Stem compensation of loop sensors for electromagnetic blood flow and vascular diameter measurement

Accuracy of electromagnetic blood flow —and vascular diameter — measurements by means of intravascular loop probes in an extracorporeal alternating magnetic field can be impaired by an error signal induced in the probe stem. The standard method of removing this artifact by twisting the lead wires is difficult to implement in this case because the wires are bifilar and are made of highly resilient metals. The solution described here makes use of a hair pin loop incorporated within the probe stem in which an e.m.f. is induced that can be used to cancel the unwanted e.m.f. induced in the lead wires of the loop of the sensor. This scheme is applicable to simple loop probes as well as to orthogonal double-loop sensors.     

The interference threshold of unipolar cardiac pacemakers in extremely low frequency magnetic fields

The effective induction loop area of implanted cardiac pacemaker (CPM) systems in magnetic fields was determined. The results were verified in a tank model placed in the centre of a Helmholtz-coil-arrangement. Both a left and a right pectorally implanted unipolar dual chamber CPM system were simulated. On this basis and with the results of benchmark-tests the interference thresholds for a collection of modern CPMs in extremely low frequency (ELF) magnetic fields were estimated. The investigations clearly showed that there are two loops, the CPM-lead-tissue-loop and the body loop, responsible for the magnitude of the disturbance voltage on the input of a cardiac pacemaker. The effective induction loop areas ranged from 100 to 221 cm2. For a left pectorally implanted, atrially controlled CPM system the interference thresholds for the magnetic induction lay between 16 and 552 µ T (RMS) for frequencies of the magnetic field between 10 and 250 Hz. Thus, there is a limited possibility for an interference of implanted CPMs by ELF magnetic fields in everyday life.     

Magnetic fields with photon beams: use of circular current loops.

Strong transverse magnetic fields can produce very large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. Through EGS4 Monte Carlo simulations, we have examined the effects of applying a magnetic field produced by a pair of circular current loops to a photon beam penetrating a water phantom of finite thickness. We have indeed found very substantial localized dose enhancements, albeit with no corresponding dose reduction just distal to the region of dose enhancement. (However, dose reduction does occur near the distal end of the phantom.) We have also observed two phenomena to be concerned with, for this configuration: significant broadening of the penumbra close to the current loop, and narrowness of the enhanced dose region in a plane parallel to the planes of the loops. We have also examined the use of a single current loop to produce the magnetic field, and have found great asymmetry in the dose distribution; this asymmetry appears to make it impossible to treat with a single circular magnet a tumor of large dimension extending below the application surface.     

Interactions between carotid sinus mechanoreceptor and chemoreceptor reflex loops

Summary Our experiments were designed to evaluate the combined influence of the mechanoreceptor and chemoreceptor control loops on the cardiovascular and respiratory systems. Both carotid sinus areas were isolated by a complete blind sac preparation in chloralose anesthetized dogs in which a flow probe had been implanted previously on the ascending aorta. The following variables were continuously monitored: central aortic pressure, heart rate, cardiac output, vascular resistance of a hind limb, tidal volume, respiratory rate, and oxygen consumption. While one sinus was submitted to step changes of pressure, the opposite sinus was kept at constant mean pressure. In addition, one sinus was perfused with either oxygenated of deoxygenated blood at constant flow rate.These studies show a definite interaction between the control loops initiated by the carotid sinus mechanoreceptors and those initiated by the chemoreceptors. The influence of the chemoreceptors is most marked in the peripheral beds, where it shifts the whole response curve of the mechanoreceptor control loop upward. This response accounts for most of the upward shift of the pressure response curve.On the other hand, the mechanoreceptor loop affects the ventilation, but this effect is range dependent, being most marked for the low pressure perfusion of the carotid sinus.This work has been supported in part by PHS Research grant HL 11747     

Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Ultra‐high‐field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low‐field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight‐fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx‐only arrays, which are larger in order to fit multi‐channel receive (Rx)‐only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high‐power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16‐channel tight‐fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx‐only vertical loops positioned along the central axis (parallel to the magnetic field B₀) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16‐element array provided 30% improvement of central signal‐to‐noise ratio (SNR) in vivo relative to a surface loop eight‐element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.     

