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Thứ Bảy, 3 tháng 9, 2011

UltraFast Doppler: kết hợp Pulsed Wave Doppler và Color Flow Imaging

SuperSonic Imagine giới thiệu UltraFast Doppler: kết hợp Pulsed Wave Doppler và Color Flow Imaging





SuperSonic Imagine đã thông báo một kỹ thuật mới, UltraFast Doppler. UltraFast Doppler kết hợp tạo hình dòng chảy màu (color flow imaging) với Doppler xung (pulsed wave doppler), bằng cách hợp nhất tạo hình dòng chảy (flow imaging) và định lượng (quantification). Điều này tạo ra các clips ultra-high frame rate color flow (dòng chảy màu có tốc độ khung hình siêu nhanh), nhanh hơn Doppler màu quy ước gấp 10 lần và điều này làm giảm artifacts.


Qua color box, dữ liệu Doppler toàn bộ có thể định lượng, bằng cách tạo ra phổ xung Doppler post-processed tại mỗi điểm của image loop lưu trữ.

UltraFast Imaging và UltraFast Doppler

Today, UltraFast imaging is being applied in new ways. As will be described in this paper, UltraFast imaging can be used to both visualize and quantify flow, hence combining the utility of color flow and PW in a single feature. In the context of flow analysis UltraFast Doppler provides high frame rates, high sensitivity and fully-quantifiable flow information over a large region of interest. The reinvention of Doppler ultrasound through UltraFast imaging has a potential major impact on physician‘s workflow, examination time and diagnostic accuracy.


UltraFast Imaging

With or without multiline capabilities, current ultrasound systems are built on a serialized architecture and images are reconstructed sequentially from several equivalent transmits.
UltraFast imaging represents a radical departure from this approach. An ultrafast imaging system is able to compute in parallel as many lines as requested and is therefore capable of computing a full image from a single transmit, irrespective of the image size and other characteristics. In such a system, the image frame rate is no longer limited by the number of lines reconstructed but by the time of flight of a single pulse to propagate through the medium and return to the transducer. There are many ways to leverage an ultrafast imaging architecture (Lu, 1998; Jensen 2005). SuperSonic Imagine’s approach is based on the use of plane wave insonification (Montaldo et al, 2009). A plane wave is generated by applying flat delays on the transmit elements of the ultrasound probe as illustrated on Figure 2. The generated wave insonifies at once the whole area of interest.

The backscattered echoes are then recorded and processed by the ultrafast scanner to compute an image of the insonified area. Plane wave imaging allows the computation of one full ultrasound image per transmit.

UltraFast imaging allows significant increase of the maximum frame rate achievable by an ultrasound system.

Table 1 lists typical frame rates for different ultrasound clinical applications using conventional and ultrafast architectures.

In UltraFast imaging, the beamforming computations must be performed on a fully parallelized architecture, typically based on a software platform.

Two technological barriers need to be overcome to build a fully software-based platform:
• The data transfer rate from the acquisition module to the processing unit. As raw (non beamformed) Radio Frequency (RF) signals are directly transferred to the PC, the data rate required to perform real time imaging is immense (several GigaBytes/s).
• The processing unit needs to be powerful enough to ensure real time beamforming. As an example, conventional gray scale imaging requires 1 to 2 Gigaflops (multiplication+addition) per second.

Figure 3 represents the architecture of an ultrafast system compared to a conventional one.

Aixplorer is the first commercially available system to break these technological barriers and allows ultrafast imaging of tissue with frame rates up to 20,000 Hz. It relies on the use of powerful graphical processing units (GPUs) from the video game industry combined with fast digital links (PCI express technology) capable of transferring massive volumes of data to these units.

The next section demonstrates how UltraFast imaging can be used to improve Doppler flow analysis by addressing important performance and workflow constraints of the currently available color and PW Doppler modes.

UltraFast Doppler Imaging

In UltraFast Doppler, several tilted plane waves are transmitted into the medium and the backscattered echoes are coherently summed to reconstruct ultrasound images (Bercoff et al, 2011). (Figure 5)

The maximum number of angles that can be used to compute an image is limited by the acquisition Pulse Repetition Frequency (PRFDoppler) needed to measure the desired Doppler velocity scale (usually the velocity scale is set by the user).


where PRFmax is the maximum PRF that can be achieved for a given imaging depth.

Using UltraFast imaging, Doppler information is continuously and simultaneously acquired across the full image. Therefore, unlike conventional color Doppler acquisitions, all pixels are sampled at a high Doppler PRF in an uninterrupted and concurrent manner, offering the unique ability to perform full flow quantification at every pixel within the UltraFast color box.

In a typical implementation of UltraFast Doppler, a single-shot acquisition can be launched from the conventional color Doppler imaging mode. A full clip of UltraFast Doppler data is acquired (typically 2 to 4 s) and the system is frozen.

