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Ulutürk
k1 All authors: Department of Radiology, Gazi University, School of Medicine, Besevler, Ankara 06510, Turkey.
Received March 17, 2003;
accepted after revision May 28, 2003.
Address correspondence to S. Ö. Oktar
(ascim{at}ttnet.net.tr).
Abstract
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SUBJECTS AND METHODS. In this study, 150 lesions in 122 randomly selected patients with various abdominal and pelvic lesions were evaluated. For each lesion, sonograms were obtained with four techniques: conventional sonography, real-time spatial compound sonography, tissue harmonic sonography, and tissue harmonic compound sonography. All images were reviewed and graded independently by two observers for overall image quality, lesion conspicuity, and elimination of artifacts.
RESULTS. Statistical analysis showed that for overall image quality, lesion conspicuity, and elimination of artifacts, tissue harmonic compound sonography was significantly superior to all of the other techniques; real-time spatial compound sonography was better than tissue harmonic sonography; and conventional sonography was the least valuable of all (p < 0.001). When data were analyzed separately according to lesion types, tissue harmonic compound sonography was significantly superior for revealing stone diseases, liver cysts, gallbladder polyps, and uterine myomas. For the remainder of lesion groups, spatial compounding was superior to tissue harmonic sonography for all aspects of evaluation, and conventional sonography was the least valuable (p < 0.05).
CONCLUSION. In abdominal and pelvic scanning, tissue harmonic compound sonography provides the best overall image quality, best lesion conspicuity, and least artifacts of all the evaluated imaging modes. Spatial compound sonography is better than tissue harmonic sonography for the evaluation of lesions in general, despite some differences among lesion groups.
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Tissue harmonic sonography is also a new gray-scale sonographic technique that was initially developed for detecting nonlinear vibrations of microbubble contrast agents [6]. It can also be used to image tissue, markedly improving sonographic contrast resolution, particularly in patients who are difficult to image with conventional techniques. Compared with conventional sonography, tissue harmonic sonography has the potential to improve image quality. Images obtained with the harmonic mode are comparatively free of most artifacts that degrade conventional sonograms. Consequently, tissue harmonic sonography improves image contrast and diagnostic accuracy [7–9].
Real-time spatial compound sonography was initially available only for use on linear array transducers. With the expansion of technology, it can now also be used on curved array transducers and, therefore, is available for abdominal and pelvic examinations. It is also possible to merge compound sonography with advanced signal processing technology, such as tissue harmonic sonography, and theoretically combine the advantages of each modality.
Initial clinical experience and in vitro studies suggest that real-time spatial compound sonography improves contrast resolution and tissue differentiation of soft tissues, such as the breast, peripheral blood vessels, and musculoskeletal system, primarily by reducing unwanted artifacts without compromising other beneficial image characteristics such as spatial resolution [10–13]. However, to our knowledge, prospective studies on abdominal and pelvic applications of compound sonography with comparison with other modes of sonographic imaging have not been reported. The purpose of our study was to compare the appropriateness of conventional gray-scale sonography, real-time compound sonography, tissue harmonic sonography, and tissue harmonic sonography merged with compound sonography for abdominal and pelvic scanning.
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A single radiologist obtained all images, which were then evaluated by two other experienced radiologists for overall image quality and further characterized as to lesion conspicuity and unwanted artifacts. Overall image quality was defined as a general assessment encompassing spatial resolution or detail, contrast of solid and fluid-filled structures, and absence of noise. Lesion conspicuity was defined as the visibility and clarity of the lesion (compared with the adjacent structures) and the distictness of the posterior echo acoustic artifact, which was considered useful. Artifacts, including speckling, side lobes, reverberation, clutter, and blurring, that degrade the image quality were also evaluated for each image. For all lesions, overall image quality, lesion conspicuity, and elimination of unwanted artifacts were assessed and categorized by grade. Observers independently graded the images from 1 (indicating the worst image) to 4 (indicating the best). They were unaware of the sonographic technique used to produce each image. Statistical analysis was performed with a commercially available statistical software program (SPSS 11.0, SPSS, Chicago, IL). A Friedman test was used for multiple statistical comparisons between the four techniques. Kappa scores were calculated to assess interobserver agreement, and a p value of less than 0.05 was considered significant.
