Blocked Elements in 1-D and 2-D Arrays—Part I: Detection and Basic Compensation on Simulated and <italic>In Vivo</italic> Targets

During a transcostal ultrasound scan, ribs and other highly attenuating and/or reflective tissue structures can block parts of the array. Blocked elements tend to limit the acoustic window and impede visualization of structures of interest. Here, we demonstrate a method to detect blocked elements and we measure the loss of image quality they introduce in simulation and in vivo. We utilize a fullwave simulation tool and a clinical ultrasound scanner to obtain element signals from fully sampled matrix arrays during simulated and in vivo transcostal liver scans, respectively. The elements that were blocked by a rib showed lower average signal amplitude and lower average nearest-neighbor cross correlation than the elements in the remainder of the 2-D aperture. The growing receive-aperture B-mode images created from the element data indicate that the signals on blocked elements are dominated by noise and that turning them OFF has a potential to improve visibility of liver vasculature. Adding blocked elements to the growing receive apertures for five in vivo transcostal acquisitions resulted in average decrease in vessel contrast and contrast to noise ratio of 19% and 10%, respectively

Acoustic Aberration in Non-Invasive Histotripsy Therapy.

Acoustic aberration effects have been extensively studied over the years for high intensity focused ultrasound (HIFU) due to the significant therapeutic disruption they can cause in thermal ablation procedures, often rendering the treatment ineffective without the implementation of aberration correction mechanisms. Histotripsy therapy uses highly energetic cavitation bubble clouds to mechanically fractionate tissue. The cavitation bubble cloud initiation is dependent on a pressure threshold mechanism, which allows this process to be controlled by the pressure amplitude available at the focus. The work presented herein explores the therapeutic effects of acoustic aberration in the scope of histotripsy therapy and investigates the feasibility of conducting non-invasive histotripsy procedures without using correction mechanisms in distinct therapeutic contexts likely to introduce high degrees of acoustic aberration. The first context investigated is transcostal therapy. It is demonstrated that histotripsy therapy is able to generate precise lesions through rib obstacles without aberration correction despite the presence of large grating lobes in the focal profile. An in vivo study follows, in which comparable porcine liver lesions are created through windows with full and no ribcage obstruction, inducing no visible damage on overlying tissues. The second context investigates transabdominal fetal therapy. It is shown that histotripsy therapy can achieve precise fetal tissue ablation in sheep models through the intact maternal abdomen without aberration correction. A long-term study on the impact of the therapy in the course of pregnancy is conducted, indicating the potential safety of this technique for non-invasive fetal applications. The third, and perhaps the most challenging therapeutic context involves transcranial therapy. A large aperture array transducer is specifically designed, and a novel sonication mechanism using extremely short pulses with large negative pressures is introduced. It is shown that precise cavitation lesions can be successfully generated transcranially by careful modulation of the focal pressure amplitude. Overall results show that the pressure threshold mechanism governing the initiation of histotripsy bubble clouds lends the therapy a considerable degree of immunity against acoustic aberration effects, a feature, which in combination with the low thermal impact of the therapy would be highly beneficial for a variety of non-invasive therapeutic applications.PhDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/100071/1/yohankim_1.pd

Thérapies ultrasonores cardiaques guidées par élastographie et échographie ultrarapides

