Pasi Karppinen

Phase-delayed laser diode array allows ultrasonic guided wave mode selection and tuning

Selecting and tuning modes are useful in ultrasonic guided wave non-destructive testing (NDT) since certain modes at various center frequencies are sensitive to specific types of defects. Ideally one should be able to select both the mode and the center frequency of the launched waves. We demonstrated that an affordable laser diode array can selectively launch either the S0 or A0 ultrasonic wave mode at a chosen center frequency into a polymer plate. A fiber-coupled diode array (4 elements) illuminated a 2 mm thick acrylic plate. A predetermined time delay matching the selected mode and frequency was employed between the output of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results imply that this array permits non-contacting guided wave ultrasonic NDT. The solution is small, affordable, and robust in comparison to conventional pulsed lasers. In addition, it does not require experienced operators.

An optoacoustic point source for acoustic scale model measurements

A massless acoustic source is proposed for scale model work. This source is generated by focusing a pulsed laser beam to rapidly heat the air at the focal point. This produces an expanding small plasma ball which generates a sonic impulse that may be used as an acoustic point source. Repeatability, frequency response, and directivity of the source were measured to show that it can serve as a massless point source. The impulse response of a rectangular space was determined using this type of source. A good match was found between the predicted and the measured impulse responses of the space.

The potential utility of high-intensity ultrasound to treat osteoarthritis.

Osteoarthritis (OA) is a widespread musculoskeletal disease that reduces quality of life and for which there is no cure. The treatment of OA is challenging since cartilage impedes the local and systemic delivery of therapeutic compounds (TCs). This review identifies high-intensity ultrasound (HIU) as a non-contact technique to modify articular cartilage and subchondral bone. HIU enables new approaches to overcome challenges associated with drug delivery to cartilage and new non-invasive approaches for the treatment of joint disease. Specifically, HIU has the potential to facilitate targeted drug delivery and release deep within cartilage, to repair soft tissue damage, and to physically alter tissue structures including cartilage and bone. The localized, non-invasive ultrasonic delivery of TCs to articular cartilage and subchondral bone appears to be a promising technique in the immediate future.

Photo-acoustic excitation and detection of guided ultrasonic waves in bone samples covered by a soft coating layer

Photo-acoustic (PA) excitation was combined with skeletal quantitative ultrasound (QUS) for multi-mode ultrasonic assessment of human long bones. This approach permits tailoring of the ultrasonic excitation and detection so as to efficiently detect the fundamental flexural guided wave (FFGW) through a coating of soft tissue. FFGW is a clinically relevant indicator of cortical thickness. An OPO laser with tunable optical wavelength, was used to excite a photo-acoustic source in the shaft of a porcine femur. Ultrasonic signals were detected by a piezoelectric transducer, scanning along the long axis of the bone, 20-50 mm away from the source. Five femurs were measured without and with a soft coating. The coating was made of an aqueous gelatin-intralipid suspension that optically and acoustically mimicked real soft tissue. An even coating thickness was ensured by using a specific mold. The optical wave length of the source (1250 nm) was tuned to maximize the amplitude of FFGW excitation at 50 kHz frequency. The experimentally determined FFGW phase velocity in the uncoated samples was consistent with that of the fundamental antisymmetric Lamb mode (A0). Using appropriate signal processing, FFGW was also identified in the coated bone samples, this time with a phase velocity consistent with that theoretically predicted for the first mode of a fluid-solid bilayer waveguide (BL1). Our results suggest that photo-acoustic quantitative ultrasound enables assessment of the thickness-sensitive FFGW in bone through a layer of soft tissue. Photo-acoustic characterization of the cortical bone thickness may thus become possible.

Ultrasonic transport of particles into articular cartilage and subchondral bone

Osteoarthritis (OA) is a debilitating musculoskeletal disease without a cure. Delivery of therapeutic compounds is a problem and localized drug therapy could enable new treatment strategies. In this study high-intensity ultrasound was used to deliver micro- and nano-particles (MNPS) into three bovine osteochondral samples: (1) control (C) that was exposed to the particles without sonication, (2) UST-1 that was sonicated prior to immersion in a MNPS and (3) UST-2 that was sonicated in the presence of MNPS. Following treatment samples were cut into 2.9 ± 0.3 mm sections and digital images were obtained by light microscopy. In the sonicated samples (UST-1 and UST-2) MNPS penetrated into articular cartilage and subchondral bone. No MNPS penetration was observed in C. The proposed technique could potentially be used for local drug treatment of OA.

