Multispectral photoacoustic microscopy of lipids using a pulsed supercontinuum laser.

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue between 1050-1714 nm using a pulsed supercontinuum laser based on a large-mode-area photonic crystal fiber. OR-PAM experiments of lipid-rich samples show the expected optical absorption peaks near 1210 and 1720 nm. These results show that pulsed supercontinuum lasers are promising for OR-PAM applications such as label-free histology of lipid-rich tissue and imaging small animal models of disease.

Near-infrared multispectral photoacoustic microscopy using a graded-index fiber amplifier.

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue using a multi-wavelength pulsed laser based on nonlinear fiber optics. 1047 nm laser pulses are converted to 1098, 1153, 1215, and 1270 nm pulses via stimulated Raman scattering in a graded-index multimode fiber. Multispectral PAM of a lipid phantom is demonstrated with our low-cost and simple technique.

Near-infrared spectroscopic photoacoustic microscopy using a multi-color fiber laser source.

We demonstrate a simple multi-wavelength optical source suitable for spectroscopic optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue. 1064 nm laser pulses are converted to multiple wavelengths beyond 1300 nm via nonlinear optical propagation in a birefringent optical fiber. OR-PAM experiments with lipid phantoms clearly show the expected absorption peak near 1210 nm. We believe this simple multi-color technique is a promising cost-effective approach to spectroscopic OR-PAM of lipid-rich tissue.

NDT of fiber-reinforced composites with a new fiber-optic pump-probe laser-ultrasound system.

Laser-ultrasonics is an attractive and powerful tool for the non-destructive testing and evaluation (NDT&E) of composite materials. Current systems for non-contact detection of ultrasound have relatively low sensitivity compared to contact peizotransducers. They are also expensive, difficult to adjust, and strongly influenced by environmental noise. Moreover, laser-ultrasound (LU) systems typically launch only about 50 firings per second, much slower than the kHz level pulse repetition rate of conventional systems. As demonstrated here, most of these drawbacks can be eliminated by combining a new generation of compact, inexpensive, high repetition rate nanosecond fiber lasers with new developments in fiber telecommunication optics and an optimally designed balanced probe beam detector. In particular, a modified fiber-optic balanced Sagnac interferometer is presented as part of a LU pump-probe system for NDT&E of aircraft composites. The performance of the all-optical system is demonstrated for a number of composite samples with different types and locations of inclusions.

A new fiber-optic non-contact compact laser-ultrasound scanner for fast non-destructive testing and evaluation of aircraft composites.

Laser ultrasonic (LU) inspection represents an attractive, non-contact method to evaluate composite materials. Current non-contact systems, however, have relatively low sensitivity compared to contact piezoelectric detection. They are also difficult to adjust, very expensive, and strongly influenced by environmental noise. Here, we demonstrate that most of these drawbacks can be eliminated by combining a new generation of compact, inexpensive fiber lasers with new developments in fiber telecommunication optics and an optimally designed balanced probe scheme. In particular, a new type of a balanced fiber-optic Sagnac interferometer is presented as part of an all-optical LU pump-probe system for non-destructive testing and evaluation of aircraft composites. The performance of the LU system is demonstrated on a composite sample with known defects. Wide-band ultrasound probe signals are generated directly at the sample surface with a pulsed fiber laser delivering nanosecond laser pulses at a repetition rate up to 76 kHz rate with a pulse energy of 0.6 mJ. A balanced fiber-optic Sagnac interferometer is employed to detect pressure signals at the same point on the composite surface. A- and B-scans obtained with the Sagnac interferometer are compared to those made with a contact wide-band polyvinylidene fluoride transducer.

On the role of intrinsic and extrinsic forces in early cardiac S-looping.

BACKGROUND: Looping is a crucial phase during heart development when the initially straight heart tube is transformed into a shape that more closely resembles the mature heart. Although the genetic and biochemical pathways of cardiac looping have been well studied, the biophysical mechanisms that actually effect the looping process remain poorly understood. Using a combined experimental (chick embryo) and computational (finite element modeling) approach, we study the forces driving early s-looping when the primitive ventricle moves to its definitive position inferior to the common atrium. RESULTS: New results from our study indicate that the primitive heart has no intrinsic ability to form an s-loop and that extrinsic forces are necessary to effect early s-looping. They support previous studies that established an important role for cervical flexure in causing early cardiac s-looping. Our results also show that forces applied by the splanchnopleure cannot be ignored during early s-looping and shed light on the role of cardiac jelly. Using available experimental data and computer modeling, we successfully developed and tested a hypothesis for the force mechanisms driving s-loop formation. CONCLUSIONS: Forces external to the primitive heart tube are necessary in the later stages of cardiac looping. Experimental and model results support our proposed hypothesis for forces driving early s-looping.

