Active Arrays

The next generation of search radar may well use the concept of the active array. This is a natural extension of the phased array. In that design, each array element is provided with its own transmitter and receiver module. This solid-state module contains a low-power transmission amplifier, receiver protec­tion, a low-noise RF preamplifier, filtering, and frequency con­version. It also contains a digitally controlled phase shifter for beam control. On transmission, all modules are driven by the same coherent signal from a frequency synthesizer. That element also provides one or more local oscillator signals for frequency conversion to IF. The IF signals are combined to form one channel for signal processing.

A chief advantage of the modular array is the elimination of a large, expensive, hard-tube transmitter. Its disadvantage is in the huge number of modules required. If 4356 elements are required, then that number of modules is also required. If the cost of each module were only $100, then the total array cost would approach $500,000. For this reason, the modular array has not found a large application. However, as technol­ogy improves and costs decrease, this approach may become very popular in future designs.

RELAXATION OSCILLATORS Ring Oscillators

Ring oscillators are a popular topology for variable fre­quency oscillators used in integrated circuits because they have broad tuning range and require no resonators. They consist of a number of amplifiers or delay stages that may be inverting or noninverting, connected in such a way that an odd number of inversions exist around the loop (Fig. 14). The loop is therefore never “happy,” and the result is that an “edge” propagates around the loop. Loops with as little as two buffers are possible, but for large number of stages it is customary to choose prime numbers to prevent the oc­currence of multiple modes of oscillation. For example, if we used six buffers, we could have three “edges” running around the loop, with the result that the frequency would sometimes be three times higher. This would happen at random after powering up the circuit.

RELAXATION OSCILLATORS Ring Oscillators

Figure 15. Diagram of a bistable oscillator.

The frequency of a ring oscillator can be varied in several ways. One popular way is to change the internal delay of each buffer by adjusting the amount of current available to charge and discharge the circuit capacitances. Another way is to vary the amount of load presented at the output of each buffer.

Phased Array Antennas

Phased array antennas are planar arrays of waveguide slots. Variable phase shifters are used to drive a group of slots and, thus, to effect electronic steering of the antenna beam. This technique can eliminate the necessity for bulky mechanical devices such as motors and gimbaled platforms.

The basic theory of phased arrays is described best by con­sidering the simplest case, which is a two-element array. The radiated fields from two adjacent sources combine in space to form a radiation pattern. When a phase shift is applied to one element, the directivity of that pattern may be altered. In this simple case, it may be shown that the relative gain of the array is

G =

2 + 2cos(n sin a – ф) 4

where a is the space angle relative to perpendicular and ф is the introduced phase shift. Maximum gain occurs when

a = sin-1 (ф/п)

Thus, for small angles, the ratio of phase shift to steered angle is approximately a factor of three. In no way does this example present the design equations for a complex array. It simply shows the effect of phase shift on boresight shift.

The concept is extended easily to linear arrays of any length, N, and to two-dimensional arrays having N X M ele­ments (9). In practice, the pattern produced by an array will
be the product of the pattern from each element and the array factor, which is determined by the element spacing. The opti­mum spacing is one-half wavelength. At wider spacing, the pattern begins to develop unwanted sidelobes called grating lobes. These can be as large as the main lobe and may cause confusion or interference. In addition, coupling between ele­ments can alter the actual antenna pattern. Phased array de­sign is a complex process.

When the array beam is steered off-axis, the beam will broaden and the gain will decrease. In general, this effect is in proportion to the cosine of the steered angle. For example, a steering angle of 45° may result in a loss of 1.5 dB relative to on-axis gain.

When the element spacing is one-half wavelength, the number of elements and required phase shifters can become quite large. For example, consider a design at X band where one wavelength is 0.03 m. An antenna 1 m on a side would require 4356 phase shifters to enable steering in both planes. The sheer cost and weight of this system might be prohibitive.

A good compromise design is one in which the antenna array is rotated mechanically in azimuth while being steered electronically in elevation. The example given previously would then require only approximately 66 phase shifters to provide elevation-only steering. This approach also allows for raster scanning. Rather than holding the elevation position constant over a full 360° rotation, the beam could be directed to visit several elevation positions during one scan. This not only reduces the time required to illuminate a given volume but, since the beam traverses the target in both dimensions, beam splitting in elevation and azimuth could be imple­mented.

An ultimate phased array design is the conformal array. Here, the array is designed as an integral part of an existing geometry. That geometry might be the fuselage or wing of an aircraft or the hull of a ship. The ideal conformal array would be a sphere or hemisphere. This design could eliminate off – axis steering loss, because the beam would always be perpen­dicular to the array surface.

