabatic excitation allow for a high slice excitation bandwidth, resulting in negligible chemical shift displacement artifacts. Exclusion of the slice selection can be considered once the field of view extends beyond the transmit field of the RF coil. The use of a transceiver at high magnetic field strengths has shown that the cLASER and nsLASER sequences are suitable for MRSI of the prostate in both phantom and in vivo validations.

Arterial Spin Labeling (ASL) fMRI: advantages, theoretical constrains, and experimental challenges in neurosciences

Cerebral blood flow (CBF) is a well-established correlate of brain function and therefore an essential parameter for studying the brain at both normal and diseased states. Arterial spin labeling (ASL) is a noninvasive fMRI technique that uses arterial water as an endogenous tracer to measure CBF. ASL provides reliable absolute quantification of CBF with higher spatial and temporal resolution than other techniques. And yet, the routine application of ASL has been somewhat limited. In this review, we start by highlighting theoretical complexities and technical challenges of ASL fMRI for basic and clinical research. While underscoring the main advantages of ASL versus other techniques such as BOLD, we also expound on inherent challenges and confounds in ASL perfusion imaging. In closing, we expound on several exciting developments in the field that we believe will make ASL reach its full potential in neuroscience research.

Measuring T₂ and T₁, and imaging T₂ without spin echoes

During adiabatic excitation, the nuclear magnetization in the transverse plane is subject to T(2) (spin-spin) relaxation, depending on the pulse length τ. Here, this property is exploited in a method of measuring T(2) using the ratio of NMR signals acquired with short and long-duration self-refocusing adiabatic pulses, without spin-echoes. This Dual-τ method is implemented with B(1)-insensitive rotation (BIR-4) pulses. It is validated theoretically with Bloch equation simulations independent of flip-angle, and experimentally in phantoms. Dual-τT(2) measurements are most accurate at short T(2) where results agree with standard spin-echo measures to within 10% for T(2) ≤ 100 ms. Dual-τ MRI performed with a long 0° BIR-4 pre-pulse provides quantitative T(2) imaging of phantoms and the human foot while preserving desired contrast and functional properties of the rest of the MRI sequence. A single 0° BIR-4 pre-pulse can provide T(2) contrast-weighted MRI and serve as a "T(2)-prep" sequence with a lower B(1) requirement than prior approaches. Finally, a Tri-τ experiment is introduced in which both τ and flip-angle are varied, enabling measurement of T(2), T(1) and signal intensity in just three acquisitions if flip-angles are well-characterized. These new methods can potentially save time and simplify relaxation measurements and/or contrast-weighted NMR and MRI.

Improving SNR and B1 transmit field for an endorectal coil in 7 T MRI and MRS of prostate cancer

Higher magnetic field strengths like 7 T and above are desirable for MR spectroscopy given the increased spectral resolution and signal to noise ratio. At these field strengths, substantial nonuniformities in B(1)(+/-) and radiofrequency power deposition become apparent. In this investigation, we propose an improvement on a conventionally used endorectal coil, through the addition of a second element (stripline). Both elements are used as transceivers. In the center of the prostate, approximately 40% signal to noise ratio increase is achieved. In fact, the signal to noise ratio gain obtained with the quadrature configuration locally can be even greater than 40% when compared to the single loop configuration. This is due to the natural asymmetry of the B(1)(+/-) fields at high frequencies, which causes destructive and constructive interference patterns. Global specific absorption rate is reduced by almost a factor of 2 as expected. Furthermore, approximately a 4-fold decrease in local specific absorption rate is observed when normalized to the B(1) values in the center of the prostate. Because of the 4-fold local specific absorption rate decrease obtained with the dual channel setup for the same reference B(1) value (20 μT at 3.5 cm depth into the prostate) as compared to the single loop, the transmission power B(1) duty cycle can be increased by a factor 4. Consequently, when using the two-element endorectal coil, the radiofrequency power deposition is significantly reduced and radiofrequency intense sequences with adiabatic pulses can be safely applied at 7 T for (1)H magnetic resonance spectroscopy and MRI in the prostate. Altogether, in vivo (1)H magnetic resonance spectroscopic imaging of prostate cancer with a fully adiabatic sequence operated at a minimum B(1)(+) of 20 μT shows insensitivity to the nonuniform transmit field, while remaining within local specific absorption rate guidelines of 10 W/kg.

