Ultrasound Contrast Agents in Vascular Disease

Types of ultrasound contrast agents

The first UCA described and widely used for sonographic evaluation of the cardiac chambers was nothing more than manually agitated saline administered into a peripheral vein to increase blood reflectivity on gray-scale and color Doppler images. Nonetheless, this primordial UCA could only be used to evaluate the right cardiac chambers or to detect right-to-left intracardiac shunts because agitated saline will not cross the pulmonary capillary circulation.

Advancements have since been made in the field of UCAs with the introduction of a series of commercially available agents. UCAs are characterized as blood-pool agents with the inherent property of strictly remaining within the vascular space. These agents consist of microbubbles, whose small size (approximately 10 µm) prevents their passage through the vascular endothelium. UCAs are capable of traversing the pulmonary capillary circulation, allowing prolonged enhancement of both arteries and veins. The first generation of UCAs included Levovist (Schering AG, Berlin, Germany), an encapsulated air bubble stabilized with palmitic acid, that was widely used and investigated but is no longer manufactured. Second-generation UCAs offer improved enhancement and longer circulation times, taking advantage of low-solubility gases that replaced air, and a more robust shell. Optison (GE Healthcare, Milwaukee WI) and SonoVue (Bracco Imaging SpA, Milan, Italy) are two examples of second generation UCAs. The latter is the most frequently used UCA in Europe, registered for vascular, cardiac, breast, and liver applications. SonoVue consists of sulfur hexafluoride gas encapsulated within a phospholipid shell that increases in vivo stability. It is approved in the United States for noncardiac applications under the commercial name Lumason (Bracco Imaging SpA, Milan, Italy). When administered intravenously, the Lumason microbubbles remain inside the blood vessels until the microbubbles rupture; the contained gas is then exhaled, and the liver metabolizes the remnants of the phospholipid shell. Because there is no renal excretion of the UCAs or their breakdown products, they can be safely used in patients with impaired renal function. All currently marketed and registered UCAs in the United States and Europe are listed in Table 35.1 .

Ultrasound contrast agent administration

A CEUS study is usually performed after an unenhanced ultrasound study. UCAs are administered via a peripheral vein. Various types of catheters or needles ranging in size from 18 to 21 gauge can be used without affecting the quality of enhancement or concentration of microbubbles. The dose required depends on the application, machine sensitivity, and transducer frequency. With SonoVue or Lumason, the recommended dose for a liver examination is 2.4 mL, but the dose may vary considerably between 1.0 and 2.4 mL depending on the type of study. The UCA is usually administered as a bolus followed by a normal saline flush of 5 to 10 mL. Vascular lumen enhancement commences a few seconds after UCA administration and lasts for several minutes. If required, a second dose of UCA may be safely administered as soon as vessel lumen enhancement decreases, or when additional imaging is needed, for example of the contralateral side during an examination of the carotid arteries. As an alternative to a bolus injection, UCAs can also be administered via slow intravenous infusion, allowing for prolonged vascular enhancement in demanding or time-consuming examinations.

Basic principles of contrast-enhanced ultrasound technique

UCAs can be visualized with conventional gray-scale and color or power Doppler techniques, termed Doppler rescue because the blood's echogenicity is increased on gray-scale imaging and the Doppler signals are significantly increased on color Doppler and duplex imaging. However, this technique results in a number of artifacts, including blooming and saturation that limit image quality. This technique is not capable of evaluating the microvasculature and tissue perfusion in solid organs, but it may be used for the evaluation of the macrovasculature, for example to distinguish between carotid occlusion and preocclusive stenosis.

A significant advancement in CEUS imaging technology was the introduction of harmonic imaging. The UCA microbubbles tend to oscillate in a nonlinear fashion when exposed to the acoustic energy of an ultrasound beam; microbubbles expand more than they contract. This asymmetric fluctuation in microbubble size leads to reflected echoes containing both the fundamental (emitted) frequency as well as harmonic and subharmonic frequencies (higher and lower frequencies). Harmonic ultrasound contrast imaging takes advantage of this phenomenon by configuring the transducer to receive and to visualize only echoes within a specified frequency range (i.e., typically double the frequency of the emitted frequency [first harmonic]). A convenient and fortunate coincidence is that the natural frequency of microbubble oscillation for commercially available contrast agents lies at approximately 3 MHz, the frequency typically used for abdominal examinations. This explains the excellent signal provided by UCAs in spite of the small doses administered. As a result, echoes reflected from microbubbles are visualized with an improved signal-to-noise ratio when compared with conventional gray-scale ultrasound imaging. Nevertheless, image quality is lowered by the simultaneous generation of harmonic echoes from static tissues. This harmonic response of static tissues is known to be proportional to the MI, one reason why low-MI should be used for CEUS examinations.

