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1 All authors: Department of Clinical Radiology, University of Muenster, Albert-Schweitzer-Str. 33, D-48129 Muenster, Germany.
Received May 1, 2000;
accepted after revision October 12, 2000.
Address correspondence to K. U. Juergens.
Abstract
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MATERIALS AND METHODS. Changes in endoluminal signal intensities and the precision of the endoluminal diameter measurement were investigated in phantom studies for different concentrations of gadopentetate dimeglumine. Before and after the Hemobahn devices had been implanted, 10 patients with peripheral arterial occlusive disease were examined on MR imaging and three-dimensional contrast-enhanced MR angiography.
RESULTS. Phantom experiments using three-dimensional MR angiography showed stent-related signal void as a dark ring in the axial image orientation, providing a precise delineation of the stent-vessel border (mean endoluminal diameter, 8.2 mm; SD, 0.6 mm). Changes in endoluminal signal intensity were evaluated quantitatively. Stent-related artifacts did not compromise diagnostic imaging quality. All Hemobahn devices were found to be patent without migration of an implanted graft. In one patient, an extensive perigraft reaction (edema and contrast-enhanced perivascular tissue) was postinterventionally detected on MR imaging and corresponded to clinically evident postimplantation symptoms.
CONCLUSION. Three-dimensional contrast-enhanced MR angiography is a suitable tool to follow up the implantation of Hemobahn devices and to detect intra- and extraluminal abnormalities.
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Because of the use of percutaneous endovascular stent placement and stent grafting after balloon angioplasty in patients with peripheral arterial occlusive disease, 3D contrast-enhanced MR angiography has developed as a reliable and noninvasive diagnostic modality for follow-up soon before and after intervention [10]. Nonferromagnetic nitinol-based endoprostheses have proven their biocompatibility [11] and are expected to be suitable for MR evaluation [12]. In previous reports, characteristics of commercially available plain and covered stents on two-dimensional and 3D MR angiography have been published [10, 13,14,15,16,17,18].
To our knowledge, no data concerning phantom experiments and consecutive in vivo studies of Hemobahn devices (Gore & Associates, Putzbrunn, Germany) using 3D contrast-enhanced MR angiography have been published.
Clinically evident perigraft inflammation due to endovascular stent graft placement has been revealed on MR imaging [19,20,21,22], but to date data have not been published about Hemobahn devices.
Thus, the purpose of our study was to investigate the precision of lumen measurements, stent-related artifacts, and the assessment of endoluminal contrast in Hemobahn devices on 3D MR angiography using phantom experiments. Furthermore, we evaluated diagnostic imaging quality, stent-related artifacts, endoluminal patency, and the appearance of perigraft reactions using MR imaging and 3D contrast-enhanced MR angiography in 10 patients with peripheral arterial occlusive disease treated by means of a Hemobahn device.
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Phantom Experiments
Phantom experiments were performed on a 1.5-T MR scanner (Magnetom Vision;
Siemens Medical Systems, Erlangen, Germany) using an extremity coil. For in
vitro MR imaging, two inflated Hemobahn devices with a labeled diameter of 6
and 10 mm (length, 5 cm) served as model for the common iliac and the femoral
artery, respectively. The stents were embedded into commercially available
candlegel (Fig. 2A) and placed
into the center of the MR scanner. The lumen of the smaller stent graft
(diameter, 6 mm) was filled with isotonic (0.9%) saline as a reference,
whereas the lumen of the wider stent graft (diameter, 10 mm) was filled with
different concentrations (1:320, 1:160, 1:80, 1:40, and 1:20) of diluted 0.5
mol/L gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany). MR
imaging was performed using a routine 3D MR angiography sequence (TR/TE,
4.6/1.8; flip angle, 30°; field of view, 293 x 390 mm; matrix, 200
x 512; section thickness, 1.67 mm) on both phantoms.
