Total variation norm for three-dimensional iterative reconstruction in limited view angle tomography.

An iterative Bayesian reconstruction algorithm for limited view angle tomography, or ectomography, based on the three-dimensional total variation (TV) norm has been developed. The TV norm has been described in the literature as a method for reducing noise in two-dimensional images while preserving edges, without introducing ringing or edge artefacts. It has also been proposed as a 2D regularization function in Bayesian reconstruction, implemented in an expectation maximization algorithm (TV-EM). The TV-EM was developed for 2D single photon emission computed tomography imaging, and the algorithm is capable of smoothing noise while maintaining edges without introducing artefacts. The TV norm was extended from 2D to 3D and incorporated into an ordered subsets expectation maximization algorithm for limited view angle geometry. The algorithm, called TV3D-EM, was evaluated using a modelled point spread function and digital phantoms. Reconstructed images were compared with those reconstructed with the 2D filtered backprojection algorithm currently used in ectomography. Results show a substantial reduction in artefacts related to the limited view angle geometry, and noise levels were also improved. Perhaps most important, depth resolution was improved by at least 45%. In conclusion, the proposed algorithm has been shown to improve the perceived image quality.

Prevention of pacemaker mediated arrhythmias.

Pacemaker mediated re-entry tachyarrhythmias and their causes are discussed. Several methods to prevent these arrhythmias are presented. It is shown that the most efficient method incorporates a long refractory period of the atrial detection after ventricular depolarization. The benefits of a short A-V delay after atrial sensing are discussed.

Spectral properties of atrial and ventricular endocardial signals.

The spectral energy distributions of signals picked up by endocardial, atrial and ventricular electrodes have been investigated. The results show similar spectral distributions for P-waves and QRS-complexes with maximum spectral densities at frequencies between 10 and 30 Hz.

The carbon tip electrode.

The development of the carbon tip electrode and its low chronic threshold behavior are described. Compared to presently used electrode material, carbon tip electrodes result in lower chronic stimulation thresholds. In a group of 43 patients, chronic pulse generator output has been favorably reduced in association with this type of electrode. Thus the use of activated carbon tip electrodes may result in the routine implantation of low output pacemakers.

An implantable pulse generator indicating asystole or extreme bradycardia.

The design of a pacemaker which indicates the occurrence of significant bradyarrhythmia is described. Significant bradyarrhythmia is defined as either asystole for 6 seconds, bradycardia with three successive RR-intervals greater than 2 seconds each, or five or more RR-intervals of 2 seconds or more over a period of 25 seconds, but without three such intervals in succession. During bradyarrhythmia the generator initially stimulates at an interval of 2 seconds (30 per minute) and when the described criteria have been fullfilled the pacemaker automatically converts to a conventional ventricular inhibited pulse generator stimulating at a rate of 70 per minute.

Pacemaker architecture: a pacemaker with an attached computer or a computer with an attached pacemaker.

Microprocessors have a major impact on cardiac pacemaker technology. The development of adaptive systems capable of responding to physiologic variables will ultimately improve the care of the individual patient. However, in order to minimize power drain, it will be necessary to incorporate certain functions into the hardware outside the microprocessor. In addition, specifically designed program languages may indeed become necessary.

Effect of externally applied focused acoustic energy on clot disruption in vitro.

Application of low-frequency ultrasound for clot disruption has been suggested as a potential therapy to enhance thrombus dissolution, but the optimal mode for delivery of ultrasound with clot-disruptive properties has not yet been extensively explored. Target-specific effects are desirable and may be accomplished by focusing the ultrasound. Adequate focusing, however, requires a short wavelength. The aim of this study was to compare the clot-disruptive effects of different modalities of focused acoustic energy. An in vitro model (10 blood clots for each modality) was used to test the clot-disruptive capacity of (i) shock waves generated in an electrohydraulic lithotriptor; (ii) focused continuous ultrasound of frequency 1.1 MHz, delivered from a specially constructed piezoelectric transducer; and (iii) focused pulse-modulated ultrasound of frequency 1.1 MHz delivered from the same transducer. Exposure to 30 s of focused pulse-modulated ultrasound caused a marked reduction (99+/-2%) in clot weight compared with 30 shock waves (11+/-5%) or 30 s exposure to focused continuous wave ultrasound (11+/-6%) (P<0.0001). The observed marked and rapid disruptive effect on blood clots of focused high-frequency ultrasound indicates an alternative approach for external ultrasound-mediated thrombus destruction in vivo. The focused pulse-modulated technique has potential to exhibit the desired effect in a well-defined target volume and provides the means for control of the average power.

Chronic threshold of stimulating electrodes: comparison of activated vitreous carbon with conventional platinum-iridium electrodes in animal tests.

Electrodes made of vitreous carbon are inert, corrosion-resistant, inactive to electrocatalytic reactions and are especially biocompatible. Upon activation, they attain a capacitance of 20 to 40 mF and become "non-polarizable". Therefore, they should be particularly suitable as stimulating and sensing electrodes for cardiac pacemakers. The connective tissue layer that develops around the electrode because of the foreign-body reaction is less than 100 micron thick. The threshold rise, through lower than that of conventional Pt-Ir or ELGILOY electrodes, cannot be attributed exclusively to the connective tissue layer that is formed. Under favourable conditions, the threshold in animal experiments remains below 525 mV. Blood-spaces adjoining the electrodes are found at higher chronic threshold values.