The accumulation of plaque deposits within a blood vessel gradually leads to a blockage or occlusion of the vessel.  The resulting abnormal narrowing of the vessel, known as stenosis, can cause several complications.  In particular, stenosis of the coronary arteries restricts blood flow to and from the heart (i.e., ischemia), potentially resulting in serious damage to the heart tissue.  The progressive thickening and hardening of the arterial wall due to plaque formation in the arterial lumen is known as atherosclerosis.  Atherosclerosis is responsible for many coronary artery diseases and syndromes, such as angina pectoris, cardiac arrest and myocardial infarction (i.e., heart attacks), as well as strokes and leg gangrene. 
Treatment of occluded blood vessels generally involves minimally invasive angioplasty procedures, which aims to physically expand the constricted artery.  Prior to the treatment, a guidewire must be advanced beyond the occluded area to permit passage of a catheter along the guidewire.  Once the guidewire is positioned beyond the blockage, the catheter, together with a balloon or stent, is passed over the guidewire and angioplasty is carried out.  Generally, a fluoroscopic procedure (e.g., angiography) is initially performed, to provide a visual representation of the occluded vessel for use by the physician to navigate the guidewire.  As blood vessels are normally apparent in X-ray images, such fluoroscopic image of the vessel is available due to dye injection into the inspected vessel, enabling the visualization of the course of the dye flow through the vessel lumen.  If the blood vessel is only partially occluded, and there is a sufficient gap within the occluded area to allow passage of a guidwire, then it is possible to advance the guidewire to the target area, and perform medical procedures, while using the guidewire for direction and support.  In the case of total occlusion, however, the guidewire is prevented from advancing through and beyond the occluded region of the artery.  The total occlusion prevents the physician from identifying the exact course of the occluded artery on the fluoroscopic images, due to the fact that there is no dye flows through the occluded portion of the vessel, therefore this portion of the vessel is not visible on the fluoroscopy.  Thus, it is very difficult to cross the entire length of the occlusion, and to determine the optimal manipulations required to correctly navigate the guidewire within the artery.  Improper manipulation of the guidewire may cause a localized dissection of the intimal or subintimal layer of the arterial wall, and even complete perforation of the arterial wall, resulting in severe complications and failure of the procedure.  Thus, minimally invasive medical procedures in cases of total occlusion of the artery have a significantly lower success rate and a higher complication rate, as compared to partial occlusion or artery narrowing. 
Existing techniques to deal with advancing a guidewire through an occluded blood vessel include, for example, cutting atherectomy catheters, which attempt to penetrate through the occlusion.  Another known technique includes pushing a guidewire into the occluded blood vessel, and trying to brake through the whole length of the total occlusion, based on an imaginary course of the occlusion.  The guidewires used for penetrating total occlusions usually exhibit stiffness, and may perforate the vessel wall when force is applied thereto, thereby generating a false lumen.  Thus, physicians may be hesitant to use such force in pushing the guidewire through the occlusion, as they do not surely follow the actual course of the occluded vessel.  If the guidewire cannot be advanced beyond the occlusion, major invasive surgery may alternately be performed, such as bypass surgery.  It is generally preferable to avoid such invasive therapeutic procedures, as they involve severe complications and trauma to the patient.  Other known techniques for treatment of an occlusion, involve laser ablation, application of radiation pulses, or administering fluid to remove the occlusion.  In some cases, a small cavity may remain open through the occluded vessel, through which a guidewire may be inserted.  However, it is usually very difficult for a physician to keep a CTO wire (a stiff wire) in the center of the vessel, due to the fact that the occluded portion of the vessel is actually not apparent in the fluoroscopic images. 
U.S. Pat.  No.  5,423,846 to Fischell entitled “Dottering auger catheter system”, is directed to a catheter system for penetrating a vessel blockage (i.e., total occlusion) in the human body, to create an initial passageway prior to a vessel opening procedure, such as balloon angioplasty or atherectomy.  The catheter system includes a centering catheter, and a dottering auger catheter (DAC).  The DAC includes a steel tube on the proximal end, a flexible catheter section, and a self-tapping auger screw on the distal end.  The screw has a conical section that tapers off to a sharp point.  The proximal end of the steel tube extends outside the body of the patient.  A handle is attached to the proximal end of the steel tube.  The centering catheter includes a balloon at its distal end.  The centering catheter has a central lumen, through which the DAC is inserted, and a second lumen, through which a fluid can be passed, to inflate the balloon. 
