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Original Report |
1 All authors: Department of Radiology, S. Maria delle Grazie Hospital, Via Domitiana Località La Schiana, Pozzuoli, Naples 80121, Italy.
Received May 19, 2003;
accepted after revision August 18, 2003.
Address correspondence to O. Catalano
(orlandcat{at}tin.it).
Abstract
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CONCLUSION. Contrast-specific sonography was used to assess eight cases of aspiration-confirmed pyogenic liver abscesses. All cases were correlated with multiphasic helical CT findings. Continuous sonographic exploration allowed recognition of morphologic details not detectable on CT images. Contrast-specific sonograms showed features including rim enhancement, arteries along abscess margins and internal septa, dense and persistent septal enhancement, absent microcirculation in fluid and necrotic components, transient arterial phase hypervascularity around abscesses, and portal phase hypovascularity around abscesses. This constellation of findings is suggestive of liver abscess.
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During the past year, we have evaluated several hepatic abscesses caused by pyogenic infection using second-generation contrast medium–enhanced real-time sonography and correlated our findings with those from contrast-enhanced helical CT.
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One patient had a cholangitic abscess, three patients had a hematogenous abscess, three patients had a cryptogenetic abscess, and one patient had a suppurated liver hematoma. Gram-positive bacteria grew in cultures from three patients, gram-negative bacteria in a culture from one patient, and polymicrobial flora in cultures from four patients.
Baseline sonography was performed with a Technos MP scanner (Esaote, Genoa, Italy) using multifrequency (2.5–5 MHz) convex probes. Contrast-enhanced studies were obtained with the contrast-devoted EsaTune unit (Esaote), using the contrast-specific technology named "contrast-tuned imaging" with a 3.5-MHz convex transducer. A sulfur hexafluoride–based microbubble contrast agent (SonoVue, Bracco, Milan, Italy) was rapidly injected into a peripheral vein through a 20-gauge needle. A volume of 4.8 mL was administered, followed by a 5-mL saline flush (using a three-way stopcock). Continuous scanning began immediately and lasted 4–5 min. A low acoustic power setting was used (40–45 kPa derated pressure, expressing a mechanical index of approximately 0.06). The ultrasound beam was focused at the deeper aspect of the lesion being examined. A timer on the sonography unit was activated at the moment of injection, and the entire examination movie was archived on the scanner. Stored video footage was subsequently sent to a PC and converted to audio-video interleave–format files for reporting and retrospective evaluation purposes.
CT studies were obtained using a single-detector scanner with 0.75-sec revolution time (Somatom Plus 4 Expert, Siemens, Erlangen, Germany). Unenhanced images were obtained in all patients. The contrast-enhanced studies were obtained with parameters of 5-mm collimation, 7.5-mm/sec table speed, 120 kVp, 180 mA, and 4-mm reconstruction interval. A nonionic contrast medium (iomeprol 350 mg I/mL, Iomeron, Bracco, Milan, Italy) was administered via a 20-gauge needle and a power injector (Angiomat 6000, Liebel-Flarsheim, Cincinnati, OH). A volume of 130 mL was injected at 2–3 mL/sec. The acquisition delay time was 40 sec for the early phase and 100 sec for the portal phase.
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All abscesses had areas of increased enhancement relative to the liver parenchyma. Liquefied lesions (10/13) showed an enhancing rim and a nonenhancing center; consequently, a clear internal margin was recognizable in these lesions (Fig. 1A, 1B). Solid-looking lesions (3/13) had diffuse but inhomogeneous contrast enhancement with a thin and irregular rim of hypervascularity.
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The enhancing rim varied greatly in thickness among different abscesses. It increased in echogenicity readily and strongly after contrast medium injection. Enhancement extended to internal septa and persisted during portal and sinusoidal phases of opacification. Large abscesses showed partial peripheral septa, and thin but complete septa could be identified in smaller lesions, giving them an overall honeycomb appearance.
No sign of contrast material microcirculation was seen in the internal fluid, debris, and necrotic components. One abscess exhibited an internal fluid–debris level.
Discrete arteries of different sizes were seen in lesion borders (8/13) and along internal septa (8/13) (Fig. 2A, 2B, 2C). Two abscesses showed high-intensity internal echoes with reverberation and back-shadowing caused by air. One of these abscesses was mostly air-filled, and only a single bubble was noted in the other. Perilesional enhancement was observed by comparison to the abscess rim, but to a lesser degree. This transient arterial phase hypervascularity around the lesion was seen in three of 13 abscesses, and a peripheral portal phase hypovascular area was present in two.
