Management of respiratory motion in extracorporeal high-intensity focused ultrasound treatment in upper abdominal organs: current status and perspectives.

Extracorporeal high-intensity focused ultrasound (HIFU) is a minimally invasive therapy considered with increased interest for the ablation of small tumors in deeply located organs while sparing surrounding critical tissues. A multitude of preclinical and clinical studies have showed the feasibility of the method; however, concurrently they showed several obstacles, among which the management of respiratory motion of abdominal organs is at the forefront. The aim of this review is to describe the different methods that have been proposed for managing respiratory motion and to identify their advantages and weaknesses. First, we specify the characteristics of respiratory motion for the liver, kidneys, and pancreas and the problems it causes during HIFU planning, treatment, and monitoring. Second, we make an inventory of the preclinical and clinical approaches used to overcome the problem of organ motion. Third, we analyze their respective benefits and drawbacks to identify the remaining physical, technological, and clinical challenges. We thereby consider the outlook of motion compensation techniques and those that would be the most suitable for clinical use, particularly under magnetic resonance thermometry monitoring.

Keep breathing! Common motion helps multi-modal mapping.

We propose an unconventional approach for transferring of information between multi-modal images. It exploits the temporal commonality of multi-modal images acquired from the same organ during free-breathing. Strikingly there is no need for capturing the same region by the modalities. The method is based on extracting a low-dimensional description of the image sequences, selecting the common cause signal (breathing) for both modalities and finding the most similar sub-sequences for predicting image feature location. The approach was evaluated for 3 volunteers on sequences of 2D MRI and 2D US images of the liver acquired at different locations. Simultaneous acquisition of these images allowed for quantitative evaluation (predicted versus ground truth MRI feature locations). The best performance was achieved with signal extraction by slow feature analysis resulting in an average error of 2.6 mm (4.2 mm) for sequences acquired at the same (a different) time.

Testing of a HIFU probe for the treatment of superficial venous insufficiency by using MRI.

A new high intensity focused ultrasound (HIFU) probe has been designed and tested by using MRI. The probe is intended to be used by physicians to correct valvular dysfunction in the saphenous vein, which is known to be the cause of superficial venous insufficiency (SVI) and varicose veins. Treating SVI with HIFU is possible, since venous tissue undergoes localized partial shrinkage when subjected to HIFU. In vitro experiments have demonstrated that diameter shrinkage should be sufficient to restore valvular function, as is done in the more aggressive approach known as external valvuloplasty. Numerical simulations and optimization have led to a probe design with two HIFU elements that focus sound uniformly over a line of length 7 mm, at a depth of 15 mm from the skin. A prototype of the probe has been constructed, with a holder that provides space for an MRI-imaging antenna. The probe has been tested by measuring pressure and temperature fields. Results are in good agreement with those predicted by an analytical approach and numerical simulations.