Improving the image quality of 3D FLAIR with a spiral MRI technique

Variable-flip-angle 3D spiral-in-out turbo spin-echo imaging using concomitant gradient compensation and echo reordering at 0.55 T

PURPOSE

To develop single-slab 3D spiral turbo spin echo (spiral SPACE) for 1-mm3 isotropic whole-brain T2-weighted imaging on a high-performance 0.55T scanner, with high scan efficiency from interleaved spiral-in-out trajectories, variable-flip-angle refocusing radiofrequency (RF) pulses, echo reordering, and concomitant-field compensation.

METHODS

A stack-of-spirals (in-out waveforms) turbo-spin-echo acquisition was implemented with T2-weighed contrast. Gradient infidelity was corrected using the gradient impulse response function (GIRF), and concomitant-field compensation was used to correct for phase errors among echoes and during the readout windows. To maintain a long echo train (˜600 ms) within each shot, variable-flip-angle refocusing RF pulses were generated using extended-phase-graph analysis. An echo-reordering scheme provided a smooth signal variation along the echo direction in k-space. Images from spiral SPACE with and without concomitant-field compensation were compared with those from Cartesian SPACE in phantoms and 6 healthy volunteers.

RESULTS

Phantom results demonstrated the improved performance of concomitant-field correction via sequence-based modifications and of GIRF-based trajectory estimation. Volunteer data showed that with concomitant-field correction and echo reordering, system imperfection associated image artifacts and blurring were substantially mitigated in spiral SPACE. Compared with Cartesian SPACE, spiral SPACE had an overall 15%-25% signal-to-noise ratio (SNR) improvement in both white matter and gray matter.

CONCLUSION

A 3D spiral-in-out SPACE acquisition with variable-flip-angles, concomitant-field compensation, and echo-reordering was demonstrated at 0.55 T, showing promising gains in SNR, compared with Cartesian SPACE.

Efficient imaging using spiral acquisitions on a portable 50-mT MR head scanner

Ultralow-field (ULF) magnetic resonance imaging (MRI) can suffer from inferior image quality because of low signal-to-noise ratio (SNR). As an efficient way to cover the k-space, the spiral acquisition technique has shown great potential in improving imaging SNR efficiency at ULF. The current study aimed to address the problems of noise and blurring cancelation in the ULF case with spiral trajectory, and we proposed a spiral-out sequence for brain imaging using a portable 50-mT MRI system. The proposed sequence consisted of three modules: noise calibration, field map acquisition, and imaging. In the calibration step, transfer coefficients were obtained between signals from primary and noise-pick-up coils to perform electromagnetic interference (EMI) cancelation. Embedded field map acquisition was performed to correct accumulated phase error due to main field inhomogeneity. Considering imaging SNR, a lower bandwidth for data sampling was adopted in the sequence design because the 50-mT scanner is in a low SNR regime. Image reconstruction proceeded with sampled data by leveraging system imperfections, such as gradient delays and concomitant fields. The proposed method can provide images with higher SNR efficiency compared with its Cartesian counterparts. An improvement in temporal SNR of approximately 23%-44% was measured via phantom and in vivo experiments. Distortion-free images with a noise suppression rate of nearly 80% were obtained by the proposed technique. A comparison was also made with a state-of-the-art EMI cancelation algorithm used in the ULF-MRI system. SNR efficiency-enhanced spiral acquisitions were investigated for ULF-MR scanners and future studies could focus on various image contrasts based on our proposed approach to widen ULF applications.

Tailored magnetic resonance fingerprinting

Neuroimaging of certain pathologies requires both multi-parametric qualitative and quantitative imaging. The role of the quantitative MRI (qMRI) is well accepted but suffers from long acquisition times leading to patient discomfort, especially in geriatric and pediatric patients. Previous studies show that synthetic MRI can be used in order to reduce the scan time and provide qMRI as well as multi-contrast data. However, this approach suffers from artifacts such as partial volume and flow. In order to increase the scan efficiency (the number of contrasts and quantitative maps acquired per unit time), we designed, simulated, and demonstrated rapid, simultaneous, multi-contrast qualitative (T₁ weighted, T₁ fluid attenuated inversion recovery (FLAIR), T₂ weighted, water, and fat), and quantitative imaging (T₁ and T₂ maps) through the approach of tailored MR fingerprinting (TMRF) to cover whole-brain in approximately four minutes. We performed TMRF on in vivo four healthy human brains and in vitro ISMRM/NIST phantom and compared with vendor supplied gold standard (GS) and MRF sequences. All scans were performed on a 3 T GE Premier system and images were reconstructed offline using MATLAB. The reconstructed qualitative images were then subjected to custom DL denoising and gradient anisotropic diffusion denoising. The quantitative tissue parametric maps were reconstructed using a dense neural network to gain computational speed compared to dictionary matching. The grey matter and white matter tissues in qualitative and quantitative data for the in vivo datasets were segmented semi-automatically. The SNR and mean contrasts were plotted and compared across all three methods. The GS images show better SNR in all four subjects compared to MRF and TMRF (GS > TMRF>MRF). The T₁ and T₂ values of MRF are relatively overestimated as compared to GS and TMRF. The scan efficiency for TMRF is 1.72 min^-1 which is higher compared to GS (0.32 min^-1) and MRF (0.90 min^-1).