Decoupling of a double‐row 16‐element tight‐fit transceiver phased array for human whole‐brain imaging at 9.4 T

One of the major challenges in constructing multi‐channel and multi‐row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4 T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight‐loop single‐row (1 × 8) human head tight‐fit TxRx array. In this work, we designed and constructed a 9.4‐T (400‐MHz) 16‐loop double‐row (2 × 8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole‐brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit‐only/receive‐only (ToRo) array, which consists of a larger Tx‐only array and a nested tight‐fit 31‐loop receive (Rx)‐only array. At the same time, the ToRo array provides greater peripheral signal‐to‐noise ratio (SNR) and better Rx parallel performance in the head–feet direction. Overall, our work provides a recipe for a simple, robust and very Tx‐efficient design suitable for parallel transmission and whole‐brain imaging at ultra‐high fields.     

The interference threshold of unipolar cardiac pacemakers in extremely low frequency magnetic fields.

The effective induction loop area of implanted cardiac pacemaker (CPM) systems in magnetic fields was determined. The results were verified in a tank model placed in the centre of a Helmholtz-coil-arrangement. Both a left and a right pectorally implanted unipolar dual chamber CPM system were simulated. On this basis and with the results of benchmark-tests the interference thresholds for a collection of modern CPMs in extremely low frequency (ELF) magnetic fields were estimated. The investigations clearly showed that there are two loops, the CPM-lead-tissue-loop and the body loop, responsible for the magnitude of the disturbance voltage on the input of a cardiac pacemaker. The effective induction loop areas rangedfrom 100 to 221 cm2. For a left pectorally implanted, atrially controlled CPM system the interference thresholds for the magnetic induction lay between 16 and 552 micro T (RMS) for frequencies of the magneticfield between 10 and 250 Hz. Thus, there is a limited possibility for an interference of implanted CPM by ELF magnetic fields in everyday life.     

The exchange of functional domains among aquaporins with different transport characteristics

 Aquaporins are transmembrane proteins that contain six bilayer-spanning domains, connected by loops A to E. The hourglass model predicts that the conserved loops B and E are essential for the formation of the water pore. To test the importance of loops B and E in the determination of the transport characteristics, we exchanged loops B and/or E between AQP0, AQP2, and AQP3. Detailed functional, immunoblot and immunocytochemical analyses of expression in Xenopus oocytes revealed that six out of the nine chimeric aquaporin proteins were not functional, because of misrouting. AQP0 with loop E of AQP2 was not impaired in its routing and revealed a low water permeability equal to that of wild-type AQP0. AQP2 with loop B of AQP0 was also routed normally and gave a high water permeability, similar to that of wild-type AQP2. AQP0 with loops B and E of AQP2 (AQP0–2BE) did not result in an increase in water permeability and was partly misrouted. However, the plasma membrane expression was high enough to expect an increase in water permeability, as loops B and E of AQP2 confer AQP2’s water permeability to AQP0. Although it is unclear for the dual chimera (AQP0–2BE), the parental water permeabilities obtained in oocytes expressing AQP0 with loop E of AQP2 or AQP2 with loop B of AQP0 indicate that, besides loops B and E, other parts of the AQP protein are important in the determination of the characteristics of the channel. Received: 24 February 1998 / Received after revision: 20 April 1998 / Accepted: 28 April 1998     

New method for evaluation of lung lymph flow rate with intact lymphatics in anaesthetized sheep

Aim: Lung lymph has commonly been studied using a lymph fistula created by tube cannulation into the efferent duct of the caudal mediastinal node in sheep. In this method, the tail region of the caudal mediastinal node is resected and the diaphragm is cauterized to exclude systemic lymph contamination, and cannulation is performed into one of the multiple efferent ducts originating from the caudal mediastinal node. Moreover, the pumping activity of lymphatics might be diminished by cannulation. Therefore, the purpose of the study was to evaluate the flow rate of lung lymph with maintenance of intact lymphatic networks around the caudal mediastinal node to the thoracic duct in sheep. Methods: An ultrasound transit‐time flow meter was used to measure lung lymph flow. The thoracic duct was clamped just above the diaphragm and the flow probe was attached to the thoracic duct just after the last junction with an efferent duct from the caudal mediastinal node. The lung lymph flow rate was measured at baseline and under conditions of lung‐oedema formation. Results: The baseline lung lymph flow rate in our model was three‐ to sixfold greater than values obtained with the cannulation method. With oedema‐formation, the lung lymph flow rate was the same as that measured using cannulation. Conclusion: The lung lymph flow was unexpectedly large under the conditions of the study, and our data suggest that the drainage effect of lymphatics is significant as a safety factor against pulmonary oedema formation.     

