How is pulmonary (lung) water density measured?

From the Guidelines

Lung water density is primarily measured using magnetic resonance imaging (MRI) techniques, particularly advanced sequences such as diffusion-weighted imaging and spectroscopy, which can calculate total lung water density, as shown in a study published in the Journal of the American College of Radiology in 2021 1.

Measurement Techniques

• MRI techniques offer improved soft-tissue contrast and can detect water content more specifically than computed tomography (CT) scans.
• Advanced MRI sequences, such as diffusion-weighted imaging and spectroscopy, can identify soft-tissue characteristics to the cellular level and calculate total lung water density.
• Other methods, including electrical impedance tomography (EIT), positron emission tomography (PET), and ultrasound, can also measure lung water density, but MRI is the most specific and accurate technique.

Clinical Applications

• Measuring lung water density is crucial for diagnosing and monitoring conditions like pulmonary edema, acute respiratory distress syndrome (ARDS), and heart failure, where excess lung water significantly impacts respiratory function and patient outcomes.
• Accurate measurement of lung water density can help guide treatment decisions and improve patient outcomes, as shown in studies published in Intensive Care Medicine in 2016 1.

Limitations and Future Directions

• While MRI is the most accurate technique for measuring lung water density, it has limitations, including low signal-to-noise ratio and low spatial resolution, which can be improved with advanced sequences and techniques.
• Future studies should focus on developing new MRI sequences and techniques to improve the accuracy and specificity of lung water density measurements, as well as exploring the clinical applications of these measurements in various respiratory and cardiovascular diseases.

From the Research

Measurement Techniques

• Lung water density can be measured using various techniques, including:
  • Gravimetry: a laboratory postmortem method considered the gold standard 2
  • Computed Tomography (CT) scans: can visualize lung lesions and quantify lung aeration, but may have limited value in quantifying pulmonary edema on a routine basis 3, 4
  • Transpulmonary thermodilution: an invasive technique that enables measurement of extravascular lung water and hemodynamic variables 3, 5
  • Echo probes: can detect an increase in lung water using B-lines (aka comet tails), but do not allow for real quantification of pulmonary edema 3
  • Lung ultrasonography (LUS): can provide a reliable, simple, and radiation-free lung densitometry in the intensive care setting 6
  • Indicator-dilution techniques: can quantify lung water, but may be expensive and difficult to implement for large-scale clinical trials or routine clinical practice 2
  • Wireless and wearable sensors: can monitor patients on hospital wards and beyond, enabling early detection of pulmonary congestion 3

Correlations and Validations

• Good correlations have been found between lung ultrasonography B-line scores and lung weight (r = 0.75, p < 0.05) and density (r = 0.82, p < 0.01) 6
• CT scans can assess lung water noninvasively, and correlated with hydrodynamic parameters, show that the increase of lung density parallels parenchymal fluid overload 4