In Vivo Intravascular MR Imaging: Transvenous
Technique for Arterial Wall Imaging
Lawrence V. Hofmann, MD, Robert P. Liddell, MD, Aravind Arepally, MD, Brian Montague, MD,
Xiaoming Yang, MD, PhD, and David A. Bluemke, MD, PhD
PURPOSE: To determine, in vivo, the potential for transvenous magnetic resonance (MR) imaging of the arterial wall
and to assess appropriate MR pulse sequences for this method.
MATERIALS AND METHODS: MR imaging was performed on 19 vessels (right renal artery, N ⴝ 9; left renal artery
N ⴝ 2; external iliac artery, N ⴝ 4; abdominal aorta, N ⴝ 4) in nine swine. The animals were either low-density
lipoprotein receptor knockout (N ⴝ 5) or Yucatan mini-pigs fed an atherogenic diet for 6 to 11 weeks (N ⴝ 4). The
intravascular MR coil/guide wire (IVMRG) (Surgi-Vision, Gaithersburg, MD) was introduced via the external iliac
vein into the inferior vena cava (IVC). The following electrocardiograph-gated MR pulse sequences were obtained:
T1-weighted precontrast with and without fat saturation and T1-weighted postcontrast with fat saturation. Two
observers scored wall signal and conspicuity and classified the vessel as normal, abnormal, or stented. Images were
compared with histopathologic findings.
RESULTS: The T1-weighted precontrast without fat saturation, T1-weighted precontrast with fat saturation, and
T1-weighted postcontrast images correlated with histopathologic findings in 12 of 15 vessels, eight of 10 vessels, and
14 of 16 vessels, respectively. Abnormal histopathologic findings included: arterial wall thickening (N ⴝ 3), arterial
dissection (N ⴝ 2), focal fibrous plaque (N ⴝ 2), adherent thrombus (N ⴝ 1). The T1-weighted postcontrast images were
not compromised by artifacts and had the highest score for vessel wall signal and conspicuity. T1-weighted precontrast images were compromised by chemical shift artifact and poor blood suppression. Negligible artifacts were
created by the platinum stent.
CONCLUSION: The T1-weighted fat saturated postcontrast pulse sequence was superior to other sequences for
transvenous MR imaging of the arterial wall.
J Vasc Interv Radiol 2003; 14:1317–1327
Abbreviations:
IVMRG ⫽ intravascular magnetic resonance guide wire, IVC ⫽ inferior vena cava
IN 1995, the American Heart Association Committee on Vascular Lesions of
the Council on Arteriosclerosis created
a histological classification for atherosclerotic lesions (1). This classification
From the Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins
Medical Institutions, Blalock 545, 600 North Wolfe
Street, Baltimore, Maryland 21287. Received May 12,
2003; revision received July 10; accepted July 13.
Presented at the 2002 SIR Annual Meeting. Address
correspondence to L.V.H.; E-mail: lhofmann@
jhmi.edu
This study was supported by a research grant from
Surgi-Vision (Gaithersburg, MD). None of the authors have identified a potential conflict of interest.
© SIR, 2003
DOI: 10.1097/01.RVI.0000092904.31640.BE
is based on the histological composition and ultrastructure of atherosclerotic lesions, which is thought to be
responsible for certain clinical syndromes in the coronary and peripheral
vascular circulation. Most atherosclerotic thromboembolic events are believed to be secondary to fibrous cap
rupture with resultant exposure of the
thrombogenic subendothelial matrix
(2).
Intense research has focused on
prospectively identifying these “at
risk” plaques. To that end, investigators have explored different imaging
modalities that would allow them to
visualize the ultrastructure of a lesion,
specifically the fibrous cap and its
thickness. Magnetic resonance (MR)
imaging, because of its superior soft
tissue resolution, has become a promising modality for this task. Most research has focused on the carotid arteries because of their superficial
location and the ability to obtain noninvasive, high-resolution images with
use of surface coils; and researchers
have been able to discriminate the fibrous cap from the lipid core and
quantify the thickness of the fibrous
cap (3– 6).