Extracorporeal magnet perivascular electromagnetic flow meters

Summary The interrupted resonance circuit proved effective in combination with an extracorporeal magnet in permitting electromagnetic blood flow measurements to be made in branch arteries of dogs by application of perivascular cuffs equipped with pick-up electrodes contacting the vessel wall. A four-turn coil, acting as a transformer secondary measures the amplitude of the magnetic field component which is effective in inducing the flow signal and thus permits calibrations for arbitrary orientations of the perivascular cuff relative to the magnetic field of the extracorporeal magnet. Recordings of mean and phasic blood flow in dogs' arteries provide illustrations of effectiveness in pharmacological studies and exhibit the reliability of the non-occlusive zero-flow base line obtained by de-energizing the magnet.     

The radiation from an electrically small circular wire loop implanted in a dissipative homogeneous spherical medium

Radio-frequency rediation properties of the implanted transmitter were studied. Two models using spheres representing the biological body were studied theoretically; one is for current loop without insulation, and the other, a loop housed in a small spherical insulation embedded in the lossy sphere. The observed anomalous phenomenon, that received rf field is increased after transmitter is embedded in a biological body, has been explained theoretically. Increase of rf field occurs when effective wavelength approaches diameter of sphere and loss tangent of medium is less than or near 1. When loop is at center of spherical body, the radiation pattern is closely the same as that of loop in free space. When loss tangent is less than 1, maximum field strength occurs if effective wavelength is equal to diameter of sphere. When loop is off-centered, orientation of the loop is found to be important in determining field strength and radiation pattern. If the magnetic dipole moment direction of loop is perpendicular to radial axis of the sphere, rf field is 5≈10 times stronger than when the dipole direction is parallel. These results will be useful in the design of implant telemetry systems.     

A counter-current system of the surface of the renal cortex of rats

Summary Using the techniques of Lissamine Green passage and micropuncture on the renal cortex of rats we were able to relate a vascular welling point (branching point of vas efferens) to one nephron. The loops of the nephrons studied on the surface were numbered sequentially in the direction of urinary flow. The distance from each loop to its vascular welling point was measured. As the distance between the loops and their glomerulus increased the distance to their welling point decreased. From these observations it was possible to conclude that the flow of blood was opposite to the urine flow. Under direct observation a counter-current flow was noticed in 72.5% of the peritubular capillaries and tubular loops; in 10.5% the flow was in the same direction, and in 17.0% no predominant direction of flow could be seen (n=131).By intravital microphotography and micropuncture as well as injection studies and maceration techniques the vas efferens was followed along the remaining portions of the nephron: The nephrons on the surface were served by their own vasa efferentia which formed their welling point near the end part of the proximal convolution. Flow in the peritubular capillaries was directed from the welling point towards the beginning of the proximal convolution; the urine flow was in the opposite direction.The physiological purpose of such a cortical counter-current system would seem to maintain a gradient of pressure between tubular and vascular system along the proximal convolution. A model is presented showing the interaction between intracapillary hydrostatic pressure, interstitial hydrostatic pressure, intracapillary colloidal osmotic pressure and interstitial colloidal osmotic pressure.with the technical assistance ofJoachim Kille     

Bacterial overgrowth and mucosal changes in isolated (Thirty-Vella) ileal loops in rabbits. Effects of intraluminal antibiotics

