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Published ahead of print on February 20, 2003, doi:10.1164/rccm.200207-665OC
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American Journal of Respiratory and Critical Care Medicine Vol 167. pp. 1704-1710, (2003)
© 2003 American Thoracic Society

Original Article

Altered Clearance of Gadolinium Diethylenetriaminepentaacetic Acid Aerosol from Bleomycin-injured Dog Lungs

Initial Observations

Kazuyoshi Suga, Yue Yuan, Nobuhiko Ogasawara, Toshinobu Tsukuda and Naofumi Matsunaga

Department of Radiology, Yamaguchi University School of Medicine, Ube, Japan

Correspondence and requests for reprints should be addressed to Kazuyoshi Suga, M.D., Department of Radiology, Yamaguchi University School of Medicine, 1-1-1 Minamikogushi, Ube, Yamaguchi 755-8505, Japan. E-mail: sugar{at}po.cc.yamaguchi-u.ac.jp


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
To characterize altered alveolar transfer to solute in bleomycin (BLM)-injured lungs, eight dogs underwent a gadolinium diethylenetriaminepentaacetic acid aerosol (Gd-AS) magnetic resonance imaging study before and on Days 7 and 40 after tracheal instillation of BLM (0.75 mg) in the left lungs. Consecutive fast-gradient echo magnetic resonance imaging was acquired during and after spontaneous inhalation of 200-mM Gd-AS. The slope (Kep) and clearance half-time (T1/2) of logarithmic regression lines for clearance curves were estimated. Histology on Day 40 was compared with that on Day 7 in another three dogs. On Days 7 and 40, Gd-AS deposition was heterogeneously reduced in the affected lungs. On Day 7 with multifocal intraalveolar exudative changes, Kep in affected areas was significantly increased compared with baseline (2.5 x 10-3 minutes-1 ± 0.3 versus 1.7 x 10-3 minutes-1 ± 0.2, p < 0.0001), with significant decrease in T1/2 (121.6 ± 19.7 minutes vs. 170.4 ± 15.8 minutes, p < 0.001). However, on Day 40 with multifocal interstitial fibrosis, Kep and T1/2 were recovered toward baseline. BLM-injured lungs can be characterized by accelerated Gd-AS clearance during the acute exudative phase and their recovery during the chronic fibrotic phase. This technique is acceptable for monitoring alveolar transfer changes in BLM-injured lungs.

Key Words: magnetic resonance imaging • aerosol • bleomycin-injured lungs • gadolinium diethylenetriaminepentaacetic acid

The structural and/or functional damage of the alveolar–capillary membrane in acute lung injury and inflammation induces an increase in the transfer rate of small molecular solutes across this membrane. Alteration of alveolar transfer to small molecular solutes associated with the damage of the alveolar–capillary membrane has been assessed by measuring the lung clearance rate of inhaled technetium-99m–labeled diethylenetriamine pentaacetic acid (Tc-99m-DTPA) radioaerosol (1–12). However, this scintigraphic method has disadvantages, such as poor spatial resolution, the use of radioactive substances, and the difficulties in obtaining cross-sectional tomographic images without overlapping of radioactivity between lesions and normal lungs. We recently developed a gadolinium-DTPA aerosol (Gd-AS) ventilation magnetic resonance (MR) imaging method in which a sufficient lung aerosol deposition can be noninvasively achieved in the spontaneously breathing animals using an open-circuit aerosol delivery system with an aerosol reservoir (13). Hydrophilic small molecular Gd-DTPA solute (molecular-weight; 742.79 daltons) deposited in the alveolar space after inhalation of Gd-DTPA aerosol was suggested to cross the intercellular pores of the alveolar epithelium by diffusing into the vascular space, similar to Tc-99m-DTPA radioaerosol (14–18). Therefore, the measurement of lung clearance of Gd-AS may allow the assessment of alveolar transfer changes to small molecular solutes associated with alveolar–capillary membrane damage on cross-sectional imaging.

