Morris, MD4 We studied the time course of changes in the Hahn spin-echo decay Hahn-T 2 in lungs of spontaneously breath-ing livbreath-ing rats at 1 hour, 3 hours, and 7 days followbre
Trang 1Technical Note
Observation of Regional Changes in the Time
Course of Oleic Acid Lung Injury
Sumie Shioya, MD,1,3* Rebecca Christman, BS,1 David C Ailion, PhD,1
Antonio G Cutillo, MD,2 K Craig Goodrich, BS,1 and Alan H Morris, MD4
We studied the time course of changes in the Hahn
spin-echo decay (Hahn-T 2 ) in lungs of spontaneously
breath-ing livbreath-ing rats at 1 hour, 3 hours, and 7 days followbreath-ing
oleic acid injection Motion artifacts were minimized by
using the motion-insensitive interleaved rapid line scan
(ILS) imaging technique Prior to injury, the lungs
exhib-ited two resolvable exponential Hahn-T 2 components.
One and 3 hours after injury the decay showed a
region-ally nonuniform behavior, which was fit with one, two, or
three exponential components The short and medium
components increased at 1 and 3 hours after injection.
The third, much longer, component is probably due to
intraalveolar pulmonary edema After 7 days the Hahn
decay was similar to that observed before injury,
proba-bly reflecting resolution of the edema Our data suggest
that Hahn-T 2 measurements can be used to characterize
the time course and regional distribution of lung injury
in living animals J Magn Reson Imaging 2000;11:
215–222 © 2000 Wiley-Liss, Inc.
Index terms: lung; pulmonary edema; Hahn-T2
INTRODUCTION
MRI IS A POTENTIAL analytical tool for the noninvasive
study of pulmonary disease In a previous in vivo study
of endotoxinduced lung injury, we observed an
in-crease in the value of the Hahn spin-echo decay time
constant (so-called Hahn-T2) but no corresponding
in-crease in proton density (1) These data suggest that
NMR techniques can detect not only pulmonary edema,
but also other experimentally induced pathologic lung
changes, including those not associated with changes
in the water content Therefore, combined Hahn-T2and water content measurements may be useful in the characterization of lung injury Because of the sensitiv-ity of the two-dimensional Fourier transform (2DFT) techniques (2) to motional artifacts, we developed a rapid version of the line scan technique [the rapid line scan (RLS)] (3), in which successive line scans in a planar image selectively involve previously unexcited regions and therefore can be obtained in rapid se-quence
All line scan techniques have a significant advantage with respect to motion artifacts in that, after the acqui-sition of data from a given line, subsequent motions of the spins in the line will have no further effect However,
in the normal (i.e., nonrapid or conventional) line scan technique, successive excitations of adjacent lines in-volve spins in previously excited regions and therefore require waiting a time of order T1, resulting in distortion due to respiratory and cardiac motions that may occur during the long T1interval The rapid line scan (or RLS technique) (3) results in significantly reduced sensitiv-ity to such motions by utilizing diagonal selective exci-tation of successive lines, so that such exciexci-tation in-volves previously unexcited spins and thus does not require a time of order T1between them Since this time can be as short as a few milliseconds, a related advan-tage of the RLS technique is that the data required to form an image of an entire plane can then be acquired much more rapidly Possible interference from spurious echoes generated from previously excited spins can be eliminated by the use of spoiler gradients (3)
One disadvantage of line scan techniques, including RLS, is that the spatial resolution is poor, in order to avoid interference from spins near the edge of a previ-ously excited line This problem can be reduced, with some improvement in the spatial resolution, by leaving the excitation order of line scans In this inter-leaved rapid line scan (or ILS) technique, enough time elapses between the excitation of spatially adjacent lines in the image so that the previously excited spins have more time to dephase, thereby allowing successive lines to be more closely spaced (4) With this technique
we obtained Hahn spin-echo decay curves from lungs of
1
Department of Physics, University of Utah, Salt Lake City, Utah
84112.
2
Division of Respiratory, Critical Care, and Occupational Pulmonary
Medicine, Department of Internal Medicine, University of Utah, Salt
Lake City, Utah 84132-0001.
3
Department of Internal Medicine, Tokai University School of Medicine,
Isehara, Kanagawa 259-1193, Japan.
