Elastography in the Liver - Virtual Touch Technologies
A technical reference guide for Elastography in the Liver Virtual Touch Technologies.
Technical Reference Guide Elastography in the Liver Virtual Touch Technologies SIEMENS siemens.com/strain Healthineers Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies Transducer Push pulse ................ ROI Fig. 1A: Virtual Touch quantification utilizes an acoustic push pulse (orange) to generate shear waves. Introduction Virtual Touch™ technologies provide an Virtual Touch technology utilizes Acoustic indication of the mechanical stiffness properties Radiation Force Impulse (ARFI) to generate of tissue. The degree of tissue stiffness has been shear waves. With Virtual Touch™ quantification shown to correlate to (VTq) these shear waves are quantified within a the etiology of various disease processes[1, 2] user-defined region of interest (ROI). This booklet will introduce you to the Fig. 1A and Fig. 1B show how VTq utilizes an technology. It is intended to guide you acoustic push pulse followed by detection through its application and provide pulses to calculate shear wave speed. recommendations for optimal technique. We at Siemens are excited to partner with you on your journey into the paradigm of mechanical stiffness assessment with ultrasound. John Benson Director, Clinical Marketing Ultrasound Siemens Medical Solutions Inc., USA 2 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide Transducer Detection pulse Shear ROI Fig. 1B: When detection pulses (orange waves arrows) interact with a passing shear wave (orange), the wave’s location at a specific time allows calculation of the shear wave speed. This value is related to tissue stiffness within the region of interest. Indications Since the introduction of ARFI technology in An overview of clinical applications are 2008 on the ACUSON S2000 ultrasound system, provided by: The Society of Radiologists in a large number of clinical studies have been Ultrasound, The World Federation for published on its clinical utility, reproducibility Ultrasound in Medicine & Biology, The and repeatability. Many studies can be found European Federation of Societies for Ultrasound at: http://www.mendeley.com/groups/ in Medicine and Biology and The Japanese 2396601/qiba-sws/papers/ Society for Ultrasound in Medicine. siemens.com/strain 3 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies 7 8 4a 2 1 4b 3 6 5 Fig. 2: Optimal patient position Fig. 3: Segment 5 and 8 landmarks Technique Best Practice for Performance of Ultrasound-based Elastography Suggested Technique Comments Fasting for 4–6 hours Food intake within 4–6 hrs prior to the ultrasound exam may artificially elevate shear wave speed values in the liver. Optimal positioning Supine or slight (30º) left lateral decubitus position with right arm elevated above head to improve intercostal access (Fig. 2). Shallow, brief breath Valsalva or deep inspiration can cause an undesired increase in central hold venous pressure, which can falsely elevate elastography values. Optimal ROI Use intercostal transducer placement to target segments 5 or 8 placement (Fig. 3). Avoid increased subcapsular reverberation by placing ROI perpendicular to at lease 2 to 3 cm below Glisson’s capsule (Fig. 4). Note: The transducer-specific lens focus is typically about 4–5 cm below the transducer, thus best measurements are in this region. Ample gel Ample gel throughout the examination (particularly during the collection of the entire set of measurements) is necessary for optimal acoustic coupling. This will ensure maximum transfer of sound. 