MR imaging of the arterial wall and
atherosclerotic plaque that involves
vessels within the abdomen and pelvis
is difficult. The relatively large distance between the artery of interest
and the surface coil, as well as increased motion artifacts, make imag-
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In Vivo Intravascular MR Imaging
JVIR
Imaging and Histopathologic Results
Animal
No.
Artery
T1
S*
C†
Image Findings
T1 Fat Saturation
S*
C†
Image Findings
1
RRA
LRA
Iliac
⫺1
⫺1
1
1
Normal
Normal
NI
NI
NI
NI
2
RRA
⫺1
1
Normal; stent visualized with negligible artifact
NI
4
Iliac
RRA
LRA
RRA
⫺1
⫺1
1
1
1
3
NI
Normal; chemical shift artifact
Normal; stent visualized with negligible artifact
Thickened wall with dissection visualized
5
RRA
1
3
6
RRA
Aorta
Iliac
RRA
Aorta
RRA
Aorta
RRA
Aorta
⫺1
0
1
2
⫺1
⫺1
⫺1
⫺1
⫺1
⫺1
1
2
1
1
1
2
3
7
8
9
Iliac
Thickened wall, dissection, perivascular
hematoma
Normal; chemical shift artifact
Normal; poor blood suppression
NI
Normal
Normal
Normal; chemical shift artifact
Normal
Normal
Normal; poor blood suppression
1
1
1
3
1
3
0
⫺1
2
1
1
0
1
1
0
1
2
3
3
1
NI
NI
Normal
NI
Thickened wall with dissection visualized
Thickened wall, dissection, perivascular
hematoma
Normal
Normal; poor blood suppression
NI
Normal
Normal; poor blood suppression
Normal
Normal
Normal
NI
NI
* S ⫽ vessel wall signal compared to adjacent muscle [⫺1 (hypointense), 0 (isointense), 1 (hyperintense)]; † C ⫽ vessel wall
conspicuity [1 (poor), 2 (fair), 3 (good), 4 (excellent)]; ⫺NI ⫽ vessel was not imaged with that sequence; RRA ⫽ right renal
artery; LRA ⫽ left renal artery; T1 ⫽ T1-weighted non–fat saturated images; T1 fat sat ⫽ T1-weighted fat saturated images; T1
postcontrast fat sat ⫽ T1-weighted postcontrast fat saturated images.
ing within these regions exceedingly
difficult. In an attempt to overcome
these limitations, investigators have
explored the use of intra-arterial MR
receiver coils (7–10). Although these
studies provided high-resolution images, the large device size (5– 8 F) and
the need to place the device within the
artery carries the risks of dissection,
embolism, and possible occlusion,
which are substantially reduced with
venous access.
In this study, a small (0.030-in diameter) intravascular MR coil/guide
wire (IVMRG; Surgi-Vision, Gaithersburg, MD) was placed in a vein adjacent to the target artery. This technique has been termed “transvenous
MR imaging.” The authors hypothesized that in certain areas of the body,
the proximity of the vein to the artery
would be sufficient to obtain high-resolution images of the arterial wall.
This technique should substantially
reduce the risk of complications asso-
ciated with intra-arterial placement of
the IVMRG. To that end, the authors
sought to determine, in vivo, the feasibility of this approach and the optimal parameters for transvenous MR
imaging of the arterial wall.
MATERIALS AND METHODS
IVMRG
The IVMRG (Surgi-Vision, Gaithersburg, MD) used in this study is a
“receive only” coil and has been previously described (9). Briefly, the device is a 75-cm-long, 0.030-inch-diameter, loopless antenna consisting of a
soft conducting wire that has an inner
conductor from a 50-ohm, 0.6-mm, coaxial cable with a polyester jacket. The
proximal end of the coaxial cable was
connected through a matching tuningdecoupling circuit to the MR scanner.
The IVMRG can function as a conventional angiography guide wire with a
performance profile similar to that of a
0.035-inch diameter nitinol guide wire.
