Cardiac Activation Mapping using Ultrasound Current Source Density Imaging.

Intracardiac ablation procedures to correct drug-resistant arrhythmias require accurate maps of cardiac activation. Conventional methods are time-consuming and have poor spatial resolution (5- 10 mm). The goal of this dissertation was to develop a new method, Ultrasound Current Source Density Imaging (UCSDI), to map biological currents. UCSDI is based on the acousto-electric (AE) effect, a modulation of the electric resistivity by acoustic pressure. If a current passes through the focal region of an ultrasound transducer, a voltage modulated at the ultrasonic frequency can be measured with a pair of electrodes located distal to the focal zone. By sweeping the focal zone, UCSDI can map a distributed current field. UCSDI has several potential advantages as a technique for mapping cardiac activation currents: high spatial resolution determined by the typically sub-mm focal characteristics of the ultrasound beam, short mapping time using electronically steered ultrasonic beams, and automatic registration with B-mode ultrasound images without sophisticated mathematical algorithms. UCSDI was first validated by mapping an artificially generated 2D current distribution. It was compared to sequential electrode mapping, computer simulation as well as to an inverse algorithm. In this study it was possible to use UCSDI to locate monopolar current sources to within 1-mm of their true locations without making any prior assumptions about the source geometry. UCSDI was then used to detect and map biological currents in an isolated rabbit heart. Both UCSDI and normal low frequency electrocardiograms (ECG) were measured simultaneously by tungsten electrodes embedded in the left ventricle. The motion of the heart was significantly reduced by perfusing it with an excitation contraction de-coupler. Measured UCSDI maps showed temporal and spatial patterns consistent with a spreading activation wave and timing consistent with normal ECG signals. UCSDI was then combined with ultrasonic strain imaging in a new method for electromechanical imaging. This combined method was used to make localized measurements of electromechanical delay. This method could be useful in cardiac resynchronization therapy for placing pacemaker leads.Ph.D.Biomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60687/1/rolafsso_1.pd

Application of acoustic-electric interaction for neuro-muscular activity mapping: A review

Acousto-electric interaction signal (AEI signal) resulting from interaction of acoustic pressure wave and electrical current field has received recent attention in biomedical field for detection and registration of bioelectrical current. The signal is very of small value and brings about several challenges when detecting it. Several observations has been done in saline solution and on nerves and tissues under controlled condition that give optimistic indication about its utilization. Ultrasound Current Source Density Imaging (UCSDI) has been introduced, that uses the AEI signal to image the current distribution. This review provides an overview of the investigations on the AEI signal and USCDI imaging that has been made, their results and several considerations on the limitations and future possibilities on using the acousto-electric interaction signal

Contactless Remote Induction of Shear Waves in Soft Tissues Using a Transcranial Magnetic Stimulation Device

This study presents the first observation of shear wave induced remotely within soft tissues. It was performed through the combination of a transcranial magnetic stimulation device and a permanent magnet. A physical model based on Maxwell and Navier equations was developed. Experiments were performed on a cryogel phantom and a chicken breast sample. Using an ultrafast ultrasound scanner, shear waves of respective amplitude of 5 and 0.5 micrometers were observed. Experimental and numerical results were in good agreement. This study constitutes the framework of an alternative shear wave elastography method

Cancellous Bone Density Evaluation using Ultrasound Backscatter from an Imaging System: Exploring the Possibility for Fetal Bone Density Evaluation

