MR imaging of lung parenchyma at 0.2 T: evaluation of imaging techniques, comparative study with chest radiography and interobserver analysis

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.

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.

Magnetic Resonance Imaging for Quantitative Assessment of Lung Aeration: A Pilot Translational Study

Background: Computed tomography is the gold standard for lung aeration assessment, but exposure to ionizing radiation limits its application. We assessed the ability of magnetic resonance imaging (MRI) to detect changes in lung aeration in ex vivo isolated swine lung and the potential of translation of the findings to human MRI scans.

Methods: We performed MRI scans in 11 isolated non-injured and injured swine lungs, as well as 6 patients both pre- and post-operatively. Images were obtained using a 1.5 T MRI scanner, with T₁ – weighted volumetric interpolated breath-hold examination (VIBE) and T₂ – weighted half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences. We scanned swine lungs, with reference samples of water and muscle, at different airway pressure levels: 0, 40, 10, 2 cmH₂O. We investigated the relations between MRI signal intensity and both lung density and gas content fraction. We analyzed patients’ images according to the findings of the ex vivo model.

Results: In the ex vivo samples, the lung T₁ – VIBE signal intensity normalized to water or muscle reference signal correlated with lung density (r² = 0.98). Thresholds for poorly and non-aerated lung tissue, expressed as MRI intensity attenuation factor compared to the deflated lung, were estimated as 0.70 [95% CI: 0.65–0.74] and 0.28 [95% CI: 0.27–0.30], respectively. In patients, dorsal versus ventral regions had a higher MRI signal intensity both pre- and post-operatively (p = 0.031). Comparing post- versus pre-operative scans, lung volume decreased (p = 0.028), while the following increased: MRI signal intensity in ventral (p = 0.043) and dorsal (p < 0.0001) regions, and percentages of non-aerated (p = 0.028) and poorly aerated tissue volumes (p = 0.028).

Conclusion: Magnetic resonance imaging signal intensity is a function of lung density, decreasing linearly with increasing gas content. Lung MRI might be useful for estimating lung aeration. Compared to CT, this technique is radiation-free but requires a longer acquisition time and has a lower spatial resolution.

Diagnostic Utility of 3T Lung MRI in Children with Interstitial Lung Disease: A Prospective Pilot Study

RATIONALE AND OBJECTIVES

The objective of this study was to assess the diagnostic utility of 3-tesla (3T) magnetic resonance imaging (MRI) of lungs in the detection of interstitial lung disease (ILD) in pediatric patients.

MATERIALS AND METHODS

Twelve children (mean: 8.5 years, range: 4-12 years) with ILD were consecutively enrolled in this prospective study. HRCT and 3T lung MRI were performed in all patients within 2 days of each other. The sensitivity, the specificity, the positive predictive value, and the negative predictive value of detecting lung abnormalities related to ILD with 3T lung MRI were calculated, with high-resolution computed tomography (HRCT) as a standard of reference. Agreement between HRCT and 3T lung MRI, as well as between two reviewers, was calculated with the kappa coefficient.

RESULTS

3T lung MRI had low sensitivity (66.67%) and high specificity (97.33%) in the detection of abnormalities related to ILD when compared to HRCT in children. Although 3T lung MRI performed well in the detection of consolidation, parenchymal bands and fissural thickening with a sensitivity of 100%, the sensitivity of 3T lung MRI in the detection of septal thickening, ground-glass opacity, nodules, and cysts was relatively low (50.0%, 50.0%, 66.67%, and 25.0%, respectively). Substantial agreement was seen between HRCT and 3T lung MRI (k = 0.7), whereas perfect agreement was seen between two reviewers in detecting abnormalities related to pediatric ILD (k = 0.9-1.0).

CONCLUSIONS

In comparison to HRCT, 3T lung MRI with routinely available MRI protocols and sequences can also well detect abnormalities such as consolidation, parenchymal bands, and fissural thickening in children with ILD. However, evaluation of septal thickening, ground-glass opacity, nodules, and cysts is limited with 3T lung MRI.

MRI in Thoracic Tuberculosis of Children

OBJECTIVE

To explore the utility of lung magnetic resonance imaging (MRI) in children with thoracic tuberculosis (TB).

METHODS

This prospective study of forty children (age range of 5 to 15 y) with thoracic TB was approved by the institutional ethics committee. Chest radiograph, lung MRI and computed tomography (CT) scan were performed in all children. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) of MRI were calculated and kappa test of agreement was used to determine the strength of agreement between the MRI and CT findings.

RESULTS

MRI performed equivalent to CT scan in detection of mediastinal and hilar lymphadenopathy, pleural effusion and lung cavitation (considered typical for TB) with sensitivity and specificity of 100%. MRI also yielded a sensitivity of 88.2% and specificity of 95.7% for nodules, with sensitivity of 100% and specificity of 92.9% for consolidation. Kappa agreement between CT and MRI in detection of each finding was almost perfect (k: 0.8-1).

CONCLUSIONS

Although CT scan is still superior to MRI in detection of smaller nodules, MRI demonstrates a very high degree of correlation and agreement with CT scan for detecting lung and mediastinal pathology related to TB, in children with thoracic TB and can be particularly useful in select population and follow-up of these children to avoid repeated radiation exposures.

MRI of the lung (3/3)-current applications and future perspectives

Background

MRI of the lung is recommended in a number of clinical indications. Having a non-radiation alternative is particularly attractive in children and young subjects, or pregnant women.