IVMRG Placement
The Animal Care and Use Committee of the institution approved all animal experiments. A total of 19 vessels
in nine swine were imaged. Five genetically engineered swine (low-density lipoprotein receptor knockout
[LDL⫺], Atlanta Cardiovascular Research Institute, Norcross, GA), were
fed standard hog chow for 9 months
(Hog Grower Chow OTC 50, PurinaMills, St. Louis, MO) before imaging.
The remaining four swine were Yucatan mini-pigs (Charles River Laboratories, Wilmington, MA) that underwent
balloon injury (11,12) of the right renal
artery and the external iliac artery. The
mini-pigs were fed an atherogenic diet
containing 20% lard, 6% cholesterol,
and 2% sodium cholate (Modified
Mini-Pig Grower Diet 5081, Purina-
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T1 Postcontrast Fat Saturation
S*
C†
1
4
1
4
1
1
4
4
1
4
1
4
1
1
1
1
1
1
1
1
1
4
4
4
4
4
4
3
3
3
1
3
Image Findings
NI
NI
Normal; stent visualized with negligible
artifact
Normal; stent visualized with negligible
artifact
Normal
Normal
NI
Thickened wall with dissection visualized
with negligible artifact
Thickened wall, dissection, perivascular
hematoma
Minimal circumferential wall thickening
Minimal asymmetric wall thickening
Normal
Normal
Normal
Normal
Normal
Normal
Thickened wall plus 3-mm mass lesion
adherent to the wall
40% vessel narrowing by marked
asymmetric wall thickening
Mills, Richmond, IN) for 6 to 11 weeks
(average 8.8 ⫾ 2.4 weeks) before
imaging.
In the fluoroscopy suite, the animals were sedated with an intramuscular injection of ketamine (22 mg/
kg), acepromazine (1.1 mg/kg), and
atropine (0.05 mg/kg). Intravenous
pentobarbital (20 mg/kg) was also administered. The animal was intubated
and ventilated with 1.5% isoflurane.
With ultrasound guidance, an 8sheath was placed in the external iliac
artery and a 6-F sheath was placed in
the external iliac vein. Abdominal and
pelvic x-ray angiography was performed. In two renal arteries, the backend of a guide wire was placed
through a reversed curve catheter
(Sos3; Angiodynamics, Queensborough, NY) to create a renal artery dissection. In four vessels, a platinum
stent (Ominflex; Angiodynamics,
Queensborough, NY) was placed (normal renal artery [N ⫽ 2], dissected
Pathology
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Diameter
(mm)
Normal
Normal
Focal 40–120 ␮m fibrous plaque; stent
5
5
9
Normal; stent
5
Focal 120–250 ␮m fibrous
Normal
Normal; stent
Dissection with intramural and perivascular
hematoma; stent
Dissection with intramural and perivascular
hematoma
Circumferential 100 ␮m intimal thickening
Asymmetric intimal thickening (500 ␮m)
Normal
No path
No path
No path
No path
Normal
3-mm area of adherent subacute thrombus
with focal intimal thickening
Marked asymmetric media thickening
because of hemorrhage, hylanization,
neovascularity
renal artery [N ⫽ 1] and external iliac
artery [N ⫽ 1]).
The IVMRG was introduced with
fluoroscopic guidance, with the aid of
a 5-F catheter, into the inferior vena
cava (IVC) or right renal vein. The animal was then transported to the MR
suite.
MR Imaging
MR images were obtained on a
1.5-T MR system (CV/I; General Electric Medical Systems, Waukesha, WI).
With use of the body coil, an initial fast
multiplanar spoiled gradient echo sequence with a 40-cm field of view
served as a scout image. Then, axial
and coronal gradient echo sequences
were obtained with three external surface coils (two posterior, one anterior)
to localize the IVMRG and to determine the course of the artery of
interest.
Double-oblique, electrocardiograph-
5
5
5
10
8
5
7.5
5
4
9
4
9
6
9
5
gated, double inversion recovery, fast
spin echo, black blood images, without breath hold or respiratory gating,
were obtained perpendicular to the
vessel lumen with use of the IVMRG.
