Abstract
Magnetomotive ultrasound (MMUS) imaging system is an ultrasound based imaging system which accompanies with magnetic nanoparticles and magnetic field, and it has the feasibility for early diagnosis. The most common MMUS uses continuous wave magnetic field to induce the magnetomotion of the magnetic nanoparticles. However, a part of energy translates into heat and causes thermal problems in the system. Such a system suffers the thermal constraints and is not suitable to generate stronger magnetic field. Under this situation, the system has a limited penetration depth or small magnetic field area (i.e., imaging field of view). To solve this problem, in this study, we propose and develop a ultrafast imaging based pulsed MMUS imaging system. We build a custom-made programmable magnetic pulser. This module can generate higher voltage and current output than general current amplifiers and it can generate 5 to10-ms magnetic pulse. With the programmable capability, this module is able to generate a coded magnetic pulse sequence. The coded high frequency magnetic pulse sequence will induce a distinct displacement profile of magnetic nanoparticles from the background tissues. After matched filtering the displacement profile at each imaging point with the designed coded magnetic pulse sequence, the background tissue motion artifacts can be significantly suppressed and thus the distribution of magnetic nanoparticles can be found via thresholding of the derived correlation coefficient and corresponding lag time at each imaging point. We experimentally test the maximum and minimum achievable magnetic pulse length of the system and magnetic field intensity, and the corresponding magnetic field distribution. We also compares the imaging results with different magnetic pulse lengths and thus optimize the most suitable magnetic pulse sequence of the system. In addition, a clinically translatable backward mode MMUS probe integrating the ultrasound transducer and the electromagnet is build and the penetration depth of the system equipped with this probe is at least 1.5 cm.