Elastography in Clinical Practice Richard G. Barr, MD, PhD, FACR, FSRUa,b,* KEYWORDS Elastography Ultrasound Breast mass Breast cancer Prostate cancer Liver fibrosis Thyroid mass
KEY POINTS Both strain and shear wave elastography have high sensitivity and specificity in characterizing breast lesions as benign or malignant. Shear wave point quantification and 2-dimensional shear wave elastography can be used to assess liver fibrosis and as a noninvasive method of monitoring disease progression or treatment response. Shear wave elastography has a high negative predictive value for assessing malignancy in the peripheral zone of the prostate. Strain elastography can be performed anywhere a good B-mode image can be obtained.
With the recent Food and Drug Administration approval for quantification using ultrasound elastography, these techniques are rapidly gaining acceptance for many clinical applications. Multiple vendors have some form of elastography available on their systems. In this article, the principles of the 2 major types of ultrasound elastography are briefly reviewed, highlighting their advantages and disadvantages. The clinical applications that are becoming widely accepted as the standard of care are discussed. Emphasis is placed on how to incorporate these techniques into your practice. A brief review of other potential applications that are not yet mature enough for routine clinical use is provided.
PRINCIPLES There are 2 types of ultrasound elastography presently available, strain elastography (SE) and shear
wave elastography (SWE).1,2 Although these both measure tissue stiffness, there are differences in the techniques and how they are used in clinical practice. SE is a qualitative (not quantitative) technique. The images obtained demonstrate the relative stiffness of the tissues within the field of view (FOV). However, the absolute stiffness of the tissue is not known; other factors, such as relative stiffness compared with normal tissue or change in size compared with B-mode, must be used to obtain clinically useful information.2,3 SE images are obtained by comparing the frame-to-frame changes of tissues when a vibratory or compression/release force is applied to the tissue. Soft tissues deform more, whereas hard tissues deform less (Fig. 1). The amount of compression/release force to obtain optimal SE images varies by vendor. In some cases, only the patients’ breathing and/or heartbeat are required to generate optimal elastograms, whereas, in others,
Disclosures: Philips Ultrasound – equipment grant, research grant, speaker’s bureau, advisory panel; Siemens Ultrasound – equipment grant, research grant, speaker’s bureau, advisory panel; SuperSonic Imagine – equipment grant, speaker’s bureau; Esaote – equipment grant; Toshiba America Medical Systems – advisory panel. a Department of Radiology, Northeastern Ohio Medical University, 4209 Ohio 44, Rootstown, OH 44272, USA; b Southwoods Imaging, Radiology Consultants Inc, 7623 Market Street, Boardman, OH 44512, USA * Southwoods Imaging, Radiology Consultants Inc, 7623 Market Street, Boardman, Ohio 44512. E-mail address: [email protected] Radiol Clin N Am 52 (2014) 1145–1162 http://dx.doi.org/10.1016/j.rcl.2014.07.002 0033-8389/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
radiologic.theclinics.com
INTRODUCTION
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Fig. 1. SE is based on how a tissue deforms when a force is applied to it. Soft tissues will deform more than stiff tissues. A simple example is if we have an almond in gelatin (A) and then apply a force with a spoon (B), the gelatin deforms significantly; however, the almond does not change shape. Therefore, the gelatin is soft, whereas the almond is hard. In SE, the system compares the frame-to-frame changes when a force is applied. The relative stiffness of the tissues is determined by how much they deform.
compression and release using the transducer are required. With practice, the optimal scanning technique can be learned and be reproducible. Some vendors have a bar, or quality measure, on the monitor that provide real-time feedback on the appropriateness of the amount of compression/ release being applied.2 The results are provided in an image that can be displayed in gray scale or in a variety of color maps. The data provided are the same regardless of which color map is used, and the map choice is often dictated by the user’s experience. It is important to remember that because SE is a relative measure of stiffness, the FOV should contain a range of tissues with varying stiffness to allow for an appropriate dynamic range in the scale for adequate interpretation.2 SWE is a technique that provides a quantitative measure of stiffness that is expressed in meters per second (the shear wave speed) or in kilopascals (Young’s Modulus).1,2 Most systems allow for either to be displayed, and they are easily converted from one to the other. In SWE, a push pulse, often referred to as acoustic radiation force impulse (ARFI), is used to generate shear waves within the tissues, which is similar to dropping a stone into a pond (the push pulse) and generating waves on the water (shear waves). Note that the shear waves are generated perpendicular to the push pulse. Conventional B-mode imaging is used to monitor the shear waves generated through the tissue and calculate the shear wave speed (Fig. 2). In this technique, either a single measurement over a small FOV can be obtained (point quantification SWE) or color mapping of a large FOV of individual pixel shear wave speeds is depicted (2-dimensional [2D] SWE). The color map used
to display the data is usually red as hard and blue as soft. The color scale can be adjusted to allow better depiction of the range of shear wave speeds within the FOV, and careful attention to the scale is advised if interpretation based on color is used. A limitation of SWE is that the push pulse is attenuated as it traverses tissue and ultimately reaches a point where it is too weak to generate shear waves. In this case, the area/tissue where adequate shear waves are not obtained is not color coded (black) on 2D SWE or a value of x.xx or 0.00 is obtained with point quantification SWE.1,2
Fig. 2. In SWE, an ARFI or push pulse is applied to the tissue. This strong acoustic pulse generates shear waves perpendicular to the applied push pulse. The shear wave speed can be measured by using B-mode imaging to detect the shear wave displacements at different positions from the pulse push. The shear wave speed varies with the stiffness of the tissue, slower for softer lesions and faster for stiffer lesions. ROI, region of interest.