Photo-acoustic phase-delayed excitation of guided waves in coated bone phantoms

Photo-acoustic skeletal quantitative ultrasound enables assessment of the fundamental flexural guided wave (FFGW) propagating in bone. This mode, consistent with the F(1,1) tube mode can now be measured through a coating of soft tissue. Interference due to ultrasound propagation in the soft tissue surrounding the bone is reduced by using phase-delayed ultrasound excitation. Photo-acoustic phase-delayed excitation was done on five axisymmetric bone phantoms (1-5 mm wall thickness), coated by a 5 mm thick soft-tissue mimicking layer. A fiber head comprising a linear array of four optical fibers (400 μm diameter), illuminated by pulsed laser diodes (905 nm wavelength) generated ultrasound. This sound was received by a small (10 mm diameter) custom-made piezo transducer from the top surface of the coating. Tuning the phase delay allowed selection of the excited mode(s). FFGW was detected in the 20-40 kHz band when the power ratio between FFGW and interference was tuned to a local maximum. This tuning removed interference and improved SNR of the FFGW mode by >10 dB. Fitting the theoretical FLC(1,1) mode of a liquid-coated (LC) tube to the measured FFGW phase-velocity provided accurate (11% ± 7% rms deviation) estimate for the cortical thickness. These results suggest that photo-acoustic phase-delayed excitation may enable in vivo assessment of cortical thickness based on FFGW.

Non-contact quantification of adhesion between a metal hemispherical shell and a polymer by guided acoustic waves

Guided acoustic waves (GAWs) are sensitive to delaminations between two materials. Certain modes can determine changes in adhesion for well-defined geometries. Considering the sensitivity of GAWs to detect delaminations, there is a paucity of attempts to quantify the adhesion between a hemispherical shell and a surrounding medium. The purpose of this study was to 1) quantify the average adhesion of a hemispherical metal shell to a polymer and 2) demonstrate that it is feasible to localize delaminations in the adhesion. A pair of custom made, needle-based transducers were used to launch GAWs into a 5 cm diameter, 1 mm thick hemispherical metal shell. The experiments were done in two parts: first, the average adhesion of a metal shell to a polymer was quantified by GAW attenuation. The shells were attached to a polymer (ultra-high molecular weight polyethylene) with various degrees of adhesion, ranging from a shell resting freely in a taylor-made polymer cavity to a shell which was adhered to the polymer with excess epoxy. In the second part, an artificial defect was generated in the interface between the metal shell and adhesive by drilling a hole into the polymer (to generate areas with no adhesion). Echoes of GAWs determined that defect localization on a curved surface is feasible. Finally, we determined that noncontacting laser ultrasonics could be used to replace the contacting needle-based transducers.

Photo-acoustic excitation and detection of fundamental antisymmetric Lamb mode in coated bone phantoms

Photo-acoustic (PA) imaging was combined with skeletal quantitative ultrasound (QUS) for multi-mode ultrasonic assessment of human long bones. This approach permits tailoring the ultrasonic excitation and detection to efficiently receive the fundamental antisymmetric Lamb mode (A0) through a coating of soft tissue. The method was tested on five axisymmetric bone phantoms of individualized wall thickness (1-5 mm) made of a composite material and coated with a layer (2.5 mm) of soft material that mimics the soft tissue. Signals were excited with a pulsed Nd:Yag laser at 532 nm wavelength and detected on the same side of the coated phantom with (i) a laser Doppler vibrometer (LDV) and for comparison also with (ii) a piezoelectric contact ultrasound receiver, scanning a source-receiver distance of 20-50 mm along the phantom. At a centre frequency of 50 kHz, a phase velocity consistent with that of the theoretically predicted A0 mode was identified in the recorded signals. Our results thus suggest that photo-a...

Detecting defects in adhesion between a metal hemisphere and a polymer base

Detecting localized defects in adhesion on a spherical shell is challenging, especially if one has access only to its rim. Our group previously quantified the spatial average of the adhesion between a metal hemisphere and a polymer base [1]. Here we report on progress towards determining the presence of localized defects, their position, size, and strength. Tests were performed on a 5 cm diameter metal hemisphere attached to a UHMWPE (Polyethylene, PE RCH 1000 D150×2000 Etralene) base. Defects in adhesion were generated by machining hexagonal holes (5 to 17.5 mm) into the polymer base prior to fixing by adhesive. These known areas without adhesion were remotely characterized with a guided ultrasonic wave (Lamb quasi-modes) generated by a Nd:YAG (1064 nm, 8 ns pulse duration) pulse from one rim of the shell and detected with a laser Doppler vibrometer. We detected the presence and position of the defects.

Selective Excitation of Guided Waves into Bone Phantoms by a Time-Delayed Laser Diode Array

We excite selectively the Ao and So plate mode into an acrylic bone phantom using a laser diode array. This is done by controlling the time delay between the signals driving the array elements. Selective mode generation is important because it can potentially increase the sensitivity of in vivo osteoporosis screening.