Relative quantification of pulmonary edema with noncontrast-enhanced MRI

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.

Acute respiratory distress syndrome induction by pulmonary ischemia-reperfusion injury in large animal models

Acute respiratory distress syndrome (ARDS) is a common critical pulmonary complication after esophagectomy and other thoracic surgeries (e.g., lung transplantation, pulmonary thromboendarterectomy). Direct pulmonary ischemia-reperfusion injury (PIRI) is known to play the main role in induction of ARDS in these cases. Large animal models are an appropriate choice for ARDS as well as PIRI study because of their physiological and anatomic similarities to the human body. With regard to large animal models, we reviewed different methods of inducing in situ direct PIRI and the commonly applied methods for diagnosing and monitoring ARDS or PIRI in an experimental research setting.

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.

Recent advances in imaging the lungs of intact small animals

A new generation of imaging devices now make it possible to generate both structural and functional images for the study of lung biology in small animals, including common laboratory mouse and rat models. "Micro" X-ray computed tomography and positron emission tomography scanners, highly sensitive cooled charge coupled device cameras for bioluminescence and fluorescence imaging, high magnetic field magnetic resonance imaging scanners, and recent advances in ultrasound system technology can be used to study such diverse processes as ventilation, perfusion, pulmonary hypertension, lung inflammation, and gene transfer, among others. Images from more than one modality can also be fused, allowing structure-function and function-function relationships to be studied on a regional basis. These new instruments, part of an emerging suite of techniques collectively known as "molecular imaging," provide an enormous potential for elucidating lung biology in intact animal models and systems.

Pulmonary edema induced by allergen challenge in the rat: noninvasive assessment by magnetic resonance imaging

The course of pulmonary edema formation after an intratracheal (i.t.) instillation of ovalbumin was followed noninvasively by magnetic resonance imaging (MRI) in actively sensitized Brown Norway (BN) rats. Changes in edema volume assessed by MRI mimicked the results from the analysis of the number and activation of inflammatory cells recovered from the broncho-alveolar lavage (BAL) fluid. Rats treated with budesonide did not develop edema following challenge with ovalbumin, and these animals showed a significant decrease in BAL fluid inflammatory cell numbers and eosinophil peroxidase and myeloperoxidase activities. Thus, following lung edema formation by MRI provides a reliable means of assessing pulmonary inflammation after allergen challenge. Unlike BAL fluid analysis, which requires killing animals at each time point, this method is noninvasive. MRI could be of importance for the noninvasive profiling of anti-inflammatory drugs in animal models of asthma and in the clinic. Magn Reson Med 45:88-95, 2001.

In vivo magnetic resonance methods in pharmaceutical research: current status and perspectives

In the last decade, in vivo MR methods have become established tools in the drug discovery and development process. In this review, several successful and potential applications of MRI and MRS in stroke, rheumatoid and osteo-arthritis, oncology and cardiovascular disorders are dealt with in detail. The versatility of the MR approach, allowing the study of various pathophysiological aspects in these disorders, is emphasized. New indication areas, for the characterization of which MR methods have hardly been used up to now, such as respiratory, gastro-intestinal and skin diseases, are outlined in a subsequent section. A strength of MRI, being a non-invasive imaging modality, is the ability to provide functional, i.e. physiological, readouts. Functional MRI examples discussed are the analysis of heart wall motion, perfusion MRI, tracer uptake and clearance studies, and neuronal activation studies. Functional information may also be derived from experiments using target-specific contrast agents, which will become important tools in future MRI applications. Finally the role of MRI and MRS for characterization of transgenic and knock-out animals, which have become a key technology in modern pharmaceutical research, is discussed. The advantages of MRI and MRS are versatility, allowing a comprehensive characterization of a diseased state and of the drug intervention, and non-invasiveness, which is of relevance from a statistical, economical and animal welfare point of view. Successful applications in drug discovery exploit one or several of these aspects. In addition, the link between preclinical and clinical studies makes in vivo MR methods highly attractive methods for pharmaceutical research.

Regional measurements of pulmonary edema by using magnetic resonance imaging

A three-dimensional magnetic resonance imaging (MRI) method to measure pulmonary edema and lung microvascular barrier permeability was developed and compared with conventional methods in nine mongrel dogs. MRIs were obtained covering the entire lungs. Injury was induced by injection of oleic acid (0.021-0.048 ml/kg) into a jugular catheter. Imaging followed for 0.75-2 h. Extravascular lung water and permeability-related parameters were measured from multiple-indicator dilution curves. Edema was measured as magnetic resonance signal-to-noise ratio (SNR). Postinjury wet-to-dry lung weight ratio was 5.30 +/- 0.38 (n = 9). Extravascular lung water increased from 2.03 +/- 1.11 to 3.00 +/- 1.45 ml/g (n = 9, P < 0.01). Indicator dilution studies yielded parameters characterizing capillary exchange of urea and butanediol: the product of the square root of equivalent diffusivity of escape from the capillary and capillary surface area (D1/2S) and the capillary permeability-surface area product (PS). The ratio of D1/2S for urea to D1/2S for butanediol increased from 0.583 +/- 0.027 to 0.852 +/- 0.154 (n = 9, P < 0.05). Whole lung SNR at baseline, before injury, correlated with D1/2S and PS ratios (both P < 0.02). By using rate of SNR change, the mismatch of transcapillary filtration flow and lymph clearance was estimated to be 0.2-1.8 ml/min. The filtration coefficient was estimated from these values. Results indicate that pulmonary edema formation during oleic acid injury can be imaged regionally and quantified globally, and the results suggest possible regional quantification by using three-dimensional MRI.