Atrophy of villi, with increases in crypt depth and Paneth cell number and size, occurs in chronically isolated (Thirty-Vella) ileal loops in rabbits. These loops are known to be heavily colonized with aerobic bacteria. To study the possible effect of the bacterial overgrowth, 2 experiments were performed. In the first study, two isolated ileal loops were created in each of 14 rabbits. The antibiotic loop was flushed with nonabsorbable antibiotics (neomycin, bacitracin, and gentamycin), whereas the control loop was flushed with saline. The antibiotic solution achieved a reduction in bacterial growth as compared to the loops flushed with saline. In the second study, a single ileal loop was created in each of 20 rabbits. Loops of 11 animals were flushed with an absorbable antibiotic (cefoxitin) and gentamycin, whereas those in 9 other rabbits were flushed with saline. This antibiotic combination achieved an essentially sterile loop. In both experiments, the Paneth cell population and crypt depth were less in antibiotic loops as compared to saline loops, whereas the degree of villus atrophy was nearly equal. These studies suggest a link between the overgrowth of bacteria seen in these isolated loops and the morphologic changes in the crypts.     

Analytical modeling provides new insight into complex mutual coupling between surface loops at ultrahigh fields

Ultrahigh‐field (UHF) (≥7 T) transmit (Tx) human head surface loop phased arrays improve both the Tx efficiency (B ₁⁺/√P ) and homogeneity in comparison with single‐channel quadrature Tx volume coils. For multi‐channel arrays, decoupling becomes one of the major problems during the design process. Further insight into the coupling between array elements and its dependence on various factors can facilitate array development. The evaluation of the entire impedance matrix Z for an array loaded with a realistic voxel model or phantom is a time‐consuming procedure when performed using electromagnetic (EM) solvers. This motivates the development of an analytical model, which could provide a quick assessment of the Z‐matrix. In this work, an analytical model based on dyadic Green's functions was developed and validated using an EM solver and bench measurements. The model evaluates the complex coupling, including both the electric (mutual resistance) and magnetic (mutual inductance) coupling. Validation demonstrated that the model does well to describe the coupling at lower fields (≤3 T). At UHFs, the model also performs well for a practical case of low magnetic coupling. Based on the modeling, the geometry of a 400‐MHz, two‐loop transceiver array was optimized, such that, by simply overlapping the loops, both the mutual inductance and the mutual resistance were compensated at the same time. As a result, excellent decoupling (below ?40 dB) was obtained without any additional decoupling circuits. An overlapped array prototype was compared (signal‐to‐noise ratio, Tx efficiency) favorably to a gapped array, a geometry which has been utilized previously in designs of UHF Tx arrays.     

Rabbit model for mucosal immunity in the bowel: I. Establishment of virus-infected ileal loops

An animal model was designed for use in studies of initial cellular immune responses to virus infection of the intestinal mucosa. The animal chosen was the New Zealand White rabbit and the mucosal site the subterminal ileum, isolated in a Thirty-Vella loop. The antigen used was parainfluenzavirus type 3, which would normally be destroyed by bile salts if ingested. Loops approximately 20 cm in length, each containing at least one Peyer's patch, were exteriorised through left paramedian stomata. Atrophic changes began to appear in the loops by 7 days, but no observable diminution in their associated lymphoid tissues was evident. The genesis of parainfluenzavirus type 3 infection in the loops was monitored by assay of sequential loop washings for infectious virus and in fluorescent antibody studies of cells from infected loop epithelia. Infectious virus was recovered for up to 13 days after inoculation and specific intracytoplasmic immunofluorescence was detected in loop epithelial cells. There was little serological evidence of systemic spread of the virus. A localised cellular immune response against parainfluenzavirus type 3 was mounted in the lymphoid tissues associated with the infected loops by day 14, but was not detected in systemic lymphoid tissues. No reactivity was detected in rabbits given inactivated virus via their loops or in those receiving infectious virus intravenously. This model appears to be capable of generating mucosal cellular responses to infection and may therefore be suitable for further studies in this field.     

A comparative study of the intestinal anatomy of deer.

