Vascular Ultrasound (eBook)

1 Principles of Physics and Technology in Diagnostic Ultrasound

Bernhard J. Arnolds, Bernhard Gaßmann, Peter-Michael Klews

1.1 Introduction

Human beings have natural receptors for light and sound. The eyes can process electromagnetic waves over just a limited range of frequencies. The ears have similar limitations when it comes to sound. To perceive frequencies outside these naturally visible or audible ranges, special technology is needed. In a sense, then, the pictures generated by this technology are “artificial” images.

The images produced by X-rays or ultrasound depend on the methods that are used for data acquisition and image processing. An image is considered “good” if it has high spatial resolution and, in the case of gray scale, has a subjectively pleasing distribution of gray levels. Another requirement is high contrast resolution, or the ability to perceive slight differences in adjacent shades of gray.

Blood flow imaging has been a topic of growing interest in diagnostic ultrasound. In 1842, C. Doppler described his eponymous effect, which states that the wavelength of light (or sound) measured by an observer depends on the relative motion between the source and receiver. This effect has been utilized in medicine since the late 1950s. Bidirectional Doppler was introduced in 1959, followed by pulsed Doppler in 1967.

The technique of color encoding of blood flow in the B-mode (gray scale) image was introduced in 1982. This technology is referred to as color duplex sonography (CDS) or color flow imaging (CFI).

The use of ultrasound contrast agents has become an established part of routine ultrasound examinations. The injection of microbubbles increases acoustic backscatter from the blood, and special signal-processing methods are used to suppress the tissue signals, resulting in images with exquisite vascular detail. One important application of this technology is in the diagnosis of intra-abdominal tumors.

Elastography is also being used in vascular ultrasound to investigate the elasticity of artery walls.

1.2 Overview of Ultrasound Techniques

Ultrasonography is used both for determining (organ) morphology and evaluating function. An ultrasound system always consists of a transducer with an application-specific shape and frequency combined with a control unit, which is the ultrasound machine itself.

The Doppler effect is useful for determining the velocity of moving objects. In medicine, the Doppler effect is most commonly used for the investigation of blood flow. Tissue Doppler is a technology that analyzes the motion of tissue structures such as the myocardial walls. Flow characteristics are displayed as either a Doppler spectrum or velocity spectrum plotted over time, or points in the B-mode image are color-encoded according to the motion measurable at those sites (Doppler shift).

All ultrasound imaging techniques described in this chapter, with the exception of continuous-wave (CW) Doppler, are based on the analysis of multiple pulse-echo cycles. The individual pulses are successively emitted from the transducer along selected ultrasound scan lines, while the echoes are continuously received and analyzed for their amplitude, phase, and frequency. Each of the continuously acquired and analyzed echoes represents a sample.

Except for CW Doppler, the ultrasound techniques described here are transit-time techniques, meaning that the depth from which echoes are received is calculated from the total pulse-echo travel time, based on the assumption of a constant sound velocity. To avoid ambiguity, the next pulse is not emitted until the transducer has received an echo from the greatest possible (or preassigned) depth. The only exception to this rule is high pulse-repetition-frequency (HPRF) Doppler, in which additional pulses are transmitted before the echo from the first transmitted pulse has been received.

All ultrasound techniques besides M-mode are sectional imaging techniques. The analysis of many consecutive scan lines, including a technique-dependent interpolation of lines between the received scan lines, results in the creation of a two-dimensional sectional image. Generally speaking, a scan line is defined as a discrete line in the ultrasound image along which the ultrasound pulse travels. It may be oriented in a perpendicular or radial direction relative to the transducer. The scan lines are idealized lines. Their thickness depends on the ultrasound wavelength and they do not take into account the true dimensions of the ultrasound beam. In some cases, as in CDS, multiple pulse-echo cycles are successively transmitted along the same scan line in order to collect the necessary echo information. Many individual scan lines are composed into a side-by-side array to produce a two-dimensional ultrasound image.

The use of multiple pulse-echo cycles per scan line does not increase the number of image increments. Only a write-zoom feature (magnified view) will increase the amount (density) of increments for a given area of interest. Thus, an ultrasound image is formed within a time period that is defined by the image depth, the number of pulse-echo cycles per line, and the number of lines per image. This is different from an ordinary photograph in which all image points are formed at the same time.

The far edges of an ultrasound image may be separated from each other by a time lag of 0.2 s or more. This may become significant, especially in the color-encoded imaging of blood flow. For example, a systolic pulse may be displayed on the left side of the image while the right side is still in diastole. This “windshield-wiper effect” depends strongly on the time required for signal acquisition and processing. The visible parameter for evaluating these temporal characteristics is the image repetition frequency called the frame rate.

The ultrasound scan lines should not be confused with the image lines on the monitor display. The number and density of image lines depend on the video standard and the area of the (ultrasound) monitor image. The number of image increments is considerably smaller than the number of image points, or video pixels; otherwise, image generation would take too long and the frame rate would be much too slow.

1.2.1 A-Mode

A-mode ultrasound (for “amplitude mode”) is rarely used nowadays but forms the basis of the B-mode technique. An A-mode image is a graphic trace of the echo amplitudes of individual scan lines (y-axis) plotted over time (x-axis). The measured transit time is converted to distance from the transducer. The deflection parallel to the y-axis on the monitor screen is proportional to the amplitude of the received echo.

1.2.2 B-Mode

B-mode ultrasound (for “brightness mode”) is the mainstay of ultrasonography and is by far the most widely used ultrasound imaging technique. The B-mode image is gray scale, meaning that it is composed entirely of different gray levels. Many successive scan lines are assembled and displayed side-by-side on the monitor to form a two-dimensional picture. The gray levels in the image are proportional to the amplitudes of the returning echoes. The greater the amplitude, the greater the brightness of the corresponding point in the image (see Fig. 2.2).

1.2.3 M-Mode

Another gray scale technique is the M-mode (for “motion mode”) or TM (“time-motion”) mode (Fig. 1.1). In this technique, different points are insonated along a single scan line. Successive acquisitions of the same scan line are displayed side by side on the monitor, although they originate from the same location in the body. The purpose of M-mode imaging is to track and display dynamic processes inside the body. It is used mainly in cardiology for evaluating the motion of the cardiac valves. M-mode is the basis for color Doppler M-mode techniques. All M-mode techniques supply functional information.

1.2.4 Color Duplex Sonography (CDS)

CDS techniques (except for tissue Doppler) work by color encoding of sites in the image where blood flow is detected. Areas devoid of blood flow are shown in gray scale. Thus, CDS or color flow mapping (CFM) superimposes areas of color-encoded motion over the B-mode image (Fig. 1.2). The reference point for defining the direction of blood flow is the transducer (or more precisely, the direction of the scan line). Only components moving toward or away from the transducer are measured. The standard practice in conventional flow-velocity-based CDS images is to encode the different flow directions in shades of red and blue. The operator can choose which flow direction is encoded in blue and which in red. The blood flow velocity indicated in all conventional CDS techniques is the intensity-weighted mean blood flow velocity or Doppler shift (phase shift of the Doppler signals). Lighter shades of color indicate higher flow velocities. The color green may be...