Spectroscopic photoacoustic microscopy using a photonic crystal fiber supercontinuum source.

Photoacoustic microscopy (PAM) provides high resolution images with excellent image contrast based on optical absorption. The compact size and high repetition rate of pulsed microchip lasers make them attractive sources for PAM. However, their fixed wavelength output precludes their use in spectroscopic PAM. We are developing a tunable optical source based on a microchip laser that is suitable for spectroscopic PAM. Pulses from a 6.6 kHz repetition rate Q-switched Nd:YAG microchip laser are sent through a photonic crystal fiber with a zero dispersion wavelength at 1040 nm. The highly nonlinear optical propagation produces a supercontinuum spectrum spanning 500-1300 nm. A tunable band pass filter selects the desired wavelength band from the supercontinuum. Our PAM system employs optical focusing and a 25 MHz spherically focused detection transducer. En-face imaging experiments were performed at seven different wavelengths from 575 to 875 nm. A simple discriminant analysis of the multiwavelength photoacoustic data produces images that clearly distinguish the different absorbing regions of ink phantoms. These results suggest the potential of this compact tunable source for spectroscopic photoacoustic microscopy.

Adaptive image reconstruction for sparse arrays using single-cycle terahertz pulses.

We demonstrate a noniterative adaptive reconstruction technique to significantly improve the imaging performance of sparse terahertz arrays employing single-cycle pulses. Grating lobe artifacts are suppressed by an adaptive weighting factor derived from the temporal coherence of signals from neighboring array elements. Image quality is further improved by a second weighting factor based on the spatial coherence of signals across the entire array. Experiments are performed with a synthetic aperture two-dimensional sparse array of 56x56 elements. Our reconstruction technique suppresses artifacts by 30 dB.

Adaptive terahertz imaging using a virtual transceiver and coherence weighting.

We demonstrate an adaptive reconstruction technique to significantly improve the depth of focus and contrast of three-dimensional reflection-mode terahertz imaging. A laterally scanned virtual transceiver element records reflections from the object of interest. A synthetic aperture focusing technique maintains fine spatial resolution over a large image depth. Measuring the spatial coherence of the received signals across the transceiver aperture provides a non-iterative self-adaptive approach to significantly improve image contrast. Test images show a spatial resolution of 0.4 mm maintained over a 16 mm depth of field, and up to a 30 dB improvement in signal-to-noise ratio.

Finite-element analysis of material and parameter effects in laser-based thermoelastic ultrasound generation.

Laser-based, thermoelastic transduction methods have potential in very high frequency (>50 MHz), high-density two-dimensional (2-D) arrays for a variety of very high-resolution superficial imaging applications, including in vivo tissue sectioning. Previous studies of these transducers generally have been based on experimental measurements or theoretical analyses using various simplifying assumptions. These theoretical models are mostly 1-D and best matched to simple geometries with a minimum number of component materials. In this work, we use a new thermoelastic solver in a commercially available finite-element analysis (FEA) software package to analyze multidimensional effects in laser-based devices of arbitrary geometry with the potential for use with arbitrary material properties. The FEA approach was verified first against experimental data. Thereafter, we explored the impact of various design variables, including laser spot size and laser penetration depth.

A high-frequency, 2-D array element using thermoelastic expansion in PDMS.

Optical generation of ultrasound is a promising alternative to piezoelectricity for high-frequency arrays. An array element is defined by the size and location of a laser beam focused on a suitable surface. Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated an increase in conversion efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. Radiation pattern measurements with an 85 MHz spherically focused transducer indicated an array element size of 20 microm. These measurements lacked the spatial resolution required to reveal fine details in the radiated acoustic field. Here we report radiation pattern measurements with a 5-microm spatial sampling, showing that the radiated acoustic field is degraded by leaky Rayleigh waves launched from the PDMS/glass interface. We demonstrate that replacing the glass with a clear PDMS substrate eliminates the leaky Rayleigh waves, producing a broad and smooth radiation pattern suitable for a two-dimensional (2-D) phased array operating at frequencies greater than 50 MHz.

Thermoelastic expansion vs. piezoelectricity for high-frequency, 2-D arrays.

Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated increased efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. In this paper we show that the radiated power from a black PDMS film is comparable to a 20 MHz piezoelectric two-dimensional (2-D) array element. Furthermore, we predict that a thermoelastic array element can produce similar acoustic power levels compared to ideal piezoelectric 2-D array elements at frequencies in the 100 MHz regime. We believe these results show that thermoelastic generation of ultrasound is a promising alternative to piezoelectricity for high-frequency, 2-D arrays.