Single-shot multi-b-value (SSMb) diffusion-weighted MRI using spin echo and stimulated echoes with variable flip angles.

Conventional diffusion-weighted imaging (DWI) sequences employing a spin echo or stimulated echo sensitize diffusion with a specific b-value at a fixed diffusion direction and diffusion time (Δ). To compute apparent diffusion coefficient (ADC) and other diffusion parameters, the sequence needs to be repeated multiple times by varying the b-value and/or gradient direction. In this study, we developed a single-shot multi-b-value (SSMb) diffusion MRI technique, which combines a spin echo and a train of stimulated echoes produced with variable flip angles. The method involves a pair of 90° radio frequency (RF) pulses that straddle a diffusion gradient lobe (GD), to rephase the magnetization in the transverse plane, producing a diffusion-weighted spin echo acquired by the first echo-planar imaging (EPI) readout train. The magnetization stored along the longitudinal axis is successively re-excited by a series of n variable-flip-angle pulses, each followed by a diffusion gradient lobe GD and a subsequent EPI readout train to sample n stimulated-echo signals. As such, (n + 1) diffusion-weighted images, each with a distinct b-value, are acquired in a single shot. The SSMb sequence was demonstrated on a diffusion phantom and healthy human brain to produce diffusion-weighted images, which were quantitative analyzed using a mono-exponential model. In the phantom experiment, SSMb provided similar ADC values to those from a commercial spin-echo EPI (SE-EPI) sequence (r = 0.999). In the human brain experiment, SSMb enabled a fourfold scan time reduction and yielded slightly lower ADC values (0.83 ± 0.26 µm2/ms) than SE-EPI (0.88 ± 0.29 µm2/ms) in all voxels excluding cerebrospinal fluid, likely due to the influence of varying diffusion times. The feasibility of using SSMb to acquire multiple images in a single shot for intravoxel incoherent motion (IVIM) analysis was also demonstrated. In conclusion, despite a relatively low signal-to-noise ratio, the proposed SSMb technique can substantially increase the data acquisition efficiency in DWI studies.

Investigating microstructural changes between in vivo and perfused ex vivo marmoset brains using oscillating gradient and b-tensor encoded diffusion MRI at 9.4 T.

PURPOSE: To investigate microstructural alterations induced by perfusion fixation in brain tissues using advanced diffusion MRI techniques and estimate their potential impact on the application of ex vivo models to in vivo microstructure. METHODS: We used oscillating gradient spin echo (OGSE) and b-tensor encoding diffusion MRI to examine in vivo and ex vivo microstructural differences in the marmoset brain. OGSE was used to shorten effective diffusion times, whereas b-tensor encoding allowed for the differentiation of isotropic and anisotropic kurtosis. Additionally, we performed Monte Carlo simulations to estimate the potential microstructural changes in the tissues. RESULTS: We report large changes (˜50%-60%) in kurtosis frequency dispersion (OGSE) and in both anisotropic and isotropic kurtosis (b-tensor encoding) after perfusion fixation. Structural MRI showed an average volume reduction of about 10%. Monte Carlo simulations indicated that these alterations could likely be attributed to extracellular fluid loss possibly combined with axon beading and increased dot compartment signal fraction. Little evidence was observed for reductions in axonal caliber. CONCLUSION: Our findings shed light on advanced MRI parameter changes that are induced by perfusion fixation and potential microstructural sources for these changes. This work also suggests that caution should be exercised when applying ex vivo models to infer in vivo tissue microstructure, as significant differences may arise.

Denoising of volumetric magnetic resonance imaging using multi-channel three-dimensional convolutional neural network with applications on fast spin echo acquisitions.

