Implementation of the FLORET UTE sequence for lung imaging

3D balanced SSFP UTE MRI for multiple contrasts whole brain imaging

PURPOSE

This study aimed to develop a new high-resolution MRI sequence for the imaging of the ultra-short transverse relaxation time (uT2) components in the brain, while simultaneously providing proton density (PD) contrast for reference and quantification.

THEORY

The sequence combines low flip angle balanced SSFP (bSSFP) and UTE techniques, together with a 3D dual-echo rosette k-space trajectory for readout.

METHODS

The expected image contrast was evaluated by simulations. A study cohort of six healthy volunteers and eight multiple sclerosis (MS) patients was recruited to test the proposed sequence. Subtraction between two TEs was performed to extract uT2 signals. In addition, conventional longitudinal relaxation time (T1) weighted, T2-weighted, and PD-weighted MRI sequences were also acquired for comparison.

RESULTS

Typical PD-contrast was found in the second TE images, while uT2 signals were selectively captured in the first TE images. The subtraction images presented signals primarily originating from uT2 components, but only if the first TE is short enough. Lesions in the MS subjects showed hyperintense signals in the second TE images but were hypointense signals in the subtraction images. The lesions had significantly lower signal intensity in subtraction images than normal white matter (WM), which indicated a reduction of uT2 components likely associated with myelin.

CONCLUSION

3D isotropic sub-millimeter (0.94 mm) spatial resolution images were acquired with the novel bSSFP UTE sequence within 3 min. It provided easy extraction of uT2 signals and PD-contrast for reference within a single acquisition.

Assessment of glenoid bone loss and other osseous shoulder pathologies comparing MR-based CT-like images with conventional CT

OBJECTIVES

To evaluate and compare the diagnostic performance of CT-like images based on a 3D T1-weighted spoiled gradient-echo sequence (T1 GRE), an ultra-short echo time sequence (UTE), and a 3D T1-weighted spoiled multi-echo gradient-echo sequence (FRACTURE) with conventional CT in patients with suspected osseous shoulder pathologies.

MATERIALS AND METHODS

Patients with suspected traumatic dislocation of the shoulder (n = 46, mean age 40 ± 14.5 years, 19 women) were prospectively recruited and received 3-T MR imaging including 3D T1 GRE, UTE, and 3D FRACTURE sequences. CT was performed in patients with acute fractures and served as standard of reference (n = 25). Agreement of morphological features between the modalities was analyzed including the glenoid bone loss, Hill-Sachs interval, glenoid track, and the anterior straight-line length. Agreement between the modalities was assessed using Bland-Altman plots, Student's t-test, and Pearson's correlation coefficient. Inter- and intrareader assessment was evaluated with weighted Cohen's κ and intraclass correlation coefficient.

RESULTS

All osseous pathologies were detected accurately on all three CT-like sequences (n = 25, κ = 1.00). No significant difference in the percentage of glenoid bone loss was found between CT (mean ± standard deviation, 20.3% ± 8.0) and CT-like MR images (FRACTURE 20.6% ± 7.9, T1 GRE 20.4% ± 7.6, UTE 20.3% ± 7.7, p > 0.05). When comparing the different measurements on CT-like images, measurements performed using the UTE images correlated best with CT.

CONCLUSION

Assessment of bony Bankart lesions and other osseous pathologies was feasible and accurate using CT-like images based on 3-T MRI compared with conventional CT. Compared to the T1 GRE and FRACTURE sequence, the UTE measurements correlated best with CT.

CLINICAL RELEVANCE STATEMENT

In an acute trauma setting, CT-like images based on a T1 GRE, UTE, or FRACTURE sequence might be a useful alternative to conventional CT scan sparing associated costs as well as radiation exposure.

KEY POINTS

• No significant differences were found for the assessment of the glenoid bone loss when comparing measurements of CT-like MR images with measurements of conventional CT images. • Compared to the T1 GRE and FRACTURE sequence, the UTE measurements correlated best with CT whereas the FRACTURE sequence appeared to be the most robust regarding motion artifacts. • The T1 GRE sequence had the highest resolution with high bone contrast and detailed delineation of even small fractures but was more susceptible to motion artifacts.

