How Much Should I Record During a CEUS Examination? Practical Aspects of the “Real-Time” Feature of a Contrast Ultrasound Study

The application of microbubble contrast agents to the ultrasound examination over the last 20 years has revolutionised the assessment of focal liver lesions [1], has been applied to many other organs in the body [2] [3] and is now an established adult application, with the use in paediatric patients increasing [4]. The technique is safe and repeatable [5] [6]. A contrast-enhance ultrasound (CEUS) examination generates a substantial amount of data for storage with the use of continuous recording, so much so that in Departments of Radiology, where a Picture Archiving System (PACS) stores data for a considerable number of imaging examinations, rumblings of discontent from the PACS manager can be heard. The misconception that the “video-clips” from a CEUS occupy too much storage is a fallacy, as many whole body computed tomography (CT) examinations from a trauma patient, with reconstruction in thin slices and 3-D, can occupy a large amount of storage space. The perception is that the “new-boy on the block” will clog up the PACS with superfluous images; perhaps better to make room for other imaging modalities.

There is no battle to be fought here; we can help ourselves. There is perhaps no real need to take long and extensive real time “video-clips” of the entire CEUS examination, but to record only the most informative sections. This is not a new concept, and it is worthwhile revisiting the background to extensive image storage as there are very good technical reasons not to perform a continuous recording, making the CEUS examination recording shorter and even more useful [7].

Characterisation of focal liver lesions with CEUS represents the single most important application of CEUS with numerous studies confirming the accuracy of this technique in comparison with CT, and near ability to match Magnetic Resonance (MR) imaging [8] [9]. The arterial phase and the late portal venous phase are crucial to the assessment for characterisation; the washout particularly important with hepatocellular carcinoma, which may be delayed as opposed to metastatic lesions where washout is relatively early [10]. With a focal liver lesion, the arterial pattern, particularly in the benign lesion is paramount to the interpretation, but this phase lasts only for 30 to 40 seconds at most, and is nearly always informative by 20 to 30 seconds [11]. The washout phase starts early for the metastatic lesion, often before 60 seconds and will continue to wash-out for the rest of the examination. A hepatocellular carcinoma will washout slowly and may not demonstrate this washout for 180 seconds or more, depending on the cell differentiation. A most unlikely scenario is that a lesion that washes out then reaccumulates contrast and enhances over time; in-filling of a benign haemangioma is an exception where the characteristic globular in-filling is diagnostic.

The essence of this is twofold; it is unlikely that extending the “video-clip” beyond 60 seconds will capture any further relevant information, and recording of washout is a prolonged effort, not needing continuous image storage. More important is the potential for microbubble “destruction” with prolonged, intense scanning over one area, with near field lesions most vulnerable, resulting in “pseudo-washout” [12] [13]. In short, continuous scanning for the first 60 seconds and then intermittent scanning after 60 seconds, taking an image on average every 30 – 60 seconds for the duration of the remainder of the examination will probably suffice. The most comfortable manner in which to perform the intermittent scanning is by “freezing” the image, keeping the transducer in the fixed appropriate position for the lesion under investigation, and re-imaging for a short period to store a single representative image – the “snapshot”.

This will be entirely different for imaging of organs without the dual blood supply of the liver, and where characterisation is less vital. The need for a prolonged “video-recording” is less imperative in many scenarios; in the kidney the abnormal vascular pattern is seen in the early arterial phase, with washout evident quite soon after, and the pyramids enhancing approximately 30 – 40 seconds after the cortex. The continuous recording need not be more than 40 seconds, followed by the intermittent single image acquisition. This application would also suffice for the pancreas or spleen, with the bowel perhaps more reliant on the increased enhancement in the arterial phase alone. When used a s a problem solving tool, in intracavitary use [14] or as part of an interventional procedure [15], short “video-clips” or just representative images is all that is required.

The acquisition of a “video-clip” should always be supplemented by representative single images of the particular diagnostic appearance e. g. the “spoke-wheel” in the arterial phase of a focal nodular hyperplasia or the “globular” peripheral lesion pattern of a haemangioma. Often during the review of the “video-clip” these representative images are saved, ready to be presented at the multi-disciplinary meetings where the case is discussed; nobody will want to sit through all the “video-clips” of your examination! Herein lies a problem, all manufacturers bar one does not allow you to obtain a single representative image during acquisition whilst the video is being recorded, which if acquired halts the video recording. This seems only to impart a restriction that delays patient flow and ease of the examination, if the operator has to review the “video-clip” after every patient. Although good practice to review the recording for any “missed” features, the ability to take an image during the dynamic phase is always rewarding; you capture the moment!

In essence, there is no need to record a full 4 – 5 minutes of a CEUS examination, a continuous recording for the first 60 seconds will suffice with single images when needed for the later phases, ostensibly looking for any form of washout pointing to potential malignancy in the liver, and elsewhere. This will reduce pressure on the storage of images on your PACS, helping to maintain a good working relationship with the PACS manager. More importantly, this practice will reduce the potential of accelerated microbubble destruction, particularly in the near-field lesion and ensure a long-lasting examination.

• References

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