Balanced steady‐state free precession thoracic imaging with half‐radial dual‐echo readout on smoothly interleaved archimedean spirals

Replication of the bSTAR sequence and open-source implementation

PURPOSE

The reproducibility of scientific reports is crucial to advancing human knowledge. This paper is a summary of our experience in replicating a balanced SSFP half-radial dual-echo imaging technique (bSTAR) using open-source frameworks as a response to the 2023 ISMRM "repeat it with me" Challenge.

METHODS

We replicated the bSTAR technique for thoracic imaging at 0.55T. The bSTAR pulse sequence is implemented in Pulseq, a vendor neutral open-source rapid sequence prototyping environment. Image reconstruction is performed with the open-source Berkeley Advanced Reconstruction Toolbox (BART). The replication of bSTAR, termed open-source bSTAR, is tested by replicating several figures from the published literature. Original bSTAR, using the pulse sequence and image reconstruction developed by the original authors, and open-source bSTAR, with pulse sequence and image reconstruction developed in this work, were performed in healthy volunteers.

RESULTS

Both echo images obtained from open-source bSTAR contain no visible artifacts and show identical spatial resolution and image quality to those in the published literature. A direct head-to-head comparison between open-source bSTAR and original bSTAR on a healthy volunteer indicates that open-source bSTAR provides adequate SNR, spatial resolution, level of artifacts, and conspicuity of pulmonary vessels comparable to original bSTAR.

CONCLUSION

We have successfully replicated bSTAR lung imaging at 0.55T using two open-source frameworks. Full replication of a research method solely relying on information on a research paper is unfortunately rare in research, but our success gives greater confidence that a research methodology can be indeed replicated as described.

Fast T2 mapping of short-T2 tissues in knee using 3D radial dual-echo balanced steady-state free precession

BACKGROUND

T2 mapping of short-T2 tissues in the knee (meniscus, tendon, and ligament) is needed to aid the clinical MRI knee diagnosis, which is hard to realize using traditional clinical methods.

PURPOSE

To accelerate the acquisition of T2 values for short-T2 tissues in the knee by analyzing the signal equation of balanced steady-state free precession (bSSFP) sequence in MRI.

METHODS

Effect of half-radial acquisition on pixel bandwidth was analyzed mathematically. A modified 3D radial dual-echo bSSFP sequence was proposed for 0.53 mm isotropic resolution knee imaging with 2 different TEs at 3 T, which alleviated the problem of off-resonance artifacts caused by traditional half-radial acquisition scheme. A novel pixel-based optimization method was proposed for efficient T2 mapping of short-T2 tissues in the knee given off-resonance values. Simulation was conducted to evaluate the sensitivity of the proposed method to other parameters. Phantom results were compared with 2D spin-echo (SE), and in vivo results were compared with SE and previously studies.

RESULTS

Simulation showed that the proposed method is insensitive to T1 and B1 variations (estimation error < 1% for T1/B1 error of ±90%), avoiding the need for separated T1 and B1 scans. High isotropic resolution knee imaging was achieved using the modified dual-echo bSSFP. The total scan time was within 3.5 min, including a separate off-resonance scan for T2 measurement. Measured mean T2 values for phantoms correlated well with SE (R2 = 0.99), and no significant difference was observed (P = 0.45). In vivo meniscus T2 measurements and ligament T2 measurements agreed with the literature, while tendon T2 measurements were much lower (31.7% lower for patellar tendon, and 13.5% lower for quadriceps tendon), which might result in its bi-component property.

CONCLUSIONS

The proposed method provides an efficient way for fast, robust, high-resolution imaging and T2 mapping of short-T2 tissues in the knee.