T₂ -weighted MRI of post-infarct myocardial edema in mice

T(2) -weighted, cardiac magnetic resonance imaging (T(2) w CMR) can be used to noninvasively detect and quantify the edematous region that corresponds to the area at risk (AAR) following myocardial infarction (MI). Previously, CMR has been used to examine structure and function in mice, expediting the study of genetic manipulations. To date, CMR has not been applied to imaging of post-MI AAR in mice. We developed a whole-heart, T(2) w CMR sequence to quantify the AAR in mouse models of ischemia and infarction. The ΔB(0) and ΔB(1) environment around the mouse heart at 7 T were measured, and a T(2) -preparation sequence suitable for these conditions was developed. Both in vivo T(2) w and late gadolinium enhanced CMR were performed in mice after 20-min coronary occlusions, resulting in measurements of AAR size of 32.5 ± 3.1 (mean ± SEM)% left ventricular mass, and MI size of 50.1 ± 6.4% AAR size. Excellent interobserver agreement and agreement with histology were also found. This T(2) w imaging method for mice may allow for future investigations of genetic manipulations and novel therapies affecting the AAR and salvaged myocardium following reperfused MI.

Gapped pulses for frequency-swept MRI

A recently introduced method called SWIFT (SWeep Imaging with Fourier Transform) is a fundamentally different approach to MRI which is particularly well suited to imaging objects with extremely fast spin-spin relaxation rates. The method exploits a frequency-swept excitation pulse and virtually simultaneous signal acquisition in a time-shared mode. Correlation of the spin system response with the excitation pulse function is used to extract the signals of interest. With SWIFT, image quality is highly dependent on producing uniform and broadband spin excitation. These requirements are satisfied by using frequency-modulated pulses belonging to the hyperbolic secant family (HSn pulses). This article describes the experimental steps needed to properly implement HSn pulses in SWIFT. In addition, properties of HSn pulses in the rapid passage, linear region are investigated, followed by an analysis of the pulses after inserting the "gaps" needed for time-shared excitation and acquisition. Finally, compact expressions are presented to estimate the amplitude and flip angle of the HSn pulses, as well as the relative energy deposited by the SWIFT sequence.

A solid-state 53Cr NMR study of chromate and dichromate salts

Solid-state (53)Cr NMR spectra of a series of chromate (CrO4(2-)) and dichromate (Cr2O7(2-)) salts have been examined by employing the stepped-frequency quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) experiment and high applied magnetic field strengths, 11.75 and 18.8 T. Cr-53 nuclear quadrupolar coupling constants, CQ(53Cr), ranging from 1.23 to 5.01 MHz for the Cr(4(2-) salts and 7.25 to 8.14 MHz for the Cr2O7(2-) salts have been measured. For the dichromate salts, this corresponds to central transition 53Cr NMR lineshapes of 200-250 kHz at 18.8 T. The use of hyperbolic secant (HS) pulses in combination with the Hahn-echo (HE) or QCPMG experiment results in significant sensitivity enhancements when acquiring 53Cr NMR spectra of magic-angle spinning (MAS) samples, provided the MAS rate is fast with respect to the second-order quadrupolar interaction. For the CrO4(2-) and Cr2O7(2-) salts, the anisotropic chromium magnetic shielding interaction is generally negligible compared to the second-order 53Cr nuclear quadrupolar interaction. No simple correlation between the structure of the CrO4(2-) and Cr2O7(2-) anions and the observed CQ(53Cr) values has been found.

