© 2000 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Characterization of Signal Properties in Atherosclerotic Plaque Components by Intravascular MRI
From the Departments of Medicine (W.J.R., Y-L.H., C.M.K., D.A.V., N.R.), Pathology (J.W.P., P.R.O.), and Surgery (D.H.B), Allegheny General Hospital, Pittsburgh, Pa.
Correspondence to Walter J. Rogers, Jr, MS, Division of Cardiology, Allegheny General Hospital, 320 East North Ave, Pittsburgh, PA 15212. E-mail wrogers{at}wpahs.org
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Abstract |
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Abstract—Magnetic resonance imaging (MRI) is capable of distinguishing between atherosclerotic plaque components solely on the basis of biochemical differences. However, to date, the majority of plaque characterization has been performed by using high-field strength units or special coils, which are not clinically applicable. Thus, the purpose of the present study was to evaluate MRI properties in histologically verified plaque components in excised human carotid endarterectomy specimens with the use of a 5F catheter–based imaging coil, standard acquisition software, and a clinical scanner operating at 0.5 T. Human carotid endarterectomy specimens from 17 patients were imaged at 37°C by use of an opposed solenoid intravascular radiofrequency coil integrated into a 5F double-lumen catheter interfaced to a 0.5-T General Electric interventional scanner. Cross-sectional intravascular MRI (156x250 µm in-plane resolution) that used different imaging parameters permitted the calculation of absolute T1and T2, the magnetization transfer contrast ratio, the magnitude of regional signal loss associated with an inversion recovery sequence (inversion ratio), and regional signal loss in gradient echo (gradient echo–to–spin echo ratio) in plaque components. Histological staining included hematoxylin and eosin, Masson’s trichrome, Kossa, oil red O, and Gomori’s iron stain. X-ray micrographs were also used to identify regions of calcium. Seven plaque components were evaluated: fibrous cap, smooth muscle cells, organizing thrombus, fresh thrombus, lipid, edema, and calcium. The magnetization transfer contrast ratio was significantly less in the fibrous cap (0.62±13) than in all other components (P<0.05) The inversion ratio was greater in lipid (0.91±0.09) than all other components (P<0.05). Calcium was best distinguished by using the gradient echo–to–spin echo ratio, which was lower in calcium (0.36±0.2) than in all plaque components, except for the organizing thrombus (P<0.04). Absolute T1 (range 300±140 ms for lipid to 630±321 ms for calcium) and T2 (range 40±12 ms for fresh thrombus to 59±21 ms for smooth muscle cells) were not significantly different between groups. In vitro intravascular MRI with catheter-based coils and standard software permits sufficient spatial resolution to visualize major plaque components. Pulse sequences that take advantage of differences in biochemical structure of individual plaque components show quantitative differences in signal properties between fibrous cap, lipid, and calcium. Therefore, catheter-based imaging coils may have the potential to identify and characterize those intraplaque components associated with plaque stability by use of existing whole-body scanners.
Key Words: atherosclerosis • catheters • MRI
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Introduction |
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Atherosclerotic plaque is an actively evolving structure with numerous components. Plaque composition and morphology may determine whether plaque rupture is likely. Spontaneous rupture of nonocclusive atherosclerotic plaque with subsequent thrombosis is the most frequent cause of acute coronary events, including myocardial infarction and unstable angina.1 2 The main components of mature atherosclerotic plaques are conveyed in the name: the soft lipid-rich atheromatous gruel and the hard, sclerotic, collagen-rich tissue.3 The process of atherosclerosis is a pathological cascade of lipid accumulation, collagenous fibrosis, ulceration, hemorrhage, thrombosis, and calcification, leading to alterations in the composition of arterial walls that cause hardening, thickening, and loss of elasticity of the vessel walls. Beginning with the deposition of lipid within the intima within macrophages and myointimal cells, the process of atherosclerosis proceeds as the lipid-laden cells rupture and release their contents. Numerous cells and cellular processes affect collagen regulation, including smooth muscle cells,4 macrophage-produced matrix metalloproteinase-1,5 matrix metalloproteinase-2,6 and calcification.7 Free lipid collects and forms lipid pools with cholesterol crystal formation. The free lipids induce a fibrotic response. Fibroblasts and collagen appear within the lesion, and a denser collagenous cap forms over the lipid pool separating the thrombogenic lipid from the coagulation factors within the blood. The destabilizing atheromatous lipid pool lacks supporting collagen, is rich in extracellular lipids (predominantly cholesterol and its esters), and is avascular and hypocellular (except at the periphery, where macrophages are often found).8 The mechanical instability of the lipid pool and the inflammatory response may lead to weakening and fracturing of the fibrous cap, resulting in hemorrhage and thrombosis of the artery.9 10
MRI at high field strength is able to detect plaque components, including lipid,11 collagen,12 13 and calcium,14 15 on the basis of biochemical properties. At present, imaging of in vivo atherosclerotic plaque is limited to the use of surface coils for superficial vessels, such as the carotid arteries.11 To discriminate plaque components within "deep" vessels, including the coronary arteries, the use of intravascular imaging coils has been proposed.16 17 18 19 20 Although there have been a number of studies showing the potential for MRI to visualize plaque components,11 21 22 23 24 25 these studies have either used high–field-strength instruments inappropriate for clinical imaging or coil assemblies not appropriate for intravascular use. The purpose of the present study was to evaluate the signal properties of plaque components by use of a 5F catheter–mounted intravascular imaging coil to attain high spatial resolution, by use of standard imaging software, and by use of a clinical (0.5-T) imaging system.