New clinical opportunities of low-field MRI: heart, lung, body, and musculoskeletal

Contemporary whole-body low-field MRI scanners (< 1 T) present new and exciting opportunities for improved body imaging. The fundamental reason is that the reduced off-resonance and reduced SAR provide substantially increased flexibility in the design of MRI pulse sequences. Promising body applications include lung parenchyma imaging, imaging adjacent to metallic implants, cardiac imaging, and dynamic imaging in general. The lower cost of such systems may make MRI favorable for screening high-risk populations and population health research, and the more open configurations allowed may prove favorable for obese subjects and for pregnant women. This article summarizes promising body applications for contemporary whole-body low-field MRI systems, with a focus on new platforms developed within the past 5 years. This is an active area of research, and one can expect many improvements as MRI physicists fully explore the landscape of pulse sequences that are feasible, and as clinicians apply these to patient populations.

Submillimeter lung MRI at 0.55 T using balanced steady-state free precession with half-radial dual-echo readout (bSTAR)

PURPOSE

To demonstrate the feasibility of high-resolution morphologic lung MRI at 0.55 T using a free-breathing balanced steady-state free precession half-radial dual-echo imaging technique (bSTAR).

METHODS

Self-gated free-breathing bSTAR (TE1 /TE2 /TR of 0.13/1.93/2.14 ms) lung imaging in five healthy volunteers and a patient with granulomatous lung disease was performed using a 0.55 T MR-scanner. A wobbling Archimedean spiral pole (WASP) trajectory was used to ensure a homogenous coverage of k-space over multiple breathing cycles. WASP uses short-duration interleaves randomly tilted by a small polar angle and rotated by a golden angle about the polar axis. Data were acquired continuously over 12:50 min. Respiratory-resolved images were reconstructed off-line using compressed sensing and retrospective self-gating. Reconstructions were performed with a nominal resolution of 0.9 mm and a reduced isotropic resolution of 1.75 mm corresponding to shorter simulated scan times of 8:34 and 4:17 min, respectively. Analysis of apparent SNR was performed in all volunteers and reconstruction settings.

RESULTS

The technique provided artifact-free morphologic lung images in all subjects. The short TR of bSTAR in conjunction with a field strength of 0.55 T resulted in a complete mitigation of off-resonance artifacts in the chest. Mean SNR values in healthy lung parenchyma for the 12:50 min scan were 3.6 ± 0.8 and 24.9 ± 6.2 for 0.9 mm and 1.75 mm reconstructions, respectively.

CONCLUSION

This study demonstrates the feasibility of morphologic lung MRI with a submillimeter isotropic spatial resolution in human subjects with bSTAR at 0.55 T.

Chest examinations in children with real-time magnetic resonance imaging: first clinical experience

BACKGROUND

Real-time magnetic resonance imaging (MRI) based on a fast low-angle shot technique 2.0 (FLASH 2.0) is highly effective against artifacts caused due to the bulk and pulmonary and cardiac motions of the patient. However, to date, there are no reports on the application of this innovative technique to pediatric lung MRI.

OBJECTIVE

This study aimed to identify the limits of resolution and image quality of real-time lung MRI in children and to assess the types and minimal size of lesions with these new sequences.

MATERIALS AND METHODS

In this retrospective study, pathological lung findings in 87 children were classified into 6 subgroups, as detected on conventional MRI: metastases and tumors, consolidation, scars, hyperinflation, interstitial pathology and bronchiectasis. Subsequently, the findings were grouped according to size (4-6 mm, 7-9 mm and ≥ 10 mm) and evaluated for visual delineation of the findings (0 = not visible, 1 = hardly visible and 2 = well visualized).

RESULTS

Real-time MRI allows for diagnostic, artifact-free thorax images to be obtained, regardless of patient movements. The delineation of findings strongly correlates with the size of the pathology. Metastases, consolidation and scars were visible at 100% when larger than 9 mm. In the 7-9 mm subgroup, the visibility was 83% for metastases, 88% for consolidation and 100% for scars in T2/T1 weighting. Though often visible, smaller pathological lesions of 4-6 mm in size did not regularly meet the expected diagnostic confidence: The visibility of metastases was 18%, consolidation was 64% and scars was 71%. Diffuse interstitial lung changes and hyperinflation, known as "MR-minus pathologies," were not accessible to real-time MRI.

