MR imaging parameters in the study of lung water. A preliminary study

In vitro measurements of water content and T2 relaxation times in lung using a clinical MRI scanner

The purpose of this study was to validate water content measurements and to determine the T2 distribution in lung on a 1.5 T clinical magnetic resonance imaging (MRI) scanner. A single-slice 16 echo pulse sequence with an echo spacing of 10 msec was employed to scan 19 healthy juvenile pig lungs. The in vitro water content of each lung was measured using MRI techniques and compared with gravimetric measurements. The mean difference between the gravimetric and MRI water contents was -4.1 +/- 7.6%, and an excellent linear correlation (R2 = 0.98) was observed between the two independent measurements. The dependence of the geometric mean T2 time upon lung water density exhibited two distinct regions (inflated and deflated) separated by a threshold density of about 0.4 g/mL.

T2* and proton density measurement of normal human lung parenchyma using submillisecond echo time gradient echo magnetic resonance imaging

OBJECTIVE

To obtain T2* and proton density measurements of normal human lung parenchyma in vivo using submillisecond echo time (TE) gradient echo (GRE) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Six normal volunteers were scanned using a 1.5-T system equipped with a prototype enhanced gradient (GE Signa, Waukausha, WI). Images were obtained during breath-holding with acquisition times of 7-16 s. Multiple TEs ranging from 0.7 to 2.5 ms were tested. Linear regression was performed on the logarithmic plots of signal intensity versus TE, yielding measurements of T2* and proton density relative to chest wall muscle. Measurements in supine and prone position were compared, and effects of the level of lung inflation on lung signal were also evaluated.

RESULTS

The signal from the lung parenchyma diminished exponentially with prolongation of TE. The measured T2* in six normal volunteers ranged from 0.89 to 2.18 ms (1.43 +/- 0.41 ms, mean +/- S.D.). The measured relative proton density values ranged between 0.21 and 0.45 (0.29 +/- 0.08, mean +/- S.D.). Calculated T2* values of 1.46 +/- 0.50, 1.01 +/- 0.29 and 1.52 +/- 0.18 ms, and calculated relative proton densities of 0.20 +/- 0.03, 0.32 +/- 0.13 and 0.35 +/- 0.10 were obtained from the anterior, middle and posterior portions of the supine right lung, respectively. The anterior-posterior proton density gradient was reversed in the prone position. There was a pronounced increase in signal from lung parenchyma at maximum expiration compared with maximum inspiration. The ultrashort TE GRE technique yielded images demonstrating signal from lung parenchyma with minimal motion-induced noise.

CONCLUSION

Quantitative in vivo measurements of lung T2* and relative proton density in conjunction with high-signal parenchymal images can be obtained using a set of very rapid breath-hold images with a recently developed ultrashort TE GRE sequence.

Differentiation of alveolitis and pulmonary fibrosis in rabbits with magnetic resonance imaging after intrabronchial administration of bleomycin

RATIONALE AND OBJECTIVES

The authors investigate the ability of magnetic resonance imaging to differentiate alveolitis and pulmonary fibrosis by correlating magnetic resonance and pathologic findings.

METHODS

Lung damage was induced in 52 rabbits by instillation of 5 mL bleomycin sulfate (10 mg/kg) into a lower-lobe bronchus using a balloon catheter. Magnetic resonance examinations were performed in a group of 7 animals 3 hours after the initial damage, and in groups of 8 animals 24 hours and 8, 14, 30, and 80 days after the initial damage. Control animals were examined 3 hours (n = 5), 24 hours, and 8 days (n = 3 for each), respectively, after the instillation of 5 mL 0.9% sodium chloride. Magnetic resonance imaging at 1.5 T included conventional T1-weighted sequences before and after injection of gadolinium-DTPA (0.1 mmol/kg), and T2-weighted fast spin echo sequences. The signal intensity and contrast enhancement of injured lung were evaluated and compared with the contralateral healthy lung and with the lungs of control animals. All animals were killed immediately after the magnetic resonance examination, and the lungs were removed and fixed before sectioning and staining.

RESULTS

There was good correlation between signal intensity and contrast enhancement with magnetic resonance imaging and histologic examination. The early phase of acute alveolitis showed lesions with high signal intensity on both T1- and T2-weighted images and marked contrast enhancement after gadolinium-DTPA administration, whereas in the late fibrotic stage the lesions displayed significantly lower signal intensity and contrast enhancement.

CONCLUSION

Magnetic resonance imaging can differentiate between alveolitis and fibrosis by means of signal intensity and contrast enhancement after gadolinium-DTPA administration.

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.

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.

Regional effects of repetition time on NMR quantitation of water in normal and edematous lungs

It is well known that pulmonary edema is, in general, spatially nonuniform. Since the NMR spin-lattice relaxation time (T1) is increased by lung edema, the spatial distribution of T1 will be nonuniform. When the repetition time (TR) is short relative to the T1 of edematous lung, lung water content will be underestimated and this underestimation will be spatially nonuniform as well. Therefore, technical artifacts which are a complex function of lung edema and its spatial distribution are expected. We compared overall and regional (topographic) lung water density measurements obtained from living rats (with normal or edematous lungs) using repetition times of 2.0 and 6.2 s (at a magnetic field of 1 T), to quantify this uneven T1 effect for normal and edematous lungs. NMR measurements at TR = 2.0 s underestimated whole lung water density (-rho H2O) TR = 6.2 s) by an average of 7.2% in normal rats and 22.5% in rats with pulmonary edema. Regional -rho H2O underestimation (%delta-rho H2O) varied from 2.2 to 8.8% (groups means) in normal lungs and from 7.3 to 30.8% in edematous lungs. As a result, the interquartile range (of the voxel distribution as a function of rho H2O) underestimated the spatial nonuniformity of lung water density by 28.0% in edematous lungs, likely because of greater loss of NMR signal from high-water-density, long-T1 lung regions. Both %delta-rho H2O and T1 were significantly correlated with -rho H2O at TR = 6.2 s.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium NMR imaging of lung water in rats

Coronal proton and sodium images of control rats and rats with either increased permeability edema produced by intravenous alloxan (300 mg/kg) or increased pressure edema produced by saline infusion (2 ml/min) were obtained. Axial chest CT images were used to monitor the development of pulmonary edema. Immediately after the imaging session compartmental lung water was measured gravimetrically. The sodium and proton imagings were done sequentially in a 31-cm-bore 1.9-T magnet without moving the animal. The anatomical boundaries of the lung on the proton images were transferred to the sodium images for calculation of the average sodium signal intensity which was determined by extrapolating the mean values from five echoes to time zero. The sodium signal intensity was correlated (r = 0.7) with the total water fraction. There was poor correlation (r = 0.56) with the extravascular water due to confounding by the sodium vascular signal.