Diffusion weighted hyperpolarized 129 Xe MRI of the lung with 2D and 3D (FLORET) spiral

PURPOSE

To enable efficient hyperpolarized ¹²⁹ Xe diffusion imaging using 2D and 3D (Fermat Looped, ORthogonally Encoded Trajectories, FLORET) spiral sequences and demonstrate that ¹²⁹ Xe ADCs obtained using these sequences are comparable to those obtained using a conventional, 2D gradient-recalled echo (GRE) sequence.

THEORY AND METHODS

Diffusion-weighted ¹²⁹ Xe MRI (b-values = 0, 7.5, 15 s/cm² ) was performed in four healthy volunteers and one subject with lymphangioleiomyomatosis using slice-selective 2D-GRE (scan time = 15 s), slice-selective 2D-Spiral (4 s), and 3D-FLORET (16 s) sequences. Experimental SNRs from b-value = 0 images ( SNR 0 EX $$ SNR{0}_{EX} $$ ) and mean ADC values were compared across sequences. In two healthy subjects, a second b = 0 image was acquired using the 2D-Spiral sequence to map flip angle and correct RF-induced, hyperpolarized signal decay at the voxel level, thus improving regional ADC estimates.

RESULTS

Diffusion-weighted images from spiral sequences displayed image quality comparable to 2D-GRE and produced sufficient SNR 0 EX $$ SNR{0}_{EX} $$ (16.8 ± 3.8 for 2D-GRE, 21.2 ± 3.5 for 2D-Spiral, 20.4 ± 3.5 for FLORET) to accurately calculate ADC. Whole-lung means and SDs of ADC obtained via spiral were not significantly different (P > 0.54) from those obtained via 2D-GRE. Finally, 2D-Spiral images were corrected for signal decay, which resulted in a whole-lung mean ADC decrease of ˜15%, relative to uncorrected images.

CONCLUSIONS

Relative to GRE, efficient spiral sequences allow ¹²⁹ Xe diffusion images to be acquired with isotropic lung coverage (3D), higher SNR $$ SNR $$ (2D and 3D), and three-fold faster (2D) within a single breath-hold. In turn, shortened breath-holds enable flip-angle mapping, and thus, allow RF-induced signal decay to be corrected, increasing ADC accuracy.

Ultra-short T2 components imaging of the whole brain using 3D dual-echo UTE MRI with rosette k-space pattern

PURPOSE

This study aimed to develop a new 3D dual-echo rosette k-space trajectory, specifically designed for UTE MRI applications. The imaging of the ultra-short transverse relaxation time (uT₂ ) of brain was acquired to test the performance of the proposed UTE sequence.

THEORY AND METHODS

The rosette trajectory was developed based on rotations of a "petal-like" pattern in the k_x -k_y plane, with oscillated extensions in the k_z -direction for 3D coverage. Five healthy volunteers underwent 10 dual-echo 3D rosette UTE scans with various TEs. Dual-exponential complex model fitting was performed on the magnitude data to separate uT₂ signals, with the output of uT₂ fraction, uT₂ value, and long-T₂ value.

RESULTS

The 3D rosette dual-echo UTE sequence showed better performance than a 3D radial UTE acquisition. More significant signal intensity decay in white matter than gray matter was observed along with the TEs. The white matter regions had higher uT₂ fraction values than gray matter (10.9% ± 1.9% vs. 5.7% ± 2.4%). The uT₂ value was approximately 0.10 ms in white matter .

CONCLUSION

The higher uT₂ fraction value in white matter compared to gray matter demonstrated the ability of the proposed sequence to capture rapidly decaying signals.

3D MR fingerprinting using Seiffert spirals

PURPOSE

Seiffert spirals were recently explored as an efficient way to traverse 3D k-space compared to traditional 3D techniques. Several studies have shown the ability of 3D MR fingerprinting (MRF) techniques to acquire T₁ and T₂ relaxation maps in a short period of time. However, these sequences do not sample across a large region of 3D k-space every TR, especially in the way that Seiffert trajectories can.