Background: Three-dimensional (3D) magnetic resonance imaging (MRI) can be acquired with a high spatial resolution with flexibility being reformatted into arbitrary planes, but at the cost of reduced signal-to-noise ratio. Deep-learning methods are promising for denoising in MRI. However, the existing 3D denoising convolutional neural networks (CNNs) rely on either a multi-channel two-dimensional (2D) network or a single-channel 3D network with limited ability to extract high dimensional features. We aim to develop a deep learning approach based on multi-channel 3D convolution to utilize inherent noise information embedded in multiple number of excitation (NEX) acquisition for denoising 3D fast spin echo (FSE) MRI. Methods: A multi-channel 3D CNN is developed for denoising multi-NEX 3D FSE magnetic resonance (MR) images based on the feature extraction of 3D noise distributions embedded in 2-NEX 3D MRI. The performance of the proposed approach was compared to several state-of-the-art MRI denoising methods on both synthetic and real knee data using 2D and 3D metrics of peak signal-to-noise ratio (PSNR) and structural similarity index measure (SSIM). Results: The proposed method achieved improved denoising performance compared to the current state-of-the-art denoising methods in both slice-by-slice 2D and volumetric 3D metrics of PSNR and SSIM. Conclusions: A multi-channel 3D CNN is developed for denoising of multi-NEX 3D FSE MR images. The superior performance of the proposed multi-channel 3D CNN in denoising multi-NEX 3D MRI demonstrates its potential in tasks that require the extraction of high-dimensional features.

Comparison of the Image Quality of Turbo Spin Echo and Echo-Planar Diffusion-Weighted Imaging of the Head and Neck Region.

BACKGROUND: Magnetic resonance imaging (MRI) of the head and neck region is notably challenging due to the complex anatomy and the critical need for high-resolution imaging to accurately diagnose various pathologies. The two prominent MRI techniques used in this context are turbo spin echo (TSE) and echo-planar diffusion-weighted imaging (EP-DWI). TSE is recognized for providing high-resolution anatomical images, whereas EP-DWI offers functional imaging that highlights the diffusion of water molecules, essential for detecting early pathological changes. This study aims to compare the image quality of TSE and EP-DWI in the head and neck region to assess their diagnostic efficacy and clinical utility. METHODS: This retrospective study was conducted at Saveetha Medical College and Hospital over six months. A total of 100 patients (50 males and 50 females, aged 18-65 years) with various head and neck pathologies were included. Patients underwent both TSE and EP-DWI sequences using a Philips MULTIVA 1.5 T scanner. Image quality was assessed based on signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), artifact presence, and lesion detection. Two experienced radiologists independently reviewed the images, with inter-observer agreement calculated using Cohen's kappa coefficient. RESULTS: The mean SNR for TSE was significantly higher than EP-DWI (45.2 vs. 28.7, p<0.01), indicating superior image clarity and detail in TSE images. TSE demonstrated a higher mean CNR compared to EP-DWI (25.4 vs. 15.8, p<0.01), suggesting better differentiation between different tissue types and pathologies. Artifacts were more frequent in EP-DWI images (45% vs. 15%), with motion artifacts being the most common. TSE detected more lesions (120 vs. 95), with more precise delineation of lesions. The inter-observer agreement was excellent for both TSE and EP-DWI, with kappa values of 0.85 and 0.80, respectively. CONCLUSION: TSE MRI provides superior image quality compared to EP-DWI for evaluating the head and neck region. The enhanced SNR and CNR in TSE images result in clearer and more detailed visualizations of anatomical structures and pathological changes, with fewer artifacts. While EP-DWI is valuable for functional imaging, its role should be complementary to TSE. The study suggests that TSE should be the preferred modality for detailed anatomical assessment in the head and neck region. Further studies with larger sample sizes and advanced imaging techniques may provide additional insights into optimizing MRI protocols for head and neck imaging.

DeepEMC-T2 mapping: Deep learning-enabled T2 mapping based on echo modulation curve modeling.

PURPOSE: Echo modulation curve (EMC) modeling enables accurate quantification of T2 relaxation times in multi-echo spin-echo (MESE) imaging. The standard EMC-T2 mapping framework, however, requires sufficient echoes and cumbersome pixel-wise dictionary-matching steps. This work proposes a deep learning version of EMC-T2 mapping, called DeepEMC-T2 mapping, to efficiently estimate accurate T2 maps from fewer echoes. METHODS: DeepEMC-T2 mapping was developed using a modified U-Net to estimate both T2 and proton density (PD) maps directly from MESE images. The network implements several new features to improve the accuracy of T2/PD estimation. A total of 67 MESE datasets acquired in axial orientation were used for network training and evaluation. An additional 57 datasets acquired in coronal orientation with different scan parameters were used to evaluate the generalizability of the framework. The performance of DeepEMC-T2 mapping was evaluated in seven experiments. RESULTS: Compared to the reference, DeepEMC-T2 mapping achieved T2 estimation errors from 1% to 11% and PD estimation errors from 0.4% to 1.5% with ten/seven/five/three echoes, which are more accurate than standard EMC-T2 mapping. By incorporating datasets acquired with different scan parameters and orientations for joint training, DeepEMC-T2 exhibits robust generalizability across varying imaging protocols. Increasing the echo spacing and including longer echoes improve the accuracy of parameter estimation. The new features proposed in DeepEMC-T2 mapping all enabled more accurate T2 estimation. CONCLUSIONS: DeepEMC-T2 mapping enables simplified, efficient, and accurate T2 quantification directly from MESE images without dictionary matching. Accurate T2 estimation from fewer echoes allows for increased volumetric coverage and/or higher slice resolution without prolonging total scan times.

