Abstract 4285: Development of a combined photoacoustic micro-ultrasound system for non-invasive detection of mouse lymph nodes and intra-tumoral oxygen saturation measurements
Authors: Heinmiller Andrew , Needles Andrew , Bates Dave , Theodoropoulos Catherine , Foster F. Stuart 
Published: July 2012

Abstract
We have developed a photoacoustic (PA) micro-ultrasound imaging system which can be used to image tumour vasculature and derive oxygen saturation measurements from within those vessels. Additionally, the system can be used to detect contrast agents such as methylene blue (MB), which is already used in the clinic to aid in sentinel lymph node detection. Here we describe the assessment of the sensitivity of the system with respect to MB detection and compare oxygen saturation (sO2) measurements derived from our system to expected values.
A modified μUS system (Vevo 2100, VisualSonics) was operated with a linear array transducer (MS-250, fc = 21 MHz), retrofitted with a housing that held rectangular fiber optic bundles (25.4 × 1.25 mm) to either side. The rectangular bundles were bifurcated ends of a single bundle that was coupled to a tuneable laser (Rainbow NIR, OPOTEK Inc., Carlsbad CA, 680-970 nm). The μUS system was synchronized with the laser and PA signals were acquired with a fluence &lt; 20 mJ/cm2, beamformed in software, and displayed at 5 Hz.
For determination of the sensitivity of the system to MB, serial dilutions of MB were performed (from 3.13E-02 mol/L to 3.13E-08 mol/L) and each dilution was imaged by drawing the solution into a 500 um diameter polyethylene tube. Signal intensity was measured at 680 nm since MB has a peak absorbance around this wavelength, and the minimum detectable concentration was determined. For determination of the presence of MB signal in vivo, PA images of the axillary lymph node were collected at 680 and 760 nm both pre- and post-infusion of MB into the forepaw of an adult CD1 mouse and a subtracted image was generated. Axillary and brachial lymph nodes were excised and imaged to verify the source of the MB signal.
sO2 measurements were assessed by imaging the jugular vein of a rat and altering breathed oxygen concentration while simultaneously measuring the oxygen partial pressure (pO2) within the same jugular vein. A real-time parametric sO2 map was generated on the system with a dual wavelength measurement of 750 and 850 nm and sO2 measurements were calculated based on a selected region of interest. Sub-cutaneous tumors were also imaged in 2D and 3D and total haemoglobin and sO2 maps were generated.
The minimum detectable concentration of MB was observed to be approximately 520 nM. We were able to see photoacoustic signal generated by MB in the mouse axial lymph node, as well as its afferent lymph vessel, and verify it through excision of the node and subsequent imaging. In vivo contrast enhancement was 16dB at 680 nm. Subtraction of 760 nm from 680 nm data yielded a contrast enhancement improvement of 8-10 dB.
In the rat jugular vein, measured sO2 correlated well with expected sO2 values (R2 = 0.832). Within a tumor, PA signal from the vasculature was detected at depths of up to 10mm and total haemoglobin and sO2 measurements were made on these signals.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4285. doi:10.1158/1538-7445.AM2011-4285