Intratracheal bleomycin (BLM) is a well established experimental model of damage of the alveolar–capillary membrane (19–30). The pathophysiology and altered alveolar transfer to Tc-99m-DTPA radioaerosol in the acute and chronic phase of BLM-injured lungs have been well characterized (19–25). In this study, we conducted an experimental study using the dog models exposed to intratracheally administered BLM to evaluate the ability of a Gd-AS MR study for characterizing the changes in the alveolar transfer rate to a small molecular solute during the acute and chronic phases of BLM-injured lungs.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animal Models
In accordance with the guideline for the care and use of laboratory animals (31) and with approval by the institution's animal use and care administrative advisory committee, a total of 11 adult beagle dogs (10.6 ± 0.8 kg) were used for this study. These animals were anesthetized with sodium pentobarbital (25 mg/kg) and were intubated using a 7-mm cuffed endotracheal tube. Small supplementary doses of sodium pentobarbital (total dose ranging from 3.2 to 6.5 mg/kg) were intermittently administered during the course of the experiment as needed to maintain adequate levels of sedation. Appropriate supplementary doses of sodium pentobarbital were carefully administered immediately before aerosol inhalation to maintain a regular and deep breathing pattern, as lung aerosol deposition was largely interrupted by fast shallow breathing (13, 32).

A Gd-AS study was performed in eight of these animals. These animals initially underwent a baseline Gd-DTPA aerosol study and after a 7-day interval received BLM instillation into the segmental bronchi of the left lower dorsal lobes. In each animal, a 6-French angiographic catheter (Cook, Bloomington, IN) was selectively inserted and wedged to the corresponding bronchial branch under fluoroscopic control, and 0.75 mg of BLM was administered into the target bronchial lumen via this catheter. Each animal then underwent a Gd-AS study on Day 7 and Day 40 after BLM instillation using exactly the same technique as in the baseline study. All of these eight animals underwent chest computed tomography (CT) scans before the MR study to evaluate the morphologic changes of the BLM-injured lungs. High-resolution CT images with a slice thickness of 2 mm were sequentially obtained using a spiral CT unit (Somatom Plus 4; Siemens-Asahi Medical, Tokyo, Japan) throughout the lungs, with an interval of 5 mm. Soon after accomplishment of the MR studies on Day 40 after BLM instillation, all of these animals were killed to investigate the histology of the affected lungs. The remaining three animals served for evaluating the histology of the acute phase of BLM-injured lungs and were killed on Day 7 in the same manner as in the previously mentioned eight animals. Light microscopic observations were made in the tissue samples of the affected and unaffected lungs stained with hematoxylin eosin.

Gd-DTPA Aerosol Generation and Delivery
Gd-AS was produced from 20 ml of 200 mmol Gd/L Gd-DTPA solute (gadopentetate dimeglumine, Magnevist; Nihon-Shering KK, Osaka, Japan) diluted with physiologic saline by an ultrasonic nebulizer (Omron NE-U12; Omron K.K., Kyoto, Japan) and was delivered to the animals via an open-circuit system with an aerosol reservoir, as described previously (13). Briefly, the ultrasonic nebulizer consisted of a canister, warmed to 29°C operating temperature, which continuously generated heterodisperse aerosols (Figure 1) . The nebulizer body was placed at a distance of 1.2 m from the MR scanner gantry edge to prevent the strong magnetic effect. It was connected to the intubated tracheal tube of the animals using a 1.2-m flexible tube of 25-mm diameter. A spherical plastic aerosol reservoir chamber of 12-cm diameter was placed at the end of this long tube. In the proximal site of this reservoir, a side airway of 20-mm diameter with an aerosol filter was opened to reduce airway resistance and to allow the animals to inhale the room air. The room air could be also in-flown into this aerosol delivery system through the fan filter of the nebulizer body, through which the aerosol could be also partly released into the surrounding air. Although the magnitude (scale) of aerosolization on the ultrasonic nebulizer could be set in the ranges from 1 to 10, it was constantly set at 8 in this experiment. At the end of a 0.4-m flexible tube of 25-mm diameter connected to the ultrasonic nebulizer, the aerodynamic median mass diameter of Gd-AS particles measured by laser diffraction scanning (Malvern 2,600 Sizur, London, UK) was 4.99 µm. A total of 20% of the particles had aerodynamic particle sizes of less than 2.2 µm. The aerosol nebulization rate measured during the initial 3 minutes was 1.9 ml/minute.