Clinical application of pulse-gated non-enhanced rapid magnetic resonance imaging in the definitive diagnosis of aortic dissection.

OBJECTIVE: Aortic Dissection (AD) is one of the most fatal acute diseases in cardiovascular diseases, with rapid onset and progression and a high fatality rate. This study aims to investigate the clinical values of non-enhancement peripheral pulse-gating rapid magnetic resonance imaging in deterministic diagnosis of AD. METHODS: Aorta magnetic resonance imaging was performed in 21 healthy volunteers at a 1.5t MR scanner sequences including cardiac-gated and peripheral pulse-gated True-FISP and HASTE were carried out separately. Acquisition Time (TA), Signal to Noise Ratio (SNR), Contrast Noise Ratio (CNR), and entirety of vessel wall blood flow artifacts were measured and compared. A total of 56 AD cases were displayed by non-enhancement peripheral pulse-gating fast MR imaging, and the results were compared with pathological findings or CTA of the aorta. The dissection rupture, tear film, true and false lumen, thrombosis, hydropericardium, and the main branches of AD were evaluated respectively. RESULTS: There were no significant differences in SNR, CNR, entirety of the vessel wall, and blood flow artifact between cardiac-gated and peripheral pulse-gated fast MR imaging. Non-enhancement pulse-gated fast scanning takes less TA time. By the pulse-gated non-enhancement fast MR imaging, the dissection rupture, tear film, true and false cavity, thrombosis, hydropericardium, and the main branches of aortic dissection were shown clearly. Multi-planar and multi-angle scans helped to show the extent of entrapment rupture, whereas partial complex tears or bi-directional tears were slightly less well visualized. CONCLUSION: Non-enhancement peripheral pulse-gated rapid magnetic resonance imaging can be used for deterministic diagnosis of AD.

Fast bias-corrected conductivity mapping using stimulated echoes.

OBJECTIVE: To demonstrate the potential of a double angle stimulated echo (DA-STE) method for fast and accurate "full" homogeneous Helmholtz-based electrical properties tomography using a simultaneous B 1 + magnitude and transceive phase measurement. METHODS: The combination of a spin and stimulated echo can be used to yield an estimate of both B 1 + magnitude and transceive phase and thus provides the means for "full" EPT reconstruction. An interleaved 2D acquisition scheme is used for rapid acquisition. The method was validated in a saline phantom and compared to a double angle method based on two single gradient echo acquisitions (GRE-DAM). The method was evaluated in the brain of a healthy volunteer. RESULTS: The B 1 + magnitude obtained with DA-STE showed excellent agreement with the GRE-DAM method. Conductivity values based on the "full" EPT reconstruction also agreed well with the expectations in the saline phantom. In the brain, the method delivered conductivity values close to literature values. DISCUSSION: The method allows the use of the "full" Helmholtz-based EPT reconstruction without the need for additional measurements. As a result, quantitative conductivity values are improved compared to phase-based EPT reconstructions. DA-STE is a fast complex- B 1 + mapping technique that could render EPT clinically relevant at 3 T.

A pilot study to identify suitable MRI protocols for preoperative planning of total hip arthroplasty.

