Biomedical subjects

M S van der Heiden

Publications and source records attributed to M S van der Heiden.

5 recordsLinked to original sources

PubMed2001

The complexity of external acoustic detection of defects in Björk-Shiley convexoconcave heart valves.

Fractures in Björk-Shiley convexoconcave (BScc) heart valves have raised questions about the feasibility of early diagnosis of technical defects by means of acoustic assessment. Three laboratory tests were conducted. To establish acoustic fingerprints, 66 valves with a defect, such as single-leg fracture (SLF) or single-leg separation (SLS), or without a defect were connected with a contact sensor and excited by dropping a small metal ball onto the outlet strut. In the second test, we simulated the valve sound propagation within the thorax. In the third test, intact, SLF, and SLS valves were placed in a mock heart immersed in a large water tank. We observed a resonance frequency corresponding with valve size and presence of defects. The second test showed that both the chest wall and the lungs created numerous reflections. This led to a substantial overlap of the original pulse frequencies and the frequencies measured. The third test confirmed that submersion of the chest in water can significantly reduce chest wall reflections. Reliable noninvasive assessment of BScc valve clicks for the presence of defects of the outlet strut is hampered by complex sound propagation within the thorax and variability of valve excitation. Acoustic fingerprints to diagnose mechanical defects should be integrated in valve design.

Heart Valve Prosthesis↗

PubMed2000

Angle matching in intravascular elastography.

Intravascular elastography is a new technique to obtain mechanical properties of the vessel wall and plaque. Mechanical information of vascular tissue is important for characterisation of different plaque components, detection of plaque vulnerability and thus choosing the proper interventional technique. The feasibility of the technique is investigated using phantoms and diseased human arteries. These studies demonstrated that elastography reveals information that is unavailable or inconclusive from the echogram alone. The technique is based on the principle that tissue strain is directly related to its mechanical properties. In intravascular elastography, the tissue is compressed using different intravascular pressures. The strain is determined using cross-correlation techniques of the radio frequency (r.f.) signals. Reliable strain estimates are only obtained when signals of corresponding tissue are correlated. Owing to catheter motion, off-centre position and non-uniform rotation of the intravascular transducer, the r.f. traces at low and at high pressure may be misaligned. Four algorithms are tested to track the corresponding ultrasound signals. Three methods (l1norm, l2norm and cross-correlation) are applied on the r.f. signal and one (l1norm) on the envelope (speckle tracking). Simulations are performed to obtain a data set with a priori knowledge of the scattering particles positions in the tissue at high and low pressure. Different positions of the catheter in the lumen, compression levels of the material and signal-to-noise ratios (SNRs) are investigated. Finally, these findings are corroborated with a phantom experiment in a water tank. From the simulations, it can be concluded that the speckle tracking algorithm has the best performance, under all circumstances. The performance decreases with larger eccentricity of the catheter and larger compression of the material. The SNR is only of minor influence. The speckle tracking algorithm has also the best performance in the phantom experiment. The performance of the speckle tracking algorithm is better than the three r.f.-based algorithms. For intravascular elastography, implementation of this method may improve the quality of the elastogram.

Algorithms↗

PubMed1995

Ultrasound backscatter at 30 MHz from human blood: influence of rouleau size affected by blood modification and shear rate.

High frequency intravascular ultrasound may show a high intensity backscatter from blood which hampers the discrimination between lumen and arterial wall. In this study, the acoustic behaviour of blood at 30 MHz in relation to rouleau size was analyzed. In a Couette viscometer, high frequency (20-40 MHz) backscatter data from normal and modified blood samples from eight volunteers were obtained at shear rates from 0 to 1000 s-1. The acoustic behaviour of blood was quantified by the integrated backscatter power and the spectral slope of the backscatter coefficient. Backscatter from blood depended on rouleau size. At a shear rate of zero, both whole blood and rouleau-enhanced blood showed a 11-dB-higher integrated backscatter power than rouleau-suppressed blood, which itself was 10 dB higher than that of hemolysed blood, the latter showing a 6-dB-higher backscatter than saline. Platelets did not contribute to the backscatter power. Plasma and saline produced no detectable integrated backscatter power other than noise. The spectral slope of whole and rouleau-enhanced blood was small (1 and 0.5, respectively), whereas rouleau-suppressed blood and hemolysed blood (both with a slope of 3.3) behaved almost like a Rayleigh scattering medium (slope = 4). The backscatter from rouleau-suppressed blood showed no shear rate dependence. At low shear rates ( < 0.8 s-1 for integrated backscatter power and < 0.2 s-1 for the spectral slope), whole blood and rouleau-enhanced blood tended to the results from the static situation (no shear). At high shear rates ( > 80 s-1 for integrated backscatter power and >11 s-1 for spectral slope), these samples tended to the results of rouleau-suppressed blood. Ultrasound backscatter at 30 MHz from human blood is only caused by red blood cells. With increasing aggregate (rouleau) size, the integrated backscatter power increased by 11 dB, and the spectral slope decreased from 3.3 to 1.

Blood↗

PubMed1995

Discrimination of intravascular lumen and dissections in single intravascular ultrasound images using subtraction, conventional averaging and saline flush.

With current 30-MHz intravascular ultrasound systems, flowing blood may cause considerable backscatter which in real-time images is characterized by dynamic speckle. However, in a single intravascular ultrasound image (still-frame) the discrimination between arterial lumen and wall may be difficult due to the frozen intraluminal speckle, particularly in the presence of dissections. We compared subtraction, averaging and saline flush as methods to improve the discrimination between arterial lumen and wall in a single image. The real-time images served as gold standard. In 22 patients who underwent peripheral balloon angioplasty, ultrasound images obtained from 84 sites were examined. The sensitivity and specificity of detecting dissections were in the subtraction image 85% and 100%, in the averaged image 57% and 96%, and in the saline flush image 58% and 86%, respectively. Subtraction is a promising method to outline the irregular lumen in a single image.

Aged↗

PubMed1993

Discrimination of the intravascular lumen and dissections in a single 30-MHz US image: use of "confounding" blood backscatter to advantage.

At 30-MHz intravascular ultrasound, the high-intensity backscatter of blood confounds discrimination of the intravascular lumen from the arterial wall. The authors developed and used a subtraction method to visualize the lumen and dissections through cancellation of backscatter from static areas. In two patients who underwent angioplasty of the superficial femoral artery, the residual hyperechoic pattern created by moving blood allowed good differentiation between the wall and lumen or dissections.

Aged↗