File Name: introduction to medical imaging physics engineering and clinical applications .zip
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Theranostics ; 7 5 Rosa M. Willmann 1. Elastography-based imaging techniques have received substantial attention in recent years for non-invasive assessment of tissue mechanical properties. These techniques take advantage of changed soft tissue elasticity in various pathologies to yield qualitative and quantitative information that can be used for diagnostic purposes.
Measurements are acquired in specialized imaging modes that can detect tissue stiffness in response to an applied mechanical force compression or shear wave.
Ultrasound-based methods are of particular interest due to its many inherent advantages, such as wide availability including at the bedside and relatively low cost. Several ultrasound elastography techniques using different excitation methods have been developed. In general, these can be classified into strain imaging methods that use internal or external compression stimuli, and shear wave imaging that use ultrasound-generated traveling shear wave stimuli.
While ultrasound elastography has shown promising results for non-invasive assessment of liver fibrosis, new applications in breast, thyroid, prostate, kidney and lymph node imaging are emerging. Here, we review the basic principles, foundation physics, and limitations of ultrasound elastography and summarize its current clinical use and ongoing developments in various clinical applications.
Ultrasound elastography USE is an imaging technology sensitive to tissue stiffness that was first described in the s [ 1 ]. It has been further developed and refined in recent years to enable quantitative assessments of tissue stiffness. Elastography methods take advantage of the changed elasticity of soft tissues resulting from specific pathological or physiological processes [ 2 ]. For instance, many solid tumors are known to differ mechanically from surrounding healthy tissues.
Similarly, fibrosis associated with chronic liver diseases causes the liver to become stiffer than normal tissues. Elastography methods can hence be used to differentiate affected from normal tissue for diagnostic applications. Conventional ultrasound US has the advantage of being an inexpensive, versatile, and widely available modality that can be used at the bedside, which also applies to USE.
USE has been explored for several clinical applications in recent years and has been introduced into clinical routine for specific applications such as liver fibrosis assessment or breast lesion characterization. Elasticity imaging by USE provides complementary information to conventional US by adding stiffness as another measurable property to current US imaging techniques [ 3 ].
In this review, we provide an overview of the principles and concepts of USE, describe various USE techniques, and discuss clinical applications of USE in the liver, breast, thyroid, kidney, prostate and lymph nodes. The following provides a brief summary of USE physics and current techniques.
More in depth reviews of elastography physics can be found elsewhere [ 2 , 4 , 5 ]. Elastography assesses tissue elasticity, which is the tendency of tissue to resist deformation with an applied force, or to resume its original shape after removal of the force.
Assuming that a material is entirely elastic and its deformation has no time dependency i. Ultrasound elastography physics, deformation models. This is applied to normal perpendicular to surface, first column , shear tangential to surface, second column , and bulk normal inward or pressure, third column forces used in ultrasound elastography.
Ultrasound elastography physics, measurement methods. In strain imaging a , tissue displacement is measured by correlation of RF echo signals between search windows boxes in the states before and after compression.
In shear wave imaging b , particle motion is perpendicular to the direction of wave propagation, with shear wave speed c S related to shear modulus G. In B-mode ultrasound c , particle motion is parallel to the direction of wave propagation, with longitudinal wave speed c L related to bulk modulus K.
While longitudinal waves are used in B-mode US, the relatively small differences in wave speed and hence K between different soft tissues do not allow adequate tissue contrast for elastography measurements.
The low wave speed in soft tissues allows for high differences in G between tissues, giving suitable tissue contrast for elastography measurements. The three types of deformations and elastic moduli are not independent, but have relationships as the solid attempts to retain its original volume, the success of which is described by the Poisson's ratio v. Although the proof is outside of the scope of this review, the relationship between Young's modulus E and shear modulus G is as follows [ 2 ]:.
Given the high-water content of soft tissue, the Poisson's ratio v is near 0. Using this with Equation 7, we obtain:. The relationships between Young's modulus E , shear modulus G , and shear wave speed c S are important because different parameters are reported according to the elastography technique and vendor.