METHODS

A 3D MRF sequence was designed using 8 Seiffert spirals rotated in 3D k-space, with flip angle modulation for T₁ and T₂ sensitivity. The sequence was compared to an MRF sequence using a 2D spiral rotated in 3D k-space using the tiny golden angle acquisition with similar resolution/readout duration. Both sequences were evaluated using simulations, phantom validation, and in vivo imaging.

RESULTS

In all experiments, the Seiffert spiral MRF sequence performed similar to if not better than the multi-axis 2D spiral MRF sequence. Strong intraclass correlation coefficients (> 0.9) were found between conventional and MRF sequences in phantoms, whereas the in vivo results showed slightly less aliasing artifact with the Seiffert trajectory.

CONCLUSION

In this study, Seiffert spirals were used within the MRF framework to acquire high-resolution T₁ and T₂ relaxation time maps in less than 2.5 min. The reduced aliasing artifacts seen with the Seiffert sequence suggests that sampling over 3D k-space evenly each TR can improve quantification or shorten scan times.

Practical protocol for lung magnetic resonance imaging and common clinical indications

Imaging speed, spatial resolution and availability have made CT the favored cross-sectional imaging modality for evaluating various respiratory diseases of children - but only for the price of a radiation exposure. MRI is increasingly being appreciated as an alternative to CT, not only for offering three-dimensional (3-D) imaging without radiation exposure at only slightly inferior spatial resolution, but also for its superior soft-tissue contrast and exclusive morpho-functional imaging capacities beyond the scope of CT. Continuing technical improvements and experience with this so far under-utilized modality contribute to a growing acceptance of MRI for an increasing number of indications, in particular for pediatric patients. This review article provides the reader with practical easy-to-use protocols for common clinical indications in children. This is intended to encourage pediatric radiologists to appreciate the new horizons for applications of this rapidly evolving technique in the field of pediatric respiratory diseases.

Bronchopulmonary dysplasia from chest radiographs to magnetic resonance imaging and computed tomography: adding value

Bronchopulmonary dysplasia (BPD) is a common long-term complication of preterm birth. The chest radiograph appearance and survivability have evolved since the first description of BPD in 1967 because of improved ventilation and clinical strategies and the introduction of surfactant in the early 1990s. Contemporary imaging care is evolving with the recognition that comorbidities of tracheobronchomalacia and pulmonary hypertension have a great influence on outcomes and can be noninvasively evaluated with CT and MRI techniques, which provide a detailed evaluation of the lungs, trachea and to a lesser degree the heart. However, echocardiography remains the primary modality to evaluate and screen for pulmonary hypertension. This review is intended to highlight the important findings that chest radiograph, CT and MRI can contribute to precision diagnosis, phenotyping and prognosis resulting in optimal management and therapeutics.

Self-gated 3D stack-of-spirals UTE pulmonary imaging at 0.55T

PURPOSE

To develop an isotropic high-resolution stack-of-spirals UTE sequence for pulmonary imaging at 0.55 Tesla by leveraging a combination of robust respiratory-binning, trajectory correction, and concomitant-field corrections.

METHODS

A stack-of-spirals golden-angle UTE sequence was used to continuously acquire data for 15.5 minutes. The data was binned to a stable respiratory phase based on superoinferior readout self-navigator signals. Corrections for trajectory errors and concomitant field artifacts, along with image reconstruction with conjugate gradient SENSE, were performed inline within the Gadgetron framework. Finally, data were retrospectively reconstructed to simulate scan times of 5, 8.5, and 12 minutes. Image quality was assessed using signal-to-noise, image sharpness, and qualitative reader scores. The technique was evaluated in healthy volunteers, patients with coronavirus disease 2019 infection, and patients with lung nodules.