Atrial fibrillation (AF) affects 2-3% of the European and North-American population, whereas ventricular tachyarrhythmia (VT) is related to an important risk of sudden death. AF and VT originate from dysfunctional electrical activity in cardiac tissues. Minimally-invasive approaches such as Radio-Frequency Catheter Ablation (RFCA) have revolutionized the treatment of these diseases; however the success rate of RFCA is currently limited by the lack of monitoring techniques to precisely control the extent of thermally ablated tissue.The aim of this thesis is to propose novel ultrasound-based approaches for minimally invasive cardiac ablation under guidance of ultrasound imaging. For this, first, we validated the accuracy and clinical viability of Shear-Wave Elastography (SWE) as a real-time quantitative imaging modality for thermal ablation monitoring in vivo. Second we implemented SWE on an intracardiac transducer and validated the feasibility of evaluating thermal ablation in vitro and in vivo on beating hearts of a large animal model. Third, a dual-mode intracardiac transducer was developed to perform both ultrasound therapy and imaging with the same elements, on the same device. SWE-controlled High-Intensity-Focused-Ultrasound thermal lesions were successfully performed in vivo in the atria and the ventricles of a large animal model. At last, SWE was implemented on a transesophageal ultrasound imaging and therapy device and the feasibility of transesophageal approach was demonstrated in vitro and in vivo. These novel approaches may lead to new clinical devices for a safer and controlled treatment of a wide variety of cardiac arrhythmias and diseases.La fibrillation atriale affecte 2-3% des européens et nord-américains, les tachycardies ventriculaires sont liées à un risque important de mort subite. Les approches minimalement invasives comme l’Ablation par Cathéter Radiofréquence (RFCA) ont révolutionné le traitement de ces maladies, mais le taux de réussite de la RFCA est limité par le manque de techniques d’imagerie pour contrôler cette ablation thermique.Le but de cette thèse est de proposer de nouvelles approches ultrasonores pour des traitements cardiaques minimalement invasifs guidés par échographie.Pour cela nous avons d’abord validé la précision et la viabilité clinique de l’Élastographie par Ondes de Cisaillement (SWE) en tant que modalité d’imagerie quantitative et temps réel pour l’ablation thermique in vivo. Ensuite nous avons implémenté la SWE sur un transducteur intracardiaque et validé la faisabilité d’évaluer l’ablation thermique in vitro et in vivo sur cœur battant de gros animal. Puis nous avons développé un transducteur intracardiaque dual-mode pour effectuer l’ablation et l’imagerie ultrasonores avec les mêmes éléments, sur le même dispositif. Les lésions thermiques induites par Ultrasons Focalisés de Haute Intensité (HIFU) et contrôlées par la SWE ont été réalisées avec succès in vivo dans les oreillettes et les ventricules chez le gros animal. Finalement la SWE a été implémentée sur un dispositif d’imagerie et thérapie ultrasonores transœsophagien et la faisabilité de cette approche a été démontrée in vitro et in vivo. Ces approches originales pourraient conduire à de nouveaux dispositifs cliniques pour des traitements plus sûrs et contrôlés d’un large éventail d’arythmies et maladies cardiaques

Focusing the Field of a HIFU Array Transducer through Human Ribs

High intensity focused ultrasound (HIFU) enables highly localised, non-invasive tissue ablation, and its efficacy in the treatment of a range of cancers, including those of the kidney, prostate and breast has been demonstrated. HIFU offers the ability to treat deep-seated tumours locally, and potentially bears fewer side effects than more established treatment modalities such as resection, chemotherapy and ionising radiation. There remain, however, a number of significant challenges which currently hinder its widespread clinical application. One of these challenges is the need to transmit sufficient energy through the ribcage to ablate tissue at the required foci whilst minimising the formation of side lobes and sparing healthy tissue. Ribs both absorb and reflect ultrasound strongly. This sometimes results in overheating of bone and overlying tissue during treatment, leading to skin burns. Successful treatment of a patient with tumours in the upper abdomen therefore requires a thorough understanding of the way acoustic and thermal energy is deposited. In this thesis, an approach which predicts the acoustic field of a multi-element HIFU array scattered by human ribs, the topology of which was obtained from CT scan data, has been developed, implemented and validated. It is based on the boundary element method (BEM). Dissipative mechanisms were introduced into the propagating medium, along with a complex surface impedance condition at the surface of the ribs. A reformulation of the boundary element equations as a constrained optimisation problem was carried out to solve the inverse problem of determining the complex surface normal velocities of a multi-element HIFU array that best fitted a required acoustic pressure distribution in a least-squares sense. This was done whilst ensuring that an acoustic dose rate parameter at the surface of the ribs was kept below a specified threshold. The methodology was tested at an excitation frequency of 1 MHz on a spherical section multi-element array in the presence of human ribs. It was compared on six array-rib topologies against other methods of focusing through the ribs, including binarised apodisation based on geometric ray tracing, phase conjugation and the DORT method (décomposition de l’opérateur de retournement temporel). The constrained optimisation approach offers greater potential than the other focusing methods in terms of maximising the ratio of acoustic pressure magnitudes at the focus to those on the surface of the ribs whilst taking full advantage of the dynamic range of the phased array