Design of improved spectral-spatial pulses for routine clinical use

Pre-excitation gradients for eddy current nulled convex optimized diffusion encoding (Pre-ENCODE)

PURPOSE

To evaluate the use of pre-excitation gradients for eddy current-nulled convex optimized diffusion encoding (Pre-ENCODE) to mitigate eddy current-induced image distortions in diffusion-weighted MRI (DWI).

METHODS

DWI sequences using monopolar (MONO), ENCODE, and Pre-ENCODE were evaluated in terms of the minimum achievable echo time (TE  min $$ {}_{\mathrm{min}} $$ ) and eddy current-induced image distortions using simulations, phantom experiments, and in vivo DWI in volunteers (N = 6 $$ N=6 $$ ).

RESULTS

Pre-ENCODE provided a shorter TE  min $$ {}_{\mathrm{min}} $$ than MONO (71.0± $$ \pm $$ 17.7ms vs. 77.6± $$ \pm $$ 22.9ms) and ENCODE (71.0± $$ \pm $$ 17.7ms vs. 86.2± $$ \pm $$ 14.2ms) in 100% $$ \% $$ of the simulated cases for a commercial 3T MRI system with b-values ranging from 500 to 3000 s/mm  2 $$ {}^2 $$ and in-plane spatial resolutions ranging from 1.0 to 3.0mm  2 $$ {}^2 $$ . Image distortion was estimated by intravoxel signal variance between diffusion encoding directions near the phantom edges and was significantly lower with Pre-ENCODE than with MONO (10.1% $$ \% $$ vs. 22.7% $$ \% $$ ,p = 6 - 5 $$ p={6}^{-5} $$ ) and comparable to ENCODE (10.1% $$ \% $$ vs. 10.4% $$ \% $$ ,p = 0 . 12 $$ p=0.12 $$ ). In vivo measurements of apparent diffusion coefficients were similar in global brain pixels (0.37 [0.28,1.45]× 1 0 - 3 $$ \times 1{0}^{-3} $$ mm  2 $$ {}^2 $$ /s vs. 0.38 [0.28,1.45]× 1 0 - 3 $$ \times 1{0}^{-3} $$ mm  2 $$ {}^2 $$ /s,p = 0 . 25 $$ p=0.25 $$ ) and increased in edge brain pixels (0.80 [0.17,1.49]× 1 0 - 3 $$ \times 1{0}^{-3} $$ mm  2 $$ {}^2 $$ /s vs. 0.70 [0.18,1.48]× 1 0 - 3 $$ \times 1{0}^{-3} $$ mm  2 $$ {}^2 $$ /s,p = 0 . 02 $$ p=0.02 $$ ) for MONO compared to Pre-ENCODE.

CONCLUSION

Pre-ENCODE mitigated eddy current-induced image distortions for diffusion imaging with a shorter TE  min $$ {}_{\mathrm{min}} $$ than MONO and ENCODE.

Design and realization of a multi-coil array for B0 field control in a compact 1.5T head-only MRI scanner

PURPOSE

To design and implement a multi-coil (MC) array for B0 field generation for image encoding and simultaneous advanced shimming in a novel 1.5T head-only MRI scanner.

METHODS

A 31-channel MC array was designed following the unique constraints of this scanner design: The vertically oriented magnet is very short, stopping shortly above the shoulders of a sitting subject, and includes a window for the subject to see through. Key characteristics of the MC hardware, the B0 field generation capabilities, and thermal behavior, were optimized in simulations prior to its construction. The unit was characterized via bench testing. B0 field generation capabilities were validated on a human 4T MR scanner by analysis of experimental B0 fields and by comparing images for several MRI sequences acquired with the MC array to those acquired with the system's linear gradients.

RESULTS

The MC system was designed to produce a multitude of linear and nonlinear magnetic fields including linear gradients of up to 10 kHz/cm (23.5 mT/m) with MC currents of 5 A per channel. With water cooling it can be driven with a duty cycle of up to 74% and ramp times of 500 μs. MR imaging experiments encoded with the developed multi-coil hardware were largely artifact-free; residual imperfections were predictable, and correctable.

CONCLUSION

The presented compact multi-coil array is capable of generating image encoding fields with amplitudes and quality comparable to clinical systems at very high duty cycles, while additionally enabling high-order B0 shimming capabilities and the potential for nonlinear encoding fields.

Phase contrast coronary blood velocity mapping with both high temporal and spatial resolution using triggered Golden Angle rotated Spiral k-t Sparse Parallel imaging (GASSP) with shifted binning

PURPOSE

High temporal and spatial resolutions are required for coronary blood flow measures. Current spiral breath-hold phase contrast (PC) MRI at 3T focus on either high spatial or high temporal resolution. We propose a golden angle (GA) rotated Spiral k-t Sparse Parallel imaging (GASSP) sequence for both high spatial (0.8 mm) and high temporal (<21 ms) resolutions.

METHODS

GASSP PC data are acquired in left anterior descending and right coronary arteries of eight healthy subjects. Binning of GA rotated spiral data into cardiac frames may lead to large k-space gaps. To reduce those gaps, the binning window is shifted and a triggered GA scheme that resets the rotation angle every heartbeat is proposed. The gap reductions are evaluated in simulations and all subjects. Peak systolic velocity (PSV), peak diastolic velocity (PDV), coronary blood flow rate, and vessel area are validated against two reference scans, and repeatability/reproducibility are determined.

RESULTS

Shifted binning reduced the mean k-space gaps of the triggered GA scheme by 14°-22° in simulations and about 20° in vivo. The k-space gap across three cardiac frames was reduced with the triggered GA scheme compared to the standard GA scheme (35.3°± 3.6° vs. 43°± 13.7°, t-test P = .04). PSV, PDV, flow rate, and area had high intra-scan repeatability (0.92 ≤ intraclass correlation coefficient [ICC] ≤ 0.99), and inter-scan (0.78 ≤ ICC ≤ 0.91) and intra-observer (0.91 ≤ ICC ≤ 0.98) reproducibility.