The user can then review the UltraFast color flow imaging clip, select the frame(s) offering best visualization of the flow properties of interest, and perform full spectral analysis at every pixel of the color box in a retrospective manner.

Retrospective UltraFast spectral analysis offers for the first time the ability to compare flow spectra and measurements from multiple locations, which have been acquired simultaneously and therefore correspond to the same cardiac cycle and exhibit perfect temporal synchronicity.

UltraFast Doppler offers:

• A significant improvement of color flow imaging performance in terms of temporal resolution and sensitivity.

• A new Doppler paradigm by merging the color Doppler and PW Doppler modes in a single fully-quantifiable acquisition, which can simplify workflow, reduce exam time, and can enable advanced measurement and visualization capabilities.









Improving Color Doppler Imaging


Conventional color Doppler imaging offers limited frame rates (typically 20 Hz) and suffers from severe trade-offs between color box size and temporal resolution. Figure 7 shows two conventional color Doppler frames from a femoral artery, plus the corresponding frames obtained by means of UltraFast Doppler.

In this example, UltraFast Doppler exhibits excellent flow sensitivity, and provides frame rates of more than 80 Hz.
On the other hand, conventional color Doppler imaging is limited to a frame rate of 19 Hz which results in insufficient sampling of the underlying flow dynamics, as illustrated by the bottom frames of Figure 7 where the reverse flow is perfectly documented in the UltraFast Doppler frame, but is completely missed by the conventional color Doppler acquisition. This example demonstrates how limited frame rates can induce a loss of information, something that can represent a major issue in the context of flow pathology.

UltraFast Doppler enables frame rate increases by a factor of 5 to 10 compared to conventional color Doppler, typically from 60 up to 200Hz, without sacrificing field of view or spatial resolution. Thanks to these improvements, complex flow dynamics and transient flow event can be visualized in a much more accurate manner, potentially leading to a more reliable hemodynamic assessment of cardiovascular diseases such as stenosis.

Compared to conventional color Doppler imaging, UltraFast Doppler acquisitions bring several advantages to the end user:

• Clips of color data can be generated with high sensitivity, and frame rates up to a factor 10 relative to conventional systems.

• The increased quality is maintained regardless of the color box size. Conventional color Doppler suffers from trade-offs between frame rate and color box size. Using plane waves, the whole area of interest can be filled with color Doppler information without any drop in frame rate.

• The flow information is consistent and synchronous throughout the imaged area, since the Doppler signals corresponding to all pixels are acquired at the same time. In contrast, conventional color Doppler lines are
sequentially acquired, so that the Doppler signals on the sides of the color box exhibit time lags that can reach several hundreds of milliseconds.

Flow Quantification Anywhere

In addition to increased flow imaging performance, UltraFast Doppler enables full quantification of flow data everywhere in the image. The user can position a sample volume anywhere within the region of interest and the system responds by instantaneously computing and displaying the PW spectrogram from the selected location.
Up to three spectrograms can be calculated and displayed simultaneously on the image as illustrated in Figure 8.

Measurements can be performed independently on all spectrograms and compared to each other with a high degree of reliability, since all spectra are computed from data acquired at the same time, on the same cardiac cycles.
It is important to note that the results of UltraFast spectral analysis are numerically equivalent to those obtained by a conventional PW Doppler exam performed under the same conditions. Figure 9 shows a comparison between peak systolic velocities (PSV) and end diastolic velocity (EDV) measurements in a flow phantom mimicking arterial flow for both techniques (Conventional PW and UltraFast Doppler). This comparison demonstrates that the PSV and EDV measurements show excellent correlation over a wide range of velocities.

As a final example, Figure 10 displays UltraFast Doppler results from a femoral artery and vein (same case as Fig 7), depicting the arterial and venous flow corresponding to a given color frame (top), and documenting the full temporal evolution of flow within each vessel in the two UltraFast spectrograms (bottom).





CONCLUSION

UltraFast imaging is a breakthrough technology that can offer significant performance improvements and innovative capabilities to the field of medical ultrasound. UltraFast Doppler is the combination of UltraFast imaging and Doppler techniques, which retain the advantages of color Doppler and PW Doppler without the respective disadvantages of each individual mode. UltraFast Doppler offers high-sensitivity and high-frame rate flow imaging allowing high-quality visualization of complex and transient flow events, plus the ability to perform accurate quantification and comparison of flow velocities through the whole image area based on full spectral analysis. Due to these characteristics, UltraFast Doppler has the potential to significantly simplify the workflow of Doppler exams and reduce the time needed to complete them. New UltraFast Doppler features and capabilities are currently under development, which will undoubtedly enhance the clinical utility of Doppler imaging even further.




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