PURPOSE: The purpose of this study is to identify suitable MRI sequences and evaluate the feasibility and performance of MRI for total hip arthroplasty (THA) preoperative planning. METHOD: A multicentric pilot study was conducted to evaluate DP TSE and T1 GRE 3D sequences. High-resolution pelvis, hip, knee and ankle images were acquired. Protocols were optimised to enhance image quality (IQ) and reduce acquisition time to fit clinical practice. The final protocol was validated with 19 healthy volunteers with variable BMIs at 1.5 and 3 Tesla. Visual assessment was performed by five radiographers and radiologists using the ViewDEX software. Visual Grading Analysis (VGA), Intraclass Correlation Coefficient (ICC), Prevalence-adjusted and bias-adjusted kappa (PABAK) and Visual Grading Characteristics (VGC) were performed to analyse data. RESULTS: VGA scores indicated that the optimised 3D DP TSE and 3D T1 GRE sequences at 3 T, as well as 3D DP TSE sequence at 1.5 T offer adequate IQ and allow a correct visualisation of the anatomy. Overall ICC analysis was moderate to good reliability at 0.749 (95 % CI 0.69-0.79) and increased from good to excellent at 0.846 (95 % CI 0.72-0.91) for DP at 3 T. PABAK shows fair agreement at 0.25 (95 % CI 0.227-0.273). VGC analysis showed that 3D DP TSE sequences performed statistically better than 3D T1 GRE at 1.5 and 3 T (p-value ≤ 0.05). Furthermore, 3 T sequences showed a statistically better performance compared to 1.5 T (p-value ≤ 0.05). CONCLUSIONS: According to the results, 3D DP and T1 MRI sequences can be considered for preoperative planning for THA. Further research is required to emphasize the clinical validation of the results.

Rapid simultaneous estimation of relaxation rates using multi-echo, multi-contrast MRI.

PURPOSE: Multi-echo, multi-contrast methods are increasingly used in dynamic imaging studies to simultaneously quantify R2∗ and R2. To overcome the computational challenges associated with nonlinear least squares (NLSQ) fitting, we propose a generalized linear least squares (LLSQ) solution to rapidly fit R2∗ and R2. METHODS: Spin- and gradient-echo (SAGE) data were simulated across T2∗ and T2 values at high (200) and low (20) SNR. Full (four-parameter) and reduced (three-parameter) parameter fits were implemented and compared with both LLSQ and NLSQ fitting. Fit data were compared to ground truth using concordance correlation coefficient (CCC) and coefficient of variation (CV). In vivo SAGE perfusion data were acquired in 20 subjects with relapsing-remitting multiple sclerosis. LLSQ R2∗ and R2, as well as cerebral blood volume (CBV), were compared with the standard NLSQ approach. RESULTS: Across all fitting methods, T2∗ was well-fit at high (CCC = 1, CV = 0) and low (CCC ≥ 0.87, CV ≤ 0.08) SNR. Except for short T2∗ values (5-15 ms), T2 was well-fit at high (CCC = 1, CV = 0) and low (CCC ≥ 0.99, CV ≤ 0.03) SNR. In vivo, LLSQ R2∗ and R2 estimates were similar to NLSQ, and there were no differences in R2∗ across fitting methods at high SNR. However, there were some differences at low SNR and for R2 at high and low SNR. In vivo NLSQ and LLSQ three parameter fits performed similarly, as did NLSQ and LLSQ four-parameter fits. LLSQ CBV nearly matched the standard NLSQ method for R2∗- (0.97 ratio) and R2-CBV (0.98 ratio). Voxel-wise whole-brain fitting was faster for LLSQ (3-4 min) than NLSQ (16-18 h). CONCLUSIONS: LLSQ reliably fit for R2∗ and R2 in simulated and in vivo data. Use of LLSQ methods reduced the computational demand, enabling rapid estimation of R2∗ and R2.

Clinical application of single-shot fast spin-echo sequence for cerebrospinal fluid flow MR imaging.

In normal-pressure hydrocephalus, disturbances in cerebrospinal fluid (CSF) circulation occur; therefore, understanding CSF dynamics is crucial. The two-dimensional phase-contrast (2D-PC) method, a common approach for visualizing CSF flow on MRI, often presents challenges owing to prominent vein signals and excessively high contrast, hindering the interpretation of morphological information. Therefore, we devised a new imaging method that utilizes T2-weighted high-signal intensification of the CSF and saturation pulses, without requiring specialized imaging sequences. This sequence utilized a T2-weighted single-shot fast spin-echo combined with multi-phase imaging synchronized with a pulse wave. Optimal imaging conditions (repetition time, presence/absence of fast recovery, and echo time) were determined using self-made contrast and single-plate phantoms to evaluate signal-to-noise ratio, contrast ratio, and spatial resolution. In certain clinical cases of hydrocephalus, confirming CSF flow using 2D-PC was challenging. However, our method enabled the visualization of CSF flow, proving to be useful in understanding the pathophysiology of hydrocephalus.