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For each sonographic technique, mean scores for overall image quality, lesion conspicuity, and elimination of unwanted artifacts were calculated using a Friedman test. Statistical analysis showed that for overall image quality, lesion conspicuity, and elimination of unwanted artifacts, tissue harmonic compound sonography was judged to be significantly superior to tissue harmonic sonography, real-time compound sonography, and conventional sonography and that real-time compound sonography was better than tissue harmonic sonography. Conventional sonography was the least valuable of all for all three criteria (p < 0.001 for all evaluated parameters).
Because the potential benefits of real-time compound sonography or tissue harmonic sonography may be different for one lesion type than for another—owing to the inherent characteristic of each type—we also analyzed the data separately by lesion type and anatomic region. Mean ranks of each technique for different lesion groups for the evaluated parameters are summarized in Table 1. To make paired comparisons between different imaging modes, Wilcoxon's signed rank test was used in lesion groups and a p value of less than 0.05 was considered significant.
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For stone diseases (urolithiasis and biliary system stones), real-time compound sonography and tissue harmonic sonography were not statistically different with regard to lesion conspicuity in which posterior acoustic properties are also taken into consideration (p = 0.073 for urolithiasis and p = 0.179 for biliary system stones). For liver cysts, differences between real-time compound sonography and tissue harmonic sonography were not statistically different with regard to lesion conspicuity (p = 0.20) and elimination of artifacts (p = 0.11). For gallbladder polyps, mean ranks for tissue harmonic sonography were greater than those for real-time compound sonography, but there was no statistically significant difference between these two imaging modes (p = 0.683). For uterine myomas, conventional sonography and tissue harmonic sonography modes were not significantly different with regard to overall image quality (p = 0.059) and lesion conspicuity (p = 0.168). For the other lesion groups, tissue harmonic compound sonography was significantly superior to real-time compound sonography and tissue harmonic sonography for all three aspects of evaluation, and real-time compound sonography was superior to tissue harmonic sonography (Table 2). Conventional sonography was the least valuable technique for all lesion types (p < 0.05 for all evaluated parameters).
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Conventional real-time sonography acquires diagnostic information by scanning lines that strike each target at a single viewing angle. Real-time compound sonography, on the other hand, uses computed beam steering technology to acquire multiple coplanar tomographic images from different viewing angles and combines them into a single compound image at real-time frame rates. The application of compounding to real-time sonography is actually not new [1–4]. However, its clinical application has been limited by the need for large amounts of computer power to generate real-time images from scanning lines at multiple angles and to maintain high frame rates. Practical implementation of this technique has recently become possible because of the considerable computational power of modern sonographic systems [10, 13].
Conventional real-time sonographic images are frequently degraded by artifacts such as shadowing and refraction. The ability of real-time compound sonography to improve image quality depends mainly on suppressing these artifacts and reinforcing the depictions of real structures. In principle, scanning from different viewing angles produces different artifact patterns. Summation of these independent frames obtained at different scanning angles substantially reduces artifacts such as clutter, speckling, glint, dropout, and refractive shadows. Also, by insonating specular reflectors at multiple angles, real-time compound sonography greatly improves the delineation of these surfaces, allowing superior definition of the margins of organs and lesions [10]. Because signals from true structures are summated, whereas random and artifactual echoes are not, the resulting image is a more realistic representation of actual tissue compared with the image obtained with conventional sonography. Speckling may be described as the artifact that is responsible for the grainy appearance on sonograms [3]. It reduces image contrast and detail resolution and diminishes the observer's ability to differentiate between normal and abnormal tissue. By suppressing the speckling, compound spatial sonography reduces the grainy appearance in sonograms and improves the signal-to-noise ratio, potentially improving the conspicuity of lesions [5, 12].
Because real-time compound sonography processes these distinct sonographic images from multiple lines of sight and compounds them in real time, the resulting images contain diagnostic information unobtainable by conventional single-line-of-sight sonography. These detailed images improve visualization of borders, interfaces, and irregular structures and have fewer artifacts.
