Visualization and quantification of breast cancer biomechanical properties with magnetic resonance elastography

Computational predictions of magnetic resonance acoustic radiation force imaging for breast cancer focused ultrasound therapy

PURPOSE

In magnetic resonance-guided focused ultrasound (MRgFUS) breast therapies, the focal location must be characterized to guide successful treatment. Focal characterization is difficult because heterogeneous breast tissues introduce phase aberrations that blur and shift the focus and traditional guidance methods do not work in adipose tissues. The purpose of this work is to evaluate numerical simulations of MRgFUS that predict the focal location. Those simulations are compared to clinical magnetic resonance acoustic radiation force imaging (MR-ARFI) data collected during in vivo treatment of breast tumors.

METHODS

The focal location was evaluated before MRgFUS treatment with MR-ARFI in five patients. The hybrid angular spectrum method (HAS) was applied to simulate pressure fields which were converted to forces, then convolved with a 3D Green's function (with time-of-arrival weighting) to produce a simulation of the MR-ARFI tissue displacement.

RESULTS

The focal locations found by the simulations and the MR-ARFI measurements were on average separated by 3.7 mm (SD: 0.9 mm). Characterization of the focal zone spatial distributions had a normalized root mean squared difference of 8.1% (SD: 2.5%). The displacement magnitudes of the simulations underestimated the MR-ARFI measurements by 82% (SD: 5.6%).

CONCLUSIONS

The agreement between MR-ARFI measurements and simulations demonstrates that HAS can predict the in vivo focal location in heterogeneous tissues, though accurate patient-specific properties are needed to improve predictions of tissue displacement magnitude. Tools developed in this study could be used to streamline MRgFUS treatment planning and optimization, for biomechanical property estimation, and in developing phase aberration correction techniques.

Magnetic Resonance Elastography of Invasive Breast Cancer: Evaluating Prognostic Factors and Treatment Response

Objectives: To assess the elasticity values in breast tissues using magnetic resonance elastography (MRE) and examine the association between elasticity values of invasive breast cancer with prognostic factors and the pathologic response to neoadjuvant systemic therapy (NST). Methods: A total of 57 patients (mean age, 54.1 years) with invasive breast cancers larger than 2 cm in diameter on ultrasound were prospectively enrolled. The elasticity values (mean, minimum, and maximum) of invasive breast cancers, normal fibroglandular tissues, and normal fat tissues were measured via MRE using a commercially available acoustic driver and compared. Elasticity values of breast cancers were compared according to prognostic factors and pathologic responses in patients who received NST before surgery. Receiver operating curve analysis was performed to evaluate the predictive efficacy of elasticity values in terms of pathological response. Results: Among the 57 patients, the mean elasticity value of invasive breast cancers was significantly higher than that of normal fibroglandular tissue and normal fat tissue (7.90 ± 5.80 kPa vs. 2.54 ± 0.80 kPa vs. 1.32 ± 0.33 kPa, all ps < 0.001). Invasive breast cancers with a large diameter (>4 cm) exhibited significantly higher mean elasticity values relative to tumors with a small diameter (≤4 cm) (11.65 ± 7.22 kPa vs. 5.87 ± 3.58 kPa, p = 0.002). Among 24 patients who received NST, mean, minimum, and maximum elasticity values significantly differed between the pathologic complete response (pCR) and non-pCR groups (all ps < 0.05). For the mean elasticity value, the area under the curve value for distinguishing pCR and non-pCR groups was 0.880 (95% confidence interval, 0.682, 0.976; p < 0.001). Conclusions: The elasticity values of invasive breast cancers measured via breast MRE showed a positive correlation with tumor size and showed potential in predicting the therapeutic response in patients receiving NST.

A mechanobiological model of bone metastasis reveals that mechanical stimulation inhibits the pro-osteolytic effects of breast cancer cells

Bone is highly susceptible to cancer metastasis, and both tumor and bone cells enable tumor invasion through a "vicious cycle" of biochemical signaling. Tumor metastasis into bone also alters biophysical cues to both tumor and bone cells, which are highly sensitive to their mechanical environment. However, the mechanobiological feedback between these cells that perpetuate this cycle has not been studied. Here, we develop highly advanced in vitro and computational models to provide an advanced understanding of how tumor growth is regulated by the synergistic influence of tumor-bone cell signaling and mechanobiological cues. In particular, we develop a multicellular healthy and metastatic bone model that can account for physiological mechanical signals within a custom bioreactor. These models successfully recapitulated mineralization, mechanobiological responses, osteolysis, and metastatic activity. Ultimately, we demonstrate that mechanical stimulus provided protective effects against tumor-induced osteolysis, confirming the importance of mechanobiological factors in bone metastasis development.

Continuous Shear Wave Elastography for Liver Using Frame-to-Frame Equalization of Complex Amplitude

This study addresses a crucial necessity in the field of noninvasive liver fibrosis diagnosis by introducing the concept of continuous shear wave elastography (C-SWE), utilizing an external vibration source and color Doppler imaging. However, an application of C-SWE to assess liver elasticity, a deep region within the human body, arises an issue of signal instability in the obtained data. To tackle this challenge, this work proposes a method involving the acquisition of multiple frames of datasets, which are subsequently compressed. Furthermore, the proposed frame-to-frame equalization method compensates discrepancies in the initial phase that might exist among multiple-frame datasets, thereby significantly enhancing signal stability. The experimental validation of this approach encompasses both phantom tests and in vivo experiments. In the phantom tests, the proposed technique is validated through a comparison with the established shear wave elastography (SWE) technique. The results demonstrate a remarkable agreement, with an error in shear wave velocity of less than 4.2%. Additionally, the efficacy of the proposed method is confirmed through in vivo tests. As a result, the stabilization of observed shear waves using the frame-to-frame equalization technique exhibits promising potential for accurately assessing human liver elasticity. These findings collectively underscore the viability of C-SWE as a potential diagnostic instrument for liver fibrosis.

Enhanced full-inversion-based ultrasound elastography for evaluating tumor response to neoadjuvant chemotherapy in patients with locally advanced breast cancer

PURPOSE

An enhanced ultrasound elastography technique is proposed for early assessment of locally advanced breast cancer (LABC) response to neoadjuvant chemotherapy (NAC).

