Contrast-enhanced harmonic endoscopic ultrasound imaging: basic principles, present situation and future perspectives

Abstract

Over the last decade, the development of stabilised microbubble contrast agents and improvements in available ultrasonic equipment, such as harmonic imaging, have enabled us to display microbubble enhancements on a greyscale with optimal contrast and spatial resolution. Recent technological advances made contrast harmonic technology available for endoscopic ultrasound (EUS) for the first time in 2008. Thus, the evaluation of microcirculation is now feasible with EUS, prompting the evolution of contrast-enhanced EUS from vascular imaging to images of the perfused tissue. Although the relevant experience is still preliminary, several reports have highlighted contrast-enhanced harmonic EUS (CH-EUS) as a promising noninvasive method to visualise and characterise lesions and to differentiate benign from malignant focal lesions. Even if histology remains the gold standard, the combination of CH-EUS and EUS fine needle aspiration (EUS-FNA) can not only render EUS more accurate but may also assist physicians in making decisions when EUS-FNA is inconclusive, increasing the yield of EUS-FNA by guiding the puncture with simultaneous imaging of the vascularity. The development of CH-EUS has also opened up exciting possibilities in other research areas, including monitoring responses to anticancer chemotherapy or to ethanol-induced pancreatic tissue ablation, anticancer therapies based on ultrasound-triggered drug and gene delivery, and therapeutic adjuvants by contrast ultrasound-induced apoptosis. Contrast harmonic imaging is gaining popularity because of its efficacy, simplicity and non-invasive nature, and many expectations are currently resting on this technique. If its potential is confirmed in the near future, contrast harmonic imaging will become a standard practice in EUS.

Keywords: Contrast agents; Contrast-enhanced harmonic endoscopic ultrasound; Gastrointestinal submucosal tumour; Microbubbles; Pancreatic tumour.

Figure 1

Schematic view of the frequency range used for filtering the second harmonic.

Figure 2

Effects of narrowing the band pulse.

Figure 3

Principle of phase inversion mode.

Figure 4

Principle of extended pure harmonic technique.

Figure 5

Pancreatic neuroendocrine tumour (left side B mode, right side CH mode). A: No tumour enhancement is observed 6 s after injecting the contrast; B: 15 s after the injection, the contrast uptake is maximal; C: 30 s after the injection, the enhancement begins to decrease.

Figure 6

Hypoenhanced pattern in a pancreatic adenocarcinoma.

Figure 7

Hyperenhanced pattern in a pancreatic neuroendocrine tumour.

Figure 8

Autoimmune pancreatitis with hypoechogenic aspect in B mode (left image) and diffuse enhancement in contrast mode (right image).

Figure 9

Hypoenhanced pancreatic adenocarcinoma with hyperenhanced peripheral foci (arrows) corresponding to inflammatory changes.

Figure 10

Gastric gastrointestinal stromal tumour displayed as hyperenhanced submucosal lesion with some macrovessels (arrows) inside the tumour.

Figure 11

Duodenal carcinoid tumour: Hyperenhancement after contrast injection.

Figure 12

Leiomyoma of the gastric cardias showing a hypoenhanced pattern.

Figure 13

Ampullary tumour with echoic material inside the distal bile duct. A: No enhancement is observed 7 s after contrast injection; B: Bile duct involvement is diagnosed after contrast enhancement 28 s after the injection.

Figure 14

Perivascular infiltration by a pancreatic adenocarcinoma displayed as a hypoechoic wall thickening of the celiac artery (arrows).