For high resolution imaging (ie, small
field of view images), the surface coils
were turned to avoid “wrap around”
artifact. T1-weighted images with and
without fat suppression were initially
obtained (TR ⫽ 1-R-R interval [average TR, 650 msec]; TE, 12 msec; TI, 350
msec; ETL, 8 –16; field of view, 8 cm; 6
signal averages; 256 ⫻ 256 resolution
interpolated to 512 matrix; 3-mm slice
thickness; 32.5-kHz bandwidth; average acquisition time, 62 seconds per
image). Because the overall signal
from the IVMRG was small, the field
of view, matrix, bandwidth, and slice
thickness were varied in the first animal to derive adequate signal while
keeping the imaging time at about 1
minute per image. A T2-weighted
pulse sequence was then performed
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(TR ⫽ 2R-R interval [average TR, 1300
msec]; effective TE, 60 msec; ETL, 24;
field of view, 8 cm; 6 NEX, 256 ⫻ 256
resolution, 3-mm slice thickness, inversion recovery blood suppression,
32.5-kHz bandwidth, 83 seconds per
image). T2-weighted images were obtained in the first three animals and
then the sequence was discontinued
because of long imaging times and
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JVIR
non-visualization of the vessel wall
secondary to poor signal-to-noise
ratio.
Gadodiamide (Omniscan; Amersham, Princeton, NJ) was then intrave-
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(100 mg/kg) and a supersaturated solution of potassium chloride. The vessels were exposed by blunt and sharp
dissection. The arterial tree, including
bilateral renal arteries, aorta, and iliac
arteries to the level of sheath insertion,
were harvested en bloc. The vessels
were placed in formalin solution.
The vessels containing stents were
embedded in methyl methacrylate,
cut, ground, and polished with use of
the EXAKT System (Werheim, Germany), and stained with hematoxylin and
eosin. Vessels without stents were embedded in paraffin and stained with
hematoxylin and eosin, Masson
trichrome, and Verhoeff elastic stains.
Histopathologic specimens were labeled as distances from known anatomic landmarks, such as the renal artery ostium or aortic bifurcation to
permit registration with the MR images. Histopathologic features were
analyzed and classified by a vascular
biologist.
Image Analysis
Figure 1. Right renal artery (RRA) dissection. The IVMRG is in the IVC and is the only
receiver coil used in the transvenous images. (a) Transvenous MR T1-weighted double
inversion recovery fast spin echo non-fat saturated image shows an axial image of the
right renal artery, posterior to the IVC, with significant thickening of the cephalad portion
of the arterial wall (short arrow) and a larger signal void that corresponds with the lumen
(long arrow) of the vessel. The IVMRG is not seen because it is in a different plane;
however, the high signal from the IVMRG can be seen in the adjacent IVC. (b) Transvenous MR T1-weighted fat saturated double inversion recovery fast spin echo sequence,
at the same location as figure part a, demonstrates an increase in wall signal compared
with figure part a. The high wall signal because of periadventitial hemorrhage is appreciated now that the perivascular fat signal is suppressed. A small lumen (arrow) is now
seen in the cephalad portion of the vessel. (c) Transvenous MR T1-weighted fat saturated
double inversion recovery fast spin echo postcontrast image, at the same location as figure
part a, shows improved wall signal and conspicuity, particularly the thin wall (arrow)
between the two lumens. (d) Summation image of all three surface coil images with use
of the same imaging parameters and acquired at the same time as figure part c. The
arterial wall is faintly seen (arrow). (e) Masson trichrome stain of the right renal artery
demonstrates the true (TL) and false lumen (FL) of an arterial dissection with significant
wall thickening and a thin wall between the true and false lumen. Postmortem thrombosis
of the false lumen compresses the true lumen. Bar ⫽ 735 ␮m.
nously administered (0.2 ␮mol/kg).
After a 5-minute delay, double inversion recovery, fast spin echo, T1weighted, fat-saturated imaging was
repeated. Inversion times (median TI,
150 msec; range, 125–200 msec) were
adjusted visually to suppress signal
from the vessel lumen. In four animals, after imaging the abdominal
vessels, the IVMRG was repositioned
in the external iliac vein to obtain postcontrast images of the external iliac
artery.