4 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide SIEMENS 6C1 HD Abdomen General TIS:0.8 E=3.70 kPa TIB: 1.7 MI: 1.6 Vs=1.11 m/s 8fps Depth=4.3 cm 2D-100% THI H4.00 MHZ 0d B/DR70 SC ON DTCEM MapD/ST2 Cooling 1 13 cm Fig. 4: Optimal Technique Suggested Technique Comments B-mode optimization Find a location where liver parenchyma returns strong echoes without increasing the B-Mode gain. Avoid large vessels and bile ducts. Use an image plane parallel to the ribs to avoid rib shadows and improve push pulse energy coupling to the body. Note: The ROI extends 1 cm above and below the in-plane ROI, so check the liver in these areas also. Acquisition of Ten measurements obtained in the same location are recommended to measurements ensure the median value is reliable. Reccomendations Fasted patient lies in a slight left lateral Place ROI optimally: • • decubitus position with right arm raised 4–6 cm deep (and at least 2 to 3 cm below · slightly above the head and perpendicular to the liver capsule) Intercostal approach within segment 5 or 8 of the liver · • Instruct patient to maintain shallow, brief • Optimize B-Mode image so liver parenchyma • is bright and large vessels, bile ducts, and rib breath hold from the time of initiating “Update” to the appearance of the value shadows are avoided on screen Ensure that the liver capsule appears sharp • Obtain 10 measurements at the same site • siemens.com/strain 5 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies Elastography Liver Assessment Label 1 Label 2 Label 3 Label 4 Indication Vs (m/s) Depth (cm) Vs (m/s) Depth (cm) Vs (m/s) Depth (cm) Vs (m/s) Depth (cm) ☑ Indication ☑ Venous Congestion Portal System ☑ Distention ☑ Enlarged Spleen ☑ Varices Median Fat Infiltration Marked Mean Focal Lesions Multiple Std Dev IQR Lesion Elasticity Soft Overall Statistics Median Std Dev Mean IQR Fig. 5: Elastography Report Fig. 6: Abdomen Report Reporting The system generates a dedicated Elastography Report (Fig. 5). This report tabulates the shear Reccomendations velocity (Vs) or the stiffness (kPA) and depth An IQR/Median Ratio is measurements (cm) associated with each site. the recommended unit for Measurements are displayed in both velocity technical accuracy. (m/s) and stiffness (kPA) on the image. The user can choose to display either kPA or m/s in the Elastography Report. The report calculates mean, median, standard deviation, interquartile range (IQR) and IQR/Median Ratio for all measurements. These statistics are important for the determination of the study’s technical accuracy (see page 8). The Elastography Report can be combined with the Liver Assessment section of the Abdomen Report page (Fig. 6). 6 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide IQR = Distance between 25th and 75th Percentiles in units of measure K K 25th 75th Percentile Percentile Lowest Median Highest Value Value Value Fig. 7: Interquartile Range (IQR) Confidence The Interquartile Range (IQR) is the range of the A Note: IQR will be higher with advancing spread of values in a repeated data set, from fibrosis. Since the IQR / Median Ratio the 25th percentile to the 75th percentile. It is normalizes for this, the IQR / Median sometimes referred to as the “middle fifty”. It is an important indicator of technical removes outlying data points to represent more accuracy. accurately the spread of values in a data set. Take the following as examples: Median Vs = 1.00 m/s Reccomendations IQR = 0.3 IQR / Median Ratio = 0.3 a. Take 10 measurements per site (note that (Technically Adequate) the report page can display the 10 Median Vs = 2.1 m/s measurements from multiple sites) IQR = 0.7 b. At least 6/10 of measurements should be IQR / Median Ratio = 0.3 “valid”. This is tabulated at the bottom of (Technically Adequate) the imaging screen Site 1: 0 Valid/0 Total Median Vs = 1.8 m/s c. An IQR / Median Ratio of ≤ 0.3 is the IQR = 1.2 recommended quality control measure IQR / Median Ratio = 0.67 for adequate technical quality. (Technically Inadequate) An IQR / Median Vs ratio of ≤ 0.3 ensures a set of measurements lie within a reasonable variability range. siemens.com/strain 7 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies Table 1: Sources of Measurement Variability Cause Category Example Patient Preparation A non-fasting state can have falsely increased stiffness values  Pre-compression As levels of pre-compression increase, inter-operator variability also increases (see Appendix 2) Technique Shallow Breath Hold Valsalva can increase stiffness values  . Patient or transducer motion can cause inaccuracy. Transducer Angle An out-of-plane transducer angle below 50˚ to the skin surface can result in artificially low shear wave velocities (see Appendix 3) Body Habitus High BMI or a thick adipose layer