Elastography in Clinical Practice In SWE, the absolute stiffness value can be used for lesion characterization or the ratio of the lesion stiffness to normal tissue stiffness can be used. Another elastography technique is available whereby ARFI is used to generate tissue deformation and the deformation is used to calculate a strain image. This method is a qualitative strain method and not an SWE method. This technique is called Virtual Touch Imaging (VTI, Siemens Ultrasound, Mountain View, CA). One factor that is critical in obtaining accurate elastograms using either the SE or SWE techniques is the amount of precompression or preload used when obtaining the elastogram.2 This precompression is the amount of pressure exerted on the tissues when the scan is taken. As tissues are compressed with the transducer (ie, using a heavy hand), the tissues become stiffer. In general, softer tissues are affected more than stiffer tissues; therefore, both the relative values in SE as well as the absolute values of SWE can be affected enough to lead to inaccurate tissue assessment. A method of applying minimal precompression has been described and found to be highly reproducible.4
BREAST ELASTOGRAPHY Both SE and SWE have been shown to have high sensitivity and specificity for characterization of breast masses as benign or malignant.5–13 Breast elastography has been recommended for breast lesion characterization in the guidelines of both the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) and the World Federation for Ultrasound in Medicine and Biology (WFUMB).14,15 Previous work in vitro has demonstrated that breast cancers are significantly stiffer than benign breast lesions with little overlap, suggesting elastography would be an excellent method for breast lesion characterization.16 Careful attention to technique is important for both types of elastography and are descripted in detail elsewhere.2,17 There have been several methods for reporting, analyzing, and interpretation of SE in breast lesion characterization. These methods include the ratio of elastogram to B-mode length (E/B ratio),5,6 a 5-point color scale (Tsukuba score),18 and the lesion-to-fat ratio or strain ratio.19 All of these methods depend on obtaining adequate strain images with minimal precompression and an FOV containing a wide range of tissue stiffness (fat, fibroglandular tissue, lesion, and pectoralis muscle). A detailed description of how to obtain optimal SE images is provided elsewhere.2
In early work on breast elastography, it was noticed that breast malignancies appear larger on the elastogram than the corresponding B-mode image, whereas benign lesions appear smaller.20 This phenomenon seems to be unique to breast lesions and can be used to characterize a lesion as benign or malignant. In a single-site pilot study using an E/B ratio of 1.0 or more as malignant and less than 1.0 as benign, Barr5 reported a sensitivity of 100% and a specificity of 99% in characterizing breast lesions as benign or malignant. In a larger multicenter, international study evaluating 651 biopsy-proven lesions, a sensitivity of 99% and a specificity of 85% was obtained.6 Additional studies have had similar results.7 Fig. 3 demonstrates an example of a benign and malignant breast lesion. To use this technique, the lesion must be accurately measured on both the B-mode image and the elastogram. Early work suggests that the E/B ratio may be reflective of tumor grade, with lowgrade malignancies (ductal carcinoma in situ or mucinous cancers) having an E/B ratio of at or near 1.0, whereas higher-grade tumors have ratios of up to 3.0 or greater.21 Some investigators have suggested using an E/B ratio of 1.2 to increase specificity but at the expense of decreased sensitivity. The 5-point color scale has been proposed for interpretation of many SE applications (Fig. 4).18,22–25 In this scale, a tissue that is entirely soft is given a score of 1; a tissue with mixed soft and stiff components is given a score of 2; a stiff lesion that is smaller than the appearance on B-mode imaging is given a score of 3; if the lesion is stiff and the same size as on B-mode, it is given a score of 4; if the lesion is stiff and larger than on B-mode, it is given a score of 5. Using this scale, Ueno9 determined a cutoff between 3 and 4 had a sensitivity of 86% and a specificity of 90% in characterizing breast lesions as benign or malignant.18 Multiple studies have reported similar results. Fig. 5 is an example of this technique. In order to semiquantitate the SE results, Itoh and colleagues18 suggested using the ratio of lesion stiffness to the stiffness of fat, the lesionto-fat ratio, or the strain ratio. In order to use this method, the lesion and fat need to be within the same image. A region of interest (ROI) is placed within the lesion and a second within fat. Most ultrasound systems allow for the calculation of the relative stiffness of the tissues. Note that precompression can significantly affect the results, as fat stiffness increases faster than lesion stiffness.4 Using this technique, Itoh and colleagues18 found a sensitivity of 77% and a
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Fig. 3. Examples of a benign (A) and malignant (B) lesion on SE. (A) A 43 year old who presented with a palpable breast mass. The B-mode image is on the left and measures 20.0 mm. The image on the right is the SE image. The lesion measures 15.8 mm on SE, giving a ratio of elastogram to B-mode length (E/B) of 0.79 suggesting a benign lesion. The lesion was a benign fibroadenoma on biopsy. (B) A 63-year-old woman had a suspicious lesion noted on screening mammography. On B-mode ultrasound (left image), there is a taller-than-wide BI-RADS category 4C lesion with a diameter of 4.5 mm. On the SE image (right), the lesion measures 10.8 mm, giving an E/B ratio of 2.4 suggestive of a malignant lesion. The lesion was an invasive ductal cancer.
specificity of 77% in breast lesion characterization.19 Several other studies have been published with a wider range of accuracy, most likely because of the variability in the techniques and effects of precompression. An example of this technique is presented in Fig. 6.
Using shear wave imaging, an absolute value of lesion stiffness can be obtained and used to characterize a breast lesion as benign or malignant.2,8,26,27 The cutoff values in various studies have varied between 60 and 80 kPa (4.5–5.2 m/s), with sensitivities ranging from 63% to 97% and Fig. 4. The 5-point color scale for lesion classification with SE. A score of 1 is given to a lesion that is entirely soft; a score of 2 is given if the lesion has both soft and stiff components; a score of 3 corresponds to a stiff lesion that is smaller than identified on B-mode; a score of 4 is given if the lesion is stiff and same size as the B-mode image; if the lesion is stiff and larger than the B-mode image, it is given a score of 5.