Methods

Provided there is sufficient expertise, magnetic resonance imaging (MRI) may be considered as the preferential modality in specific clinical conditions such as cystic fibrosis and acute pulmonary embolism, since additional functional information on respiratory mechanics and regional lung perfusion is provided. In other cases, such as tumours and pneumonia in children, lung MRI may be considered an alternative or adjunct to other modalities with at least similar diagnostic value.

Results

In interstitial lung disease, the clinical utility of MRI remains to be proven, but it could provide additional information that will be beneficial in research, or at some stage in clinical practice. Customised protocols for chest imaging combine fast breath-hold acquisitions from a “buffet” of sequences. Having introduced details of imaging protocols in previous articles, the aim of this manuscript is to discuss the advantages and limitations of lung MRI in current clinical practice.

Conclusion

New developments and future perspectives such as motion-compensated imaging with self-navigated sequences or fast Fourier decomposition MRI for non-contrast enhanced ventilation- and perfusion-weighted imaging of the lung are discussed.

Main Messages

• MRI evolves as a third lung imaging modality, combining morphological and functional information.

• It may be considered first choice in cystic fibrosis and pulmonary embolism of young and pregnant patients.

• In other cases (tumours, pneumonia in children), it is an alternative or adjunct to X-ray and CT.

• In interstitial lung disease, it serves for research, but the clinical value remains to be proven.

• New users are advised to make themselves familiar with the particular advantages and limitations.

[Oxygen-enhanced functional MR lung imaging]

Current diagnostic tools for the assessment of lung function are limited by global measurements or the need for radioactive tracers. Ideally, these tools should allow quantitative, regional distinct analyses without exposure to radiation. The current paper presents oxygen-enhanced functional MRI for assessment of lung ventilation. First applied in humans in 1996, a considerable amount of experience is now available on 1.5T scanners. The generation of quantitative T1-maps shows a high clinical potential. Low-field MR scanners, which are mostly open-designed, are especially interesting for functional lung imaging. The open design has advantages in respect to patient comfort by lower noise production and easy access to the patients and the costs are lower (no need for helium cooling). Lower signal-to-noise ratios can be overcome by changing the relaxation times. New navigator techniques allow further compensations. This article focuses on the presentation of low-field scanners and the application of T1 and T2(*) maps is described for healthy volunteers and first patients.

Magnetic resonance lung function--a breakthrough for lung imaging and functional assessment? A phantom study and clinical trial

Background

Chronic lung diseases are a major issue in public health. A serial pulmonary assessment using imaging techniques free of ionizing radiation and which provides early information on local function impairment would therefore be a considerably important development. Magnetic resonance imaging (MRI) is a powerful tool for the static and dynamic imaging of many organs. Its application in lung imaging however, has been limited due to the low water content of the lung and the artefacts evident at air-tissue interfaces. Many attempts have been made to visualize local ventilation using the inhalation of hyperpolarized gases or gadolinium aerosol responding to MRI. None of these methods are applicable for broad clinical use as they require specific equipment.

Methods

We have shown previously that low-field MRI can be used for static imaging of the lung. Here we show that mathematical processing of data derived from serial MRI scans during the respiratory cycle produces good quality images of local ventilation without any contrast agent. A phantom study and investigations in 85 patients were performed.

Results

The phantom study proved our theoretical considerations. In 99 patient investigations good correlation (r = 0.8; p ≤ 0.001) was seen for pulmonary function tests and MR ventilation measurements. Small ventilation defects were visualized.

Conclusion

With this method, ventilation defects can be diagnosed long before any imaging or pulmonary function test will indicate disease. This surprisingly simple approach could easily be incorporated in clinical routine and may be a breakthrough for lung imaging and functional assessment.

Hyperpolarized HHe 3 MRI of the lung in cystic fibrosis: assessment at baseline and after bronchodilator and airway clearance treatment

RATIONALE AND OBJECTIVES

The purpose of this study is to determine hyperpolarized helium 3 (HHe) magnetic resonance (MR) findings of the lung in patients with cystic fibrosis (CF) compared with healthy subjects and determine whether HHe MR can detect changes after bronchodilator therapy or mechanical airway mucus clearance treatment.

MATERIALS AND METHODS

Thirty-one subjects, 16 healthy volunteers and 15 patients with CF, underwent HHe lung ventilation MR imaging and spirometry at baseline. Eight patients with CF then were treated with nebulized albuterol, after which a follow-up HHe MR scan was obtained. Subsequently, recombinant human deoxyribonuclease (DNase) treatment and chest physical therapy were performed in these eight subjects, followed by a third HHe MR scan. For each MR study, the number of ventilation defects was scored by a human reader.

RESULTS

Patients with CF had significantly more HHe MR ventilation defects per image than healthy subjects (mean, 8.2 defects in patients with CF vs 1.6 defects in healthy subjects; P < .05). Even the four subjects with CF with a normal forced expiratory volume in 1 second had significantly more ventilation defects than healthy subjects (mean, 6.5 defects in these patients with CF; P = .0002). After treatment with albuterol, there was a small, but statistically significant, decrease in number of ventilation defects (mean, 9.6-8.0 defects; P = .025). After DNase and chest physical therapy, there was a trend toward increasing ventilation defects (mean, 8.3 defects; P = .096), but with a residual net improvement relative to baseline.

CONCLUSION

In patients with CF, HHe MR ventilation defects correlate with spirometry, change with treatment, and are elevated in number in patients with CF with normal spirometry results. Thus, HHe MR appears to possess many of the characteristics required of a biomarker for pulmonary CF and may be useful in the evaluation of CF pulmonary disease severity or progression.