Another advantage of phased arrays is their capability for instant target verification. An initial detection could be fol­lowed by freezing the beam in the direction of the target de­tection. Then, a longer dwell could be chosen to both reduce measurement error and increase confidence level. The time savings relative to scan-to-scan verification could be sig­nificant.

A final application of phased arrays is in platform motion compensation. When the radar is carried by an aircraft or ship, it is desirable that the beam position be maintained rel­ative to earth coordinates independent of platform motion. This is implemented easily using beam steering and its use eliminates the necessity for complex motor-gimbal apparatus.

IMPEDANCE CARDIOGRAPHY

Impedance cardiography (ICG) is the noninvasive mea­surement of physiologically and clinically relevant pa­rameters of the heart and circulation, based on electrical impedance measurements of the thorax during the car­diac cycle. Recent reviews have been presented by (7) and

. The technique uses a low-current (0.5 mA to 4 mA), high-frequency (50 kHz to 100 kHz), alternating current across the thorax, not perceivable to the subject. The re­sulting impedance changes associated with the cardiac cy­cle (impedance decreases by about 0.2 Q from diastole to systole) provide information on stroke volume, cardiac out­put, pulmonary capillary wedge pressure (9), and systolic time intervals (by calculating the first time derivative of the impedance waveform). In 1969, (10) suggested an index of cardiac function based on particular calculations applied to the impedance tracing. This so-called Heather index was shown to correlate with the severity of cardiac pathology (7).

Among other techniques to measure cardiac size (e. g., X ray, ultrasound, magnetic resonance imaging), external impedance cardiography has the advantages of being non­invasive, requiring only relatively low-cost equipment, and permitting continuous monitoring of signals originating from the beating heart, even during exercise (11). Major shortcomings result from the fact that a sound physical model and a comprehensive theory still need to be devel­oped. (12, 13), and (14) were among the first researchers to study the feasibility of the method. Chest impedance is determined by the relatively constant electrical con­duction properties of all tissues concerned, plus a mod­ulated component caused by the combination of respira­tion, thoracic dimensional changes, and a cardiovascualr size-related factor. The latter component is due to cyclic changes of size (i. e., the geometry changes with contrac­tion) of the four compartments of the heart and the major blood vessels, as well as to the periodic alignment and de­formation of the erythrocytes in the flow. Ejection of blood from the heart distends the walls of the arteries, thus in­creasing their blood volume and resulting in an impedance decrease, besides a decrease of lung resistivity owing to blood perfusion. During relaxation of the cardiac ventri­cles, blood in the systemic circulation travels downstream, causing a reduction of the arterial diameter while the ery­throcytes lose their orientation owing to the lower veloc­ity, all leading to an increase of the thoracic impedance. Using suitable filtering techniques, it turns out that the contributions derived from breathing and locomotion can be eliminated. Obviously, respiratory components are sim­ply removed if patients are instructed to temporarily hold their breath, but in animals this approach would not be feasible. Various filtering procedures have been developed, including Fourier linear combiner (FLC) and event-related transversal types. A major problem is that many physio­logical signals are quasiperiodic; that is, they have a mean period with a small random variation around this mean at each interval. (15) introduced a scaling factor to enhance flexibility when choosing filter parameters in the FLC ap­proach. They successfully applied their method to ICG – derived stroke volume (SV) in a volunteer during exercise. Alternatively, one may apply an ensemble averaging tech­nique to 20 beats, thus eliminating respiratory influences (16). The number and position of the electrodes employed may vary depending on the specific purpose of the study or the particular geometrical model assumed. Basically, a four-electrode (tetrapolar) arrangement is employed: two current injecting electrodes form the outer pair, and the inner two are the sensing electrodes. This setup overcomes not only impedance problems related to electrode polariza­tion but also the relatively high skin impedance. A typical configuration for ICG is illustrated in Fig. 1.

The amount of blood pumped per minute by one side of the heart is termed cardiac output (CO) and equals the product of heart rate (HR) and SV. In ICG it is the pas­sage of blood in the major arterial vessel (called the aorta) during the ejection phase of the ventricle that mainly de­termines the changes in electrical impedance. In contrast, the electrocardiogram (ECG) is a recording of the electri­cal wavefront as it spreads over the cardiac tissues, and this signal provides no information on the amount ofblood pumped nor does it give any insight into the size of the ventricle or the strength of contraction. Therefore it is im­portant to emphasize that the actual (internal) source of electricity in the heart, namely, the action potentials that cause a periodic voltage change on the order of 100 mV over the cell membranes of the heart, are unrelated to the electrical impedance variations resulting from the exter­nal stimulation electrodes and as recorded by the sensing

Figure 1. Electrode configuration for thoracic impedance cardio­graphy: V, the voltage recording electrodes on the lateral base of the neck and another pair on the chest at the level of the xyphoid; C, constant current-injecting electrodes on forehead and at ab­domen placed 15 cm caudally from the voltage electrodes.

electrodes. It may be concluded that the ECG and the tho­racic impedance signal provide complementary informa­tion on the activity of the heart: the ECG on the internally generated electricity, and the thoracic impedance on the hemodynamic changes owing to the mechanical action of the heart.

