Fatemi, Mostafa

Medical ultrasound scanners use high-energy pulses to probe the human body. The radiation force resulting from the impact of such pulses on an object can vibrate the object, producing a localized high-intensity sound in the audible range. Here, a theoretical model for the audio sound generated by ultrasound scanners is presented. This model describes the temporal and spectral characteristics of the sound. It has been shown that the sound has rich frequency components at the pulse repetition frequency and its harmonics. Experiments have been conducted in a water tank to measure the sound generated by a clinical ultrasound scanner in various operational modes. Results are in general agreement with the theory. It is shown that a typical ultrasound scanner with a typical spatial-peak pulse-average intensity value at 2 MHz may generate a localized sound-pressure level close to 100 dB relative to 20 μPa in the audible (<20 kHz) range under laboratory conditions. These findings suggest that fetuses may become exposed to a high-intensity audio sound during maternal ultrasound examinations. Therefore, contrary to common beliefs, ultrasound may not be considered a passive tool in fetal imaging.

The propagation speed of shear waves is related to frequency and the complex stiffness (shear elasticity and viscosity) of the medium. A method is presented to solve for shear elasticity and viscosity of a homogeneous medium by measuring shear wave speed dispersion. Harmonic radiation force, introduced by modulating the energy density of incident ultrasound, is used to generate cylindrical shear waves of various frequencies in a homogeneous medium. The speed of shear waves is measured from phase shift detected over the distance propagated. Measurements of shear wave speed at multiple frequencies are fit with the theoretical model to solve for the complex stiffness of the medium. Experiments in gelatin phantoms show promising results validated by an independent method. Practical considerations and challenges in possible medical applications are discussed.

Coupled vibration of arterial tubes is analyzed with the wave propagation approach and first-order shear deformation theory. Both the interior and exterior fluids are considered as compressible so that acoustic waves can be generated and propagated in the fluids. Results obtained using the theory have been evaluated against those available in the literature and the agreement has been found to be good. The theory can be used for future research on the vibration and acoustics of arterial walls. Vibration experiments were carried out on a silicone rubber tube in a water tank with a novel ultrasound stimulated optical vibrometry system. This system uses the radiation force of ultrasound to vibrate the tube at low frequency and records the resulting response by a laser vibrometer. Both the excitation and measurement are remote and noncontact. The silicone rubber tube was chosen because it has mechanical properties close to those of arteries. The fundamental frequency is well excited by the radiation force and measured with the laser. The measured fundamental frequency is in good agreement with the present theory.

A quantitative model is presented for a sphere vibrated by two ultrasound beams of frequency ω1 and ω2. Due to the interference of two sound beams, the radiation force has a dynamic component of frequency ω2−ω1. The radiation impedance and mechanical impedance of the sphere are then used to compute the vibration speed of the sphere. Vibration speed versus vibration frequency is measured by laser vibrometer on several spheres, both in water and in gel phantom. These experimental results are used to verify the model. This method can be used to estimate the material properties of the medium (e.g., shear modulus) surrounding the sphere.

An ultrasound method based on radiation force is presented for imaging the acoustic response of a material to mechanical excitation. Acoustic energy was emitted from solids and tissues in response to an oscillatory radiation force produced by interfering focused beams of ultrasound. Frequency spectra of ultrasound-stimulated acoustic emission exhibited object resonances. Raster-scanning the radiation force over the object and recording the amplitude and phase of the emitted sound resulted in data from which images related to the elastic compositions of the acoustically emitting objects could be computed. Acoustic emission signals distinguished tuning-fork resonances, submillimeter glass spheres, and calcification in excised arteries and detected object motions on the order of nanometers.

We describe theoretical principles of an imaging modality that uses the acoustic response of an object to a highly localized dynamic radiation force of an ultrasound field. In this method, named ultrasound-stimulated vibro-acoustography (USVA), ultrasound is used to exert a low-frequency (in kHz range) force on the object. In response, a portion of the object vibrates sinusoidally in a pattern determined by its viscoelastic properties. The acoustic emission field resulting from object vibration is detected and used to form an image that represents both the ultrasonic and low-frequency (kHz range) mechanical characteristics of the object. We report the relation between the emitted acoustic field and the incident ultrasonic pressure field in terms of object parameters. Also, we present the point-spread function of the imaging system. The experimental images in this report have a resolution of about 700 μm, high contrast, and high signal-to-noise ratio. USVA is sensitive enough to detect object motions on the order of nanometers. Possible applications include medical imaging and material evaluation.