Sodium MRI with coated magnetite: measurement of extravascular lung water in rats

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.

Superparamagnetic iron oxide MION as a contrast agent for sodium MRI in myocardial infarction

An intravascular iron-based contrast agent was used as a sodium (23Na) MRI T2 relaxant in an effort to suppress the blood signal from the ventricular cavities in normal and infarcted canine myocardium in vivo. 23Na MRI signal decreases in blood were attributed to decreases in the fast (T2f) and slow (T2s) transverse relaxation components, which were quantified as a function of dose and MRI echo time (TE). In vivo 23Na MRI signal decreases up to 65% were noted in ventricular blood when imaging under dose and TE conditions of 10 mg/kg body weight and 5 ms, respectively. Contrast injection followed by subsequent 23Na MRI in canine myocardial infarction led to a clear delineation of the location of the injured tissue, as identified by postmortem triphenyltetrazolium chloride staining, and to an improvement in the contrast-to-noise ratio between the blood in the ventricular chamber and the infarcted tissue that was as high as 3.3-fold in the postcontrast images in comparison to the precontrast images.

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.

MR contrast agents in acute experimental cerebral ischemia: potential adverse impacts on neurologic outcome and infarction size

The purpose of the study was to investigate the effects of magnetic resonance (MR) contrast agents on neurologic outcome and infarction size in a rat stroke model. Focal cerebral ischemia was induced in 80 rats using an endovascular occlusion technique of the middle cerebral artery. Four hours after occlusion, 64 animals (4 groups of 16 each) received gadodiamide (Gd-DTPA-BMA) in a single (0.1 mmol/kg) or triple (0.3 mmol/kg) clinical dose or the ultrasmall superparamagnetic iron oxide particles contrast agent NC 100150 in a single (0.03 mmol/kg, 1.5 mg Fe(2+)/kg) or double (0.06 mmol/kg, 3.0 mg Fe(2+)/kg) clinical dose, respectively. Sixteen animals received equivolumetric saline (control group). Neurologic score and body weight were recorded every 8 hours. Twenty-four hours after vessel occlusion, infarction size was measured by 2,3, 5-triphenyl-tetrazolium-chloride (TTC) staining. Neither the normal nor the triple clinical dose of gadodiamide or NC 100150 in the single or double dose had any statistically significant effects on infarction volume, mortality, body weight, or neurologic outcome (P > 0.05). Our results suggest that bolus injection of gadodiamide and the ultrasmall superparamagnetic iron oxide particles NC 100150 in clinically relevant doses does not significantly affect infarction volume and clinical outcome of acute cerebral ischemia.

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.

New techniques and developments in physiologic imaging of airways

Recently, noninvasive imaging methods for assessing regional and individual airway responsiveness have been developed in animal models and applied to humans with obstructive lung disease. This article examines the technique of high-resolution CT scan and MR imaging and their applications to the imaging of airways.

Relative quantification of pulmonary edema with noncontrast-enhanced MRI

Pulmonary edema is a debilitating effect of acute respiratory distress syndrome. The ability to measure it noninvasively with high sensitivity and in three dimensions could be useful in not only detection but also in assessment and guidance of treatment. To this end, a three-dimensional MRI pulse sequence to measure the formation of edema was developed and tested. Another sequence was tested to measure blood flow in distal pulmonary arteries. Pulmonary edema was induced in nine dogs via venous injections of oleic acid. Edema was verified by wet-to-dry weight ratio (5.30 +/- .38) and extra-vascular lung water at baseline (2.03 +/- 1.12 ml/g dry lung weight) versus postinjury (3.00 +/- 1.45 ml/g) (P < .005). The signal-to-noise ratio within the lungs increased from 5.47 +/- 1.00 at baseline to 7.51 +/- 1.96 (P < .005), and the time course of edema formation was resolved. Results from MR phase-contrast blood flow measurements were variable. The authors conclude that the three-dimensional scan provides a sensitive relative quantification of pulmonary edema formation without the use of contrast agents or ionizing radiation.

MR microscopy of the rat lung using projection reconstruction

Projection reconstruction has been implemented with self-refocused selection pulses on a small bore, 2.0 T MR microscope, to allow imaging of lung parenchyma. Scan synchronous ventilation and cardiac gating have been integrated with the sequence to minimize motion artifacts. A systematic survey of the pulse sequence parameters has been undertaken in conjunction with the biological gating parameters to optimize resolution and signal-to-noise (SNR). The resulting projection images with effective echo time of < 300 microseconds allow definition of lung parenchyma with an SNR improvement of approximately 15 x over a more conventional 2DFT short echo gradient sequence.