Vessel Analysis
After MR imaging, the animals
were killed with use of pentobarbital
MR images were transferred to a
personal computer (Dell, Austin, TX)
with eFilm Workstation 1.5.3 (eFilm
Medical, Toronto, Canada) software.
Standard settings were set to “linear”
and “auto.” Two observers (L.V.H.,
D.A.B.), blinded to the histopathologic
results, reviewed by consensus 41 separate images of 19 vessels. The observers compared the signal of the vessel
wall to that of the adjacent muscle as
hypointense [-1], isointense [0], or hyperintense [1]. In addition, the overall
conspicuity of the vessel wall and image quality was scored as poor [1], fair
[2], good [3], or excellent [4]. For each
image, the observers classified the arterial wall as either normal or abnormal. If the vessel wall was classified as
abnormal, the observers described the
extent and nature of the abnormality.
In addition, the presence of artifacts
that could compromise interpretation
was recorded. The presence or absence
of a stent was recorded. The distance
from the center of the vein to the center of the artery was measured on the
images and recorded.
The image data were compared to
histopathologic specimens for 15 vessels. The images were matched to the
histopathologic specimens by comparison of distances from the known anatomic landmarks on the MR images
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and gross specimens. Pathologic findings were not available in four vessels because of difficulty with image/gross specimen registration. In
these vessels, all three pulse sequences were concordant and used
as a surrogate for histopathology.
RESULTS
Forty-one images (T1-weighted
precontrast without fat saturation, N
⫽ 15); T1-weighted precontrast with
fat saturation, N ⫽ 10; T1-weighted
postcontrast with fat saturation, N ⫽
16) of 19 arteries were reviewed. The
Table lists the imaging and histopathologic results. Vessels ranged in
size from 4 to 10 mm (average size, 6.6
⫾ 2.01 mm). The average distance
from the center of the artery to the
center of the vein was 11.6 ⫾ 6.0 mm.
T1-weighted Precontrast without Fat
Saturation
The T1-weighted precontrast images without fat saturation (normal
vessel, N ⫽ 8; stented vessel, N ⫽ 2;
abnormal vessel, N ⫽ 5) exhibited the
poorest wall signal and conspicuity.
The observers correctly identified all
images of normal and stented vessels.
The stent struts were faintly seen as
small focal areas of signal void.
On the T1-weighted precontrast images without fat saturation, the observers were able to correctly identify
2 of 5 abnormal arteries. These two
vessels had the highest wall signal and
conspicuity scores of all vessels imaged with this sequence, due primarily to periadventitial hemorrhage that
produced a thick wall with high signal; two lumens were identified, corresponding to the true and false lumen
(Fig 1). However, in one of the dissected vessels, a stent in the false lumen could not be visualized with this
pulse sequence (Fig 2).
The three arteries that were incorrectly classified as normal on T1weighted precontrast images without
fat saturation were compromised by
poor blood suppression (N ⫽ 2) or
chemical shift artifact (N ⫽ 1). Poor
blood suppression prevented the observers from identifying focal areas of
wall thickening in one artery and adherent subacute thrombus in the other
artery. Chemical shift artifact compromised interpretation of a third abnor-
mal artery with 100 ␮m circumferential intimal thickening.
The chemical shift between fat and
water results in misregistration of fat
and water signal in the frequency encoding direction. For the parameters
used in this study, the chemical shift
between fat and water was 1.1 pixels.
The chemical shift obscured the arterial wall in the frequency encoding direction in three of 15 vessels on the
non-fat suppressed pulsed sequence.
The appearance was that of wall thickening on one side of the vessel wall
with diminished wall thickness on the
opposite side. This effect was most
prominent for the smaller vessels (eg,
renal arteries) and disappeared on the
fat suppression sequences, confirming
our hypothesis that this was chemical
shift artifact.
