Contrast enhanced ultrasound imaging

FROM BENCH TO IMAGING

Contrast enhanced ultrasound imaging

Steven B. Feinstein, MD, FACC,a Blai Coll, MD, PhD,b Daniel Staub, MD,c

Dan Adam, PhD,d Arend F. L. Schinkel, MD, PhD,e Folkert J. ten Cate, MD,e

and Kai Thomenius, PhDf

HISTORIC DEVELOPMENT

Introduction

The origins of contrast enhanced ultrasound imag-

ing (CEUS) date to the earliest observations of Claude

Joyner and publications of Gramiak and Shah in 1968.1

Interest in the development of ultrasound contrast agents

and associated clinical applications continues today,

nearly 40 years after the first reports.

The use of blood pool agents for enhancement of

cardiovascular structures is ubiquitous, current clinical

diagnostic imaging modalities utilize contrast agents to

define anatomy and quantify tissue perfusion. Specifi-

cally, with regard to ultrasound contrast agents, the

unique physical properties of air-filled, microspheres

serving as true intravascular indicators provide an

unparalleled access to the intrinsic spatial and temporal

heterogeneity of tissue perfusion.2

Importantly, though initially developed for diag-

nostic applications, novel therapeutic applications for

the ultrasound contrast agents are forthcoming. Thus,

while the use of ultrasound contrast agents today pro-

vides important clinical information regarding chamber

enhancement and myocardial perfusion, in the near

future these agents will provide therapeutic options for

site-specific, drug/gene delivery. Ultimately, ultrasound

contrast agents may provide the opportunity to dramat-

ically alter both the diagnosis and subsequent treatment

of numerous diseases.

Background

Today, the clinical applications of CEUS occupy a

unique position in the non-invasive imaging field as

defined by these parameters:

• Use of non-ionizing, acoustic energy

• Unparalleled spatial and temporal resolution

• Real-time processing

• Established user base

• Portability and economy

During the formative years of CEUS development,

researchers focused on the development of novel agents

and the requisite validation of the concept that these air-

filled, microspheres represented true, intravascular, non-

diffusible indicators. The list of scientists/clinicians

performing this pioneering work in the 1960-1980’s

include many of the following names: Gramiak and

Shah,1 Reale et al,3 Meltzer et al,4 DeMaria et al,5,6

Reisner, Schwartz, Kremkau,7 Ziskin et al,8 Bove et al,9

Bommer et al,10 McKay and coworkers,11 Kort and

Kronzon,12 Goldberg13 to list but a few of the innova-

tors. The scientific investigations provided a foundation

for the ensuing clinical applications of CEUS.

Later, a second wave of CEUS development was

spearheaded by the efforts of Armstrong et al,14 Tei

et al,15 Feinstein et al,16 Powsner et al,17 Kaul et al,18

Porter et al,19 Kemper et al,20 Zwehl et al,21 to list only a

few. Of note, the initial efforts designed to quantify the

novel ‘‘contrast effect’’ can be attributed to DeMaria

and Bommer,6 Ong et al,22 Meltzer et al.23

Ultimately, the efforts culminated in the production

of commercial ultrasound contrast agents (Levovist in

Europe, Berlex, Schering, and Albunex in the USA,

Molecular Biosystems Inc.).

Today, implementation of sophisticated harmonic

imaging systems and lower mechanical index imaging

provide prolonged in vivo persistence and markedly

enhanced, signal to noise ratios. Importantly, the

appropriate and clinically indicated clinical uses of

CEUS are reimbursed through third party insurers based

on the proven safety and efficacy of use.

The development of ‘‘second’’ generation con-

trast agents generally utilized high molecular weight,

low soluble gases, and resulted in the prolonged

From the Rush University Medical Center,a Chicago, IL; DETMA,b

Hospital Universitari Arnau de Vilanova, Lleida, Spain; Division

of Angiology,c University Hospital Basel, Basel, Switzerland;

Department of Biomedical Engineering,d Technion - Israel Institute

of Technology, Haifa, Israel; The Thoraxcenter,e Rotterdam, The

Netherlands; GE Global Research,f Niskayuna, NY.

Reprint requests: Steven B. Feinstein, MD, FACC, Rush University

Medical Center, Suite 1015 Jelke, 1653 West Congress Parkway,

Chicago, IL 60612; [email protected].