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Methods |
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Seventeen human carotid endarterectomy specimens were included in the present study. Specimens were frozen immediately after excision at 4°C. It has been previously shown that the relaxation properties of atherosclerotic plaque do not change when specimens are stored at 4°C for up to 6 days.25
Intravascular MRI
Imaging was performed on a General Electric 0.5-T interventional
scanner. Details regarding this instrument have been previously
described in detail.26 Briefly, the design permits the
operator access to the center of the scanner for the purpose of surgery
or other interventions. This system requires the use of surface coils
for radiofrequency transmission and reception. In the present
study, endarterectomy specimens were positioned
with the long axis parallel to the static magnetic field and patient
couch. At physiological temperatures,
atheromatous lipids often exist near phase
transition.27 Magnetic resonance signal properties will be
affected by the temperature of the ex vivo specimen. Thus,
specimens were warmed to 37°C in a saline solution and were
maintained at this temperature (±0.2°C) throughout imaging by use of
a recirculating heater (Polyscience). After the catheter coil was
positioned within the lumen of the vessel segment, the vessel specimen
was wrapped in saline-saturated gauze and placed in an airtight plastic
case for imaging. The cassette maintained the spatial relationship
between the specimen and coil. Bags of saline (500 mL, 150 mEq/L) were
placed above and below the sample to minimize temperature changes and
act as a "load" for the transmit coil. The transmit coil contained
two 24-cm square elements positioned above and below the saline bags
and specimen. The temperature of the specimen was verified between
scans by use of a digital thermometer (model 08403, Cole-Parmer) with
an accuracy of ±0.2°C.
The receiver coil was an opposed solenoid design described by Hurst et
al.17 In contrast to standard coil designs, which make
images of the volume enclosed by the radiofrequency coil, the opposed
solenoid is an "inside-out" design, which permits imaging of the
walls of the carotid specimen with the coil positioned within the
lumen. The coil included two 10-turn solenoid elements wound in
opposite directions and separated by a 2-mm gap (Figure
I, which can be
accessed online at http://atvb.ahajournals.org). The coil was
integrated into the distal end of a 5F dual-lumen catheter without
changing the catheter diameter. The miniaturized tuning circuit was
located immediately proximal to the coil inside one of the catheter
lumens. This lumen also contained a coaxial cable, which terminated in
a miniature connector fit into a Luer port at the proximal catheter
end. A second Luer port permitted access to the second lumen for
delivery of fluids or guidewire introduction. The coil was tuned to
resonate at the Larmor frequency (21.3 MHz) of the system, and coils
including attached coaxial cable had an average Q value of
28.9±2.1.
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Imaging Protocol
After a stable temperature was achieved, axial and sagittal
scout images were used to identify the location of the coil and vessel
section. A scout image in the coronal plane showed the relationship
between the opposed solenoid coil and the overlying carotid artery
specimen. Regions of signal void occurring at each coil element
provided a means of registering the location of the cross-sectional
image plane to the specimen. This scout was also used to ensure that
the image plane was perpendicular to the vessel segment. A 3D imaging
sequence was used to acquire multiple short-axis images of the
specimen. Examination of these data verified that the atherosclerotic
plaque was centered within the imaging volume of the catheter coil.