4
Pulmonary Division, Department of Internal Medicine, LSD Hospital,
Salt Lake City, Utah 84143.
Contract grant sponsor: National Institutes of Health; Contract grant
numbers: HL31216 and CA44972.
*Address reprint requests to: S.S., Department of Internal Medicine,
Tokai University School of Medicine, Isehara, Kanagawa 259-1193,
Japan.
Received May 27, 1999; Accepted September 17, 1999.
© 2000 Wiley-Liss, Inc 215
Trang 2normal living rats and resolved these curves into two
exponential components (4)
In a subsequent comparative study (5), we found no
significant differences between the values of the
Hahn-T2components obtained, respectively, from live,
spontaneously breathing rats and from excised rat
lungs Furthermore, we measured the Hahn-T2 in
whole excised rat lungs in both the imaging mode
(us-ing the interleaved RLS technique) and the
conven-tional nonimaging mode and found no significant
dif-ferences between the results with these two techniques
In the present study, we investigated the in vivo
Hahn-T2changes in oleic acid-injured lungs at various
times following the injury Our goal was to determine
whether the Hahn-T2measured by in vivo NMR imaging
can monitor and characterize the time course of oleic
acid lung injury This type of experimental lung injury
is one of the most widely used animal models of
in-creased permeability pulmonary edema and is of
clini-cal interest because of its pathophysiologiclini-cal
similari-ties (especially in the acute phase) with the acute
respiratory distress syndrome (ARDS) (6) The oleic acid
injury model has been used extensively to investigate
the effects of pulmonary edema on lung fluid balance,
hemodynamics, mechanical properties, and gas
ex-change as well as to test the response of lung injury to
various therapeutic strategies (6)
MATERIALS AND METHODS
Experimental Protocol
In all, 23 imaging experiments were performed using 14 female Sprague-Dawley rats (250 –300 g) Details of the experimental protocol are in Table 1 The rats were anesthetized by intraperitoneal injection of sodium pentobarbital with an initial dose of 60 mg/kg During the time course study, an additional dose (30 mg/kg intramuscularly) was given 3 hours after the initial dose The rats received a bolus of 0.12 mg/kg oleic acid through the tail vein Control data were obtained in seven normal rats In four of these seven rats, the Hahn-T2was measured both before oleic acid injection and 3 hours after injection After the acquisition of the 3-hour MRI data, one of the four rats was sacrificed for histologic examination, and three were allowed to sur-vive in order to obtain measurements again 7 days later In four additional rats, the Hahn-T2 was mea-sured at 1 hour after oleic acid injection Two of these four rats died after the 1-hour MRI measurements, and two were again studied 3 hours after injury Another three rats were added for Hahn-T2measurements at 3 hours after injection and for histology examination A larger number of animals were studied at 3 hours after injury because at this stage we observed the most marked changes in proton density and in the Hahn-T2 decay (Table 2) On the whole, histological data were
Table 1
Experimental Protocol*
No of animals
Hahn-T2measurement Control
(n⫽7)
Injection (n⫽11)
1 hour (n⫽4)
3 hours (n⫽9)
7 days (n⫽3)
*E⫽euthanasia, D⫽natural death, H⫽histology,s⫽Hahn-T2measurement,2⫽oleic acid injection
Table 2
Time Course of Hahn-T2Response in Oleic Acid-Injured Lungs
T2component
Data are expressed as mean⫾standard deviation ROI⫽region of interest, proton density⫽relative signal intensity of lung tissue at an echo time of 16 msec expressed as a percentage of the signal intensity of a water phantom, Mo⫽relative contribution of medium and long
T components to the total signal intensity *P ⬍ 0.001.**P ⬍ 0.01.***P ⬍ 0.05,compared with the corresponding control values
Trang 3obtained from three normal control rats and from two
rats at 1 hour, six rats at 3 hours, and three rats at 7
days after injury
NMR Imaging
The NMR imaging studies were performed using a 2.35
T, 33 cm bore superconducting magnet (Oxford
Instru-ments, Oxford, England), operating at 0.94 T (40.10
MHz) A 5-cm-diameter high-pass birdcage coil (7) was
used The images were generated by the
motion-insen-sitive ILS technique, as described earlier (4) Because of
its essential features (diagonal excitation of the 90° and
180° selected planes, use of spoiler gradients to
dephase spurious echoes, and interleaved excitation
order), the ILS technique overcomes several limitations
associated with lung imaging, artifacts induced by
re-spiratory and cardiac motion, long imaging time