After angiography is performed to indicate a blockage in an artery of the patient, a guide wire is advanced toward the artery until the distal end of the guide wire is adjacent to the proximal surface of the blockage.  The centering catheter is then advanced over the guide wire, until the distal end of the centering catheter contacts the proximal surface of the blockage.  The guide wire is removed, and the balloon of the centering catheter is inflated, thereby centering the distal end of the centering catheter within the artery (and reducing the possibility that the screw will penetrate through the wall of the artery).  A contrast medium is injected through the central lumen of the centering catheter to verify the position of the centering catheter of the artery, and the length of the blockage.  The DAC is advanced through the centering catheter, until the distal end of the DAC contacts the proximal surface of the blockage.  The auger screw is advanced beyond the length of the blockage, by simultaneously applying a rotational torque and a push force to the DAC via the handle.  The auger screw is removed, and a contrast medium is injected through the central lumen of the centering catheter to verify that the DAC created a pathway through the blockage.  Another guide wire is advanced through the centering catheter and the created pathway.  The centering catheter is removed, and a balloon angioplasty or atherectomy procedure is performed. 
U.S. Pat.  No.  6,210,408 to Chandrasekaran et al entitled “Guide wire system for RF recanalization or vascular blockages”, is directed to a method and system for recanalizing an occluded blood vessels within the vasculature of a patient.  The system includes a centering catheter, a guide wire, and a radio frequency (RF) generator.  The guide wire includes an ablation tip on the distal end.  The centering catheter includes an elongate catheter body having a guide wire lumen, and a centering mechanism (e.g., an elongated, inflatable balloon).  The guide wire is coupled with the RF generator.  The RF generator is further coupled with a patient return electrode, and with a footswitch. 
The return electrode (e.g., a pad with a substantially large area) is attached to the patient, to maximize the delivery of the RF energy to the target tissue.  The guide wire is inserted and routed through the patient vasculature, until the ablation tip is disposed proximal to the total occlusion.  The centering catheter is advanced over the guide wire, until the centering mechanism (i.e., balloon) is disposed adjacent to the total occlusion.  A contrast agent is conveyed into the deflated balloon, to enable easier fluoroscope detection of the balloon.  An inflation medium is conveyed into the balloon, inflating the balloon until it is in secure contact with the blood vessel.  The balloon maintains the guide wire along the centerline of the blood vessel, such that the ablation tip is substantially centered as it contacts the occlusion.  The RF generator is then activated by depressing the footswitch, delivering RF energy to the ablation tip.  A sufficiently high voltage potential is produced to initiate a spark erosion process, thereby ionizing the liquid contained in the occlusive material.  The ionization converts the occlusive material into a plasma state, and the resultant particulate matter is safely absorbed by the blood stream.  After the spark erosion process is initiated, a lower voltage potential is applied to maintain plasma conversion.  The output power of the RF energy is a function of the relative impedance between the ablation tip and the load impedance.  The voltage, impedance and electrode geometry is selected such that the spark erosion process is initiated when a load impedance that indicates occlusive material is reached (i.e., above the impedance of blood or healthy vessel tissue).  The ablation tip includes at least one discontinuous feature (e.g., an edge or point), to facilitate sparking between the ablation tip and the tissue.  As the RF energy is applied, the guide wire is distally advanced through the center of the occlusion.  The centering catheter is removed, and a therapeutic device for treatment of the occlusion (e.g., a PCTA catheter) is introduced over the guide wire. 