Subsequent 2D images were acquired at the center of the coil. MRI
parameters were selected to permit calculation of absolute
T1 and T2 of the plaque components. All 2D acquisitions had a 3-mm
slice thickness, 4-cm field of view, a bandwidth of 7.81 kHz per pixel,
and 160 phase and 256 frequency lines resulting in in-plane resolution
of 250 µm in the phase direction and 156 µm in the
frequency direction. Two signal averages were obtained, and images were
interpolated to 256x256. Regional T1 was based on 6 image acquisitions
with use of a constant time to echo (TE, 17 ms) and repetition times
(TRs) of 300, 400, 500, 800, 1000, and 2000 ms. Regional T2 was based
on 5 image acquisitions with use of a fixed 1000-ms TR and TEs of 20,
40, 60, 80, and 100 ms. Magnetization transfer contrast (MTC) was
selected to identify connective tissue.22
Three-dimensional magnetization transfer sequences included a
saturation pulse 1200 Hz off resonance applied for 14 ms during each TR
period. Three-dimensional parameters included a TR of 220
ms, TE of 4.2 ms, bandwidth of 15.6 kHz per pixel, and a 1.5-mm
effective slice thickness. Inversion recovery (TE 17 ms, TR 1000 ms,
and time to inversion 350 ms), with use of a time to inversion
calculated to saturate the signal from water, was used to distinguish
water from lipid. Gradient echo sequences (TE 17 ms, TR 500 ms, and
flip angle 60°) were used to highlight regions of calcium deposition,
as has been previously described.14 A ratio of
region-of-interest (ROI) signal intensity between gradient echo and a
similar spin echo (SE) sequence (TR 500 ms, TE 17 ms), defined as the
GRE-SE ratio, was used to express the sensitivity of each plaque
component to GRE imaging. Total imaging time for each specimen was 
3
hours.
Histology
Endarterectomy specimens of atherosclerotic
plaque were refrozen at 4°C after MRI. Whole specimens were digitally
photographed and x-rayed. Each specimen was then cut at the location of
MRI that had been identified by an indelible marker at the time of
intravascular MRI (IV-MRI). The coil center was identified on the
specimen cassette and permitted accurate transfer of the imaging
location to the specimen before processing. Three-millimeter-thick
cross sections on either side of the imaging site were frozen to
-20°C in Tissue-Tek OCT water-soluble embedding medium, and 6-µm
tissue sections were cut from each block, stored overnight in 10%
formalin vapor, and stained with oil red O stain for
lipids.28 The tissue blocks were thawed, fixed in 10%
formalin solution for 4 hours, and partially decalcified in a formic
acid (leaving calcium salts but reducing large solid calcium deposits),
formaldehyde, methanol, and water solution for 8 hours. The tissue
blocks were subjected to standard tissue processing28 and
embedded in paraffin. Sections (4 µm) were cut and stained with
hematoxylin and eosin (H&E), Masson’s trichrome, Gomori iron, and
Kossa calcium stains.28
Endarterectomy removes the atherosclerotic luminal lesion with a plane of dissection usually within the innermost vascular media. The resected specimens, therefore, consisted primarily of the diseased intima and very few smooth muscle cells of the media present at the periphery.