(re-quired by the conventional line scan technique), and
low signal-to-noise ratio (SNR) due to the low proton
density of lungs, at the price of lower spatial resolution
(3,4)
In the present experiments, each ILS image consisted
of 16 lines with 128 voxels in each line The lines were
positioned so that all lines included some lung
informa-tion (i.e., regions that contained only chest wall were
not imaged to save imaging time) Using a raised cosine
radiofrequency pulse of 1.4 msec total width, we
ob-tained a spatial resolution of 1.5 ⫻ 1.5 ⫻ 2.5 mm per
voxel Since lung tissue has a short T2, the pulse was
made as narrow as possible to allow for a short echo
time for the Hahn-T2determination while still providing
adequate spatial resolution Each line in an image was
repeated and averaged 32 times to improve the SNR
The repetition time was 8 seconds, resulting in an
ac-quisition time of 4.3 minutes for each image or 70
min-utes for a series of 16 images During the imaging
stud-ies, the anesthetized animals breathed spontaneously
Because of the reduced sensitivity of our imaging
tech-nique to motion artifacts, respiratory gating was not used Figure 1a provides an example of an ILS image obtained from a normal rat lung The image shows no motion artifacts even in the absence of cardiac or respi-ratory gating Figure 1b presents, for comparison, an MRI image of the same lung acquired using the 2DFT technique
It should be noted that the lower spatial resolution of the ILS images compared with that achievable by other techniques is not a significant limitation for the present quantitative MRI studies These studies did not require the high spatial resolution needed for conventional de-scriptive MRI because they were designed to obtain quantitative data from relatively large lung regions (see below) Furthermore, the use of a larger voxel size for our MRI measurements offered the advantage of pro-viding more NMR signal per voxel in less time
Measurements of Hahn T 2
Hahn-decay curves were obtained from a series of
16 –18 spin-echo images with echo times ranging from
16 to 250 msec The echo time intervals were 4 –50 msec The Hahn-decay time constant (8), measured in the present study, is called Hahn T2to distinguish it from the T2 measurements obtained from the Carr-Purcell-Meiboom-Gill (CPMG) technique (9,10) In the presence of diffusion across inhomogeneous magnetic fields (11,12), the Hahn-decay time constant may be much shorter than the CPMG T2 (which reflects the
“true” T2if the 180° pulse spacing is sufficiently short) (13,14) In this case the Hahn decay is a measure of the rate of diffusion as well as the strength of the magnetic gradients, whereas the CPMG decay will reflect only the underlying T2 if the pulses are sufficiently closely spaced If, however, the pulses are selective and not particularly closely spaced, then it is possible that CPMG will also reflect diffusion However, the shortness
of the underlying T2 in lung (4) along with the time
Figure 1 a: ILS image of a normal rat lung, consisting of 32 lines b: The image is compared with an image of the same lung
obtained by the conventional two-dimensional Fourier transform (2DFT) technique Note the absence of motion artifacts in the ILS image (acquired without cardiac or respiratory gating)
Trang 4required for the external imaging gradients would make
it considerably more difficult to perform CPMG
mea-surements in the imaging mode in the lung For these
reasons, we chose in this study to examine the
re-sponse of the Hahn echo decay time constant (rather
than CPMG T2) to oleic acid injury
For each image, the magnetization was determined
from regions of interest (ROIs) in the lung tissue As
shown in Figs 2a and 3a, we chose ROIs in the
periph-eral lung tissue to avoid signal contributions from large
blood vessels and the chest wall Each ROI consisted of
10 – 40 voxels and sampled a relatively large portion of
the lung (⬃1 cm2cross-sectional area) Accordingly, it
seems likely that the relatively small displacements
(about 2 mm) due to the breathing motion resulted in
very small errors due to lack of repeatability of the exact
position of each voxel in the lung Each ROI was used for a series of 16 –18 images required for the Hahn-T2 measurements For the normal control and 7-day mea-surements, two ROIs were chosen for each rat, one in the right and one in the left lung, except for two exper-iments performed 7 days after oleic acid injury, in which three ROIs were selected (Fig 3a) At 1 and 3 hours after oleic acid injection, Hahn-T2data were gen-erally obtained from four ROIs in each rat, two in the right lung and two in the left, avoiding large vessels The use of two ROIs for each image acquired in the control studies was considered to be adequate because under these experimental conditions the Hahn-T2 behavior was regionally uniform (The Hahn-T2decay curve was always characterized by two exponential components,