MR elastography reports the magnitude of the complex shear modulus G , which has both elastic and viscous components and is calculated from phase-contrast multiphase pulse sequence data [ 5 ]. Ultrasound shear wave imaging directly measures shear wave speed c S , which is either reported or converted to Young's modulus E.
While it is technically easy to convert between E and G via equation 9, estimations of these values depend on the used frequency of excitation, making comparison of E reported in USE and G in MR elastography difficult.
Ultrasound Elastography Techniques. Currently available USE techniques can be categorized by the measured physical quantity: 1 strain imaging left , and 2 shear wave imaging right. From these principles, the different currently available USE techniques can be classified by the measured physical quantity Figure 3 :. Shear waves created by the excitation are measured perpendicular to the acoustic radiation force application or parallel to the 1D transient elastography excitation; the shear wave speed c s is reported, or Young's modulus E is computed and reported using Equation 9.
Strain imaging was the first introduced USE technique [ 7 ] and there are two approaches for strain imaging using ultrasound: Strain elastography SE and acoustic radiation force impulse ARFI strain imaging Figure 3. Manual compression works fairly well for superficial organs such as the breast and thyroid but is challenging for assessing elasticity in deeper located organs such as the liver [ 8 ].
Since this method is not dependent on superficially applied compression, it may be used to assess deeper located organs [ 1 ]. The induced tissue displacement in the same direction as the applied stress is then measured by a number of different methods dependent on the manufacturer, including radiofrequency RF echo correlation-based tracking, Doppler processing, or a combination of the two methods.
We review RF echo correlation-based tracking, one of the most common and simplest methods. The strain measurements are displayed as a semitransparent color map called an elastogram, which is overlaid on the B-mode image. Typically, low strain stiff tissue is displayed in blue, and high strain soft tissue is displayed in red, although the color scale can vary depending on the ultrasound vendor [ 1 , 10 ].
A pseudo-quantitative measurement called the strain ratio can be used, which is the ratio of strain measured in adjacent usually normal reference tissue region of interest ROI to strain measured in a target lesion ROI. This is an alternative approach for measuring strain. In this technique a short-duration 0. The displacement within a specified ROI is subsequently measured by the same methods as in strain elastography. Also, similar to strain elastography, the displacements may be displayed as an elastogram overlaid on the B-mode image [ 13 ].
In contrast to strain imaging, which measures physical tissue displacement parallel to the applied normal stress, SWI employs a dynamic stress to generate shear waves in the parallel or perpendicular dimensions.
Measurement of the shear wave speed results in qualitative and quantitative estimates of tissue elasticity. The main characteristics of each method are summarized in Figure 4. It is the most widely used and validated technique for assessment of liver fibrosis, and it is often used by clinicians in the office. The Fibroscan TM probe is a single device that contains both an ultrasound transducer and a mechanical vibrating device.
The operator selects the imaging area using time-motion ultrasound based on multiple A-mode lines in time at different proximal locations assembled to form a low quality image to locate a liver portion 2.
The same probe then uses A-mode US to measure the shear wave speed and Young's modulus E is calculated [ 15 ]. The entire exam takes approximately 5 minutes [ 17 ]. Unlike ARFI strain imaging, the tissue displacement itself is not measured. Instead, a portion of the longitudinal waves generated by ARFI is intra-converted to shear waves through the absorption of acoustic energy [ 12 ].
The speed of the shear waves perpendicular to the plane of excitation c s are measured, which are either directly reported or converted Young's modulus E and reported to provide a quantitative estimate of tissue elasticity. First, the operator can use B-mode US to directly visualize the liver to select a uniform area of liver parenchyma without large vessels or dilated bile ducts.