T1-weighted Precontrast with Fat
Saturation
Ten arteries were imaged with a
T1-weighted precontrast fat saturation
pulse sequence. This sequence demonstrated an improved wall signal score
compared to the T1-weighted precontrast non-fat saturation pulse sequences (P ⫽ .02; ␹2 analysis). As was
seen in the T1-weighted precontrast
non–fat saturation images, the dissected arterial walls demonstrated
high wall signal and conspicuity. The
stent in the dissected vessel was not
seen on this pulse sequence.
Two abnormal vessels were incorrectly classified as normal. One vessel
with asymmetric intimal thickening
(500-␮m thick) was incorrectly categorized as normal because of poor blood
suppression. Circumferential intimal
thickening (100-␮m thick) was not appreciated on the image of the second
vessel.
T1-weighted Postcontrast with Fat
Saturation
T1-weighted postcontrast images
were obtained of 16 vessels. This was
the only sequence in which all vessels
received a vessel wall signal score of 1
(vessel wall signal greater than adjacent muscle signal). In addition, all
wall conspicuity scores were good or
excellent. All postcontrast images
were superior in conspicuity to non–
contrast-enhanced pulse sequences.
Fourteen of 16 images demonstrated
JVIR
Figure 2. Right renal artery dissection
with a platinum stent in false lumen. The
intravascular MR guide wire (bright signal) is in the IVC. (a) Transvenous MR
T1-weighted double inversion recovery
fast spin echo non-fat saturated image
shows an axial image of the right renal
artery with two lumens (arrows) and significant wall thickening. (b) Transvenous
MR T1-weighted fat saturated double inversion recovery fast spin echo sequence,
at the same location as figure part a, demonstrates an increase in wall signal compared with figure part a because of the
high signal from the perivadventitial hemorrhage. Two lumens are again appreciated (arrows). (c) Transvenous MR T1weighted fat saturated double inversion
recovery fast spin echo postcontrast image,
at the same location as figure part a, demonstrates improved wall signal and conspicuity compared with the non– contrast-enhanced images. Faint signal voids from the
stent struts are seen (arrows). (d) Hematoxylin and eosin stain of the right renal artery
demonstrates both the true lumen (TL) and
a stent within the false lumen (FL). Bar ⫽
850 ␮m.
™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™3
changes that correlated with histopathologic findings. The platinum
stent was faintly visible in all images
and was not thought to compromise
image interpretation. Two vessels had
focal fibrous plaques less than 250 ␮m
in thickness that were not appreciated
on the IVMRG images.
Arterial wall abnormalities and
stents were best detected on the postcontrast images. For example, in the
vessel with an arterial dissection and a
stent, the stent struts were perceptible
only after contrast administration (Fig
2). In another vessel, a 3-mm adherent
subacute thrombus was not seen on
the non– contrast-enhanced images,
partly because of poor blood suppression, but was readily seen after the
administration of contrast material
(Fig 3). A third vessel wall was incorrectly characterized as normal on the
precontrast images, but properly classified as thickened on the postcontrast
images.
DISCUSSION
Recently, investigators have used
electron-beam CT, multi-detector CT,
transcutaneous ultrasound, intravascular ultrasound, and MR imaging to
evaluate the arterial wall. Electron-
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Figure 3. Subacute adherent aortic thrombus. (a) Transvenous MR T1-weighted non-fat saturated double inversion recovery fast spin
echo image with the intravascular MR guide wire (long arrow) in the inferior vena cava, shows an axial image of the aorta (short arrow),
compromised by poor blood suppression. (b) Transvenous MR T1-weighted fat saturated double inversion recovery fast spin echo
postcontrast image, at the same location as figure part a, demonstrates a 3-mm high signal mass (arrow) adherent to the vessel wall, not
seen in figure part a. (c) Gross specimen of the aorta at the same location as figure part a, shows a 3-mm mass adherent to the aortic
wall. (d) Hematoxylin and eosin stain of figure part c shows a subacute thrombus adherent to a focal area of intimal thickening. Bar ⫽
1,250 ␮m.
