Determination of lung water content and distribution by nuclear magnetic resonance imaging

Lung Magnetic Resonance Imaging: Technical Advancements and Clinical Applications

ABSTRACT

Since lung magnetic resonance imaging (MRI) became clinically available, limited clinical utility has been suggested for applying MRI to lung diseases. Moreover, clinical applications of MRI for patients with lung diseases or thoracic oncology may vary from country to country due to clinical indications, type of health insurance, or number of MR units available. Because of this situation, members of the Fleischner Society and of the Japanese Society for Magnetic Resonance in Medicine have published new reports to provide appropriate clinical indications for lung MRI. This review article presents a brief history of lung MRI in terms of its technical aspects and major clinical indications, such as (1) what is currently available, (2) what is promising but requires further validation or evaluation, and (3) which developments warrant research-based evaluations in preclinical or patient studies. We hope this article will provide Investigative Radiology readers with further knowledge of the current status of lung MRI and will assist them with the application of appropriate protocols in routine clinical practice.

Lung water estimation on cardiac magnetic resonance imaging for predicting adverse cardiovascular outcomes in patients with heart failure

OBJECTIVES

Pulmonary congestion is a central feature of heart failure (HF) seen in acute decompensated state as well as in chronic stable disease. The present study sought to determine whether simplified cardiac magnetic resonance imaging (CMR)-derived lung water density (LWD) measurement has prognostic relevance in predicting adverse cardiovascular outcomes in patients with HF and left ventricular ejection fraction (LVEF)<50%.

METHODS

Eighty consecutive patients referred for CMR with HF and LVEF<50% along with 22 healthy age- and sex-matched controls were prospectively recruited. LWD was the lung-to-liver signal intensity ratio multiplied by 70% (estimated hepatic water density). The primary endpoint was composite of all-cause mortality or HF-related hospitalization within 6 months from CMR.

RESULTS

The mean LWD was significantly higher in HF patients compared to healthy controls (19.78 ± 6.1 vs 13.6 ± 2.3; p < 0.001). The mean LWD was significantly different among patients with NYHA class I/II and NYHA class III/IV (17.88 ± 4.8 vs 21.77 ± 1.08; p = 0.004). At 6 months, the primary endpoint was reached in 12 (15%) patients. Patients with "wet lungs" (LWD > 18.1%) had higher incidence of adverse cardiovascular outcomes compared to patients with "dry lungs". LWD was an independent predictor of adverse cardiovascular outcomes in multivariable analysis. At the optimal cut-off of LWD > 23.38%, the sensitivity and specificity were 91.67 and 91.18%, respectively, to predict adverse cardiovascular outcomes.

CONCLUSION

LWD on CMR is independently associated with increased risk of mortality and HF-related hospitalization in HF patients with LVEF<50%.

ADVANCES IN KNOWLEDGE

Non-invasive quantitative estimation of LWD on CMR can improve risk stratification and guide management in HF patients.

Measuring lung water adds prognostic value in heart failure patients undergoing cardiac magnetic resonance

To assess whether a simplified cardiac magnetic resonance (CMR)–derived lung water density (LWD) quantification predicted major events in Heart Failure (HF). Single-centre retrospective study of consecutive HF patients with left ventricular ejection fraction (LVEF) < 50% who underwent CMR. All measurements were performed on HASTE sequences in a parasagittal plane at the right midclavicular line. LWD was determined by the lung-to-liver signal ratio multiplied by 0.7. A cohort of 102 controls was used to derive the LWD upper limit of normal (21.2%). The primary endpoint was a composite of time to all-cause death or HF hospitalization. Overall, 290 patients (mean age 64 ± 12 years) were included. LWD measurements took on average 35 ± 4 s, with good inter-observer reproducibility. LWD was increased in 65 (22.4%) patients, who were more symptomatic (NYHA ≥ III 29.2 vs. 1.8%; p = 0.017) and had higher NT-proBNP levels [1973 (IQR: 809–3766) vs. 802 (IQR: 355–2157 pg/mL); p < 0.001]. During a median follow-up of 21 months, 20 patients died and 40 had ≥ 1 HF hospitalization. In multivariate analysis, NYHA (III–IV vs. I–II; HR: 2.40; 95%-CI: 1.30–4.43; p = 0.005), LVEF (HR per 1%: 0.97; 95%-CI: 0.94–0.99; p = 0.031), serum creatinine (HR per 1 mg/dL: 2.51; 95%-CI: 1.36–4.61; p = 0.003) and LWD (HR per 1%: 1.07; 95%-CI: 1.02–1.12; p = 0.007) were independent predictors of the primary endpoint. These findings were mainly driven by an association between LWD and HF hospitalization (p = 0.026). A CMR-derived LWD quantification was independently associated with an increased HF hospitalization risk in HF patients with LVEF < 50%. LWD is a simple, reproducible and straightforward measurement, with prognostic value in HF.

