Image Fusion and Electromagnetic Needle Tracking for the Biopsy of Pelvic Lesions – Report of 2 Cases

Introduction

Percutaneous or transvaginal biopsy is often performed under ultrasound (US) guidance, which has the advantage of real-time images and no ionizing radiation. However, US may be challenging in obese patients, and areas covered by air may be difficult to visualize, for instance in the pelvis. These challenges may be overcome by using image fusion and electromagnetic needle tracking.

Image fusion involving US is a technique, in which a previously recorded CT or MRI examination is co-registered with real-time US images. It works by means of software in the system and a magnetic positioning system – a magnet placed beside the patient and magnetic sensors attached to or incorporated in the transducer – and is available from most commercial high-end systems. The images are aligned by co-registration of corresponding points visible on US and CT (or MRI), for instance anatomical landmarks as the umbilicus and the aortic bifurcation. The time spent on the co-registration decreases with experience and is approximately 5 min. Accuracy depends on the landmarks, positioning of the patient and respiration, but has been shown to be as low as a few millimeters in patients (Hakime A et al. Cardiovasc Intervent Radiol. 2011; 34: 338–344). When the images are co-registered, the CT or MRI images are reformatted in a projection to fit the real-time US images (Ewertsen C et al. Am J Roentgenol. 2013; 200: W249–255).

Electromagnetic needle tracking is also based on a magnetic positioning system and can be used separately or together with image fusion. An electromagnetic sensor is embedded in the tip of the needle. The tip of the needle and the puncture line are marked electronically and simultaneously on the US and CT (or MRI) images, instead of the usual mechanical registration of the echoes from the needle tip (Venkatesan AM et al. Radiology. 2011; 260: 848–856). These new techniques enable percutaneous US-guided intervention in areas with a poor US overview. In patients with a previous history of cancer or with suspicion of actual cancer disease, biopsy is crucial in order to establish a correct diagnosis and treatment plan.

We present 2 cases in which image fusion and electromagnetic needle tracking have been used to locate and biopsy poorly defined lymph nodes in the pelvis.