can grossly attenuate shear waves and result in out of tolerance (x.xxm/s) or values with high data set variability Patient Factors Liver Function Elevated aminotransferase can elevate elastography results Comorbidities Congestive Heart Failure (CHF), extrahepatic cholestasis, significant portal hypertension, elevated amniotransferase levels and liver fibrosis can cause high Vs measurements Technical Accuracy Technical accuracy of a VTq examination is 2. High Data Set Variability reliant on the reproduction of consistent Erroneous outliers can negatively impact calculated values. the IQR. Sources of measurement variability Measurement variability aggregates as two are broadly attributable to factors relating types of readings: to technique, and the patient. These are 1. Out of Tolerance Measurements (x.xxm/s) summarized in Table 1. A variance in measurements across vendors has also If the system displays the Vs as x.xx m/s, been observed. internal measurement quality thresholds were not met. The most common reason for ! Note: because of the potential variability this is low signal to noise ratio (SNR) of the of the co-morbidities listed in Table 1, it is shear wave. Low SNR is typically caused by imperative to be aware of the patient‘s poor scan technique or patient dependent medical history prior to performing a VTq factors, such as a thick adipose layer, high examination. BMI, or significant fibrosis and/ or steatosis that inhibits the generation of shear waves within the liver. 8 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide Table 1: Sources of Measurement Variability Cause Category Example Patient Preparation A non-fasting state can have falsely increased stiffness values  Pre-compression As levels of pre-compression increase, inter-operator variability also increases (see Appendix 2) Shallow Breath Hold Technique Valsalva can increase stiffness values  . Patient or transducer motion can cause inaccuracy. Transducer Angle An out-of-plane transducer angle below 50˚ to the skin surface can result in artificially low shear wave velocities (see Appendix 3) Body Habitus High BMI or a thick adipose layer can grossly attenuate shear waves and result in out of tolerance (x.xxm/s) or values with high data set variability Patient Factors Liver Function Elevated aminotransferase can elevate elastography results Comorbidities Congestive Heart Failure (CHF), extrahepatic cholestasis, significant portal hypertension, elevated amniotransferase levels and liver fibrosis can cause high Vs measurements Reccomendations Familiarity with these two independent clinical studies measuring technical reproducibility of shear wave velocity measurements. Guzman-Aroca, F, Reus M, Berna-Serna J, • Serrano L, Serrano C, Gilabert A, Cepero A. “Reproducibility of Shear Wave Velocity Measurements by Acoustic Radiation Force Impulse Imaging of the Liver.” J Ultrasound Med 2011; 30:975-979. Kurabayashi T, Taketomi-Takahashi A, • Endo K. “Reliable Measurement Procedure of Virtual Touch Tissue Quantification with Acoustic Radiation Force Impulse Imaging.” J Ultrasound Med 2011; 30:745-751. siemens.com/strain 9 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies 6 SIEMENS 6C1 HD Abdomen General 5 E=6.84 kPa TIS:0.8 TIB: 1.7 Vs=1.51 m/s UI: 1.6 4 Depth=4.3 cm 2D-100% THI 14.00 MHZ CdB/DR70 SC Off DTCEM 3 MapD/ST2 2 Displacement (µm) 1 0 -1 0 1 2 3 4 5 6 7 8 13cm Time (msec) Fig. 8B: Shear wave displacement signals resulting Fig. 8A: Poor scan technique from poor technique landmarks Data Set Variability Low Signal-to-Noise Ratio (SNR) A common cause of measurement variability is low signal to noise ratio (SNR) of the shear wave displacement signal. This results in noisy displacement profiles that make it difficult to detect the true shear wave peaks at each detection beam line. Fig. 8 and 9 demonstrate the difference between the shear wave displacement signals from a poor technique (Fig. 8A and 8B) compared to that of a better technique (Fig. 9A and 9B). 