as shown in the Results section and in Table 2.) In
Figure 2 a: ILS image of an oleic acid-injured lung 3 hours after injection Since signal intensity was regionally nonuniform,
measurements were performed in five different regions of interests (ROI) We chose ROIs in the peripheral lung tissue to avoid signal contributions from large blood vessels and the chest wall For the measurement of both the Hahn-T2decay and proton density, the ROIs were chosen so that the signal intensity within each ROI was approximately uniform b: Hahn-T2decay curves obtained from five different ROIs (A–E) in the lung shown in Fig 2a The Hahn-T2behavior is nonuniform even between regions having the same signal intensity Curves C and D consisted of three exponential components (Hahn-T2 value for each component: 16.6, 49.6, and 114 msec for C and 14.7, 81.1, and 158 msec for D), whereas A, B, and E consisted of two components (36.6 and 97.7 msec for A, 42.5 and 65.8 msec for B, and 13.9 and 39.6 msec for E)
Figure 3 a: ILS image 7 days after oleic acid injection The signal intensity is now regionally more uniform b: Hahn-T2decay curves obtained from three different ROIs (A–C) in the lung shown in Fig 3a The Hahn-T2behavior is uniform, the decay curves consisting of two components (Hahn-T2value for each component, A: 9.3 and 43.4 msec; B: 11.0 and 47 msec; C: 9.3 and 62.2 msec)
Trang 5contrast, the Hahn-T2behavior of the lungs studied at
1 and 3 hours after injury varied regionally (the decay
curves being characterized by one, two, or three
com-ponents) Therefore, a larger number of ROIs was
re-quired to obtain adequately representative data
To determine the average signal intensity for an ROI,
the real and imaginary parts of the complex signal were
first averaged separately (15), and then the magnitude
was calculated This procedure avoids errors due to
noise rectification, which occur when signal and noise
are of comparable magnitude Thus the complex
aver-aged signal reflects the actual magnetization, since
the complex-averaged noise should approach zero
when the number of voxels is large Using this method,
we were able to determine the longest exponential
component of the Hahn-T2decay curve with improved
accuracy
The transverse magnetization decay obtained by the
Hahn spin-echo method was described as a
multiexpo-nential function,
M⫽ 冘A iexp共⫺k i t兲,
where M denotes the actual magnetization at time t,
k i⫺ 1is the time constant, and A iis the relative
magne-tization characterizing each different component i The
Hahn decay curves from ROIs placed on the lung tissue
were resolved into one, two, or three Hahn-T2
compo-nents (5) In the presence of three compocompo-nents, the
third component was subtracted from the original data,
and the subtracted data were then fit with
two-expo-nent functions
Measurement of Proton Density
To detect and quantify the changes in lung water
con-tent due to oleic acid injury, we determined the proton
density by measuring the complex-averaged
magneti-zation over each of the ROIs used for the Hahn-T2
mea-surements The relative magnetization at an echo time
of 16 msec for peripheral lung tissue was obtained from
the lung signal intensity and expressed as a percentage
of the signal intensity of a water phantom The water
phantom consisted of a plexiglass cylinder (45 mm in
diameter, which was close to the rat’s chest size) filled
with water doped with CuSO4 to decrease T1 and
thereby prevent saturation The data from the water
phantom were obtained each time using the same coil
just before acquiring the Hahn-T2 data from the rat
lungs The T1and T2values for the water phantom were
approximately 350 and 250 msec, respectively
Our proton density data were not corrected for signal
losses due to the T2decay and therefore do not quantify
absolute lung water content Back-extrapolation of the
Hahn-T2 decay curves to correct for T2 losses would
cause significant errors because of the
multiexponenti-ality of these curves and because magnetic field
inho-mogeneity has different effects on the signals from lung
and phantom On the other hand, several studies have
shown that uncorrected proton density measurements
closely reflect relative changes in lung water content, as
demonstrated by the close correlation observed
be-tween these measurements and lung water content or
