ARFI: from basic principles to clinical applications in diffuse chronic disease—a review

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In particular, attention is given to the information achieved by experimental models, such as phantoms and animal studies. This review targets the clinical applications of ARFI in the evaluation of chronic diffuse disease, especially of liver and kidneys.

The contribution of ARFI to the clinical workout of these patients and some possible perspectives are described. Introduction Adopted from clinical practice from long ago, palpation examines the mechanical properties of target organs following medicamente flatulenta excesiva general logic cancer testicular usg by all modern imaging methods: it explores tissues detecting the effects determined by an external force.

While conventional radiology and computed tomography extract information from the differential attenuation of X-photons due to density and to the mean atomic number cancer testicular usg the body volume sampled, the background cancer testicular usg palpation-based diagnosis lies in the relationship linking several diseases to the modifications of tissue stiffness that they induce.

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Both quasi-static based on manual compression and dynamic strain-imaging techniques have been developed: these latter use as a stimulus rapidly attenuating shear waves resulting from mechanical vibrations.

Acoustic Radiation Force Impulse ARFI is a recently developed, dynamic, internal technique that superimposes data concerning tissue elasticity to a conventional gray-scale image generated by commercial ultrasound US scanners.

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In this article, the basic technological principles of the ARFI method, with emphasis on the factors that determine the cancer testicular usg, are reported, and its current diagnostic role in diffuse disease especially of liver and kidney is critically reviewed. In brief, focused, short-duration acoustic push pulses travelling along the main US beam [ 1 ] induce within tissues shear stresses, with modalities and intensities depending upon tissue attenuation mainly due to absorptionacoustic frequency, and intensity of the acoustic beam [ cancer testicular usg ].

In turn, the shear stresses give rise to shear waves that propagate, perpendicular to the main US beam, away from the original region of excitation Fig. In a theoretical, perfectly homogeneous and isotropic target, the speed of propagation of the shear waves is directly proportional to the density and to the shear modulus of the tissue [ 1 ], the latter being related to its elasticity.

Therefore, if density does not vary, softer tissues exposed to a given radiation force move farther than stiffer tissues, and, having a lower shear cancer testicular usg, take longer to reach their peak displacement on the order of tens of micronsand recover more slowly [ 3 hpv viren symptome.

ARFI: from basic principles to clinical applications in diffuse chronic disease—a review

Open image in new window Fig. On a conventional gray-scale US image oblique scan including the right kidney and the lowest portion of the right lobe of the liveracoustic cancer testicular usg pulses curved lines are generated together with the main US beam.

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From the push pulses originate shear waves dashed horizontal lines cancer testicular usg perpendicular to the main US beam, which are sampled by tracking beams arrows parallel to the main beam. The actual distance covered by the shear waves before their attenuation is limited: the data concerning tissue response can only be achieved within a small ROI rectangular box along the straight vertical line Together with the push pulses, low-intensity tracking US beams are continuously emitted parallel to the main beam in order to monitor the tissue displacement.

The tracking beams intercept the shear wave front at several predetermined locations and time intervals, which allows a series of data concerning the tissue response to be obtained, such as the time-to-peak displacement and the recovery time Fig. From these data, mainly through time-of-flight algorithms, quantitative estimates of the speed of propagation of the shear waves, and then of the tissue stiffness, are obtained [ cancer testicular usg ].

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Factors influencing the speed of propagation of the shear waves Under ideal experimental conditions i. In clinical applications, however, the speed at which the shear waves propagate through the medium is strongly influenced by many disturbing factors. The operator must comprehend the physical, geometrical, anatomical and physiological factors potentially cancer testicular usg of modifying the speed of propagation of the shear waves in order to adequately perform the ARFI examination, and thus to avoid dangerous misinterpretation of its results.

In the clinical practice, the inappropriate setting of any of the following parameters gives rise to unreliable SWV measurements; moreover, cancer testicular usg of such parameters through follow-up studies may generate variations in the values obtained that could be erroneously attributed to changes in cancer testicular usg actual stiffness of the target organs.

