Since the 1960s, when ultrasound became established as a medical imaging mode, there have been significant and steady breakthroughs brought about by new technological introductions. In the 1960s, there was an advancement with real-time imaging through mechanical scanning. In the 1970s, multichannel electronic transducer arrays emerged. Doppler modes appeared in the following decade, which enabled flow analysis. The modality as a whole saw an evolution with the miniaturization of ultrasound into portable units, but there have been no significant imaging breakthroughs since Doppler in the 1980s.
Today, with the advent of massive parallel computing capabilities first seen in the video gaming industry, where new graphical processing units allow the simultaneous processing of thousands of channels, medical imaging has taken another giant step forward. The adoption of this quantum leap in processing power has enabled a completely new level of ultrasound called ultrafast imaging.
Ultrafast imaging can acquire image information at frame rates of up to several thousand Hz, an increase by a factor of 100 relative to conventional ultrasound systems. The increase in computing power has resulted in two major new imaging innovations. The first is ShearWave Elastography, which provides real-time true tissue elasticity in a color-coded map, and the second is Ultrafast Doppler, which unites color flow imaging with pulsed wave Doppler. Ultrafast Doppler renders ultra-high frame rate color flow clips that are up to 10 times faster than conventional color Doppler. With such high frame rates, high sensitivity, and fully quantifiable flow information over a large region of interest, Ultrafast Doppler has the potential to have a major impact on a physician’s workflow, examination time, and diagnostic accuracy.
Conventional Doppler: Two Modes Back and Forth
An indispensable tool in the diagnosis and assessment of cardiovascular diseases and cancer, Doppler is well established in ultrasound imaging for flow analysis and quantification. There are currently two different methods of displaying Doppler information: Color Doppler flow velocity and/or power and spectral Doppler analysis (continuous wave [CW] and pulsed wave [PW]). Color Doppler imaging is used to spatially locate a flow region of interest, while PW spectral Doppler performs the quantitative measurements in that flow region.
Color Doppler helps to overcome the limited spatial sampling of PW Doppler by sacrificing the detailed quantitative analysis in order to transmit ultrasound firings and acquire Doppler signals over a relatively large 2D region of interest. The information displayed is the mean flow velocity and/or Doppler power estimated from a small number of firings. Color Doppler images are displayed in real time at frame rates that are usually around 10 to 20 Hz. Color Doppler suffers from a trade-off between frame rate and color box size (field of view).
Pulsed Wave Doppler
PW provides detailed Doppler spectral analysis of flow characteristics by acquiring signals within a limited spatial region at very high sampling rates. Flow quantification is then typically available only at a single location (sample volume).
The challenge is that physicians performing a typical Doppler exam need to successively analyze with PW spectral Doppler the locations identified by color flow, necessitating continuous switching back and forth between the modes. Due to the continuous switching, this examination typically takes 20 minutes or more, depending on the exam and its degree of difficulty. In addition, patient breathing and motion presents another challenge in maintaining sample location and could have an effect on the quality of the Doppler spectrum and the derived diagnosis.
Impact on Daily Workflow
Doppler analysis is one of the most demanding features of an ultrasound system. The complexity causes technical limitations, including low frame rates and small regions of quantification, and has a significant negative workflow impact on the user and facility. The limitations of the conventional Doppler examinations are due to the fact that current ultrasound systems image using sequential insonification with focused beams and successive reconstruction of image lines, limiting the frame rate. Limited frame rate results in insufficient sampling of the underlying flow dynamics. By radical contrast, as the image frame rate is not limited by the number of lines reconstructed, ultrafast imaging is able to take a single transmit and compute a full image—with plane waves able to insonify whole areas of interest at once regardless of the image size. Thus, Ultrafast Doppler can provide frame rates of up to 300 Hz and display excellent flow sensitivity.
Examination Efficiency, Clinical Benefits
In the last several months, the French ultrasound company, SuperSonic Imagine, announced that it had used its UultraFast imaging platform to unite color flow imaging with pulsed wave Doppler. The same technology also acquires fully quantifiable Doppler data throughout the color box, enabling the generation of postprocessed pulsed wave Doppler spectra from multiple locations in the same image.
Retrospective Ultrafast Doppler spectral analysis offers the ability to compare flow spectra and measurements from multiple locations, which have been acquired simultaneously and therefore correspond to the same cardiac cycle, exhibiting perfect temporal synchronicity. In the context of vascular diseases, a tool with these abilities has the potential to increase diagnostic accuracy. With these improvements, complex flow hemodynamics and transient flow events can be visualized in a much more accurate manner, potentially leading to a more reliable hemodynamic assessment of vascular diseases. One, for instance, is stenosis grading through the comparisons of proximal, distal, and stenotic velocities or calculation of resistance indexes in renal arteries.
Analyzing Flow in Seconds Versus Minutes
Ultrafast Doppler saves significant time in two fundamental ways. First, all information necessary for analyzing flow within a given area is acquired in a few seconds versus conventional Doppler methods, which typically take several minutes or more. Second, quantification and analysis can potentially be performed offline, freeing the ultrasound system for the next patient. In addition, several other automatic tools are available to aid physicians in analysis such as immediate peak velocity display. This displays the peak velocities throughout the cardiac cycle, which allows selection of the best area for Doppler analysis. With the combination of reduced acquisition time and efficient analysis, Ultrafast Doppler has the potential to reduce exam duration and increase patient throughput.
Gordana Ivanac, MD, PhD, is a radiologist in the department of radiology at the University Hospital Dubrava in Zagreb, Croatia. For more information, contact .