CONCLUSION

GASSP enables single breath-hold coronary PC MRI with high temporal and spatial resolutions. Shifted binning and a triggered GA scheme reduce k-space gaps. Quantitative coronary flow metrics are highly reproducible, especially within the same scanning session.

Diffusion-weighted magnetic resonance imaging of the oral and maxillofacial region: optimal fat suppression method

OBJECTIVE

To compare 3 fat suppression methods-water excitation (WE), chemical shift selective (CHESS), and short T1 inversion recovery (STIR)-for optimal image quality and apparent diffusion coefficient (ADC) values with magnetic resonance imaging (MRI) using diffusion-weighted imaging (DWI) of the oral and maxillofacial region.

STUDY DESIGN

In total, 53 patients with 73 lesions were enrolled in this study. MRI using DWI protocols with the 3 fat suppression methods were performed in addition to a conventional MRI protocol. The diagnostic image quality of lesions, image uniformity, degree of image artifacts, and ADC values of the lesions were evaluated. Average visual scores and ADC values were compared, and post hoc pairwise comparisons were performed, with the level of significance set at P < .0167.

RESULTS

Diagnostic image quality was not significantly different among the fat suppression methods (P ≥ .042). Image uniformity was significantly higher (P < .001), and the degree of image artifacts was significantly lower (P < .001), in images using the STIR method. Mean ADC values did not differ significantly among the 3 methods.

CONCLUSIONS

The STIR method was the most useful fat suppression method for DWI of the oral and maxillofacial region because of its high level of image uniformity and few image artifacts.

Comparison of 2D Fat Suppressed Proton Density (FS-PD) and 3D (WATS-c) MRI pulse sequences in evaluation of chondromalacia patellae

Background

Comparing the diagnostic performance of widely used 2D FSE technique (fat-suppressed proton density; FS-PD) and the 3D technique (water-selective cartilage scan; WATS-c) in evaluation of the chondromalacia patella by using arthroscopy as reference standard

Results

Seventy-five adult patients were enrolled in this study. They underwent MRI examinations then arthroscopy done in 2–4 days after it. MRI was done using 2D (FS-PD) and 3D (WATS-c) sequences and MR images were compared by two radiologists separately, then grading of the cartilage lesions was performed according to modified Noyes grading system and comparison between grade 0–1, 2, and 3 lesions was done using arthroscopic findings as a reference. A false-negative result is considered if there was undergrading of chondromalacia and false-positive result if chondromalacia was overgraded. Each sequence sensitivity, specificity, and accuracy was calculated by both readers.

For reader 1, the sensitivity is 69% for WATS-c and 80% for FS-PD and the accuracy is 90% for WATS-c and 92% for FS-PD and for reader 2, the sensitivity is 56% for WATS-c and 84% for FS-PD and the accuracy is 88% for WATS-c and 94% for FS-PD.

Conclusion

2D FS-PD images showed better diagnostic performance than 3D WATS-c images for evaluating chondromalacia patella.

Interleaved spatial/spectral encoding in ultrafast 2D NMR spectroscopy

The possibility to record a full 2D spectrum in less than a second using ultrafast 2D NMR (UF2DNMR) is beneficial in many applications. However, the spatial encoding process on which UF2DNMR is based sets specific constraints on the spectral width and resolution of the resulting spectra. To overcome these limitations, a tailored encoding method using spatial/spectral pulses (SPSP) can be employed as an alternative to the traditional linear spatial encoding of interactions. Here we analyze and further develop this alternative spatial encoding strategy. We first carry out numerical simulations to describe the features of bidimensional SPSP pulses. Sidebands are identified along the spectral dimension of the excitation profile. An interleaved excitation scheme is then developed and implemented experimentally to suppress the unwanted signals that arise from these harmonic sidebands. Two examples are shown to illustrate the potential of the proposed approach. An ultrafast selective TOCSY spectrum is recorded to access sub-spectra and fully assign ¹H NMR resonances of individual residues of cyclosporin A. An ultrafast HSQC spectrum of a mixture of metabolites is recorded with an optimized spectral width in the spatially encoded dimension.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

A minimum-phase Shinnar-Le Roux spectral-spatial excitation RF pulse for simultaneous water and lipid suppression in 1H-MRSI of body extremities

PURPOSE

To develop a spectral-spatial (SPSP) excitation RF pulse for simultaneous water and lipid suppression in proton (1H) magnetic resonance spectroscopic imaging (MRSI) of body extremities.

METHODS

An SPSP excitation pulse is designed to excite Creatine (Cr) and Choline (Cho) metabolite signals while suppressing the overwhelming water and lipid signals. The SPSP pulse is designed using a recently proposed multidimensional Shinnar-Le Roux (SLR) RF pulse design method. A minimum-phase spectral selectivity profile is used to minimize signal loss from T2⁎ decay.

RESULTS

The performance of the SPSP pulse is evaluated via Bloch equation simulations and phantom experiments. The feasibility of the proposed method is demonstrated using three-dimensional, short repetition-time, free induction decay-based 1H-MRSI in the thigh muscle at 3T.

CONCLUSION

The proposed SPSP excitation pulse is useful for simultaneous water and lipid suppression. The proposed method enables new applications of high-resolution 1H-MRSI in body extremities.

