Frequency dependence of tissue attenuation measured by acoustic microscopy

Basic concept and clinical applications of quantitative ultrasound (QUS) technologies

In the field of clinical ultrasound, the full digitalization of diagnostic equipment in the 2000s enabled the technological development of quantitative ultrasound (QUS), followed by multiple diagnostic technologies that have been put into practical use in recent years. In QUS, tissue characteristics are quantified and parameters are calculated by analyzing the radiofrequency (RF) echo signals returning to the transducer. However, the physical properties (and pathological level structure) of the biological tissues responsible for the imaging features and QUS parameters have not been sufficiently verified as there are various conditions for observing living tissue with ultrasound and inevitable discrepancies between theoretical and actual measurements. A major issue of QUS in clinical application is that the evaluation results depend on the acquisition conditions of the RF echo signal as the source of the image information, and also vary according to the model of the diagnostic device. In this paper, typical examples of QUS techniques for evaluating attenuation, speed of sound, amplitude envelope characteristics, and backscatter coefficient in living tissues are introduced. Exemplary basic research and clinical applications related to these technologies, and initiatives currently being undertaken to establish the QUS method as a true tissue characterization technology, are also discussed.

High-Frequency Time-Resolved Scanning Acoustic Microscopy for Biomedical Applications

High-frequency focused ultrasound has emerged as a powerful modality for both biomedical imaging and elastography. It is gaining more attention due to its capability to outperform many other imaging modalities at a submicron resolution. Besides imaging, high-frequency ultrasound or acoustic biomicroscopy has been used in a wide range of applications to assess the elastic and mechanical properties at the tissue and single cell level. The interest in acoustic microscopy stems from the awareness of the relationship between biomechanical and the underlying biochemical processes in cells and the vast impact these interactions have on the onset and progression of disease. Furthermore, ultrasound biomicroscopy is characterized by its non-invasive and non-destructive approach. This, in turn, allows for spatiotemporal studies of dynamic processes without the employment of histochemistry that can compromise the integrity of the samples. Numerous techniques have been developed in the field of acoustic microscopy. This review paper discusses high-frequency ultrasound theory and applications for both imaging and elastography.

Photoacoustic microscopy: principles and biomedical applications

Photoacoustic microscopy (PAM) has become an increasingly popular technology for biomedical applications, providing anatomical, functional, and molecular information. In this concise review, we first introduce the basic principles and typical system designs of PAM, including optical-resolution PAM and acoustic-resolution PAM. The major imaging characteristics of PAM, i.e. spatial resolutions, penetration depth, and scanning approach are discussed in detail. Then, we introduce the major biomedical applications of PAM, including anatomical imaging across scales from cellular level to organismal level, label-free functional imaging using endogenous biomolecules, and molecular imaging using exogenous contrast agents. Lastly, we discuss the technical and engineering challenges of PAM in the translation to potential clinical impacts.

Improved High-Frequency Ultrasound Corneal Biometric Accuracy by Micrometer-Resolution Acoustic-Property Maps of the Cornea

Purpose

Mapping of epithelial thickness (ET) is useful for detection of keratoconus, a disease characterized by corneal thinning and bulging in which epithelial thinning occurs over the apex. In prior clinical studies, optical coherence tomography (OCT) measurements of ET were systematically thinner than those obtained by 40-MHz high-frequency ultrasound (HFU) where a constant speed of sound (c) of 1636 m/s was used for all corneal layers. The purpose of this work was to study the acoustic properties, that is, c, acoustic impedance (Z), and attenuation (α) of the corneal epithelium and stroma independently using a scanning acoustic microscope (SAM) to investigate the discrepancy between OCT and HFU estimates of ET.

Methods

Twelve unfixed pig corneas were snap-frozen and 6-μm sections were scanned using a custom-built SAM with an F-1.08, 500-MHz transducer and a 264-MHz bandwidth. Two-dimensional maps of c, Z, and α with a spatial resolution of 4 μm were derived.

