MR Image–guided Endovascular Procedures with the Ultrasmall Superparamagnetic Iron Oxide SH U 555 C as an Intravascular Contrast Agent: Study in Pigs

The incorporation of RF coils into the tips of intravascular devices has been shown to enable the localization of catheters and guidewires under MR guidance. Furthermore, such coils can be used for endoluminal imaging. The long cable required to connect the coil with the scanner input inadvertently acts as a dipole antenna which picks up RF energy from the body coil during transmit. Currents are induced on the cable which can lead to localized heating of surrounding tissue. Cables of various lengths were measured to determine if a resonance in the heating as a function of cable length could be found. Coaxial chokes with a length of lambda/4 were added to coaxial cables to reduce the amplitude of the currents induced on the cable shield. A 0.7-mm diameter triaxial cable, small enough to fit into a standard intravascular device, was developed and measured both with and without a coaxial choke. It is demonstrated that resonant heating does occur and that it can be significantly reduced by avoiding a resonant length of cable and by including coaxial chokes on the cable.

To develop and evaluate a sequence for breath-hold three-dimensional gadolinium-enhanced magnetic resonance (MR) angiography of the abdominal aorta. In 63 patients, the abdominal aorta and its branches were imaged for 29, 43, or 58 seconds with breath holding. A fast spoiled gradient-echo sequence was used at 1.5 T during infusion of 42 mL of a gadolinium chelate. Correlation with conventional angiography was performed in 19 patients. MR image quality (signal-to-"total" noise ratio [S/N*]) with breath holding was compared with that with free breathing (104 patients). With breath-hold gadolinium-enhanced MR angiography, renal, celiac, and superior mesenteric artery occlusive disease was graded appropriately in 15 of 19 patients, and 10 of 11 accessory renal arteries were depicted correctly. Renal artery branches were visualized in 86 of 95 kidneys on breath-hold images compared with only 84 of 236 kidneys with free breathing (P < .001). Distal renal artery S/N* was 3.1 with breath holding and 2.1 with free breathing (P < .001). Breath holding statistically significantly improves three-dimensional gadolinium-enhanced MR angiography of the renal, celiac, and superior mesenteric arteries.

Percutaneous transluminal renal angioplasty is a safe and effective treatment for nonostial stenoses of the renal arteries, but it has proved to be disappointing for ostial stenoses. Therefore, we prospectively studied the use of intravascular stents for the treatment of critical ostial stenoses after unsuccessful balloon angioplasty. Stainless-steel endoprostheses were placed across 74 renal-artery stenoses located within 5 mm of the aortic lumen in 68 patients with hypertension. Twenty patients had mild or severe renal dysfunction. The indications for stent placement were elastic recoil (63 arteries) or dissection (1 artery) of the vessel after angioplasty, or restenosis after initially successful balloon angioplasty (10 arteries). Patients were followed for a mean of 27 months with measurements of blood pressure and serum creatinine, duplex sonography, and intraarterial angiography. Initial technical success was achieved in all patients. Minor complications (local hematomas) occurred in only three patients; there were no major complications. Eighty-four percent of the patients were free of primary occlusion 60 months after the procedure. Restenosis of more than 50 percent of the vessel diameter occurred in 8 of 74 arteries (11 percent). Reintervention resulted in a secondary patency rate of 92 percent. Long-term normalization of blood pressure was achieved in 11 patients (16 percent). Serum creatinine levels did not change significantly after successful stent implantation in patients with previously impaired renal function. Accurate placement of renal-artery stents is technically feasible without major complications. The favorable early and long-term results suggest that primary stent placement is an effective treatment for renal-artery stenosis involving the ostium.