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  		Figure 1. A gadolinium diethylenetriaminepentaacetic acid aerosol (Gd-AS nebulizer and delivery unit. A spherical plastic aerosol reservoir with a 12-cm diameter is placed at the end of the 1.2-m tube with 25-mm diameter connected to the ultrasonic nebulizer. At the proximal site of this reservoir, a tubing of 20-mm diameter with an aerosol filter is connected to reduce airway resistance and to allow the animals to inhale the air (arrow). Air can also enter into this delivery system through an aerosol filter attached to the aerosol generator (arrow).

 
Gd-DTPA Aerosol Ventilation MR Study
All of the MR studies were performed using 1.5 Tesla MR scanner (CSI; Yokogawa-General Electric, Tokyo, Japan). The animals were placed within a quadrature knee coil on the MR table throughout the MR experiment. Initially, midcoronal localizer fast-gradient T1-weighted images were acquired without a breath hold. Three consecutive precontrast MR images covering the BLM-injured lower lungs were then obtained with an interval time of 1 minute. The image acquisition was performed using a three-dimensional fast gradient echo technique (fast spoiled gradient recalled acquisition in the steady state), with the following sequence parameters: repetition time/echo time of 3.5 milliseconds/0.9 milliseconds; flip angle of 15°; matrix size = 256 x 128; a field of view of 360 x 200 mm; sampling bandwidth of ± 83.3 kHz; one number of excitation. The acquisition volumes were prescribed on the basis of findings on coronal gradient-recalled-echo (6.9/2.0, 60° flip angle) two-dimensional MR images. Twenty-six images covering almost entire lungs, including the lower lungs, were acquired with a reconstructed slice thickness of 4 mm after Fourier transformation, using Zero interpolation and a field of view of 360 x 200 mm. Thereafter, the long tube of the aerosol delivery system was connected to the intubated tube of the animals, and aerosol inhalation was started. After these precontrast images, 20 consecutive MR images during aerosol inhalation were obtained with a 1-minute interval for 20 minutes. Then the long tube of the aerosol delivery system was quickly disconnected, and the following 15 clearance phase images were also continuously and consecutively acquired with a 5-minute interval for 75 minutes. Respiratory triggering for image acquisition was not used because the preexisting body wrapping-type triggering device interrupted sufficient respiratory chest wall motion of the dogs. However, during the acquisition of each of the clearance phase MR images, a transient breath hold of the animals was performed with the lungs inflated with 150 ml of air immediately after spontaneous expiration of the animals, using an air bag unit connected to an intubated tracheal tube.

Measurement of Lung Gd-DTPA Aerosol Deposition and Clearance Rate
The reconstructed transaxial MR images were analyzed on a workstation (VISART/EX; Toshiba Medical, Shibaura, Japan). Regional lung signal intensity (SI) was measured using the region of interest (ROI) placed in the selected, single level of the affected left lung and in the symmetrical portion of the unaffected contralateral lung. The ROI for the affected lung was initially placed in the MR study on Day 7 after BLM instillation and selectively in the lung areas with significant Gd-AS deposition in each animal, which corresponded to the lung areas with relatively slight increases in CT attenuation on chest CT images. The ROI was manually placed along the peripheral lung portions to minimize any contribution from the large vessels/bronchi and from the noticeable artifacts caused by fast cardiac motion to obtain pure alveolar ROI (i.e., alveoli only) (Figure 2) . In the MR studies of the baseline and Day 40 after BLM instillation, the ROI was placed at the same locations as in the Day 7 study as possible in each animal. The areas of the ROIs in the lung parenchyma varied from 43.2 to 136.4 mm2 (96.3 ± 35.2 mm2). The relative lung SI increases against the precontrast lung SI (%{Delta}SI) at each time point was calculated using this formula: ([postcontrast SI - precontrast SI]/precontrast SI) x 100%. The precontrast lung SI was determined by averaging the measurements of the three precontrast images. The maximum %{Delta}SI in each lung region was then determined by averaging the SI increases on the last three inhalation phase images, as the maximum enhancement was always noted on these images. The clearance of Gd-DTPA solute from regional lungs was analyzed according to a monoexponential model, where the logarithmic regression line of the decreased %{Delta}SI as a function of time during a clearance phase was fitted by the least-squares method and the slope of the regression line (the decline in %{Delta}SI per minute [Kep]) and the clearance half-time (T1/2 in minutes) were calculated. The correlation coefficient for the fit ranged from 0.684 to 0.973 (average, 0.87 ± 0.06).