METHODS

The proposed elastography technique inputs ultrasound radiofrequency data obtained through tissue quasi-static stimulation and adapts a strain refinement algorithm formulated based on fundamental principles of continuum mechanics, coupled with an iterative inverse finite element method to reconstruct the breast Young's modulus (E) images. The technique was explored for therapy response assessment using data acquired from 25 LABC patients before and at weeks 1, 2, and 4 after the NAC initiation (100 scans). The E ratio of tumor to the surrounding tissue was calculated at different scans and compared to the baseline for each patient. Patients' response to NAC was determined many months later using standard clinical and histopathological criteria.

RESULTS

Reconstructed E ratio changes obtained as early as one week after the NAC onset demonstrate very good separation between the two cohorts of responders and non-responders to NAC. Statistically significant differences were observed in the E ratio changes between the two patient cohorts at weeks 1 to 4 after treatment (p-value < 0.001; statistical power greater than 97%). A significant difference in axial strain ratio changes was observed only at week 4 (p-value = 0.01; statistical power = 76%). No significant difference was observed in tumor size changes at weeks 1, 2 or 4.

CONCLUSION

The proposed elastography technique demonstrates a high potential for chemotherapy response monitoring in LABC patients and superior performance compared to strain imaging.

Design, Construction, and Implementation of a Magnetic Resonance Elastography Actuator for Research Purposes

Magnetic resonance elastography (MRE) is a technique for determining the mechanical response of soft materials using applied harmonic deformation of the material and a motion-sensitive magnetic resonance imaging sequence. This technique can elucidate significant information about the health and development of human tissue such as liver and brain, and has been used on phantom models (e.g., agar, silicone) to determine their suitability for use as a mechanical surrogate for human tissues in experimental models. The applied harmonic deformation used in MRE is generated by an actuator, transmitted in bursts of a specified duration, and synchronized with the magnetic resonance signal excitation. These actuators are most often a pneumatic design (common for human tissues or phantoms) or a piezoelectric design (common for small animal tissues or phantoms). Here, we describe how to design and assemble both a pneumatic and a piezoelectric MRE actuator for research purposes. For each of these actuator types, we discuss displacement requirements, end-effector options and challenges, electronics and electronic-driving requirements and considerations, and full MRE implementation. We also discuss how to choose the actuator type, size, and power based on the intended material and use. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Design, construction, and implementation of a convertible pneumatic MRE actuator for use with tissues and phantom models Basic Protocol 2: Design, construction, and implementation of a piezoelectric MRE actuator for localized excitation in phantom models.

Fast magnetic resonance elastography with multiphase radial encoding and harmonic motion sparsity based reconstruction

Objective. To achieve fast magnetic resonance elastography (MRE) at a low frequency for better shear modulus estimation of the brain. Approach. We proposed a multiphase radial DENSE MRE (MRD-MRE) sequence and an improved GRASP algorithm utilizing the sparsity of the harmonic motion (SH-GRASP) for fast MRE at 20 Hz. For the MRD-MRE sequence, the initial position encoded by spatial modulation of magnetization (SPAMM) was decoded by an arbitrary number of readout blocks without increasing the number of phase offsets. Based on the harmonic motion, a modified total variation and temporal Fourier transform were introduced to utilize the sparsity in the temporal domain. Both phantom and brain experiments were carried out and compared with that from multiphase Cartesian DENSE-MRE (MCD-MRE), and conventional gradient echo sequence (GRE-MRE). Reconstruction performance was also compared with GRASP and compressed sensing. Main results. Results showed the scanning time of a fully sampled image with four phase offsets for MRD-MRE was only 1/5 of that from GRE-MRE. The wave patterns and estimated stiffness maps were similar to those from MCD-MRE and GRE-MRE. With SH-GRASP, the total scan time could be shortened by additional 4 folds, achieving a total acceleration factor of 20. Better metric values were also obtained using SH-GRASP for reconstruction compared with other algorithms. Significance. The MRD-MRE sequence and SH-GRASP algorithm can be used either in combination or independently to accelerate MRE, showing the potentials for imaging the brain as well as other organs.

Time-of-flight and noise-correlation-inspired algorithms for full-field shear-wave elastography using digital holography

SIGNIFICANCE

Quantitative stiffness information can be a powerful aid for tumor or fibrosis diagnosis. Currently, very promising elastography approaches developed for non-contact biomedical imaging are based on transient shear-waves imaging. Transient elastography offers quantitative stiffness information by tracking the propagation of a wave front. The most common method used to compute stiffness from the acquired propagation movie is based on shear-wave time-of-flight calculations.

AIM

We introduce an approach to transient shear-wave elastography with spatially coherent sources, able to yield full-field quantitative stiffness maps with reduced artifacts compared to typical artifacts observed in time-of-flight.

APPROACH

A noise-correlation algorithm developed for passive elastography is adapted to spatially coherent narrow or any band sources. This noise-correlation-inspired (NCi) method is employed in parallel with a classic time-of-flight approach. Testing is done on simulation images, experimental validation is conducted with a digital holography setup on controlled homogeneous samples, and full-field quantitative stiffness maps are presented for heterogeneous samples and ex-vivo biological tissues.

RESULTS

The NCi approach is first validated on simulations images. Stiffness images processed by the NCi approach on simulated inclusions display significantly less artifacts than with a time-of-flight reconstruction. The adaptability of the NCi algorithm to narrow or any band shear-wave sources was tested successfully. Experimental testing on homogeneous samples demonstrates similar values for both the time-of-flight and the NCi approach. Soft inclusions in agarose sample could be resolved using the NCi method and feasibility on ex-vivo biological tissues is presented.

CONCLUSIONS

The presented NCi approach was successful in computing quantitative full-field stiffness maps with narrow and broadband source signals on simulation and experimental images from a digital holography setup. Results in heterogeneous media show that the NCi approach could provide stiffness maps with less artifacts than with time-of-flight, demonstrating that a NCi algorithm is a promising approach for shear-wave transient elastography with spatially coherent sources.

Axially- and torsionally-polarized radially converging shear wave MRE in an anisotropic phantom made via Embedded Direct Ink Writing

Magnetic Resonance Elastography (MRE) is a non-invasive imaging method to quantitatively map the shear viscoelastic properties of soft tissues. In this study, Embedded Direct Ink Writing is used to fabricate a muscle mimicking anisotropic phantom that may serve as a standard for imaging studies of anisotropic materials. The technique allowed us to obtain a long shelf life silicone-based phantom expressing transverse isotropic mechanical properties. Another goal of the present investigation is to introduce a torsionally-polarized, radially-converging shear wave actuation method for MRE. The implemented design for this novel setup was first validated via its application to isotropic and homogeneous gelatin phantoms. Then, a comparison of the resulting complex wave images from axially- and torsionally-polarized MRE on the developed anisotropic phantom and on a skeletal muscle murine sample is presented, highlighting the value of using multiple actuation and motion encoding polarization directions when studying anisotropic materials.