Enhancing fluid signal in driven-equilibrium short-TI inversion-recovery imaging with short TR times: A feasibility study.

PURPOSE: Fluid-sensitive turbo spin echo (TSE) MRI with short-TI inversion-recovery preparation for fat suppression (STIR) plays a critical role in the diagnostics of the musculoskeletal system (e.g., close to metal implants). Potential advantages of 3D acquisitions, however, are difficult to exploit due to long acquisition times. Shortening the TR incurs a signal loss, and a driven-equilibrium (DE) extension reduces fluid signal even further. METHODS: The phase of the flip-back pulse was changed by 180° relative to the conventional implementation (i.e., 90° along the positive x-axis (90°x) instead of -90°x). After signal modeling and numerical simulations, the modification was implemented in STIR-TSE sequences and tested on a clinical 3T system. Imaging was performed in the lumbar spine, and long-TR images without DE were acquired as reference. CSF SNR and fluid-muscle contrast were measured and compared between the sequences. Imaging was repeated in a metal implant phantom. RESULTS: A shortening of TR by 43%-57% reduced the CSF SNR by 39%-59%. A conventional DE module further reduced SNR to 26%-40%, whereas the modified DE recovered SNR to 59%-108% compared with the long-TR acquisitions. Fluid-tissue contrast was increased by about 340% with the modified DE module compared with the conventional extension. Similar results were obtained in implant measurements. CONCLUSIONS: The proposed DE element for TSE-STIR sequences has the potential to accelerate the acquisition of fluid-sensitive images. DE-STIR may work most efficiently for 3D acquisitions, in which no temporo-spatial interleaving of inversion and imaging pulses is possible.

Efficient standardization of clinical T2-weighted images: Phase-conjugacy e-CAMP with projected gradient descent.

PURPOSE: To standardize T 2 $$ {}_2 $$ -weighted images from clinical Turbo Spin Echo (TSE) scans by generating corresponding T 2 $$ {}_2 $$ maps with the goal of removing scanner- and/or protocol-specific heterogeneity. METHODS: The T 2 $$ {}_2 $$ map is estimated by minimizing an objective function containing a data fidelity term in a Virtual Conjugate Coils (VCC) framework, where the signal evolution model is expressed as a linear constraint. The objective function is minimized by Projected Gradient Descent (PGD). RESULTS: The algorithm achieves accuracy comparable to methods with customized sampling schemes for accelerated T 2 $$ {}_2 $$ mapping. The results are insensitive to the tunable parameters, and the relaxed background phase prior produces better T 2 $$ {}_2 $$ maps compared to the strict real-value enforcement. It is worth noting that the algorithm works well with challenging T 2 $$ {}_2 $$ w-TSE data using typical clinical parameters. The observed normalized root mean square error ranges from 6.8% to 12.3% over grey and white matter, a clinically common level of quantitative map error. CONCLUSION: The novel methodological development creates an efficient algorithm that allows for T 2 $$ {}_2 $$ map generated from TSE data with typical clinical parameters, such as high resolution, long echo train length, and low echo spacing. Reconstruction of T 2 $$ {}_2 $$ maps from TSE data with typical clinical parameters has not been previously reported.

Feasibility and performance of spin-echo EPI MR elastography at 3 Tesla for staging hepatic fibrosis in the presence of hepatic iron overload.