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  		Figure 2. Chest computer tomography (CT) (top) and fast gradient echo T1-weighted magnetic resonance (MR) (bottom) images in a dog that received transtracheal bleomycin (BLM) instillation into the left lower lung. (A) The chest CT image on Day 7 after BLM instillation (top middle) shows condense consolidation in the affected lung, which is partly resolved on Day 40 (top right) (arrows). On the MR image at the same lung level as in CT image, abnormally high signal intensities are seen in the affected areas on Day 7 after BLM instillation (bottom middle), which is partly resolved on Day 40 (bottom right) (arrows). (B) Precontrast (left), the first (middle) and last (right) clearance phase MR images (at the same lung level as in Figure 2A) on Day 7 after BLM instillation show significant lung aerosol deposition and subsequent aerosol clearance in both lungs. The regions of interest (ROIs) are placed along the peripheral areas of the affected lung and in the symmetrical portion of the contralateral unaffected lung (left), and they are placed at the same locations as possible throughout the MR studies in each animal. (C) The time-SI curves throughout the inhalation and clearance phases in the baseline study. After 20 minutes of inhalation of Gd-AS, the right and left dorsal lungs are sufficiently enhanced with the maximum %{Delta}signal intensity (SI) of 362.5% and 233.3%, respectively. The right and left dorsal normal lungs have the decline in %{Delta}SI per minute (Kep) and T1/2 of 1.9 x 10-3 minutes-1 and 165.8 minutes and 1.5 x 10-3 minutes-1 and 184.1 minutes, respectively. (D) The time-SI curves on Day 7 after BLM administration. Although some parts of the affected left lung show Gd-AS deposition, the maximum %{Delta}SI of 132.4% in this area is less than that of 230.5% in the unaffected contralateral lung. The Kep of 2.7 x 10-3 minutes-1 in this affected lung was greater than that of 1.6 x 10-3 minutes-1 in the unaffected contralateral lung, and T1/2 of 110.3 minutes in this affected lung was less than that of 182.6 minutes in the unaffected contralateral lung. (E) The time-SI curves on Day 40 after BLM administration. Although some parts of the affected left lung show Gd-AS deposition, the maximum %{Delta}SI of 132.4% in this area is still low compared with that of 230.5% in the unaffected contralateral lung. There are significant recoveries of Kep and T1/2 toward the baseline values in the affected area, 1.2 x 10-3 minutes-1 and 243.0 minutes, respectively. The Kep and T1/2 in the unaffected lung are 1.7 x 10-3 minutes-1 and 171.4 minutes, respectively.