Off-resonance artifact correction for MRI: A review

In magnetic resonance imaging (MRI), inhomogeneity in the main magnetic field used for imaging, referred to as off-resonance, can lead to image artifacts ranging from mild to severe depending on the application. Off-resonance artifacts, such as signal loss, geometric distortions, and blurring, can compromise the clinical and scientific utility of MR images. In this review, we describe sources of off-resonance in MRI, how off-resonance affects images, and strategies to prevent and correct for off-resonance. Given recent advances and the great potential of low-field and/or portable MRI, we also highlight the advantages and challenges of imaging at low field with respect to off-resonance.

Quantitative Analysis for the Delineation of the Subthalamic Nuclei on Three-Dimensional Stereotactic MRI Before Deep Brain Stimulation Surgery for Medication-Refractory Parkinson’s Disease

Delineation of the subthalamic nuclei (STN) on MRI is critical for deep brain stimulation (DBS) surgery in patients with Parkinson’s disease (PD). We propose this retrospective cohort study for quantitative analysis of MR signal-to-noise ratio (SNR), contrast, and signal difference-to-noise ratio (SDNR) of the STN on pre-operative three-dimensional (3D) stereotactic MRI in patients with medication-refractory PD. Forty-five consecutive patients with medication-refractory PD who underwent STN-DBS surgery in our hospital from January 2018 to June 2021 were included in this study. All patients had whole-brain 3D MRI, including T2-weighted imaging (T2WI), T2-weighted fluid-attenuated inversion recovery (FLAIR), and susceptibility-weighted imaging (SWI), at 3.0 T scanner for stereotactic navigation. The signal intensities of the STN, corona radiata, and background noise were obtained after placing regions of interest (ROIs) on corresponding structures. Quantitative comparisons of SNR, contrast, and SDNR of the STN between MR pulse sequences, including the T2WI, FLAIR, and SWI. Subgroup analysis regarding patients’ sex, age, and duration of treatment. We used one-way repeated measures analysis of variance for quantitative comparisons of SNR, contrast, and SDNR of the STN between different MR pulse sequences, and we also used the dependent t-test for the post hoc tests. In addition, we used Mann–Whitney U test for subgroup analyses. Both the contrast (0.33 ± 0.07) and SDNR (98.65 ± 51.37) were highest on FLAIR (all p < 0.001). The SNR was highest on SWI (276.16 ± 115.5), and both the SNR (94.23 ± 31.63) and SDNR (32.14 ± 17.23) were lowest on T2WI. Subgroup analyses demonstrated significantly lower SDNR on SWI for patients receiving medication treatment for ≥13 years (p = 0.003). In conclusion, on 3D stereotactic MRI of medication-refractory PD patients, the contrast and SDNR for the STN are highest on FLAIR, suggesting the optimal delineation of STN on FLAIR.

Clinical Value of 3D-FLAIR MRI in Idiopathic Sudden Sensorineural Hearing Loss

BACKGROUND

Three-dimensional fluid-attenuated inversion recovery sequence magnetic resonance imaging (3D-FLAIR MRI) has been used in the diagnosis of inner ear diseases. However, the relevance of 3D-FLAIR MRI appearances with multiple features and prognosis of patients with idiopathic sudden sensorineural hearing loss (ISSNHL) remains unclear.

METHODS

This study was a retrospective trial. We recruited 1300 patients with unilateral ISSNHL hospitalized from May 2017 to January 2019. They were divided into four groups according to their 3D-FLAIR MRI appearances: normal (n = 739), inner ear hemorrhage (n = 218), increased protein content (n = 288), and blood-labyrinth barrier damage (n = 55). The correlation between 3D-FLAIR MRI appearances and the degree or type of deafness of the participants was analyzed.

RESULTS

There was a statistical difference in the deafness side (p < 0.001) and vestibular dysfunction (p < 0.001) among the four groups. There was a statistical difference in the duration of treatment (p < 0.001) and the incidence of dizziness or vertigo (p < 0.001) for patients among these groups. The degree of deafness in the patients in the inner ear hemorrhage group was significantly more severe than that of the patients in the other three groups (p < 0.001).

CONCLUSION

3D-FLAIR MRI appearances were correlated with the prognosis of patients with ISSNHL.

Spiral 2D T2-Weighted TSE Brain MR Imaging: Initial Clinical Experience

BACKGROUND AND PURPOSE

Spiral MR imaging may enable improved image quality and higher scan speeds than Cartesian trajectories. We sought to compare a novel spiral 2D T2-weighted TSE sequence with a conventional Cartesian and an artifact-robust, non-Cartesian sequence named MultiVane for routine clinical brain MR imaging.