10 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide 20 SIEMENS 6C1 HD Abdomen General E=3.70 kPa 15 Vs=1.11 m/s 20-100% Depth=4.3 cm THE H4.00 MHZ OUB/DR70 sc off 10 DTCEM MapD/ST2 5 Displacement (µm) 0 -5 Cooling 1 0 1 2 3 4 5 6 7 8 13cm Time (msec) Fig. 9B: Shear wave displacement signals resulting from Fig. 9A: Good scan technique good technique Data Set Variability Improving Displacement Magnitude with Technique Fig. 8 and 9 demonstrate the difference between amplitude and low SNR leading to artificially the shear wave displacement signals from a elevated shear wave velocities in a normal poor technique compared to that of a better liver (Fig. 8A and 8B). A better scan technique technique. includes a ROI perpendicular to Glisson’s A poor technique commonly involves the capsule and good coupling of the transducer to proximity of the ROI to rib shadow, and poor the skin surface. This will result in much higher coupling between the transducer and skin shear wave amplitude and SNR, and more surface. This results in low shear wave appropriate shear wave velocities for a normal liver (Fig. 9A and 9B). siemens.com/strain 11 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies 4 cm 4cm Fig. 10: Attenuation of push pulse due to propagation Fig. 11: Good quality push pulse due to propagation in in a challenging medium (e.g. liver with steatosis) a non-challenging medium (e.g. normal liver tissue) Data Set Variability Beam Attenuation Another common cause of measurement In Fig. 11, all scan parameters are consistent variability is beam attenuation caused by with Fig 10. However, the attenuation is low propagation in challenging tissue. This is because the push pulse is propagating in illustrated in Fig. 10 and 11, which display the push beam using a ‘sensor’ map for less-challenging tissue, and so good quality displacement. shear waves are the result. This is typical in liver tissue without the presence of steatosis. In Fig. 10, the attenuation is high. This is typical in livers with steatosis, or in patients A Note: As a result, co-morbidities with high BMI. The push pulse simply cannot and body habitus are significant penetrate deeply enough to generate good considerations in the quality of measurement results. quality shear waves. 12 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide 4C1 4V1 5 kPa 103 103 A 10 kPa X 20 kPa O 40 kPa 80 kPa Center Frequency (Hz) Center Frequency (Hz) 102 102 0 2 4 6 8 0 2 4 6 8 Depth (cm) Depth (cm) 6C1HD 9L4 103 103 Center Frequency (Hz) Center Frequency (Hz) 102 102 0 2 4 6 8 0 2 4 6 8 Depth (cm) Depth (cm) Fig. 12: How transducer center frequency can influence shear wave velocity Effect of Frequency on Shear Wave Velocity VTq uses an excitation pulse that generates a The following equation describes the relatively wide range of shear wave frequencies. dependence of shear wave speed on frequency The velocity of the shear wave can be when a homogeneous Voigt model is assumed: influenced by the shear wave frequency, as shown in Figure 12. c,(,) =. Reccomendations where are the shear wave speed, shear wave angular frequency, shear elasticity, To ensure consistency of the patient shear viscosity, and material density. Since follow-up, it is recommended to perform these parameters can vary in visco-elastic serial scans with consistent transducer human tissue, shear wave angular frequency selections. can have an effect on shear wave velocity measurements and should be considered in interpreting shear wave velocity measurements. siemens.com/strain 13 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies Fig. 13: ACUSON S3000 Ultrasound System, HELX Evolution with Touch Control Steps to Success Step 1: Step 3: Ensure measurements are acquired according Apply knowledge of sources of variability in to the recommendations in this guide. mechanical stiffness (both physiological and pathological) to each individual patient, when Step 2: variable measurements are produced. Ensure validity criteria is met for each patient examination according to recommendations in this guide. Remember: IQR / Median Ratio of ≤ 0.3 is the recommended data point for technical accuracy assessment. 