Novel Methods for Detecting Fractures in Prosthetic Heart Valves

This paper describes the use of two simple electromagnetic methods for detecting strut fractures in prosthetic heart valves. The first method involves immersing the heart valve in a uniform time varying electromagnetic field and measuring the perturbation of the field in regions proximate to the strut. In vitro tests done to date indicate that the method is capable of discriminating between intact and fractured struts. The second method that is currently being investigated involves the use of electromagnetic-acoustic transduction methods for exciting the resonant modes of the outlet strut. Differences between the frequencies associated with the resonant modes of intact and fractured struts are exploited to diagnose the state of the valve. Introduction: Heart valves play a critical role in regulating blood flow through the cardiovascular system. Diseases of the heart valve can either be congenital or caused by infections such as rheumatic fever or endocarditis. Problems could also arise simply as a consequence of aging. The disease can lead to either stenosis, where the valve fails to open completely, or regurgitation, where the valve fails to close completely. In instances where the damage cannot be repaired through surgery, replacement with a prosthetic device is often indicated. One of the more common popular mechanical devices that was implanted extensively between 1979 and 1986 is the Bjork-Shiley Convexo-Concave (BSCC) valve. The BSCC valve consists of a pyrolytic carbon disc that serves as an occluder to block the flow of blood in one direction but allows flow in the other direction. The valve employs two struts as shown in Figure 1 to hold the disc in place. The outlet strut is TIG welded to the suture ring while the inlet strut is integral to the ring. Although the exact reason has not been determined, a combination of circumstances including fatigue and perhaps stress corrosion, causes one of the welds, anchoring the outlet strut, to fracture [1-2]. The failure of the other weld can cause the strut to separate from the suture ring, thereby allowing the disc to detach from the valve. This is usually fatal and consequently there is considerable interest in methods that can detect separation of one of the legs of the strut from the suture ring [3-7]. This paper describes a simple electromagnetic method that allows the detection of single leg separation (SLS) failures in BSCC valves. Figure 1 Björk-Shiley Convexo-Concave (BSCC) Heart Valve. Gradiometer Based Method: The method involves immersion of the heart valve in a spatially uniform time varying field as shown in Figure 2. The field, which is established by exciting a pair of large Helmholtz coils, is oriented in a direction that is parallel to the plane of the suture ring. Consequently the current induced in the ring is minimal. Since the plane of the outlet strut is not parallel to the plane of the suture ring, eddy currents are induced in the ring. These eddy currents perturb the field in the vicinity of the strut. If the strut weld is fractured, the eddy current path is interrupted and consequently the perturbation of the field, if any, is minimal. Thus the presence of a perturbation in the field is an indication of an intact outlet strut (IOS) while the absence of a perturbation indicates an SLS condition. The perturbation is detected by measuring the gradient of the field using a gradiometer. The gradiometer is mounted in a catheter which is inserted into the femoral artery and threaded through the arterial system to position it close to the outlet strut. Figure 3 shows the catheter assembly containing the gradiometer at the tip while Figure 4 shows a prototype of the Helmholtz coil assembly together with a surgery table with a metal-free bed that was specially built. The coil pair is oriented at an angle to ensure that the field produced by the pair is parallel to the suture ring in sheep. These animals are currently being used to test the viability of the approach. Figure 5 shows the block diagram of the associated instrumentation. The signal generator produces the excitation waveform that is amplified to drive the excitation coils. An impedance matching circuit, shown in Figure 6 ensures that the total load impedance of the circuit and the excitation coils is matched to that of the amplifier output stage at the excitation frequency. The pair of excitation coils generates a uniform magnetic field around the heart valve during the test. Figure 2: Gradiometer Based Method for Detecting SLS Failures in BSCC Valves The gradiometer consists of two coaxial pancake coils mounted on a nonmagnetic and electrically non-conducting spindle. The signal from the gradiometer is applied to a lock-in amplifier (LIA) that uses the excitation signal as the reference signal to improve the signal-tonoise ratio. The LIA provides the amplitude (V) and phase (θ) (relative to the reference signal) as well as the inphase (X) and quadrature components (Y) of the difference in the voltage induced in the two coils comprising the gradiometer. Figure 3: Catheter tip containing the gradiometer (left) and the complete catheter assembly (right) Figure 4: Excitation Coil Assembly and the Surgery Table Signal generator Power amplifier x 2 Impedance