lung tissue volume density values obtained by gravi-metric or morphogravi-metric techniques (16 –18) Since the purpose of our proton density measurements was to document relative changes in lung water content, rather than measure absolute lung water content, our procedure was quite adequate for the present study
Histology
After completion of the imaging studies, lungs were resected en bloc The trachea was cannulated The lungs were fixed by instillation of a 10% formaldehyde solution at 20 cm H2O fixative pressure Light micros-copy samples were prepared by hematoxylin-eosin staining
Statistical Analysis
The changes in Hahn-T2and proton density measured
at various times during the experiments were evaluated statistically by analysis of variance with the Scheffe’s multiple comparison test and by linear regression
anal-ysis (19) A value of P ⬍ 0.05 was considered signifi-cant
RESULTS
Normal control values for proton density and Hahn-T2 were obtained from 14 ROIs in seven normal rats prior
to the injection of oleic acid SNR values for normal peripheral lung tissue were 100 and 3 at echo times of
16 and 90 msec, respectively The Hahn-T2decay for each ROI was fit with a biexponential curve as de-scribed in previous studies (1,4) There were no signif-icant differences in the two Hahn-T2components be-tween the right and left lungs Signal intensity was uniform, with 9%⫾ 2% (mean ⫾ standard deviation) of the signal intensity for pure water for all ROIs in the normal peripheral tissue (Table 2)
In contrast to the normal control lungs, which showed uniform signal intensity, the signal intensity of the lung tissue after oleic acid injection was regionally nonuniform The spatial variations in lung signal inten-sity appeared 1 hour after oleic acid injection Figure 2a shows an ILS image of oleic acid-injured lung acquired
3 hours after injection As in the other lungs, the re-gions of interest used for the Hahn-T2and proton-den-sity measurements were selected so that signal inten-sity was relatively uniform within each ROI Hahn-T2 decay curves obtained from five ROIs (A–E) in the lungs shown in Fig 2a are presented, as an example, in Fig 2b As shown in Fig 2b, Hahn-T2 measurements ob-tained 1 and 3 hours after injection showed a regionally nonuniform behavior, characterized by one, two, or three T2 components One hour after injection, the Hahn-T2decay was monoexponential for 2 of 15 ROIs, biexponential for 10, and triexponential for 3 Three hours after injection, the Hahn-T2decay was monoex-ponential for 6 of 35 ROIs, biexmonoex-ponential for 23, and triexponential for 6
Seven days after oleic acid injury, the regional non-uniformity in signal intensity was no longer present, and the lungs appeared normal (Fig 3a) At this stage,
Trang 6the Hahn-T2decay curves obtained from different ROIs
were similar, as illustrated in Fig 3b, which shows the
decay curves obtained from 3 ROIs (A–C) in the lungs
presented in Fig 3a Monoexponential and
triexponen-tial Hahn-T2decay curves were not detected: in all eight
ROIs sampled at this stage, the decay curves were
biex-ponential
Figure 4a and b shows examples of monoexponential
and biexponential Hahn-T2 decays with exponential
component fit The biexponential decay shown in Fig
4b was obtained from ROI A in Fig 2a In the
biexpo-nential Hahn-T2decays, the value of the short T2
com-ponent increased 3 hours after the oleic acid injection
(P ⬍ 0.01, compared with baseline and 1-hour values)
(Table 2) The medium Hahn-T2component increased
at 1 and 3 hours after injection (P ⬍ 0.001, compared
with the control) The monoexponential Hahn-T2value
was similar to the medium Hahn-T2value for normal
lung The short and medium components of the
triex-ponential decays were not significantly different from the corresponding control values
The proton density (Table 2) increased significantly at
3 hours compared with the control and then returned to nearly the control value after 7 days As shown in Table
2, at each time after injury (1 hour, 3 hours, and 7 days) there was no significant difference in proton density among the groups characterized, respectively, by a one-, two- or three-component Hahn-T2 decay How-ever, at 3 hours after oleic acid injection there was a correlation between the proton density and the short Hahn-T2 component (n ⫽ 29, correlation coefficient:
r ⫽ 0.534; P ⬍ 0.005) and between the proton density
and the medium Hahn-T2component (n ⫽ 35, r ⫽ 0.360; P ⬍ 0.05) This correlation was not observed at
1 hour and 7 days
Figure 5a and b presents microscopic views of the lung tissue 3 hours and 7 days after oleic acid injection One hour after injection, we observed diffusely
