Clinically compatible subject-specific dynamic parallel transmit pulse design for homogeneous fat saturation and water-excitation at 141657T: Proof-of-concept for 14165CEST MRI of the brain

PURPOSE

To evaluate the benefits and challenges of dynamic parallel transmit (pTx) pulses for fat saturation (FS) and water-excitation (WE), in the context of CEST MRI.

METHODS

"Universal" k_T -points (for FS) and spiral non-selective (for WE) trajectories were optimized offline for flip angle (FA) homogeneity. Routines to optimize the pulse shape online, based on the subject's fields maps, were implemented (target FA of 110°/0° for FS, 0°/5° for WE at fat/water frequencies). The pulses were inserted in a CEST sequence with a pTx readout. The different fat suppression schemes and their effects on CEST contrasts were compared in 12 volunteers at 7T.

RESULTS

With a 25%-shorter pulse duration, pTx FS largely improved the FA homogeneity (root-mean-square-error (RMSE) = 12.3° vs. 53.4° with circularly-polarized mode, at the fat frequency). However, the spectral selectivity was degraded mainly in the cerebellum and close to the sinuses (RMSE = 5.8° vs. 0.2° at the water frequency). Similarly, pTx WE showed a trade-off between FA homogeneity and spectral selectivity compared to pTx non-selective pulses (RMSE = 0.9° and 1.1° at the fat and water frequencies, vs. 4.6° and 0.5°). In the brain, CEST metrics were reduced by up to 31.9% at -3.3 ppm with pTx FS, suggesting a mitigated lipid-induced bias.

CONCLUSION

This clinically compatible implementation of dynamic pTx pulses improved the fat suppression homogeneity at 7T taking into account the subject-specific B₀ heterogeneities online. This study highlights the lipid-induced biases on the CEST z-spectrum. The results are promising for body applications where B₀ heterogeneities and fat are more substantial.

Time resolved DCE-MRI of the kidneys: Evaluation of the renal vasculatures and tumors using F-DISCO with and without compressed sensing in normal and wide-bore 3T systems

Dynamic contrast-enhanced MR imaging (DCE-MRI) has been widely used for the evaluation of renal arteries. This method is also useful for tumor and renal parenchyma characterization. The very fast MRI may provide stable and precise information regarding vasculature and soft tissues. The purpose of this study was to evaluate the ability of DCE-MRI to assess renal vasculatures and tumor perfusions using Differential subsampling with Cartesian ordering with spectrally selected inversion recovery with adiabatic pulses (F-DISCO) with and without compressed sensing (CS) in normal and wide-bore 3T systems. Fifty-one patients who underwent DCE-MRI using F-DISCO with or without CS for evaluation of renal or adrenal regions were included. Image quality, artifacts, fat saturation, and selective visual recognition of renal vasculatures were assessed by using a 5-point scale. Tumor recognition was verified by using a 5-point scale of confidence level. Signal intensities of each structure were also measured. In all cases, the temporal resolution of each phase for DCE-MRI was 1.9 to 2.0 seconds. Image quality, artifacts, fat saturation, and selective visual recognition of vasculatures were all acceptable (mean score 4.2-4.9). The selective visualization of renal arteries and veins was successfully accomplished (mean score 4.0-4.9). Contrast media perfusion for renal vasculature, renal parenchyma, and tumors was also recognized. DCE-MRI for the evaluation of renal vasculatures and tumors using F-DISCO with or without CS can be performed with high temporal and spatial resolutions in normal and wide-bore 3T systems. This information can be obtained in a stable fashion throughout the dynamic contrast study. CS can additionally provide benefits that the total imaging time may be shorter than without CS.

On the fat saturation effect in quantitative ultrashort TE MR imaging

PURPOSE

To investigate the effect of fat saturation (FatSat) on quantitative UTE imaging of variable knee tissues on a 3T scanner.