PURPOSE: To assess the feasibility and performance of MR elastography (MRE) for quantifying liver fibrosis in patients with and without hepatic iron overload. METHODS: This retrospective single-center study analyzed 139 patients who underwent liver MRI at 3 Tesla including MRE (2D spin-echo EPI sequence) and R2* mapping for liver iron content (LIC) estimation. MRE feasibility and diagnostic performance between patients with normal and elevated LIC were compared. RESULTS: Patients with elevated LIC (21%) had significantly higher MRE failure rates (24.1% vs. 3.6%, p < 0.001) compared to patients with normal LIC (79%). For those with only insignificant to mild iron overload (LIC < 5.4 mg/g; 17%), MRE failure rate did not differ significantly from patients without iron overload (8.3% vs. 3.6%, p = 0.315). R2* predicted MRE failure with fair accuracy at a threshold of R2* ≥ 269 s-1 (LIC of approximately 4.6 mg/g). MRE showed good diagnostic performance for detecting significant (≥ F2) and severe fibrosis (≥ F3) in patients without (AUC 0.835 and 0.900) and with iron overload (AUC 0.818 and 0.889) without significant difference between the cohorts (p = 0.884 and p = 0.913). For detecting cirrhosis MRE showed an excellent diagnostic performance in both groups (AUC 0.944 and 1.000, p = 0.009). CONCLUSION: Spin-echo EPI MRE at 3 Tesla is feasible in patients with mild iron overload with good to excellent performance for detecting hepatic fibrosis with a failure rate comparable to patients without iron overload.

Feasibility of deep learning-reconstructed thin-slice single-breath-hold HASTE for detecting pancreatic lesions: A comparison with two conventional T2-weighted imaging sequences.

Objective: The objective of this study was to evaluate the clinical feasibility of deep learning reconstruction-accelerated thin-slice single-breath-hold half-Fourier single-shot turbo spin echo imaging (HASTEDL) for detecting pancreatic lesions, in comparison with two conventional T2-weighted imaging sequences: compressed-sensing HASTE (HASTECS) and BLADE. Methods: From March 2022 to January 2023, a total of 63 patients with suspected pancreatic-related disease underwent the HASTEDL, HASTECS, and BLADE sequences were enrolled in this retrospectively study. The acquisition time, the pancreatic lesion conspicuity (LCP), respiratory motion artifact (RMA), main pancreatic duct conspicuity (MPDC), overall image quality (OIQ), signal-to-noise ratio (SNR), and contrast-noise-ratio (CNR) of the pancreatic lesions were compared among the three sequences by two readers. Results: The acquisition time of both HASTEDL and HASTECS was 16 s, which was significantly shorter than that of 102 s for BLADE. In terms of qualitative parameters, Reader 1 and Reader 2 assigned significantly higher scores to the LCP, RMA, MPDC, and OIQ for HASTEDL compared to HASTECS and BLADE sequences; As for the quantitative parameters, the SNR values of the pancreatic head, body, tail, and lesions, the CNR of the pancreatic lesion measured by the two readers were also significantly higher for HASTEDL than for HASTECS and BLADE sequences. Conclusions: Compared to conventional T2WI sequences (HASTECS and BLADE), deep-learning reconstructed HASTE enables thin slice and single-breath-hold acquisition with clinical acceptable image quality for detection of pancreatic lesions.

Evaluating the Efficacy of Volume Isotropic Turbo Spin Echo Acquisition Versus T2-Weighted Turbo Spin Echo Imaging in the Diagnosis of Nerve Root and Perineuronal Pathologies in Spinal Disorders.

Background While two-dimensional (2D) turbo spin echo (TSE) sequences offer better through-plane resolution than three-dimensional (3D) isotropic TSE sequences images, with a narrower thickness of the slice, 3D isotropic TSE sequences are known to have a weaker in-plane resolution as well as blurring of the image. These elements may make it more difficult to distinguish between nearby structures that may affect nerve roots and small nerve roots during spinal imaging. This study aimed to analyze the accuracy of T2 TSE sequence and volumetric isotropic TSE acquisition in determining the indentation of nerve roots and perineural diseases such as nerve sheath tumors and Tarlov cysts. Methods Fifty patients who attended the Department of Radiodiagnosis for magnetic resonance (MR) spine participated in this prospective study. Routine MR lumbosacral (LS) spine sequences, such as survey, coronal T2 short-tau inversion recovery (STIR), sagittal T2 TSE, sagittal T1 TSE, and axial T2 TSE, were carried out after a localizer was acquired. More sequences from volume isotropic turbo spin echo acquisition (VISTA) were acquired. For both 2D and 3D sequences, the visibility ratings for perineural cysts, spinal canal stenosis, and nerve root indentation were evaluated. Visibility ratings ranged from zero to four. Results In the cases of perineural cyst, spinal canal stenosis, and nerve root impingement, the mean difference between the VISTA and T2 TSE visibility scores was 0.04, 0.54, and 0.56, respectively. The VISTA and T2 TS had standard deviation differences of 0.006, 0.026, and 0.06, respectively. The "t" values for nerve root impingement, spinal canal stenosis, and perineural cysts were, in order, 50, 180, and 70. Because the p-value was <0.01, a statistically significant variation has been observed. Conclusion In the diagnosis of neural and perineuronal disorders, the visibility scores for 3D T2 TSE (VISTA) were considerably better than those for 2D T2 TSE in identifying perineural cysts, spinal canal stenosis, and nerve root indentation.