State-of-the-art MR Imaging for Thoracic Diseases

Since thoracic MR imaging was first used in a clinical setting, it has been suggested that MR imaging has limited clinical utility for thoracic diseases, especially lung diseases, in comparison with x-ray CT and positron emission tomography (PET)/CT. However, in many countries and states and for specific indications, MR imaging has recently become practicable. In addition, recently developed pulmonary MR imaging with ultra-short TE (UTE) and zero TE (ZTE) has enhanced the utility of MR imaging for thoracic diseases in routine clinical practice. Furthermore, MR imaging has been introduced as being capable of assessing pulmonary function. It should be borne in mind, however, that these applications have so far been academically and clinically used only for healthy volunteers, but not for patients with various pulmonary diseases in Japan or other countries. In 2020, the Fleischner Society published a new report, which provides consensus expert opinions regarding appropriate clinical indications of pulmonary MR imaging for not only oncologic but also pulmonary diseases. This review article presents a brief history of MR imaging for thoracic diseases regarding its technical aspects and major clinical indications in Japan 1) in terms of what is currently available, 2) promising but requiring further validation or evaluation, and 3) developments warranting research investigations in preclinical or patient studies. State-of-the-art MR imaging can non-invasively visualize lung structural and functional abnormalities without ionizing radiation and thus provide an alternative to CT. MR imaging is considered as a tool for providing unique information. Moreover, prospective, randomized, and multi-center trials should be conducted to directly compare MR imaging with conventional methods to determine whether the former has equal or superior clinical relevance. The results of these trials together with continued improvements are expected to update or modify recommendations for the use of MRI in near future.

Texture analysis using proton density and T2 relaxation in patients with histological usual interstitial pneumonia (UIP) or nonspecific interstitial pneumonia (NSIP)

Objectives

The purpose of our study was to assess proton density (PD) and T2 relaxation time of usual interstitial pneumonia (UIP) and nonspecific interstitial pneumonia (NSIP) and to evaluate their utility in differentiating the two patterns. Furthermore, we aim to investigate whether these two parameters could help differentiate active-inflammatory and stable-fibrotic lesions in NSIP.

Methods

32 patients (mean age: 69 years; M:F, 1:1) with pathologically proven disease (UIP:NSIP, 1:1), underwent thoracic thin-section multislice CT scan and 1.5T MRI. A total of 437 regions-of-interest (ROIs) were classified at CT as advanced, moderate or mild alterations. Based on multi-echo single-shot TSE sequence acquired at five echo times, with breath-holding at end-expiration and ECG-triggering, entire lung T2 and PD maps were generated from each subject. The T2 relaxation time and the respective signal intensity were quantified by performing a ROI measurement on the T2 and PD maps in the corresponding CT selected areas of the lung.

Results

UIP and NSIP regional patterns could not be differentiated by T2 relaxation times or PD values alone. Overall, a strong positive correlation was found between T2 relaxation and PD in NSIP, r = 0.64, p<0.001; however, this correlation was weak in UIP, r = 0.20, p = 0.01. T2 relaxation showed significant statistical difference between active-inflammatory and stable-fibrotic NSIP regions at all levels, p<0.05, while for the analysis of ventral lesions PD proved no statistical difference, p>0.05.

Conclusions

T2 relaxation times and PD values may provide helpful quantitative information for differentiating NSIP from UIP pattern. These parameters have the potential to differentiate active-inflammatory and stable-fibrotic lesions in NSIP.

Ultrashort echo time MRI of pulmonary water content: assessment in a sponge phantom at 1.5 and 3.0 Tesla

PURPOSE

We aimed to develop a predictive model for lung water content using ultrashort echo time (UTE) magnetic resonance imaging (MRI) and a sponge phantom.

MATERIALS AND METHODS

Image quality was preliminarily optimized, and the signal-to-noise ratio (SNR) of UTE was compared with that obtained from a three-dimensional fast gradient echo (FGRE) sequence. Four predetermined volumes of water (3.5, 3.0, 2.5, and 2.0 mL) were soaked in cellulose foam sponges 1.8 cm3 in size and were imaged with UTE-MRI at 1.5 and 3.0 Tesla (T). A multiple echo time experiment (range, 0.1-9.6 ms) was conducted, and the T2 signal decay curve was determined at each volume of water. A three-parameter equation was fitted to the measured signal, allowing for the calculation of proton density and T2*. The calculation error of proton density was determined as a function of echo time. The constants that allowed for the determination of unknown volumes of water from the measured proton density were calculated using linear regression.

RESULTS

UTE-MRI provided excellent image quality for the four phantoms and showed a higher SNR, compared to that of FGRE. Proton density decreased proportionally with the decreases in both lung water and field strength (from 3.5 to 2.0 mL; proton density range at 1.5 T, 30.5-17.3; at 3.0 T, 84.2-41.5). Minimum echo time less than 0.6 ms at 1.5 T and 1 ms at 3.0 T maintained calculation errors for proton density within the range of 0%-10%. The slopes of the lines for determining the unknown volumes of water with UTE-MRI were 0.12±0.003 at 1.5 T and 0.05±0.002 at 3.0 T (P < 0.0001).

