Leonardo G. Montilla

Real-time photoacoustic and ultrasound imaging: a simple solution for clinical ultrasound systems with linear arrays

Recent clinical studies have demonstrated that photoacoustic imaging (PAI) provides important diagnostic information during a routine breast exam for cancer. PAI enhances contrast between blood vessels and background tissue, which can help characterize suspicious lesions. However, most PAI systems are either not compatible with commercial ultrasound systems or inefficiently deliver light to the region of interest, effectively reducing the sensitivity of the technique. To address and potentially overcome these limitations, we developed an accessory for a standard linear ultrasound array that optimizes light delivery for PAI. The photoacoustic enabling device (PED) exploits an optically transparent acoustic reflector to help direct laser illumination to the region of interest. This study compares the PED with standard fiber bundle illumination in scattering and non-scattering media. In scattering media with the same incident fluence, the PED enhanced the photoacoustic signal by 18 dB at a depth of 5 mm and 6 dB at a depth of 20 mm. To demonstrate in vivo feasibility, we also used the device to image a mouse with a pancreatic tumor. The PED identified blood vessels at the periphery of the tumor, suggesting that PAI provides complementary contrast to standard pulse echo ultrasound. The PED is a simple and inexpensive solution that facilitates the translation of PAI technology to the clinic for routine screening of breast cancer.

Real-time, contrast enhanced photoacoustic imaging of cancer in a mouse window chamber.

A clinical ultrasound scanner and 14 MHz linear array collected real-time photoacoustic images (PAI) during an injection of gold nanorods (GNRs) near the region of a mature PC-3 prostate tumor in mice implanted with a skin flap window chamber. Three dimensional spectroscopic PAI (690-900 nm) was also performed to investigate absorption changes near the tumor and enhance specific detection of GNRs. Whereas GNRs improved PAI contrast (+18 dB), the photoacoustic spectrum was consistent with the elevated near infrared absorption of GNRs. The versatile imaging platform potentially accelerates development of photoacoustic contrast agents and drug delivery for cancer imaging and therapy.

3-D photoacoustic and pulse echo imaging of prostate tumor progression in the mouse window chamber

Understanding the tumor microenvironment is critical to characterizing how cancers operate and predicting their response to treatment. We describe a novel, high-resolution coregistered photoacoustic (PA) and pulse echo (PE) ultrasound system used to image the tumor microenvironment. Compared to traditional optical systems, the platform provides complementary contrast and important depth information. Three mice are implanted with a dorsal skin flap window chamber and injected with PC-3 prostate tumor cells transfected with green fluorescent protein. The ensuing tumor invasion is mapped during three weeks or more using simultaneous PA and PE imaging at 25 MHz, combined with optical and fluorescent techniques. Pulse echo imaging provides details of tumor structure and the surrounding environment with 100-μm(3) resolution. Tumor size increases dramatically with an average volumetric growth rate of 5.35 mm(3)/day, correlating well with 2-D fluorescent imaging (R = 0.97, p < 0.01). Photoacoustic imaging is able to track the underlying vascular network and identify hemorrhaging, while PA spectroscopy helps classify blood vessels according to their optical absorption spectrum, suggesting variation in blood oxygen saturation. Photoacoustic and PE imaging are safe, translational modalities that provide enhanced depth resolution and complementary contrast to track the tumor microenvironment, evaluate new cancer therapies, and develop molecular contrast agents in vivo.

Comparisons of Planar and Tubular Biaxial Tensile Testing Protocols of the Same Porcine Coronary Arteries

To identify the orthotropic biomechanical behavior of arteries, researchers typically perform stretch-pressure-inflation tests on tube-form arteries or planar biaxial testing of splayed sections. We examined variations in finite element simulations (FESs) driven from planar or tubular testing of the same coronary arteries to determine what differences exist when picking one testing technique vs. another. Arteries were tested in tube-form first, then tested in planar-form, and fit to a Fung-type strain energy density function. Afterwards, arteries were modeled via finite element analysis looking at stress and displacement behavior in different scenarios (e.g., tube FESs with tube- or planar-driven constitutive models). When performing FESs of tube inflation from a planar-driven constitutive model, pressure–diameter results had an error of 12.3% compared to pressure-inflation data. Circumferential stresses were different between tube- and planar-driven pressure-inflation models by 50.4% with the planar-driven model having higher stresses. This reduced to 3.9% when rolling the sample to a tube first with planar-driven properties, then inflating with tubular-driven properties. Microstructure showed primarily axial orientation in the tubular and opening-angle configurations. There was a shift towards the circumferential direction upon flattening of 8.0°. There was also noticeable collagen uncrimping in the flattened tissue.

