Blood Flow Imaging with Ultrafast Doppler

Summary

This protocol shows how to apply ultrafast ultrasound Doppler imaging to quantify blood flows. After a 1 s long acquisition, the experimenter has access to a movie of the full field of view with axial velocity values for each pixel every ≈0.3 ms (depending on the ultrasound time of flight).

Abstract

The pulsed-Doppler effect is the main technique used in clinical echography to assess blood flow. Applied with conventional focused ultrasound Doppler modes, it has several limits. Firstly, a finely tuned signal filtering operation is needed to distinguish blood flows from surrounding moving tissues. Secondly, the operator must choose between localizing the blood flows or quantifying them. In the last two decades, ultrasound imaging has undergone a paradigm shift with the emergence of ultrafast ultrasound using unfocused waves. In addition to a hundredfold increase in framerate (up to 10000 Hz), this new technique also breaks the conventional quantification/localization trade-off, offering a complete blood flow mapping of the field of view and a simultaneous access to fine velocities measurements at the single-pixel level (down to 50 µm). This data continuity in both spatial and temporal dimensions strongly improves the tissue/blood filtering process, which results in an increase sensitivity to small blood flow velocities (down to 1 mm/s). In this method paper, we aim to introduce the concept of ultrafast Doppler as well as its main parameters. Firstly, we summarize the physical principles of unfocused wave imaging. Then, we present the Doppler signal processing main steps. Particularly, we explain the practical implementation of the critical tissue/blood flow separation algorithms and on the extraction of velocities from these filtered data. This theoretical description is supplemented by in vitro experiences. A tissue phantom embedding a canal with flowing blood-mimicking fluid is imaged with a research programmable ultrasound system. A blood flow image is obtained and the flow characteristics are displayed for several pixels in the canal. Finally, a review of in vivo applications is proposed, showing examples in several organs such as carotids, kidney, thyroid, brain and heart.

Introduction

Ultrasound imaging is one of the most commonly used imaging techniques in clinical practice and research activities. The combination of ultrasound wave emission in the biological tissues followed by the recording of the backscattered echoes allows the reconstruction of anatomical images, the so-called “B-Mode”. This method is perfectly adapted for soft tissue imaging, such as biological tissues, which typically permit the penetration of ultrasound over several centimeters, with a propagation speed of ≈1540 m/s. Depending on the center frequency of the ultrasound probe, images with a resolution from 30 µm to 1 mm are obtained. Furthermore, it is ....

Protocol

1. Doppler phantom preparation setup (Figure 2A)

1. Connect the peristaltic pump, the blood mimicking fluid reservoir, the pulse dampener and the Doppler flow phantom with the plastic tubes.
2. Choose the canal with a 4 mm diameter.
3. Program the pump to eject 720 mL/min of fluid for 0.3 s and then to eject 50 mL/min for 0.7 s to respectively mimic the systole and diastole cardiac phases
4. Run the pump and gently shake the pipes to expel potential air bubbles.
   NOTE: The operator can choose a different canal diameter and different pump rate but will have to ensure that the ultrasound sequence is fast ....

Results

The quality of the acquisition and the post-processing is firstly assessed by visual inspection. The shape of the canal must be clearly visible in the power Doppler image, and the tissue area must appear dark. If the power Doppler signal is not restricted to the canal, it can mean that either the clutter filter step went wrong (SVD threshold is too low), or the probe experienced a strong movement during the acquisition.

After visual inspection, the study of the spectrogram inside the canal can.......

Discussion

Several variations are possible around the main frame of this protocol.

Hardware concerns
If the user supplies its custom host computer, the motherboard and the computer’s case must have an available PCI express slot. The CPU must also have enough PCIe lanes to handle all the devices.

Probe selection
The ultrasound probe (also named transducer) is chosen according to the spatial resolution needed and to the geometr.......

Materials

Name	Company	Catalog Number	Comments
Blood-mimicking fluid	CIRS Inc, Norfolk, Virginia, USA	069DTF
Doppler flow phantom	CIRS Inc, Norfolk, Virginia, USA	ATS523A
Matlab	MathWorks, Natick, Massachusetts, United States
Peristaltic pump / Doppler flow pump	CIRS Inc, Norfolk, Virginia, USA	769	Include tubings and pulse dampener
Transducer adpter	Verasonics, Kirkland, Washington, USA	UTA 408-GE
Ultrafast ultrasound research scanner	Verasonics, Kirkland, Washington, USA	Vantage 256
Ultrasound probe/transducer	GE Healthcare	GE 9L-D