 
Statistical Analysis
Values were expressed as means with SD. Significance of the differences of the data comparisons was assessed using a paired or unpaired Student's t test, and statistical significant levels were accepted with a p value of less than 0.05. A linear regression analysis was performed to assess the linear dependence of the maximum %{Delta}SI and the precontrast SI in the affected lungs using commercially available software (StatView 4.02 SE + Graphics; Abacus Concepts, Berkeley, CA). Statistical significant levels were accepted with a p value of less than 0.05.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Baseline Study
In the baseline MR study of the eight animals, there were no significant differences in the precontrast, arbitrary SI per pixel between the symmetrical portions of the right and left dorsal lungs (11.0 ± 1.5 versus 10.0 ± 1.8; p = NS) (Figure 2A). During the 20-minute period of Gd-AS inhalation, the lung SI was gradually increased with time despite the respiratory SI changes, without any hyperdeposition of aerosol in the central airways (Figures 2B and 2C). The mean maximum %{Delta}SI of both dorsal lungs was 218.8 ± 46.5% (range from 151.0 to 311.0%), without any significant difference between the right and left lungs (227.0 ± 51.5% versus 210.7 ± 42.7%, p = NS). The mean Kep and the mean T1/2 of both dorsal lungs were 1.7 x 10-3 minutes-1 ± 0.2 (range from 1.3 x 10-3 minutes-1 to 2.0 x 10-3 minutes-1) and 170.9 ± 17.1 minutes (range from 141 to 199 minutes) without any significant difference between the right and left lungs (1.7 x 10-3 minutes-1 ± 0.2 versus 1.6 x 10-3 minutes-1 ± 0.2, and 170.4 ± 15.8 minutes versus 171.4 ± 19.3 minutes, both p = NS).

BLM-Injured Animals
On Day 7 after BLM instillation, the chest CT images showed heterogeneous opacities from ground-glass attenuation to condense consolidation with air bronchograms in the affected left dorsal lungs in all eight animals (Figure 2A). The precontrast MR images also showed significantly higher SI in these affected lungs compared with that in the unaffected right dorsal lungs (23.3 ± 3.0 versus 10.8 ± 1.1, p < 0.0001) (Figure 2A). During Gd-AS inhalation, the unaffected lungs were enhanced with the mean maximum %{Delta}SI of 230.5 ± 51.3%, without a significant difference from the baseline (p = NS). In contrast, the affected lungs showed heterogeneous enhancement. Some lung portions with markedly increased SI on the precontrast MR images were not enhanced, but some portions with relatively lower SI on these MR images were obviously enhanced with the mean maximum %{Delta}SI of 132.4 ± 59.8%, although it was significantly lower than that of the unaffected lungs (230.5 ± 51.3%) and the matched baseline value (218.8 ± 46.5%) (p < 0.01 and p < 0.001, respectively) (Figures 2B–2D). The maximum %{Delta}SI was significantly and inversely correlated with the precontrast lung SI in the affected lungs, as Y = 521.684 - 16.653 x X (r = 0.854, p < 0.01). The mean Kep of the affected lungs was significantly greater than that of the unaffected lungs and the baseline (p < 0.01 and p < 0.0001, respectively) (Figures 2D– 4) . The mean Gd-AS clearance time (T1/2) was also significantly lower than that of the unaffected areas and the baseline (p < 0.01 and p < 0.001, respectively) (Figures 2D– 4). However, the mean Kep and T1/2 of the unaffected lungs were not significantly different from the baseline values (p = NS).



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  		Figure 4. The changes of T1/2 before and after BLM instillation in the 8 dogs. (The same symbol represents the same animal. The red boxes represent the mean values of T1/2 of the unaffected right lungs in these animals and the blue boxes for the affected left lungs. The vertical bars represent the SD of the mean.) There is no significant change in T1/2 of the unaffected right lungs before and after BLM instillation (p = NS). In contrast, T1/2 of the affected left lungs is apparently decreased in every animal on Day 7 after BLM instillation, with a significant difference between that of the contralateral unaffected lung (p = 0.0005) and that of the baseline study (p < 0.0001). On Day 40, T1/2 of the affected left lungs has recovered toward the baseline values in the majority of these animals, and it is rather much prolonged compared with the baseline value in two animals. Overall, there is no significant difference in the mean T1/2 of the affected lungs and that of the contralateral unaffected lungs and the baseline value (both p = NS). This value of the affected lungs on Day 40 is significantly greater than that of the affected lungs on Day 7 (p < 0.01).

