Thoratec transcutaneous energy transformer system: a review and update

A transcutaneous energy transformer system (TETS) can provide power to an implanted ventricular assist device (VAD) across an unbroken layer of skin. A TETS includes a subcutaneous secondary coil, which traditionally connects to remote power conditioning circuitry located to avoid eddy current losses and heating that occur in metal near operating TETS coils. Litz wire, used to construct the coil and connect it to that circuitry, efficiently conducts the high frequency alternating current but is bulky and stiff. A novel concept (US Patent No. 6,327,504 B1) packages the secondary coil's output power conditioning circuitry within the unused aperture of the coil while minimizing eddy current losses. The concept allows use of a more flexible cable for its direct current power output. The result is improved reliability, functionality, and efficiency along with decreased implant volume and a thinner, more flexible lead system to interconnect to the VAD. This in turn enhances system versatility by expanding sites available for module implantation. A TETS using this concept has demonstrated efficiency exceeding 80% and peak power outputs of 45 W with good tissue compatibility in the bovine model after a 30 day implant.     

Feedback control of TET system with variable coupling coefficients for a novel artificial anal sphincter

For treating severe faecal incontinence, the authors developed an intelligent artificial anal sphincter system (AASS) equipped with a feedback sensor that utilized a transcutaneous energy transfer system (TETS). To deliver the correct amount of power (i.e. to match the load demand under variable coupling conditions caused by changes in positioning between the coils due to fitting and changes in posture), a regulating method to stabilize output voltage with a closed loop variable-frequency controller was developed in this paper. The method via which the voltage gain characteristics of a voltage-fed series-tuned TETS were derived is also described. The theoretical analysis was verified by the results of the experiment. A numerical analysis method was used as a control rule with respect to the relationship between operating frequency and output voltage. To validate the feedback control rules, a prototype of the TET charging system was constructed, and its performance was validated with the coupling variation between 0.12–0.42. The results show that the output voltage of the secondary side can be maintained at a constant 7?V across the whole coupling coefficient range, with a switching frequency regulation range of 271.4–320.5?kHz, and the proposed controller has reached a maximal end-to-end power efficiency of 67.5% at 1?W.     

Improvement of wireless power transmission efficiency of implantable subcutaneous devices by closed magnetic circuit mechanism

Induction coils were fabricated based on flexible printed circuit board for inductive transcutaneous power transmission. The coil had closed magnetic circuit (CMC) structure consisting of inner and outer magnetic core. The power transmission efficiency of the fabricated device was measured in the air and in vivo condition. It was confirmed that the CMC coil had higher transmission efficiency than typical air-core coil. The power transmission efficiency during a misalignment between primary coil and implanted secondary coil was also evaluated. The decrease of mutual inductance between the two coils caused by the misalignment led to a low efficiency of the inductive link. Therefore, it is important to properly align the primary coil and implanted secondary coil for effective power transmission. To align the coils, a feedback coil was proposed. This was integrated on the backside of the primary coil and enabled the detection of a misalignment of the primary and secondary coils. As a result of using the feedback coil, the primary and secondary coils could be aligned without knowledge of the position of the implanted secondary coil.     

Automatic detection of linear artifacts in medical images

Our purpose in this study is to describe an algorithm for the automatic detection of linear artifacts in medical images. Linear artifacts arise as a result of many different forms of tissues and tissue boundaries within the imaging volume. Additionally, linear artifacts can arise for artificial structures such as radioactive seeds and radioactive linear sources. It is the purpose of the described algorithm to automatically detect linear artifacts of a certain length and diameter. The algorithm was written and compiled on a Pentium-4 based computer in the Microsoft Visual C/C++ language. Inert coils supplied by Radiomed Inc. were implanted into a standard prostate ultrasound phantom. Transaxial ultrasound images of the implanted phantom were obtained at 2 mm increments. The coded algorithm was then applied to the ultrasound imaging volume to automatically segment out the implanted coils. Thirteen coils were implanted in the prostate phantom. Thirteen coils were automatically identified in the imaging volume. An algorithm was developed to automatically determine the position and orientation of radioactive coils within an imaging volume. The algorithm successfully identified thirteen coils implanted in an ultrasound prostate phantom.     