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Two of our patients (Figs. 3A, 3B, 3C and 4A, 4B, 4C, 4D) were thought to have a metastasis. In one (Fig. 3A, 3B, 3C), the main lesion and two satellite lesions appeared solid on CT images, with inhomogeneous hypoattenuation and irregular walls. Contrast-specific sonography showed irregular enhancing septa running through necrotic areas and a discontinuous peripheral rim. These findings were consistent with the hypothesis of a nonliquefied abscess that was confirmed at subsequent aspiration. The other patient (Fig. 4A, 4B, 4C, 4D) had an initial diagnosis of solitary metastasis on the basis of conventional sonography findings and the absence of a known extrahepatic tumor. At first glance, an inhomogeneously hypoechoic and ill-defined lesion appeared metastatic. Nevertheless, contrast-specific sonography revealed several findings, including an unsuspected internal honeycomb architecture that led to the proper diagnosis.
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In 4–5 min of recorded video footage, images obtained at certain time intervals were particularly helpful. Abscess arteries and rim enhancement were especially noted 15–40 sec after injection; perilesional hyperechogenicity, 30–50 sec after injection; internal abscess septa, 30–120 sec after injection; and perilesional hypoechogenicity, 1–3 minutes after injection.
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Conventional (fundamental) sonographic features of pyogenic liver abscess have been previously reported [6, 9, 10]. Gray-scale imaging is a main area of advancement for contrast-enhanced sonography. Detection and characterization of hepatic lesions can be greatly improved with both intermittent and continuous contrast-specific technologies [3]. Intermittent techniques require microbubble destruction to reach adequate signal enhancement; low mechanical index technologies such as contrast-tuned imaging allow nondestructive stimulation of second-generation contrast media and the unique possibility of continuous scanning [1, 2]. Contrast-tuned imaging software allows selective tuning of the sonographic scanner with the contrast medium signal: having a specific transmitting and receiving resonance frequency prevents interference from tissue signals.
Sonography has limitations in imaging liver abscesses. Compared with conventional sonography, CT offers greater contrast resolution of the abscess, mainly because of the contrast enhancement of uninvolved parenchyma [4]. In our series, contrast-specific sonography achieved the same degree of abscess conspicuity. Subtle lesions on baseline sonography showed high lesion-to-parenchyma contrast on SonoVue-enhanced studies.
Typical pyogenic abscesses appeared on contrast-specific sonography as partially enhancing lesions with a thin or thick rim of dense opacification and a persistently hypoechoic center. Pulsatile vessels could be seen in the rim and along the internal septa. In comparison with baseline images, the contrast-specific images showed that the external margin between the abscess and the uninvolved parenchyma and the internal margin between the solid rim and liquefactive center become sharper after contrast medium injection.
The internal structure and septa were more conspicuous than they were on conventional sonography or CT. Conventional sonography may detect internal debris and septa better than CT and show a complex structure, whereas CT identifies a relatively homogeneous lesion [4]. Contrast-specific sonography brings out even more differences.
The liver parenchyma around the abscess may show changes that are not detectable on conventional sonography. A transient arterial phase hypervascularity around the abscess was seen in our series as a consequence of perifocal hyperemia, which we already identified on CT and MRI [11]. A hypovascular area peripheral (distal) to the abscess could also be observed, probably caused by perifocal edema or defective venous perfusion [11]. Color Doppler sonography may detect reversal flow in portal veins adjacent to liver abscesses [12], and these hemodynamic changes may also be detected on contrast-specific sonography.
The spectrum of contrast-specific sonography findings in our series was, in the end, similar to those described in the literature from contrast-enhanced CT [4, 6, 10] and gadolinium-enhanced MRI [11].
In summary, we have reported the contrast-specific sonographic findings in pyogenic liver abscess. To our knowledge, ours is the first report on the subject. Our study is limited by the small number of patients, but it shows how contrast-specific sonography can be used to identify typical features of pyogenic abscess that dynamically correlate with features recognizable on contrast-enhanced CT and MRI. We believe that the radiologist's confidence in the diagnosis and staging of liver abscess may be greatly improved using this sonographic technique.
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