Measuring viscoelastic parameters in Magnetic Resonance Elastography: a comparison at high and low magnetic field intensity

Magnetic Resonance Elastography (MRE) is a non-invasive imaging technique which involves motion-encoding MRI for the estimation of the shear viscoelastic properties of soft tissues through the study of shear wave propagation. The technique has been found informative for disease diagnosis, as well as for monitoring of the effects of therapies. The development of MRE and its validation have been supported by the use of tissue-mimicking phantoms. In this paper we present our new MRE protocol using a low magnetic field tabletop MRI device at 0.5 T and sinusoidal uniaxial excitation in a geometrical focusing condition. Results obtained for gelatin are compared to those previously obtained using high magnetic field MRE at 11.7 T. A multi-frequency investigation is also provided via a comparison of commonly used rheological models: Maxwell, Springpot, Voigt, Zener, Jeffrey, fractional Voigt and fractional Zener. Complex shear modulus values were comparable when processed from images acquired with the tabletop low field scanner and the high field scanner. This study serves as a validation of the presented tabletop MRE protocol and paves the way for MRE experiments on ex-vivo tissue samples in both normal and pathological conditions.

Modulated Start-Up Mode of Cancer Cell Migration Through Spinophilin-Tubular Networks

Spinophilin (SPL) is a multifunctional actin-binding scaffolding protein. Although increased research on SPL in cancer biology has revealed a tumor suppressive role, its modulation in cancer biology, and oncological relevance remains elusive. Thus, we determined the role of SPL in the modulation of the junctional network and cellular migration in A549 lung cancer cell line. Knockdown of SPL promoted cancer cell invasion in agarose spot and scratch wound assays. Attenuation of SPL expression also enhanced invadopodia, as revealed by enhanced vinculin spots, and enhanced sodium bicarbonate cotransporter NBC activity without enhancing membranous expression of NBCn1. Disruption of the tubular structure with nocodazole treatment revealed enhanced SPL expression and reduced NBC activity and A549 migration. SPL-mediated junctional modulation and tubular stability affected bicarbonate transporter activity in A549 cells. The junctional modulatory function of SPL in start-up migration, such as remodeling of tight junctions, enhanced invadopodia, and increased NBC activity, revealed here would support fundamental research and the development of an initial target against lung cancer cell migration.

Breast Tumor Diagnosis Using Finite-Element Modeling Based on Clinical in vivo Elastographic Data

OBJECTIVES

This study exploited finite-element modeling (FEM) to simulate breast tissue multicompression during ultrasound elastography to classify breast tumors based on their nonlinear biomechanical properties.

METHODS

Numeric simulations were first calculated by using 3-dimensional (3D) virtual models with an assumed tumor's geometric dimensions but with actual material properties to test and validate the FEM. Further numeric simulations were used to construct 3D models based on in vivo experimental data to verify our models. The models were designed for each individual in vivo case, emphasizing the geometry, position, and biomechanical properties of the breast tissue. At different compression levels, tissue strains were analyzed between the tumors and the background normal tissues to explore their nonlinearity and classify the tumor type. Tumor classification parameters were deduced by using a power-law relationship between the applied compressive forces and strain differences.

RESULTS

Classification parameters were compared between benign and malignant tumors, for which they were found to be statistically significant in classifying the tumor types (P < .05) by both the validation and verification of FEM. We compared the classification parameters between the in vivo and FEM classifications, for which they were found to be strongly correlated (R = 0.875; P < .001), with no statistical differences between their outcomes (P = .909).

CONCLUSIONS

Good agreement between the model outcomes and the in vivo diagnostics was reported. The implemented models were validated and verified. The introduced 3D modeling method may augment elastographic methods to preliminary classify breast tumors at an early stage.

Protective Role of IRBIT on Sodium Bicarbonate Cotransporter-n1 for Migratory Cancer Cells

IP₃ receptor-binding protein released with IP₃ (IRBIT) interacts with various ion channels and transporters. An electroneutral type of sodium bicarbonate cotransporter, NBCn1, participates in cell migration, and its enhanced expression is related to cancer metastasis. The effect of IRBIT on NBCn1 and its relation to cancer cell migration remain obscure. We therefore aimed to determine the effect of IRBIT on NBCn1 and the regulation of cancer cell migration due to IRBIT-induced alterations in NBCn1 activity. Overexpression of IRBIT enhanced cancer cell migration and NBC activity. Knockdown of IRBIT or NBCn1 and treatment with an NBC-specific inhibitor, S0859, attenuated cell migration. Stimulation with oncogenic epidermal growth factor enhanced the expression of NBCn1 and migration of cancer cells by recruiting IRBIT. The recruited IRBIT stably maintained the expression of the NBCn1 transporter machinery in the plasma membrane. Combined inhibition of IRBIT and NBCn1 dramatically inhibited the migration of cancer cells. Combined modulation of IRBIT and NBCn1 offers an effective strategy for attenuating cancer metastasis.

Broadband Time Domain Diffuse Optical Reflectance Spectroscopy: A Review of Systems, Methods, and Applications

This review presents recent developments and a wide overview of broadband time domain diffuse optical spectroscopy (TD-DOS). Various topics including physics of photon migration, advanced instrumentation, methods of analysis, applications covering multiple domains (tissue chromophore, in vivo studies, food, wood, pharmaceutical industry) are elaborated. The key role of standardization and recent studies in that direction are discussed. Towards the end, a brief outlook is presented on the current status and future trends in broadband TD-DOS.

A new technique for motion encoding gradient-less MR elastography of the psoas major muscle: A gradient-echo type multi-echo sequence

The present study aimed to develop vibration techniques for magnetic resonance (MR) elastography (MRE) of the psoas major muscle (PM). Seven healthy volunteers were included. MRE was performed with motion-encoding gradient (MEG)-less multi-echo MRE sequence, which allows clinicians to perform MRE using conventional MR imaging. In order to transmit mechanical vibration of the pneumatic type to the PM, a long narrow vibration pad was designed using a 3D printer, and the optimum vibration techniques were verified. The vibration pad was placed under the lower back, with the volunteers in the supine position. The results indicated that the PM vibrated well through the transmitted vibration from the lumbar spine, which suggests that the placement of a narrow vibration pad under the supine body, along the lumbar spine, allows the vibration of the PM. The shear modulus of the PM (n = 7) was 1.23 ± 0.09 kPa (mean ± SEM) on the right side and 1.22 ± 0.15 kPa on the left side, with no significant difference (t-test, P > 0.05). Increased stiffness of the muscle due to continuous local contraction may be an important cause of non-specific low back pain (LBP). The present vibration techniques for MRE of the PM provide a quantitative diagnostic tool for changes in muscle stiffness associated with non-specific LBP.