Spiral Perfusion Imaging With Consecutive Echoes (SPICE™) for the Simultaneous Mapping of DSC- and DCE-MRI Parameters in Brain Tumor Patients: Theory and Initial Feasibility

Dynamic contrast-enhanced (DCE) and dynamic susceptibility contrast (DSC) magnetic resonance imaging (MRI) are the perfusion imaging techniques most frequently used to probe the angiogenic character of brain neoplasms. With these methods, T₁- and T₂/T₂*-weighted imaging sequences are used to image the distribution of gadolinium (Gd)-based contrast agents. However, it is well known that Gd exhibits combined T₁, T₂, and T₂* shortening effects in tissue, and therefore, the results of both DCE- and DSC-MRI can be confounded by these opposing effects. In particular, residual susceptibility effects compete with T₁ shortening, which can confound DCE-MRI parameters, whereas dipolar T₁ and T₂ leakage and residual susceptibility effects can confound DSC-MRI parameters. We introduce here a novel perfusion imaging acquisition and postprocessing method termed Spiral Perfusion Imaging with Consecutive Echoes (SPICE) that can be used to simultaneously acquire DCE- and DSC-MRI data, which requires only a single dose of the Gd contrast agent, does not require the collection of a precontrast T₁ map for DCE-MRI processing, and eliminates the confounding contrast agent effects due to contrast extravasation. A detailed mathematical description of SPICE is provided here along with a demonstration of its utility in patients with high-grade glioma.

3D single point imaging with compressed sensing provides high temporal resolution R 2* mapping for in vivo preclinical applications

OBJECTIVE

Purely phase-encoded techniques such as single point imaging (SPI) are generally unsuitable for in vivo imaging due to lengthy acquisition times. Reconstruction of highly undersampled data using compressed sensing allows SPI data to be quickly obtained from animal models, enabling applications in preclinical cellular and molecular imaging.

MATERIALS AND METHODS

TurboSPI is a multi-echo single point technique that acquires hundreds of images with microsecond spacing, enabling high temporal resolution relaxometry of large-R ₂* systems such as iron-loaded cells. TurboSPI acquisitions can be pseudo-randomly undersampled in all three dimensions to increase artifact incoherence, and can provide prior information to improve reconstruction. We evaluated the performance of CS-TurboSPI in phantoms, a rat ex vivo, and a mouse in vivo.

RESULTS

An algorithm for iterative reconstruction of TurboSPI relaxometry time courses does not affect image quality or R ₂* mapping in vitro at acceleration factors up to 10. Imaging ex vivo is possible at similar acceleration factors, and in vivo imaging is demonstrated at an acceleration factor of 8, such that acquisition time is under 1 h.

CONCLUSIONS

Accelerated TurboSPI enables preclinical R ₂* mapping without loss of data quality, and may show increased specificity to iron oxide compared to other sequences.

Metabolite-selective hyperpolarized (13)C imaging using extended chemical shift displacement at 9.4T

PURPOSE

To develop a technique for frequency-selective hyperpolarized (13)C metabolic imaging in ultra-high field strength which exploits the broad spatial chemical shift displacement in providing spectral and spatial selectivity.

METHODS

The spatial chemical shift displacement caused by the slice-selection gradient was utilized in acquiring metabolite-selective images. Interleaved images of different metabolites were acquired by reversing the polarity of the slice-selection gradient at every repetition time, while using a low-bandwidth radio-frequency excitation pulse to alternatingly shift the displaced excitation bands outside the imaging subject. Demonstration of this technique is presented using (1)H phantom and in vivo mouse renal hyperpolarized (13)C imaging experiments with conventional chemical shift imaging and fast low-angle shot sequences.

RESULTS

From phantom and in vivo mouse studies, the spectral selectivity of the proposed method is readily demonstrated using results of chemical shift spectroscopic imaging, which displayed clearly delineated images of different metabolites. Imaging results using the proposed method without spectral encoding also showed effective separation while also providing high spatial resolution.

CONCLUSION

This method provides a way to acquire spectrally selective hyperpolarized (13)C metabolic images in a simple implementation, and with potential ability to support combination with more elaborate readout methods for faster imaging.

Fat-suppressed alternating-SSFP for whole-brain fMRI using breath-hold and visual stimulus paradigms

PURPOSE

To achieve artifact-suppressed whole-brain pass-band-balanced steady-state free precession functional MRI from a single functional magnetic resonance imaging (fMRI) scan.

METHODS

A complete and practical data acquisition sequence for alt-SSFP fMRI was developed. First, multishot flyback-echo-planar imaging (EPI) and echo-time shifting were used to achieve data acquisition that was robust against eddy currents, gradient delays, and ghosting artifacts. Second, a steady-state catalyzation scheme was implemented to reduce oscillations in the transient signal when catalyzing in and out of alternate steady states. Next, a short spatial-spectral radiofrequency (RF) pulse was designed to achieve excellent fat-suppression while maintaining a repetition time <15 ms to sensitize functional activation toward smaller vessels and capillaries. Lastly, parallel imaging was used to achieve whole-brain coverage and sufficiently high temporal resolution.

RESULTS

Breath-hold experiments showed excellent fat-suppression and alt-SSFP's capability to recover functional sensitivity from signal dropout regions of conventional gradient-echo and banding artifacts from conventional pass-band-balanced steady-state free precession. Applying fat-suppression resulted in improved activation maps and increased temporal SNR. Visual stimulus functional studies verify the proposed method's excellent functional sensitivity to neuronal activation.

CONCLUSION

Artifact-suppressed images are demonstrated, showing a practical pass-band-balanced steady-state free precession fMRI method that permits whole-brain imaging with excellent blood oxygen level-dependent sensitivity and fat suppression.

Design of multidimensional Shinnar-Le Roux radiofrequency pulses

PURPOSE

To generalize the conventional Shinnar-Le Roux method for the design of multidimensional radiofrequency pulses.

METHODS

Using echo-planar gradients, the multidimensional radiofrequency pulse design problem was converted into a series of one-dimensional polynomial design problems. Each of the one-dimensional polynomial design problems was solved efficiently. B0 inhomogeneity compensation and design of spatial-spectral pulses were also considered.

RESULTS

The proposed method was used to design two-dimensional excitation and refocusing pulses. The results were validated through Bloch equation simulation and experiments on a 3.0 T scanner. Large-tip-angle, equiripple-error, multidimensional excitation was achieved with ripple levels closely matching the design specifications.