con-Figure 4 Examples of monoexponential (a) and biexponential (b) Hahn-T2decays with exponential component fit The curve in
b corresponds to decay A in Fig 2b obtained from ROI (A) in Fig 2a E, original data; F, short component; Œ, medium component The dotted line in 4b, which is the reconstructed line from two exponential components, fits the experimental data well As explained in the text, in the triexponential Hahn-T2decay curves the third component was subtracted from the original data, and the subtracted data were then fitted with a two-exponential function
Figure 5 a: Microscopic view of a lung 3 hours after oleic acid injection (⫻25, hematoxylin-eosin staining) The lung tissue shows nonuniform lung injury characterized by massive pulmonary edema at the periphery b: Microscopic view of a lung 7 days after oleic acid injection The lung shows disappearance of pulmonary edema with focal pulmonary fibrosis
Trang 7gested alveolar capillaries, irregularly distributed
alve-olar edema, hemorrhage, and septal necrosis At 3
hours (Fig 5a), capillary congestion, edema, and septal
necrosis became more marked Seven days after
injec-tion (Fig 5b), all acute lesions disappeared, with the
exception of mild edema Some fibrotic areas were
dis-seminated throughout the lungs
DISCUSSION
Lung injury induced by oleic acid, an 18-carbon
unsat-urated fatty acid with a single double bond, is one of the
most widely used animal models of acute permeability
pulmonary edema and fatty embolism (6) The early
stage of oleic acid-induced lung injury is characterized
by the formation of thrombosis and cellular necrosis
The presence of severe interstitial and alveolar
pulmo-nary edema is a typical feature of the injury during the
acute stage The repair stage is characterized by
prolif-eration of type 2 cells and fibrotic foci in the subpleural
areas of the lung (20) The histologic changes observed
in the present study are consistent with those
previ-ously reported for the acute and repair stages of oleic
acid lung injury
In two previous studies, investigators have attempted
to characterize the acute stage of oleic acid-induced
injury using MRI Schmidt et al (21) detected
pulmo-nary edema induced by oleic acid in live rats and
esti-mated T2 using two images acquired with repetition
times of 2.0 seconds and echo times of 28 and 56 msec
In another study, Phillips et al (22) estimated T2 in
mechanically ventilated cats after oleic acid lung injury
by fitting a semilogarithmic straight line to four echo
data points In these studies, they only estimated
changes in T2 Using the ILS imaging technique, we
measured the multiexponential Hahn spin-echo decay
using 16 –18 MR images in living rats Our data
de-scribe in detail the regional changes of oleic acid injury
over the whole time course of the injury, including the
late recovery stage
In the present study, a regionally uneven Hahn-T2
was detected as early as 1 hour after injection The
Hahn-T2at 1 and 3 hours after injection was
nonuni-form throughout the lung and was characterized by
one, two, or three T2-component exponential decays
This finding may reflect the typical spatial
nonunifor-mity of oleic acid lung injury, which was detectable in
our histologic sections However, the observed spatial
differences in the Hahn-T2decay are not due to
varia-tions in lung water content because proton density,
which reflects lung water content (16,23), did not vary
significantly between lung regions exhibiting
nonuni-form Hahn-T2 behavior As indicated above, proton
density and Hahn-T2measurements for each group (1
hour, 3 hours, and 7 days) were obtained from the same
ROIs The present data differ from our previous
obser-vations of the Hahn-T2behavior over the time course of
endotoxin lung injury (1) In endotoxin-injured lung
tissue, the Hahn-T2decays were exclusively
biexponen-tial, and the changes in T2were uniform throughout the
lung at each measurement time (1) Seven days after
oleic acid injury, the Hahn-T2behavior was again
sim-ilar to that observed before injection
The monoexponential Hahn decay in oleic acid-in-duced lung injury may reflect the presence of extremely severe tissue damage, such as the necrosis observed in the histologic sections Under these conditions, the en-vironment for lung tissue water may become more uni-form due to severe disruption of the tissue architecture The biexponential Hahn-T2 decays measured 1 hour after injection show a significant increase in the me-dium T2component with no corresponding increase in proton density This observation is of interest because it indicates that the early stage of lung injury prior to the development of overt pulmonary edema may be detect-able by Hahn-T2measurements Our data are in agree-ment with the results of previous in vitro and in vivo studies of endotoxin-injured lungs, which showed sig-nificant Hahn-T2changes with no corresponding vari-ations in lung water content (1,24)