CONCLUSION

The method provides motion robust images of the lung and thorax. However, the lower sensitivity for small lung lesions is a major limitation for routine use of this technique. Currently, the method is adequate for diagnosing inflammatory lung diseases, atelectasis, effusions and lung scarring in children with irregular breathing patterns or bulk motion on sedation-free MRI. A medium-term goal is to improve the diagnostic accuracy of small nodules and interstitial lesions.

MRI lung lobe segmentation in pediatric cystic fibrosis patients using a recurrent neural network trained with publicly accessible CT datasets

Purpose

To introduce a widely applicable workflow for pulmonary lobe segmentation of MR images using a recurrent neural network (RNN) trained with chest CT datasets. The feasibility is demonstrated for 2D coronal ultrafast balanced SSFP (ufSSFP) MRI.

Methods

Lung lobes of 250 publicly accessible CT datasets of adults were segmented with an open‐source CT‐specific algorithm. To match 2D ufSSFP MRI data of pediatric patients, both CT data and segmentations were translated into pseudo‐MR images that were masked to suppress anatomy outside the lung. Network‐1 was trained with pseudo‐MR images and lobe segmentations and then applied to 1000 masked ufSSFP images to predict lobe segmentations. These outputs were directly used as targets to train Network‐2 and Network‐3 with non‐masked ufSSFP data as inputs, as well as an additional whole‐lung mask as input for Network‐2. Network predictions were compared to reference manual lobe segmentations of ufSSFP data in 20 pediatric cystic fibrosis patients. Manual lobe segmentations were performed by splitting available whole‐lung segmentations into lobes.

Results

Network‐1 was able to segment the lobes of ufSSFP images, and Network‐2 and Network‐3 further increased segmentation accuracy and robustness. The average all‐lobe Dice similarity coefficients were 95.0 ± 2.8 (mean ± pooled SD [%]) and 96.4 ± 2.5, 93.0 ± 2.0; and the average median Hausdorff distances were 6.1 ± 0.9 (mean ± SD [mm]), 5.3 ± 1.1, 7.1 ± 1.3 for Network‐1, Network‐2, and Network‐3, respectively.

Conclusion

Recurrent neural network lung lobe segmentation of 2D ufSSFP imaging is feasible, in good agreement with manual segmentations. The proposed workflow might provide access to automated lobe segmentations for various lung MRI examinations and quantitative analyses.

Morphologic and Functional Assessment of Sarcoidosis Using Low-Field MRI

Online supplemental material is available for this article.

Repeatability of dynamic 3D phase-resolved functional lung (PREFUL) ventilation MR Imaging in patients with chronic obstructive pulmonary disease and healthy volunteers

BACKGROUND

A previous study has demonstrated the feasibility of 3D phase-resolved functional lung (PREFUL) MRI in healthy volunteers and patients with chronic pulmonary disease. Before clinical use, the repeatability of the ventilation parameters derived from 3D PREFUL MRI must be determined.

PURPOSE

To evaluate repeatability of 3D PREFUL and to compare with pulmonary functional lung testing (PFT).

STUDY TYPE

Prospective.

POPULATION

Fifty-three healthy subjects and 13 patients with chronic obstructive pulmonary disease (COPD).

FIELD STRENGTH/SEQUENCE

A prototype 3D stack-of-stars spoiled-gradient-echo sequence at 1.5 T.

ASSESSMENT

Study participants underwent repeated MRI examination (median time interval between scans COPD/healthy subjects [interquartile range]: 7/0 days [6-8/0-0 days]) and one PFT carried out at the time of the baseline MRI. For 3D PREFUL, regional ventilation (RVent) and flow-volume loops were computed and rated by cross-correlation (CC). Also, ventilation time-to-peak (VTTP) was computed. Ventilation defect percentage (VDP) maps were obtained for RVent and CC.

STATISTICAL TESTS

Repeatability of 3D PREFUL parameters was evaluated using Bland-Altman analysis, coefficient of variation (COV) and intraclass correlation coefficient (ICC). The relation between 3D PREFUL and PFT measures (forced expiratory volume in 1 second (FEV₁ ) and forced vital capacity (FVC) was assessed using the Pearson correlation coefficient (r).