CONCLUSION

The conventional Shinnar-Le Roux method can be extended to design multidimensional radiofrequency pulses. The proposed method achieves almost equiripple excitation errors, allows easy control of the tradeoff among design parameters, and is computationally efficient.

Diffusion-weighted echo-planar imaging of the head and neck using 3-T MRI: Investigation into the usefulness of liquid perfluorocarbon pads and choice of optimal fat suppression method

PURPOSE

To investigate whether image quality can be improved using liquid perfluorocarbon pads (Sat Pad) and clarify the optimal fat-suppression method among chemical shift selective (CHESS), water excitation (WEX), and short TI inversion recovery (STIR) methods in diffusion-weighted imaging (DWI) of the head and neck using 3-T magnetic resonance imaging. Correlations between results of visual inspection and quantitative analysis were also examined.

MATERIAL AND METHODS

This study was approved by our Institutional Review Board and informed consent was waived. DWI was performed on 25 subjects with/without Sat Pad and using three fat-suppression methods (6 patterns). Image quality was evaluated visually (4-point scales and lesion-depiction capability) and by quantitative analysis (signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR)). Two-way repeated-measures analysis of variance (ANOVA) was used to detect significant differences in scores of visual evaluation, SNR, and CNR.

RESULTS

Mean visual evaluation scores were significantly higher with Sat Pad using STIR than without Sat Pad for all fat-suppression methods (P<0.05). DWI with Sat Pad using STIR tended to be useful for depicting lesions. DWI using STIR showed reduced W-SNR (W: whole area of depicted structure) and CNR (between semispinalis capitis muscle and subcutaneous fat) due to fewer artifacts and uniform fat suppression.

CONCLUSION

Combining Sat Pad with STIR provides good image quality for visual inspections. When numerous artifacts are present and fat suppression is insufficient, higher SNR and CNR do not always provide good diagnostic image quality.

Direct design of 2D RF pulses using matrix inversion

Multi-dimensional pulses are frequently used in MRI for applications such as targeted excitation, fat-water separation or metabolic imaging with hyperpolarised (13)C compounds. For the design, the problem is typically separated into the different dimensions. In this work, a method to directly design two-dimensional pulses using the small-tip angle approximation is introduced based on a direct matrix representation. The numerical problem is solved in a single step directly in two dimensions by matrix inversion. Exemplary spectral-spatial excitation and spatio-temporal encoding (SPEN) pulses are designed and validated. The main benefits of the direct design approach include a reduction of artefacts in case of spectral-spatial pulses, a simple and straightforward computer implementation and high flexibility in the pulse design.

Homogenous fat suppression for bilateral breast imaging using independent shims

PURPOSE

To demonstrate the capability of incorporating independent shims into a dual-band spectral-spatial excitation and to compare fat suppression between standard global shims and independent shims for in vivo bilateral breast imaging at 1.5T.

METHODS

A dual-band spectral-spatial excitation pulse was designed by interleaving two flyback spectral-spatial pulses, playing one during positive gradient lobes and the other during negative gradient lobes. Each slab was enabled to have an independent spatial offset, spectral offset, and slab-phase modulation by modulating radiofrequency phase, and independent linear shims were incorporated by playing extra shim gradients. Phantom experiments were performed to demonstrate the functionality of the pulse, and in vivo experiments were performed for 10 healthy volunteers to compare fat suppression between standard shims and independent shims.

RESULTS

The phantom experiments confirmed that the dual-band pulse can provide independent spectral and spatial offsets and linear shims to the two slabs. Independent shims provided qualitatively more homogeneous fat suppression than standard shims in seven out of 10 subjects, with equivalent fat suppression in two of the other three subjects.

CONCLUSION

Incorporating independent shims into the dual-band spectral-spatial excitation can provide homogeneous fat suppression in bilateral breast imaging.

Robust selective signal suppression using binomial off-resonant rectangular (BORR) pulses

PURPOSE

To study the selective signal suppression capability of a binomial off-resonant rectangular (BORR) radiofrequency pulse method.

MATERIALS AND METHODS

The BORR pulse consists of two consecutive rectangular pulses with a phase difference of π. The exact solution of the Bloch equations was used to simulate its frequency response. The BORR pulse was implemented in a gradient echo sequence and tested on phantoms, the knee, and the breast.

RESULTS

The frequency response of the BORR pulse acquired on the phantom confirmed the theory. Broad suppression bands ensured high suppression efficiency and robustness in both in vitro and in vivo scans compared with other saturation pulses.

CONCLUSION

The BORR pulse method provides a simple, efficient, and robust selective signal suppression alternative for three-dimensional short TR (repetition time) imaging.

Reduced field-of-view excitation using second-order gradients and spatial-spectral radiofrequency pulses

The performance of multidimensional spatially selective radiofrequency (RF) pulses is often limited by their long duration. In this article, high-order, nonlinear gradients are exploited to reduce multidimensional RF pulse length. Specifically, by leveraging the multidimensional spatial dependence of second-order gradients, a two-dimensional spatial-spectral RF pulse is designed to achieve three-dimensional spatial selectivity, i.e., to excite a circular region-of-interest in a thin slice for reduced field-of-view imaging. Compared to conventional methods that use three-dimensional RF pulses and linear gradients, the proposed method requires only two-dimensional RF pulses, and thus can significantly shorten the RF pulses and/or improve excitation accuracy. The proposed method has been validated through Bloch equation simulations and phantom experiments on a commercial 3.0T MRI scanner.