RESULTS

The technique provided diagnostic quality images with parenchymal lung SNR of 3.18 ± 0.0.60, 4.57 ± 0.87, 5.45 ± 1.02, and 5.89 ± 1.28 for scan times of 5, 8.5, 12, and 15.5 minutes, respectively. The respiratory binning technique resulted in significantly sharper images (p < 0.001) as measured with relative maximum derivative at the diaphragm. Concomitant field corrections visibly improved sharpness of anatomical structures away from iso-center. The image quality was maintained with a slight loss in SNR for simulated scan times down to 8.5 minutes. Inline image reconstruction and artifact correction were achieved in <5 minutes.

CONCLUSION

The proposed pulmonary imaging technique combined efficient stack-of-spirals imaging with robust respiratory binning, concomitant field correction, and trajectory correction to generate diagnostic quality images with 1.75 mm isotropic resolution in 8.5 minutes on a high-performance 0.55 Tesla system.

Hyperpolarized 129Xe magnetic resonance spectroscopy in a rat model of bronchopulmonary dysplasia

Premature infants often require mechanical ventilation and oxygen therapy, which can result in bronchopulmonary dysplasia (BPD), characterized by developmental arrest and impaired lung function. Conventional clinical methods for assessing the prenatal lung are not adequate for the detection and assessment of long-term health risks in infants with BPD, highlighting the need for a noninvasive tool for the characterization of lung microstructure and function. Theoretical diffusion models, like the model of xenon exchange (MOXE), interrogate alveolar gas exchange by predicting the uptake of inert hyperpolarized (HP) ¹²⁹Xe gas measured with HP ¹²⁹Xe magnetic resonance spectroscopy (MRS). To investigate HP ¹²⁹Xe MRS as a tool for noninvasive characterization of pulmonary microstructural and functional changes in vivo, HP ¹²⁹Xe gas exchange data were acquired in an oxygen exposure rat model of BPD that recapitulates the fewer and larger distal airways and pulmonary vascular stunting characteristics of BPD. Gas exchange parameters from MOXE, including airspace mean chord length (L_m), apparent hematocrit in the pulmonary capillaries (HCT), and pulmonary capillary transit time (t_x), were compared with airspace mean axis length and area density (MAL and ρ_A) and percentage area of tissue and air (PTA and PAA) from histology. L_m was significantly larger in the exposed rats (P = 0.003) and correlated with MAL, ρ_A, PTA, and PAA (0.59<|ρ|<0.66 and P < 0.05). Observed increase in HCT (P = 0.012) and changes in t_x are also discussed. These findings support the use of HP ¹²⁹Xe MRS for detecting fewer, enlarged distal airways in this rat model of BPD, and potentially in humans.

Optimizing trajectory ordering for fast radial ultra-short TE (UTE) acquisitions

PURPOSE

Additional spoiler gradients are required in 3D UTE sequences with random view ordering to suppress magnetization refocusing. By leveraging the encoding gradient induced spoiling effect, the spoiler gradients could potentially be reduced or removed to shorten the TR and increase encoding efficiency. An analysis framework is built that models the gradient spoiling effects and a new ordering scheme is proposed for fast 3D UTE acquisition.

THEORY AND METHODS

UTE signal evolution and spatial encoding gradient induced spoiling effect are derived from the Bloch equations. And the concept is validated in 2D radial UTE simulation. Then an optimized ordering scheme, named reordered 2D golden angle (r2DGA) scheme, for 3D UTE acquisition is proposed. The r2DGA scheme is compared to the sequential and 3D golden angle schemes in both phantom and volunteer studies.

RESULTS

The proposed r2DGA ordering scheme was applied to two applications, single breath-holding and free breathing 3D lung MRI. With r2DGA ordering scheme, breath-holding lung MRI scan increased 60% scan efficiency by removing the spoiler gradients and the free breathing scan reduced 20% scan time compared to the 3D golden angle scheme by reducing the spoiler gradients.

CONCLUSIONS

The proposed r2DGA ordering scheme UTE acquisition reduces the need of spoiler gradients and increases the encoding efficiency, and shows improvements in both breath-holding and free breathing lung MRI applications.