In our study, we found compound sonography advantageous for obtaining superior detail, delineation of lesion boundaries, and good penetration and for eliminating unwanted artifacts. One potential limitation of spatial compound sonography is image blurring from motion. This blurring is due to the longer time required for averaging the frames obtained from multiple pulses and displaying the summated image. The number of frames and steering angles vary depending on the transducer characteristics and the clinical application. In general, the more frames in the compound acquisition sequence, the better the compound image quality. In our sonographic equipment, target and survey modes were available. The survey mode minimizes the blurring and allows rapid scanning by producing three coplanar images in the compound acquisition sequence; and the target mode maximizes the image quality by producing nine coplanar images, but it has a greater likelihood of motion blurring. The tradeoff between improving image quality and minimizing motion blurring can be optimized for different clinical applications [14]. In our experience, we have noticed that with the addition of tissue harmonic sonography to compound sonography, blurring diminishes.
Another limitation of compound sonography is that speckle reduction dramatically alters the familiar appearance of the B-mode sonogram. On the other hand, some studies have shown that removing speckle noise improves lesion margin definition and enhances lesion conspicuity [5, 12]. With speckle reduction, the usual sonographic appearance of structures changes, but there is no loss of important anatomic detail in the smoothed images.
Tissue harmonic sonography is also a relatively new sonographic technique that is based on the nonlinear interaction of an acoustic signal as it propagates through the body. In conventional sonography, echoes are transmitted and received at the same frequency, whereas harmonic sonography uses only the second harmonic frequency for imaging [7, 15, 16]. The current technology uses the second harmonic, which is twice the transmitted frequency, because the higher harmonic necessitates extremely wide bandwidth transducers. The harmonic band is generated by tissue itself, with the distorted and scattered energy much weaker than the transmitted energy, generating much weaker harmonics. As a result, tissue harmonic sonography contains minimal noise and successfully eliminates some image-degrading artifacts, such as side lobes and reverberation. The resultant image is clearer and relatively free of artifacts compared with the fundamental image. Meanwhile, posterior acoustic shadow, which can be considered a useful artifact, becomes more conspicuous. That difference may be related to the higher receiving frequency and narrower dynamic range used in tissue harmonic sonography [7, 8]. Tissue harmonic sonograms also have greater contrast compared with the sonograms generated by conventional sonography. This difference can be advantageous for evaluating and characterizing fluid-containing areas.
Previously, series evaluations have shown that tissue harmonic sonography has significant diagnostic advantages in deep abdominal and pelvic imaging [7, 17–19]. It improves image quality by increasing signal-to-noise ratio and enhancing lateral resolution because of a narrow transmitting frequency bandwidth, with respect to that of the fundamental beam [7, 15, 18]. However, tissue harmonic images were also found to have less penetration than the images produced with conventional sonography [20], a finding that was also noted in our study. Despite this limitation, we found that tissue harmonic sonography improved our evaluation of most of the abdominal and pelvic lesions.
In this prospective study, we compared conventional sonography, real-time compound sonography, tissue harmonic sonography, and tissue harmonic compound sonography of various abdominal and pelvic lesions and explored whether theoretical advantages of compound sonography and tissue harmonic sonography improved diagnostic image quality.
The results of our study show that for overall image quality, lesion conspicuity, and elimination of unwanted artifacts, tissue harmonic compound sonography is superior to other sonographic modes for abdominal and pelvic imaging in general. Real-time compound sonography is better than tissue harmonic sonography, and conventional sonography is the least valuable mode for all three criteria. These findings show close correspondence with the theoretical advantages of real-time compound sonography and tissue harmonic sonography.
Central acoustic shadowing or enhancement is a useful sonographic artifact in lesion recognition and conspicuity. In compound acquisition of a lesion with central shadowing by averaging the frames, the nonoverlapping portions of the shadows are diminished, and the overlapping shadows immediately behind the lesion are concentrated in a triangular region. Thus, posterior echoes are preserved in the central portion and reduced in periphery (Fig. 1A, 1B, 1C, 1D). We observed that this preservation leads to clearer visualization of the tissue behind the lesion without manipulation of the transducer and that it may contribute to the increased overall image quality in real-time compound sonography. This property could be considered a potential disadvantage of real-time compound sonography. However, any disadvantage could be negated by a user's ability to rapidly switch to conventional real-time or harmonic sonography, which might be necessary for the evaluation of posterior echo pattern.