In vivo detection of microscopic anisotropy using quadruple pulsed-field gradient (qPFG) diffusion MRI on a clinical scanner

We report our design and implementation of a quadruple pulsed-field gradient (qPFG) diffusion MRI pulse sequence on a whole-body clinical scanner and demonstrate its ability to non-invasively detect restriction-induced microscopic anisotropy in human brain tissue. The microstructural information measured using qPFG diffusion MRI in white matter complements that provided by diffusion tensor imaging (DTI) and exclusively characterizes diffusion of water trapped in microscopic compartments with unique measures of average cell geometry. We describe the effect of white matter fiber orientation on the expected MR signal and highlight the importance of incorporating such information in the axon diameter measurement using a suitable mathematical framework. Integration of qPFG diffusion-weighted images (DWI) with fiber orientations measured using high-resolution DTI allows the estimation of average axon diameters in the corpus callosum of healthy human volunteers. Maps of inter-hemispheric average axon diameters reveal an anterior-posterior variation in good topographical agreement with anatomical measurements reported in previous post-mortem studies. With further technical refinements and additional clinical validation, qPFG diffusion MRI could provide a quantitative whole-brain histological assessment of white and gray matter, enabling a wide range of neuroimaging applications for improved diagnosis of neurodegenerative pathologies, monitoring neurodevelopmental processes, and mapping brain connectivity.

Comparison between different implementations of the 3D FLASH sequence for knee cartilage quantification

OBJECTIVE

To compare several sequence implementations of the 3D FLASH sequence in the context of quantitative cartilage imaging.

MATERIALS AND METHODS

Test-retest coronal fast low angle shot (FLASH) sequences with water excitation were acquired in knees of 12 healthy participants, using two 1.5 T scanners from the same manufacturer. On one of the scanners, the FLASH was additionally compared with a FLASH VIBE, 75% with 100% slice resolution, a non-selective with a conventional spatial pulse, and "asymmetric echo allowed" with "not allowed".

RESULTS

Implementations of the FLASH showed systematic differences of up to 3.3%, but these were not statistically significant. Precision errors were similar between protocols, but tended to be smallest for the FLASH VIBE with 100% slice resolution (0.6-6.7%). In the medial tibia cartilage volume and thickness differed significantly (P < 0.01; 6.2 and 5.9%) between the two scanners.

CONCLUSION

Using a validated FLASH sequence, one can reduce slice resolution to 75% and allow asymmetric echo without sacrificing precision, in order to reduce the total acquisition time. However, in longitudinal studies, the scanner and the specific sequence implementation should be kept constant between baseline and follow-up, in order to avoid systematic off-sets in the measurements.

Nonuniform and multidimensional Shinnar-Le Roux RF pulse design method

The Shinnar-Le Roux (SLR) radiofrequency (RF) pulse design algorithm is widely used for designing slice-selective RF pulses due to its intuitiveness, optimality, and speed. SLR is limited, however, in that it is only capable of designing one-dimensional pulses played along constant gradients. We present a nonuniform SLR RF pulse design framework that extends most of the capabilities of classical SLR to nonuniform gradient trajectories and multiple dimensions. Specifically, like classical SLR, the new method is a hard pulse approximation-based technique that uses filter design relationships to produce the lowest power RF pulse that satisfies target magnetization ripple levels. The new method is validated and compared with methods conventionally used for nonuniform and multidimensional large-tip-angle RF pulse design.

Robust fat suppression at 3T in high-resolution diffusion-weighted single-shot echo-planar imaging of human brain

Single-shot echo-planar imaging is the most common acquisition technique for whole-brain diffusion tensor imaging (DTI) studies in vivo. Higher field MRI systems are readily available and advantageous for acquiring DTI due to increased signal. One of the practical issues for DTI with single-shot echo-planar imaging at high-field is incomplete fat suppression resulting in a chemically shifted fat artifact within the brain image. Unsuppressed fat is especially detrimental in DTI because the diffusion coefficient of fat is two orders of magnitude lower than that of parenchyma, producing brighter appearing fat artifacts with greater diffusion weighting. In this work, several fat suppression techniques were tested alone and in combination with the goal of finding a method that provides robust fat suppression and can be used in high-resolution single-shot echo-planar imaging DTI studies. Combination of chemical shift saturation with slice-select gradient reversal within a dual-spin-echo diffusion preparation period was found to provide robust fat suppression at 3 T.

Fat-water selective excitation in balanced steady-state free precession using short spatial-spectral RF pulses

Fat suppression is important but challenging in balanced steady-state free precession (bSSFP) acquisitions, for a number of clinical applications. In the present work, the practicality of performing fat-water selective excitations using spatial-spectral (SPSP) RF pulses in bSSFP sequence is examined. With careful pulse design, the overall duration of these SPSP pulses was kept short to minimize detrimental effects on TR, scan time and banding artifact content. Fat-water selective excitation using SPSP pulses was demonstrated in both phantom and human bSSFP imaging at 3T, and compared to results obtained using a two-point Dixon method. The sequence with SPSP pulses performed better than the two-point Dixon method, in terms of scan time and suppression performance. Overall, it is concluded here that SPSP RF pulses do represent a viable option for fat-suppressed bSSFP imaging.

Evaluation of the chondromalacia patella using a microscopy coil: comparison of the two-dimensional fast spin echo techniques and the three-dimensional fast field echo techniques

OBJECTIVE

We wanted to compare the two-dimensional (2D) fast spin echo (FSE) techniques and the three-dimensional (3D) fast field echo techniques for the evaluation of the chondromalacia patella using a microscopy coil.

MATERIALS AND METHODS

Twenty five patients who underwent total knee arthroplasty were included in this study. Preoperative MRI evaluation of the patella was performed using a microscopy coil (47 mm). The proton density-weighted fast spin echo images (PD), the fat-suppressed PD images (FS-PD), the intermediate weighted-fat suppressed fast spin echo images (iw-FS-FSE), the 3D balanced-fast field echo images (B-FFE), the 3D water selective cartilage scan (WATS-c) and the 3D water selective fluid scan (WATS-f) were obtained on a 1.5T MRI scanner. The patellar cartilage was evaluated in nine areas: the superior, middle and the inferior portions that were subdivided into the medial, central and lateral facets in a total of 215 areas. Employing the Noyes grading system, the MRI grade 0-I, II and III lesions were compared using the gross and microscopic findings. The sensitivity, specificity and accuracy were evaluated for each sequence. The significance of the differences for the individual sequences was calculated using the McNemar test.