14 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide Glossary of Terms Acoustic Radiation Force Impulse. A “push pulse” produced by the ARFI transducer which causes tissue displacement Shear Wave Transverse waves produced by an ARFI “push pulse”, the speed of which indicate tissue stiffness Stiffness A descriptive term for tissue elasticity to depict how hard or soft an area of tissue is Displacement The amount of tissue movement that occurs in response to an ARFI push pulse. This indicates tissue stiffness ROI The user-defined region of interest box within which shear waves are detected and quantified Pre-compression The amount of pressure a user places on the transducer during acquisition Inter-Quartile Range. This is the distance between 25th and 75th IQR percentiles in units of measure. It gives a representation of the distribution, or spread, of a set of data points and is used as a criteria for validity Median The middle of a given data set IQR / Median Velocity This is the ratio used to indicate validity. The recommended value is ≤ 0.3 Mean The average value of a given data set siemens.com/strain 15 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies References  Wellman, et.al. Breast Tissue Stiffness in Compression  Masatoshi Kudo et al. JSUM Ultrasound Elastography is Correlated to Histological Diagnosis. Harvard Practice Guidelines: Liver Journal of Medical BioRobotics Laboratory Technical Report, 1999. Ultrasonics Sep 2013; 40(4) https://biorobotics.harvard.edu/pubs/1999/ mechprops.pdf  Barr RG et al. Elastography Assessment of Liver Fibrosis: Society of Radiologists in Ultrasound  Rosen J, Brown J, De S, Sinanan M, Hannford B. Consensus Conference Statement. Radiology 2015; Biomechanical Properties of Abdominal Organs In 276(3): 845-61 Vivo and Postmortem Under Compression Loads. Journal of Biomechanical Engineering. 2008; Vol. 130,  Mederacke I, Wursthorn K, Kirschner J, et al. Food 021020-1. intake increases liver stiffness in patients with chronic or resolved hepatitis C virus infection. Liver Int  Barr, et.al. Elastography Assessment of Liver Fibrosis: 2009;29(10):1500-1506. Society of Radiologists in Ultrasound Consensus Statement. Radiology 2015 Sep 16;276(3):845-61.  Horster S, Mandel P, Zachoval R, Clevert DA. Comparing acoustic radiation force impulse imaging  Ferraioli G et al. WFUMB Guidelines and to transient elastography to assess liver stiffness in Recommendations for Clinical Use of Ultrasound healthy volunteers with and without Valsalva Elastography: Part 3: Liver Ultrasound Med Biol. manoeuvre. Clin Hemorheol Microcirc 2010;46(2-3): 2015 May; 41(5): 1161-79 159-168.  Cosgrove D et al. EFSUMB Guidelines and Recommendations on the Clinical Use of Ultrasound Elastography Ultraschall in Med 2013; 34: 238–253 16 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide Appendices Appendix 1: Differences in Right and Left Lobe of Liver in Normal Subjects Differences in Right and Left Lobe of Differences in Right and Left Lobe of Liver in Normal Subjects Liver in Chronic Liver Disease Jaffer, et al. reported that with trained According to Toshima, et al. shear wave velocity operators, ARFI (Virtual Touch quantification) measurements in the right lobe of the liver were is a reliable and reproducible method of liver more reliable than measurements in the left stiffness quantification in segments 5/6 and lobe. 7/8, but acquisitions in segment 3 should be avoided. Values obtained deep to the liver References capsule allow more reliable liver stiffness Toshima T, Shirabe K, Takeishi K, Motomura M, quantification. Mano Y, Uchiyama H, Yoshizumi T, Soejima Y, Taketomi A, Maehara Y. “New method for References assessing liver fibrosis based on acoustic Jaffer OS, Lung PFC, Bosanac D, Patel VM, radiation force impulse: a special reference to Ryan SM, Heneghan MA, Quaglia A, Sidhu PS. the difference between right and left liver.” “Acoustic radiation force impulse quantification”: J. Gastroenterology (2011) 46:705-711. repeatability measurements in selected liver segments and influence of age, body mass index and liver to capsule box distance. The British Journal of Radiology, 85 (2012), e858-e863. siemens.com/strain 17 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies 4 3.5 3 * 2.5 2 O 6C1HD Shear wave speed (m/s) O 4C1 1.5 4V1 x 9L4 1 0 10 20 30 40 50 Percentage of compression (%) Fig. 14: Effect of pre-compression on shear wave velocity