 


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  		Figure 3. The changes of Kep before and after BLM instillation in the eight dogs. (The same symbol represents the same individual animal. The red boxes represent the mean values of Kep of the unaffected right lungs in these animals and the blue boxes for the affected left lungs. The vertical bars represent the SD of the mean.) There is no significant change in Kep of the unaffected right lungs before and after BLM instillation (p = NS). In contrast, Kep of the affected left lungs is apparently increased in every animal on Day 7 after BLM instillation, with significant differences in the mean value from that of the contralateral unaffected lungs (p = 0.0001) and from that of the baseline study (p < 0.0001). On Day 40, Kep is recovered toward the baseline value in the majority of the affected areas, and it is significantly decreased compared with the baseline value in two animals. Overall, there is no significant difference in the mean Kep of the affected left lungs compared with that of the unaffected right lungs and baseline value (both p = NS). This value of the affected lungs on Day 40 is significantly lower than that of the affected lungs on Day 7 (p < 0.001).

 
On Day 40 after BLM instillation, the abnormal CT opacities in the affected lungs were partly resolved in all animals (Figure 2A). On the precontrast MR images, the high SI of the affected lungs was also attenuated at this time, although the mean SI of these lungs was still greater than that of the unaffected lungs (16.3 ± 3.0 versus 10. 4 ± 1.3, p < 0.0001) (Figure 2A). During Gd-AS inhalation, there was no significant difference in the mean maximum %{Delta}SI of the affected lungs compared with that on Day 7 (127.1 ± 85.6% versus 132.4 ± 59.8%, p = NS) (Figure 2E). The maximum %{Delta}SI was also inversely correlated with the precontrast lung SI in the affected lungs, as Y = 528.202 - 22.596 x X (r = 0.922, p < 0.01). Although there were some variations in each animal, the Kep and T1/2 in the affected lungs had shown recovery toward the baseline without significant differences from the matched baseline values (both p = NS) (Figures 3 and 4). There were significant differences in the mean Kep and T1/2 of the affected lungs compared with those on Day 7 (p < 0.001 and p < 0.01, respectively).

The total volume of Gd-DTPA solute aerosolized during Gd-AS inhalation time of 20 minutes was 13.8 ± 0.4 ml on Day 7 and 13.5 ± 0.7 ml on Day 40, respectively, which were not significantly different from that of 13.7 ± 0.4 ml in the baseline (both p = NS). The mean breathing rate of the animals throughout the aerosol study was 14.7 ± 2.8 per minute on Day 7 and 14.3 ± 2.5 per minute on Day 40, respectively, which were also not significantly different from that of 13.7 ± 2.4 per minute in the baseline (p = NS).

The histology of the affected lungs in the three animals killed on Day 7 showed focal exudative changes in the alveolar spaces with round cell and macrophage infiltration, hyaline membranes, detachment of the epithelium, and edematous thickening of the interstitial space (Figure 5A) . These changes appeared patchy rather than diffuse. In contrast, the eight animals killed on Day 40 showed focal fibrosis in association with inflammatory cell infiltration and thickening of the interstitial space predominantly in the peribronchiolar areas (Figure 5B). Dilated bronchiole and honeycombing changes caused by destruction of the alveolar structures were also seen. Moreover, the alveolar capillaries were obliterated by marked fibrous tissues. However, there were also intervening normal-appearing lung tissues within the affected lungs.




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  		Figure 5. Microphotographs of the BLM-injured lungs (hematoxylin and eosin stain, magnification x 480). (A) The histology of the affected areas in one of the three animals that was killed on Day 7 after BLM instillation shows focal exudative changes in the alveolar spaces, with round cell and macrophage infiltration, detachment of the epithelium, and edematous thickening of the interstitial space. (B) In contrast, the histology of the affected areas on Day 40 after BLM instillation shows widespread but focal fibrosis with thickening of the interstitial space. Honeycombing changes caused by destruction of the alveolar structures are seen. The alveolar capillaries are obliterated due to replacement by marked fibrous tissues.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In this preliminary study, a sufficient lung Gd-AS deposition could be obtained in spontaneously breathing dogs, which allowed the estimation of the Gd-AS clearance rate from the BLM-injured lungs. The results revealed an acceleration of the Gd-AS clearance rate from the acute, exudative, phase of BLM-injured lungs on Day 7, as was assessed from the Kep and T1/2 values. However, in the chronic, fibrotic phase on Day 40, this accelerated clearance had shown recovery toward the baseline. Although these results were obtained only from the lower dorsal dog lungs with anatomic and physiologic characteristics different from human lungs, this MR technique appears to have potential for characterizing the alterations of Gd-AS clearance over time in the BLM-injured lungs.