RESULTS

The gross and microscopic findings demonstrated 167 grade 0-I lesions, 40 grade II lesions and eight grade III lesions. Iw-FS-FSE had the highest accuracy (sensitivity/specificity/accuracy = 88%/98%/96%), followed by FS-PD (78%/98%/93%, respectively), PD (76%/98%/93%, respectively), B-FFE (71%/100%/93%, respectively), WATS-c (67%/100%/92%, respectively) and WATS-f (58%/99%/89%, respectively). There were statistically significant differences for the iw-FS-FSE and WATS-f and for the PD-FS and WATS-f (p < 0.01).

CONCLUSION

The iw-FS-FSE images obtained with a microscopy coil show best diagnostic performance among the 2D and 3D GRE images for evaluating the chondromalacia patella.

Subject-specific water-selective imaging using parallel transmission

Spectral-spatial excitation pulses are an efficient means of achieving water- or fat-only imaging and can be used in conjunction with a variety of pulse sequences. However, the approach lacks reliability since its performance is dependent on the homogeneity of the static magnetic field. Sensitivity to static magnetic field variation can be reduced by designing pulses with wider frequency stop bands, but these require longer pulse durations. In the proposed method, spectral-spatial pulses are optimized on a subject-dependent basis to take into account measured subject-specific static magnetic field variation. Extra control of the radiofrequency (RF) field from multichannel transmission is used to achieve this without increasing the length of the pulses. The method characterizes RF pulses using relatively few parameters and has been applied to abdominal imaging at 3 T with an eight-channel system. In a comparison of standard and subject-specific pulses on five healthy volunteers, the latter improved fat suppression in all subjects, with a reduction in RF power of 13% +/- 6%. A forward model suggests that the mean flip angle in fat was reduced from 0.72 degrees +/- 0.55 degrees to 0.12 degrees +/- 0.04 degrees for a 20 degrees excitation; uniformity of water excitation also improved, with the standard deviation divided by mean reduced from 0.26 +/- 0.05 to 0.16 +/- 0.05.

Reduced field-of-view single-shot fast spin echo imaging using two-dimensional spatially selective radiofrequency pulses

PURPOSE

To demonstrate reduced field-of-view (RFOV) single-shot fast spin echo (SS-FSE) imaging based on the use of two-dimensional spatially selective radiofrequency (2DRF) pulses.

MATERIALS AND METHODS

The 2DRF pulses were incorporated into an SS-FSE sequence for RFOV imaging in both phantoms and the human brain on a 1.5 Tesla (T) whole-body MR system with the aim of demonstrating improvements in terms of shorter scan time, reduced blurring, and higher spatial resolution compared with full FOV imaging.

RESULTS

For phantom studies, scan time gains of up to 4.2-fold were achieved as compared to the full FOV imaging. For human studies, the spatial resolution was increased by a factor of 2.5 (from 1.7 mm/pixel to 0.69 mm/pixel) for RFOV imaging within a scan time (0.7 s) similar to full FOV imaging. A 2.2-fold shorter scan time along with a significant reduction of blurring was demonstrated in RFOV images compared with full FOV images for a target spatial resolution of 0.69 mm/pixel.

CONCLUSION

RFOV SS-FSE imaging using a 2DRF pulse shows advantages in scan time, blurring, and specific absorption rate reduction along with true spatial resolution increase compared with full FOV imaging. This approach is promising to benefit fast imaging applications such as image guided therapy.

Maximum linear-phase spectral-spatial radiofrequency pulses for fat-suppressed proton resonance frequency-shift MR Thermometry

Conventional spectral-spatial pulses used for water-selective excitation in proton resonance frequency-shift MR thermometry require increased sequence length compared to shorter wideband pulses. This is because spectral-spatial pulses are longer than wideband pulses, and the echo time period starts midway through them. Therefore, for a fixed echo time, one must increase sequence length to accommodate conventional spectral-spatial pulses in proton resonance frequency-shift thermometry. We introduce improved water-selective spectral-spatial pulses for which the echo time period starts near the beginning of excitation. Instead of requiring increased sequence length, these pulses extend into the long echo time periods common to PRF sequences. The new pulses therefore alleviate the traditional tradeoff between sequence length and fat suppression. We experimentally demonstrate an 11% improvement in frame rate in a proton resonance frequency imaging sequence compared to conventional spectral-spatial excitation. We also introduce a novel spectral-spatial pulse design technique that is a hybrid of previous model- and filter-based techniques and that inherits advantages from both. We experimentally validate the pulses' performance in suppressing lipid signal and in reducing sequence length compared to conventional spectral-spatial pulses.

Design of spectral-spatial outer volume suppression RF pulses for tissue specific metabolic characterization with hyperpolarized 13C pyruvate

[1-(13)C] pyruvate pre-polarized via DNP has been used in animal models to probe changes in metabolic enzyme activities in vivo. To more accurately assess the metabolic state and its change from disease progression or therapy in a specific region or tissue in vivo, it may be desirable to separate the downstream (13)C metabolite signals resulting from the metabolic activity within the tissue of interest and those brought into the tissue by perfusion. In this study, a spectral-spatial saturation pulse that selectively saturates the signal from the metabolic products [1-(13)C] lactate and [1-(13)C] alanine was designed and implemented as outer volume suppression for localized MRSI acquisition. Preliminary in vivo results showed that the suppression pulse did not prevent the pre-polarized pyruvate from being delivered throughout the animal while it saturated the metabolites within the targeted saturation region.