MATERIALS AND METHODS

Thirty-one patients were scanned with all 3 sequences (Cartesian, 4 minutes 14 seconds; MultiVane, 2 minutes 49 seconds; spiral, 2 minutes 12 seconds) on a standard clinical 1.5T MR scanner. Three readers described the presence and location of abnormalities and lesions and graded images qualitatively in terms of overall image quality, the presence of motion and pulsation artifacts, gray-white matter differentiation, lesion conspicuity, and subjective preference. Image quality was objectivized by measuring the SNR and the coefficients of variation for CSF, GM, and WM.

RESULTS

Spiral achieved a scan time reduction of 51.9% and 21.9% compared with Cartesian and MultiVane, respectively. The number and location of lesions were identical among all sequences. As for the qualitative analysis, interreader agreement was high (Krippendorff α > .75). Spiral and MultiVane both outperformed the Cartesian sequence in terms of overall image quality, the presence of motion artifacts, and subjective preference (P < .001). In terms of the presence of pulsation artifacts, gray-white matter differentiation, and lesion conspicuity, all 3 sequences performed similarly well (P > .15). Spiral and MultiVane outperformed the Cartesian sequence in coefficient of variation WM and SNR (P < .01).

CONCLUSIONS

Spiral 2D T2WI TSE is feasible for routine structural brain MR imaging and offers high-quality, artifact-robust brain imaging in short scan times.

Technical note: A spiral fluid-attenuated inversion recovery magnetic resonance imaging technique for stereotactic radiosurgery treatment planning for trigeminal neuralgia

PURPOSE

Magnetic resonance imaging (MRI) is commonly used in treatment planning for stereotactic radiosurgery (SRS) of trigeminal neuralgia (TN). With current MRI techniques, the delineation of the trigeminal nerve root entry zone (REZ) may be degraded due to poor contrast and artifacts. The purpose of this work is to develop an MRI technique with better delineation of the trigeminal nerve REZ to improve SRS treatment planning for TN.

METHODS

A spiral fluid-attenuated inversion recovery (FLAIR) MRI technique was developed to improve image quality by improving tissue contrast, fluid suppression, artifact reduction, and signal-to-noise ratio (SNR). A concomitant-phase compensation method based on spiral gradient waveforms was implemented to minimize artifacts due to magnetic field change induced by the metal frame used in Gamma Knife treatment planning. The image quality of spiral FLAIR was assessed in four healthy volunteers. The geometric accuracy was quantitatively evaluated by registering spiral FLAIR to computed tomography (CT) images and comparing it with existing MRI techniques.

RESULTS

The spiral FLAIR technique demonstrated better delineation of the trigeminal nerve REZ, improved tissue contrast of the brain stem, and minimized flow artifacts, compared to steady-state free precession (SSFP) MRI. Spiral FLAIR also improved fluid suppression, SNR, and artifacts, which contributed to better delineation of the trigeminal nerve REZ compared to conventional Cartesian FLAIR. The measured mean (± standard deviation) distance between spiral FLAIR and CT images is 0.98 ± 0.56 mm, comparable to 0.40 ± 0.26 mm in 3T T1 spoiled gradient echo (T1-SPGR), 0.59 ± 0.25 mm in 3T SSFP, 0.66 ± 0.38 mm in 1.5T T1-SPGR, and 0.61 ± 0.25 mm in 1.5T Cartesian FLAIR.

CONCLUSION

A spiral FLAIR technique with improved image quality and good geometric accuracy provides a potential alternative for treatment planning in SRS for TN patients.

Mono-planar T-Hex: Speed and flexibility for high-resolution 3D imaging

Purpose

The aim of this work is the reconciliation of high spatial and temporal resolution for MRI. For this purpose, a novel sampling strategy for 3D encoding is proposed, which provides flexible k‐space segmentation along with uniform sampling density and benign filtering effects related to signal decay.

Methods

For time‐critical MRI applications such as functional MRI (fMRI), 3D k‐space is usually sampled by stacking together 2D trajectories such as echo planar imaging (EPI) or spiral readouts, where each shot covers one k‐space plane. For very high temporal and medium to low spatial resolution, tilted hexagonal sampling (T‐Hex) was recently proposed, which allows the acquisition of a larger k‐space volume per excitation than can be covered with a planar readout. Here, T‐Hex is described in a modified version where it instead acquires a smaller k‐space volume per shot for use with medium temporal and high spatial resolution.

Results

Mono‐planar T‐Hex sampling provides flexibility in the choice of speed, signal‐to‐noise ratio (SNR), and contrast for rapid MRI acquisitions. For use with a conventional gradient system, it offers the greatest benefit in a regime of high in‐plane resolution <1 mm. The sampling scheme is combined with spirals for high sampling speed as well as with more conventional EPI trajectories.

Conclusion

Mono‐planar T‐Hex sampling combines fast 3D encoding with SNR efficiency and favorable depiction characteristics regarding noise amplification and filtering effects from T2∗ decay, thereby providing flexibility in the choice of imaging parameters. It is attractive both for high‐resolution time series such as fMRI and for applications that require rapid anatomical imaging.