Browsing by Subject "Acoustic output"

Ultrasound imaging is one of the most widely used diagnostic imaging modalities. Abdominal ultrasound is typically used for screening liver diseases, and it is the recommended modality for six month screening in patients at risk for hepatocellular carcinoma (HCC). The major drawback of abdominal ultrasound is poor image quality that is insufficient for diagnosis, which is reported in 25-60% of patients, and is often correlated with obesity. Tissue harmonic imaging (THI) has become the default imaging mode for most abdominal imaging exams. It has also been applied in motion tracking in ultrasound elastrography. THI provides better image quality through decreased sidelobe energy and decreased reverberation clutter in the abdominal wall compared to fundamental imaging. However, THI can be both signal-to-noise ratio (SNR) and penetration-depth limited during clinical imaging, resulting in decreased diagnostic utility.

A logical approach to increase the SNR of harmonic imaging is to increase the acoustic source pressure, but the acoustic output of diagnostic imaging has been subject to a de facto upper limit based upon the Food and Drug Administration (FDA) guideline for the Mechanical Index (MI < 1.9). This value was derived from historic values, rather than being linked to scientific evidence of bioeffects. A recent report from the American Institute of Ultrasound in Medicine (AIUM) concluded that exceeding the recommended maximum MI given in the FDA guidance up to an estimated in situ value of 4.0 could be warranted without concern for increased risk of cavitation in non-fetal tissues without gas bodies, if there there were concurrent improvement in diagnostic utility.

This thesis presents the preliminary work of evaluating the potential diagnostic benefit of employing acoustic output beyond the FDA guideline of MI = 1.9 in the context of hepatic imaging. Three clinical studies were performed with goals of: (1) quantifying the image quality improvement of using elevated acoustic output in B-mode harmonic imaging; (2) assessing penetration depth changes in harmonic imaging; and assessing elevated MI in (3) harmonic motion tracking and in (4) acoustic radiation force impulse (ARFI) excitation of shear wave elasticity imaging (SWEI). High MI B-mode harmonic imaging resulted in modest increases in the contrast-to-noise ratio of hypoechoic hepatic vessels. Difficult-to-image patients who suffer from poor ultrasound image quality demonstrated larger improvement than easy-to-image subjects. The imaging penetration depth increased linearly with increasing MI, on the order of 4 - 8 cm per unit MI increase for a given focal depth. High MI harmonic motion tracking resulted in considerable increase in shearwave speed (SWS) estimation yield, by 27% and 37% at focal depths of 5 cm and 9 cm respectively, due to improved harmonic tracking data quality through increased SNR and decreased jitter of tissue motion data. In addition, SWS estimation yield was shown to be linearly proportional to the push (ARFI excitation) energy. SWEI measurements with elevated push energy were successful in patients for whom standard push energy levels failed. These studies suggested that liver capsule depth could be used prospectively to identify patients who would benefit from elevated output in push energy. These results indicate that using elevated acoustic output has the potential to provide clinical benefit for diagnostic ultrasound.

The US Food and Drug Administration (FDA) provides guidelines for maximum acoustic output for diagnostic ultrasound imaging through metrics such as intensity, Mechanical Index (MI), and Thermal Index (TI). However, even within these guideline values, if the acoustic exposure levels used do not benefit image quality, they represent an unnecessary risk to patient safety. Ultrasound users have control over many settings, including ones that directly and indirectly change the acoustic output, and the user is largely responsible for deciding how to manage the safety risks based on on-screen displays of MI and TI. The FDA and professional societies advise users to observe the ALARA (as low as reasonably achievable) principle with regard to acoustic exposure, but several studies have shown that the majority of ultrasound users do not monitor safety indices. To address this discrepancy, an adaptive ultrasound method has been developed that could be used to automatically adjust acoustic exposure in real-time in response to image quality feedback.

In this work, MI was used as the measure of acoustic output, and lag-one coherence (LOC) was the image quality feedback parameter. LOC is the average spatial correlation between backscattered echoes received on neighboring ultrasound transducer array elements. Previous work has shown that LOC is predictive of local signal-to-noise ratio (SNR), and that it is sensitive to incoherent acoustic clutter and temporally-incoherent noise. During B-mode ultrasound imaging, LOC was monitored as MI was adjusted, and the data consistently formed a sigmoid shape. At lower MI values, LOC increased quickly with increasing output, but at higher MI values, increases in acoustic output often did not translate to increased image quality. This relationship was consistent for other image quality metric-versus-MI data, including contrast, contrast-to-noise ratio (CNR), and generalized contrast-to-noise ratio (gCNR).

