Contrast-enhanced ultrasonography: abdominal applications

Mirko D’Onofrio
MD PhD
Radiologist
Department of Radiology
GB Rossi Hospital
University of Verona
Italy

Ultrasound contrast agents are based on gas-filled microbubbles (eg, sulphur hexafluoride-filled microbubble with phospholipid peripheral shell: SonoVue(®), Bracco, Milan, Italy) with harmonic responses at low ultrasound acoustic pressure (mechanical index <0.2) and mean diameter around 3 μm, resulting in exclusively intravascular distribution (blood pool contrast agent).(1)

The first types of agents were extremely hard to handle because the imaging methods were based on destructive techniques. But progressive and important advances have been made in the field of ultrasound contrast media and related detection techniques in recent years. As a result, they have become more and more easy to use, so that today they have increasingly wider application in clinical practice.

Standardisation of contrast-enhanced ultrasonography (CEUS) images can be obtained by using external offline softwares, thus obtaining comparable images from different studies.(2) Internal standardisation is expected in the future, giving the possibility to store the raw data from an automatic scan (dedicated probes) instead of manual scan in order to improve the reproducibility of the study and overcome the subjectivity.(2)

Guidelines for the use of CEUS already exist,(3) with an update expected in 2008.

Liver applications
Ultrasound is often the first examination performed for liver study. Focal liver lesions frequently cause diagnostic problems. Hypoechoic focal liver lesions are indeterminate, and the Doppler study shows low accuracy.(4,5) The demonstration of tumour macrocirculation (eg, during Doppler study) can fail in the prediction of tumour perfusion. In order to skip all these problems and to overcome the limitation of ultrasound in focal liver lesions study, the evaluation of the perfusion throughout the injection of
contrast agents is now possible.

CEUS has been shown to be useful in the detection and characterisation of focal liver lesions.(5–11)

Correct tumour characterisation is often related to the demonstration on imaging of lesion vascularisation, such as hypervascularity for hepatocellular carcinoma.(2)

In ultrasound the use of microbubbles improves the detection and characterisation of liver metastases (see Figure 1). The improvement in the detection of metastases means that CEUS is similar in accuracy to contrast multidetector computed tomography.(1)

[[HHE07_fig1_R10]]

CEUS can also characterise benign focal liver lesions with indeterminate appearance incidentally detected giving immediate diagnosis, the possibility to avoid further imaging modalities and therefore consequent cost-saving and improvements in patient management.

The efficacy of percutaneous ablation procedures can be improved by evaluating the results immediately after the treatment with CEUS.(12) In particular, only the complete disappearance of vascularised viable tissue within the tumour proves the complete treatment of the lesion.

Other abdominal applications
In current literature, reports of other abdominal applications of CEUS (eg, kidney and pancreas) are growing in number. CEUS can help, for instance, in the study of pseudotumours and complex cysts of the kidney.(13)

Regarding the pancreatic pathology, the use of microbubbles improves the ultrasonographic characterisation of pancreatic ductal adenocarcinoma (see Figure 2).(14–16)

[[HHE07_fig2_R12]]

In addition to intravenous injection, intracavity administration of ultrasound contrast agents has been discussed, for example, in the study of vescicoureteric reflux.(17)

Moreover, other abdominal applications have been proposed, such as abdominal trauma and study of the great vessels.

Future applications
CEUS is highly accurate in demostrating tumour vascularisation, with reported excellent correlation with mean vascular density (MVD).(14) CEUS is the only imaging modality with dynamic observation of the contrast-enhanced phase, allowing real-time study of tumour vascularisation. Moreover, microbubbles are “blood pool” contrast agents, thus improving imaging of the tumoral vascular network. Thus CEUS could be used as a routine application in oncologic follow-up of patients under chemotherapy. Other future applications of CEUS are connected to the development of antiangiogenic therapies during which an accurate analysis of tumoral residual vascularisation is necessary. For this reason, CEUS is expected to be a valid onco-imaging modality.(14)

Conclusions
Contrast-enhanced ultrasonography has radically changed the role of ultrasound in many diagnostic algorithms, becoming not only a first-level but also a second-level diagnostic tool.

CEUS can provide important information in the workup of diagnostic problems and complementary information to other imaging modalities such as multidetector computed tomography and magnetic resonance imaging.

References

1. Cosgrove DO. Eur J Gastroenterol Hepatol 2007;19:1-2.
2. D’Onofrio M, et al. J Digit Imaging 2007;20:256-62.
3. EFSUMB Study Group. Ultraschall Med 2004;25:249-56.
4. Harvey CJ, Albrecht T. Eur Radiol 2001;11:1578-93.
5. D’Onofrio M, et al. J Clin Ultrasound 2005;33(4):164-72.
6. Solbiati l, et al. Eur Radiol 2001;11 Suppl 3:E15.
7. Wilson SR, et al. Radiology 2000; 215:153-61.
8. Leen E. Eur Radiol 2001;11 Suppl 3:E27.
9. Albrecht T, et al. Am J Roentgenol 2001;176:1191-8.
10. Quaia E, et al. Eur Radiol 2006;16:1599-1609.
11. Quaia E, et al. Radiology 2004;232:420-30.
12. Solbiati L, et al. Eur Radiol 2004;14 Suppl 8:P34-42.
13. Correas JM, et al. Ultrasound Q 2006;22:53-66.
14. D’Onofrio M, et al. Pancreatology 2005;5:398-402.
15. D’Onofrio M, et al. World J Gastroenterol 2006;12:4181-4.
16. D’Onofrio M, et al. Abdominal Imaging 2007;32:171-81.
17. Darge K, et al. Pediatr Radiol 2005;35:73-8.