Drainage of Air-Containing Cavities Guided by Image Fusion involving Ultrasound and Electromagnetic Needle Tracking

Introduction

Percutaneous drainage of abscesses is often performed guided by ultrasound (US), which has the advantage of real time images and can be done bed-side. However, ultrasound is unable to visualize structures behind air and bone.

Software for image fusion, where a previously made CT- or MRI examination can be co-registered with live US, is commercially available. These systems work by means of an electromagnetic tracking system – a magnetic transmitter placed close to the patient and one or two magnetic sensors attached to the transducer – and software in the US system. By adding the results from CT or MRI, US-guided punctures of anatomical areas invisible to ultrasound become possible (Crocetti L. Invest Radiol 2008; 43: 33 – 39; Ewertsen C. Ultraschall in Med 2011; 32: 191 – 197).

The co-registration is performed by initially identifying identical planes perpendicular to the umbilicus in both imaging modalities. Afterwards, common points in the area of interest are identified in both modalities. The accuracy may be checked by adding electronical markers (GPS markers), which are shown on both modalities. If the markers are displayed in identical structures in different planes, the co-registration is accurate. If not, more points are added until the alignment is accurate.

Very recently, electromagnetic needle tracking has become available to such systems and can be used together with image fusion. An electromagnetic sensor (diameter 0.9 mm) is embedded in the needle tip. The needle is connected to the US system and works in cooperation with the magnetic positioning system ([Fig. 1]). The needle tip and its virtual route of puncture are electronically and simultaneously displayed on both the US and the CT (or MRI) image (Ewertsen C. Ultraschall in Med 2012; 33: 1 – 3)

Fig. 1 a Set-up of the system. Magnetic transmitter (upper arrow) is placed close to the patient and magnetic sensors (lower arrow) are attached to the transducer. b The needle for electromagnetic needle tracking; a magnetic sensor (upper arrow) is embedded in a trochar needle (middle arrow) covered in stylet (lower arrow).

Abb. 1 a Systemaufbau. Der magnetische Sender (oberer Pfeil) ist in der Nähe des Patienten angebracht und die magnetischen Sensoren (unterer Pfeil) sind am Schallkopf angebracht. b Die Nadel für das elektromagnetische Nadel-Tracking; ein magnetischer Sensor (oberer Pfeil) ist in eine Trocharnadel eingebettet (mittlerer Pfeil), die im Stilett umhüllt ist (unterer Pfeil).

Air-containing cavities due to leakage from gastrointestinal anastomoses or from air-filled abscesses may be a complication to colorectal surgery (Kirat HT. Int J Colorectal Dis 2011; 26: 1469 – 1974). A preferred treatment of these patients may be percutaneous drainage in local anaesthesia in order to avoid a stressful open surgical procedure.

We present three patient cases, where air-filled intraabdominal cavities developed following re-laparotomies after initial laparoscopic hemi-colectomies. The air-filled cavities were successfully treated by percutaneous US-guided drainage using image fusion and electromagnetic needle tracking, thus saving the patient from re-surgery.