Balloon overstretch dilation of the in-frarenal aorta was performed successfully in all 11 animals. Perforation/rupture of the aorta occurred in two, but both survived for the duration of the nonsurvival experimental protocol. At the widest, dilated segments were up to twice the reference aortic diameters. In all cases, aneurysms persisted throughout the experimental protocol, up to 6 h, usually without significant recoil.
In vitro all active devices showed enhanced signal in the immediately surrounding region. However, the receiver coil on the active-marker/passive-stent device () coupled inductively with the nitinol stent, resulting in increased signal along the entire device rather than discretely at each end-marker coil. There was marked signal drop-off within 2 to 3 mm of the distal end of the active-stent devices (), a known limitation of the loopless antenna design (20
). The active-stent/active-marker design () overcame this limitation by placing a loop coil (27
) to act as an edge marker distal to the stent.
Heating was only noted at the tip. The maximum temperature increase was 2.0°C in the in vitro static phantom, and 2.2°C in vivo.
Signal-to-noise profiles of the final endograft design, combining an active stent with active edge markers, was higher than for all previous designs, and contrast-to-noise for the final device was different primarily at the endograft edges (data not shown).
Real-time MR guidance of endograft procedures
Real-time imaging with SSFP provided adequate temporal resolution for device navigation, positioning, and deployment. Spatial resolution was sufficient to visualize important visceral and branch artery origins necessary for precise device placement.
In one of the animals with aortic rupture, the rapidly accumulating retroperitoneal hematoma obscured important anatomy. This problem was circumvented by thick-slice, real-time angiography with a hand injection of dilute (30 mM) gadolinium contrast during image acquisition. Active devices were easily visualized within the aorta using color highlighting and individual channel gain adjustment.
Multislice imaging provided reference coronal and sagittal slices of the infrarenal aorta with renal and iliac vessels while allowing interactive adjustment of axial slices to verify and adjust device position. The slices were displayed individually and in combination after real-time 3D rendering (). A point marking system allowed precise delineation of important anatomic references, including proximal and distal aneurysm extent, with display on the 3D image to guide device positioning and deployment ().
Figure 3 Real-time multislice imaging and three-dimensional rendering during endograft positioning. In the left column, real-time magnetic resonance imaging multislice axial, sagittal, and coronal images shown simultaneously facilitate precise device positioning. (more ...)
The balance between spatial resolution, the number of image slices, and temporal resolution was adjusted iteratively, in combination with temporal filtering and multipla-nar imaging. Optimal anatomic guidance seemed to be provided by multiple (3
) non-orthogonal slices, each fully refreshed only approximately once per second.
Performance of different MRI-endograft designs
The commercial endograft system (, one tested) was visualized passively by its marked susceptibility artifact, but this made it very difficult to differentiate the stent itself from the delivery shaft. We attempted to improve device visualization by inserting a quarter-wave active antenna-guidewire through the lumen of the endograft system. This enhanced local signal but did not satisfactorily delineate the device.
The active-marker/passive-stent device (, one tested) showed bright signal along the length of the device rather than discretely at the markers only. The distal edge of the stent could not definitively be identified. Additionally, the tip of the delivery system, extending 2 cm beyond the distal active marker, was not conspicuous because of volume averaging.
The loopless coil design of the active-stent device (, two tested) provided good signal except along the distal several millimeters of the actual endograft, reducing operator confidence during positioning and deployment.
The final device iteration combined an active stent with an active distal loop coil at the distal tip of the delivery catheter (, seven tested). This design produced the most reproducible signal pattern and provided satisfactory operator confidence in device position.
The two procedural failures were attributed to the fragility of these homemade prototypes, built primarily for imaging rather than mechanical characteristics. Of the two procedural failures, one was related to shifting of the self-expanding endograft during unsheathing, and the other was related to migration during withdrawal of the detachable antenna connection.
MR assessment of procedural success
Nine of 11 en-dograft procedures were successful under rtMRI guidance. The two failures were identified using first-pass MRA. No attempt was made to correct acute endoleaks using adjunctive balloon or stent devices.
Stent strut apposition at the proximal and distal target segments of the aorta was convincingly shown by high-resolution axial SSFP, fast spin echo, and 3D gradient echo scans ().
Figure 4 Stent strut apposition. (A) Fast spin echo image shows nitinol stent well apposed to target proximal infrarenal aorta (arrows show signal void from stent struts). (B) Spin-echo axial image at level of aneurysm, showing excluded sac (dashed outline). Orientation (more ...)
Repeat MRA in 9 of 11 cases showed both aneurysm exclusion by the endograft () and patency of the renal arteries. Iliac arteries were also patent by angiography. Successful exclusion was further corroborated by lack of contrast accumulation in the aneurysm sac during late-phase angiographic and real-time SSFP scans. In the other two cases, MR contrast-angiography revealed procedural failure with evidence of gadolinium within the aneurysm sac.
Figure 5 Magnetic resonance angiogram (maximum-intensity projection) before and after endograft delivery. (A) Conventional contrast-enhanced magnetic resonance angiography shows infrarenal abdominal aortic aneurysm after balloon overstretch. (B) Abdominal aortic (more ...)
High-resolution phase-contrast studies with vector- and color-flow mapping showed reduction of in-plane and through-plane turbulence consistent with restoration of laminar flow (). In the two procedural failures, phase contrast imaging was not specifically conducted to identify flow jets associated with endoleaks identified by contrast MRA.
Figure 6 Phase-contrast flow assessments before and after endograft deployment. Axial overlays of in-plane (vector flow map) and through-plane (color map) flow within ruptured experimental aneurysm. Before endovascular repair, there is marked turbulence and evidence (more ...)
X-ray angiography and pathology
Digital subtraction angiography corroborated MRA findings in all nine cases deemed successful by MRI. Direct inspection and palpation of the resected abdominal aorta in six successful cases confirmed that the renal arteries were not involved.