Reliable preparation of agarose phantoms for use in quantitative magnetic resonance elastography

Agarose phantoms are one type of phantom commonly used in developing in vivo brain magnetic resonance elastography (MRE) sequences because they are inexpensive and easy to work with, store, and dispose of; however, protocols for creating agarose phantoms are non-standardized and often result in inconsistent phantoms with significant variability in mechanical properties. Many magnetic resonance imaging (MRI) and ultrasound studies use phantoms, but often these phantoms are not tailored for desired mechanical properties and as such are too stiff or not mechanically consistent enough to be used in MRE. In this work, we conducted a systematic study of agarose phantom creation parameters to identify those factors that are most conducive to producing mechanically consistent agarose phantoms for MRE research. We found that cooling rate and liquid temperature affected phantom homogeneity. Phantom stiffness is affected by agar concentration (quadratically), by final liquid temperature and salt content in phantoms, and by the interaction of these two metrics each with stir rate. We captured and quantified the implied relationships with a regression model that can be used to estimate stiffness of resulting phantoms. Additionally, we characterized repeatability, stability over time, impact on MR signal parameters, and differences in agar gel microstructure. This protocol and regression model should prove beneficial in future MRE development studies that use phantoms to determine stiffness measurement accuracy.

Non-invasive in vivo quantification of human skin tension lines

Human skin is a composite tissue that exhibits anisotropic mechanical properties. This anisotropy arises primarily from the alignment of collagen and elastin fibers in the dermis, which causes the skin to exhibit greater tension in one direction, making it appear stiffer. A diverse number of skin tension guidelines have been developed to assist surgeons in making incisions that produce the least conspicuous scars. However, skin anisotropy is believed to vary from subject to subject, and no single guideline is universally recognized as the best to implement for surgical applications. To date, no system exists that can rapidly and non-invasively measure lines of skin tension in vivo. In this article, we evaluate the ability of a new aspiration system to measure the anisotropy of human skin. The device painlessly applies a radial stress of 17 kPa to a region of skin, and captures radially asymmetric skin deformations via a dermal camera. These deformations are used to quantify orientations of strain extrema and the direction of greatest skin stiffness. The ratio of these asymmetric strains varies between 1 and -0.75. A simple 2D transverse isotropic model captures this behavior for multiple anatomical sites. Clinical trials reveal that skin tension line orientations are comparable with existing skin tension maps and generally agree across subjects, however orientations statistically differ between individuals. As such, existing guidelines appear to provide only approximate estimates of skin tension orientation. STATEMENT OF SIGNIFICANCE: Skin tension lines (STL) in human skin arise primarily from collagen fiber alignment in the dermis. These lines are used by surgeons to guide incisions that produce the least conspicuous scars. While numerous anatomical STL maps exist, no single guideline is universally recognized as the most reliable. Moreover, manual methods of quantifying STL are imprecise. For the first time, we have developed a device capable of rapidly and non-invasively measuring STL orientations in vivo, using a single test. Our results are used to establish a simple constitutive model of mechanical skin anisotropy. Clinical trials further reveal STL orientations are comparable with existing maps, but statistically differ between individuals. Existing guidelines therefore appear to provide only approximate estimates of STL orientation.

Detection of inhomogeneity by the observation of the surface of the material simulating biological tissues

This paper presents a research which involves the observation of the movement of points presented on a material surface under the influence of mechanical extortion. Tests were performed using two 15 mm silicone layers, one of which contained 1 mm thick elements of nitrile-butadiene rubber. Analysed materials were structurally heterogeneous tissue phantoms. Test results that were obtained indicated that the developed method allows detecting inhomogeneity and its approximate location, what may be used in pathological state prevention.

Application of Imaging Technologies in Breast Cancer Detection: A Review Article

One of the techniques utilised in the management of cancer in all stages is multiple biomedical imaging. Imaging as an important part of cancer clinical protocols can provide a variety of information about morphology, structure, metabolism and functions. Application of imaging technics together with other investigative apparatus including in fluids analysis and vitro tissue would help clinical decision-making. Mixed imaging techniques can provide supplementary information used to improve staging and therapy planning. Imaging aimed to find minimally invasive therapy to make better results and reduce side effects. Probably, the most important factor in reducing mortality of certain cancers is an early diagnosis of cancer via screening based on imaging. The most common cancer in women is breast cancer. It is considered as the second major cause of cancer deaths in females, and therefore it remained as an important medical and socio-economic issue. Medical imaging has always formed part of breast cancer care and has used in all phases of cancer management from detection and staging to therapy monitoring and post-therapeutic follow-up. An essential action to be performed in the preoperative staging of breast cancer based on breast imaging. The general term of breast imaging refers to breast sonography, mammography, and magnetic resonance tomography (MRT) of the breast (magnetic resonance mammography, MRM).

Further development in technology will lead to increase imaging speed to meet physiological processes requirements. One of the issues in the diagnosis of breast cancer is sensitivity limitation. To overcome this limitation, complementary imaging examinations are utilised that traditionally includes screening ultrasound, and combined mammography and ultrasound. Development in targeted imaging and therapeutic agents calls for close cooperation among academic environment and industries such as biotechnological, IT and pharmaceutical industries.

Magnetic resonance elastography of the human brain using a multiphase DENSE acquisition

Purpose

In magnetic resonance elastography (MRE), a series of time‐shifted images is acquired at specific phase offsets in relation to an induced mechanical excitation. To efficiently gather the set of phase offset images and to overcome limitations due to prolonged TEs and related susceptibility artifacts at low‐frequency MRE, we developed an improved displacement encoding with a stimulated echoes (DENSE) method.

Methods

The proposed multiphase DENSE‐MRE acquisition scheme allows full sampling of the wave propagation in 1 encoding direction during each TR using multiple readouts at specific phase offsets. With this approach, all phase offsets can be imaged in 1 TR without the need for whole sequence repetitions at time‐shifted offsets relative to the excitation motion. We tested this technique in phantom experiments with 60 Hz and in the brain of 4 volunteers using 20‐Hz harmonic excitation.