新型人工肛门括约肌系统及初步动物实验研究

目的研究可用于治疗肛门失禁的植入式人工肛门括约肌系统（artificial anal sphincter system,AASS）。方法系统由体内执行、体外控制和经皮能量传输3个模块构成,体内外通过无线模式通信。当体内直肠壁上压力超出阈值后,向体外发送排便报警信号,患者通过体外控制模块控制体内执行模块打开或关闭直肠,采用经皮能量传输模块为体内电池充电。通过1例猪的动物实验,评估在活体环境下系统各模块的可靠性和稳定性。15d后将动物解剖,观察组织变化并进行体内的腹泻实验和血供实验。结果植入体内后,体外控制模块可实时控制体内执行模块的各项操作,通过经皮能量传输充电3h,在体内可持续工作24h。拟腹泻状态下,对肠壁施加7．16kPa的压力（正常肛肠静息压为6．26～9．47kPa）,能有效抑制肠道内容物渗漏。结论AASS可以模拟正常人体肛门括约肌功能,为严重肛门失禁的患者提供了一种新的治疗方法。     

The use of an MEG device as 3D digitizer and motion monitoring system

An algorithm is described that localizes a set of simultaneously activated coils using MEG detectors. These coil positions are used for continuous or intermittent head position registration during long MEG sessions, to coregistrate MR and MEG data and to localize EEG electrodes attached to the scalp, when EEG and MEG are recorded simultaneously. The algorithm is based on a mathematical model in which the coils are described as stationary magnetic dipoles with known source time functions. This knowledge makes it possible to detect and remove bad channels automatically. It is also assumed that the source time functions are orthogonal. Therefore, the localization problem splits into independent localization problems. for each coil. The method is validated in a phantom experiment, where the relative coil positions were known. From this experiment it is found that the average error is 0.25 cm. An error of 0.23 cm was found in an experiment where 64 electrode positions were measured four times independently. Examples of the applications of the method are presented. Our method eliminates the use of an external 3D digitizer and maps the MEG directly onto other modalities. This is not only a practical advantage, but it also reduces the gross registration error. Furthermore, head motions can be monitored and MEG data can be corrected for these motions.     

Changes in EEG associated with sleep-awake behavior in young adult versus aged adult Fischer-344 rats

Age-related spontaneous cortical and hippocampal EEG changes associated with level of arousal in rats were investigated. The EEG of rats with chronically implanted electrodes were recorded using bipolar leads and simultaneously analyzed with a computer to obtain the power spectra. In the awake stage, EEGs in the aged rats consisted predominantly of slow waves when compared with those in the young rats. The cortical EEG activity of the aged rats exhibited specific irregular burst waves which consisted of two spectral peaks at 8-9 Hz and 15-16 Hz during the drowsy period. The relative powers of the cortical and hippocampal waves in the aged rats differed from those of the young rats during the slow-wave sleep stage. These findings suggest that the slowing of the EEG in the rats during the awake stage may be related to decreased brain activity associated with aging, and that the irregular burst waves in the cortical EEG in the aged rats appear to correlate with the changes observed in age-related human sleep patterns.     

Frames of reference for gaze saccades evoked during stimulation of lateral intraparietal cortex