Results

SAM showed that the value of c in the epithelium (i.e., 1548 ± 18 m/s) is substantially lower than the value of c in the stroma (i.e., 1686 ± 33 m/s).

Conclusion

SAM results demonstrated that the assumption of a constant value of c for all corneal layers is incorrect and explains the prior discrepancy between OCT and HFU ET determinations.

Translational Relevance

The findings of this study have important implications for HFU-based ET measurements and will improve future keratoconus diagnosis by providing more-accurate ET estimates.

Grueneisen relaxation photoacoustic microscopy in vivo

Grueneisen relaxation photoacoustic microscopy (GR-PAM) can achieve optically defined axial resolution, but it has been limited to ex vivo demonstrations so far. Here, we present the first in vivo image of a mouse brain acquired with GR-PAM. To induce the GR effect, an intensity-modulated continuous-wave laser was employed to heat absorbing objects. In phantom experiments, an axial resolution of 12.5  μm was achieved, which is sixfold better than the value achieved by conventional optical-resolution PAM. This axial-resolution improvement was further demonstrated by imaging a mouse brain in vivo, where significantly narrower axial profiles of blood vessels were observed. The in vivo demonstration of GR-PAM shows the potential of this modality for label-free and high-resolution anatomical and functional imaging of biological tissues.

A Novel Quantitative 500-MHz Acoustic Microscopy System for Ophthalmologic Tissues

OBJECTIVE

This paper describes development of a novel 500-MHz scanning acoustic microscope (SAM) for assessing the mechanical properties of ocular tissues at fine resolution. The mechanical properties of some ocular tissues, such as lamina cribrosa (LC) in the optic nerve head, are believed to play a pivotal role in eye pathogenesis.

METHODS

A novel etching technology was used to fabricate silicon-based lens for a 500-MHz transducer. The transducer was tested in a custom-designed scanning system on human eyes. Two-dimensional (2-D) maps of bulk modulus (K) and mass density (ρ) were derived using improved versions of current state-of-the-art signal processing approaches.

RESULTS

The transducer employed a lens radius of 125 μm and had a center frequency of 479 MHz with a -6-dB bandwidth of 264 MHz and a lateral resolution of 4 μm. The LC, Bruch's membrane (BM) at the interface of the retina and choroid, and Bowman's layer (BL) at the interface of the corneal epithelium and stroma, were successfully imaged and resolved. Analysis of the 2-D parameter maps revealed average values of LC, BM, and BL with KLC = 2.81 ±0.17; GPa, KBM = 2.89 ±0.18; GPa, KBL = 2.6 ±0.09 ; GPa, ρ LC = 0.96 ±0.03 g/cm³; ρ BM = 0.97 ±0.04 g/cm³; ρ BL = 0.98 ±0.04 g/cm³.

SIGNIFICANCE

This novel SAM was shown to be capable of measuring mechanical properties of soft biological tissues at microscopic resolution; it is currently the only system that allows simultaneous measurement of K, ρ, and attenuation in large lateral scales (field area >9 mm²) and at fine resolutions.

Volumetric imaging of erythrocytes using label-free multiphoton photoacoustic microscopy

Photoacoustic microscopy (PAM) is an imaging modality well suited to mapping vasculature and other strong absorbers in tissue. However, one of the primary drawbacks to PAM when used for high-resolution imaging is the relatively poor axial resolution due to the inverse dependence on the transducer bandwidth. While submicron lateral resolution PAM can be achieved by tightly focusing the excitation light, the axial resolution is fundamentally limited to 10s of microns for typical transducer frequencies. Here we present a multiphoton PAM technique called transient absorption ultrasonic microscopy (TAUM), which results in a completely optically resolved voxel with an experimentally measured axial resolution of 1.5 microns. This technique is demonstrated by imaging individual red blood cells in three dimensions in blood smear and ex vivo tissues. To the best of our knowledge, this is the first demonstration of fully resolved, volumetric photoacoustic imaging of erythrocytes.