Bimodal Floquet description of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

A theoretical treatment of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is presented here based on bimodal Floquet theory. The conditions necessary for good heteronuclear decoupling are derived. An analysis of a few of the decoupling schemes implemented until date is presented with regard to satisfying such decoupling conditions and efficiency of decoupling. Resonance conditions for efficient heteronuclear dipolar decoupling are derived with and without the homonuclear (1)H-(1)H dipolar couplings and their influence on heteronuclear dipolar decoupling is pointed out. The analysis points to the superior efficiency of the newly introduced swept two-pulse phase-modulation (SW(f)-TPPM) sequence. It is shown that the experimental robustness of SW(f)-TPPM as compared to the original TPPM sequence results from an adiabatic sweeping of the modulation frequencies. Based on this finding alternative strategies are compared here. The theoretical findings are corroborated by both numerical simulations and representative experiments.

Sensitivity enhancement of NMR spectra of half-integer spin quadrupolar nuclei in solids using hyperbolic secant pulses

The experimental factors influencing the enhancements achievable for the central NMR transition, m(I)=1/2-->m(I)=-1/2, of spin-3/2 and spin-5/2 nuclei in the solid state using hyperbolic secant, HS, pulses for population transfer are investigated. In the case of powder samples spinning at the magic angle, it is found that the spinning frequency, the bandwidth and the frequency offset of the HS pulse play a crucial role in determining the maximum enhancements. Specifically, the bandwidth must be set to the spinning frequency for maximum signal enhancements. The (87)Rb NMR enhancement obtained for RbClO(4) using HS pulses was relatively insensitive to the magic angle spinning frequency; however, in the case of Al(acac)(3), the (27)Al enhancement increased with MAS frequency. In order to obtain an adiabatic HS sweep, one should optimize the rf field for a given pulse duration or optimize the pulse duration for a given rf field.

Fast magnetization-driven preparation for imaging of contrast-enhanced coronary arteries during intra-arterial injection of contrast agent

PURPOSE

To implement a short-duration magnetization preparation sequence, which consists of a saturation followed by multiple inversion pulses, for imaging of short-T1 species and suppression of long-T1 species.

MATERIALS AND METHODS

Computer optimizations were performed to derive preparation schemes that 1) suppress long-T1 background species with T1>or=250 msec, 2) maximize the MZ of contrast-enhanced (CE) structures with T1<or=50 msec, and 3) have a preparation duration in the range of 200 msec. The optimized sequences were tested on a phantom and a pig model instrumented with an intracoronary catheter for infusion of contrast media.

RESULTS

Computer simulations generated preparation schemes with durations of 165-225 msec depending on the number of preparation pulses used, which generated saturation of over 98% for T1>250 msec, and about a 30% reduction for 20 msec<T1<50 msec. The phantom studies validated the performance of the optimized sequences. Coronary artery angiograms (380 msec for preparation and image acquisition) demonstrated signal-to-noise ratios (SNRs) in the range of 13-15.5 and contrast-to-noise ratios (CNRs) in the range of 6.3-7.1 in the CE coronary vessels.

CONCLUSION

This work demonstrates that fast magnetization-driven preparation schemes can be implemented for fast imaging of CE coronary vessels with efficient saturation of background species.

Factors influencing flip angle mapping in MRI: RF pulse shape, slice-select gradients, off-resonance excitation, andB0 inhomogeneities

To understand the various effects that influence actual flip angles, and correct for these effects, it is important to precisely quantify the MRI parameters (such as T1, T2, and perfusion). In this paper actual flip angle maps are calculated using a conventional gradient-echo (GRE) sequence with different radiofrequency (RF) pulse shapes (Gaussian, sinc, and truncated-sinc), slice-selection gradients, off-resonance excitations, and B0 field inhomogeneities. The experimental results demonstrate that RF pulse shapes significantly affect the flip angle distribution and calibration factors. Off-resonance RF excitations, B0 nonuniformities, and slice-selection gradients can lead to degradations in the signal intensities of the images used to map the flip angle, and potentially introduce a bias and increased variance in the measured flip angles.