MISSTEC-S: A fast 1H pulse calibration from spectra simultaneously produced by a spin echo and a stimulated echo.

Radio-Frequency (RF) pulse calibration is an essential step in guaranteeing both optimum acquisition quality in multi-pulse NMR and accurate results in quantitative experiments. Most existing methods are based on a series of spectra for which the flip angle of one or more pulses is progressively incremented, implying a significant experiment time. In order to circumvent this drawback, we have previously proposed an approach based on the acquisition of a spin echo and a stimulated echo - the MISSTEC sequence - which requires only 8 s to determine the PW90-1H, while it is several minutes in the case of the use of a nutation curve. In this work, a new sequence for RF calibration is presented: MISSTEC-S. It is derived from the previously proposed MISSTEC sequence, but the observation of echoes in presence of magnetic field gradient is replaced by the observation of FIDs. This modification allows both spectra to be phased, while imposing a strong constraint on the Mixing Time (TM). However, the relationship used to calculate the flip angle is only correct when TM is small enough to neglect longitudinal relaxation during this delay. In order to reduce TM, the first FID is truncated during acquisition and subsequently lengthened using points from the second FID. Results obtained with MISSTEC-S were compared to those obtained from a complete nutation curve and an excellent correlation was observed, although the experimental time to obtain the PW90 is dramatically reduced.

Assessing the radiofrequency shielding effect of titanium mesh on diffusion-weighted imaging: a comparative study of the twice-refocused spin-echo and Stejskal-Tanner sequences.

This study compared twice-refocused spin-echo sequence (TRSE) and Stejskal-Tanner sequence (ST) to evaluate their respective effects on the image quality of magnetic resonance (MR) diffusion-weighted imaging in the presence of radiofrequency (RF) shielding effect of titanium mesh in cranioplasty. A 1.5-T MR scanner with a Head/Neck coil 20 channels and a phantom simulating the T2 and apparent diffusion coefficient (ADC) value of the human brain were used. Imaging was performed with and without titanium mesh placed on the phantom in TRSE and ST, and normalized absolute average deviation (NAAD), Dice similarity coefficient (DSC), and ADC values were calculated. The NAAD values were significantly lower for TRSE than for ST in the area below the titanium mesh, and the drop rates due to titanium mesh were 14.1% for TRSE and 9.8% for ST. The DSC values were significantly lower for TRSE than for ST. The ADC values were significantly higher for TRSE than for ST without titanium mesh. The ADC values showed no significant difference between TRSE and ST with titanium mesh. The ST had a lower RF shielding effect of titanium mesh than the TRSE.

Can Three-Dimensional T1-weighted MR Imaging Visualize the Vascular Lumen after Carotid Artery Stenting?

PURPOSE: We aimed to evaluate the quality of various 3D T1-weighted images (T1WIs) of the stent lumen using a carotid stent phantom and determine the suitable T1WI sequence for visualization of the stent lumen after carotid artery stenting. METHODS: The carotid stent phantom consisted of polypropylene tubes that mimicked common carotid arteries with and without stenting. On 1.5T and 3.0T MRI scanners, transverse T1WIs of the carotid stent phantom were obtained using 3D turbo spin-echo (TSE), 3D fast field-echo (3D-FFE), and 3D turbo field echo volumetric interpolated breath-hold examination (VIBE) under clinical conditions. The signal intensity ratio (SIR) was determined using the mean signal intensity of the stent lumen (SIstent) divided by the lumen without a stent in each T1WI. The SNR of the stent lumen (SNRstent) was calculated from SIstent divided by the standard deviation of the uniform region near the stent lumen. RESULTS: The 3D-FFE and VIBE had higher SNRstent than other T1WIs and clearly visualized the stent lumen. The 3D-TSE had the lowest SIR and SNRstent, preventing stent lumen visualization. CONCLUSION: T1WIs obtained using 3D-FFE and VIBE allows stent lumen visualization.