Ultrahigh resolution photoacoustic microscopy via transient absorption

We have developed a novel, hybrid imaging modality, Transient Absorption Ultrasonic Microscopy (TAUM), which takes advantage of the optical nonlinearities afforded by transient absorption to achieve ultrahigh-resolution photoacoustic microscopy. The theoretical point spread function for TAUM is functionally equivalent to confocal and two-photon fluorescence microscopy, potentially enabling cellular/subcellular photoacoustic imaging. A prototype TAUM system was designed, built, and used to image a cross-section through several capillaries in the excised cheek pouch of a Syrian Hamster. The well-resolved capillaries in the TAUM image provided experimental evidence of the spatial resolution. These results suggest that TAUM has excellent potential for producing volumetric images with cellular/subcellular resolution in three dimensions deep inside living tissue.

Off-axis photoacoustic microscopy

Photoacoustic microscopy (PAM) is a high-contrast, high-resolution imaging modality, used primarily for imaging hemoglobin and melanin. Important applications include mapping of the microvasculature and melanoma tumor margins. We demonstrate a novel PAM design that markedly simplifies the implementation by separating the optical illumination from the acoustic detection path. This modification enables the use of high-quality commercial optics and transducers, and may be readily adapted to commercial light microscopes. The designed PAM system is only sensitive to signals generated in the overlap of the illumination and detection solid angles, providing the additional benefit of quasi-dark-field detection. An off-axis PAM system with a lateral resolution of 26 microm and a modest axial resolution of 410 microm has been assembled and characterized using tissue samples. The axial resolution is readily scaled down to tens of micrometers within the same design, by utilizing commercially available high-frequency acoustic transducers.

Investigation of elasto-mechanical properties of alginate microcapsules by scanning acoustic microscopy

High-frequency scanning acoustic microscopy (SAM) was used for investigation of acoustic impedance and 3D-surface topography of full alginate microspheres that act as model of artificial biological cells. Elasto-mechanical properties of the investigated specimens have been characterized by acoustic impedance. Mean surface impedance of microspheres (diameter: 300 microm) was measured with SAM at 900 MHz with a spatial resolution of 1.5 microm. The sensitivity and reproducibility of SAM had to be increased considerable to receive and quantify signals in the very low impedance region. The multilayer analysis method was used to get quantitative data of acoustic impedance with SAM at a microscopic level. 3D images show details of structure and surface topography. As a reference, bulk measurements were performed on full alginate cylinders. The acoustical impedance and the mechanical stiffness c(11) were obtained from mass density and longitudinal ultrasound velocity at 6 MHz. The impedances received with both methods are in close agreement. The results demonstrate the SAM as a powerful tool for characterizing mechano-elastical parameters as well as surface structure and topography of microspheres with high spatial resolution.

A 100-200 MHz ultrasound biomicroscope

The development of higher frequency ultrasound imaging systems affords a unique opportunity to visualize living tissue at the microscopic level. This work was undertaken to assess the potential of ultrasound imaging in vivo using the 100-200 MHz range. Spherically focused lithium niobate transducers were fabricated. The properties of a 200 MHz center frequency device are described in detail. This transducer showed good sensitivity with an insertion loss of 18 dB at 200 MHz. Resolution of 14 /spl mu/m in the lateral direction and 12 /spl mu/m in the axial direction was achieved with f/1.14 focusing. A linear mechanical scan system and a scan converter were used to generate B-scan images at a frame rate up to 12 frames per second. System performance in B-mode imaging is limited by frequency dependent attenuation in tissues. An alternative technique, zone-focus image collection, was investigated to extend depth of field. Images of coronary arteries, the eye, and skin are presented along with some preliminary correlations with histology. These results demonstrate the feasibility of ultrasound biomicroscopy In the 100-200 MHz range. Further development of ultrasound backscatter imaging at frequencies up to and above 200 MHz will contribute valuable information about tissue microstructure.