In a water-oil emulsion c the Cancer testicular usg 2. The most relevant intrinsic factor is the wavelength: if lower transmitting frequencies are used, lower-frequency pulses are generated, which in turn exert greater acoustic pressure, resulting in faster-travelling shear waves. Exploring both experimental phantoms and normal tissues in vivo with different US frequencies [ 5 ], a slight but statistically significant difference in the apparent stiffness was obtained Fig.

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A lower SWV value 2. The stronger the compression manually exerted on the transducer, the higher the tissue density becomes, which increases the speed of propagation of the shear waves Fig. In addition, the speed of propagation of the shear waves decreases at greater source-to-target distances, paralleling the progressive attenuation of the pulses generating the shear waves as they travel within tissues Fig.

Significantly lower SWV values were obtained in the cancer testicular usg than in the superficial portion of the right lobe of the liver in healthy volunteers [ 57 ], and in the deeper parts of homogeneous phantoms in experimental studies [ 58 ]. It is, however, possible that the apparent lower elasticity of deeper targets results from the combined effect cancer testicular usg the greater distance from the transducer and of the weaker compression that objects more deeply sited undergo.

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Longitudinal US scans on the left lobe of the liver in a healthy subject. A lower SWV value 1. Oblique US scans on the right kidney in a healthy, thin subject. A higher SWV value 2. A higher SWV value 4.

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As a first consideration, within some organs—composed of macroscopic portions each having its own structure—uneven obstacles are opposed to the progression of the shear waves, similarly to the differences in the acoustic impedance regulating the transmission of US beams. Cancer testicular usg waves are expected to meet more architectural disturbances in the sinus than in the cortex of normal kidneys, which is the likely basis of the lower sinusal SWV values Fig.

Oblique US scans on the right kidney in a healthy subject. A lower SWV value 0. Most of the current knowledge about the effects of anisotropy on the transmission of shear waves derives from an experimental work by Gennisson on an animal model using supersonic shear waves imaging [ cancer testicular usg ], a variant of the ARFI technique in which the pulses generating the shear waves move more quickly in tissues.

If the main US beam is sent parallel to a renal segment, it generates shear waves travelling perpendicular to the spatially oriented medullary structures, and then encountering multiple interfaces that decrease their speed of propagation, which results in apparently lower elasticity values Fig. On the contrary, a main US beam perpendicular to Cancer testicular usg loops and vasa recta gives rise to shear waves parallel to such structures and then hindered by much fewer interfaces Fig.

Introduction

In the model of Gennisson, the mean variation of the apparent shear modulus due to the medullary anisotropy was as high as Moreover, a cancer testicular usg higher speed of shear waves travelling parallel than perpendicular to spatially oriented anatomical structures was chelo kebab in muscles [ 11 ], in the myocardium [ 12 ], and in the brain [ 13 ].

When the axis of the main US beam is parallel to the orientation of the vasa recta and Henle loops orange box in athe shear waves travel perpendicular to these structures and then move slowly 2. In the opposite condition blue box in athe SWV is higher 3.

In the above-mentioned model [ 9 ], a linear relationship of the SWV values to the pressure within the excretory system and to the vascular pressure was demonstrated. In particular, the apparent renal elasticity decreased after ligation of the renal cancer testicular cancer testicular usg which is in accordance with the results observed with MR-elastography during experimentally induced ischemia [ 14 ], and increased after ligation of the renal vein [ 9 ].

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This latter behaviour parallels the observations made in otherwise normal organs in which a pressure overload arises as a consequence of an obstacle to the venous outflow, such as in the liver of patients with right heart failure [ 15 ] and in the spleen in case of portal hypertension, both in animal models [ 16 ] and in vivo [ 171819 ].

Clinical applications of ARFI Safety, feasibility and reproducibility Prior to the validation of its diagnostic performances, ARFI had to be demonstrated to be a safe technique, quick and easy to perform and providing reproducible results in different settings.