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beam CT and multi-detector CT,
which have been used for calcium
scoring of the coronary arteries, primarily identifies calcified plaques,
(AHA type Vb) (1) but has a lower
sensitivity in identifying earlier, less
advanced lesions (13). Transcutaneous
ultrasound is able to not only quantify
the plaque burden (14,15), but also is
able to evaluate the surface characteristics and echogenicity of plaque (16).
However, this technique is limited because only superficial vessels such as
the carotid and superficial femoral artery can be examined. Intravascular
ultrasound is able to image deep vessels; however, it is often difficult to
interpret, poorly characterizes soft tissue, and is unable to image “behind” a
calcified plaque (17). Intravascular MR
imaging does not have these limitations (7). However, it is invasive and
the small field of view currently available suggests that screening of large
arterial segments is not practical. Imaging times are also quite long (approximately 1 minute per image). Because this technique is in its infancy,
further advances in MR technology
will hopefully diminish its current
disadvantages.
Transvenous MR imaging, compared with intra-arterial intravascular
MR imaging, significantly reduces the
risk of a limb-threatening complication during IVMRG placement and
imaging. Martin et al (18) proposed
imaging the artery by placing an 8-F
looped MR receiver coil in the adjacent
vein in a swine model. However, with
their large diameter receiver coil, they
found arterial imaging from the venous system was severely limited because placement of their coil was timeconsuming and ghosting artifacts
often compromised their images.
provided the greatest wall signal and
conspicuity scores.
Administration of intravenous gadolinium-based contrast material provided full circumferential enhancement of the arterial wall. The etiology
of this enhancement is unknown, but
is believed to be associated with the
degree of development of the vasa vasorum (19,20). The enhancement provides the necessary signal for transvenous MR imaging with a loopless
antenna. In this coil design, signal decreases at a rate proportional to 1/r,
where r is the distance from the receiver coil to the area of interest (21).
Therefore, the increased signal from
gadolinium-related wall enhancement
in part compensates for the signal loss
because of the distance from the
IVMRG to the arterial wall.
Our results demonstrate that a T1weighted fat saturated pulse sequence,
without contrast enhancement, was
not sufficient to increase wall conspicuity compared to the T1-weighted
non–fat saturated pulse sequence. Although there was a significant increase
in the wall signal score compared with
the adjacent muscle, the overall decreased signal of the entire image limited wall conspicuity. In addition, the
T2-weighted sequence was abandoned
after the first three animals because of
lack of sufficient MR signal. Better signal could have been obtained by prolonging the imaging time, but we felt
that image acquisition times substantially longer than 1 minute would
have limited clinical application. T1
images after gadolinium administration and without fat suppression were
not assessed because of the high signal
of both the arterial wall and surrounding fat and thus low contrast to noise
ratio.
Arterial Wall Visualization
Artifacts
Despite respiratory motion and arterial pulsation, arterial wall visualization with the IVMRG was straightforward with no significant limitations.
Based on these features, transvenous
MR visualization of the adjacent arterial wall was successful in not only
identifying normal vessels but also in
providing high-resolution imaging of
pathologic states in most vessels. Of
the transvenous MR pulse sequences
studied, this study found the T1weighted postcontrast pulse sequence
Chemical shift misregistration of fat
and water signal and poor blood suppression compromised interpretation
of the non– contrast-enhanced images.
On the T1-weighted non–fat saturation precontrast images, chemical shift
misregistration of fat and water signal
simulated focal wall thickening in
three vessels. This artifactual wall
“thickening” could be proved by either swapping the phase and frequency encoding directions or by reduction of thickening on fat
•
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suppressed images. This effect was noticed typically in smaller vessels (eg,
renal arteries) when the normal wall
thickness was similar to the degree of
chemical shift misregistration.
Poor blood suppression compromised the interpretation of four precontrast images, two without fat saturation and two with fat saturation. The
MR pulse sequence used incorporates
a 180° nonselective pulse that inverts
the spins of blood signal outside the
imaging plane. After an appropriate
delay, related to the T1 time of blood
signal, the blood flowing into the imaging plane is “nulled.” However, this
was not successful in all cases, likely
because of slow flow along the vessel
wall. Lack of blood suppression can
simulate vessel wall thickening. Additional modification to the MR pulse
sequence may reduce this effect.
