Ultrasound backscatter microscopy for imaging of oral carcinoma

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide.1,2 Approximately 37,000 men and women had a new diagnosis of HNSCC in 2011 in the United States.3 The 5-year survival rate is only 50% to 60%.1 Most patients with HNSCC typically present with advanced disease in the clinic despite the fact that oral or pharyngeal tumors are easily identifiable within the mouth and throat by direct visualization or simple awake endoscopy.4,5 Given the advanced stage of oral carcinomas at detection, current treatment approaches such as radiotherapy and surgical resection can result in substantial morbidity and impairment of speech, swallowing, taste, and facial appearance.6 As such, there is an urgent clinical need for techniques that allow for early diagnosis, optimal staging, and restaging and for monitoring responses following therapy for HNSCC. Ultrasound (US) imaging is one such technique currently used for the diagnosis, assessment, and surgical management of HNSCC.7–10 The corresponding US probes, typically with 5–15-MHz transducers, provide sufficient imaging penetration depth but are constrained by limited spatial resolution, which may hamper their effectiveness in detecting neoplastic features characteristic of HNSCC such as early invasive growth and disruptions of subepithelial layers and the basement membrane in suspected lesions. Other imaging modalities such as optical coherence tomography have been developed to overcome the resolution limitations associated with clinical US imaging. Studies in animal models and patients have yielded images of suspicious oral lesions with axial and lateral resolutions of less than 10 μm, allowing for the visualization of individual tissue layers of the oral mucosa. The spatial resolution afforded by optical coherence tomography allows for the detection of precancerous changes such as epithelial dysplasia in addition to the disruption of healthy tissue layers associated with more advanced neoplastic stages. However, optical coherence tomography is limited to a superficial interrogation of tissue surfaces with a maximum penetration depth ranging from 1 to 3 mm, with commercial systems providing a penetration depth of 1.5 mm and an imaging focal depth of 1.0 mm.11–14 High-frequency (>20-MHz) ultrasound backscatter microscopy (UBM) systems may show promise for oral cancer detection by producing high-resolution images at a reasonable penetration depth that may better delineate tissue structures. Specifically, with operating frequencies of 40 to 60 MHz, a spatial resolution of less than 0.1 mm and up to 6 mm of depth penetration with reasonable contrast to noise ratios can be obtained from these systems.15–18 Commercial systems with these characteristics (eg, Vevo series; VisualSonics, Toronto, Ontario, Canada) have also now become readily available.18 The sizes of the imaging probes of these commercial systems, however, has made their use in oral applications impractical. Recently, our group built a compact and portable high-frequency (41-MHz) UBM system that uses a single-element transducer.19 The size and shape of the probe (specifications in “Materials and Methods”) render it suitable for interrogation of oral tissue in vivo in human patients at up to 6 to 7 mm in depth with high axial resolution (<40 μm).20,21 In this study, we made use of this system to evaluate whether UBM is sensitive to morphologic changes, such as epithelial and mucosal hypertrophy, lesion development, and destruction of the structural integrity of oral tissue layers, occurring during oral carcinoma initiation and progression. We used our UBM system on a hamster buccal pouch model that mimics human pathologic characteristics.22–24 We provide correlative analyses between the disease status as assessed by tissue histologic analysis and UBM-based imaging findings.