Intravascular magnetic resonance imaging and special contrast agents

Intravascular magnetic resonance imaging and special contrast agents

Magnetic resonance imaging can assess the finer details of atherosclerotic lesions in blood vessels – i.e., plaque component characterization. But currently this is limited to the evaluation of large or superficial vessels like aorta and carotid arteries because of the drop in signal to noise ratio while imaging deeper vessels using an external coil. Hence intra vascular MRI probes are being developed. Systems based on 0.030 inch and 0.014 inch guide wires have been developed. Most of these systems need an intravascular probe as well as an external MRI scanner. The devices have been made using nitinol tubing and the intravascular probe has an MRI receiver coil. Potential concerns exist about tissue heating in a radio frequency field in the presence of metallic structures. Newer probes with both magnets and coils within the probe are also being developed, allowing stand alone catheter based imaging without the aid of an external MRI scanner. The advantage would be obvious as an MRI scanner makes cath lab environment complicated as every equipment there needs to be MRI compatible. The prototype of this device can provide colour coded tissue component map. These may have a role in identifying the vulnerable plaque, especially along with targeting contrast agents like supermagnetic iron oxide (SPIO). Catheter tip magnet producing 0.2 Tesla magnetic field has been tested. A preliminary study presented at the Transcatheter Cardiovascular Therapeutics (TCT) 2007 had evaluated 104 patients from ten centres in United States of America, Europe and Israel. The study was designed to assess the device safety and functional performance of the intravascular MRI system. There were no deaths or perforations and the major adverse cardiac event rate was 0.96% at twenty four hours and 1.9% at one month. Success of the procedural performance was 88% and at least one intravascular magnetic resonance imaging measurement could be obtained in 95% of patients.

Water diffusion coefficients are different for lipid rich core and collagenous cap of the atherosclerotic plaque. Intravascular MRI catheter makes use of this difference in determining the extend and location of vascular lipid infiltration. It is well known that a thin fibrous cap makes a plaque vulnerable while high extracellular matrix content or smooth muscle rich tissue within the plaque makes it stable. Intravascular MRI probe can provide a radial resolution of 250 micrometer [J Am Coll Cardiol, 2006; 47:48-56].
Ultra small particles of SPIO given intravenously are retained in the macrophages within the plaques. While this uptake is 75% for ruptured or vulnerable plaques, it is only 7% for stable plaques. Use of labeled LDL helps in the detection of LDL receptors within the liver. Studies are underway to see if LDL labeled particles can be used determine the presence of LDL rich lesions and whether these can be used to deliver targeted therapy. Gadolinium-labeled human HDL nanoparticles are also being evaluated as novel contrast agents as they can enter the atherosclerotic plaques without any additional targeting mechanism.