Results

Three‐dimensional wave propagation could be acquired in 7 minutes 30 seconds. Following background phase elimination, clear wave images were obtained, showing the propagation of the waves over time. Calculated shear modulus maps of the phantom matched well to the maps obtained by conventional gradient‐echo MRE. In the brain, low‐frequency DENSE‐MRE images were free of susceptibility‐induced artifacts and the calculated maps showed a median global complex shear modulus magnitude of 0.72 kPa and phase angle of 1.03 rad across volunteers.

Conclusion

The proposed multiphase DENSE approach allows efficient low‐frequency MRE with short TEs and is well‐suited for low‐frequency MRE of the human brain.

Altered Placental Chorionic Arterial Biomechanical Properties During Intrauterine Growth Restriction

Intrauterine growth restriction (IUGR) is a pregnancy complication due to placental dysfunction that prevents the fetus from obtaining enough oxygen and nutrients, leading to serious mortality and morbidity risks. There is no treatment for IUGR despite having a prevalence of 3% in developed countries, giving rise to an urgency to improve our understanding of the disease. Applying biomechanics investigation on IUGR placental tissues can give important new insights. We performed pressure-diameter mechanical testing of placental chorionic arteries and found that in severe IUGR cases (RI > 90th centile) but not in IUGR cases (RI < 90th centile), vascular distensibility was significantly increased from normal. Constitutive modeling demonstrated that a simplified Fung-type hyperelastic model was able to describe the mechanical properties well, and histology showed that severe IUGR had the lowest collagen to elastin ratio. To demonstrate that the increased distensibility in the severe IUGR group was related to their elevated umbilical resistance and pulsatility indices, we modelled the placental circulation using a Windkessel model, and demonstrated that vascular compliance (and not just vascular resistance) directly affected blood flow pulsatility, suggesting that it is an important parameter for the disease. Our study showed that biomechanics study on placenta could extend our understanding on placenta physiology.

Breast magnetic resonance elastography: a review of clinical work and future perspectives

This review on magnetic resonance elastography (MRE) of the breast provides an overview of available literature and describes current developments in the field of breast MRE, including new transducer technology for data acquisition and multi-frequency-derived power-law behaviour of tissue. Moreover, we discuss the future potential of breast MRE, which goes beyond its original application as an additional tool in differentiating benign from malignant breast lesions. These areas of ongoing and future research include MRE for pre-operative tumour delineation, staging, monitoring and predicting response to treatment, as well as prediction of the metastatic potential of primary tumours.

An analytical poroelastic model of a non-homogeneous medium under creep compression for ultrasound poroelastography applications - Part I

An analytical theory for the unconfined creep behavior of a cylindrical inclusion (simulating a soft tissue tumor) embedded in a cylindrical background sample (simulating normal tissue) is presented and analyzed in this paper. Both the inclusion and the background are considered as fluid-filled, porous materials, each of them being characterized by a set of mechanical properties. Specifically, in this paper, the inclusion is considered to be less permeable than the background. The cylindrical sample is compressed using a constant pressure within two frictionless plates and is allowed to expand in an unconfined way along the radial direction. Analytical expressions for the effective Poisson's ratio (EPR) and fluid pressure inside and outside the inclusion are derived and analyzed. The theoretical results are validated using finite element models (FEM). Statistical analysis shows excellent agreement between the results obtained from the developed model and the results from FEM. Thus the developed theoretical model can be used in medical imaging modalities such as ultrasound poroelastography to extract the mechanical parameters of tissues and/or to better understand the impact of the different mechanical parameters on the estimated displacements, strains, stresses and fluid pressure inside a tumor and in the surrounding tissue.

Complex mechanics of the heterogeneous extracellular matrix in cancer

The extracellular matrix (ECM) performs many critical functions, one of which is to provide structural and mechanical integrity, and many of the constituent proteins have clear mechanical roles. The composition and structural characteristics of the ECM are widely variable among different tissues, suiting diverse functional needs. In diseased tissues, particularly solid tumors, the ECM is complex and influences disease progression. Cancer and stromal cells can significantly influence the matrix composition and structure and thus the mechanical properties of the tumor microenvironment (TME). In this review, we describe the interactions that give rise to the structural heterogeneity of the ECM and present the techniques that are widely employed to measure ECM properties and remodeling dynamics. Furthermore, we review the tools for measuring the distinct nature of cell-ECM interactions within the TME.

Broadband diffuse optical characterization of elastin for biomedical applications

Elastin is a key structural protein of dynamic connective tissues widely found in the extracellular matrix of skin, arteries, lungs and ligaments. It is responsible for a range of diseases related to aging of biological tissues. The optical characterization of elastin can open new opportunities for its investigation in biomedical studies. In this work, we present the absorption spectra of elastin using a broadband (550-1350nm) diffuse optical spectrometer. Distortions caused by fluorescence and finite bandwidth of the laser source on estimated absorption were effectively accounted for in measurements and data analysis and compensated. A comprehensive summary and comparison between collagen and elastin is presented, highlighting distinct features for its accurate quantification in biological applications.

Supersonic transient magnetic resonance elastography for quantitative assessment of tissue elasticity

Non-invasive, quantitative methods to assess the properties of biological tissues are needed for many therapeutic and tissue engineering applications. Magnetic resonance elastography (MRE) has historically relied on external vibration to generate periodic shear waves. In order to focally assess a biomaterial or to monitor the response to ablative therapy, the interrogation of a specific region of interest by a focused beam is desirable and transient MRE (t-MRE) techniques have previously been developed to accomplish this goal. Also, strategies employing a series of discrete ultrasound pulses directed to increasing depths along a single line-of-sight have been designed to generate a quasi-planar shear wave. Such 'supersonic' excitations have been applied for ultrasound elasticity measurements. The resulting shear wave is higher in amplitude than that generated from a single excitation and the properties of the media are simply visualized and quantified due to the quasi-planar wave geometry and the opportunity to generate the wave at the site of interest. Here for the first time, we extend the application of supersonic methods by developing a protocol for supersonic transient magnetic resonance elastography (sst-MRE) using an MR-guided focused ultrasound system capable of therapeutic ablation. We apply the new protocol to quantify tissue elasticity in vitro using biologically-relevant inclusions and tissue-mimicking phantoms, compare the results with elasticity maps acquired with ultrasound shear wave elasticity imaging (US-SWEI), and validate both methods with mechanical testing. We found that a modified time-of-flight (TOF) method efficiently quantified shear modulus from sst-MRE data, and both the TOF and local inversion methods result in similar maps based on US-SWEI. With a three-pulse excitation, the proposed sst-MRE protocol was capable of visualizing quasi-planar shear waves propagating away from the excitation location and detecting differences in shear modulus of 1 kPa. The techniques demonstrated here have potential application in real-time in vivo lesion detection and monitoring, with particular significance for image-guided interventions.