The average maximum %{Delta}SI of 218.8 ± 46.5% in the normal dog lungs was greater than the previously reported values obtained using a mechanical aerosol delivery system (14–17). This sufficient Gd-AS deposition may be due to the effect of the spherical reservoir (13). However, the amount of Gd-AS deposited in the lungs is estimated to be only approximately 1% of the nebulized Gd-DTPA solute (11, 12). Furthermore, the lung is the most difficult organ to be imaged by MR because of the unique tissue composition with low proton density and great magnetic susceptibility effect caused by the large air and tissue interface (14, 17, 18). Nevertheless, the strongly T1-weighted MR imaging with very short echo time provides good sensitivity for the lung aerosol enhancement by minimizing inherent signal loss from the short T2*of lung parenchyma and the T2 effect of the contrast agent. This rapid image acquisition also minimizes respiratory motion artifacts. The normal-appearing homogeneous Gd-AS deposition without hyperdeposition in the central airways is consistent with the findings of previous radioaerosol and Gd-AS MR studies in mechanically ventilated animals (15, 17). Similar to Tc-99m-DTPA solute, hydrophilic small molecular weight Gd-DTPA solute deposited in the alveolar space may cross the intercellular pores of the epithelium by diffusing into the vascular space (13, 15, 16, 33). However, the mean clearance T1/2 of 170.9 ± 17.1 minutes in the normal dog lungs is much longer than that of Tc-99m-DTPA of 26.3 ± 6.5 minutes in the mechanically ventilated dogs (11, 12). This slow clearance is consistent with the findings of previous MR studies and may be partly due to the differences in molecular weight of solute or concentration gradient across the alveolar–capillary membrane (14, 16, 17).

Despite this slow clearance of Gd-AS, our study clearly showed the accelerated clearance from the affected lungs on Day 7 after BLM instillation. This accelerated clearance during the acute phase is consistent with the findings of previous radioaerosol studies (7, 10, 19, 21, 26). It may be related to injury of the alveolar–capillary membrane (3, 4, 7, 10, 19). As seen in the three animals killed on Day 7, the morphologic changes with multifocal epithelial denudation should increase the permeability and area for transfer of intraalveolar Gd-DTPA solute to the microvascular circulation. The previous electron microscopic studies also showed the degeneration of the alveolar–capillary membrane, with detachment of the pulmonary epithelium and denudation of the basement membrane during the acute phase (10, 21). Alveolar epithelial transfer to much larger molecular weight proteins was also reported to be increased during the acute phase (19, 34).

The normal Gd-AS clearance during the chronic phase is also consistent with the findings of previous radioaerosol studies (19, 21, 26, 35). This reversibility may be related to the lung repair process, as indicated by resolution of the dense infiltrate on CT images and of the precontrast high SI and by the histologic appearance with the intervening normal-appearing lung tissues. Lung remodeling with a newly formed epithelial cell gap junction and basement membrane were observed as early as within several weeks in BLM-treated lungs (19, 21–24). Improvement of alveolar transfer factor for CO was also noted in the survivors of BLM-induced pneumonitis (35), although in the limited numbers of dogs, the Kep and T1/2 values were relatively delayed on Day 40 compared with the baseline. This may be caused by an increased distance of the diffusion pathway of intraalveolar Gd-DTPA solute because of interstitial fibrotic thickening and by the decreased transfer area due to the honeycombing changes and obliterated alveolar capillaries (10–12, 21, 36, 37). The previous histologic studies further revealed multifocal hyperplasia of alveolar type II epithelium with thickening and duplication of the basement membrane during the chronic phase after necrosis of type 1 epithelium (19, 22, 24, 34). These morphologic changes should also increase the distance of the Gd-DTPA solute diffusion pathway and retard Gd-DTPA transfer from the alveolar space.

