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H J Zweifel

Publications and source records attributed to H J Zweifel.

15 recordsLinked to original sources
PubMed1998

Impact of Doppler guidewire size and flow rates on intravascular velocity profiles.

Coronary blood flow velocity measurements by conventional intravascular catheter-based Doppler devices are well known to be affected by catheter size. Moreover, it is of clinical importance that the assessment of maximum vasodilator capacity, i.e., the coronary reserve, might be considerably affected by a shape change of the velocity profile under hyperemia. Therefore, the present in vitro study aimed to assess the impact of a small-size Doppler guidewire on the velocity profiles interrogated in tubes with diameters corresponding to the epicardial coronary arteries at clinically relevant flow rates. A 0.014" guidewire was inserted into four serially connected silicone tubes of known diameter, which were perfused with discarded human whole blood by means of a roller pump. In order to determine the effect of the Doppler guidewire on the velocity profile antegrade and retrograde perfusion were carried out in each vessel segment. Vm, the true mean velocity, was calculated from the time collected flow divided by the corresponding vessel cross-sectional area. Average peak velocity (APV) measurements were obtained by pulling back the Doppler device across each of the four vessel segments with given flow rates ranging from 1.14-5.88 ml/s in antegrade and retrograde direction. The shape factor of the recorded velocity profile (fp) is defined as vm/APV and is generally assumed to be close to 0.5. Antegrade perfusion: APV = 1.63 vm + 5.35 (R2 = 0.98); fp = 0.001 vm + 0.51. Retrograde perfusion: APV = 1.71 vm + 3.33 (R2 = 0.98); fp = 0.001 vm + 0.52. Due to the constant relationship between APV and vm, velocity profiles within vessels of epicardial coronary artery size are not substantially disturbed by the presence of the Doppler guidewire. The slight, but significant increase of experimental fp with increasing flow is at variance with the theoretically expected fp values.

Blood Flow Velocity↗
PubMed1997

[Recent developments in the area of Doppler ultrasound imaging].

Developments in Doppler Sonography: Review of duplex Sonography with and without ultrasonic contrast media, power Doppler, transcranial Doppler sonography. Duplex sonography for investigating the extracranial brain circulation should always be used in conjunction with unidimensional direction sensitive Doppler sonography.

Blood Flow Velocity↗
PubMed1992

[Gait analysis--a prerequisite for general movement analysis].

Our system for analysis of gait is a combined visualisation of angular velocities in any joint with dynamic measurements of pressure at the sole while walking in a natural surrounding. Infrared reflectors are attached to chosen parts of the body. Their position in space is sampled over time and transferred to an easily manageable personal computer that visualizes motion in progression as angular velocities. Pressure at the sole is measured by pressure sensors. The magnitude of force is expressed as calibrated circular areas following the dynamic course of gait. The system is accurate and easily operated. The time necessary for measurements and evaluation of data as well as the costs render an application in daily clinical work acceptable.

Foot↗
PubMed1990

[CT/MRI correlated stereometry with ultrasound in brain surgery].

Today the accuracy of medical imaging systems lies between +/- 0.5 mm (CT) and +/- 2.5 mm (MRI). In neurosurgery the overall uncertainty may reach the dimension centimetres. A non-tactile ultrasonic measurement system is described that allows three-dimensional measurement with a theoretical accuracy of +/- 0.1 mm in a frequency range between 50 and 70 kHz. In practice the accuracy is better than +/- 1 mm with a repetition rate of 10 per second for the x,y,z-data sets.

Angiography, Digital Subtraction↗
PubMed1990

[Technical aspects of ultrasound aspiration].

Technical, metallurgical and ergonomic problems which were encountered during a three-year development period of a surgical ultrasonic aspirator are reported. The aim of the construction of a lighter, handier, piezo-driven handpiece with a durable titanium tip was achieved. A first version was extensively tested during more than 40 resections of brain tumours. A second prototype of a handpiece with an integrated aspiration and flushing facility will be soon evaluated clinically. Further applications and perspectives of ultrasonic aspiration are discussed.

Brain Neoplasms↗
PubMed1990

Interactive sonar-operated device for stereotactic and open surgery.

An accuracy of +/- 1 mm in surgical localization of brain targets was reliably attained with a new sonar-operated system, provided that air turbulence was minimized. Since a conventional frame construction with coordinate settings has been disposed of, apart from a halo ring head holder, free access to the operating site is guaranteed. Using offset probes, measurements in depth can be realized with a relatively simple hardware arrangement. A small, 'floating' locator probe can be attached to all kinds of surgical instruments; its spatial position is computed in real time and immediately referred to computed tomography/magnetic resonance imaging scans. Although this work is still in progress, promising applications both in stereotactic and in open surgery are envisaged.

Angiography, Digital Subtraction↗
PubMed1990

Physical and technical foundations of diagnostic ultrasound.

The physical and technical foundations of investigations with diagnostic ultrasound are discussed. Two questions need to be answered: what is ultrasound, and how does an ultrasonic device operate? The mechanical properties of the human body are described by physical laws, such as the propagation of ultrasound waves, the interaction of ultrasound with tissue, attenuation, reflection, refraction, etc. Today high-technology ultrasonic devices are driven by computers. Imaging of the same kidney with different devices shows the progress made in image quality over a period of about 15 years. Details of tissue and biometry are largely enhanced. Powerful tools allow different forms of precise analysis of online measurements. Advanced handling of information gives further opportunities to image processing. The final question is the extent to which the physician and surgeon want to work with such as sophisticated high-technology imaging system and learn to manage its hard- and software.

Humans↗
PubMed1988

[Technical safety in ultrasound diagnosis].

Technical devices and systems are getting more and more complex in diagnostic ultrasound. Safety aspects of ultrasound equipment are defined as "electrical safety", "safety of ultrasound" and "functional safety". Proposals are made to increase the safety of ultrasound and its interactions with mammalian cell cultures in order to establish a biological standard. A model of safety control is discussed. In future it will be necessary to pay increasing attention to questions on the efficiency of ultrasonic diagnosis and the responsibility of the examiner, since safe equipment alone cannot guarantee safe results if the examiner is unable to use the equipment properly and to interpret the resulting data correctly.

Equipment Design↗
PubMed1982

[Training and quality assurance in ultrasound diagnosis].

Medical ultrasound equipment is mainly used for diagnostic purposes. Examiner, patient, measuring equipment and ultrasound-based diagnosis form the ultrasound diagnostic system which is characterised by a multitude of internal and external relations. By extending the scope of the system, quality assurance of the ultrasound diagnostic procedures is improved with regard to the physical performance of the system. The performance standards should be established internationally. Even the best of equipment is only as good as the user's experience In this respect, the standards have to be met theoretically and practically on all levels of medical education. The most appropriate settings would be represented by national associations of ultrasound. The introduction of ultrasound diagnostic procedures on the level GP everyday work should lead to a decrease in cost and an increase in speed and efficiency by eliminating routine x-ray examinations.

Curriculum↗