Elastography in Clinical Practice Fig. 5. A 55-year-old woman presented with a new lesion on screening mammography. On B-mode imaging (left) the lesion has angular borders and is taller than wide BI-RADS category 4C lesion. On the color-coded SE image (red is soft and blue is hard), the lesion is mostly blue with occasional green that corresponds to a score of 4 consistent with a malignant lesion. On image-guided core biopsy, the lesion was an invasive ductal cancer.
specificities ranging from 83% to 100%. Some breast cancers do not allow for the generation of measureable shear waves for accurate assessment of the cancer stiffness. When this occurs, the cancer is not color coded or has a low-quality map.28 There may be a ring of high velocity surrounding the lesion. Breast cancers tend to have increased heterogeneity on SWE. Examples of benign and malignant lesions are presented in Fig. 7. In a large multicenter study,8 the evaluation of SWE signal homogeneity and lesion-to-fat ratios were the best differentiators of benign and malignant. The addition of SWE improved the characterization of breast lesions compared with using Breast Imaging-Reporting and Data System
(BI-RADS) alone, with a sensitivity and specificity of 93% and 59% for BI-RADS and 92% and 76% with the addition of SWE. The investigators comment that the major value of the addition of SWE is in BI-RADS 3 and 4a lesions whereby the SWE results are used to upgrade or downgrade the lesion. In a study comparing SE and SWE in the same patient population, both demonstrated high sensitivity (98% and 93% respectively) and specificity (95% and 89% respectively) of breast lesion characterization as benign or malignant when a quality map is used with SWE to aid in lesion characterization.29 With some vendors, SE a bull’s-eye artifact is identified in benign simple and complicated
Fig. 6. A 75-year-old woman presented with an abnormal screening mammogram. To calculate the lesion-to-fat ratio, or strain ratio, a region of interest (ROI) (yellow circles) is placed in the lesion and a second ROI is placed in fat. The ratio of the relative lesion stiffness can then be calculated. In this example, the same patient in Fig. 5 has a strain ratio of 6.37, that is, the lesion is 6.37 times stiffer than fat. This ratio (>4.5) is suggestive of a malignancy consistent with this invasive ductal carcinoma.
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Fig. 7. Examples of a benign (A) and malignant (B) lesion on SWE. (A) A 43-year-old woman presented with a palpable mass. The SWE image is the upper image and the B-mode image is the lower image. The lesion has a maximum kilopascal of 27.7 (3.0 m/s) (yellow circles) consistent with a benign lesion. This lesion was a benign fibroadenoma on biopsy. (B) A 68-year-old woman presented with an abnormal screening mammogram. The velocity map of an SWE study is presented here. The lesion is color coded red with a shear wave velocity (Vs) of 6.8 m/s (135 kPa) consistent with a malignant lesion. The lesion was an invasive ductal cancer on biopsy.
cysts.30 This artifact is characterized by a white central signal within a black outer signal and a bright spot posterior to the lesion (Fig. 8). This artifact has been used to decrease the number of biopsies performed in breast lesions.30
LIVER Focal Liver Lesions Focal liver lesions have been evaluated with both SE and SWE techniques. Because SE is
qualitative, a lesion can be compared with normal liver to determine its relative stiffness. However, this technique is limited in that the background liver may have variable stiffness depending on the degree of fibrosis. Another confounding factor is that both benign and malignant lesions can be soft or hard compared with normal liver. Using SWE, a true measurement of stiffness is obtained. However, because of the wide variability of a given pathologic conditions stiffness, characterization of a lesion as benign or malignant is
Elastography in Clinical Practice
Fig. 8. The bull’s-eye artifact is identified with Siemens (Mountain View, CA) and Philips (Bothell, WA) SE imaging. The artifact has been shown to be highly specific for benign simple and complicated cysts. In this example of a 45-year-old woman who presented with a palpable mass, the bull’s-eye artifact is identified at the site of the palpable abnormality. On B-mode imaging, the lesion is an isoechoic, complicated cyst that would not have been identified without the use of elastography.
problematic. In a series by Yu and Wilson,31 hemangioma had a range of shear wave velocity (Vs) of 0.87 to 4.01 m/s with an average of 0.71 m/s, whereas hepatocellular carcinomas had a range of 0.77 to 4.34 m/s with an average of 1.01 m/s. Although the overall difference in Vs of malignant 2.57 1.01 m/s and benign lesions 1.73 0.8 was statistically significant (P2) on biopsy. The cutoff values for predicting cirrhosis (F 5 4) range
Elastography in Clinical Practice between 11.0 and 13.6 kPa.39,40 Although TE is less accurate in distinguishing between contiguous stages of fibrosis, it can differentiate absence and mild fibrosis from significant fibrosis and cirrhosis. A meta-analysis in recurrent hepatitis C posttransplantation demonstrated 98% sensitivity and 84% specificity of TE for predicting cirrhosis.41 The use of TE in chronic hepatitis C has been endorsed in the recommendations for the management of viral hepatitis by the European Association for the Study of the Liver.61 Studies performed in patients with nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) have found steatosis does not seem to affect LS measurements.38,44,45 TE has been used to assess many other diffuse liver diseases with similar good results.47 TE has been evaluated for predicting complications of cirrhosis, such as portal hypertension and mortality. The area under the receiver operating characteristic curves (AUROCs) for predicting clinically significant portal hypertension (hepatic venous pressure gradient of >12 mm Hg) were 0.94 to 0.99 for cutoffs ranging from 13.6 to 21.0 kPa.48,49 Measurements of splenic stiffness have also been evaluated and showed better correlation with portal pressure (r 5 0.89) in patients with cirrhosis than LS.62 In a large prospective study of patients with chronic hepatitis C, TE was more accurate than liver biopsy in predicting prognosis. Patients with TE greater than 9.5 kPa had a significantly reduced 5-year survival.50 Recommendations for routine clinical use of TE have been published by the EFSUMB.14
Shear Wave Speed Measurement With increasing fibrosis, the liver becomes stiffer, which can be monitored using SWE.63,64 Fig. 11
shows the results from a 27 year old with chronic hepatitis C. The stiffness average of 4.77 kPa is consistent with the patient’s liver biopsy result of mild fibrosis. Fig. 12 is the image from a patient with marked cirrhosis demonstrating a markedly elevated stiffness of 66 kPa. How this new technology can be best used in clinical practice is under extensive investigation.63 The use of this technique may be able to decrease the number of liver biopsies performed for evaluation of chronic liver disease. Potential uses of this technique include liver cirrhosis suspected but not obvious on B-mode ultrasound, evaluation of patients with chronic hepatitis C, follow-up of patients to detect progression of liver disease, and to determine when to initiate treatment. SWE can be performed in a small ROI, point quantification SWE (pSWE), or with a colorcoded map of the shear wave speed (in either meters per second or kilopascals) over a larger ROI, 2D SWE. Interobserver variability with pSWE has been shown to be good with an intraclass correlation coefficient of 0.87.51,52 Interoperator reproducibility has also been reported to be good using 2D SWE.58 Cutoff values of 1.21 to 1.34 m/s have been shown to predict significant fibrosis (F 5 >2) with an AUROC of 0.85 to 0.89.53,65 For diagnosis of cirrhosis, SWE cutoff values range from 1.55 to 2.0 m/s with AUROCs of 0.89 to 0.93.53,54,65 In a recent metaanalysis of 518 patients with chronic liver disease,55 the AUROC was 0.87 for predicting significant fibrosis (F 5 >2), 0.91 for severe fibrosis (F 5 >3), and 0.93 for cirrhosis. The values for 2D SWE are similar to pSWE with AUROCs of 0.95 to 0.98 for F 5 >2, 0.96 for F 5 >3, and 0.97 to 0.98 for F 5 4.56,57 The accuracy of SWE for the assessment of liver fibrosis is similar to TE.53,66 Preliminary findings using SWE show promising results
Fig. 11. Point SWE of a 28 year old with chronic hepatitis C infection. The stiffness value of 4.11 kPa is consistent with the patient’s mild fibrosis on liver biopsy. The white rectangle is the FOV where the measurement is taken.