The intestinal anatomy of fallow deer (Dama dama), white-tailed deer (Odocoileus virginianus), reindeer (Rangifer tarandus), and elk (Alces alces) was studied. Special attention was given to the spiral loop of the ascending colon and the proportional length of the small and large intestines. The spiral loop of the ascending colon is wound 3-3 1/2 turns in the fallow deer, 2-3 turns in the white-tailed deer, 5 1/2-7 turns in the reindeer and 3-4 turns in the elk. The distal half of the last centrifugal turn of the spiral loop, regularly has one additional loop in the ventro-caudal quadrant of the mesenterial disc in the white-tailed deer, and one, or sometimes two such loops in the reindeer. In the elk this part of the gut usually constitutes a greater part of the spiral loop than the coil proper, and runs in several irregular loops and small coils between the coil proper and the jejunal attachment. Irregular patterns of the coil proper are common, and occur in all four species. The mean relative length of the large intestine was about 52% of the small intestine in the reindeer and 55-57% in the other species, which indicates, that significant differences in gut proportions do not exist in the deer species studied, perhaps with an exception for the reindeer.     

Flowmeter for slow-flowing physiological liquids

We have developed a device to measure continuously the slow flow of fluid secreted by the fungs of foetal lambs. Typically this fluid flows along the trachea at 5–10 ml kg⁻¹h⁻¹ Intermittent retardation of flow is thought to facilitate prenatal lung development. The flowmeter is inserted into an extracorporeal loop of tubing inserted into the foetal trachea. A differential U-tube pressure sensor, employing an infra-red optical system to detect movement of a meniscus, is inserted into the loop. The optical detector controls a miniature, reversible peristaftic pump which is connected in paralfel with the U-tube. The pump is activated in such a way as to correct deviations in the level of the meniscus and flow measurements are thus obtained from the rate and direction of pump motor rotation. Instantaneous flow rate and integrated flow can be recorded simultaneously. The device has proven reliable over long periods of use and could be used to measure the movement of many slow-flowing physiological liquids     

Chemical control stability in the elderly

The prevalence of central apnoea and periodic breathing is increased in the elderly. This implies that the chemical control of breathing might become less stable with ageing. To investigate this, we measured loop gain in healthy elderly individuals using proportional assist ventilation. Loop gain is an engineering term that describes the stability of a system controlled by feedback loops, such as the respiratory control system. A loop gain close to zero indicates a stable system, whereas a loop gain close to or greater than one indicates an unstable system. Eleven healthy elderly subjects were studied with a mean ± s.d. age and body mass index (BMI) of 71 ± 5 years and 25 ± 3 kg m⁻², respectively. We also studied a small group of elderly individuals with obstructive sleep apnoea (OSA) for comparison ( n = 3, age 68 ± 1 years, BMI 32 ± 11 kg m⁻²). Comparisons were made with previously studied young individuals (age 27 ± 4 years, BMI 23 ± 1 kg m⁻²). We found significantly lower loop gains in the healthy elderly group (loop gain ≤ 0.25) compared with the young group (loop gain ≤ 0.47, P = 0.001). Also, we found quite low loop gains in the elderly OSA group (loop gain ≤ 0.26). We conclude that the chemical control of breathing does not become unstable with ageing and is thus an unlikely cause of central (and possibly obstructive) apnoeas in this population.     

磁场对果蝇唾腺染色体的影响

本文探讨了磁场对果蝇唾腺染色体的影响，研究的结果表明：经不同强度的恒磁场或脉冲磁场作用过的果黾三龄幼虫，其唾腺染色体在右2染色体的上端，靠近染色中心的部位，易产生缺失环，随着磁场强度的增加，产生缺失环的频率升高，这种现象可以由亲代遗传给子一代，子二代，同时又表现出自身修复的能力。     

Measurement of electric fields due to time-varying magnetic field gradients using dipole probes

The operation of dipole probes in measuring electric fields in conductive media exposed to temporally varying magnetic fields is discussed. The potential measured by the probe can be thought of as originating from two contributions to the electric field, namely the gradient of the scalar electric potential and the temporal derivative of the magnetic vector potential. Using this analysis, it is shown that the exact form of the wire paths employed when using electric field probes to measure the effects of temporally varying magnetic fields is very important and this prediction is verified via simple experiments carried out using different probe geometries in a cylindrical sample exposed to a temporally varying, uniform magnetic field. Extending this work, a dipole probe has been used to measure the electric field induced in a cylindrical sample by gradient coils as used in magnetic resonance imaging (MRI). Analytic solutions for the electric field in an infinite cylinder are verified by comparison with experimental measurements. Deviations from the analytic solutions of the electric field for the x-gradient coil due to the finite length of the sample cylinder are also demonstrated.