Stepped frequency ultrasound computed tomography with waveform inversion

Routine US (ultrasound) scans for breast imaging run on a conventional console suffer from machine and operator dependence and are subject to personal interpretation. Recently, the framework of USCT (ultrasound computed tomography) has emerged as a safe, powerful and operator independent alternative to diagnostic US scans and x-rays mammography. The most known systems employ one circular array or a combination of transmitters and receivers by exploiting reflection, diffraction and transmission data. These systems are based on a pulsed transmission. Following propagation in tissue, the signals are usually recorded with a direct RF sampling scheme and stored as digital time-series. Image reconstruction is performed in the frequency domain in the 400 kHz-1 MHz bandwidth over a limited number of discrete frequencies. In this paper, we propose a new architecture based on the stepped frequency continuous waveform (SFCW) principle. In this scheme, the transmission is a continuous one and the received waveforms undergo a homo-dyne stage. By sequentially transmitting single tones at different frequencies, data can be collected directly in the frequency domain at specific frequencies, with programmable frequency steps and with any desired SNR. We describe in detail the transmitter and the receiver paths and compare with a conventional pulsed USCT architecture. Finally, we highlight the benefits of a SFCW-USCT device and comment on SNR, absorbed power, data fidelity and data storage.