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PROSTATE ELASTOGRAPHY
Fig. 12. In this 64-year-old man with advanced alcohol-induced cirrhosis, the 2D shear wave elastogram confirms a markedly stiff liver with a max kilopascal value of 102 (5.8 m/s). The white circle in the FOV is where the measurement of stiffness is taken.
in patients with NAFLD and NASH67,68 and posttransplantation.69 The EFSUMB14 and WFUMB70 have guidelines for the use of SWE in assessment of liver fibrosis, particularly in hepatitis C. At this time, the cutoff values for the various manufactures are different; the appropriate cutoff values for the system should be used when assessing the degree of liver fibrosis.
THYROID ELASTOGRAPHY Thyroid nodules are a common finding. Fineneedle aspiration is the most widely used method of lesion characterization. B-mode imaging and color Doppler have been used to characterize lesions, and guidelines have been published elsewhere.71 Both SE and SWE have been used to characterize thyroid lesions as benign or malignant. Preliminary results are mixed, with some investigators obtaining good results, whereas others are less encouraging. Asteria and colleagues72 used a 4-point color scale in SE and found a sensitivity and specificity for thyroid cancer of 94% and 81%, respectively. Ragu and colleagues,73 using a 5-point color-scale with SE, found a sensitivity of 97% and a specificity of 100%. Bojunga and colleagues24 performed a meta-analysis of SE and found a mean sensitivity of 92% and mean specificity of 90%. Sebag and colleagues,74 using SWE and a cutoff of 65 kPa, found a sensitivity of
Prostate cancer (PC) is the most common malignancy in American men (excluding skin cancers) and is second only to lung cancer as a cause of cancer-related death.75 Serum prostate-specific antigen (PSA) has been used as a screening test for PC, with PSA levels greater than 4.0 ng/mL considered abnormal. In addition to adenocarcinoma, a variety of benign diseases of the prostate, such as acute prostatitis and benign prostatic hypertrophy, as well as prostatic instrumentation will elevate PSA levels; false-positive PSA results are common. About 20% of PC will present with a PSA less than 4.0 ng/mL. A commonly used screening protocol for PC combines serum PSA measurement with digital rectal examination (DRE). The positive predictive value (PPV) of this combination is quoted as 61%, an improvement over DRE alone (PPV 31%) or PSA alone (PPV 42%). Transrectal ultrasound–guided sextant prostate biopsies are then used to evaluate those patients identified as abnormal by this screening protocol. Only 25% of these patients will have at least one positive biopsy, whereas the biopsy technique will fail to detect tumors in at least 25% of patients proven subsequently to have PC.76 Multi-parametric MRI is increasingly being used for tumor detection and localization. However, although its sensitivity is high, its specificity is low, particularly for the detection of small lesions with Gleason scores less than 6. Contrastenhanced ultrasound is also being evaluated for detection and characterization of PC. Reports of the use of both SE and SWE to detect PC have been published.
STRAIN ELASTOGRAPHY When using SE, the FOV should cover the entire prostate gland and surrounding tissue. A strain ratio between normal and abnormal tissue can be used for semiquantitative analysis. Minimal amounts of pressure should be used in performing the examination. Studies using SE have demonstrated a sensitivity and specificity of 55% to 70%, PPV of 57% to 85%, and a negative predictive value (NPV) of 72% to 87%. There is controversy regarding the inability to differentiate PC from chronic prostatitis. SE has been reported to improve biopsy guidance; a well-designed study did not confirm these results.77–80
Elastography in Clinical Practice
Fig. 13. The SE (A) of a 53-year-old man with a thyroid nodule demonstrates that the lesion is stiffer (blue) than the surrounding normal thyroid tissue. The 2D SWE (B) can estimate the stiffness of the nodule that in, this case, is a maximum kilopascal of 16.9 (2.4 m/s) (circles).
SHEAR WAVE ELASTOGRAPHY A small single-center prospective study of 53 patients using SWE found excellent results in lesion detection and characterization in the peripheral zone.76 The kilopascal value of cancers ranged from 30 to 110, with a mean value of 58.0 20.7, whereas benign lesions had kilopascal values ranging from 9 to 107, with a mean value of 21.5 11.5. Correas and colleagues81,82 have obtained similar results. Based on the ROC curve, a value of 37 kPa was used as the cutoff between benign and malignant. This cutoff produced a sensitivity of 96.0%, specificity of 96.0%, PPV of 69.0%, and NPV of 99.6%.76 Most false positives
were secondary to calcifications noted on B mode in benign tissue. Nodules identified on B-mode imaging had a 12% positive biopsy rate. Using SWE to characterize B-mode–detected nodules, a sensitivity of 100%, specificity of 96%, PPV of 75%, and NPV of 100% were observed.76 To obtain 100% sensitivity at the sextant level, a cutoff value of 30 kPa was required. Using this technique, a 140% increase in the positive biopsy rate could have been obtained.76 Correas and colleagues82 found that the Vs ratio of the lesion to background prostate tissue was more discriminatory, as it takes into account the increased stiffness of the peripheral zone from
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KEY POINTS Both strain and shear wave elastography have high sensitivity and specificity in characterizing breast lesions as benign or malignant. Shear wave point quantification and 2-dimensional shear wave elastography can be used to assess liver fibrosis and as a noninvasive method of monitoring disease progression or treatment response. Shear wave elastography has a high negative predictive value for assessing malignancy in the peripheral zone of the prostate. Strain elastography can be performed anywhere a good B-mode image can be obtained.
