The application of adaptive beamforming to biomedical ultrasound imaging has been an active research area in recent years. Adaptive beamforming techniques have the capability of achieving excellent resolution and sidelobe suppression, thus improving the quality of the ultrasound images. This quality improvement, however, comes at a high computational cost. The work presented in this thesis aims to answer the following basic question: Can we reduce the computational complexity of adaptive beamforming without a significant degradation of the image quality? Our objective is to explore a combination of low-complexity non-adaptive beamforming, such as the conventional Delay-and-Sum (DAS) method, with high-complexity adaptive beamforming, such as the standard Minimum-Variance Distortionless Response (MVDR) method implemented using the Generalized Sidelobe Canceller (GSC). Such a combination should have the lower computational complexity than adaptive beamforming, but it should also offer the image quality comparable to that obtained using adaptive beamforming. In addition to the adaptive GSC-based MVDR beamforming method, we also investigate the performance of the so-called Adaptive Single Snapshot Beamformer (ASSB), which is relatively unexplored in the ultrasound imaging literature. The main idea behind our approach to combining a non-adaptive beamformer with an adaptive one is based on the use of the data-dependent variable known as the coherence factor. The resulting hybrid beamforming method can be summarized as follows: For each input snapshot to be beamformed, calculate the corresponding coherence factor; if the coherence factor is below a certain threshold, use non-adaptive DAS beamforming, otherwise use adaptive (GSC-based or ASSB-based) beamforming. We have applied this simple switching scheme to the simulated B-mode ultrasound images of the 12-point and point-scatterer-cyst phantoms that are commonly used in the ultrasound imaging literature to evaluate the image quality. Our simulation results show that, in comparison to optimal high-complexity always-adaptive beamforming, our hybrid beamformer can yield significant computational savings that range from 59% to 99%, while maintaining the image quality (measured in terms of resolution and contrast) within a 5% degradation margin. / Graduate

Thesis (Ph.D.)--Tufts University, 1995. / Submitted to the Dept. of Electrical Engineering. Includes bibliographical references (leaves 237-253). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Thesis (Ph.D.). / Written for the Dept. of Electrical and Computer Engineering. Title from title page of PDF (viewed 2008/02/12). Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xiii, 195 p.; also includes graphics (some col.) Includes bibliographical references (p. 190-195). Available online via OhioLINK's ETD Center

Thesis (Ph. D.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 158-167).

Thesis (PhD)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: Tomas Gedeon. Includes bibliographical references (leaves 120-131).

Thesis (Ph.D.)--City University of Hong Kong, 2005. / "Submitted to Department of Mathematics in partial fulfillment of the requirements for the degree of Doctor of Philosophy" Includes bibliographical references (leaves 121-133)

Thesis (Ph.D.)--City University of Hong Kong, 2009. / "Submitted to Department of Manufacturing Engineering and Engineering Management in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves [169]-205)

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 146-158). Available also in a digital version from Dissertation Abstracts.