Previous studies suggest that stimulation of lateral intraparietal cortex (LIP) evokes saccadic eye movements toward eye- or head-fixed goals, whereas most single-unit studies suggest that LIP uses an eye-fixed frame with eye-position modulations. The goal of our study was to determine the reference frame for gaze shifts evoked during LIP stimulation in head-unrestrained monkeys. Two macaques (M1 and M2) were implanted with recording chambers over the right intraparietal sulcus and with search coils for recording three-dimensional eye and head movements. The LIP region was microstimulated using pulse trains of 300 Hz, 100-150 microA, and 200 ms. Eighty-five putative LIP sites in M1 and 194 putative sites in M2 were used in our quantitative analysis throughout this study. Average amplitude of the stimulation-evoked gaze shifts was 8.67 degrees for M1 and 7.97 degrees for M2 with very small head movements. When these gaze-shift trajectories were rotated into three coordinate frames (eye, head, and body), gaze endpoint distribution for all sites was most convergent to a common point when plotted in eye coordinates. Across all sites, the eye-centered model provided a significantly better fit compared with the head, body, or fixed-vector models (where the latter model signifies no modulation of the gaze trajectory as a function of initial gaze position). Moreover, the probability of evoking a gaze shift from any one particular position was modulated by the current gaze direction (independent of saccade direction). These results provide causal evidence that the motor commands from LIP encode gaze command in eye-fixed coordinates but are also subtly modulated by initial gaze position.     

Primary side control of load voltage for transcutaneous energy transmission

Transcutaneous energy transmission (TET) is considered as a good way to wirelessly power the implanted devices in human bodies. The load voltage provided from the TET to the implanted device should be kept stable to ensure the device working well, which however, is easily affected by the required power variation for different body movements and coil-couple malposition accompanying skin peristalsis. Commonly, the load voltage applied onto the device should be measured and feedback for power is regulated by implanting sensing and communication units into the body, which causes additional energy cost, increased size and weight of the implanted device. This paper takes the TET for artificial heart as an example to propose a novel primary side control method of the load voltage for TET, which does not require any additional implanted components. In the method, sensing coils are used to measure the malposition between the transmitter coil (TC) and receiver coil, and the magnitude of the TC current outside the human body. The measurement results are used to estimate the load voltage inside the body through calculation, whose value provide a base to develop a PI control system to regulate the input power of TET for the load voltage stability. The proposed method is experimentally validated on an actual TET for artificial heart by varying its load in a wide range under serious coil-couple malposition. With applying the primary side control, the variation of the load voltage is reduced to only 25 % of that without the control.     

Location estimation of an in vivo robotic capsule relative to arrayed power transmission coils

A tracking method is presented here for an in vivo robotic capsule with power supplied from one of the multiple power transmission coils. The proposed method aims to select the best coupled coil among the array of power transmission coils. It relies on the fact that the driving current of the power transmitter increases with inductive coupling of the receiver coil inside the capsule with the transmitter coil. Investigation of the current increase characteristic according to its location relative to the transmission coils allows development of a strategy for the in vivo robotic capsule. This study shows results with two transmission coils and a two-dimensional power receiver. Experimental results present the possibility of selecting the best coil by estimating the relative location of the capsule.     

Maintenance of the circulation in vivo by an implanted automatically controlled artificial heart

An artificial heart in which the output is automatically controlled by the venous return, when implanted into dogs in the orthotopic position was found to be highly sensitive to changes in the hemodynamic conditions (a decrease in the venous return, hypo- and hypervolemia, increased peripheral resistance) and prevented the development of critical situations. For 24 h the principal vital functions of the experimental animals were maintained.     

Decibel Attenuation of Pulsed Electromagnetic Field (PEMF) in Blood and Cortical Bone Determined Experimentally and from the Theory of Ohmic Losses

We studied the PEMF power attenuation in tissues representative of clinical applications (blood and cortical bone) to determine the amount of power available for PEMF purported biological effects. The experimental system consisted of a pair of nearly circular, parallel and coaxial coils separated by a distance of one coil diameter. The power attenuation was measured using a small search coil connected to a digital oscilloscope. The coils were powered by a voltage switch operating at two different frequencies (3.8 and 63 kHz) producing bursts of pulses (numbering 21 and 1619) and triggered at two different frequencies (1.5 and 15 Hz, respectively). The tissue samples were placed inside the coils so as to expose them to either transverse electric field (at the center of coils) or the transverse magnetic field (at the coil wire). The cylindrical coil geometry yielded closed-form expressions for power attenuation based on magnetic diffusion equation and ohmic losses due to bulk tissue magnetic permeability and electrical conductivity. The measured power attenuation at these PEMF frequencies of not more than one decibel (1 dB) was well explained by the theory for the 3.8 kHz but less so for the 63 kHz frequency PEMF. The results provide important insights regarding physical mechanism of weak PEMF power dissipation in tissues.     