Comparative study of shear wave-based elastography techniques in optical coherence tomography

We compare five optical coherence elastography techniques able to estimate the shear speed of waves generated by one and two sources of excitation. The first two techniques make use of one piezoelectric actuator in order to produce a continuous shear wave propagation or a tone-burst propagation (TBP) of 400 Hz over a gelatin tissue-mimicking phantom. The remaining techniques utilize a second actuator located on the opposite side of the region of interest in order to create three types of interference patterns: crawling waves, swept crawling waves, and standing waves, depending on the selection of the frequency difference between the two actuators. We evaluated accuracy, contrast to noise ratio, resolution, and acquisition time for each technique during experiments. Numerical simulations were also performed in order to support the experimental findings. Results suggest that in the presence of strong internal reflections, single source methods are more accurate and less variable when compared to the two-actuator methods. In particular, TBP reports the best performance with an accuracy error < 4.1 % . Finally, the TBP was tested in a fresh chicken tibialis anterior muscle with a localized thermally ablated lesion in order to evaluate its performance in biological tissue.

Evaluation of impact of an external breast shield (FlexiShield) in electronic brachytherapy for breast IORT: A phantom study

PURPOSE

To investigate Axxent (iCAD, Inc., San Jose, CA) electronic brachytherapy balloon deformation and its dosimetric impact because of an external flexible shield (FlexiShield [FS]; iCAD, Inc.).

METHODS AND MATERIALS

Prostheses breast tissue phantom overlaid three spherical balloon applicators to simulate three clinical scenarios depending on minimum skin-to-balloon surface spacing (SS): balloon with SS of 2 cm, 1 cm, and balloon with 1 cm SS and touching the chest wall. Two sets of megavoltage CT (MVCT) scans were obtained with or without FS for 15 different sizes of balloons. For 45 pairs of MVCT scans, balloon deformation was measured in superior-inferior (d_SI) dimension on coronal and sagittal planes and anterior-posterior (d_AP) and lateral (d_LAT) dimensions on the equatorial plane of balloon. SS was also compared. A treatment plan was made on each MVCT scan. Doses at four balloon surface points and skin were compared. Conformity index value was also compared to evaluate three-dimensional dose distribution. Clinically, 20 Gy was prescribed to the surface of balloon.

RESULTS

Balloon deformation was observed with compression in SI and AP dimensions and expansion in lateral dimension. Average SI compression was 0.5 mm. Average d_Lat - d_AP was 2.4 mm, which resulted in elevated point doses at AP dimension by 10.8% of prescribed dose and reduced point doses at lateral dimension by 4.6%. FS decreased SS by 1.8 mm, increasing skin dose by 1.2 Gy, on average. Conformity index value was decreased from 0.922 to 0.908, on average.

CONCLUSIONS

This phantom study demonstrates that use of skin shielding during breast intraoperative radiation therapy can cause balloon deformation and SS reduction, resulting in dosimetric changes that are disregarded in current practice.

Molecular imaging and cancer gene therapy

Gene therapy is known as one of the most advanced approaches for therapeutic prospects ranging from tackling genetic diseases to combating cancer. In this approach, different viral and nonviral vector systems such as retrovirus, lentivirus, plasmid and transposon have been designed and employed. These vector systems are designed to target different therapeutic genes in various tissues and cells such as tumor cells. Therefore, detection of the vectors containing therapeutic genes and monitoring of response to the treatment are the main issues that are commonly faced by researchers. Imaging techniques have been critical in guiding physicians in the more accurate and precise diagnosis and monitoring of cancer patients in different phases of malignancies. Imaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are non-invasive and powerful tools for monitoring of the distribution of transgene expression over time and assessing patients who have received therapeutic genes. Here, we discuss most recent advances in cancer gene therapy and molecular approaches as well as imaging techniques that are utilized to detect cancer gene therapeutics and to monitor the patients' response to these therapies worldwide, particularly in Iranian Academic Medical Centers and Hospitals.Cancer Gene Therapy advance online publication, 18 November 2016; doi:10.1038/cgt.2016.62.

Imaging techniques: new avenues in cancer gene and cell therapy

Cancer is one of the world's most concerning health problems and poses many challenges in the range of approaches associated with the treatment of cancer. Current understanding of this disease brings to the fore a number of novel therapies that can be useful in the treatment of cancer. Among them, gene and cell therapies have emerged as novel and effective approaches. One of the most important challenges for cancer gene and cell therapies is correct monitoring of the modified genes and cells. In fact, visual tracking of therapeutic cells, immune cells, stem cells and genetic vectors that contain therapeutic genes and the various drugs is important in cancer therapy. Similarly, molecular imaging, such as nanosystems, fluorescence, bioluminescence, positron emission tomography, single photon-emission computed tomography and magnetic resonance imaging, have also been found to be powerful tools in monitoring cancer patients who have received therapeutic cell and gene therapies or drug therapies. In this review, we focus on these therapies and their molecular imaging techniques in treating and monitoring the progress of the therapies on various types of cancer.

Functional magnetic resonance imaging techniques and their development for radiation therapy planning and monitoring in the head and neck cancers

Radiation therapy (RT), in particular intensity-modulated radiation therapy (IMRT), is becoming a more important nonsurgical treatment strategy in head and neck cancer (HNC). The further development of IMRT imposes more critical requirements on clinical imaging, and these requirements cannot be fully fulfilled by the existing radiotherapeutic imaging workhorse of X-ray based imaging methods. Magnetic resonance imaging (MRI) has increasingly gained more interests from radiation oncology community and holds great potential for RT applications, mainly due to its non-ionizing radiation nature and superior soft tissue image contrast. Beyond anatomical imaging, MRI provides a variety of functional imaging techniques to investigate the functionality and metabolism of living tissue. The major purpose of this paper is to give a concise and timely review of some advanced functional MRI techniques that may potentially benefit conformal, tailored and adaptive RT in the HNC. The basic principle of each functional MRI technique is briefly introduced and their use in RT of HNC is described. Limitation and future development of these functional MRI techniques for HNC radiotherapeutic applications are discussed. More rigorous studies are warranted to translate the hypotheses into credible evidences in order to establish the role of functional MRI in the clinical practice of head and neck radiation oncology.