Osteoporosis is a common chronic disease and a well known major source of morbility and mortality among the elderly. Low bone density also occurs in infants and small children during development and can be problematically excessive if the fetus experiences issues during pregnancy such as malnutrition, lack of vitamin D and smoking. Currently the only available methodologies for fetal bone density evaluation are Dual-energy X-ray Absorptiometry (DEXA) or Magnetic Resonance Imaging (MRI). Both are sensitive to movement artifacts. DEXA exposes the subjects to significant radiation so is not suggested during pregnancy. Quantitative MRI is noisy, expensive, slow (8-20 mins) and the effects of high field strengths on the developing fetus is unknown. Therefore, the goal of this study is to find a fast, accurate and non-ionizing method for the evaluation of fetal bone density. In this study, the quantitative ultrasound backscatter coefficient (BSC) was chosen to evaluate bone density using the B-mode ultrasound system. Compared with the speed of sound and ultrasound attenuation in the traditional ultrasound measurement for bone density, the backscatter method is more accessible to central sites such as the human spine and fetal femur bone. Additionally, it has a rapid path to commercialization with the potential to be added as a new feature in the current commercial ultrasound imaging systems for bone density evaluation. The contributions of this work are: 1. A simulation study was accomplished that compared backscatter coefficients from a single element transducer, a linear array transducer, and a curved array transducer with the change of trabecular thickness and trabecular spacing. An overall similar Pearson correlation (single: R = 0.94, SD = 10.84dB, linear: R = 0.92, SD = 6.6dB, curved: R=0.95, SD=6.89dB) between the BSC and porosity was found from three transducers, but the standard deviation (SD) was smaller from the two array probes. This improved standard deviation may result from the wider spatial range of the array transducers. 2. A simulation model using COMSOL for the fetal bone density evaluation was built based on the Biot’s poroelastic theory and the backscatter coefficient. The theoretical backscatter coefficient from the Biot model was calculated with the best available biomechanical parameters from the human femoral cancellous bone and the geometrical features of the fetal femur. This work also proposed a method for compensating the ultrasound signal attenuation from abdominal tissue, femur tissue, amniotic fluid between the probe and fetal femur. The result showed good correlation of BSC (R = -0.9970, P = 2.0058e^-04, SD = 10.21%) and apparent integrated backscatter (AIB) (R = -0.9469, P = 0.0146, SD = 10.62%) with the porosity. This suggests in vivo ultrasound bone evaluation could be implemented in the current commercial ultrasound B-mode systems. 3. An in vitro study was conducted that compared the backscatter coefficient (BSC), the apparent integrated backscatter (AIB) and the Spectrum Centroid Shift (SCS) from the fundamental backscatter signal and the second harmonics of the ultrasound imaging system. The result from the second harmonics (R : BSC = 0.7374, AIB = 0.6243, SCS =-0.6421) showed better correlation than the fundamental backscatter (R : BSC = 0.7055, AIB = 0.5393, SCS = -0.5858) with a gold standard bone mineral density obtained from DEXA scans of the same samples. An analysis from the Farran cylindrical model and the second harmonics of a rigid cylinder showed the second harmonics has less noise and showed better performance than the fundamental backscatter approach. In conclusion, the backscatter coefficient from ultrasound imaging showed good correlation in both the simulation studies and the in vitro study. It has the potential to be a convenient, fast, cheap methodology for adult and fetal bone density evaluation

Current Density Imaging through Acoustically Encoded Magnetometry: A Theoretical Exploration

The problem of determining a current density confined to a volume from measurements of the magnetic field it produces exterior to that volume is known to have non-unique solutions. To uniquely determine the current density, or the non-silent components of it, additional spatial encoding of the electric current is needed. In biological systems such as the brain and heart, which generate electric current associated with normal function, a reliable means of generating such additional encoding, on a spatial and temporal scale meaningful to the study of such systems, would be a boon for research. This paper explores a speculative method by which the required additional encoding might be accomplished, on the time scale associated with the propagation of sound across the volume of interest, by means of the application of a radially encoding pulsed acoustic spherical wave

Gradient-based quantitative image reconstruction in ultrasound-modulated optical tomography: first harmonic measurement type in a linearised diffusion formulation

Ultrasound-modulated optical tomography is an emerging biomedical imaging modality which uses the spatially localised acoustically-driven modulation of coherent light as a probe of the structure and optical properties of biological tissues. In this work we begin by providing an overview of forward modelling methods, before deriving a linearised diffusion-style model which calculates the first-harmonic modulated flux measured on the boundary of a given domain. We derive and examine the correlation measurement density functions of the model which describe the sensitivity of the modality to perturbations in the optical parameters of interest. Finally, we employ said functions in the development of an adjoint-assisted gradient based image reconstruction method, which ameliorates the computational burden and memory requirements of a traditional Newton-based optimisation approach. We validate our work by performing reconstructions of optical absorption and scattering in two- and three-dimensions using simulated measurements with 1% proportional Gaussian noise, and demonstrate the successful recovery of the parameters to within +/-5% of their true values when the resolution of the ultrasound raster probing the domain is sufficient to delineate perturbing inclusions.Comment: 12 pages, 6 figure