With the recent Food and Drug Administration approval for quantification using ultrasound elastography, these techniques are rapidly gaining acceptance for many clinical applications. Multiple vendors have some form of elastography available on their systems. In this article, the principles of the 2 major types of ultrasound elastography are briefly reviewed, highlighting their advantages and disadvantages. The clinical applications that are becoming widely accepted as the standard of care are discussed. Emphasis is placed on how to incorporate these techniques into your practice. A brief review of other potential applications that are not yet mature enough for routine clinical use is provided.
PRINCIPLES There are 2 types of ultrasound elastography presently available, strain elastography (SE) and shear
wave elastography (SWE).1,2 Although these both measure tissue stiffness, there are differences in the techniques and how they are used in clinical practice. SE is a qualitative (not quantitative) technique. The images obtained demonstrate the relative stiffness of the tissues within the field of view (FOV). However, the absolute stiffness of the tissue is not known; other factors, such as relative stiffness compared with normal tissue or change in size compared with B-mode, must be used to obtain clinically useful information.2,3 SE images are obtained by comparing the frame-to-frame changes of tissues when a vibratory or compression/release force is applied to the tissue. Soft tissues deform more, whereas hard tissues deform less (Fig. 1). The amount of compression/release force to obtain optimal SE images varies by vendor. In some cases, only the patients’ breathing and/or heartbeat are required to generate optimal elastograms, whereas, in others,
Disclosures: Philips Ultrasound – equipment grant, research grant, speaker’s bureau, advisory panel; Siemens Ultrasound – equipment grant, research grant, speaker’s bureau, advisory panel; SuperSonic Imagine – equipment grant, speaker’s bureau; Esaote – equipment grant; Toshiba America Medical Systems – advisory panel. a Department of Radiology, Northeastern Ohio Medical University, 4209 Ohio 44, Rootstown, OH 44272, USA; b Southwoods Imaging, Radiology Consultants Inc, 7623 Market Street, Boardman, OH 44512, USA * Southwoods Imaging, Radiology Consultants Inc, 7623 Market Street, Boardman, Ohio 44512. E-mail address: [email protected] Radiol Clin N Am 52 (2014) 1145–1162 http://dx.doi.org/10.1016/j.rcl.2014.07.002 0033-8389/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
radiologic.theclinics.com
INTRODUCTION
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Fig. 1. SE is based on how a tissue deforms when a force is applied to it. Soft tissues will deform more than stiff tissues. A simple example is if we have an almond in gelatin (A) and then apply a force with a spoon (B), the gelatin deforms significantly; however, the almond does not change shape. Therefore, the gelatin is soft, whereas the almond is hard. In SE, the system compares the frame-to-frame changes when a force is applied. The relative stiffness of the tissues is determined by how much they deform.
compression and release using the transducer are required. With practice, the optimal scanning technique can be learned and be reproducible. Some vendors have a bar, or quality measure, on the monitor that provide real-time feedback on the appropriateness of the amount of compression/ release being applied.2 The results are provided in an image that can be displayed in gray scale or in a variety of color maps. The data provided are the same regardless of which color map is used, and the map choice is often dictated by the user’s experience. It is important to remember that because SE is a relative measure of stiffness, the FOV should contain a range of tissues with varying stiffness to allow for an appropriate dynamic range in the scale for adequate interpretation.2 SWE is a technique that provides a quantitative measure of stiffness that is expressed in meters per second (the shear wave speed) or in kilopascals (Young’s Modulus).1,2 Most systems allow for either to be displayed, and they are easily converted from one to the other. In SWE, a push pulse, often referred to as acoustic radiation force impulse (ARFI), is used to generate shear waves within the tissues, which is similar to dropping a stone into a pond (the push pulse) and generating waves on the water (shear waves). Note that the shear waves are generated perpendicular to the push pulse. Conventional B-mode imaging is used to monitor the shear waves generated through the tissue and calculate the shear wave speed (Fig. 2). In this technique, either a single measurement over a small FOV can be obtained (point quantification SWE) or color mapping of a large FOV of individual pixel shear wave speeds is depicted (2-dimensional [2D] SWE). The color map used
to display the data is usually red as hard and blue as soft. The color scale can be adjusted to allow better depiction of the range of shear wave speeds within the FOV, and careful attention to the scale is advised if interpretation based on color is used. A limitation of SWE is that the push pulse is attenuated as it traverses tissue and ultimately reaches a point where it is too weak to generate shear waves. In this case, the area/tissue where adequate shear waves are not obtained is not color coded (black) on 2D SWE or a value of x.xx or 0.00 is obtained with point quantification SWE.1,2
Fig. 2. In SWE, an ARFI or push pulse is applied to the tissue. This strong acoustic pulse generates shear waves perpendicular to the applied push pulse. The shear wave speed can be measured by using B-mode imaging to detect the shear wave displacements at different positions from the pulse push. The shear wave speed varies with the stiffness of the tissue, slower for softer lesions and faster for stiffer lesions. ROI, region of interest.