Fast large-tip-angle multidimensional and parallel RF pulse design in MRI

Large-tip-angle multidimensional radio-frequency (RF) pulse design is a difficult problem, due to the nonlinear response of magnetization to applied RF at large tip-angles. In parallel excitation, multidimensional RF pulse design is further complicated by the possibility for transmit field patterns to change between subjects, requiring pulses to be designed rapidly while a subject lies in the scanner. To accelerate pulse design, we introduce a fast version of the optimal control method for large-tip-angle parallel excitation. The new method is based on a novel approach to analytically linearizing the Bloch equation about a large-tip-angle RF pulse, which results in an approximate linear model for the perturbations created by adding a small-tip-angle pulse to a large-tip-angle pulse. The linear model can be evaluated rapidly using nonuniform fast Fourier transforms, and we apply it iteratively to produce a sequence of pulse updates that improve excitation accuracy. We achieve drastic reductions in design time and memory requirements compared to conventional optimal control, while producing pulses of similar accuracy. The new method can also compensate for nonidealities such as main field inhomogeneties.

Spectral-spatial pulse design for through-plane phase precompensatory slice selection in T2*-weighted functional MRI

T(2)*-weighted functional MR images suffer from signal loss artifacts caused by the magnetic susceptibility differences between air cavities and brain tissues. We propose a novel spectral-spatial pulse design that is slice-selective and capable of mitigating the signal loss. The two-dimensional spectral-spatial pulses create precompensatory phase variations that counteract through-plane dephasing, relying on the assumption that resonance frequency offset and through-plane field gradient are spatially correlated. The pulses can be precomputed before functional MRI experiments and used repeatedly for different slices in different subjects. Experiments with human subjects showed that the pulses were effective in slice selection and loss mitigation at different brain regions.

Pulse sequence for dynamic volumetric imaging of hyperpolarized metabolic products

Dynamic nuclear polarization and dissolution of a (13)C-labeled substrate enables the dynamic imaging of cellular metabolism. Spectroscopic information is typically acquired, making the acquisition of dynamic volumetric data a challenge. To enable rapid volumetric imaging, a spectral-spatial excitation pulse was designed to excite a single line of the carbon spectrum. With only a single resonance present in the signal, an echo-planar readout trajectory could be used to resolve spatial information, giving full volume coverage of 32 x 32 x 16 voxels every 3.5s. This high frame rate was used to measure the different lactate dynamics in different tissues in a normal rat model and a mouse model of prostate cancer.

3T MR of the prostate: reducing susceptibility gradients by inflating the endorectal coil with a barium sulfate suspension

Most prostate MRI/MRS examinations are performed with an endorectal coil inflated with air, leading to an air-tissue interface that induces magnetic susceptibility gradients within the gland. Inflation of the coil with a barium sulfate suspension is described and compared to inflation with air or liquid perfluorocarbon (PFC). The B(0) field in the prostate gland was mapped for five healthy volunteers when the endorectal coil was inflated with each of the three agents. A marked decrease in the posterior-anterior (P-A) field gradient and a significant improvement in field homogeneity were evident in the presence of a barium suspension and PFC relative to air. MRS data acquired from the prostate gland in the presence of air, PFC, and a barium suspension in the endorectal coil showed similar trends, demonstrating improvement in line-widths and spectral resolution when the barium suspension or the PFC were inflating the endorectal coil. On this basis we conclude that a barium suspension provides an available, cheap, and safe alternative to PFC, and we suggest that inflating the endorectal coil with a barium suspension should be considered for prostate MR studies, especially at high field strengths (such as 3T).

Simultaneous outer volume and blood suppression by quadruple inversion-recovery

A new method has been developed for reduced field-of-view (FOV) imaging with simultaneous blood suppression. This method combines suppression of signals from the outer volume and inflowing blood by using a small-FOV quadruple inversion-recovery (SFQIR) preparative pulse sequence consisting of two double-inversion pulse pairs separated by appropriate delays. Within each pair, inversion pulses are successively applied to the imaged slice and the slab orthogonal to the imaging plane with the thickness equal to the FOV size in the phase-encoding direction. Each double inversion results in the reinversion of the magnetization in the central part of the FOV, while the outer areas of the FOV and inflowing blood remain inverted. The SFQIR module was implemented for single- and multislice acquisition with a fast spin-echo readout sequence. Based on a theoretical model of the signal, the timing parameters of the sequence corresponding to the maximal suppression efficiency can be found by minimizing the variation of the normalized signal over the entire range of T1's that occur in tissues. The method was tested for black-blood imaging of the aorta and carotid arteries, and the results demonstrated its ability to eliminate motion and flow artifacts, reduce scan time, and improve spatial resolution.

Single breath-hold whole-heart MRA using variable-density spirals at 3t

Multislice breath-held coronary imaging techniques conventionally lack the coverage of free-breathing 3D acquisitions but use a considerably shorter acquisition window during the cardiac cycle. This produces images with significantly less motion artifact but a lower signal-to-noise ratio (SNR). By using the extra SNR available at 3 T and undersampling k-space without introducing significant aliasing artifacts, we were able to acquire high-resolution fat-suppressed images of the whole heart in 17 heartbeats (a single breath-hold). The basic pulse sequence consists of a spectral-spatial excitation followed by a variable-density spiral readout. This is combined with real-time localization and a real-time prospective shim correction. Images are reconstructed with the use of gridding, and advanced techniques are used to reduce aliasing artifacts.

Strategies for shimming the breast

There is evidence in the literature indicating a significant static field inhomogeneity in the human breast. A nonhomogenous field results in line broadening and frequency shifts in MRS and can cause intensity loss and spatial errors in MRI. Thus, there is a clear rationale for determining the regional variations in the static field homogeneity in the breast and providing strategies to correct them. Herein, the nature and extent of the static magnetic field at 3 T were measured in central planes of the human breast using both phase maps and multivoxel MRS techniques. In addition, the effect of first- and high-order shimming and of spatial saturation pulses on the static field inhomogeneity was evaluated. Both the theoretical and the measured field were found to be primarily linear in nature, with a reduction of 300 Hz from the nipple to the chest wall. First-order shimming reduced this inhomogeneity by 65%. Interestingly, the combination of spatial saturation pulses and first-order shimming was more effective than high-order shim alone. Since many clinical scanners do not have either higher-order shim or automated higher shimming algorithms that work in the presence of fat, the suggested combination provides an effective means to correct inhomogeneities in the breast.