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Endoluminal signal intensities resulting from different concentrations of
contrast medium (SICM) and from the saline phantom
(SINaCl) were determined on axial image orientation in the middle
of each stent graft by drawing the largest possible user-defined endoluminal
region of interest. The region of interest was consistent in size from sample
to sample. Results are expressed as mean value ± the standard deviation
of five measurements of each concentration of contrast medium and of the
saline phantom. Standardized on SINaCl, relative enhancement (RE)
was calculated as follows:
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In Vivo MR Imaging
Ten patients (one woman [age, 62 years] and nine men [age range, 48-78
years; mean age ± SD, 58.8 ± 12.6 years) with peripheral
arterial occlusive disease, clinically classified as Fontaine IIB and III,
were included. In five patients the superficial femoral artery, in three
patients the common iliac artery, and in one patient each a common femoral and
popliteal artery was treated by means of a Hemobahn device. MR imaging was
performed on a 1.5-T scanner using a body phased array coil 1 day before and
after implantation of a Hemobahn stent graft. The device was percutaneously
placed after diagnostic digital subtraction angiography and successful
percutaneous balloon angioplasty. In one patient, two devices were implanted
because of an extended stenosis in the distal common femoral and popliteal
artery. Therefore, 11 Hemobahn devices, with a length of 5 cm (n =
4), 6 cm (n = 1), 10 cm (n = 5), or 15 cm (n = 1)
were placed. The stent grafts were 6 mm (n = 6), 7 mm (n =
4), and 8 mm (n = 1) in diameter.
The study was approved by the review board at our institution. After informed consent was obtained from each patient, the patient was positioned within the MR scanner in the supine position. Axial and paracoronal scout images (turbo fast long-angle shot [FLASH] sequence: 11/4.2; flip angle, 15°) were obtained to localize the stent graft. Axial and paracoronal fat-saturated T1-weighted (450/15.0; flip angle, 90°; field of view, 290 x 390 mm; matrix, 256 x 256) and axial proton density (3220/22; flip angle, 180°) and T2-weighted spin-echo sequences (4500/99; flip angle, 180°) were acquired.
The aim of determining the individual time delay (scan delay
[Tsd]) is to maximize endovascular contrast by achieving optimal
arterial gadolinium concentration before the middle of k-space data
acquisition, because in the central parts of the k-space image contrast is
obtained [7]. Therefore, a test
bolus of 1 mL gadopentetate dimeglumine was administered by IV injection (flow
rate, 2 mL/sec) followed by a flush of 30 mL of saline to measure the time
delay from the start of IV injection to peak arterial contrast enhancement.
Simultaneously, a turbo FLASH sequence providing 60 consecutive images was
started to calculate bolus arrival time (BAT) by means of signal intensity
measurements within a region of interest at the proximal end of the Hemobahn
device. We used the following equation, which is based on the work of Boos et
al. [23], to examine the iliac
arteries:
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Breath-hold 3D contrast-enhanced MR angiography (4.6/1.8; flip angle, 30°; field of view, 345 x 460 mm; matrix, 200 x 320; linear k-space acquisition; phase encode and slice selection gradient in two orthogonal axis [y- and z-axes], readout or frequency encoding along the x-axis) was performed with automated IV bolus injection of 0.2 mmol/kg body weight gadopentetate dimeglumine at a flow rate of 2 mL/sec using an MR-compatible power injector (SHS 200; Medrad, Pittsburgh, PA). After a subsequent flush of the IV line with 30 mL of saline, contrast-enhanced paracoronal and sagittal fat-saturated T1-weighted spin-echo sequences were performed.
On the MR workstation, MR data sets before and after gadolinium injection were subtracted and maximal intensity projections from three different view angles around the long axis were obtained using the implemented software package (Numaris). Axial and paracoronal MR images and maximal intensity projections based on MR angiographic data were analyzed by three experienced radiologists in consensus. The results of the angiograms were known to each radiologist. On hard copy, image quality was assessed with a 4-point scale (excellent, good, fair, poor). Stent graft localization and possible postinterventional migration, endoluminal patency, and the appearance of perigraft reactions were qualitatively analyzed.
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In Vivo MR Imaging
In vivo, all MR angiograms were successfully obtained without any technical
problems and were suitable for analysis. Image quality was rated as excellent
(n = 7) or good (n = 3). Artifacts displaying the stent as a
dark ring in the axial orientation did not compromise diagnostic imaging
quality. All Hemobahn devices were found to be patent on the date of 3D
contrast-enhanced MR angiography (Fig.
4A,4B,4C).
No migration of an implanted device was diagnosed.
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One of the 10 patients with a stenosis of the left superficial femoral artery developed an extensive perigraft reaction 3 weeks after the implantation of the Hemobahn device. MR imaging revealed edema and contrast enhancement with hyperintense perivascular signal around the left superficial femoral artery corresponding to a clinically evident postimplantation syndrome. Deep vein thrombosis was excluded by the venous phase of 3D contrast-enhanced MR angiography (Juergens KU, unpublished data).