Elastography in Clinical Practice In SWE, the absolute stiffness value can be used for lesion characterization or the ratio of the lesion stiffness to normal tissue stiffness can be used. Another elastography technique is available whereby ARFI is used to generate tissue deformation and the deformation is used to calculate a strain image. This method is a qualitative strain method and not an SWE method. This technique is called Virtual Touch Imaging (VTI, Siemens Ultrasound, Mountain View, CA). One factor that is critical in obtaining accurate elastograms using either the SE or SWE techniques is the amount of precompression or preload used when obtaining the elastogram.2 This precompression is the amount of pressure exerted on the tissues when the scan is taken. As tissues are compressed with the transducer (ie, using a heavy hand), the tissues become stiffer. In general, softer tissues are affected more than stiffer tissues; therefore, both the relative values in SE as well as the absolute values of SWE can be affected enough to lead to inaccurate tissue assessment. A method of applying minimal precompression has been described and found to be highly reproducible.4
BREAST ELASTOGRAPHY Both SE and SWE have been shown to have high sensitivity and specificity for characterization of breast masses as benign or malignant.5–13 Breast elastography has been recommended for breast lesion characterization in the guidelines of both the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) and the World Federation for Ultrasound in Medicine and Biology (WFUMB).14,15 Previous work in vitro has demonstrated that breast cancers are significantly stiffer than benign breast lesions with little overlap, suggesting elastography would be an excellent method for breast lesion characterization.16 Careful attention to technique is important for both types of elastography and are descripted in detail elsewhere.2,17 There have been several methods for reporting, analyzing, and interpretation of SE in breast lesion characterization. These methods include the ratio of elastogram to B-mode length (E/B ratio),5,6 a 5-point color scale (Tsukuba score),18 and the lesion-to-fat ratio or strain ratio.19 All of these methods depend on obtaining adequate strain images with minimal precompression and an FOV containing a wide range of tissue stiffness (fat, fibroglandular tissue, lesion, and pectoralis muscle). A detailed description of how to obtain optimal SE images is provided elsewhere.2
In early work on breast elastography, it was noticed that breast malignancies appear larger on the elastogram than the corresponding B-mode image, whereas benign lesions appear smaller.20 This phenomenon seems to be unique to breast lesions and can be used to characterize a lesion as benign or malignant. In a single-site pilot study using an E/B ratio of 1.0 or more as malignant and less than 1.0 as benign, Barr5 reported a sensitivity of 100% and a specificity of 99% in characterizing breast lesions as benign or malignant. In a larger multicenter, international study evaluating 651 biopsy-proven lesions, a sensitivity of 99% and a specificity of 85% was obtained.6 Additional studies have had similar results.7 Fig. 3 demonstrates an example of a benign and malignant breast lesion. To use this technique, the lesion must be accurately measured on both the B-mode image and the elastogram. Early work suggests that the E/B ratio may be reflective of tumor grade, with lowgrade malignancies (ductal carcinoma in situ or mucinous cancers) having an E/B ratio of at or near 1.0, whereas higher-grade tumors have ratios of up to 3.0 or greater.21 Some investigators have suggested using an E/B ratio of 1.2 to increase specificity but at the expense of decreased sensitivity. The 5-point color scale has been proposed for interpretation of many SE applications (Fig. 4).18,22–25 In this scale, a tissue that is entirely soft is given a score of 1; a tissue with mixed soft and stiff components is given a score of 2; a stiff lesion that is smaller than the appearance on B-mode imaging is given a score of 3; if the lesion is stiff and the same size as on B-mode, it is given a score of 4; if the lesion is stiff and larger than on B-mode, it is given a score of 5. Using this scale, Ueno9 determined a cutoff between 3 and 4 had a sensitivity of 86% and a specificity of 90% in characterizing breast lesions as benign or malignant.18 Multiple studies have reported similar results. Fig. 5 is an example of this technique. In order to semiquantitate the SE results, Itoh and colleagues18 suggested using the ratio of lesion stiffness to the stiffness of fat, the lesionto-fat ratio, or the strain ratio. In order to use this method, the lesion and fat need to be within the same image. A region of interest (ROI) is placed within the lesion and a second within fat. Most ultrasound systems allow for the calculation of the relative stiffness of the tissues. Note that precompression can significantly affect the results, as fat stiffness increases faster than lesion stiffness.4 Using this technique, Itoh and colleagues18 found a sensitivity of 77% and a
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Fig. 3. Examples of a benign (A) and malignant (B) lesion on SE. (A) A 43 year old who presented with a palpable breast mass. The B-mode image is on the left and measures 20.0 mm. The image on the right is the SE image. The lesion measures 15.8 mm on SE, giving a ratio of elastogram to B-mode length (E/B) of 0.79 suggesting a benign lesion. The lesion was a benign fibroadenoma on biopsy. (B) A 63-year-old woman had a suspicious lesion noted on screening mammography. On B-mode ultrasound (left image), there is a taller-than-wide BI-RADS category 4C lesion with a diameter of 4.5 mm. On the SE image (right), the lesion measures 10.8 mm, giving an E/B ratio of 2.4 suggestive of a malignant lesion. The lesion was an invasive ductal cancer.
specificity of 77% in breast lesion characterization.19 Several other studies have been published with a wider range of accuracy, most likely because of the variability in the techniques and effects of precompression. An example of this technique is presented in Fig. 6.
Using shear wave imaging, an absolute value of lesion stiffness can be obtained and used to characterize a breast lesion as benign or malignant.2,8,26,27 The cutoff values in various studies have varied between 60 and 80 kPa (4.5–5.2 m/s), with sensitivities ranging from 63% to 97% and Fig. 4. The 5-point color scale for lesion classification with SE. A score of 1 is given to a lesion that is entirely soft; a score of 2 is given if the lesion has both soft and stiff components; a score of 3 corresponds to a stiff lesion that is smaller than identified on B-mode; a score of 4 is given if the lesion is stiff and same size as the B-mode image; if the lesion is stiff and larger than the B-mode image, it is given a score of 5.