The electrical impedance signal is obtained from a spe­cial arrangement of disposable spot or band electrodes placed on the skin of the head, neck, and chest. A set of current-injecting electrodes is driven by a constant sinu­soidal current of less than 1 mA root mean square at a frequency ranging from 50 kHz to 100 kHz, while another set of electrodes senses the resulting voltage from which the impedance signal is calculated. The peripheral ECG is usually recorded from the limb leads (Fig. 2).

Thoracic impedance ( Z) has a baseline component Z0 and a time-variable component ( dZ). Z0 depends on pos­ture, tissue composition, and the volume of fluids within the chest. Cardiac edema causes a decrease of the value for Z0 (17). In noncardiac edema (e. g., in the adult respira­tory distress syndrome), Z0 can increase or decrease owing to capillary leakage of proteins (18). The component dZ cor­responds to the volume change in the thoracic aorta during the cardiac cycle. The maximum value of the time deriva­tive of tZ is proportional to the peak ascending aortic blood flow. (13) developed a formula to derive SV (in mL) from the thoracic ICG:

SV — pTgiL/ZofidZ/dpnin

where p is the specific resistivity of blood (about 135 Qcm and 150 Qcm for women and men, respectively), Tet the duration of the ejection period (s), L the distance between both recording electrodes (cm), and Z0 the baseline com-

С

Figure 2. Left ventricular (LV) volume as obtained by intraven – tricular impedance catheter in the open-chest dog. Also shown is the flow in the aorta resulting from these volume changes during an episode of irregular heart rhythm. (From Ref. (35) with permis­sion.)

ponent of the thoracic impedance (Q). This approach has been widely applied, sometimes after modification of the expression to account for body build. Underlying assump­tions for Kubicek’s equation and subsequent modifications are that the tissues are modeled as a homogeneous elec­trical conductor with the shape of a cylinder or truncated cone, and that the impedance variations observed in syn­chrony with the heartbeat exclusively reflect the time – varying volumetric changes of the cardiovascular system in the specific model considered. For the cylinder model, the length is related to the height of the chest, while the cross-sectional area reflects the thoracic circumference; the values of these parameters are obviously characteristic for each individual at a given time. (19) determined the inac­curacy of Kubicek’s one-cylinder model, or more generally of any geometrical model with a uniform cross section, both in a theoretical study and using in vivo experiments. The results indicate that the one-cylinder model is not valid and must be replaced by a model with two serially placed cylinders, which appropriately exhibit a length-dependent behavior corresponding to differences in body height and mass. It was concluded that corrections for the Kubicek equation are required, along with the incorporation of a two-cylinder model. Interestingly, (20) found for the trun­cated cone model no gender effect on SV when normalized for body surface, because the differences in the component

Z0 were counterbalanced by changes in dZ. However, both in females and in males they documented a decline of nor­malized SV with age (range 20 years to 69 years).

As an extension of the routine approach to estimating SV by thoracic impedance and using the Kubicek equation,

also found a high correlation with the clinically impor­tant pulmonary capillary wedge pressure (PCWP), which corresponds to left atrial pressure and thus reflects left ventricular filling. They also compared the noninvasively obtained SV with values determined by thermodilution. The study population consisted of 24 patients with cardiac problems, including coronary artery disease and various types of valvular defects. The PCWP as determined with a pulmonary artery catheter correlated ( r = 0.92,p < .0001) with the ratio obtained by dividing the amplitude of the ICG during diastole (the O-wave) by the maximum value during systole. It was hypothesized that this ratio reflects efficiency of the left ventricle (LV) because the O-wave cor­responds with preload, whereas the peak of dZ/dt is related to afterload. A comparison between the two methods (i. e., impedance versus thermodilution) to estimate SV was less encouraging ( r = 0.69, p < .05), particularly for patients with valvular disease. As an extension, (21) assessed right ventricular diastolic function in patients with chronic ob­structive pulmonary disease by means of region of inter­est analysis applied to electrical impedance tomography, as will be described later.

