TECH EXPLANATION

When an ultrasound pulse impacts a soft tissue scatterer, a single echo is created. When an ultrasound pulse impacts a needle tip, the needle tip absorbs energy from the pulse, and subsequently releases the energy over time by radiating a continuous wave.

Left drawing: an echo from a scatterer propagates toward a transducer. Right drawing: wavefronts in a continuous wave radiated by a needle tip propagate toward a transducer.

In B-mode imaging, pulse transmission and the receive period start at the same time. As a result, echoes from soft tissue obscure the continuous wave radiated by the needle tip. In Time Delay Imaging, there is a time delay between the start of pulse transmission and the start of the receive period. During the time delay, soft tissue echoes dissipate, while needle tip continuous wave radiation persists. As a result, the continuous wave is visualized as a ringdown artifact.

When a wavefront in the continuous wave is received by the transducer, it is recorded as a curve-shaped signature in the radiofrequency data. The shape of the signature is governed by the radius of the wavefront, which is equal to the depth of the needle tip. Therefore, each wavefront in the continuous wave creates a signature with the same shape, such that the continuous wave signal consists of the signature repeated along the time axis of the radiofrequency data. Because the time delay is longer than the round trip time between the transducer and the needle tip, the needle tip signal is present from the start of the radiofrequency data.

Receive beamforming uses a sampling curve to select radiofrequency data to reconstruct a pixel. Sampling curve shape becomes flatter as time value increases to account for increased point source depth. For a needle tip signal, the changing sampling curve shape matches the constant signature shape at the time value equivalent to the depth of the needle tip, resulting in visualization of a point, which is the narrowest part of the ringdown artifact. At smaller and larger time values, the increasing mismatch between sampling curve shape and signature shape results in visualization of an increasing width curve, thus creating a ringdown artifact which increase in width as it advances toward the top and bottom of the image.

Left drawing: radiofrequency data set. Groups of letters "X" represent signatures in a needle tip signal. Dashed lines represent sampling curves. Right drawing: image with ringdown artifact reconstructed from the needle tip signal in the left drawing.

For a needle tip in soft tissue, a Time Delay Image visualizes a ringdown artifact created by the needle tip, and does not visualize soft tissue. A separately acquired B-mode image visualizes soft tissue, and does not visualize the ringdown artifact. The Time Delay Image and the B-mode image are combined, creating a compound image, which visualizes both the ringdown artifact and the soft tissue, thus indicating the position of the needle tip relative to surrounding soft tissue structures.

Images of a needle tip in soft tissue. Left drawing: Time Delay Image. Center drawing: B-mode image. Right drawing: compound image combining the Time Delay Image and the B-mode image.