Rescaled Local Interaction Simulation Approach for Shear Wave Propagation Modelling in Magnetic Resonance Elastography

Properties of soft biological tissues are increasingly used in medical diagnosis to detect various abnormalities, for example, in liver fibrosis or breast tumors. It is well known that mechanical stiffness of human organs can be obtained from organ responses to shear stress waves through Magnetic Resonance Elastography. The Local Interaction Simulation Approach is proposed for effective modelling of shear wave propagation in soft tissues. The results are validated using experimental data from Magnetic Resonance Elastography. These results show the potential of the method for shear wave propagation modelling in soft tissues. The major advantage of the proposed approach is a significant reduction of computational effort.

In vitro cancer cell-ECM interactions inform in vivo cancer treatment

The general progression of cancer drug development involves in vitro testing followed by safety and efficacy evaluation in clinical trials. Due to the expense of bringing candidate drugs to trials, in vitro models of cancer cells and tumor biology are required to screen drugs. There are many examples of drugs exhibiting cytotoxic behavior in cancer cells in vitro but losing efficacy in vivo, and in many cases, this is the result of poorly understood chemoresistant effects conferred by the cancer microenvironment. To address this, improved methods for culturing cancer cells in biomimetic scaffolds have been developed; along the way, a great deal about the nature of cancer cell-extracellular matrix (ECM) interactions has been discovered. These discoveries will continue to be leveraged both in the development of novel drugs targeting these interactions and in the fabrication of biomimetic substrates for efficient cancer drug screening in vitro.

General review of magnetic resonance elastography

Magnetic resonance elastography (MRE) is an innovative imaging technique for the non-invasive quantification of the biomechanical properties of soft tissues via the direct visualization of propagating shear waves in vivo using a modified phase-contrast magnetic resonance imaging (MRI) sequence. Fundamentally, MRE employs the same physical property that physicians utilize when performing manual palpation - that healthy and diseased tissues can be differentiated on the basis of widely differing mechanical stiffness. By performing “virtual palpation”, MRE is able to provide information that is beyond the capabilities of conventional morphologic imaging modalities. In an era of increasing adoption of multi-parametric imaging approaches for solving complex problems, MRE can be seamlessly incorporated into a standard MRI examination to provide a rapid, reliable and comprehensive imaging evaluation at a single patient appointment. Originally described by the Mayo Clinic in 1995, the technique represents the most accurate non-invasive method for the detection and staging of liver fibrosis and is currently performed in more than 100 centers worldwide. In this general review, the mechanical properties of soft tissues, principles of MRE, clinical applications of MRE in the liver and beyond, and limitations and future directions of this discipline -are discussed. Selected diagrams and images are provided for illustration.

Automatic Segmentation of Mechanically Inhomogeneous Tissues Based on Deformation Gradient Jump

Variations in properties, active behavior, injury, scarring, and/or disease can all cause a tissue's mechanical behavior to be heterogeneous. Advances in imaging technology allow for accurate full-field displacement tracking of both in vitro and in vivo deformation from an applied load. While detailed strain fields provide some insight into tissue behavior, material properties are usually determined by fitting stress-strain behavior with a constitutive equation. However, the determination of the mechanical behavior of heterogeneous soft tissue requires a spatially varying constitutive equation (i.e., one in which the material parameters vary with position). We present an approach that computationally dissects the sample domain into many homogeneous subdomains, wherein subdomain boundaries are formed by applying a betweenness based graphical analysis to the deformation gradient field to identify locations with large discontinuities. This novel partitioning technique successfully determined the shape, size and location of regions with locally similar material properties for: (1) a series of simulated soft tissue samples prescribed with both abrupt and gradual changes in anisotropy strength, prescribed fiber alignment, stiffness, and nonlinearity, (2) tissue analogs (PDMS and collagen gels) which were tested biaxially and speckle tracked (3) and soft tissues which exhibited a natural variation in properties (cadaveric supraspinatus tendon), a pathologic variation in properties (thoracic aorta containing transmural plaque), and active behavior (contracting cardiac sheet). The routine enables the dissection of samples computationally rather than physically, allowing for the study of small tissues specimens with unknown and irregular inhomogeneity.

Concurrent Visualization of Acoustic Radiation Force Displacement and Shear Wave Propagation with 7T MRI

Manual palpation is a common and very informative diagnostic tool based on estimation of changes in the stiffness of tissues that result from pathology. In the case of a small lesion or a lesion that is located deep within the body, it is difficult for changes in mechanical properties of tissue to be detected or evaluated via palpation. Furthermore, palpation is non-quantitative and cannot be used to localize the lesion. Magnetic Resonance-guided Focused Ultrasound (MRgFUS) can also be used to evaluate the properties of biological tissues non-invasively. In this study, an MRgFUS system combines high field (7T) MR and 3 MHz focused ultrasound to provide high resolution MR imaging and a small ultrasonic interrogation region (~0.5 x 0.5 x 2 mm), as compared with current clinical systems. MR-Acoustic Radiation Force Imaging (MR-ARFI) provides a reliable and efficient method for beam localization by detecting micron-scale displacements induced by ultrasound mechanical forces. The first aim of this study is to develop a sequence that can concurrently quantify acoustic radiation force displacements and image the resulting transient shear wave. Our motivation in combining these two measurements is to develop a technique that can rapidly provide both ARFI and shear wave velocity estimation data, making it suitable for use in interventional radiology. Secondly, we validate this sequence in vivo by estimating the displacement before and after high intensity focused ultrasound (HIFU) ablation, and we validate the shear wave velocity in vitro using tissue-mimicking gelatin and tofu phantoms. Such rapid acquisitions are especially useful in interventional radiology applications where minimizing scan time is highly desirable.

Multi-axis dose accumulation of noninvasive image-guided breast brachytherapy through biomechanical modeling of tissue deformation using the finite element method

PURPOSE

Noninvasive image-guided breast brachytherapy delivers conformal HDR (192)Ir brachytherapy treatments with the breast compressed, and treated in the cranial-caudal and medial-lateral directions. This technique subjects breast tissue to extreme deformations not observed for other disease sites. Given that, commercially-available software for deformable image registration cannot accurately co-register image sets obtained in these two states, a finite element analysis based on a biomechanical model was developed to deform dose distributions for each compression circumstance for dose summation.

MATERIAL AND METHODS

The model assumed the breast was under planar stress with values of 30 kPa for Young's modulus and 0.3 for Poisson's ratio. Dose distributions from round and skin-dose optimized applicators in cranial-caudal and medial-lateral compressions were deformed using 0.1 cm planar resolution. Dose distributions, skin doses, and dose-volume histograms were generated. Results were examined as a function of breast thickness, applicator size, target size, and offset distance from the center.