Over the past decade children have been facing increasingly large deficits towards their ability to pay attention. The inability to pay attention impacts children in every facet of their lives from focusing at school, in their home life, after school activities, etc. Inattention may impact children’s school performance, formation of social bonds, and the inability to pick up on appropriate social cues. Recently, more research has been conducted looking at attention processes in children and how they impact their daily functioning. However, a void lies in the treatments to help increase these deficits. The impact inattention has on children may be potentially harmful to their development and overall well-being. One type of treatment that may be beneficial in decreasing attention deficits is mindfulness. Mindfulness is the non-judgmental observation of the ongoing stream of internal and external stimuli as they arise. Mindfulness is a treatment that has been recently gaining popularity in youth populations. Mindfulness is a type of treatment that differs from traditional approaches. The aim of mindfulness is to modify how a person interacts with their environment, teaching them to be aware of the stimuli in their environment, being in contact with their present moment, and helping to increase their overall attention. Experiment I sought to examine the effects of a 6 session, 120-minute Mindfulness Based Intervention utilizing mindfulness activities for children versus that of a Control Group when engaging in various attention tasks. The tasks targeted different attention processes (i.e. sustained attention and inhibitory control) and were presented to the participants at pre and posttest. Participants were selected for the Mindfulness Group or the Control Group based on age. Each participant completed four attention tasks (CPT-X Task, Go-No/Go Task, Visual Cancellation Task, and Crossword Puzzle Task) at the beginning of the study. After completion of the tasks, the Mindfulness Group participated in mindfulness activities and the Control Group continued with treatment as usual (their standard after school/summer curriculum). After the mindfulness training, all participants completed the attention tasks again. Experiment I results were indicative of statistically significant differences with respect to a number of dependent measures across groups. On the CPT-X task, a significant difference was shown at posttest in the experimental (M=1.90, SD=3.12) compared to the control (M=6.4, SD=5.18) group; t (40)=(-3.32), p= .002 on the total false alarms and targets missed. A significant difference was also shown at posttest in the experimental (M=1.55, SD=2.98) compared to the control (M=4.95, SD=4.75) group; t (40)= (-2.71), p= .01 for false alarms in the CPT-X task. When analyzing the Visual Cancellation Task outcomes, a significant difference was shown at posttest in the experimental (M=1.90, SD=3.12) compared to the control (M=6.4, SD=5.18) group; t (40)=(-3.32), p= .002 for total missed. A significant difference was also shown at posttest in the experimental (M=1.55, SD=2.98) compared to the control (M=4.95, SD=4.75) group; t (40)=(-2.71), p= .01 for false alarms on the Visual Cancellation Task. When both sustained attention tasks were compared to determine task outcomes, significant correlations were found between False Alarms on the CPT-X Task and the Visual Cancellation Task: r= .675, p= < .01. Significant correlations were also found between Total Missed on the CPT-X Task and the Visual Cancellation Task: r= .487, p< .05. These results suggest that on multiple attention tasks, sustained attention increased after exposure to mindfulness. Taken altogether, these results indicate that the introduction of a Mindfulness Based Intervention for children served to increase sustained attention. In order to compare the impact of a mindfulness-based training approach on impulsiveness, a multiple probe experimental design across participants was utilized in Experiment II with five participants formally diagnosed with ADHD. During baseline, the participants participated in the delay-discounting task and completed two questionnaires (AFQ-Y and CAMM). The training phases included mindfulness activities from Dixon (2014), ACT for Children with Autism and Emotional Disorders. After participating in the mindfulness activity, the participants were presented with the discounting task and the two questionnaires. Overall, increases in the participant’s discounting scores as measured by their AUC (Area Under the Curve) improved. This suggests that after receiving the mindfulness activities, the participants made less impulsive decisions towards monetary outcomes. Moderate changes on the CAMM self-report measure occurred for 3 out of 5 participants. The introduction of the mindfulness activities showed to increase self-control decision making and mindful awareness. Experiment III sought to examine the effects of a 5 day, 6 hour intensive mindfulness based camp utilizing mindfulness activities versus that of a control group. Participants for the experimental group were recruited via the community and the control group participants were age matched with the experimental group. Participants in the mindfulness camp participated in mindfulness activities across a 5 day period and completed two questionnaires (AFQ-Y and CAMM) before the camp began and again at the end of the last day. The AFQ-Y and CAMM questionnaires are empirically validated questionnaires used to measure psychological flexibility and mindful awareness outcomes with children. The control group did not receive any mindfulness training and completed the same two questionnaires 5 days apart. Results of Experiment III were indicative of statistically significant differences with respect to both dependent measures (AFQ-Y and CAMM) when analyzed within and across the experimental and control group. When analyzing the CAMM outcomes, a significant difference was shown at posttest in the experimental (M=29.66, SD=7.16) compared to the control (M=21.26, SD=8.22) group, t (30)= (2.98), p=.006. Additionally, a significant difference at posttest was shown in the experimental (M=17.13 SD=7.64) compared to the control (M=27.4, SD=12.30) group, t (30) = (-2.74), p= .010 on the AFQ-Y. Visual analysis of the AFQ-Y measure within the mindfulness group from pretest to posttest denotes a decrease in scores across 80% of the participants suggesting increases in psychological flexibility. Visual analysis of the CAMM measure within the mindfulness group from pretest to posttest shows an increase in scores across 67% of the participants suggesting increases in mindful awareness. Previous research has investigated the efficacy of mindfulness interventions with children with the total training contact hours varying. Results of the current study showed that in Experiment I, after a total of 120 minutes of mindfulness training, increases in attention were demonstrated across the experimental group. Experiment II demonstrated that mindfulness training was effective in altering decision making in children with ADHD and Experiment III results showed that after 30 hours of the mindfulness training, mindfulness served to facilitate increases in psychological flexibility and mindful awareness. Additionally, across experiments I and III, a greater influence on attention, mindful awareness, and psychological flexibility was shown compared to the control groups, suggesting the importance of mindfulness training with children.