Pulse-inversion harmonic imaging allows visualization of echoes reflected from UCAs while suppressing those arising from static tissues. To understand how this technique works, it is important to appreciate that static tissues are mainly linear reflectors, meaning that they emit echoes with the same frequency as that transmitted by the transducer. Static tissues may also respond nonlinearly when high-MI waves are used, but this effect is minimized because the current devices use low-MI pulses. UCAs respond to the ultrasound wave in a nonlinear fashion because of asymmetric microbubble oscillation and will emit echoes in the harmonic frequencies. In pulse-inversion imaging, the transducer emits a sequence of two pulses identical in terms of frequency and amplitude, but the second pulse is 180 degrees out of phase compared with the first. The returning echoes from these pulses cancel one another when they encounter soft tissues that act as linear reflectors. However, when these pulses encounter nonlinear reflectors such as UCAs, the reflected harmonic components are combined to produce signals of higher intensity. As a result, this technique has the potential to visualize returning signals from microbubbles while erasing static tissue signals in real-time. This technique can be used to produce both two-dimensional (2D) and three-dimensional (3D) images.

Another reason low-MI real-time sonography is particularly important is that high-MI ultrasound waves rupture microbubbles, resulting in a decrease in microbubble enhancement. This adverse effect is avoided with low-MI imaging, leading to optimized and prolonged visualization of UCAs thereby increasing the clinical utility of CEUS. However, the destruction of microbubbles by high-MI pulses can be used for diagnostic purposes by assessing the perfusion pattern within a targeted organ or soft tissue. A brief and short-lived high-MI pulse can be applied to purposely cause the destruction of microbubbles lying within the imaging field. Reversion to low-MI imaging can then image reperfusion as the microbubbles return into the imaging field. This technique can be useful in the evaluation of tumor neovascularity or the vascular pattern of focal liver lesions. Temporal maximum intensity projection (MIP) imaging is another CEUS technique making use of high-MI pulses to cause microbubble destruction in order to monitor the subsequent replenishment of intact microbubbles within the targeted soft tissue and to create cumulative images, similar to the spatial MIP images generated for CTA and MRA. In temporal MIPs, the ultrasound device acts as an open-shutter camera recording, aggregating the bright echoes originating from intact microbubbles as they enter the imaging field that was left devoid of microbubbles after their destruction by a high-MI pulse. This process is valuable in evaluating the architecture of the microvasculature of liver tumors or for characterizing vascular lesions such as extracranial carotid artery stenosis. These contrast-specific techniques have been extensively studied in clinical trials and are the dominant methods for CEUS examinations in every day clinical practice.

Another CEUS scanning technique used to limit microbubble destruction is intermittent imaging. In this imaging mode, the ultrasound transducer is configured to emit and to receive an ultrasound pulse in a fixed and operator-defined time interval or in a predefined part of the electrocardiogram when imaging the heart. This method of imaging allows time for microbubbles to enter the scanning field and increases reflectivity, but sacrifices real-time information. Depending on the manufacturer, ultrasound devices have the potential for combining real-time low-MI imaging with intermittent high-MI imaging in a dual-image display pattern.

Quantification of contrast enhancement

CEUS examinations are qualitatively evaluated so that a subjective impression can be rendered. An advantage of CEUS when compared with conventional sonography is its potential to provide quantification of the contrast enhancement. The main advantage of this type of analysis is deriving quantitative variables that describe the pharmacokinetic curve of UCA enhancement, allowing reproducibility and increased interobserver and intraobserver agreement. These variables include time-to-peak enhancement, duration of enhancement, and wash-in and wash-out times and are computed from the time-intensity curves sampled within a region-of-interest. Quantification of CEUS examinations can be performed at the time of examination with software incorporated in the ultrasound device or with software for offline analysis. VueBox (Bracco Suisse SA, Geneva, Switzerland) is a widely used software application for quantification of CEUS examinations. Quantification analysis of CEUS examinations can be simplified by analyzing only a selection of frames and not the entire examination video loop. Examples where the quantification of CEUS examination has been valuable include tumor response to biologic therapy, intraplaque neovascularization in carotid plaques, and bowel wall vascularity in patients suffering from Crohn disease.

Artifacts

Like any ultrasound technique, CEUS has inherent artifacts. Use of UCAs generates additional technical artifacts that need to be appreciated by the sonographer. For example, continuous scanning may lead to the degradation of image quality as a result of the destruction of microbubbles. The blooming artifact in the Doppler rescue technique refers to the excessive signal enhancement that causes color Doppler signals to appear outside the vessel lumen and lead to the erroneous perception of blood flow. This artifact is similar to that seen with conventional color Doppler examinations and can be resolved by reducing the color gain setting or setting a higher pulse repetition frequency.

Spectral Doppler artifacts related to the administration of microbubbles include spectral broadening and the presence of high-intensity spikes scattered throughout the waveform caused by microbubble disruption (bubble noise). Spectral waveform Doppler analysis should be performed prior to the administration of microbubbles because UCAs may increase the measured peak velocity.

The pseudoenhancement artifact refers to the presence of echo signals situated in the vascular wall or within atherosclerotic plaques lying distally to the transducer and is created by the nonlinear propagation and distortion of ultrasound pulses through the cloud of microbubbles in the vascular lumen. This artifact mimics the enhancement demonstrated by intraplaque neovascularization. This issue is being addressed by the development of new ultrasound pulse sequences. The term incomplete suppression artifact refers to the visualization of hyperechoic areas lying outside the vascular lumen on images produced by contrast-specific techniques. These signals are caused by the reflection of ultrasound by heavily reflective objects and can be differentiated from true enhancement because such echoes appear constantly static, whereas echoes originating from microbubbles are moving.