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In a preliminary study, the stent graft material polytetrafluoroethylene, which is used in Hemobahn devices, has shown a promising 1-year patency rate in the superficial femoral arteries [13]. In the present study, Hemobahn-related artifacts did not compromise the diagnostic imaging quality of 3D contrast-enhanced MR angiography. Thurnher et al. [24] hypothesized that metallic stent-related artifacts would hamper the analysis of postinterventional complications. However, susceptibility artifacts did not significantly impair diagnostic quality of in vivo MR imaging. Therefore, the results of our study support previous 3D contrast-enhanced MR angiographic findings on covered nitinol-based stents by Hilfiker et al. [17], who reported a clear visualization of the contained lumen of three stents (Cragg, Cragg EndoPro System, Mintec, Freeport, Bahamas; Passenger, Boston Scientific, Natick, MA). Kellner et al. [25] performed MR imaging in 26 patients after balloon dilatation and consecutive placement of covered and uncovered stent grafts. Based on the experiences of these researchers, nitinol-based devices caused "black ring" susceptibility artifacts. However, these artifacts did not have a negative effect on diagnostic imaging quality. Furthermore, these artifacts did not impair the evaluation of the perivascular space in regard to the assessment of soft-tissue changes after stent graft placement with MR angiography [25].
We performed phantom measurements using a 3D MR angiographic sequence from the routine clinical setup (flip angle, 30°). Therefore, the results of the phantom study are limited to the MR parameters used in this study. An increase of the flip angle would have accenuated the T1 effects, whereas a decrease in the flip angle would have increased the T2 effects.
In our study, the stent-vessel borders were delineated precisely, and the inner luminal diameter of a labeled 10-mm Hemobahn was assessed with a standard deviation of 0.6 mm. However, the labeled stent graft diameter was underestimated on 3D contrast-enhanced MR angiography. These results are consistent with those of Hilfiker et al. [18], who reported an overestimation of wall thickness and an underestimation of luminal diameter of a nitinol-based abdominal aortic stent graft on a 3D MR angiography data set.
The size of stent-related susceptibility artifacts depends on the field strength and the length of the TE [12, 17, 26]. We have performed 3D contrast-enhanced MR angiography at 1.5 T using a short, but constant, TE of 1.8 msec. This value is comparable with the TE commonly used in 3D MR angiographic sequences in a clinical setting. Therefore, our study is limited in concluding a shorter TE might have decreased susceptibility artifacts. However, our results show clinically sufficient visualization of a nitinol-based device on 3D MR angiography. High-end MR imaging scanners are currently becoming available that provide ultrafast 3D MR angiographic sequences with a TE of less than 1 msec.
In regard to noninvasive diagnostic followup of endovascular stent grafts, the lack of iodinated contrast medium, ionizing radiation, and invasive arterial catheterization are major advantages of 3D contrast-enhanced MR angiography. Gadopentetate dimeglumine as an example of an extracellular paramagnetic contrast agent has proven not to be nephrotoxic [27, 28] and, therefore, is a suitable alternative for patients with impaired renal function and a contraindication to iodinated contrast agents [9]. Based on results of their own dilution series, Hilfiker et al. [17] performed their in vitro MR studies using a concentration of 1:20 of diluted 0.5 mol/L gadopentetate dimeglumine. In our phantom experiments we used the same concentration of this agent, thus confirming the ability to assess changes in intraluminal signal intensity due to different concentrations of gadopentetate dimeglumine using 3D MR angiography.
Despite the limited number of patients, our study showed that MR imaging enables noninvasive evaluation of Hemobahn devices. However, further studies enrolling a larger number of patients will have to be performed to confirm our results. In the future, blood pool contrast agents providing longer blood half-life and stronger reduction of T1-relaxation than gadopentetate dimeglumine will be widely available to improve diagnostic imaging quality of contrast-enhanced MR angiography.
We conclude that 3D contrast-enhanced MR angiography is a suitable tool to follow up the implantation of Hemobahn devices and to detect intra- and extraluminal abnormalities—for instance, perigraft reactions correlating to a clinically present postimplantation syndrome.
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) and in saline-filled phantom
(
) by three-dimensional MR angiography.