measurements Appendices Appendix 2: Effect of Pre-compression on Shear Wave Velocity Measurements Effect of Pre-compression on Shear As levels of pre-compression increase, inter- Wave Velocity Measurements operator variability also increases. Using minimal, mild and moderate compression Using a CIRS 049 elasticity phantom, a with the same test phantom setup had similar test of the effect of transducer pre-load shear wave velocity measurements at minimal- (pre-compression) on shear wave velocity to-mild compression. There was significant measurements indicates that shear wave speed variability at moderate compression in breast increases as compression level increases. The scanning. mean shear wave speeds are within ± 20% of the elasticity phantom reference value at 0% Note: Always use minimal-to-mild to 30% compression. These results indicate compression. that minimal-to-mild compression (5% to 15%) is not likely to cause significant bias on Virtual Touch quantification measurements. 18 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide 1 90-degree transducer angle 5 40° 1 60-degree transducer angle 50° 4 60° 1 2 70° 80° +EH 3 E 90° 00.000 2 Shear wave speed (m/s) 1 0 40 50 60 70 80 90 Scanning angle (degrees) Fig. 15: Transducer angle on a flat surface Fig. 16: Shear wave speed versus scan angle Appendices Appendix 3: Effect of Transducer Angle on Shear Wave Velocity Measurements / Results of In-Plane Transducer Angle Effect of In-Plane and Out-of-Plane Results of Out-of Plane Transducer Angle Transducer Angle on Shear Wave An out-of-plane transducer angle below Velocity Measurements 50 degrees to the skin surface can result The effect of transducer angle was tested on in artificially low shear wave velocities tissue equivalent elasticity phantoms using a (indicated in the rightmost graph) due to loss 9L4 linear array transducer. of transducer contact with the flat scanning surface. Note: In general, use a transducer angle of 90 degrees to the skin surface for the .- highest measurement accuracy. siemens.com/strain 19 Technical Reference Guide | Elastography in the Liver Virtual Touch Technologies SIEMENS 914 Thyroid Genera TIB: 0.7 E=16.6 kPa MI: 1.6 Vs=2.35 m/s 13fps 2D -- 100% Depth=1.7 cm THI H8.00 MHZ 4dB/DR70 SC Off DTCEM MapD/ST3 32JP3 4cm Fig. 17: Example of a shear wave reflection artifact Appendices Appendix 4: Potential Shear Wave Reflection Artifacts Shear wave reflections can cause artificially interference. These conditions can result in low shear wave velocity values if a focal lesion misinterpretation by the Virtual Touch shear and surrounding tissue have very high Young’s wave velocity algorithm as an artificially Modulus differences. This can result in an low velocity due to the extra time interval interface that reflects shear waves and creates between the first shear wave peak and the a “false peak” from constructive shear wave false, second shear wave peak. 20 siemens.com/strain Elastography in the Liver Virtual Touch Technologies | Technical Reference Guide 7 Line 12 6 Line 28 5 False Peaks 4 True Peaks 3 Displacement (µs) 2 1 Fig. 18: Relationship of shear 0 0 0.002 0.004 0.006 0.008 0.01 0.012 wave displacement versus time Time (ms) demonstrating false peaks from shear wave reflections siemens.com/strain 21 The products/features mentioned in this document may not be commercially available in all countries. Due to regulatory reasons their future availability cannot be guaranteed. Please contact your local Siemens organization for further details. Standalone clinical images may have been cropped to better visualize pathology. ACUSON S2000 and Virtual Touch are trademarks of Siemens Medical Solutions USA, Inc. Siemens Healthineers Headquarters Legal Manufacturer Siemens Healthcare GmbH Siemens Medical Solutions USA, Inc. Henkestr. 127 Ultrasound 91052 Erlangen, Germany 685 East Middlefield Road Phone: +49 9131 84-0 Mountain View, CA 94043, USA siemens.com/healthineers Phone: +1-888-826-9702 siemens.com/ultrasound Published by Siemens Healthcare GmbH · Order No. A91US-450-1C-4A00 · Printed in Germany · 5188 08171.5 · © Siemens Medical Solutions USA, Inc., 2017
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