METHODS

Three quantitative UTE imaging techniques, including the UTE multi-echo sequence for T 2 ∗ measurement, the adiabatic T_1ρ prepared UTE sequence for T_1ρ measurement, and the magnetization transfer (MT)-prepared UTE sequence for MT ratio (MTR) and macromolecular proton fraction (MMF) measurements were used in this study. Twelve samples of cartilage and twelve samples of meniscus, as well as six whole knee cadaveric specimens, were imaged with the three above-mentioned UTE sequences with and without FatSat. The difference, correlation, and agreement between the UTE measurements with and without FatSat were calculated to investigate the effects of FatSat on quantification.

RESULTS

Fat was well-suppressed using all three UTE sequences when FatSat was deployed. For the small sample study, the quantification difference ratio (QDR) values of all the measured biomarkers ranged from 0.7% to 12.6%, whereas for the whole knee joint specimen study, the QDR values ranged from 0.2% to 12.0%. Except for T_1ρ in muscle and MMF in meniscus (p > 0.05), most of the measurements showed statistical differences for T_1ρ , MTR, and MMF (p < 0.05) between FatSat and non-FatSat scans. Most of the measurements for T 2 ∗ showed no significant differences (p > 0.05). Strong correlations were found for all the biomarkers between measurements with and without FatSat.

CONCLUSION

The UTE biomarkers showed good correlation and agreement with some slight differences between the scans with and without FatSat.

Differential Effects of Monounsaturated and Polyunsaturated Fats on Satiety and Gut Hormone Responses in Healthy Subjects

The difference between fat saturation on postprandial hormone responses and acute appetite control is not well understood. The aim of this study was to compare the postprandial ghrelin, gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP1) response and subjective appetite responses after isoenergetic high-fat meals rich in either monounsaturated (MUFAs) or polyunsaturated fatty acids (PUFAs) in healthy Chinese males. A randomized, controlled, single-blinded crossover study was conducted in 13 healthy Chinese men. Two high-fat meals (64% of energy) rich in MUFAs or PUFAs were tested. Total ghrelin, GIP and active GLP1 and visual analog scale (VAS) were measured over 4 h. Ghrelin was reduced greater after MUFA compared to PUFA at the beginning of the meal (at 30 and 60 min) and was significantly negatively correlated with subjective VAS for preoccupation for both MUFA and PUFA meals. No significant difference for ghrelin 240 min incremental area under the curve (iAUCs) were found. MUFA induced higher GIP response than PUFA. GIP was associated with all the VAS measurements except preoccupation for MUFA meal. No difference was found for GLP1 between two meals, nor was GLP1 associated with VAS. In conclusion, the results demonstrate that ghrelin, GIP and VAS respond differently to MUFA and PUFA meals. Ghrelin and GIP, but not GLP1, were associated with acute appetite control, especially after MUFA meal.

Fat suppression for ultrashort echo time imaging using a single-point Dixon method

PURPOSE

In ultrashort echo time (UTE) imaging, fat suppression can improve short T₂ * contrast but can also reduce short T₂ * signals. The conventional two-point Dixon (2p-Dixon) method does not perform well due to short T₂ * decay. In this study, we propose a new method to suppress fat for high contrast UTE imaging of short T₂ tissues, utilizing a single-point Dixon (1p-Dixon) method.

METHODS

The proposed method utilizes dual-echo UTE imaging, where UTE is followed by the second TE, chosen flexibly. Fat is estimated by applying a 1p-Dixon method to the non-UTE image after correction of phase errors, which is used to suppress fat in the UTE image. In vivo ankle and knee imaging were performed at 3 T to evaluate the proposed method.

RESULT

It was observed that fat and water signals in tendons were misestimated by the 2p-Dixon method due to signal decay, while the 1p-Dixon method showed reliable fat and water separation not affected by the short T₂ * signal decay. Compared with the conventional chemical shift based fat saturation technique, the 1p-Dixon based approach showed much stronger signal intensities in the Achilles, quadriceps, and patellar tendons, with significantly improved contrast to noise ratios (CNRs) of 11.8 ± 2.2, 16.0 ± 1.6, and 26.8 ± 1.3 with the 1p-Dixon method and 0.6 ± 0.2, 4.6 ± 1.0, and 17.5 ± 1.4 with regular fat saturation, respectively.