Of crucial importance, of course, is a quantitative vali­dation of the thoracic impedance method, preferably using a gold standard for the determination of SV. Unfortunately, an ideal reference technique does not exist, and there­fore researchers often rely on methods that are feasible and minimally traumatic. For example, (22) recently per­formed a comparison of hemodynamic parameters derived from transthoracic electrical bioimpedance with those ob­tained by thermodilution and ventricular angiography. These investigators studied 24 human patients with coro­nary artery disease while employing the three indepen­dent methods and concluded that impedance cardiogra­phy should not replace invasive hemodynamic monitoring. However, an accompanying editorial moderated this pes­simistic conclusion by pointing to the expected impact from emerging noninvasive echocardiographic Doppler technol­ogy.

As mentioned before, the physical size and shape of an individual are related to the dimensions considered in any geometrical model such as the cylinder or the truncated cone. For a given adult these values may be expected not to change very much, unless a disproportionate dimensional change occurs within a relatively short time frame. In preg­nancy, for example, maternal blood volume increases by about 40%, while CO reportedly rises by some one-third within nine months. Therefore, in longitudinal studies in­volving pregnant women, certain precautions may be indi­cated. (23) addressed the question of whether thoracic elec­trical bioimpedance is suitable for monitoring SV during pregnancy. It was demonstrated that the Kubicek formula for SV needs to be modified by a multiplication factor, which for each woman is to be determined at the beginning of the pregnancy period. Subsequently, (24) explored the possibil­ity of relating CO to body surface area in women during the course of their pregnancy. Based on the poor correlations found for both impedance and Doppler echocardiographic data, the authors concluded that normalized CO does not offer any additional information relevant for comparative evaluations during pregnancy and may even be mislead­ing.

The availability of ICG as a measurement technique that is noninvasive and permits continuous registration ev­idently opens the way to investigations of circadian effects. (25) took advange of these properties of ICG and studied sleep and circadian influences on the cardiac autonomic nervous system activity. They found that parasympathetic activity (derived from respiratory sinus arrhythmia ob­served in the ECG) is mostly influenced by the circadian system, whereas sympathetic nervous system activity (as­sessed with the pre-ejection period of the impedance signal) is predominantly affected by the sleep system.

Unfortunately, there exists no “gold standard” for car­diac size determination, because most techniques require barely verified assumptions concerning anatomical geom­etry. A similar shortcoming also applies to SV, which refers to the difference between diastolic and systolic volumes. Another approach would be to compare SV as measured in the aorta using a flow probe with the ICG signal; such as experiment will be feasible in the chronically instrumented animal and therefore deserves due attention.

Two types of problems related to application of the ICG will be discussed. First is the type of electrodes employed, that is, band (or tape type, consisting ofa6 mm conductive foil strip) versus spot (or regional) electrodes. The band signal is made up of different signals from various regions and may reflect changes in SV (rather than the absolute value), whereas spot electrodes may be important in mea­suring regional physiological activities of the central cir­culation (11, 26). Also, the anatomical position of the elec­trode and movement can cause large artifacts that may override the signal of primary interest (27). Second, the commonly made assumption that aortic distension during the ventricular ejection phase is the predominant compo­nent is not justified, because concomitant changes of lung resistivity, ventricular contraction, and other factors may interfere with comparable magnitude (26, 28). Obviously, the current limitations preclude wide clinical acceptance and certainly require further measurement refinements of the ICG method, but recently progress has been made in this field (19-29).

BIOIMPEDANCE

As an introduction to electrical impedance and conduc­tance in biology, a review of the relevant terminology is given and the scope of the discipline is presented. The area of bioimpedance is broad, including, for example, impedance cardiography, electrode impedance, impedance spectroscopy, intraluminal conductance, and impedance to­mography. The field of bioimpedance deals with the elec­trical conduction properties of biological materials as a response to the injection of current. It has been known for more than two centuries that biological structures dis­play the phenomenon of electrical conduction. Later, it was found that the precise electrical properties of tissues depend on their cellular composition and their coupling. These characteristics imply that the voltage changes at a particular site may provide valuable information regard­ing the biological materials and processes concerned.

However, to date, our understanding of the electrical impedance of biological tissues and their changes, as far as they are associated with physiological activity, is still limited. This article discusses the application of electrical impedance in medicine. Briefly, bioimpedance can be used to quantitate extracellular fluid, to assess volume changes, and as an imaging tool similar to ultrasonography. Reviews can be found in (1-3), and (4).

In order to meet the requirements of specific applica­tions, electrodes for delivering or recording electrical po­tentials in biological structures appear in a variety of ma­terials, sizes, and shapes. The interface between electrode and tissue has been studied extensively (5, 6).