Pulsatile electrical impedance response from cerebrally dead adult patients is not a reliable tool for detecting cerebral perfusion changes.

The original objective of this work was to verify the possibility of using electrical pulsatile cerebral impedance measurements as a diagnostic aid for assessing the brain-death condition in adults; a subordinate target was to validate a simple method for detecting perfusional changes in the brain. To this end, impedance signals were recorded, for a comparative study, from both live subjects and brain-dead patients, using a simple four-electrode arrangement. Rather unexpectedly, pulsatile transcephalic impedance waveforms exhibiting a temporal dependance similar to those of live subjects were detected in artificially ventilated, cerebrally dead, adult subjects; distributions of the time delays between impedance peaks and ECG peaks were also recorded for the two groups (dead and live subjects). These data provided no evidence, at the 1% significance level, against the hypothesis that the two sample groups are drawn from identical populations. The detection of impedance variations from brain-dead patients can be explained by the residual persistence of blood flow through the scalp, by mechanical variations synchronous with the heart beat and by the presence of the oscillating flow and the systolic spikes that precede the final blood flow arrest. The fact that impedance variations can be traced back to a multiplicity of causes, unrelated to the normal unidirectional flow, renders the transcephalic impedance method inappropriate for detecting cerebral perfusion changes in adults. This conclusion is also strengthened by some theoretical results recently derived from a multilayer model of the head.

Real-time spectral analysis of HRV signals: an interactive and user-friendly PC system.

We present a real-time system, built around a PC and a low-cost data acquisition board, for the spectral analysis of the heart rate variability signal. The Windows-like operating environment on which it is based makes the computer program very user-friendly even for non-specialized personnel. The Power Spectral Density is computed through the use of a hybrid method, in which a classical FFT analysis follows an autoregressive finite-extension of data; the stationarity of the sequence is continuously checked. The use of this algorithm gives a high degree of robustness of the spectral estimation. Moreover, always in real time, the FFT of every data block is computed and displayed in order to corroborate the results as well as to allow the user to interactively choose a proper AR model order.

Cerebral blood-flow monitor for use in neonatal intensive care units.

The implementation of a real-time multichannel system for monitoring cerebral blood-flow is described. The instrument relies on a completely modular architecture and is based on the principle of measuring the electrical impedance between a number of periodically sensed electrode pairs positioned around the subject's head. The whole setup is controlled by a host computer that performs several functions, such as real-time acquisition, analysis, display and data logging. Two operating options can be chosen by the user: a normal mode that allows continuous monitoring and a triggered mode in which the measurement cycle is automatically started by the occurrence of a preset condition in some other circulatory signal, e.g. the permanently available ECG signal. The design is considerably user-friendly and embodies a number of special safety precautions to take account of the peculiar condition of patients, usually newborn infants hospitalized in intensive care units.

An instrument for real-time spectral estimation of heart rate variability signals.

A Digital Signal Processor (DSP)-based instrument is proposed for estimating and displaying the Heart Rate Variability (HRV) spectrum in real-time. It consists of an intelligent module which is properly interfaced to an IBM PC and whose operations are independent from the computer's other tasks. In this way, the simultaneous recording of the ECG sequence, needed for the more complete off-line analysis, can be performed by the same host. The employed hybrid spectral estimator (in which a classical FFT analysis follows the autoregressive extrapolation of data) appears to be the most apt for the present fixed point arithmetics implementation. The reliability of the instrument and its accuracy are checked both with suitable test signals and by comparison with the results obtained through off-line analysis of the same ECG tracks. The instrument is presently used for cardiovascular investigations, in particular for quickly picking patients with cardiac autonomic neuropathy (CAN) out of a population of diabetic subjects.

Measurement of zero-count probability in photoelectron statistics.

The probability of zero-count P(0)(T) (as a function of the counting interval T) is one of the most interesting functions characterizing a light field. Experimentally, P(0)(T) is usually obtained by measuring successively the zero-count probability for a set of different intervals. This procedure exposes the measurement of P(0)(T) to errors imputable to drift. We present a simple zero-counter which is essentially free from drift effects and displays P(0)(T) directly on the CRT of an oscilloscope for sixteen values of T. Another advantage of the instrument is a conspicuous reduction of the overall measuring time.

Triggering techniques in triggered photon-counting.

In the measurement of triggered photon-counting distributions (TPCDs) the triggering pulse which starts a collecting interval may be picked up according to different random procedures. We investigate, both theoretically and experimentally, the extent to which TPCDs are affected by the choice of the triggering pulses. It turns out that in ordinary conditions the measured distribution may be significantly influenced by the triggering procedure. This result is another manifestation of the bunching effect and indicates that a great deal of attention should be exercised in performing and interpreting triggered photon-counting.

Multipurpose analyzers for photoelectron statistics: implementation and use.

The statistical properties of variously scattered laser light can be derived from photocount data through the estimate of different functions. Even if the second-order correlation usually plays the main role, other function (e.g., triggered and nontriggered distributions, moments of various order, and higher-order correlations) may give more appropriate results in many experimental conditions. We present a multifunction analyzer whose working principle is based on the acquisition of a long sequence of interpulse intervals (through a circuitally simple personal-computer front-end interface), which is followed by the off-line calculation of one or more of the functions for which an algorithm is available. Up to 5 × 10(5) photopulse intervals can be recorded at a maximum rate of approximately 2 × 10(5) data points/s. A short description of the algorithms used to calculate the different functions is given together with some useful hints and a table of typical processing times.

Edge-detection schemes highly suitable for hardware implementation.

Two schemes for edge detection of real images based on gradient maxima are presented. Images are filtered with narrow filters to increase localization. Experimental results and theoretical considerations suggest that the exact shape of the filter is not critical for good performance of the algorithm. Therefore a filter can be chosen to allow for a highly efficient hardware implementation, for example, a binary filter or a 4-bit finite-impulse-response filter. Because the digitized values of a binary filter are powers of 2, the hardware implementation does not require time-consuming computations, such as multiplication and time shift, but just appropriate addressings. The performance of this scheme, or a similar scheme using 4-bit filters, is as satisfactory as that of more sophisticated schemes. Therefore these low-cost schemes are likely to be more suitable for hardware implementation.

Measurements of surface roughness: use of a CCD camera to correlate doubly scattered speckle patterns.

We describe an instrument, built around a commercial CCD camera and some fast image-processing boards, that evaluates roughness height by measuring the average size of doubly scattered speckle patterns. The device is a variant of a recent proposal that was based on the use of a spatial modulator to perform the Fourier transform of a speckle image. In the present setup, the Fourier transform is replaced by the direct evaluation of a second-order correlation function. Strictly speaking, the device proposed in this paper is not a real-time device but its response time (approximately 10 s) is sufficiently short to be of practical value for many applications. Updated CCD cameras that will significantly improve the performance of our prototype are already on the market.