We report an ultrasound contrast agent for which we engineered the shell structure to impart much better stability under intense stress and deformation.

Contrast-enhanced ultrasound with microbubbles has shown promise in detection of prostate cancer (PCa), but sensitivity and specificity of detection remain challenging. Targeted nanoscale contrast agents with improved capability to accumulate in tumors may result in prolonged signal enhancement and improved detection of PCa with ultrasound. Here we report on a new nanobubble contrast agent that specifically targets prostate specific membrane antigen (PSMA) overexpressed in most prostate tumors. The PSMA-targeted bubbles (PSMA-NB) were utilized to simultaneously image dual flank PCa tumors (PSMA-positive PC3pip and PSMA-negative PC3flu) to examine whether the biomarker can be successfully detected and imaged using this probe in a mouse model. Results demonstrate that active targeting of NBs to PSMA rapidly and selectively enhances tumor accumulation and is critical for tumor retention of the contrast agent. Importantly, these processes could be visualized and quantified, in real time, with standard clinical ultrasound. Such demonstration of the immense yet underutilized potential of ultrasound in the area of molecular imaging can open the door to future opportunities for improving sensitivity and specificity of cancer detection using parametric NB-enhanced ultrasound imaging.Despite significant efforts, prostate cancer (PCa) is still the second most common leading cause of cancer-related deaths worldwide, with 180,000 new cases diagnosed in the USA in 2018 [1][2] . Accurate diagnosis of PCa is a crucial step necessary for informing the clinical management of the disease, yet conventional options leave much space for improvement. Currently, men with an abnormal digital rectal exam and/or increased levels of prostate serum antigen (PSA) are considered at high risk for cancer and are referred for a prostate biopsy to assess if PCa is present.The standard PCa biopsy procedure uses transrectal ultrasound (US) guidance to determine the prostate gland orientation, but the delineation of tumors within the prostate using US is unclear. Accordingly, biopsies are performed in a systematic manner by selecting 6-12 or more area from the peripheral zone of the prostate. These cores represent only 1% of prostate tissue and are a gross under sampling of prostate gland tissue, and biopsies performed using this conventional procedure result in significant false negatives of up to 50% [3][4][5] . Concern over the lack of pathological data in

A resonant mass measurement technique simultaneously distinguishes and characterizes (size and concentration) buoyant and non-buoyant particles in a bubble sample.

Current commercially available ultrasound contrast agents are gas-filled, lipid- or protein-stabilized microbubbles larger than 1 μm in diameter. Because the signal generated by these agents is highly dependent on their size, small yet highly echogenic particles have been historically difficult to produce. This has limited the molecular imaging applications of ultrasound to the blood pool. In the area of cancer imaging, microbubble applications have been constrained to imaging molecular signatures of tumor vasculature and drug delivery enabled by ultrasound-modulated bubble destruction. Recently, with the rise of sophisticated advancements in nanomedicine, ultrasound contrast agents, which are an order of magnitude smaller (100-500 nm) than their currently utilized counterparts, have been undergoing rapid development. These agents are poised to greatly expand the capabilities of ultrasound in the field of targeted cancer detection and therapy by taking advantage of the enhanced permeability and retention phenomenon of many tumors and can extravasate beyond the leaky tumor vasculature. Agent extravasation facilitates highly sensitive detection of cell surface or microenvironment biomarkers, which could advance early cancer detection. Likewise, when combined with appropriate therapeutic agents and ultrasound-mediated deployment on demand, directly at the tumor site, these nanoparticles have been shown to contribute to improved therapeutic outcomes. Ultrasound's safety profile, broad accessibility and relatively low cost make it an ideal modality for the changing face of healthcare today. Aided by the multifaceted nano-sized contrast agents and targeted theranostic moieties described herein, ultrasound can considerably broaden its reach in future applications focused on the diagnosis and staging of cancer.