A method of preparing a suspension of microbubbles for use in a carrier liquid, wherein the microbubbles have a gas core and a liquid shell, said liquid shell comprising magnetic nanoparticles, and wherein the method comprises the steps of: (a) determining a size of the shell relative to the gas core and an amount of nanoparticles in the shell of a microbubble that satisfy the following conditions: (i) the force due to buoyancy ( F BW) of the microbubble in the carrier liquid is greater than the weight of the microbubble; (ii) the magnetic force ( F M) on the microbubble due to a magnetic field applied to the carrier liquid is greater than the combined weight and force due to buoyancy ( F BW) of the microbubble; (iii) said magnetic force ( F M) on the microbubble is greater than the force due to viscous drag ( F D) on the microbubble due to flow of the carrier liquid; and (iv) the scattering cross section (σ scat) of the microbubble to ultrasound allows the microbubble to be detectable and rupturable on exposure to ultrasound; (b) using the amount of nanoparticles in the shell and the relative size of the shell to the gas core of the microbubble to determine the amount of magnetic nanoparticles and the amount of liquid for the shell needed to prepare a suspension of the microbubbles; and (c) preparing a suspension of the microbubbles using the amount of magnetic nanoparticles and the amount of liquid for the shell needed to prepare the suspension. (a) determining a size of the shell relative to the gas core and an amount of nanoparticles in the shell of a microbubble that satisfy the following conditions: (i) the force due to buoyancy ( F BW) of the microbubble in the carrier liquid is greater than the weight of the microbubble; (ii) the magnetic force ( F M) on the microbubble due to a magnetic field applied to the carrier liquid is greater than the combined weight and force due to buoyancy ( F BW) of the microbubble; (iii) said magnetic force ( F M) on the microbubble is greater than the force due to viscous drag ( F D) on the microbubble due to flow of the carrier liquid; and (iv) the scattering cross section (σ scat) of the microbubble to ultrasound allows the microbubble to be detectable and rupturable on exposure to ultrasound; (i) the force due to buoyancy ( F BW) of the microbubble in the carrier liquid is greater than the weight of the microbubble; (ii) the magnetic force ( F M) on the microbubble due to a magnetic field applied to the carrier liquid is greater than the combined weight and force due to buoyancy ( F BW) of the microbubble; (iii) said magnetic force ( F M) on the microbubble is greater than the force due to viscous drag ( F D) on the microbubble due to flow of the carrier liquid; and (iv) the scattering cross section (σ scat) of the microbubble to ultrasound allows the microbubble to be detectable and rupturable on exposure to ultrasound; (b) using the amount of nanoparticles in the shell and the relative size of the shell to the gas core of the microbubble to determine the amount of magnetic nanoparticles and the amount of liquid for the shell needed to prepare a suspension of the microbubbles; and (c) preparing a suspension of the microbubbles using the amount of magnetic nanoparticles and the amount of liquid for the shell needed to prepare the suspension.