The components of the arterial wall and the atherosclerotic
plaque were distinguished by histomorphology and histochemistry. In the
present study, we choose 7 categories of
histological findings of atherosclerotic vessels to
correlate with the magnetic resonance images: (1) loose, edematous,
fibrous tissue, (2) dense fibrous cap, (3) smooth muscle cells, (4)
lipid, (5) fresh hemorrhage, (6) organizing thrombus, and
finally (7) calcification (Figure
II, which can be accessed online at
http://atvb.ahajournals.org). The 7 categories of
histological findings were identified by the following
methods. The densely collagenous fibrous cap was identified by intense
green staining of fibrous tissue by Masson’s trichrome stain
surrounding the vascular lumen and isolating the
atheromatous lipid (Figure IIA). The collagenous
portion of the plaque that stained less intensely green on Masson’s
trichrome was examined for lipid content, which was visualized
histologically by the presence of foam cells (Figure
IID) and cholesterol clefts (Figure IIE) on H&E and
Masson’s trichrome and as red intracellular droplets and extracellular
lipid pools on oil red O stain (Figure IIF and IIG). Areas of decreased
density of fibrous tissue not containing lipid were interpreted as
edematous (Figure IIB). Smooth muscle cells of the vascular media were
stained red with the trichrome stain (Figure IIC). Fresh
hemorrhage consisted of collections of intact erythrocytes
mixed with fibrin in the plaque lesion. Organizing thrombosis was
identified by the presence of red fibrin on trichrome (Figure II, panel
I), fibrovascular ingrowth (Figure II, panel K), and blue
hemosiderin deposits with Gomori iron stain (Figure II,
panel J) from the breakdown of erythrocytes. Calcium deposits were
identified as acellular purple crystals by H&E (Figure IIL) and
confirmed as brown crystals by Kossa stain (Figure IIM). A color-coded
digital parametric image (Figure 1) was created that mapped the various
histological components of the atherosclerotic
endarterectomy cross section at the site of MRI for
correlation with the corresponding MRI images. Regions of fibrous cap,
edematous collagen, smooth muscle, lipid, hemorrhage,
organizing thrombus, and calcium were color-coded and used to guide the
localization of ROIs on magnetic resonance images.
Image Processing
Magnetic resonance images were transferred to a SUN Ultra Sparc
workstation for processing. Registration between IV-MRI and digitized
histological images was accomplished in 2 steps. IV-MR
images were centered between coil-induced signal voids. This same
location was transferred as an indelible mark on each specimen by
carefully opening the specimen cassette and transferring the location
of the coil center to the specimen. Rotational registration used the
longitudinal incision made in the vessel at the time of the procedure.
This cut was visible on IV-MRI and histology. On the basis of the
results of histology, circular ROIs were positioned within locations
containing predominantly 1 of the 7 plaque component categories. Custom
software was used to compute T1 and T2 values for specific plaque
components. Intensity curves were generated across the sampled TRs for
T1 calculations and TEs for T2 calculations. The relaxation time of a
selected region was estimated by least squares fitting of its intensity
curve with a nonlinear function by using the method of gradient
expansion (IDL software, Research Systems). The inversion ratio (INV
ratio) was defined as the ratio of signal intensities in an ROI for an
SE sequence with and without a 180° inversion pulse. A similar ratio,
the GRE-SE ratio, was constructed comparing the signal from GRE and SE
sequences.
Statistical Analysis
Data are expressed as mean±SD. Differences among the 7 plaque
component categories for T1, T2, INV ratio, MTC ratio, and GRE-SE ratio
were analyzed by 1-way ANOVA (when data were normally
distributed) or nonparametric Kruskal-Wallis ANOVA on
ranks. Individual differences were analyzed by the Student
t test for independent samples or Mann-Whitney rank sum test
when data were not normally distributed.
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Results |
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The average length of vessel segment visualized by use of the constructed coils was 4.2±0.2 mm. Seven plaque components were evaluated in the specimens: a fibrous cap in 94% (16 of 17), smooth muscle cells in 100% (16 of 16), organizing thrombus in 59% (10 of 17), fresh thrombus in 47% (8 of 17), edema in 35% (6 of 17), lipid in 59% (10 of 17), and calcium in 59% (10 of 17). Figure 2
displays the regional signal changes in
cross-sectional IV-MRI by using different imaging sequences. Proton
density (Figure 2A) and T1 images (Figure 2B) provide
overall plaque morphology with reduced regional contrast compared with
H&E (Figure 2F) and trichrome (Figure 2G). Inversion
recovery (Figure 2C) shows a loss of signal in regions of edema
and loose connective tissue (10 to 2 o’clock). T2-weighted imaging
(Figure 2D) shows signal loss in regions of connective tissue
and lipid, as verified by oil red O (Figure 2H) and trichrome
(Figure 2G). GRE shows multiple focal regions of signal loss
(Figure 2E) associated with calcium as seen by Kossa staining
(Figure 2J).
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Plaque characterization parameters, derived from IV-MRI
signal intensity, are presented in the
Table. There was significant
heterogeneity between INV ratio values for the studied
components (ANOVA, P<0.0005). Lipid regions displayed
little signal reduction after an inversion pulse adjusted to saturate
the water signal (0.91±0.06). This resulted in a higher INV ratio in
lipid compared with fibrous cap (0.62±0.16, P<0.05),
smooth muscle cells (0.35±0.004, P<0.001), organizing
thrombus (0.40±0.10, P<0.001), edema (0.25±0.07,
P<0.001), fresh thrombus (0.41±0.07, P<0.001),
or calcium 0.28±0.05, P<0.001). A specimen containing a
lipid pool is displayed in Figure 3.