The MI value at which the LOC began to approach an asymptote was denoted the "ALARA MI.” In this work, ALARA MI values were calculated for a range of obstetric imaging targets that are scanned during anatomy exams, including placenta, fetal abdomen, heart, kidney, bladder, stomach, ventricles, and extremities. The placenta data had the lowest median ALARA MI (0.59) and the fetal heart data had the highest (0.83). There was considerable variation in the ALARA MI values, even for the same participant, so frequent updates to the acoustic output settings would be recommended during live scanning. Additionally, the correlation between the ALARA MI and the LOC achieved at that setting was found to be very weak.

Initially, a fixed region of interest (ROI) was used for acoustic output optimization. This would require the structure to be aligned with the ROI and the optimization process to be manually initiated. Considering the demands on the sonographer during clinical ultrasound scanning, it would not be feasible to add these steps every time a new imaging window is used. An automated ROI-selection algorithm was developed that would allow the entire adaptive acoustic output selection process to happen without user input. This algorithm used envelope-detected B-mode image data that are readily available on clinical scanners to identify where to perform the optimization. Testing on clinical placenta and fetal abdomen data showed that it reliably recommended good regions for acoustic output optimization.

The results of this work suggest that near-maximum image quality can be achieved with a lower acoustic output level than is currently used clinically, and automated acoustic output adjustments could enable more consistent observation of the ALARA principle. In the future, this could be extended to other ultrasound modes, such as Doppler imaging, and additional acoustic output metrics could be incorporated.

Preliminary assessment of temporal SNR was performed, and a wide range of temporal SNR levels is associated with the ALARA MI settings found in this study. Future work may also investigate using a temporal SNR threshold to determine the ALARA output level. Spatial coherence measurements, such as LOC, reflect the degradation in image quality from acoustic clutter and electronic noise, and temporal coherence is affected by motion and electronic noise. Although motion is an important factor in clinical imaging, temporal coherence does not require access to channel data, so these calculations would be easier to implement on existing scanners. These trade-offs are important to consider when attempting to capture the underlying electronic noise level to inform an automated ALARA ultrasound system.

Ultrasound can be used for a variety of therapeutic purposes. High-intensity focused ultrasound (HIFU) has progressed over the past decade to become a viable therapeutic method and is valuable as a non-invasive alternative to many surgical procedures. Ultrasonic thermal therapies can also be used to release thermally sensitive liposomes encapsulating chemotherapeutic drugs. In the brain, the permeability of the blood-brain barrier to drugs, antibodies, and gene transfer can be increased with a mechanical mechanism using ultrasound and contrast agent.

The work presented in this dissertation tests the hypothesis that a diagnostic system can be used for combined imaging and therapeutic applications. In order to evaluate the effectiveness of a diagnostic system for use in therapeutic applications, a set of non-destructive tests is developed that can predict the potential for high acoustic output. A rigorous, nondestructive testing regimen for standard, diagnostic transducers to evaluate their potential for therapeutic use is formulated. Based on this work, transducer heating is identified as the largest challenge. The design and evaluation of several custom diagnostic transducers with various modifications to reduce internal heating are described. These transducers are compared with diagnostic controls using image contrast, face heating, hydrophone, and ARFI displacement measurements. From these results, we conclude that the most promising design is a passively and actively cooled, PZT-4 multilayer composite transducer, while the acoustically lossless lens and capactive micro-machined transducers evaluated herein are determined to be ineffective.

Three therapeutic applications are evaluated for the combined system. Image-guided spot ablations, such as in the treatment of early stage liver cancers, could not be successfully performed; however, the additional acoustic output requirements are determined to be on the order of 2.4 times those that can be currently produced without transducer damage in a clinically relevant amount of time (10-20 seconds per spot). The potential of a diagnostic system for a hyperthermia application is shown by producing temperatures for the duration necessary to release chemotherapeutic agents from thermally-activated liposomes without damage to the transducer. Finally, a mechanically-based therapeutic method for opening the BBB with ultrasonic contrast agent and specialized sonication regimes under ultrasonic B-mode guidance is demonstrated.

These studies indicate that a diagnostic system is capable of both moderate thermal and mechanical therapeutic applications under co-registered image-guidance.