Wide-band acoustic spectroscopy of biological material based on a laser-induced grating technique

A laser-induced transient grating technique enables fast noncontact acoustic measurements on transparent biological materials in a frequency range from tens of megahertz to 1 GHz. We have applied this method to the characterization of bovine vitreous and found high-frequency acoustic attenuation values to be close to those of water, with a quadratic dependence on frequency, in contrast to low-frequency data. The potential of the technique for studying other biological materials, such as human stratum corneum, is demonstrated.

Picosecond grating spectroscopy for characterizing the acoustic properties of biological material

We present measurements of the ultrasound attenuation and sound velocity of a number of liquids, transparent biological materials (the vitreous and lens of the bovine eye), and biological fluids (whole blood) at frequencies between 925 and 1020 MHz by using a picosecond thermal grating. Sound velocity and attenuation measurements of liquids (e.g., methanol and ethanol) agree very well with those reported in the literature. The sound velocity in the biological materials studied also agrees with the reported values in the literature. In contrast, the attenuation coefficients measured for biological materials, 2000-5000 dB/cm, are much higher than would be extrapolated from published low-frequency data.

A new method to correlate acoustic spectroscopic microscopy (30 MHz) and light microscopy

A powerful new method is used to investigate the correlation between light microscopic and acoustic properties of biological tissues. Specimens of liver were sectioned into successive slices, 250 micrometers and 10 micrometers thick. The thick sections were investigated acoustically, the thin sections by means of light microscopy. Markers that could be detected and located, both optically and acoustically, were used to find and reconstruct corresponding regions in the acoustic and optical sections (2.5 x 2.5 mm). Parameter images were reconstructed from the sections investigated acoustically. The acoustic parameters were attenuation at 30 MHz, the slope of the attenuation spectrum (between 10 and 50 MHz), backscattering at 30 MHz, the slope of the backscattering spectrum (between 10 and 50 MHz) and the local ultrasound velocity. Acoustic images were obtained in the frequency range from 10 to 50 MHz, yielding a lateral resolution of about 50 micrometers. The sections for light microscopy were stained according to the Goldner trichrome staining technique. The histological composition was determined quantitatively, using digital image segmentation techniques. The percentage of collagen-rich fibrous tissue, luminal structure and interstitial spaces, and the number of nuclei were calculated for regions of 250 x 250 micrometers. These histological features were correlated with the acoustic parameters obtained from the corresponding regions in adjacent sections. It was thus possible to find the histological components responsible for acoustic parameters.

Quantitative acoustic microscopy of individual living human cells

The elastic properties of cells can be measured with microscopic resolution by acoustic microscopy. By measuring the waveform of very short pulses, the thickness, and the acoustic velocity, impedance and attenuation can be determined from the two separate signals reflected from the top and the bottom of the cell.

Effects of tissue processing techniques in acoustical (1.2 GHz) and light microscopy

In this study the influence of various tissue processing and staining techniques on the acoustical properties of liver tissue was investigated. A qualitative study was performed using ultrasound attenuation as the imaged parameter of a combined optical/acoustical microscope with a 1.2 GHz transducer. Images were made of three sets of adjacent liver sections (6 microns in thickness) which were prepared in ten different ways: fixed by alcohol or formalin; stained by hematoxylin-eosin (HE), toluidine blue (TB) or non-stained; sectioned by a cryostat or by a paraffin microtome. It was concluded that the images obtained from cryostat sections were of much higher quality than those from paraffin sections. Images obtained from sections that were sectioned while embedded in paraffin displayed no detail at all. No consistent effect was noticed with respect to staining by HE or TB. Alcohol fixed sections gave more detailed images than formalin fixed sections. Formalin fixation in combination with cryostat sectioning yielded many cytoplasmic vacuoles.