RESULTS

Over the range of examined thicknesses, target size increased several millimeters as compression thickness decreased. This trend increased with increasing offset distances. Applicator size minimally affected target coverage, until applicator size was less than the compressed target size. In all cases, with an applicator larger or equal to the compressed target size, > 90% of the target covered by > 90% of the prescription dose. In all cases, dose coverage became less uniform as offset distance increased and average dose increased. This effect was more pronounced for smaller target-applicator combinations.

CONCLUSIONS

The model exhibited skin dose trends that matched MC-generated benchmarking results within 2% and clinical observations over a similar range of breast thicknesses and target sizes. The model provided quantitative insight on dosimetric treatment variables over a range of clinical circumstances. These findings highlight the need for careful target localization and accurate identification of compression thickness and target offset.

Preliminary in vivo breast vibro-acoustography results with a quasi-2-d array transducer: a step forward

We previously investigated the application of a novel imaging modality, vibro-acoustography (VA), using an annular confocal transducer (confocal VA) integrated into a clinical prone stereotactic mammography system, to detect various breast abnormalities. To shorten the scanning time and provide improved coverage of the breast, we have evolved our imaging system by implementing VA on a clinical ultrasound scanner equipped with a ‘‘quasi-2-D’’ array transducer. We call this technique ‘‘quasi-2-D vibro-acoustography’’ (Q2-DVA). A clinical ultrasound scanner (GE Vivid 7) was modified to perform both ultrasound imaging and VAusing an array transducer consisting of a matrix of 12 rows by 70 columns of ultrasound elements. The newly designed system was used to perform VA on patients with either benign or cancerous lesions. Our results indicate that benign and malignant solid breast lesions were easily detected using our newly modified VA system. It was also possible to detect microcalcifications within the breast. Our results suggest that with further development, Q2-DVA could provide high-resolution diagnostic information in the clinical setting and may be used either as a stand-alone or as a complementary tool in support of other clinical imaging modalities.

Anisotropic polyvinyl alcohol hydrogel phantom for shear wave elastography in fibrous biological soft tissue: a multimodality characterization

Shear wave elastography imaging techniques provide quantitative measurement of soft tissues elastic properties. Tendons, muscles and cerebral tissues are composed of fibers, which induce a strong anisotropic effect on the mechanical behavior. Currently, these tissues cannot be accurately represented by existing elastography phantoms. Recently, a novel approach for orthotropic hydrogel mimicking soft tissues has been developed (Millon et al 2006 J. Biomed. Mater. Res. B 305-11). The mechanical anisotropy is induced in a polyvinyl alcohol (PVA) cryogel by stretching the physical crosslinks of the polymeric chains while undergoing freeze/thaw cycles. In the present study we propose an original multimodality imaging characterization of this new transverse isotropic (TI) PVA hydrogel. Multiple properties were investigated using a large variety of techniques at different scales compared with an isotropic PVA hydrogel undergoing similar imaging and rheology protocols. The anisotropic mechanical (dynamic and static) properties were studied using supersonic shear wave imaging technique, full-field optical coherence tomography (FFOCT) strain imaging and classical linear rheometry using dynamic mechanical analysis. The anisotropic optical and ultrasonic spatial coherence properties were measured by FFOCT volumetric imaging and backscatter tensor imaging, respectively. Correlation of mechanical and optical properties demonstrates the complementarity of these techniques for the study of anisotropy on a multi-scale range as well as the potential of this TI phantom as fibrous tissue-mimicking phantom for shear wave elastographic applications.

Magnetomotive optical coherence tomography for the assessment of atherosclerotic lesions using αvβ3 integrin-targeted microspheres

PURPOSE

We investigated the early-stage fatty streaks/plaques detection using magnetomotive optical coherence tomography (MM-OCT) in conjunction with αvβ3 integrin-targeted magnetic microspheres (MSs). The targeting of functionalized MSs was investigated by perfusing ex vivo aortas from an atherosclerotic rabbit model in a custom-designed flow chamber at physiologically relevant pulsatile flow rates and pressures.

PROCEDURES

Aortas were extracted and placed in a flow chamber. Magnetic MS contrast agents were perfused through the aortas and MM-OCT, fluorescence confocal, and bright field microscopy were performed on the ex vivo aorta specimens for localizing the MSs.

RESULTS

The results showed a statistically significant and stronger MM-OCT signal (3.30 ± 1.73 dB) from the aorta segment perfused with targeted MSs, compared with the nontargeted MSs (1.18 ± 0.94 dB) and control (0.78 ± 0.41 dB) aortas. In addition, there was a good co-registration of MM-OCT signals with confocal microscopy.

CONCLUSIONS

Early-stage fatty streaks/plaques have been successfully detected using MM-OCT in conjunction with αvβ3 integrin-targeted magnetic MSs.

A regularization-free elasticity reconstruction method for ultrasound elastography with freehand scan

BACKGROUND

In ultrasound elastography, reconstruction of tissue elasticity (e.g., Young's modulus) requires regularization and known information of forces and/or displacements on tissue boundaries. In practice, it is challenging to choose an appropriate regularization parameter; and the boundary conditions are difficult to obtain in vivo. The purpose of this study is to develop a more applicable algorithm that does not need any regularization or boundary force/displacement information.

METHODS

The proposed method adopts the bicubic B-spline as the tissue motion model to estimate the displacement fields. Then the estimated displacements are input to the finite element inversion scheme to reconstruct the Young's modulus of each element. In the inversion, a modulus boundary condition is used instead of force/displacement boundary conditions. Simulation and experiments on tissue-mimicking phantoms are carried out to test the proposed method.

RESULTS

The simulation results demonstrate that Young's modulus reconstruction of the proposed method has a relative error of -3.43 ± 0.43% and root-squared-mean error of 16.94 ± 0.25%. The phantom experimental results show that the target hardening artifacts in the strain images are significantly reduced in the Young's modulus images. In both simulation and phantom studies, the size and position of inclusions can be accurately depicted in the modulus images.

CONCLUSIONS

The proposed method can reconstruct tissue Young's modulus distribution with a high accuracy. It can reduce the artifacts shown in the strain image and correctly delineate the locations and sizes of inclusions. Unlike most modulus reconstruction methods, it does not need any regularization during the inversion procedure. Furthermore, it does not need to measure the boundary conditions of displacement or force. Thus this method can be used with a freehand scan, which facilitates its usage in the clinic.