IV-MRI proton density weighting shows a region (4 to 8 o’clock) of
slightly increased signal (Figure 3A), which is unaffected by
water signal suppression in an inversion recovery image (Figure 3B). H&E staining at low power (Figure 3C) and high power
(Figure 3E) verifies the presence of lipid and
cholesterol crystals. Overall differences were also found
between components for the MTC ratio (Table 1; ANOVA,
P<0.005). The fibrous cap showed the greatest signal change
after off-resonance magnetization transfer pulses (0.62±0.13). This
was significantly different from smooth muscle cells (0.87±12,
P<0.05), organizing thrombus (0.81±0.06,
P<0.001), edema (0.87±0.21, P<0.001), fresh
thrombus (0.78±0.06, P<0.03), or calcium (0.85±0.24,
P<0.05). The dense fibrous cap overlying a region of
organizing thrombus is well visualized by IV-MRI sequences
(Figure 4). The GRE-SE ratio
(Table 1) indicated that calcium was particularly sensitive to
GRE imaging, displaying a GRE-SE ratio of 0.36±0.21. This reduction
was greater than in smooth muscle cells (0.75±0.11,
P<0.006), edema (0.88±0.05, P<0.001), fresh
thrombus (1.01±0.17, P<0.004), and lipid 0.69±0.10,
P<0.04). Organizing thrombus (0.66±0.22) showed a
borderline difference (P=0.08). The effect of different MRI
sequences on intraplaque calcium is shown in Figure 2E and
confirmed by histology in Figure 2J. Absolute T1 values ranged
from 300±147 ms for lipid to 625±323 ms for calcium. There was a wide
range in observed T1 values in calcium (230 to 970 ms). Although
regional differences in plaque component relaxation times provided
contrast within acquired images, there was no significant difference in
absolute T1 values between plaque components. Absolute T2 values ranged
from 40±12 ms for fresh thrombus to 59±21 ms for smooth muscle cells.
T2 values were not statistically different between plaque
components.
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Discussion |
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In the present study, intravascular imaging coils, pulse sequences, and MRI equipment with potential for clinical application were used to characterize the signal properties of histologically validated plaque components in excised human carotid endarterectomy specimens. A number of imaging approaches were applied to determine which approaches best discriminated between plaque components. We found that magnetization transfer contrast, inversion recovery, and gradient imaging approaches were time efficient and were also able to distinguish the major atherosclerotic plaque components associated with plaque stability.
The stability of atherosclerotic lesions is dependent on the presence and geometry of specific plaque components rather than simply on plaque volume. The presence of a lipid pool creates focal regions of increased stress due to the interface between the soft lipid pool and stiff surrounding sclerotic material.29 MRI is sensitive to both of these materials. It is likely that differences in the amount of lipid in the evaluated image or sample volume alter the measured T2 value. The majority of lipids in the present study were found to be mixed within organizing thrombus or connective tissue and were best distinguished from the fibrous cap by the use of a water-saturating inversion pulse rather than by differences in T2. The lipid inversion ratio was 0.91±0.01 versus 0.62±0.16 for the fibrous cap (P<0.05).
The presence of large proteins, such as collagen, fibronectin, and elastin, may be detected by use of imaging sequences that include an off-resonance saturation pulse. These pulses preferentially affect the water molecules bound to macromolecules versus those in the free water pool. Images acquired in the presence of such pulses have reduced signal intensity in proportion to the amount of collagen or other macromolecules. Kim et al12 have previously shown that the ratio of signal intensity between acquisitions with off-resonance saturation to those without saturation (Ms/Mo) varies dramatically between tissue type. For example, they showed that lipid had an Ms/Mo ratio of 1.0, whereas articular cartilage, whose macromolecular structure is primarily type I and type II collagen, had an MTC ratio of 0.25. In the present study, we calculated an MTC ratio of 0.62±13 for the fibrous cap versus 0.93 for regions of lipid. The actual percentage or the type of collagen or other macromolecules in these regions is unknown, and the type of imaging scheme used would not be expected to produce the same magnitude of signal change as that used by Kim et al.12
Regions of intraplaque calcium were frequently observed in the
present specimens. Although the role and regulation of calcium in
plaque stability has not yet been fully determined, its presence as
detected by electron beam CT has recently been evaluated as a indicator
of underlying atherosclerosis.31
GRE imaging is more sensitive in the detection of calcium than is SE
imaging. This is caused by low proton density and by dephasing of the
local water molecules by calcium. Henkelman and
Kucharczyk14 have shown a linear relationship between
calcium in the form of calcium hydroxyapatite and T1 relaxation time.