J Nucl Cardiol 2010;17:106–15.

1071-3581/$34.00

Copyright � 2009 by the American Society of Nuclear Cardiology.

doi:10.1007/s12350-009-9165-y

106

in vivo persistence. These second generation agents

fulfilled the required clinical expectations for safe,

efficient, and economical non-invasive imaging of the

left-sided cardiac chambers (i.e., left ventricular

opacification, Doppler enhancement, and myocardial

perfusion).

The ‘‘third’’ generation of contrast agents may be

considered as ‘‘designer’’ agents for molecular imaging.

These agents are uniquely and specifically labeled

and, as such, are designed to provide quantitative,

physiologic localization (‘‘molecular imaging’’) of

inflammation, and related disease states. The leaders in

this field include Jonathan Lindner, Lisa Villanueva,

Thomas Porter, Evan Unger, Samuel Wickline, David

McPherson, to name but a few.

Ultimately, the ‘‘fourth generation’’ CEUS appli-

cations break new ground in the area of therapy. These

agents are designed as ultrasound-directed, site-specific

drug/gene therapeutic systems. The pioneers in this

field include Ishihara,24 Unger et al,25 Grayburn and

coworkers,26 and an ever-expanding cadre of

researchers.

Reflecting back to the early years of CEUS, it is

clear that the dedicated work of many clinicians/

researchers cannot be underestimated. Their research

efforts directly contributed to the development of a

novel, safe, and efficacious non-invasive imaging

modality which improves patient care and reduces

downstream testing expenses and risk.

Table 1 (Ultrasound contrast agents) lists the cur-

rent ultrasound agents.

CEUS CLINICAL APPLICATIONS: LEFTVENTRICULAR OPACIFICATION

In the USA, two FDA ultrasound contrast agents are

currently approved for clinical use: Optison, GE Medi-

cal Diagnostics, Princeton, NJ; Definity (Lantheus

Medical Imaging, Billerica, MA). Several additional

agents are approved for clinical use outside of the USA

and include: Sonazoid, Sonovue, and Levovist.

Generally, direct visualization of the left ventricular

chamber and endocardial surfaces permits sonographers

and physicians to make clinical judgments regarding left

ventricular systolic function, filling status, and intra-

cavitary anatomy. Fundamentally, if health care pro-

fessionals cannot visualize the full extent of the left

ventricle, the diagnostic accuracy of the test is limited

and the physician’s confidence is reduced.

Therefore, ultrasound contrast agents are indicated in

patients who possess technically limited, suboptimal

echocardiograms. The American Society of Echocardiog-

raphy in 200027 and 200828 and the European Association

of Echocardiography 200929 recognized the clinical value

of using ultrasound contrast agents and issued position

papers providing guidelines. The following list of indica-

tions was abstracted from the ASE and EAE guidelines:

• Improve endocardial visualization: The resting state

echocardiography revealed reduced image quality (i.e.,

two contiguous left ventricular endocardial segments

were not observed in the non-contrast images).

• Reduce variability and increase accuracy in assessing

LV volume and LV ejection fraction.

Table 1. Ultrasound contrast agents (present and past agents)

Manufacturer Name Type Development stage

Accusphere Polymer/perfluorocarbon Clinical development

Alliance/Schering Imavist Encapsulated perfluorocarbon Clinical development

Andaris Quantison Albumin/low solubility gas Clinical development?

Bracco Sonovue Lipid/sulfur hexafluoride Approved for clinical use

Byk-Gulden BY963 Lipid/air (BY963) Clinical development

Cavcon Filmix Lipid/air Pre-clinical development

Lantheus Medical Imaging Definity Pentane/octafluoropropane Approved for clinical use

GE Healthcare Optison Sonicated albumin/octafluoropropane Approved for clinical use

GE Healthcare Sonazoid Lipid/perfluorocarbon Approved for clinical use

Point Biomedical Bisphere Perfluorocarbon/polymer bilayer Clinical development

Porter MD/University of

Nebraska

PESDA Sonicated albumin/perfluoropropane Not commercially available

Schering Echovist Approved for clinical use

Schering Levovist Lipid/air Approved for clinical use

Schering Sonavist Polymer/air Clinical development

Sonus Echogen Surfactant/perfluorocarbon Withdrawn from development

Journal of Nuclear Cardiology Feinstein et al 107

Volume 17, Number 1;106–15 Contrast enhanced ultrasound imaging

• Increase reader confidence for the interpretation of

left ventricle functional, structure, and filling status;

at rest and in stress echocardiography

• Confirm or exclude left ventricular structural abnor-

malities: apical variant of hypertrophic cardiomyop-

athy, ventricular non-compaction, apical thrombus,

aneurysm, pseudoaneurysm, myocardial rupture, and

intracardiac masses (tumors and thrombi).