CONCLUSION

In a sponge phantom imaged at 1.5 and 3.0 T, unknown volumes of water can be predicted with high accuracy using UTE-MRI.

Methods of Measuring Lung Water

Pulmonary oedema can result from both cardiogenic and non-cardiogenic aetiologies and is a cause of considerable morbidity and mortality. Accurate methods of quantifying pulmonary oedema are needed for both clinical and research purposes. Applications could include early recognition, and thus prevention, of impending decompensation in heart failure patients, guidance of fluid management in patients with established pulmonary oedema, and as a pharmacodynamic outcome measure for early clinical trials of drugs for the treatment of pulmonary oedema. Magnetic resonance imaging, computed tomography, positron emission tomography, electrical impedance, and thermodilution methods have all been used with the aim of measuring lung water. These methods differ in their accuracy, cost, ionising radiation dose, invasiveness, portability, and ability to provide dynamic measures. To date, none have been established as a ‘gold standard’ clinical measurement to improve clinical outcomes or to assist drug development. This review aims to discuss each of these methods in turn, focussing on advantages, limitations, and possible future development and applications.

The measurement of lung water

INTRODUCTION: In this review, we compare the spectrum of currently available methods for quantifying pulmonary edema in patients. REVIEW: Imaging and indicator dilution techniques comprise the most common strategies for measuring lung water at the bedside. The most accurate (within 10% of the gravimetric gold standard) and most reproducible (< 5% between-test variation) are also, unfortunately, the most expensive and most difficult to implement for purposes of large-scale clinical trials or for routine clinical practice. CONCLUSION: The standard chest radiograph remains the best screening test for the detection of pulmonary edema. Indicator-dilution techniques are probably the best available method at present for quantitation in patient groups.

1H NMR measurements of wet/dry ratio and T1, T2 distributions in lung

Proton-magnetic-resonance measurements have been carried out on juvenile porcine peripheral lung parenchyma. The free-induction-decay signal contained a motionally restricted component which decayed in a few tens of microseconds and a mobile component with a T2 time greater than 1 ms. The average second moment, M2, for the motionally restricted signal was found to be 3.42 +/- (0.25) x 10(9) s-2. The T2 distribution for the mobile signal consistently showed four resolvable components of T2 range: 2-6, 10-40, 80-110, and 190-400 ms. The 2-6 ms component was present in a fully dehydrated preparation and was therefore assigned to a nonaqueous lung constituent. The motionally restricted FID component had a T1 = 0.772 +/- 0.11 s and the mobile component had a T1 = 0.967 +/- 0.02 s. The hydrogen content per unit mass for lung parenchyma and water were estimated in two ways: (1) on the basis of chemical content and (2) on the basis of comparison of restricted and mobile signals to the gravimetric (G) water content for a lung sample studied at a wide range of water contents. Lung wet/dry weight ratios were estimated from the free-induction decays and compared with gravimetric measurement. The ratio of (wet/dry)NMR/(wet/dry)G was 1.00 +/- 0.08 and 1.00 +/- 0.05 for the two methods of estimation.

Acute respiratory distress syndrome: diagnosis and management

This review will attempt to put together the voluminous studies and concepts that have been published during the past 25 years following the description of the acute respiratory distress syndrome (ARDS) regarding diagnosis and management. The initial discussion will focus on how to clinically diagnose ARDS based recommendations. This also gives the current definition of acute lung injury and when to call it ARDS. The radiographic and hemodynamic characteristics are discussed including oxygenation parameters. The management outlines the conventional as well as new therapies intended to improve survival of this devastating disease.

The value of portable chest roentgenography in adult respiratory distress syndrome. Comparison with computed tomography

In 17 patients with adult respiratory distress syndrome, we used data derived from computed tomographic (CT) scan densitometric analysis to validate the value of portable chest roentgenograms in objectively estimating the amount of pulmonary edema. Chest roentgenograms and CT scans were taken in the same ventilatory conditions (apnea at 10 cm H2O of positive end-expiratory pressure [PEEP]); blood gas samples and hemodynamic parameters were collected at the same time. Roentgenographic analysis was undertaken by independent observers using two standardized scoring systems proposed in the literature. CT scan analysis was performed using the CT number frequency distribution and the gas lung volume (measured by helium dilution technique) to estimate quantitatively the lung density, the lung weight, and the percentage of normally aerated and nonaerated tissue. Knowing the mean CT number of the pulmonary parenchyma in a group of normal subjects, we also inferred the ideal lung weight expected in the study population and computed the excess tissue mass as the difference between actual and ideal lung weight. Both the roentgenographic scoring systems showed direct correlation with the pulmonary impairment as detected by CT scan densitometric analysis (CT number, percentage of nonaerated tissue, lung weight, and excess tissue mass; p less than 0.01) and inverse relation with the percentage of normally aerated tissue (p less than 0.01). We also found a relationship between roentgenographic scores and the impairment in gas exchange as detected by shunt fraction (p less than 0.05). We conclude that standardized reading of portable chest roentgenograms by means of scoring tables is a valuable tool in estimating the amount of pulmonary edema in a patient with adult respiratory distress syndrome.