Stents
Because stents have become indispensable in the treatment of vascular
disease, the authors were interested in
the intravascular MR appearance of a
platinum stent and the adjacent vessel
wall. In this study, a balloon-expandable platinum stent (Angiodynamics
Inc., Queensbury, NY) was used because platinum is weakly ferromagnetic and a previous study concluded
that stainless steel and most nitinol
stents cause significant MR ferromagnetic artifacts (22).
For the cross-sectional images of
the vessel lumen, black blood fast spin
echo images were obtained. This pulse
sequence minimizes susceptibility artifacts compared with the gradient
echo imaging technique. It is interesting to note that on the transvenous
MR images it was somewhat difficult
to visualize the struts of the platinum
stent. Only a small focal area of decreased signal similar in size to the
stent strut was seen. There was no obvious degradation of image quality beyond the expected boundary of the
stent. Because only four vessels with
stents were studied, further investigation of the intravascular MR appearance of this platinum stent is required.
In animals, the IVMRG has been
used for real-time guidance and monitoring of angioplasty and stent deployment (23,24). However, in previous studies the use of a nitinol stent
prevented visualization of the arterial
1326
•
In Vivo Intravascular MR Imaging
wall. The results of this animal study
demonstrates the feasibility of transvenous MR imaging of a platinum
stent, a potential approach to follow-up of patients after renal artery
stent placement without the risks of
conventional angiography or the administration of iodinated contrast material. More interestingly, by combining MR contrast agents with drugs
and/or genetic material, the delivery
of these agents into the arterial wall
can be monitored (25). This ability
would ensure proper and accurate delivery of therapeutic agents.
Limitations of the study design include the lack of quantitative image
analysis and lack of assessment of interobserver variability. The complex
composition of human atherosclerotic
plaque is difficult to duplicate in animal models, therefore the goal of this
study was only to determine feasibility of vessel wall imaging, before commencing human transvenous MR imaging studies. The authors of this
study are currently developing software that would permit quantitative
analysis of the arterial wall. Human
studies will involve these important
metrics of atherosclerotic plaque.
With the current IVMRG design
and its inherent limitations, image acquisition is slow, requiring 50 – 60 seconds per image. Different techniques,
such as coronal and sagittal views,
would be necessary to screen long vessel segments. Further advances in coil
technology would allow shorter imaging times. Although this study has
demonstrated the feasibility of the
transvenous MR, the invasive nature
of this technique must be justified by
the information gained.
The relative roles of transvenous
and intra-arterial MR imaging has not
been clearly defined. However, the authors believe that transvenous MR imaging will be superior and used for
real-time monitoring of arterial interventions. The IVMRG could be placed
in the vein adjacent to the target artery
and used for high-resolution imaging.
The main reason for advocating the
transvenous imaging approach during
an intervention is that the IVMRG is
produced in one standard length (100
cm) with a 3.5-mm diameter distal hub
to connect to the MR scanner. This
short length and distal hub makes
catheter exchanges impossible, and
thus impractical as a working wire.
Additionally, increasing the length of
the IVMRG, to accommodate catheter
exchanges, would degrade image
quality. For example, increasing its
length to 135 cm would decrease the
signal-to-noise ratio by 15% (Viohl I,
personal communication, 2002).
Future studies could employ MR
guidance instead of fluoroscopic guidance for the placement of the IVMRG.
MR guidance was not evaluated in
this experiment because the primary
goal was arterial wall imaging.
In conclusion, the T1-weighted fat
saturated postcontrast pulse sequence
was the optimum sequence for transvenous MR imaging of the arterial
wall. Furthermore, arterial wall visualization was not compromised by the
presence of a platinum stent. In the
future it is possible that this guide
wire could be used to identify arterial
pathology (unstable atherosclerotic
plaque) and then direct conventional
therapies (angioplasty/stent placement) and/or molecular therapies
(gene/drug delivery).
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