Fat quantification using three-point dixon technique: in vitro validation

RATIONALE AND OBJECTIVES

To test the repeatability, reproducibility and accuracy of the three-point Dixon (3PD) sequence for estimating true fat volume ratios using a fat/water phantom.

MATERIALS AND METHODS

A phantom, constructed from test tubes of varying fat content, was imaged using the 3PD sequence on a 1.5T MRI scanner by two operators four times each. Fat volume ratios were calculated from these images and compared with true fat volumes.

RESULTS

Measures of fat volume ratios calculated from the 3PD MR images correlated strongly with values for true fat volumes (r = 0.96).

CONCLUSION

The 3PD technique was found to be highly reproducible and accurate, and may be useful for in vivo quantification of fat in lean tissues, such as the liver, pancreas or skeletal muscle.

Real-time cardiac MRI at 3 tesla

Real-time cardiac and coronary MRI at 1.5T is relatively "signal starved" and the 3T platform is attractive for its immediate factor of two increase in magnetization. Cardiac imaging at 3T, however, is both subtly and significantly different from imaging at 1.5T because of increased susceptibility artifacts, differences in tissue relaxation, and RF homogeneity issues. New RF excitation and pulse sequence designs are presented which deal with the fat-suppression requirements and off-resonance issues at 3T. Real-time cardiac imaging at 3T is demonstrated with high blood SNR, blood-myocardium CNR, resolution, and image quality, using new spectral-spatial RF pulses and fast spiral gradient echo pulse sequences. The proposed sequence achieves 1.5 mm in-plane resolution over a 20 cm FOV, with a 5.52 mm measured slice thickness and 32 dB of lipid suppression. Complete images are acquired every 120 ms and are reconstructed and displayed at 24 frames/sec using a sliding window. Results from healthy volunteers show improved image quality, a 53% improvement in blood SNR efficiency, and a 232% improvement in blood-myocardium CNR efficiency compared to 1.5T.

Fat suppression strategies in enhanced MR imaging of the breast: comparison of SPIR and water excitation sequences

PURPOSE

To compare two fat suppression techniques of spectrally-selective inversion pulse (spectral presaturation with inversion recovery-SPIR) and spectral-spatial excitation pulse of water excitation (WE) for contrast-enhanced MR imaging of the breast.

MATERIALS AND METHODS

Forty women with histologically-proven breast cancer were examined. Both pulse types were applied to postcontrast, axial, three-dimensional field echo sequence. Contrast noise ratios (CNR) of lesion-to-breast parenchyma, lesion-to-fat, and parenchyma-to-fat were determined. Qualitative image analysis using a four-point scale was also performed by two observers.

RESULTS

All the CNR values of obtained with WE techniques were significantly higher than those with SPIR. Qualitative analysis indicated that the WE images were statistically superior for the lesion-to-breast parenchyma contrast while being slightly inferior to the SPIR images for fat suppression homogeneity without statistical significance.

CONCLUSION

Compared to SPIR, the WE technique suppressed the subcutaneous fat signal more potently and improved the contrast of the enhanced breast lesion against the parenchyma and the subcutaneous fat. WE will be a powerful fat suppression strategy for enhanced MR imaging of the breast.

Direct monitoring of coronary artery motion with cardiac fat navigator echoes

Navigator echoes (NAVs) provide an effective means of monitoring physiological motion in magnetic resonance imaging (MRI). Motion artifacts can be suppressed by adjusting the data acquisition accordingly. The standard pencil-beam NAV has been used to detect diaphragm motion; however, it does not monitor cardiac motion effectively. Here we report a navigator approach that directly measures coronary artery motion by exciting the surrounding epicardial fat and sampling the signal with a k-space trajectory sensitized to various motion parameters. The present preliminary human study demonstrates that superior-inferior (SI) respiratory motion of the coronary arteries detected by the cardiac fat NAV highly correlates with SI diaphragmatic motion detected by the pencil-beam NAV. In addition, the cardiac fat navigator gating is slightly more effective than the diaphragmatic navigator gating in suppressing motion artifacts in free-breathing 3D coronary MR angiography (MRA).

Lipid content in the musculature of the lower leg assessed by fat selective MRI: intra- and interindividual differences and correlation with anthropometric and metabolic data

PURPOSE

To assess the muscular lipid content (LC) in different muscle groups of the lower leg by a magnetic resonance imaging technique working with chemical shift selective excitation, and comparison with anthropometric and metabolic data.

MATERIALS AND METHODS

Examinations were performed in 67 volunteers (54 male/13 female, age 29 +/- seven years) on a 1.5 T whole body imager, applying a highly selective spectral-spatial technique for fat selective MRI. LC was measured in six calf muscles and correlated with body mass index (BMI), percent body fat (PFAT), and insulin sensitivity (IS) of the subjects.

RESULTS

Mean muscular LC of all subjects was between 2.0% (Tibialis posterior [TP]) and 3.8% (Peroneus muscles) with female subjects showing a significantly higher LC in all muscle groups (P < 0.05 each). LCs correlated moderately with BMI (R between 0.39 [TP] and 0.53 [GM]) and with PFAT (R between 0.38 [TP] and 0.62 [GM]). Insulin-resistant subjects showed slightly but not significantly increased LC compared to insulin-sensitive subjects in BMI-matched subgroups.

CONCLUSION

The fat-selective MRI technique allows a reliable non-invasive measure of muscular lipids - even in muscle groups with inherent low LC - within a relatively short measurement time of about three minutes. The presented data reveal interesting interrelationships between LC and anthropometric and metabolic data, and therefore provide new insight into muscular fat metabolism.