Also, unlike 1D-TE where the shear waves are produced by excitation at the body surface, pSWE produces shear waves which originate locally inside the liver, making pSWE less affected by ascites and obesity [ 5 , 6 , 18 ]. Instead of a single focal location as in ARFI strain imaging and pSWE, multiple focal zones are interrogated in rapid succession, faster than the shear wave speed. This creates a near cylindrical shear wave cone, allowing real-time monitoring of shear waves in 2D for measurement of shear wave speed or Young's modulus E and generation of quantitative elastograms [ 4 ].
The advantages of this technique include real-time visualization of a color quantitative elastogram superimposed on a B-mode image [ 19 ], enabling the operator to be guided by both anatomical and tissue stiffness information [ 20 ]. With a growing clinical interest in developing new USE applications, or refining existing ones, it is essential to understand current technical limitations that hinder reproducibility of measurements.
Several technical confounders are known to affect USE. A number of these can be traced back to general sonography limitations such as shadowing, reverberation, and clutter artifacts, or the operator-dependent nature of free-hand ultrasound systems [ 6 , 21 ]. Similarly, tissue attenuation decreases ultrasound signal as a function of depth, limiting accurate assessment of deeper tissue or organs.
Fluid or subcutaneous fat also attenuates propagation of the external stimulus applied at the skin surface i. Fibroscan TM , which can invalidate measurements in the setting of obesity or abdominal ascites [ 21 ]. System settings and parameters i. In addition, the lack of uniformity of commercial system design and settings makes comparing measurements from one manufacturer system to another a difficult task [ 4 , 6 , 19 , 21 , 22 ].
However, efforts are underway to address some of these concerns; for example, an initiative by the Quantitative Imaging Biomarkers Alliance QIBA is attempting to use phantoms to standardize quantitative measurements from different USE techniques [ 22 , 23 ]. Of the USE methods described above, measurements from methods that utilize external stimuli, such as strain elastography, are the most challenging to reproduce.
Measurements in these modes are highly subjective since the magnitude of the applied stress is difficult to control with operator dependent manual compression and the inherent variability of physiologic motion when used as a stimulus. Selection of the ROI is also operator dependent and can introduce variability [ 18 ].
In addition, the extent of stress induced by an operator can result in strain concentration artifacts around specific structures, which then distort the strain field and generate artifacts in images or erroneous measurements [ 6 , 19 , 24 ]. As a result, SE methods only allow semi-quantitative assessments of stiffness that are difficult to compare longitudinally.
Elastography in general is also susceptible to internal sources of stress i. For example in liver applications, it is preferable to measure stiffness in the right lobe over the left lobe to minimize internal stimulations generated by the nearby palpating heart which can result in erroneous measurements.
In the case of elastography modes that utilize internal sources of excitation stress i. Core assumptions are that the tissue is:. To date, these assumptions have held in specific clinical scenarios i.
In principle, these assumptions violate conventional models that describe soft tissue mechanical properties as complex and heterogeneous materials that have both a viscous and an elastic mechanical response when probed [ 21 ].
The first half addresses the physics of important imaging modalities now in use: ultrasound, CT, MRI, and the recently emerging flat panel x-ray detectors and their application to mammography. Dvorak, Pavel. Webster, E. Our graduates go on to exciting roles in research, clinical science and the NHS, and industry. They lie at the crossroads of frontier re-search in physics, biology, chemistry, and medicine. History of the department.
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. Smith and A. Smith , A. Webb Published Engineering. General image characteristics, data acquisition and image reconstruction 2. X-ray planar radiology and computed tomography 3.
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues physiology. Medical imaging seeks to reveal internal structures hidden by the skin and bones, as well as to diagnose and treat disease. Medical imaging also establishes a database of normal anatomy and physiology to make it possible to identify abnormalities. Although imaging of removed organs and tissues can be performed for medical reasons, such procedures are usually considered part of pathology instead of medical imaging.
Ignoreeri ja kuva leht. Alates Teised raamatud teemal : Medical imaging - Hetkel poes: 11 nimetust.
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Nanotheranostics ; 5 3 : International Journal of Biological Sciences. International Journal of Medical Sciences. Journal of Cancer. Journal of Genomics.
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