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Acoustic shadowing from stones was better detected in harmonic mode than in fundamental mode. In our study, for stone diseases (urinary tract and biliary stones), real-time compound sonography and tissue harmonic sonography were not statistically different for lesion conspicuity in which posterior acoustic shadowing was also taken into consideration (p > 0.05). We also observed that when tissue harmonic sonography was combined with compound sonography, posterior acoustic artifact data were preserved for accurate characterization of the lesion while preserving the improved image quality (Fig. 2A, 2B, 2C, 2D). Because tissue harmonic sonography is efficient for evaluation of posterior echo pattern, it can be used as a complementary imaging method for some types of lesions or could be added to compound sonography.
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Real-time compound sonography also makes it easier to observe cystic lesions of the abdomen and the pelvic region. With real-time compound sonography, insonating the walls of the cyst at optimal angle by the multiple beams improves continuity of specular reflectors and reveals well-defined walls [10]. Also, decreasing artifactual echoes in cysts may improve the solid–cystic differentiation of the lesion and change the final assessment. Reduced noise and speckling in compound sonography substantially eliminates artifacts in liquid cavities, which appear much darker and cleaner on images. True echoes from debris in complex cysts, septa, or calcifications should be more conspicuous on real-time compound sonography because of reduced speckle noise, and therefore, confidence in diagnosis increases.
We observed that cystic lesions of abdominal and pelvic regions in general were best shown on compound sonography merged with tissue harmonic sonography (Fig. 3A, 3B, 3C, 3D). Tissue harmonic sonography also characterizes cystic lesions significantly more accurately than does conventional sonography. However, cystic lesions look clearer on real-time compound sonography than on tissue harmonic sonography and least clear on conventional sonography (Fig. 4A, 4B, 4C, 4D). When we analyzed the lesion groups separately, we found that for renal and ovarian cysts, real-time compound sonography was better than tissue harmonic sonography for all evaluated parameters (p < 0.05). On the other hand, for liver cysts, real-time compound sonography and tissue harmonic sonography were not statistically different from each other for lesion conspicuity and elimination of artifacts. Perhaps elimination of reverberation artifacts and a markedly diminished defocusing effect of abundant subcutaneous fat tissue on tissue harmonic sonography permit a more reliable depiction of the region [7, 9], which may also be the reason that the mean score of tissue harmonic sonography for gallbladder polyps was greater than that of real-time compound sonography, even without any statistical significant difference (p = 0.683).
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An important limitation of our study was that although the observers evaluating the images were unaware of image type while they graded the sonograms, absolute blinding was difficult to attain because the conventional, spatial compound, and harmonic sonograms differed so considerably in quality. This study was performed to compare different sonographic imaging modes mainly in terms of overall image quality. Each mode has its own limitations, such as blurring and decreased posterior echo characteristics in compound sonography and decreased penetration in tissue harmonic sonography. The use of compound sonography and tissue harmonic sonography in combination helps to reduce these artifacts.
We conclude that combining real-time spatial compounding with tissue harmonic sonography improves overall image quality and lesion conspicuity and eliminates unwanted artifacts and therefore produces the best results in abdominal and pelvic scanning. We believe tissue harmonic compound sonography can make examinations more successful, especially the examination of lesions that are difficult to image. Additional research is required to determine if tissue harmonic compound sonography can improve diagnostic accuracy by affecting sensitivity or specificity. In addition, although we found real-time compound sonography to be better than tissue harmonic sonography for evaluation of lesions in general, differences in image quality among separate lesion groups need further investigation.
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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k E, Öner Y,
Ik S. Benefits of tissue harmonic imaging in evaluation of
abdominal and pelvic lesions. Abdom Imaging2003; 28:103
–109[Medline]
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