New Technology – Fusion Imaging and Interventional Ultrasound

Image fusion can be carried out between all image modalities provided they are geometrically consistent. Fusion ultrasound (FUS) is the application of image fusion, where dynamic ultrasound images are presented simultaneously with corresponding images obtained from a previously acquired CT, CT/PET or MRI volume. Fusion of a live on-going US-examination with a previously acquired US-examination may be performed as well. In the former EJU-newsletter, Dr Caroline Ewertsen and Dr Adrian Săftoiu presented a fine overview of this new technique. In the following, we report and illustrate some of our experience with FUS and interventional procedures.

Technical Requirements for FUS.

The first step on a FUS procedure is to load a dataset (CT, CT/PET or MRI) into the US-system by means of an USB-memory stick, a CD-ROM, a hard drive, or via the network cable and a DICOM query/retrieve connection to PACS (Picture Archiving and Communication System). Obviously, the CT-scan is obtained under other conditions than the US-scan regarding parameters such as patient position and respiration depth and the examiner must consider these limitations for a successful outcome of the image fusion including the interventional procedure.

A magnetic field is created around the patient by a magnetic transmitter and position sensors mounted on the US-transducer enable definition of the actual three-dimensional transducer position (Fig. [ 1 ]).

Fig. 1 Fusion imaging (US and CT). The magnetic transmitter (covering the face of the "patient") and the position sensors (mounted on the transducer) are seen. On the monitor, real time US and "fusion" CT are seen side by side.

Definition of three points defines a plane, by which alignment (registration) can be established between an US-image and the corresponding slice from the uploaded dataset of the previously acquired CT-exam. The US-exam is carried out as usual and images are shown in real-time (master) side-by-side and simultaneously with the corresponding dynamic virtual CT-image (slave). The two images can also be shown superimposed on each other. CEUS (Contrast enhanced ultrasound) may be conducted as CEUS-CT fusion (Fig. [ 2 ]).

Fig. 2 US-guided biopsy of a liver metastasis.
a. US-CT-fusion. The liver metastasis is located near the dome and is clearly visualized on CT (right image), but partly invisible on US (left image) due to pleural air artifact.
b. CEUS-CT-fusion. On CEUS (left image) the metastasis becomes more visible.

FUS and Biopsy

Cancer patients are often followed with CT after the primary treatment. In many institutions including our own department, when CT shows sign of recurrence and histopathology confirmation is necessary, the next step usually is an US guided biopsy. Small liver metastases or small retroperitoneal lymph nodes, however, might be difficult to find with US, and in this situation FUS in our experience can often provide a helpful guide to the correct lesion area (Fig. [ 3 ]). Furthermore, FUS enables biopsy and MIT ablations, even if a tumor is inconspicuous or part of the tumor is invisible on US (Fig [ 2 ]). Finally, in some cases, a biopsy might be avoided either due to agreement between a suspicious tumor finding on CT and subsequent fusion with CEUS that confirms the tumor or because the subsequent fusion with CEUS provides a benign explanation to the CT finding (Fig. [ 4 ]).

Fig. 3 US-CT-fusion. Patient with a history of testis cancer and suspicious lymph node on CT (right image, arrow) on follow up. Fusion was helpful by guiding US to identify the lymph node (left image, arrow) prior to USguided biopsy.

Fig. 4 CEUS-CT-fusion. On CT (right image) a suspicious lesion was found, however CEUS (left image) showed a typical post-ablation zone and 4 GPS markers outline the zone.

Use of GPS Markers in Volume Navigation

Position marking is often referred to as "GPS marking". The examiner may tag any position of interest by a small cross-mark. If the tagged point at any time is outside the scan plane it will change appearance (shape and color) to inform the examiner about distance and direction from the scan plane to the marked/tagged position. We use this feature in our planning of percutaneous ablation of liver metastases for an exact mapping of metastases regarding location and size (Fig. [ 5 ]). GPS markers are also available in CEUS (Fig. [ 4 ]). Furthermore, we use GPS markers real-time during the ablation procedure by marking the borders of the metastasis with e.g. 4 markers immediate prior to ablation (Fig. [ 6 ]). During thermal ablation, microbubbles are produced that usually disturb the US visualization of the metastasis. The 4 GPS markers, however, remain at their position at the tumor border, thereby enabling evaluation of the extension of the heat-coagulation-zone with respect to the original location of the metastasis. Furthermore, correction of the ablation needle can be performed guided by the GPS markers.

Fig. 5 Three liver metastases have been tagged with the GPS system. GPS marker no 1 is in the actual scan plane (small square) whereas GPS markers 2 and 3 are located out of the scan plane (bigger squares).

Fig. 6 Volume navigation and US-guided ablation. a and b The magnetic transmitter and the position sensors are seen in a surgical environment together with the ablation needle (microwaves).
c The liver metastasis have been tagged with 4 GPS markers.

Needle Tracking Device

A tracking device is a trocar-needle system with a build-in position sensor for definition of the needle tip position. The needle-tip position may be superimposed on the US-image as well as on any other image modality after fusion. The tracking device can be used in two different ways: The In-Plane procedure, where the scan plane and the needle plane are identical (traditional US-guided puncture) or the Off-Plane procedure, where the scan plane and the needle plan are different. Unfortunately, the sensor has to be removed just prior to the biopsy (since they use the same cannula) and the biopsy is therefore performed more or less "blind". The technique might have a potential use in performing some US-guided procedures such as for instance percutaneous nephrostomies.

Perspectives

Multi-treated patients with complex metastatic disease have become a more coon challenge due to the improvements of oncological and surgical therapies. CT-PET frequently presents unclear findings in these patients with a considerable risk of false positive or false negative errors. The use of FUS and FUS-guided biopsy may reduce these errors. Widespread use of FUS depends on availability of the fusion technology on a broad range of US-machines plus, integration of the technique in the clinical routines. In addition, rational selection of patient for FUS in both the radiological and the clinical specialties is mandatory.

Bjørn Skjoldbye
Christian P. Nolsøe
Torben Lorentzen

Department of Gastroenterology
Ultrasound Section
Herlev Hospital
University of Copenhagen
Denmark

The authors.