In addition to these Gd-AS clearance rate changes, lung Gd-AS deposition was significantly decreased during both acute and chronic phases, with the correlation with the precontrast lung SI changes. During the acute phase, the precontrast high SI may be caused by the intraalveolar exudate and interstitial edema (38). Alveolar Gd-AS deposition should be also decreased by these pathophysiologies and by impaired alveolar mechanics associated with the increased extracellular matrix and altered surfactant components (11, 23, 28, 30, 32). During the chronic phase, the attenuation of the precontrast high SI may be caused by the resolution of intraalveolar exudate and interstitial edema, the focal fibrosis with collagen deposition, and the dilated bronchiole and honeycombing changes (38). These pathophysiologies should disturb regional ventilation and reduce Gd-AS deposition, despite the resolution of the exudate and edema (11, 12, 23, 32, 36). As Gd-AS deposition remained significantly decreased despite the trend to recovery of abnormal precontrast SI and Gd-AS clearance on Day 40 in these dogs, the assessment of Gd-AS deposition may be a good indicator for detecting the pathophysiology especially during the chronic phase. However, assessment of Gd-AS clearance may be more sensitive for detecting the functional changes during the acute phase, as this clearance rate change appeared more prominent on Day 7.

This MR technique appears to have a potential for characterizing and monitoring the alterations of aerosol deposition and clearance in BLM-injured lungs, similar to Tc-99m-DTPA radioaerosol (7, 10, 19, 21, 26). Compared with scintigraphic images acquired during breathing, the high temporal and spatial resolution cross-sectional MR images permit a more accurate analysis of regional Gd-AS kinetics and are easily comparable with chest CT images. It also has the advantages of no ionizing radiation exposure and its wide availability (12, 14–18). Alveolar epithelial permeability can be also assessed by bronchoalveolar lavage or the CO diffusing capacity for the lungs as a whole (27, 39). However, by using this MR technique, this assessment can be performed much simply at any selected cross-sectional lung regions. Although Gd-DTPA is not yet approved for humans in the form of an inhaled aerosol, previous studies did not show any acute hemodynamic and toxic effects in animals (13, 16). The small amount of Gd-AS deposited in the alveoli is redistributed via the circulatory system followed by quick elimination by the kidneys (13–15, 17). Although some Gd-AS particles might be swallowed, gastrointestinal Gd-DTPA administration was reported to be safe (17, 40, 41).

As drawbacks of this MR study, the results might have been affected by Gd-AS deposition in the medium and small airways within the peripheral ROI because these airways cannot be distinguished from the alveoli because of the limited spatial resolution of MR images. Acquisition of the same respiratory phase images using a navigator system and a two-component analysis of the fast and slow phases for the clearance curves acquired with shorter imaging intervals may allow more accurate assessment (4, 10, 11, 42). Although a relatively long inhalation time was required to obtain sufficient aerosol deposition, adequate breathing pattern for sufficient lung aerosol deposition can be conducted in patients and will reduce the inhalation time and dose of Gd-AS. This study lacked the comparisons with the established radioaerosol and CO diffusing capacity methods. However, similar to radioaerosol methods, this MR technique is expected to be a sensitive indicator of subclinical BLM-induced pneumonitis and other interstitial diseases, disease progression, and therapeutic effects (3, 8–10, 19, 21, 43). It may also be used for assessing the distribution and effects of the inhaled drugs that inhibit fibrogenesis (11, 44, 45) and for simultaneous assessment of regional ventilation and alveolar epithelial permeability in various lung disorders (1–4). Further studies are required to clarify these issues compared with radioaerosol and CO diffusing capacity methods.


   FOOTNOTES
 
Supported in part by a Grant for Scientific Research (11670891) from the Japanese Ministry of Education, Science, Sports and Culture.

Received in original form July 8, 2002; accepted in final form February 18, 2003


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
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