Measuring human cardiac tissue sodium concentrations using surface coils, adiabatic excitation, and twisted projection imaging with minimal T2 losses

PURPOSE

To measure tissue sodium concentrations in the human heart with (23)Na MRI using a surface coil, thereby eliminating the effects of inhomogeneous excitation by surface coils and minimizing T(1) and T(2) relaxation.

MATERIALS AND METHODS

We combined fully relaxed, very short-echo, (23)Na twisted projection imaging (TPI) with adiabatic half passage (AHP) excitation and external referencing on subjects and comparing with a concentration reference phantom scan to quantify TSC with surface coils. (23)Na signal losses during hard (square), composite, and tanh/tan amplitude/frequency-modulated AHP excitation pulses were analyzed over a wide range of RF field strengths and T(2short) values.

RESULTS

AHP excitation yielded a homogeneous excitation flip angle and negligible losses compared to a 90 degrees hard pulse wherever the B1 field exceeded the adiabatic threshold, rendering this sequence suitable for applications that use surface coil excitation. An AHP (23)Na TPI sequence was used with a surface coil at 1.5 T to noninvasively quantify myocardial TSC in 10 normal volunteers. The mean TSC was 43 +/- 4, 53 +/- 12, and 17 +/- 4 micromol/g in the left ventricular (LV) free wall, septum, and adipose tissue, respectively, consistent with prior invasive measurements on biopsy and autopsy specimens.

CONCLUSION

It is now possible to noninvasively quantify TSC in the human heart with surface coil (23)Na MRI.

Improving the amplitude-modulated control experiment for multislice continuous arterial spin labeling

The use of an amplitude-modulated radiofrequency (RF) pulse for a control experiment is a proven method to control for off-resonance effects in multislice continuous arterial spin labeling (CASL) experiments. This method is also known as double adiabatic inversion. The adiabaticity factor of a single half-pulse, beta(1/2), and a new dimensionless parameter alpha, which is obtained from the labeling parameters and the flow velocity, are introduced. This makes it possible to distinguish three distinct cases: 1) With low alpha, a double inversion occurs. 2) With alpha > or = approximately 4, the efficiency with which the longitudinal magnetization is returned to the z-axis depends on the phase of the amplitude modulation at the time the spins cross the center of the labeling plane. 3) In the intermediate region, the efficiency shows undesirable fluctuations. In a Bloch equation simulation, three optimized parameter sets are determined. Near ideal performance should always be achieved by combinations of parameters for which beta(1/2) > or = approximately 2 and alpha approximately pi/beta(1/2). The efficiency increases were realized in a volunteer study, showing the practical application of the suggested optimization.

Magnetic resonance imaging of the coronary vessel wall at 3 T using an obliquely oriented reinversion slab with adiabatic pulses

Three-dimensional methods offer volumetric coverage in coronary vessel wall imaging, in addition to high signal-to-noise ratios (SNR). To increase SNR further, it is desirable to implement such 3D methods at 3 T. At this field strength, the pulse sequence must be robust to main field and RF inhomogeneities. To achieve this, the double inversion-recovery (DIR) preparation was adapted to use adiabatic pulses, with a slab-selective reinversion replacing the previously used 2D pencil-beam. The slab was oriented obliquely, in order to avoid upstream blood (e.g., left ventricle) or the navigator beam. Phantom experiments suggest that at 3 T, this approach improves both the net profile of the DIR pulse pair and the restoration of magnetization in the navigator region. Using this method, the feasibility of 3D coronary vessel wall imaging was demonstrated at 3 T. Fourteen healthy subjects were scanned using a segmented gradient-echo sequence with prospective navigator gating. Good-quality images of left and right coronary arteries were obtained, with SNR values of 29.7 +/- 7.5 (vessel wall); 10.5 +/- 4.4 (blood); 14.3 +/- 5.2 (fat); and 45.6 +/- 18.0 (myocardium). No problems occurred with ECG-gating or power deposition (SAR) limits.

