Synthetic aperture ultrasound imaging

Abstract

The paper describes the use of synthetic aperture (SA) imaging in medical ultrasound. SA imaging is a radical break with today’s commercial systems, where the image is acquired sequentially one image line at a time. This puts a strict limit on the frame rate and the possibility of acquiring a sufficient amount of data for high precision flow estimation. These constrictions can be lifted by employing SA imaging. Here data is acquired simultaneously from all directions over a number of emissions, and the full image can be reconstructed from this data. The paper demonstrates the many benefits of SA imaging. Due to the complete data set, it is possible to have both dynamic transmit and receive focusing to improve contrast and resolution. It is also possible to improve penetration depth by employing codes during ultrasound transmission. Data sets for vector flow imaging can be acquired using short imaging sequences, whereby both the correct velocity magnitude and angle can be estimated. A number of examples of both phantom and in vivo SA images will be presented measured by the experimental ultrasound scanner RASMUS to demonstrate the many benefits of SA imaging.

Introduction

The paper gives a review of synthetic aperture (SA) techniques for medical ultrasound with a description of the current status and the obstacles towards obtaining real-time SA imaging. Synthetic aperture techniques were originally conceived for radar systems in the 1950s and were initially implemented using digital computers in the late 1970s and more advanced techniques were introduced in the late 1980s [1]. There are many similarities between Radar and ultrasound systems, but there are also very significant differences. A SA Radar system usually employs one transmitter and receiver, and the aperture in synthesized by moving the antenna over the region of interest in an airplane or satellite. In medical ultrasound, the array has a fixed number of elements and is usually stationary. The synthesizing is performed by acquiring data from parts of the array to reduce the amount of electronic channels. For Radar, the object is most often in the far-field of the array, whereas the object always is in the near-field of a medical ultrasound system, which complicates the reconstruction. Since the medical array is stationary, it is possible to repeat measurements rapidly, which is not the case for a SA Radar systems. The position between the different elements is also fixed in ultrasound, whereas the deviations from a straight flight path for airplane often have to be compensated for in Radar systems. A vital difference is also that the dynamic range in a Radar image is significantly less than the 40–80 dB dynamic range in ultrasound images.

All these factors affect the implementation of a medical SA ultrasound system and many details have to be changed compared to SA Radar systems to obtain a successful implementation. This paper will describe some of the choices to be made to make a complete SA system that includes vector flow estimation.

Synthetic aperture imaging has been investigated in ultrasonics since the late 1960 and early 1970 [2], [3]. In the 1970s and 1980s, it was primarily explored for nondestructive testing (NDT) using a more or less direct implementation of the SA principle, known today as monostatic synthetic aperture imaging [4]. With the introduction of transducer arrays in the 1970s, focus was gradually directed towards this application area to pursue real-time implementations [5], [6], [7].

Until the beginning of 1990, the idea of applying the synthetic aperture imaging approach for medical ultrasound imaging had only been considered occasionally [3], [8]. In 1992, O’Donnell and Thomas published a method intended for intravascular imaging based on synthetic aperture imaging utilizing a circular aperture [9]. To overcome the problem with low SNR and impedance matching between the transducer and receiver circuit, the single element transmission was replaced by simultaneous excitation of a multi-element subaperture. Due to the circular surface of the transducer, the subaperture generated a spherical wave with limited angular extend at each emission, thus, permitting synthetic aperture focusing to be applied. This was the first direct attempt to apply synthetic aperture imaging for medical ultrasound imaging. Since then, the application of multi-element subapertures to increase the SNR of synthetic aperture imaging has been investigated using phased array transducers by Karaman and colleagues for small scale systems [10], [11], by Lockwood and colleagues for sparse synthetic aperture systems with primary focus on 3D imaging applications [12], [13], and by Nikolov and colleagues for recursive ultrasound imaging [14]. In all cases, the multi-element subaperture was used to emulate the radiation pattern of the single element transmission by applying de-focusing delays in such a way that a spherical wave with limited angular extend was produced. The definition of synthetic transmit aperture (STA) imaging was introduced by Chiao and colleagues in [15]. This paper also considered the feasibility of applying spatial encoding to enable transmission on several elements simultaneously, while separating the individual transmissions in the receiver using addition and subtraction of the received signals. A third approach, which utilizes orthogonal Golay codes to increase the SNR, while transmitting simultaneously on several elements, was also considered by Chiao and Thomas in [16].

The influence of motion in STA imaging and methods for compensation have been investigated in several publications [17], [18], [19], [20], [21]. Commonly it is reported that axial motion is the dominant factor causing image quality degradation due to the significantly higher spatial frequency in this dimension. The presented motion estimation methods are generally based on time-domain cross-correlation of reference signals to find the shift in position in the axial dimension. Since tissue motion is inherently three dimensional, it is however likely, that to retain the advantages of STA imaging, at least two dimensional (2D) motion correction to compensate successfully for scan plane tissue motion is required.