Proton density increased 3 hours after oleic acid-induced injury This change was associated with an increase in the values of the two Hahn-T2exponential components A third much longer component was ob-served in the Hahn-T2decays obtained 1 and 3 hours after injury The value of this component did not change between 1 and 3 hours Similarly, in a previous in vitro study of oleic acid-injured lungs, we observed a third T2 component that had a value close to that measured in exudates obtained from the edematous lungs (25) In the histologic sections obtained from the injured lungs, irregularly distributed alveolar edema was observed 1 hour after oleic acid injection and became more marked
3 hours after injection Therefore, the third T2 compo-nent measured in vivo may reflect the development of a third water space, probably alveolar edema
Regional changes in lung density in oleic acid-injury have been demonstrated by computed tomography (CT) Hedlund et al (26) reported that oleic acid infusion
in dogs produced a patchy and predominantly periph-eral increase in lung density 1– 4 hours after infusion (26) In accordance with the results of conventional CT studies, our ILS images of oleic acid-injured lungs also showed regional variations in signal intensity The res-olution of MR images obtained using the ILS technique
is much lower than that of high-resolution CT (27) However, while the resolution achievable by the ILS technique is adequate for quantitative lung MR imag-ing, as discussed above, this technique provides other criteria, in addition to proton density, for the assess-ment of lung injury, for example, the measureassess-ments of Hahn-T2decay Using the ILS technique, lung regions with the same NMR signal intensity can be further characterized on the basis of their Hahn-T2behavior Because the time needed to acquire the Hahn data by the imaging strategy adopted for the present study was long (about 70 minutes), we studied the course of oleic acid injury at relatively wide time intervals (1 hour, 3 hours, and 7 days after injury) Our data (see, for ex-ample, Table 2) indicate that the severity of the oleic acid-induced injury differed substantially between the stages selected for the present study Therefore, we believe that the time resolution of our technique was adequate for the purposes of our study However, more rapid Hahn-T2 measurements would be desirable for practical applications and to monitor lung injury at
Trang 8more closely spaced time intervals Substantial
reduc-tion in the imaging time can be obtained by using fewer
but larger voxels (at the expense of spatial resolution),
by reducing the repetition time somewhat (with
corre-sponding loss of signal due to T1effects), or by
decreas-ing the number of averages (with corresponddecreas-ing loss of
SNR)
In conclusion, the present data suggest that in vivo
Hahn-T2measurements can detect and follow the time
course and regional distribution of experimentally
duced pulmonary edema This application further
in-creases the potential of NMR as a noninvasive approach
to the characterization of lung injury in intact living
animals Furthermore, the present in vivo Hahn-T2
measurements represent an additional step toward the
application of MRI techniques to the assessment of
pul-monary edema in humans Presumably, the Hahn-T2
changes reported in the present study are not strictly
specific of oleic acid lung injury but reflect the
struc-tural abnormalities detectable in pulmonary edema at
comparable stages of evolution As discussed above,
our data suggest that Hahn-T2measurements in
dis-eased lungs may provide independent information that
complements the results of current measurements of
lung water content Significant Hahn-T2 changes are
detectable when conventional (proton density) NMR
measurements of lung water content are still within
normal range (Table 2) Therefore, MRI methods for
measuring Hahn-T2 might facilitate the detection of
edemagenic pulmonary disease at an early stage
How-ever, the application of T2measurements to the
detec-tion and characterizadetec-tion of clinical pulmonary disease
requires further investigation and, in particular, the
implementation of faster imaging techniques
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