T1 ranged from 500 ms at a concentration of 50 mg calcium per
milliliter of agarose gel to >1500 ms at a 350 mg/mL concentration. We
measured T1s in the region of calcium between 230 and 970 ms. This
range may reflect differences in regional calcium concentration. In the
present study, intraplaque calcium was detected by greater regional
signal loss in GRE versus SE and reported as a GRE-SE ratio. The
quantitative association between the magnitude of signal loss and
amount of calcium was not performed, and it should be noted that other
components in plaque, such as iron, would show a similar signal loss on
GRE images. Previous investigators have qualitatively detected the
presence of calcium by local signal loss.11 22
A number of IV-MRI coils have been proposed.16 17 18 19 20 All seek to capitalize on the increased signal to noise and associated potential increase in spatial resolution, gained through the proximity of the receiver coil to the vessel segment being imaged. The thickness of the fibrous cap is an important determinant of plaque stability, and the ability to resolve its thickness may define IV-MRI spatial resolution requirements. The differences between imaging coils presently under development are as follows: radial signal homogeneity, visualized vessel length, and the ability to produce signal when not aligned along the axis of the static magnetic field. The opposed-solenoid design17 used in the present study provides excellent radial signal homogeneity. This is important in identification of plaque components based on regional changes in signal intensity. However, this design images only short segments of the vessel (4.2±0.2 mm in the present study). Additionally, performance in this design is reduced as the coil moves off axis from the main magnetic field. Predicted loss of sensitivity at 45° off axis is 30%. Hurst et al17 also evaluated simple loop, 4-wire birdcage, 4-wire multipole, and 4-wire center return designs. Increasing the number of conductors in these designs improves radial signal homogeneity but causes the signal to fall off more rapidly as one moves away from the coil. Atalar et al19 proposed a catheter design that permits the imaging of longer vessel segments and better performance when the coil is not parallel to the main magnetic field, at the expense of reduced radial signal homogeneity. In all currently proposed intravascular coil designs, there must be a close match between the coil and vessel diameter because of the fall off of the rapid radial signal.
Determination of absolute T1 and T2 from acquired images was time consuming and did not discriminate between plaque components in the present study. However, pulse sequences that take advantage of specific differences in biochemical structure of individual plaque components, including magnetization transfer contrast, inversion recovery, and GRE, can be acquired rapidly and are successfully discriminated on the basis of changes in image signal intensity and may provide a more efficient method of MRI plaque characterization.
For IV-MRI to achieve clinical application, development must occur in a number of areas beyond imaging catheters themselves. Many of the routine devices used in catheter interventions, such as guiding catheters, introducers, and guidewires, must be modified to be magnetic resonance compatible and visible. Positioning MRI catheters within either peripheral or coronary arteries requires tracking32 33 and visualization techniques.34 To efficiently survey longer vessel segments, 2 approaches are currently under development. Atalar et al19 makes use of a catheter coil that images an extended vessel segment. This design trades axial signal inhomogeneity for a longer length of visualized vessel. Rivas et al35 have proposed the use of the opposed-solenoid coil with a rapid "real-time" imaging technique to overcome the limited length of vessel seen with the use of this coil design. It is likely that both methods will find clinical application.
Results of the present study indicate that IV-MRI with clinically compatible catheter-based receiver coils, hardware, and standard pulse sequences is capable of discriminating major atherosclerotic plaque components, including lipids, fibrous cap, and calcium, on the basis of inherent biochemical differences. Pulse sequences that take advantage of differences in biochemical structure within plaque components, including inversion recovery; MTC, and GRE imaging combined with intravascular imaging coils, may permit in vivo evaluation of plaque components and characterization of its stability.
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Acknowledgments |
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This study was supported by a Sponsored Research Grant from Cordis/Johnson & Johnson.
Received November 10, 1999; accepted March 20, 2000.
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