Xingwang Yong

Purpose: Balanced steady‐state free‐precession (bSSFP) enables rapid fetal brain acquisition but suffers from inherently low tissue contrast. This study aims to develop an age‐specific optimization strategy for bSSFP to enhance gray matter–white matter (GM–WM) contrast in the developing fetal brain. Methods: An optimization pipeline was designed for 2D and 3D bSSFP sequences, integrating the Transition Into Driven Equilibrium (TIDE) model with the extended phase graph (EPG) algorithm. Using T1 and T2 relaxation times measured from fetal brains, the pipeline derived age‐specific variable flip angle (VFA) trains to maximize GM–WM contrast while constraining signal‐to‐noise ratio (SNR) and point‐spread function. Performance was benchmarked against default bSSFP and T2‐weighted half‐Fourier acquisition single‐shot turbo spin echo (HASTE) sequences based on relative contrast and segmentation accuracy. Results: The contrast enhancement was evaluated in a cohort of 18 fetal brains from 24 to 37 gestational weeks. The optimized 2D and 3D bSSFP sequences yielded significantly higher relative cortical contrast compared with the default bSSFP ( p < 0.001 for 2D and p < 0.002 for 3D), achieving average improvements of 4.13‐fold and 4.06‐fold, respectively. Furthermore, in the segmentation tasks, the optimized images yielded substantially higher segmentation accuracy compared with HASTE images (DICE = 0.93 for optimized 2D bSSFP and 0.89 for optimized 3D bSSFP, vs. 0.79 for HASTE). Conclusions: The proposed age‐specific bSSFP optimization strategy enhanced the characterization of GM–WM contrast in fetuses of different gestational ages. This readily translatable method holds potential to advance both clinical assessments and neurodevelopmental studies of the fetal brain.

Purpose: To improve the signal‐to‐noise ratio (SNR) and mitigate T ₂ ‐decay‐related blurring in whole‐brain chemical exchange saturation transfer (CEST) imaging by optimizing the refocusing flip angles of a turbo spin echo (TSE) readout, also known as the SPACE‐CEST sequence. Methods: The SNR of the SPACE‐CEST sequence was modeled as a function of flip angles, and a resolution penalty term was added to control blurring. Flip angles were optimized under two modes: Resolution‐only and SNR+Resolution mode. The optimized flip angles were compared to conventional variable/constant flip angles regarding transverse magnetization usage efficiency, longitudinal magnetization storage, and specific absorption rate (SAR). The SNR and resolution benefits of the proposed method were validated in a phantom, healthy volunteers, and brain tumor patients. Results: The optimized flip angles in the Resolution‐only mode generated a constant signal, validating the framework. The optimized flip angles in SNR+Resolution mode exhibited higher transverse magnetization usage efficiency and greater longitudinal storage. The SNR+Resolution mode produced signals comparable to those from conventional constant flip angles but with significantly lower flip angles. Notably, the SNR+Resolution mode demonstrated high transverse magnetization usage efficiency even with 20% B ₁ inhomogeneity and across a wide range T ₁ and T ₂ values, along with a 32.9% decrease in SAR. On average, the SNR+Resolution mode showed a 14.2% increase in SNR in humans and a 3.6% improvement in spatial resolution. Conclusion: Higher transverse magnetization usage efficiency can be achieved by optimizing refocusing flip angles of the SPACE‐CEST sequence, leading to increased SNR and decreased SAR.

Purpose: A differential analysis with fitted magnetization transfer and lipid signals (DIGITAL) method is proposed for eliminating fat artifacts and accurately extracting amide proton transfer (APT) contrast in lipid‐rich regions. Theory and Methods: The DIGITAL method integrates a two‐pool water‐magnetization transfer (MT) Bloch‐McConnell (BM) model with a six‐peak fat Bloch model. Renormalization is employed to correct fat‐induced Z‐spectrum distortion, and APT ^# signals are calculated by subtracting the fitted Z‐spectrum from the raw Z‐spectrum. The proposed DIGITAL method was evaluated using numerical simulations, phantoms, and breast tumor data, with its performance compared against prior multi‐pool Lorentzian (MPL) and numerical fitting of extrapolated semisolid magnetization transfer reference signals (NEMR) methods. We also developed a voxel‐wise reliability map to assess data and fit quality. Results: In simulations and experiments, renormalization effectively corrected fat‐induced signal scaling and restored the Z‐spectrum value at 0 ppm to near 0. The proposed DIGITAL method demonstrated superior uniformity in APT ^# maps compared to MPL and NEMR. Moreover, DIGITAL was minimally affected by fat, exhibiting high robustness against variations in fat concentrations and a strong linear relationship with amide proton concentrations. In addition, the reliability map functioned as an essential quality assurance tool, enabling the identification of outliers and improving the overall precision of APT analysis. Conclusion: The proposed DIGITAL method can effectively eliminate fat artifacts and extract accurate APT signals without requiring sequence modifications, offering a promising solution to the long‐standing challenge of lipid contamination in body APT imaging.