Lung MRI in Children: The Road Less Travelled

Magnetic resonance imaging (MRI) of the lungs is one of the most underutilized imaging modality when it comes to imaging of thoracic diseases in children. This is largely due to less-than-optimal image quality and multiple technical challenges involved with MRI of the lungs. Advances in MRI technology along with increased awareness about optimization of MR protocol have led to it being viewed as a feasible option for evaluation of various chest diseases in children. This short review article takes the reader to the road less travelled to explore newer horizons for applications of this rapidly evolving magnetic resonance technique in the field of thoracic diseases in children.

Quantification of lung water density with UTE Yarnball MRI

PURPOSE

An efficient Yarnball ultrashort-TE k-space trajectory, in combination with an optimized pulse sequence design and automated image-processing approach, is proposed for fast and quantitative imaging of water density in the lung parenchyma.

METHODS

Three-dimensional Yarnball k-space trajectories (TE = 0.07 ms) were designed at 3 T for breath-hold and free-breathing navigator acquisitions targeting the lung parenchyma (full torso spatial coverage) with minimal T₁ and T 2 ∗ weighting. A composite of all solid tissues surrounding the lungs (muscle, liver, heart, blood pool) was used for user-independent lung water density signal referencing and B₁ -inhomogeneity correction needed for the calculation of relative lung water density images. Sponge phantom experiments were used to validate absolute water density quantification, and relative lung water density was evaluated in 10 healthy volunteers.

RESULTS

Phantom experiments showed excellent agreement between sponge wet weight and imaging-derived water density. Breath-hold (13 seconds) and free-breathing (~2 minutes) Yarnball acquisitions in volunteers (2.5-mm isotropic resolution) had negligible artifacts and good lung parenchyma SNR (>10). Whole-lung average relative lung water density values with fully automated analysis were 28.2 ± 1.9% and 28.6 ± 1.8% for breath-hold and free-breathing acquisitions, respectively, with good test-retest reproducibility (intraclass correlation coefficient = 0.86 and 0.95, respectively).

CONCLUSIONS

Quantitative lung water density imaging with an optimized Yarnball k-space acquisition approach is possible in a breath-hold or short free-breathing study with automated signal referencing and segmentation.

Modern pulmonary imaging of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a complex and serious cardiopulmonary morbidity in infants who are born preterm. Despite advances in clinical care, BPD remains a significant source of morbidity and mortality, due in large part to the increased survival of extremely preterm infants. There are few strong early prognostic indicators of BPD or its later outcomes, and evidence for the usage and timing of various interventions is minimal. As a result, clinical management is often imprecise. In this review, we highlight cutting-edge methods and findings from recent pulmonary imaging research that have high translational value. Further, we discuss the potential role that various radiological modalities may play in early risk stratification for development of BPD and in guiding treatment strategies of BPD when employed in varying severities and time-points throughout the neonatal disease course.

Current state of the art MRI for the longitudinal assessment of cystic fibrosis

Pulmonary MRI can now provide high-resolution images that are sensitive to early disease and specific to inflammation in cystic fibrosis (CF) lung disease. With specificity and function limited via computed tomography (CT), there are significant advantages to MRI. Many of the modern MRI techniques can be performed throughout life, and can be employed to understand changes over time, in addition to quantification of treatment response. Proton density and T1 /T2 contrast images can be obtained within a single breath-hold, providing depiction of structural abnormalities and active inflammation. Modern radial and/or spiral ultrashort echo-time (UTE) techniques rival CT in resolution for depiction and quantification of structure, for both airway and parenchymal abnormalities. Contrast perfusion MRI techniques are now utilized routinely to visualize changes in pulmonary and bronchial circulation that routinely occur in CF lung disease, and noncontrast techniques are moving closer to clinical translation. Functional information can be obtained from noncontrast proton images alone, using techniques such as Fourier decomposition. Hyperpolarized-gas MRI, increasingly using 129 Xe, is now becoming more widespread and has been demonstrated to have high sensitivity to early airway obstruction in CF via ventilation MRI. The sensitivity of 129 Xe MRI promises future use in personalized medicine, management of early CF lung disease, and in future clinical trials. By combining structural and functional techniques, with or without hyperpolarized gases, regional structure-function relationships can be obtained, giving insight into the pathophysiology of disease and improved clinical management. This article reviews the modern MRI techniques that can routinely be employed for CF lung disease in nearly any large medical center. Level of Evidence: 4 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2019.

