Current Medical Imaging - Volume 8, Issue 1, 2012

During the last decade, numerous methods and devices have been developed implementing elastography or mechanical imaging in various medical applications such as the differentiation of benign and malignant lesions in the liver, breast and prostate as well as the characterization of vaginal wall elasticity. Elastography is a noninvasive method in which stiffness or strain of soft tissue are used to detect or classify tumors. Mechanical imaging is a tactile or stress imaging based on the visualization of tissue internal structures in terms of their elasticity modulus. A tumor or a suspicious cancerous growth is normally 5-28 times stiffer than the background of normal tissue. When a mechanical compression or vibration is applied, the tumor deforms less than the surrounding tissue. The strain in the tumor is less than the surrounding tissue. Elastography has been shown to be affected by the degree of adherence of the tumor to its surroundings, indicating a potential to extend its method to tumor mobility characterization to improve diagnostic accuracy and surgical guidance. Ultrasonic imaging is the most common medical imaging technique producing elastograms, and it has the advantages of being cheaper, faster and more portable than other technique. Magnetic resonance elastogram combines MRI with sound waves to create a visual map, showing the elasticity of body tissue, and is used primarily to detect hardening of the liver. This issue present updated informations about the techniques and clinical applications of new emerging technology by world experts. Aglyamov et al. discuss the applications of elasticity imaging and sensing using targeted motion using laser-induced gas microbubbles as well as magnetic nanoparticles, and also report their recent experimental results. Konofagou et al. review the applications of harmonic motion imaging for tumor detection as well as its relevance in thermal treatment. Urban et al. provide an overview of viscoelasticity and development as well as application of shear wave velocity dispersion ultrasound vibrometry. Hansen et al. describe the basics, background and results of ultrasound strain imaging using cross-correlation. Litwiller et al. summarize the fundamental techniques and applications of magnetic resonance elastography, especially for the staging of hepatic cirrhosis. Sinkus et al. focus on the dynamic 3D approach via MRI and report on a large patient collective that mechanical parameters are very pertinent for the differentiation between low-grade and mid/high-grade hepatic fibrosis. Finally Sarvazyan and Artann laboratories review the biomechanical basis of mechanical imaging and its applications of evaluating breast cancer, prostate lesions and vaginal wall elasticity.

From times immemorial, medical practitioners have used palpation for detection and characterization of pathologies. Recently, a new diagnostic imaging modality has emerged, called elasticity imaging (EI), which is an extension of the ancient art of palpation. Various imaging modalities may be used for EI, including ultrasound, MRI or even X-ray, to visualize the elasticity (stiffness) of soft tissue structures. Elasticity imaging is a cutting-edge diagnostic modality that can be adapted for a very wide range of medical applications. Because many diseases are accompanied by significant changes in tissue mechanical properties and various tumors, especially malignancies, have elastic properties that are often markedly different from surrounding tissues, EI could provide a significant adjunct to current diagnostic methods. Lesions in the breast, liver, thyroid, prostate, pancreas, and lymph nodes have been successfully imaged using EI. Elasticity imaging has the potential to differentiate benign and malignant lesions, to detect steatohepatitis in patients with fatty liver disease, to monitor ablation and other therapeutic lesion treatments. EI has demonstrated its effectiveness in assessing the progress of ablation therapy. In this two-volume special issue on hot topics, some of the world's leading research groups present reviews of many different approaches to EI. The first issue begins with an overview of the historical and theoretical basis of EI. A review of ultrasound methods for elasticity imaging using external vibrators is then presented. That article is followed by two articles where the ultrasound transducer is used as the source of deformation. The first volume of this two-issue series closes with three articles that involve acoustic radiation force techniques for EI. The second issue begins with three more articles that involve acoustic radiation force in EI. The final ultrasound-based EI approach described here uses physiological motion as the source of deformation to image the elastic properties of vasculature. Two papers then review the developments and status of elasticity imaging based on magnetic resonance imaging methods. The final paper of the second volume of this special issue reviews a method for elasticity imaging based on measurements of the surface stress distribution - a method that closely approximates manual palpation by humans. This is a rapidly developing field and, despite efforts to provide a comprehensive review, inevitably some new developments have been left out. We apologize in advance for this and suppose that there will be future reviews where recent meritorious work will be included. We take this opportunity to provide special thanks to the authors of these review articles. Each group has made important contributions to the field of elasticity imaging, has provided their expertise in reporting the current state of affairs and provided enthusiastic support for this project. We are deeply grateful.