CONCLUSION

The proposed 1p-Dixon based fat suppression allows more flexible selection of imaging parameters and more accurate fat and water separation over the conventional 2p-Dixon in UTE imaging. Moreover, the proposed method provides much improved CNR for short T₂ tissues over the conventional fat saturation method.

Simultaneous fat saturation and magnetization transfer contrast imaging with steady-state incoherent sequences

PURPOSE

This work combines an n-dimensional fat sat(uration) radiofrequency (RF) pulse with steady-state incoherent (SSI) pulse sequences, e.g., spoiled gradient-echo sequence, to simultaneously produce B0 insensitive fat suppression and magnetization transfer (MT) contrast. This pulse is then referred to as "fat sat and MT contrast pulse."

THEORY

We discuss the features of the fat sat and MT contrast pulse and the MT sensitivities of the SSI sequences when combining with fat sat. Moreover, we also introduce an adapted RF spoiling scheme for SSI sequences with fat sat.

METHODS

Simulations and phantom experiments were conducted to demonstrate the adapted RF spoiling. Fat suppression and MT effects are shown in 3T phantom experiments and in vivo experiments, including brain imaging, cartilage imaging, and angiography.

RESULTS

To ensure that the sequence reaches steady state, the adapted RF spoiling is required for fat sat SSI sequences. Fat sat and MT contrast pulse works robustly with field inhomogeneity and also produces MT contrasts.

CONCLUSION

SSI sequences with fat sat and MT contrast pulse and adapted RF spoiling can robustly produce fat suppressed and MT contrast images in the presence of field inhomogeneity.

Four dimensional spectral-spatial fat saturation pulse design

PURPOSE

The conventional spectrally selective fat saturation pulse may perform poorly with inhomogeneous amplitude of static (polarizing) field (B0 ) and/or amplitude of (excitation) radiofrequency field (B1 ) fields. We propose a four dimensional spectral-spatial fat saturation pulse that is more robust to B0/B1 inhomogeneity and also shorter than the conventional fat saturation pulse.

THEORY

The proposed pulse is tailored for local B0 inhomogeneity, which avoids the need of a sharp transition band in the spectral domain, so it improves both performance and pulse length. Furthermore, it can also compensate for B1 inhomogeneity. The pulse is designed sequentially by small-tip-angle approximation design and an automatic rescaling procedure.

METHODS

The proposed method is compared to the conventional fat saturation in phantom experiments and in vivo knee imaging at 3 T for both single-channel and parallel excitation versions.

RESULTS

Compared to the conventional method, the proposed method produces superior fat suppression in the presence of B0 and B1 inhomogeneity and reduces pulse length by up to half of the standard length.

CONCLUSION

The proposed four dimensional spectral-spatial fat saturation suppresses fat more robustly with shorter pulse length than the conventional fat saturation in the presence of B0 and B1 inhomogeneity.

Parallel excitation for B-field insensitive fat-saturation preparation

Multichannel transmission has the potential to improve many aspects of MRI through a new paradigm in excitation. In this study, multichannel transmission is used to address the effects that variations in B(0) homogeneity have on fat-saturation preparation through the use of the frequency, phase, and amplitude degrees of freedom afforded by independent transmission channels. B(1) homogeneity is intrinsically included via use of coil sensitivities in calculations. A new method, parallel excitation for B-field insensitive fat-saturation preparation, can achieve fat saturation in 89% of voxels with M(z) ≤ 0.1 in the presence of ± 4 ppm B(0) variation, where traditional CHESS methods achieve only 40% in the same conditions. While there has been much progress to apply multichannel transmission at high field strengths, particular focus is given here to application of these methods at 1.5 T.