Elastography in Clinical Practice Fig. 5. A 55-year-old woman presented with a new lesion on screening mammography. On B-mode imaging (left) the lesion has angular borders and is taller than wide BI-RADS category 4C lesion. On the color-coded SE image (red is soft and blue is hard), the lesion is mostly blue with occasional green that corresponds to a score of 4 consistent with a malignant lesion. On image-guided core biopsy, the lesion was an invasive ductal cancer.
specificities ranging from 83% to 100%. Some breast cancers do not allow for the generation of measureable shear waves for accurate assessment of the cancer stiffness. When this occurs, the cancer is not color coded or has a low-quality map.28 There may be a ring of high velocity surrounding the lesion. Breast cancers tend to have increased heterogeneity on SWE. Examples of benign and malignant lesions are presented in Fig. 7. In a large multicenter study,8 the evaluation of SWE signal homogeneity and lesion-to-fat ratios were the best differentiators of benign and malignant. The addition of SWE improved the characterization of breast lesions compared with using Breast Imaging-Reporting and Data System
(BI-RADS) alone, with a sensitivity and specificity of 93% and 59% for BI-RADS and 92% and 76% with the addition of SWE. The investigators comment that the major value of the addition of SWE is in BI-RADS 3 and 4a lesions whereby the SWE results are used to upgrade or downgrade the lesion. In a study comparing SE and SWE in the same patient population, both demonstrated high sensitivity (98% and 93% respectively) and specificity (95% and 89% respectively) of breast lesion characterization as benign or malignant when a quality map is used with SWE to aid in lesion characterization.29 With some vendors, SE a bull’s-eye artifact is identified in benign simple and complicated
Fig. 6. A 75-year-old woman presented with an abnormal screening mammogram. To calculate the lesion-to-fat ratio, or strain ratio, a region of interest (ROI) (yellow circles) is placed in the lesion and a second ROI is placed in fat. The ratio of the relative lesion stiffness can then be calculated. In this example, the same patient in Fig. 5 has a strain ratio of 6.37, that is, the lesion is 6.37 times stiffer than fat. This ratio (>4.5) is suggestive of a malignancy consistent with this invasive ductal carcinoma.
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Fig. 7. Examples of a benign (A) and malignant (B) lesion on SWE. (A) A 43-year-old woman presented with a palpable mass. The SWE image is the upper image and the B-mode image is the lower image. The lesion has a maximum kilopascal of 27.7 (3.0 m/s) (yellow circles) consistent with a benign lesion. This lesion was a benign fibroadenoma on biopsy. (B) A 68-year-old woman presented with an abnormal screening mammogram. The velocity map of an SWE study is presented here. The lesion is color coded red with a shear wave velocity (Vs) of 6.8 m/s (135 kPa) consistent with a malignant lesion. The lesion was an invasive ductal cancer on biopsy.
cysts.30 This artifact is characterized by a white central signal within a black outer signal and a bright spot posterior to the lesion (Fig. 8). This artifact has been used to decrease the number of biopsies performed in breast lesions.30
LIVER Focal Liver Lesions Focal liver lesions have been evaluated with both SE and SWE techniques. Because SE is
qualitative, a lesion can be compared with normal liver to determine its relative stiffness. However, this technique is limited in that the background liver may have variable stiffness depending on the degree of fibrosis. Another confounding factor is that both benign and malignant lesions can be soft or hard compared with normal liver. Using SWE, a true measurement of stiffness is obtained. However, because of the wide variability of a given pathologic conditions stiffness, characterization of a lesion as benign or malignant is
Elastography in Clinical Practice
Fig. 8. The bull’s-eye artifact is identified with Siemens (Mountain View, CA) and Philips (Bothell, WA) SE imaging. The artifact has been shown to be highly specific for benign simple and complicated cysts. In this example of a 45-year-old woman who presented with a palpable mass, the bull’s-eye artifact is identified at the site of the palpable abnormality. On B-mode imaging, the lesion is an isoechoic, complicated cyst that would not have been identified without the use of elastography.
problematic. In a series by Yu and Wilson,31 hemangioma had a range of shear wave velocity (Vs) of 0.87 to 4.01 m/s with an average of 0.71 m/s, whereas hepatocellular carcinomas had a range of 0.77 to 4.34 m/s with an average of 1.01 m/s. Although the overall difference in Vs of malignant 2.57 1.01 m/s and benign lesions 1.73 0.8 was statistically significant (P2) on biopsy. The cutoff values for predicting cirrhosis (F 5 4) range
Elastography in Clinical Practice between 11.0 and 13.6 kPa.39,40 Although TE is less accurate in distinguishing between contiguous stages of fibrosis, it can differentiate absence and mild fibrosis from significant fibrosis and cirrhosis. A meta-analysis in recurrent hepatitis C posttransplantation demonstrated 98% sensitivity and 84% specificity of TE for predicting cirrhosis.41 The use of TE in chronic hepatitis C has been endorsed in the recommendations for the management of viral hepatitis by the European Association for the Study of the Liver.61 Studies performed in patients with nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) have found steatosis does not seem to affect LS measurements.38,44,45 TE has been used to assess many other diffuse liver diseases with similar good results.47 TE has been evaluated for predicting complications of cirrhosis, such as portal hypertension and mortality. The area under the receiver operating characteristic curves (AUROCs) for predicting clinically significant portal hypertension (hepatic venous pressure gradient of >12 mm Hg) were 0.94 to 0.99 for cutoffs ranging from 13.6 to 21.0 kPa.48,49 Measurements of splenic stiffness have also been evaluated and showed better correlation with portal pressure (r 5 0.89) in patients with cirrhosis than LS.62 In a large prospective study of patients with chronic hepatitis C, TE was more accurate than liver biopsy in predicting prognosis. Patients with TE greater than 9.5 kPa had a significantly reduced 5-year survival.50 Recommendations for routine clinical use of TE have been published by the EFSUMB.14
Shear Wave Speed Measurement With increasing fibrosis, the liver becomes stiffer, which can be monitored using SWE.63,64 Fig. 11
shows the results from a 27 year old with chronic hepatitis C. The stiffness average of 4.77 kPa is consistent with the patient’s liver biopsy result of mild fibrosis. Fig. 12 is the image from a patient with marked cirrhosis demonstrating a markedly elevated stiffness of 66 kPa. How this new technology can be best used in clinical practice is under extensive investigation.63 The use of this technique may be able to decrease the number of liver biopsies performed for evaluation of chronic liver disease. Potential uses of this technique include liver cirrhosis suspected but not obvious on B-mode ultrasound, evaluation of patients with chronic hepatitis C, follow-up of patients to detect progression of liver disease, and to determine when to initiate treatment. SWE can be performed in a small ROI, point quantification SWE (pSWE), or with a colorcoded map of the shear wave speed (in either meters per second or kilopascals) over a larger ROI, 2D SWE. Interobserver variability with pSWE has been shown to be good with an intraclass correlation coefficient of 0.87.51,52 Interoperator reproducibility has also been reported to be good using 2D SWE.58 Cutoff values of 1.21 to 1.34 m/s have been shown to predict significant fibrosis (F 5 >2) with an AUROC of 0.85 to 0.89.53,65 For diagnosis of cirrhosis, SWE cutoff values range from 1.55 to 2.0 m/s with AUROCs of 0.89 to 0.93.53,54,65 In a recent metaanalysis of 518 patients with chronic liver disease,55 the AUROC was 0.87 for predicting significant fibrosis (F 5 >2), 0.91 for severe fibrosis (F 5 >3), and 0.93 for cirrhosis. The values for 2D SWE are similar to pSWE with AUROCs of 0.95 to 0.98 for F 5 >2, 0.96 for F 5 >3, and 0.97 to 0.98 for F 5 4.56,57 The accuracy of SWE for the assessment of liver fibrosis is similar to TE.53,66 Preliminary findings using SWE show promising results
Fig. 11. Point SWE of a 28 year old with chronic hepatitis C infection. The stiffness value of 4.11 kPa is consistent with the patient’s mild fibrosis on liver biopsy. The white rectangle is the FOV where the measurement is taken.
