The extracranial duplex examination allows assessment of the VA mid-cervical portion (V2), the proximal VA and its origin (V0/V1), and the most distal portion accessible on the neck (V3). A linear transducer with 4–7 MHz frequency range is commonly used. To optimize the gray scale image on B-mode, set the dynamic range to 40–50 dB and the time-gain compensation (TGC) as appropriate to the depth of the vertebral arteries. Alternatively, automatic image optimization functions, provided by most of new-generation ultrasound systems, yield high-quality imaging results. The B-mode image helps with the assessment of the diameter of the VA (which is usually about 3–4 mm). It may also detect atherosclerotic plaque (particularly at the origin of the VA), intraluminal clots, or even a double lumen in case of an arterial dissection. However, these findings are less prevalent than in patients with carotid artery disease.

The intracranial color-coded duplex examination allows assessment of the patency and flow velocities of the cerebral portion of the VA (V4) in the axial plane through the subforaminal acoustic window. A phased-array transducer with 1.5–2.5 MHz carrier frequency is required. The maximum insonation depth is usually set at 10–12 cm.

Patients should be placed in a supine position with the neck extended. To find the VA, firstly visualize the common carotid artery in a longitudinal projection on B-mode with transducer position anterior to the sternocleidomastoid muscle. Slide the probe posteriorly to image one or more levels of the V2 segment of the VA as it courses through the transverse processes of the vertebrae (“acoustic shadows”) (Fig. 14.1). Confirm that the direction of flow in the VA is the same as the common carotid artery. Then trace the V2 segment proximally and “heel-toe” the probe above the clavicle to image the pre-vertebrae portion and the origin of the VA as it arises from the SA. It is important to examine this part as V0/V1 is the most common site of atherosclerotic disease [22]. Finally follow the V2 segment of the VA more distal trying to visualize the V3 segment as it surrounds the axis. However, direct assessment of the V3 segment may be technically difficult and the diagnosis of vertebral obstruction at this level is based on color-flow and Doppler findings proximal or distal to this segment. If color or power flow images are difficult to obtain, spectral interrogation of areas with typical B-mode appearance of surrounding bony structures should still be performed.

As opposed to the extracranial examination, the intracranial portion of the VA is examined in the axial plane. Patients should be placed in a sitting position with the head bent forward as much as possible. In patients not able to sit (e.g., due to an acute neurological deficit), the examination can be performed in the side posture instead, but the head still bent forward. Place the transducer approximately 2–3 cm below the occipital protuberance in the midline of the neck, and slide the transducer’s sound beam slightly toward the bridge of the nose. Once the foramen magnum is visualized as hypoechoic rotund structure close to the transducer on B-mode, color Doppler imaging (for optimized settings see below) allows visualization of the vertebrobasilar vasculature (ideally in the form of a Y – “Y-sign”) (Fig. 14.1). Commonly, the VA cannot be visualized along its entire course on a single plane, thus requiring stepwise adjustment of the transducer’s angulation (and even positioning by moving laterally) to trace the proximal, middle, and distal segments of the intracranial VA.

Choose the appropriate color pulse repetition frequency (PRF) by setting the color velocity scale for the expected velocities in the vessel. For normal adult arteries, the peak systolic velocity range is usually between 40 and 50 cm/s. The zero baseline of the color bar is set at approximately 2/3 of the range with the majority of frequencies allowed in the red direction (for flow toward the brain). Adjust the scale further to avoid systolic aliasing (low PRF) or diastolic flow gaps (high PRF or filtering) in normal vessels.

Initially the color or power mode gain should be adjusted to an optimized level, with color displayed throughout the lumen of the vessel and no “bleeding” of color into the surrounding tissue. This is the level at which all color images should be assessed. In situations where there is very low flow, or questionable occlusion, an “over gained” level may be advantageous to show any flow that might be present, e.g., total occlusion versus a near-occlusion or critical stenosis.

The size of the color box should cover the entire vessel diameter and at least 1–2 cm of its length. Large or wide color boxes will slow down frame rates and resolution of the imaging system. Align the box, i.e., select an appropriate color-flow angle correction, according to the vessel geometry and course.

With regard to the intracranial portion of the VA, the color box should be enlarged to display the entire intracranial vertebrobasilar vasculature (e.g., size of the color box half of the sector image). However, in most cases, the distal portion of the basilar artery cannot be visualized properly, and a smaller color box yields better scanning results for assessment of the VA and the vertebrobasilar junction. The color PRF should be optimized by adjusting the color velocity scale to ±10–20 cm/s. Increase the color gain until “bleeding” of color occurs into the surrounding tissue.

Display the longitudinal image of the extracranial VA. Use color-flow image as a guide for Doppler examination. Begin the examination using a Doppler sample volume size of 1.5 mm positioned in the middle of the vessel. The insonation angle should be parallel to the color blood flow and lower or equal than 60° in every segment. Adjust the Doppler spectral power and gain to optimize the quality of the signal return. Slowly sweep the sample volume throughout the different inter-vertebrae visualized segments. Identify regions of flow disturbance or absence. Additionally, include Doppler spectral waveforms proximal, within, and distal to all areas where flow abnormalities were observed. Locate the origin, proximal, and distal segment of the extracranial and, when deemed necessary, the intracranial portion of the VA. Record flow patterns paying careful attention to flow direction. Follow accessible cervical segments of the VA. Change angulations of the color box and Doppler sample along with the course of the artery.

The Doppler spectral evaluation of flow dynamics of the intracranial portion of the VA is described in Chap. 15: “Transcranial Doppler Sonography,” in this book.

In the following we describe established clinical indications for color duplex scanning of the extracranial VA in routine clinical practice and specific diagnostic criteria derived from previous studies and our own experience (Table 14.1).