The methods used to estimate tissue elasticity based on the motion of local inhomogeneities such as laser-induced gas microbubbles, magnetic nanoparticles and other targets under an externally applied force are reviewed. The theoretical bases of the motion of a target in a viscoelastic medium are described. Given various targets differing in size, these methods allow for elasticity measurements at different levels of spatial resolution. Short acoustic radiation force and magnetic field pulses were used to initiate the motion of various test objects (solid spheres, laser-induced microbubbles and magnetic nanoparticles) with sizes ranging from several millimeters to tens of nanometers. The induced motion was monitored by conventional pulse-echo ultrasound systems adapted for these measurements. The elasticity of the medium was evaluated based on a theoretical model describing the motion of a particle in a viscoelastic medium. Shear elasticity, evaluated using the developed approach, agrees well with independent measurements of mechanical properties of the medium. In this paper we review our recent theoretical and experimental results and discuss the applications of elasticity imaging and sensing using targeted motion, i.e., motion induced via the external forces acting on local inhomogeneities.

Palpation is an established screening procedure for the detection of several superficial cancers including breast, thyroid, prostate, and liver tumors through both self and clinical examinations. This is because solid masses typically have distinct stiffnesses compared to the surrounding normal tissue. In this paper, the application of Harmonic Motion Imaging (HMI) for tumor detection based on its stiffness as well as its relevance in thermal treatment is reviewed. HMI uses an amplitude-modulated, focused ultrasound (FUS) beam to generate an oscillatory acoustic radiation force for an internal, non-contact palpation to internally estimate relative tissue hardness. HMI studies have dealt with the estimation of tissue dynamic motion in response to an oscillatory force at the same frequency, and have been shown feasible in simulations, phantoms, ex vivo human and bovine tissues as well as animals in vivo. Since it uses a FUS beam, HMI can also be used in an ideal integration setting with thermal ablation using High-Intensity Focused Ultrasound (HIFU), which also leads to an alteration in the tumor stiffness. In this paper, a short review of HMI is provided that encompasses the findings in all the aforementioned areas. The findings presented herein demonstrate that the HMI displacement can depict the underlying tissue stiffness, and the HMI image of the relative stiffness could accurately detect and characterize the tumor or thermal lesion based on its distinct properties. HMI may thus constitute a non-ionizing, cost-efficient and reliable complementary method for tumor detection, localization, diagnosis and treatment monitoring.

Measurement of tissue elasticity has emerged as an important advance in medical imaging and tissue characterization. However, soft tissue is inherently a viscoelastic material. One way to characterize the viscoelastic material properties of a material is to measure shear wave propagation velocities within the material at different frequencies and use the dispersion of the velocities, or variation with frequency, to solve for the material properties. Shearwave Dispersion Ultrasound Vibrometry (SDUV) is an ultrasound-based technique that uses this feature to characterize the viscoelastic nature of soft tissue. This method has been used to measure the shear elasticity and viscosity in various types of soft tissues including skeletal muscle, cardiac muscle, liver, kidney, prostate, and arterial vessels. This versatile technique provides measurements of viscoelastic material properties with high spatial and temporal resolution, which can be used for assessing these properties in normal and pathologic tissues. The goals of this paper are to 1) give an overview of viscoelasticity and shear wave velocity dispersion, 2) provide a history of the development of the SDUV method, and 3) survey applications for SDUV that have been previously reported.