Purpose: To prospectively accelerate whole‐brain CEST acquisition by joint k‐space and image‐space parallel imaging (KIPI) with a proposed golden‐angle view ordering technique (GAVOT) in Cartesian coordinates. Theory and Methods: The T ₂ ‐decay effect will vary across frames with variable acceleration factors (AF) in the prospective acquisition using sequences with long echo trains. The GAVOT method uses a subset strategy to eliminate the T ₂ ‐decay inconsistency, where all frames use a subset of shots from the calibration frame to form their k‐space view ordering. The golden‐angle rule is adapted to ensure uniform k‐space coverage for arbitrary AFs in Cartesian coordinates. Phantom and in vivo studies were conducted on a 3 T scanner. Results: The GAVOT view ordering yielded a higher g‐factor than conventional uniformly centric ordering, whereas the noise propagation in amide proton transfer (APT) weighted images was similar between different view ordering. Compared to centric ordering, GAVOT successfully eliminated the T ₂ ‐decay inconsistency across all frames, resulting in fewer image artifacts for both KIPI and conventional parallel imaging methods. The synergy of GAVOT and KIPI mitigated strong aliasing artifacts and achieved high‐quality reconstruction of prospective variable‐AF datasets. GAVOT‐KIPI reduced the scan time to 2.1 min for whole‐brain APT weighted imaging and 4.7 min for quantitative APT signal (APT ^# ) mapping. Conclusion: GAVOT makes the prospective variable AF strategy flexible and practical, and, in conjunction with KIPI, ensures high‐quality reconstruction from highly undersampled data, facilitating the clinical translation of whole‐brain CEST imaging.

Purpose: In 0–2‐year‐old brains, the T₂‐weighted (T2w) contrast between white matter (WM) and gray matter (GM) is weaker compared with that in adult brains and rapidly changes with age. This study aims to design variable‐flip‐angle (VFA) trains in 3D fast spin‐echo sequence that adapt to the dynamically changing relaxation times to improve the contrast in the T2w images of the developing brains.Methods: T₁ and T₂ relaxation times in 0–2‐year‐old brains were measured, and several age groups were defined according to the age‐dependent pattern of T₂ values. Based on the static pseudo‐steady‐state theory and the extended phase graph algorithm, VFA trains were designed for each age group to maximize WM/GM contrast, constrained by the maximum specific absorption rate and overall signal intensity. The optimized VFA trains were compared with the default one used for adult brains based on the relative contrast between WM and GM. Dice coefficient was used to demonstrate the advantage of contrast‐improved images as inputs for automatic tissue segmentation in infant brains.Results: The 0–2‐year‐old pool was divided into groups of 0–8 months, 8–12 months, and 12–24 months. The optimal VFA trains were tested in each age group in comparison with the default sequence. Quantitative analyses demonstrated improved relative contrasts in infant and toddler brains by 1.5–2.3‐fold at different ages. The Dice coefficient for contrast‐optimized images was improved compared with default images (p < 0.001).Conclusion: An effective strategy was proposed to improve the 3D T2w contrast in 0–2‐year‐old brains.

Purpose: To propose a novel Numerical fitting method of the Extrapolated semisolid Magnetization transfer Reference (NEMR) signal for quantifying the CEST effect.Theory and Methods: Modified two‐pool Bloch‐McConnell equations were used to numerically fit the magnetization transfer (MT) and direct water saturation (DS) signals at far off‐resonance frequencies, which was subsequently extrapolated into the frequency range of amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) pools. Then the subtraction of the fitted two‐pool z‐spectrum and the experimentally acquired z‐spectrum yielded APT^# and NOE^# signals mostly free of MT and DS contamination. Several strategies were used to accelerate the NEMR fitting. Furthermore, the proposed NEMR method was compared with the conventional extrapolated semisolid magnetization transfer reference (EMR) and magnetization transfer ratio asymmetry (MTR_asym) methods in simulations and stroke patients.Results: The combination of RF downsampling, MT lineshape look‐up table, and conversion of MATLAB code to C code accelerated the NEMR fitting by over 2700‐fold. Monte‐Carlo simulations showed that NEMR had higher accuracy than EMR and eliminated the requirement of the steady‐state condition. In ischemic stroke patients, the NEMR maps at 1 μT removed hypointense artifacts seen on EMR and MTR_asym images, and better depicted stroke lesions than EMR. For NEMR, NOE^# yielded significantly (p < 0.05) stronger signal contrast between stroke and normal tissues than APT^# at 1 μT.Conclusion: The proposed NEMR method is suitable for arbitrary saturation settings and can remove MT and DS contamination from the CEST signal for improved detection of ischemic stroke.

The measurements of magnetic flux density ( $\emph {B}_{\emph {z}}$ ) measurement needed in magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance current density imaging (MRCDI) techniques often utilize spin echo (SE)-based sequences for data acquisition. The low imaging speed of SE-based methods significantly hampers the clinical applications of MREIT and MRCDI. Here, we propose a new sequence for substantially accelerating the acquisition of $\emph {B}_{\emph {z}}$ measurements. A skip-echo acquired turbo spin echo (SATE) imaging sequence based on the conventional turbo spin echo (TSE) technique was proposed by adding a skip-echo module in front of the TSE acquisition module. The skip-echo module consisted of a series of refocusing pulses without acquisition. In SATE, amplitude-modulated crusher gradients were used to remove the stimulated echo pathways, and the radiofrequency (RF) pulse shape was specially selected to preserve more signals. In efficiency evaluation experiments performed on a spherical gel phantom, we demonstrated that SATE had improved measurement efficiency compared to conventional TSE and multi-echo injection current nonlinear encoding (ME-ICNE) sequences via skipping one echo before acquiring signals. The accuracy of the $\emph {B}_{\emph {z}}$ measurements by SATE was validated against those by the ME-ICNE method, while SATE was able to accelerate the data acquisition up to 10-fold. Volumetric coverage of $\emph {B}_{\emph {z}}$ maps obtained in the phantom, pork, and human calf illustrated that SATE can reliably measure volumetric $\emph {B}_{\emph {z}}$ distributions within clinically acceptable time. The proposed SATE sequence provides a fast and effective approach for volumetric coverage of $\emph {B}_{\emph {z}}$ measurements, greatly facilitating the clinical applications of MREIT and MRCDI techniques.