U.S. Pat.  No.  6,643,533 to Knoplioch et al entitled “Method and apparatus for displaying images of tubular structures”, is directed to a method and apparatus for the display and analysis of vascular images acquired through a medical imaging system.  The method may be used for displaying a stenosis of a vessel in the patient body, determining the smallest cross-section of the vessel.  The method includes the step of first identifying a centerline of the vessel.  The next step involves selecting a local center point on the centerline.  The following step involves obtaining a cross-section plane normal to the local center point, and identifying a contour of the vessel within the cross-sectional plane.  The next step involves sequentially measuring the lengths of various segments across the contour, where each segment intersects the local center point.  The shortest segment is identified out of all the taken measurements.  The next step involves determining an imaging plane showing the stenosis.  The imaging plane is defined by the shortest segment and a local axis tangent to the centerline at the local center point.  Subsequently, the imaging plane is displayed, showing a cross-section of the vessel which indicates the stenosis.  An image acquisition may then be performed relative to the imaging plane.  For example, an X-ray acquisition may be performed, with the perpendicular to the imaging plane as a line of sight and the local center point as a target.  A magnetic resonance (MR) system may be used to acquire image slices, with the location of the slice being the imaging plane or another plane that is translated from the imaging plane by a selected distance. 
U.S. Pat.  No.  6,824,550 to Noriega et al entitled “Guidewire for crossing occlusions or stenosis”, is directed to a system and method for crossing stenosis, partial occlusions or total occlusions in a body lumen.  The system includes a hollow guidewire, a drive shaft, a housing, and a drive motor.  The drive shaft moveably extends within the axial passage of the guidewire.  The drive motor is coupled with the drive shaft.  The drive motor is further electrically coupled with a control system and a power supply.  The proximal end of the guidewire is coupled with the housing, which is attached to an input device.  The input device controls the rotation and axial movement of the drive shaft.  The distal tip of the drive shaft has a shaped profile.  The shape may be configured optimally for the type of occlusion to be penetrated.  The distal tip may be shaped or deflected from the longitudinal axis of the guidewire, such that the rotation of the drive shaft creates a path radius that is larger than, the same, or smaller than the radius of the distal end of the guidewire. 
A user advances the hollow guidewire along the body lumen, to the target site.  The user activates the drive motor to rotate and advance the drive shaft, from an axially retracted position to an axially extended position, thereby creating a path through the occlusion.  The user may also rotate the drive shaft manually for slow speed rotation.  As the distal tip is rotated, the distal tip macerates the clot at the target site, separating the clot from the wall of the body lumen.  The user may aspirate the macerated clot through the guidewire working channel, or deliver a thrombylatic fluid to dissolve the macerated clot.  The guidewire may further include an access or support system, such as an infusion or aspiration catheter, to aspirate the target site or to infuse therapeutic or diagnostic materials therein.  The hollow guidewire may also be used to advance an atherectomy device into or adjacent to the path of the occlusion.  The distal portion of the drive shaft may be radiopaque, to allow a physician to track the position of the drive shaft via fluoroscopy. 
U.S. Pat.  No.  6,911,026 to Hall et al entitled “Magnetically Guided atherectomy”, is directed to a magnetically guided catheter for treating a totally occluded arterial vasculature.  An energy source is coupled to the distal tip of the catheter.  The distal tip of the catheter includes a magnetically active element.  The catheter is guided to the treatment site via a guide wire and sheath, which also includes a magnetic element.  The catheter may be a thermal catheter, which is heated by an RF source, in a bipolar or monopolar configuration.  Alternatively, the catheter is resistance heated or laser heated.  The catheter may include a lumen, through which an imaging wire may be inserted, to visualize and locate the occlusion (e.g., using ultrasound imaging or fluorescence spectroscopy).  A contrast agent may be injected between the sheath and catheter body, allowing the catheter to be viewed in the patient body.  A cooling fluid may be injected to the catheter tip to regulate the temperature distribution. 
The patient undergoes a preoperative scan (e.g., using MRI, CT, ultrasound imaging), and the preoperative data is loaded into a workstation console.  During the treatment, an X-ray machine provides real-time biplane X-ray data of the patient, to the workstation.  The catheter includes a fiducial marker, allowing the preoperative scan data and real-time scan data to be merged.  The physician may select the location of the treatment site on the workstation, and the workstation computes the magnetic fields and gradients required to navigate the catheter to the selected location.  An external magnet generates magnetic forces on the catheter tip, and the applied field and gradient orients the tip direction toward the selected location.  The physician (or a robotic element) pushes on the proximal end of the catheter to advance the guidewire and sheath.  The physical motion together with the magnetic orientation of the tip serves to position the catheter at the selected location.