RESULTS

In healthy subjects and COPD patients, no significant bias (all P range: 0.09-0.77) and a moderate to good repeatability of RVent, VTTP, and VDP_RVent were found (COV range: 0.1%-18.2%, ICC range: 0.51-0.88). For CC and VDP_CC moderate repeatability was found (COV range: 0.6%-43.6%, ICC: 0.38-0.60). CC, VDP_RVent , and VDP_CC showed a good correlation with FEV₁ (all |r| > 0.58, all P < 0.05) and FEV₁ /FVC ratio (all |r| > 0.62, all P < 0.05).

DATA CONCLUSION

3D PREFUL provided a good repeatability of RVent, VTTP, and VDP_RVent and moderate repeatability of CC and VDP_CC in healthy volunteers and COPD patients, and correlated well with FEV₁ and FEV₁ /FVC.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 2.

3D phase-resolved functional lung ventilation MR imaging in healthy volunteers and patients with chronic pulmonary disease

PURPOSE

To test the feasibility of 3D phase-resolved functional lung (PREFUL) MRI in healthy volunteers and patients with chronic pulmonary disease, to compare 3D to 2D PREFUL, and to investigate the required temporal resolution to obtain stable 3D PREFUL measurement.

METHODS

Sixteen participants underwent MRI using 2D and 3D PREFUL. Retrospectively, the spatial resolution of 3D PREFUL (4 × 4 × 4 mm³ ) was decreased to match the spatial resolution of 2D PREFUL (4 × 4 × 15 mm³ ), abbreviated as 3D_lowres . In addition to regional ventilation (RVent), flow-volume loops were computed and rated by a cross-correlation (CC). Ventilation defect percentage (VDP) maps were obtained. RVent, CC, VDP_RVent , and VDP_CC were compared for systematic differences between 2D, 3D_lowres , and 3D PREFUL. Dividing the 3D PREFUL data into 4- (≈ 20 phases), 8- (≈ 40 phases), and 12-min (≈ 60 phases) acquisition pieces, the ventilation parameter maps, including the heterogeneity of ventilation time to peak, were tested regarding the required temporal resolution.

RESULTS

RVent, CC, VDP_RVent , and VDP_CC  presented significant correlations between 2D and 3D PREFUL (r = 0.64-0.94). CC and VDP_CC  of 2D and 3D_lowres  PREFUL were significantly different (P < .0113). Comparing 3D_lowres  and 3D PREFUL, all parameters were found to be statistically different (P < .0045).

CONCLUSION

3D PREFUL MRI depicts the whole lung volume and breathing cycle with superior image resolution and with likely more precision compared to 2D PREFUL. Furthermore, 3D PREFUL is more sensitive to detect regions of hypoventilation and ventilation heterogeneity compared to 3D_lowres  PREFUL, which is important for early detection and improved monitoring of patients with chronic lung disease.

3D Magnetic Resonance Spirometry

Spirometry is today the gold standard technique for assessing pulmonary ventilatory function in humans. From the shape of a flow-volume loop measured while the patient is performing forced respiratory cycles, the Forced Vital Capacity (FVC) and the Forced Expiratory Volume in one second (FEV1) can be inferred, and the pulmonologist is able to detect and characterize common respiratory afflictions. This technique is non-invasive, simple, widely available, robust, repeatable and reproducible. Yet, its outcomes rely on the patient's cooperation and provide only global information over the lung. With 3D Magnetic Resonance (MR) Spirometry, local ventilation can be assessed by MRI anywhere in the lung while the patient is freely breathing. The larger dimensionality of 3D MR Spirometry advantageously allows the extraction of original metrics that characterize the anisotropic and hysteretic regional mechanical behavior of the lung. Here, we demonstrated the potential of this technique on a healthy human volunteer breathing along different respiratory patterns during the MR acquisition. These new results are discussed with lung physiology and recent pulmonary CT data. As respiratory mechanics inherently support lung ventilation, 3D MR Spirometry may open a new way to non-invasively explore lung function while providing improved diagnosis of localized pulmonary diseases.