21 Novel and Upcoming Ultrasound Techniques

As already mentioned in Chapter 2, section B-Flow Imaging, B-flow is a non-Doppler subtraction imaging technique (patented by General Electric [GE]). It must not be forgotten that color flow imaging (CFI) and B-flow display vascularization, while contrast-enhanced ultrasound (CEUS) images the perfusion. The advantages of B-flow are mentioned in Chapter 2.

By adding contrast material at a low acoustic output (8–10), only moving targets are displayed, while stationary echoes are cancelled out.

This can be advantageous in characterizing focal liver diseases (Fig. 21.1a, b) or in highly advanced metastatic liver disease to demonstrate only the enhanced vasculature.

Other examples of B-flow CEUS can be found in Chapter 13, section Tumor Vasculature and Perfusion, Fig. 13.10c; Chapter 14, Fig. 14.20d, while Fig. 14.20f was performed using B-flow CEUS; and Chapter 15, Fig. 15.23a– c (all are B-flow CEUS images).

21.2 Superb Microvascular Imaging (SMI)

SMI uses a multidimensional filter to separate flow signals from clutter, thus removing only the clutter and preserving the slow flow signals.¹ Thus, it is capable of depicting low flow states with a high spatial resolution. A color box is still needed (Fig. 21.1c,d). Its sensitivity in picking tumor vasculature is reported to be higher when compared to color Doppler imaging (CDI).^{2 – 6}

A comparable Doppler-based technique might be the MicroV^TM imaging (patented by Esaote), using algorithms to eliminate clutter signals. As MicroV^TM is new on the market, there are no validated multicenter studies published so far.

21.3 Plane Wave Imaging, Ultrafast Doppler, Vector Flow Imaging (VFI)

Plane wave imaging is a novel imaging technique with a frame rate of between 400 and 2000 fps, enhancing not only B-mode image quality but can also be used to visualize the character of blood flow using a conventional pulse wave Doppler technique with up to three sample volumes positioned and calculated simultaneously at the same time. Thus, time differences between the onset of peaks and nadirs can be calculated (Fig. 21.2).

Vector flow imaging (VFI) (GE, Mindray^TM) represents another field of use for plane wave imaging. No focal zone must be set as focusing is exclusively performed on the receive site. In examining the peripheral or superficial abdominal vessels, successive tilted plane wave emissions enable reconstruction of the velocity fields. Blood flow velocity is represented as arrows of different lengths and colors; the arrowheads point toward the flow direction. Flow information is stored after a recording time of 1.5 s using a color box with limited size. Only linear probes enable velocity calculations on the basis of transverse oscillation in the return echoes by an automatic alignment of the angle cursor in the direction of blood motion.

Helical blood flow (rotation around axis of flow) was observed in 97% in a volunteer group, which was already demonstrated by Doppler techniques (Fig. 21.3).¹³

In the early arterial phase, the helical flow is the predominant flow pattern in all peripheral and abdominal arteries (e.g., carotid¹⁰ and femoral arteries¹² or infrarenal aorta⁹). During diastole flow vectors may be parallel to the vessel wall. Magnetic resonance imaging (MRI) can prove a helical flow pattern in coronary arteries also. So, it looks like a principal pattern of blood flow due to reflection in bending arteries, veins,¹¹ at bifurcations, or change in the vessel diameter¹⁰ (Fig. 21.4a−c, Fig. 21.5a,b).

It is further believed that helical flow suppresses flow disturbances, thus having a potentially protective influence against building up atherosclerotic plaques^{7 ,​ 8} (Fig. 21.2b, Fig. 21.3b).

21.4 Novel Calculation Techniques for Arterial Stiffness

Velocity profiles in VFI are helpful in demonstrating^{7 – 14} the physiologic (Fig. 21.4b,c), i.e. helical (Fig. 21.5b), flow pattern and in calculating velocities with the highest local resolution in blood flow imaging so far. It further allows calculation of wall shear stress (WSS), especially at bifurcation or bending arteries. WSS, the frictional force of the blood on the vessel wall, plays a crucial role in atherosclerotic plaque development. Low WSS has been associated with plaque growth.^{7 – 14} As there are different ways to measure WSS, more research is needed in this field⁷ (Fig. 21.6).

Even in the coronary system, MRI could prove a helical blood flow character, which is believed to be physiologic and even has an atheroprotective nature. The link between helical flow and WSS provides the clinical relevance to further investigate helical flow patterns.^{7 – 14}

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