Coil optimization for magnetic brain stimulation

Further development of magnetic brain stimulation requires smaller coils, smaller power consumption, and less coil heating. This study addresses the optimization of the complete stimulator and in particular the coil. We describe the coil structure in terms of simple mathematical functions and examine the influence of changes in the structure on several figures of merit. A few optimal coil geometries suitable for repetitive brain stimulation are depicted. It is demonstrated that today’s coils are far from optimal and that, for instance, the power consumption can be reduced remarkably from the level of today's equipment. Improvements may act as a springboard toward new applications.     

基于射频感应控制的掌指人工关节研究

针对目前临床上存在大量掌指臂丛神经损伤患者无法医治的状况，提出了通过植入带有驱动电机的掌指人工关节来行使掌指功能的治疗方案。文中研究了植入式人工关节的结构、机械传动、控制信号的获取和传输等。为解决驱动电机的电源问题，作者提出了运用电磁感应的经皮能量传输技术来驱动植入电机的旋转，利用反映掌指动作意识的肌电信号来控制射频磁场的发射，使得体内线圈感应体外射频磁场后驱动电机转动。最后研究了一套临床关节植入的手术方案，并在三例尸体上进行了功能实验，验证了植入的人工关节能满足人体掌指的基本功能。     

矩量法及其在磁共振射频线圈中的应用

目的:基于矩量法,实现对具体射频线圈的电磁仿真分析。方法:对不同射频线圈进行建模,基于具体的线圈模型,选取合适的基函数对未知函数进行展开,并代入电磁场下的相应算子方程,从而实现运用矩量法对射频线圈的电磁仿真分析。本文首先通过推导给出矩量法的电磁场计算公式,再将其进行必要的化简,最后通过Matlab工具实现对具体线圈模型的仿真计算。结果:通过对头部线圈,头颈联合线圈以及术中多通道射频接收线圈的仿真,得到电磁场分布结果。结论:与基于比奥-萨法尔定律传统方法计算磁场分布相比,矩量法能够更方便地分析复杂拓扑结构的射频线圈的磁场分布,应用范围更广。同时,该方法比FDTD等方法需要的计算资源少很多,更适合工程设计人员使用。     

Measuring V1 receptive fields despite eye movements in awake monkeys

One difficulty with measuring receptive fields in the awake monkey is that even well-trained animals make small eye movements during fixation. These complicate the measurement of receptive fields by blurring out the region where a response is observed, causing underestimates of the ability of individual neurons to signal changes in stimulus position. In simple cells, this blurring may severely disrupt estimates of receptive field structure. An accurate measurement of eye movements would allow correction of this blurring. Scleral search coils have been used to provide such measurements, although little is known about their accuracy. We have devised a range of approaches to address this issue: implanting two coils into a single eye, exploiting the small size of V1 receptive fields and developing maximum-likelihood fitting techniques to extract receptive field parameters in the presence of eye movements. All our investigations lead to the same conclusion: our scleral search coils (which were not sutured to the globe) are subject to an error of approximately the same magnitude as the small eye movements which occur during fixation: SD approximately 0.1 degrees. This error is large enough to explain the SD of measured vergence in the absence of any real changes in vergence state. This, and a variety of other arguments, indicate that the real variation in vergence is much smaller than coil measurements suggest. These results suggest that monkeys, like humans, maintain very stable vergence. The error has a slower time course than fixational eye movements so that search coils report the difference in eye position between two consecutive trials more accurately than the eye position itself on either trial. Receptive field estimates are unlikely to be improved by assuming the coil record is veridical and correcting for eye position accordingly. However, receptive field parameters can reliably be determined by a fitting technique that allows for eye movements. It is possible that suturing coils to the globe reduces the artifacts, but no method has been available to demonstrate this. These receptive field measurements provide a general means by which the reliability of eye-position measurements can be assessed.     