Fast and High-Resolution T2 Mapping Based on Echo Merging Plus k-t Undersampling with Reduced Refocusing Flip Angles (TEMPURA) as Methods for Human Renal MRI.

PURPOSE: To develop a highly accelerated multi-echo spin-echo method, TEMPURA, for reducing the acquisition time and/or increasing spatial resolution for kidney T2 mapping. METHODS: TEMPURA merges several adjacent echoes into one k-space by either combining independent echoes or sharing one echo between k-spaces. The combined k-space is reconstructed based on compressed sensing theory. Reduced flip angles are used for the refocusing pulses, and the extended phase graph algorithm is used to correct the effects of indirect echoes. Two sequences were developed: a fast breath-hold sequence; and a high-resolution sequence. The performance was evaluated prospectively on a phantom, 16 healthy subjects, and two patients with different types of renal tumors. RESULTS: The fast TEMPURA method reduced the acquisition time from 3-5 min to one breath-hold (18 s). Phantom measurements showed that fast TEMPURA had a mean absolute percentage error (MAPE) of 8.2%, which was comparable to a standardized respiratory-triggered sequence (7.4%), but much lower than a sequence accelerated by purely k-t undersampling (21.8%). High-resolution TEMPURA reduced the in-plane voxel size from 3 × 3 to 1 × 1 mm2, resulting in improved visualization of the detailed anatomical structure. In vivo T2 measurements demonstrated good agreement (fast: MAPE = 1.3%-2.5%; high-resolution: MAPE = 2.8%-3.3%) and high correlation coefficients (fast: R = 0.85-0.98; high-resolution: 0.82-0.96) with the standardized method, outperforming k-t undersampling alone (MAPE = 3.3-4.5%, R = 0.57-0.59). CONCLUSION: TEMPURA provides fast and high-resolution renal T2 measurements. It has the potential to improve clinical throughput and delineate intratumoral heterogeneity and tissue habitats at unprecedented spatial resolution.

Accelerated 3D multi-echo spin-echo sequence with a subspace constrained reconstruction for whole mouse brain T 2 mapping.

PURPOSE: To accelerate whole-brain quantitative T 2 $$ {\mathrm{T}}_2 $$ mapping in preclinical imaging setting. METHODS: A three-dimensional (3D) multi-echo spin echo sequence was highly undersampled with a variable density Poisson distribution to reduce the acquisition time. Advanced iterative reconstruction based on linear subspace constraints was employed to recover high-quality raw images. Different subspaces, generated using exponential or extended-phase graph (EPG) simulations or from low-resolution calibration images, were compared. The subspace dimension was investigated in terms of T 2 $$ {\mathrm{T}}_2 $$ precision. The method was validated on a phantom containing a wide range of T 2 $$ {\mathrm{T}}_2 $$ and was then applied to monitor metastasis growth in the mouse brain at 4.7T. Image quality and T 2 $$ {\mathrm{T}}_2 $$ estimation were assessed for 3 acceleration factors (6/8/10). RESULTS: The EPG-based dictionary gave robust estimations of a large range of T 2 $$ {\mathrm{T}}_2 $$ . A subspace dimension of 6 was the best compromise between T 2 $$ {\mathrm{T}}_2 $$ precision and image quality. Combining the subspace constrained reconstruction with a highly undersampled dataset enabled the acquisition of whole-brain T 2 $$ {\mathrm{T}}_2 $$ maps, the detection and the monitoring of metastasis growth of less than 500 µ m 3 $$ \mu {\mathrm{m}}^3 $$ . CONCLUSION: Subspace-based reconstruction is suitable for 3D T 2 $$ {\mathrm{T}}_2 $$ mapping. This method can be used to reach an acceleration factor up to 8, corresponding to an acquisition time of 25 min for an isotropic 3D acquisition of 156 µ $$ \mu $$ m on the mouse brain, used here for monitoring metastases growth.