Real-time pulse echo and photoacoustic imaging using an ultrasound array and in-line reflective illumination

Recent clinical studies have demonstrated that photoacoustic (PA) imaging, in conjunction with pulse echo (PE) ultrasound is a promising modality for diagnosing breast cancer. However, existing devices are unwieldy and are hard to integrate into the clinical environment. In addition, it is difficult to illuminate thick samples because light must be directed around the transducer. Conventional PA imaging designs involve off-axis illumination or transillumination through the object. Whereas transillumination works best with thin objects, off-axis illumination may not uniformly illuminate the region of interest. To overcome these problems we have developed an attachment to an existing clinical linear array that can efficiently deliver light in line with the image plane. This photoacoustic enabling device (PED) exploits an optically transparent acoustic reflector to co-align the illumination with the acoustic waves, enabling realtime PA and PE imaging. Based on this concept, we describe results from three types of PEDs in phantoms and rat tissue. The most recent version is fabricated by rapid prototyping, and attached to a 10 MHz linear array. Real-time PA and PE images of a 127-μm diameter wire were consistent with our expectations based on the properties of the ultrasound transducer. Comparisons with and without the PED of another test phantom printed on transparency demonstrated that the PED does not appreciably degrade or distort image quality. The PED offers a simple and inexpensive solution towards a real-time dual-modality imaging system for breast cancer detection. It could also be adapted for virtually any kind of ultrasound transducer array and integrated into routine ultrasound exams for detection of cancerous lesions within 1-2 cm from the probe surface.

In vivo multi-modality photoacoustic and pulse echo tracking of prostate tumor growth using a window chamber

Understanding the tumor microenvironment is critical to characterizing how cancers operate and predicting how they will eventually respond to treatment. The mouse window chamber model is an excellent tool for cancer research, because it enables high resolution tumor imaging and cross-validation using multiple modalities. We describe a novel multimodality imaging system that incorporates three dimensional (3D) photoacoustics with pulse echo ultrasound for imaging the tumor microenvironment and tracking tissue growth in mice. Three mice were implanted with a dorsal skin flap window chamber. PC-3 prostate tumor cells, expressing green fluorescent protein (GFP), were injected into the skin. The ensuing tumor invasion was mapped using photoacoustic and pulse echo imaging, as well as optical and fluorescent imaging for comparison and cross validation. The photoacoustic imaging and spectroscopy system, consisting of a tunable (680-1000nm) pulsed laser and 25 MHz ultrasound transducer, revealed near infrared absorbing regions, primarily blood vessels. Pulse echo images, obtained simultaneously, provided details of the tumor microstructure and growth with 100-μm3 resolution. The tumor size in all three mice increased between three and five fold during 3+ weeks of imaging. Results were consistent with the optical and fluorescent images. Photoacoustic imaging revealed detailed maps of the tumor vasculature, whereas photoacoustic spectroscopy identified regions of oxygenated and deoxygenated blood vessels. The 3D photoacoustic and pulse echo imaging system provided complementary information to track the tumor microenvironment, evaluate new cancer therapies, and develop molecular imaging agents in vivo. Finally, these safe and noninvasive techniques are potentially applicable for human cancer imaging.

Tracking contrast agents using real-time 2D photoacoustic imaging system for cardiac applications

Photoacoustic (PA) imaging is a rapidly developing imaging modality that can detect optical contrast agents with high sensitivity. While detectors in PA imaging have traditionally been single element ultrasound transducers, use of array systems is desirable because they potentially provide high frame rates to capture dynamic events, such as injection and distribution of contrast in clinical applications. We present preliminary data consisting of 40 second sequences of coregistered pulse-echo (PE) and PA images acquired simultaneously in real time using a clinical ultrasonic machine. Using a 7 MHz linear array, the scanner allowed simultaneous acquisition of inphase-quadrature (IQ) data on 64 elements at a rate limited by the illumination source (Q-switched laser at 20 Hz) with spatial resolution determined to be 0.6 mm (axial) and 0.4 mm (lateral). PA images had a signal-to-noise ratio of approximately 35 dB without averaging. The sequences captured the injection and distribution of an infrared-absorbing contrast agent into a cadaver rat heart. From these data, a perfusion time constant of 0.23 s-1 was estimated. After further refinement, the system will be tested in live animals. Ultimately, an integrated system in the clinic could facilitate inexpensive molecular screening for coronary artery disease.