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PROSTATE ELASTOGRAPHY
Fig. 12. In this 64-year-old man with advanced alcohol-induced cirrhosis, the 2D shear wave elastogram confirms a markedly stiff liver with a max kilopascal value of 102 (5.8 m/s). The white circle in the FOV is where the measurement of stiffness is taken.
in patients with NAFLD and NASH67,68 and posttransplantation.69 The EFSUMB14 and WFUMB70 have guidelines for the use of SWE in assessment of liver fibrosis, particularly in hepatitis C. At this time, the cutoff values for the various manufactures are different; the appropriate cutoff values for the system should be used when assessing the degree of liver fibrosis.
THYROID ELASTOGRAPHY Thyroid nodules are a common finding. Fineneedle aspiration is the most widely used method of lesion characterization. B-mode imaging and color Doppler have been used to characterize lesions, and guidelines have been published elsewhere.71 Both SE and SWE have been used to characterize thyroid lesions as benign or malignant. Preliminary results are mixed, with some investigators obtaining good results, whereas others are less encouraging. Asteria and colleagues72 used a 4-point color scale in SE and found a sensitivity and specificity for thyroid cancer of 94% and 81%, respectively. Ragu and colleagues,73 using a 5-point color-scale with SE, found a sensitivity of 97% and a specificity of 100%. Bojunga and colleagues24 performed a meta-analysis of SE and found a mean sensitivity of 92% and mean specificity of 90%. Sebag and colleagues,74 using SWE and a cutoff of 65 kPa, found a sensitivity of
Prostate cancer (PC) is the most common malignancy in American men (excluding skin cancers) and is second only to lung cancer as a cause of cancer-related death.75 Serum prostate-specific antigen (PSA) has been used as a screening test for PC, with PSA levels greater than 4.0 ng/mL considered abnormal. In addition to adenocarcinoma, a variety of benign diseases of the prostate, such as acute prostatitis and benign prostatic hypertrophy, as well as prostatic instrumentation will elevate PSA levels; false-positive PSA results are common. About 20% of PC will present with a PSA less than 4.0 ng/mL. A commonly used screening protocol for PC combines serum PSA measurement with digital rectal examination (DRE). The positive predictive value (PPV) of this combination is quoted as 61%, an improvement over DRE alone (PPV 31%) or PSA alone (PPV 42%). Transrectal ultrasound–guided sextant prostate biopsies are then used to evaluate those patients identified as abnormal by this screening protocol. Only 25% of these patients will have at least one positive biopsy, whereas the biopsy technique will fail to detect tumors in at least 25% of patients proven subsequently to have PC.76 Multi-parametric MRI is increasingly being used for tumor detection and localization. However, although its sensitivity is high, its specificity is low, particularly for the detection of small lesions with Gleason scores less than 6. Contrastenhanced ultrasound is also being evaluated for detection and characterization of PC. Reports of the use of both SE and SWE to detect PC have been published.
STRAIN ELASTOGRAPHY When using SE, the FOV should cover the entire prostate gland and surrounding tissue. A strain ratio between normal and abnormal tissue can be used for semiquantitative analysis. Minimal amounts of pressure should be used in performing the examination. Studies using SE have demonstrated a sensitivity and specificity of 55% to 70%, PPV of 57% to 85%, and a negative predictive value (NPV) of 72% to 87%. There is controversy regarding the inability to differentiate PC from chronic prostatitis. SE has been reported to improve biopsy guidance; a well-designed study did not confirm these results.77–80
Elastography in Clinical Practice
Fig. 13. The SE (A) of a 53-year-old man with a thyroid nodule demonstrates that the lesion is stiffer (blue) than the surrounding normal thyroid tissue. The 2D SWE (B) can estimate the stiffness of the nodule that in, this case, is a maximum kilopascal of 16.9 (2.4 m/s) (circles).
SHEAR WAVE ELASTOGRAPHY A small single-center prospective study of 53 patients using SWE found excellent results in lesion detection and characterization in the peripheral zone.76 The kilopascal value of cancers ranged from 30 to 110, with a mean value of 58.0 20.7, whereas benign lesions had kilopascal values ranging from 9 to 107, with a mean value of 21.5 11.5. Correas and colleagues81,82 have obtained similar results. Based on the ROC curve, a value of 37 kPa was used as the cutoff between benign and malignant. This cutoff produced a sensitivity of 96.0%, specificity of 96.0%, PPV of 69.0%, and NPV of 99.6%.76 Most false positives
were secondary to calcifications noted on B mode in benign tissue. Nodules identified on B-mode imaging had a 12% positive biopsy rate. Using SWE to characterize B-mode–detected nodules, a sensitivity of 100%, specificity of 96%, PPV of 75%, and NPV of 100% were observed.76 To obtain 100% sensitivity at the sextant level, a cutoff value of 30 kPa was required. Using this technique, a 140% increase in the positive biopsy rate could have been obtained.76 Correas and colleagues82 found that the Vs ratio of the lesion to background prostate tissue was more discriminatory, as it takes into account the increased stiffness of the peripheral zone from
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