The rupture of atherosclerotic plaques is considered to be the main cause of cardiovascular events like stroke and myocardial infarction. Early detection of atherosclerotic plaques that have a high risk to rupture is desired to be able to intervene before the cardiovascular event occurs. During the cardiac cycle the vessel wall and plaque are deformed by the pulsating blood. The resulting strains can be estimated locally by ultrasound strain imaging/elastography. Studies with intravascular ultrasound (IVUS) elastography of the coronary arteries have showed that the presence of high strains is highly related to plaque vulnerability. However, although IVUS elastography has shown to be quite successful, its invasiveness limits the technique to being applied to patients that already are in the cath-lab. A noninvasive version of the technique is desired. With the increase in quality of ultrasonic equipment and improved accuracy of strain imaging methods, noninvasive vascular strain imaging has become within reach. Multiple research groups have and are developing methods to perform noninvasive strain imaging of the carotid arteries by ultrasound. Most methods derive strain estimates by cross-correlating ultrasound data that were acquired at different intraluminal pressures. Others apply image registration methods to estimate strains. This paper provides a description of the basics and backgrounds of ultrasound strain imaging using cross-correlation. Furthermore, advanced implementations of cross-correlation based and other techniques are discussed together with their pro's and con's. At the end, an overview is presented of the results that have been obtained with the various approaches until now.

Often compared to the practice of manual palpation, magnetic resonance elastography is an emerging technology for quantitatively assessing the mechanical properties of tissue as a basis for characterizing disease. The potential of MRE as a diagnostic tool is rooted in the fact that normal and diseased tissues often differ significantly in terms of their intrinsic mechanical properties. MRE uses magnetic resonance imaging (MRI) in conjunction with the application of mechanical shear waves to probe tissue mechanics. This process can be broken down into three essential steps: 1) inducing shear waves in the tissue, 2) imaging the propagating shear waves with MRI, and 3) analyzing the wave data to generate quantitative images of tissue stiffness MRE has emerged as a safe, reliable and noninvasive method for staging hepatic liver fibrosis, and is now used in some locations as an alternative to biopsy. MRE is also being used in the ongoing investigations of numerous other organs and tissues, including, for example, the spleen, kidney, pancreas, brain, heart, breast, skeletal muscle, prostate, vasculature, lung, spinal cord, eye, bone, and cartilage. In the article that follows, some fundamental techniques and applications of MRE are summarized.

Elasticity imaging is a rather recent non-invasive imaging modality which provides in-vivo data about the viscoelastic properties of tissue. With manual palpation being an integral part of many diagnostic procedures, it is obvious that elasticity imaging has many interesting and promising potentials in medical imaging, i.e. from tissue/lesion characterization over therapy follow-up to guidance during interventions which involve ablation. The general concept of this method is to displace the material mechanically and infer from displacement measurements the intrinsic local viscoelastic properties. Many different technical realizations exist (static, dynamic, transient) utilizing different imaging modalities (MRI, ultrasound) which all probe different frequency domains. Since viscoelastic properties of tissue change strongly with frequency, care must be taken when interpreting the data in terms of elastic and viscous component. In this review we will focus on the dynamic 3D approach via MRI, i.e. a mono-frequent mechanical excitation and a volumetric assessment of the displacement field. This allows overcoming several physical difficulties: firstly compressional waves can be properly suppressed via the application of the curl-operator, secondly waveguide effects are eliminated and finally the calculation of the complex shear modulus does not necessitate any assumption of the underlying rheological model. Clinical results on a large patient collective show that mechanical parameters are very pertinent for the differentiation between low-grade and mid/high-grade liver fibrosis. They outperform the well establish classical APRI blood test as well as a 1D ultrasound-based approach for elasticity imaging.

Mechanical Imaging (MI) is a branch of Elastography. MI differs from conventional ultrasonic and MR elastography in that it evaluates soft tissue mechanical structure using stress data rather than dynamic or static strain data. MI closely mimics manual palpation because the MI probe with a force sensor array attached to its tip acts as a palpating finger. MI is intrinsically a three-dimensional imaging modality because the surface stress patterns obtained at different levels of tissue compression are defined by three-dimensional mechanical structure of the tissue. This review presents the biomechanical basis of MI and its applications for breast cancer screening, and the differentiation of benign and malignant lesions, the visualization and evaluation of prostate conditions, and for the characterization of vaginal wall elasticity.