Efficiency of flow-driven adiabatic spin inversion under realistic experimental conditions: a computer simulation

Continuous arterial spin labeling (CASL) using adiabatic inversion is a widely used approach for perfusion imaging. For the quantification of perfusion, a reliable determination of the labeling efficiency is required. A numerical method for predicting the labeling efficiency in CASL experiments under various experimental conditions, including spin relaxation, is demonstrated. The approach is especially useful in the case of labeling at the carotid artery with a surface coil, as consideration of the experimental or theoretical profile of the B(1) field is straightforward. Other effects that are also accounted for include deviations from a constant labeling gradient, and variations in the blood flow velocity due to the cardiac cycle. Assuming relevant experimental and physiological conditions, maximum inversion efficiencies of about 85% can be obtained.

Self-diffusion measurements with chemical shift resolution in inhomogeneous magnetic fields

A methodology for chemical shift resolved molecular self-diffusion measurements in time-independent static and radiofrequency field gradients is demonstrated. Diffusion encoding is provided by a stimulated echo sequence with additional z-storage that allows for a change of diffusion time without affecting the relaxation weighting. The signal is acquired stroboscopically between the pulses of a train of adiabatic double passages that induces a z-rotation counteracting the phase spread resulting from precession in the inhomogeneous static field, as demonstrated in recent approaches to the goal of high-resolution "ex situ" NMR. Simulations of the pulse sequence show that the acquired signal results from the desired coherence pathway. Successful demonstrations of the experiment were performed on a mixture of water and isopropanol.

Improved signal to noise in high-resolution magic angle spinning total correlation spectroscopy studies of prostate tissues using rotor-synchronized adiabatic pulses

A rotor-synchronized WURST-8 adiabatic pulse scheme was compared to the conventional MLEV-17 hard pulse scheme for isotropic mixing in total correlation spectroscopy (TOCSY) studies of intact human prostate tissues under high-resolution magic angle spinning (HR-MAS) conditions. Both mixing schemes were extremely sensitive to the rotational resonance condition and dramatic reductions in signal to noise were observed when pulse durations deviated from 1/(spin rate). A significant increase in cross-peak intensities was observed using rotor-synchronized WURST-8 adiabatic pulses versus those observed using the rotor-synchronized MLEV-17 hard pulse scheme in both solution and tissue. In tissue, absolute signal intensities ranged from 1.5x to 10.5x greater (average: 4.75x) when WURST-8 was used in place of MLEV-17. Moreover, the difference was so dramatic that several metabolite cross peaks observed using WURST-8 pulses were not observed using MLEV-17 pulses, including cross peaks corresponding to many of the choline- and ethanolamine-containing metabolites. Due to the complex modulation of TOCSY cross peaks for multiply coupled spins and the shorter T(2) relaxation times of tissue metabolites, maximum cross-peak intensities occurred at shorter mixing times than predicted by theory. In summary, a WURST-8 adiabatic mixing scheme produced significantly greater absolute cross-peak signal intensities than MLEV-17 hard pulse mixing, and maximum cross-peak intensity versus mixing time must be established for specific spin systems and T(2) relaxation times.

Broadband phase modulation by adiabatic pulses

The use of inhomogeneous but spatially correlated static and radiofrequency (RF) magnetic fields offers a potential methodology for performing magnetic resonance spectroscopy of samples placed outside the bore of the magnet. However, its practical implementation still presents challenging problems, among them the control of nuclear spins over broad frequency offset intervals. The present study introduces an efficient method of encoding the phase of the magnetization when the variation of the static field along the sample is much larger than the RF amplitude. The procedure is based on the use of consecutively applied full-passage adiabatic pulses. The induced phase modulation is broadband and selective because it does not depend on the offset relative to the central frequency and the limits can be sharply defined. Finally, the encoded phase depends almost linearly on the local RF amplitude. All these features enable the recovery of an inhomogeneity-free spectrum with amplitudes close to the theoretically attainable maximum.