Proton MRI of the Lung: How to Tame Scarce Protons and Fast Signal Decay

Pulmonary proton MRI techniques offer the unique possibility of assessing lung function and structure without the requirement for hyperpolarization or dedicated hardware, which is mandatory for multinuclear acquisition. Five popular approaches are presented and discussed in this review: 1) oxygen enhanced (OE)-MRI; 2) arterial spin labeling (ASL); 3) Fourier decomposition (FD) MRI and other related methods including self-gated noncontrast-enhanced functional lung (SENCEFUL) MR and phase-resolved functional lung (PREFUL) imaging; 4) dynamic contrast-enhanced (DCE) MRI; and 5) ultrashort TE (UTE) MRI. While DCE MRI is the most established and well-studied perfusion measurement, FD MRI offers a free-breathing test without any contrast agent and is predestined for application in patients with renal failure or with low compliance. Additionally, FD MRI and related methods like PREFUL and SENCEFUL can act as an ionizing radiation-free V/Q scan, since ventilation and perfusion information is acquired simultaneously during one scan. For OE-MRI, different concentrations of oxygen are applied via a facemask to assess the regional change in T₁ , which is caused by the paramagnetic property of oxygen. Since this change is governed by a combination of ventilation, diffusion, and perfusion, a compound functional measurement can be achieved with OE-MRI. The known problem of fast T₂ * decay of the lung parenchyma leading to a low signal-to-noise ratio is bypassed by the UTE acquisition strategy. Computed tomography (CT)-like images allow the assessment of lung structure with high spatial resolution without ionizing radiation. Despite these different branches of proton MRI, common trends are evident among pulmonary proton MRI: 1) free-breathing acquisition with self-gating; 2) application of UTE to preserve a stronger parenchymal signal; and 3) transition from 2D to 3D acquisition. On that note, there is a visible convergence of the different methods and it is not difficult to imagine that future methods will combine different aspects of the presented methods.

Improved pulmonary 129 Xe ventilation imaging via 3D-spiral UTE MRI

PURPOSE

Hyperpolarized ¹²⁹ Xe MRI characterizes regional lung ventilation in a variety of disease populations, with high sensitivity to airway obstruction in early disease. However, ventilation images are usually limited to a single breath-hold and most-often acquired using gradient-recalled echo sequences with thick slices (~10-15 mm), which increases partial-volume effects, limits ability to observe small defects, and suffers from imperfect slice selection. We demonstrate higher-resolution ventilation images, in shorter breath-holds, using FLORET (Fermat Looped ORthogonally Encoded Trajectories), a center-out 3D-spiral UTE sequence.

METHODS

In vivo human adult (N = 4; 2 healthy, 2 with cystic fibrosis) ¹²⁹ Xe images were acquired using 2D gradient-recalled echo, 3D radial, and FLORET. Each sequence was acquired at its highest possible resolution within a 16-second breath-hold with a minimum voxel dimension of 3 mm. Images were compared using ¹²⁹ Xe ventilation defect percentage, SNR, similarity coefficients, and vasculature cross-sections.

RESULTS

The FLORET sequence obtained relative normalized SNR, 40% greater than 2D gradient-recalled echo (P = .012) and 26% greater than 3D radial (P = .067). Moreover, the FLORET images were acquired with 3-fold-higher nominal resolution in a 15% shorter breath-hold. Finally, vasculature was less prominent in FLORET, likely due to diminished susceptibility-induced dephasing at shorter TEs afforded by UTE sequences.

CONCLUSION

The FLORET sequence yields higher SNR for a given resolution with a shorter breath-hold than traditional ventilation imaging techniques. This sequence more accurately measures ventilation abnormalities and enables reduced scan times in patients with poor compliance and severe lung disease.