Ten-year NEDO BVAD development program: moving forward to the clinical arena

Since 1995, the Baylor Group has been developing a totally implantable NEDO BVAD system. This 10-year program was completed in March 2005, and preparation for clinical trials is underway. This article summarizes the entire 10-year NEDO program and describes the strategy for clinical trials. The project aimed to achieve: (1) dual centrifugal pumps with the ability of full biventricular support, (2) a compact system implantable into small adults, (3) a totally implantable system with transcutaneous energy transmission system (TETS), (4) a durable system with a lifetime of over 5 years, and (5) a system free of thrombus and with minimal hemolysis. The final goals are to complete preclinical system evaluations and commence the clinical trials in the near future. In vitro studies have demonstrated a pump capacity of over 8.5 l/min and an Index of Hemolysis of <0.004 g/100 l. The pump-bearing life expectancy was over 5 years. To date, eight pumps endured in vivo studies of over 3 months without complications, including thromboembolic events. The in vitro endurance studies of eight pumps are longer than 1 year. There were no mechanical malfunctions or pump failure. A stepwise clinical trial is being planned: Step1, a wearable BVAD/VAD will be clinically studied; Step 2, the BVAD/VAD will be implanted intracorporeally without TETS; and, Step 3, a totally implantable system will be clinically evaluated. The NEDO BVAD system has completed preclinical testing. Clinical trial preparation is underway.     

A Green's function approach to local rf heating in interventional MRI

Current safety regulations for local radiofrequency (rf) heating, developed for externally positioned rf coils, may not be suitable for internal rf coils that are being increasingly used in interventional MRI. This work presents a two-step model for rf heating in an interventional MRI setting: (1) the spatial distribution of power in the sample from the rf pulse (Maxwell's equations); and (2) the transformation of that power to temperature change according to thermal conduction and tissue perfusion (tissue bioheat equation). The tissue bioheat equation is approximated as a linear, shift-invariant system in the case of local rf heating and is fully characterized by its Green's function. Expected temperature distributions are calculated by convolving (averaging) transmit coil specific absorption rate (SAR) distributions with the Green's function. When the input SAR distribution is relatively slowly varying in space, as is the case with excitation by external rf coils, the choice of averaging methods makes virtually no difference on the expected heating as measured by temperature change (deltaT). However, for highly localized SAR distributions, such as those encountered with internal coils in interventional MRI, the Green's function method predicts heating that is significantly different from the averaging method in current regulations. In our opinion, the Green's function method is a better predictor since it is based on a physiological model. The Green's function also elicits a time constant and scaling factor between SAR and deltaT that are both functions of the tissue perfusion rate. This emphasizes the critical importance of perfusion in the heating model. The assumptions made in this model are only valid for local rf heating and should not be applied to whole body heating.     

Single-Step 3-D Image Reconstruction in Magnetic Induction Tomography: Theoretical Limits of Spatial Resolution and Contrast to Noise Ratio

Magnetic induction tomography (MIT) is a low-resolution imaging modality for reconstructing the changes of the complex conductivity in an object. MIT is based on determining the perturbation of an alternating magnetic field, which is coupled from several excitation coils to the object. The conductivity distribution is reconstructed from the corresponding voltage changes induced in several receiver coils. Potential medical applications comprise the continuous, non-invasive monitoring of tissue alterations which are reflected in the change of the conductivity, e.g. edema, ventilation disorders, wound healing and ischemic processes. MIT requires the solution of an ill-posed inverse eddy current problem. A linearized version of this problem was solved for 16 excitation coils and 32 receiver coils with a model of two spherical perturbations within a cylindrical phantom. The method was tested with simulated measurement data. Images were reconstructed with a regularized single-step Gauss–Newton approach. Theoretical limits for spatial resolution and contrast/noise ratio were calculated and compared with the empirical results from a Monte-Carlo study. The conductivity perturbations inside a homogeneous cylinder were localized for a SNR between 44 and 64 dB. The results prove the feasibility of difference imaging with MIT and give some quantitative data on the limitations of the method.