A high-resolution 3D ultrasonic system for rapid evaluation of the anterior and posterior segment

BACKGROUND AND OBJECTIVE Traditional ultrasound imaging systems for ophthalmology employ slow, mechanical scanning of a single-element ultrasound transducer. The goal was to demonstrate rapid examination of the anterior and posterior segment with a three-dimensional (3D) commercial ultrasound system incorporating high-resolution linear probe arrays. MATERIALS AND METHODS The 3D images of the porcine eye were generated in approximately 10 seconds by scanning one of two commercial linear arrays (25- and 50-MHz). Healthy enucleated pig eyes were compared with those with induced injury or placement of a foreign material (eg, metal). Rapid, volumetric imaging was also demonstrated in one human eye in vivo. RESULTS The 50-MHz probe provided exquisite volumetric images of the anterior segment at a depth up to 15 mm and axial resolution of 30 μm. The 25-MHz probe provided a larger field of view (lateral X depth: 20 × 30 mm), sufficient for capturing the entire anterior and posterior segments of the pig eye, at a resolution of 60 μm. A 50-MHz scan through the human eyelid illustrated detailed structures of the Meibomian glands, cilia, cornea, and anterior segment back to the posterior capsule. CONCLUSION The 3D system with its high-frequency ultrasound arrays, fast data acquisition, and volume rendering capability shows promise for investigating anterior and posterior structures of the eye.

In vivo photoacoustic and pulse echo imaging of a pancreatic tumor using a hand held device

Ultrasonography and computed tomography are often used to diagnose pancreatic cancer. Using similar equipment as ultrasonography, photoacoustic (PA) imaging can provide vascular information over the same region of interest. Information about the vascularity in and around a lesion can be used to aid in the characterization and diagnosis of various cancers. Current PA imaging setups are restricted to bulky bench-top setups, limiting its practicality for clinical research. An ideal imaging platform would be non-invasive, real-time, portable, and inexpensive. We designed and fabricated an attachment to a clinical ultrasound probe which houses an optically transparent acoustic reflector in water. The design enables laser illumination in-line with the acoustic propagation path. The detector array simultaneously captures unbeamformed data on 64 elements at frequencies up to 10 MHz. We used the device to image in vivo a subcutaneous tumor in a SCID mouse implanted with Capan-2 pancreatic cancer cells and also a mouse pancreas embedded in a gel. Laser pulses (5 ns, 13 mJ/cm2) were transmitted through an optical window in the device, providing line illumination below the skin surface. Realtime 2D PA images between 700 nm and 960 nm were captured along with conventional pulse echo (PE) ultrasound to examine different sources of contrast in the tumor. The PE image identifies the acoustic window, skin surface, and variation in acoustic impedance within the tumor. PA images visualized areas of near infrared light absorption 4 mm deep within the tumor. Co-registered PE images provided an anatomical reference. This in vivo study demonstrates how a simple adapter to a clinical ultrasound array can be used for real-time simultaneous PE and PA imaging of cancer. With this efficient and practical design, cancer research can capitalize on noninvasive optical contrast below the tissue surface. With an optimized design, clinicians can incorporate PA imaging with routine ultrasound exams.

Polarization acquisition using a commercial Fourier transform spectrometer in the MWIR

A spectropolarimeter utilizing an Oriel MIR8000 Fourier Transform Spectrometer in the MWIR is demonstrated. The use of the channeled spectral technique, originally developed by K. Oka, is created with the use of two AR coated Yttrium Vanadate (YVO4) crystal retarders with a 2:1 thickness ratio. A basic mathematical model for the system is presented, showing that the Stokes parameters are directly present in the interferogram. Theoretical results are then compared with real data from the system, an improved model is provided to simulate the effects of absorption within the crystal, and error between reconstructions with phase-corrected and raw interferograms is analyzed.