In 2000-2002, shortly after the initial FDA approval

of ultrasound contrast agent, the initial clinical reports

described the value of CEUS for identifying left-sided,

cardiac chambers, particularly in patients with techni-

cally limited echocardiogram examinations.30-32 These

early studies provided a strong clinical base from which

future guidelines were developed.

Today, the use of CEUS is an accepted standard of

care. Notwithstanding the implementation of harmonic

imaging systems, experts generally agree that approxi-

mately 10-30% of all transthoracic echo images are

considered technically difficult or uninterpretable. In

fact, the value of CEUS is increasingly relevant in

today’s healthcare climate where efficiency, safety, and

utility are at a premium.

As an example, in a recent study from Senior et al,33

the authors used CEUS for determination of left ven-

tricular remodeling after acute myocardial infarction.

The addition of CEUS to the clinical study provided an

independent, incremental value for the prediction of late

mortality. This recent study reinforced the concept that

contrast echocardiography provides clinical utility for

the determination of left ventricular function and clinical

outcomes following an acute myocardial infarction.

Importantly, in 2009 Kurt et al,34 published a

landmark study in which they reported that the routine

use of CEUS for left ventricular chamber enhancement

significantly impacted diagnostic accuracy and resource

utilization; directly benefiting patient management. In

this large, prospective study, ultrasound contrast agents

were clinically indicated in 14.5% of the cohort (632/

4362). The impact of CEUS imaging was reflected in a

change in therapy (drugs), procedures or both in 35.6%

while the most benefit accrued to those patients who

were in the surgical intensive care unit. In this critically

ill population, the authors noted a change in therapy and

procedures in 62.7% of the patients. Additionally, the

authors commented on a reduction in subsequent testing

which included exposure to ionizing radiation and

invasive testing.

CEUS SAFETY

In September 2007, following the passage of House

Resolution (H.R. 3580), the FDA officials were provided

with additional authority for monitoring of phase-IV,

post-approval surveillance of ethical drugs. Subse-

quently, in October of 2008, following a series of self-

reported adverse events, the FDA officials issued a

‘‘Black Box’’ warning for Perflutren ultrasound contrast

agents affecting two previously FDA approved ultra-

sound contrast agents: Optison (approved in 1997; GE

Medical Diagnostics, Princeton, NJ) and Definity

(approved in 2001; Lantheus Medical Imaging, Bille-

rica, MA). The revised product label included new

contraindications. The sequence of events leading up to

these labeling were identified as follows:

• Post-marketing reports of *190 serious adverse

events and four deaths shortly following administra-

tion of the contrast agents (all self-reported cases)

• ‘‘Safety signal’’ appeared to be identified in animal

study resembling serious cardiopulmonary reactions

observed in humans

• The safety issues associated with Sonvue in Europe

• Lack of pulmonary hemodynamic data in humans

• A pre-marketing database that generally excluded

patients with unstable cardiopulmonary conditions

• Lack of a systematic risk assessment and management

plan

• Failure of a manufacturer to initiate an ‘‘important

post-marketing clinical study commitment to assess

its product’s safety.’’

Shortly following the revised product labeling,

an international grassroots organization of physicians,

sonographers, nurses, and interested parties strongly

requested reconsideration of the newly applied restric-

tions. Additionally, professional guilds (the American

Society of Echocardiography and the European Asso-

ciation of Echocardiography) similarly voiced concern

over the new labeling limitations on the ultrasound

contrast agents.

In direct response to the October 2007 FDA label

changes, clinicians promptly responded with a series of

peer-reviewed, publications focused on the proven

clinical safety record of ultrasound contrast agents. As

of May 2009, published reports cited over 228,611

patient cases in which ultrasound contrast agents were

safely used.

In May 2008, the FDA officials revised the labeling

changes to reflect the well-established, clinical safety

record of ultrasound contrast agents.

Continuing today, the clinical community, grass-

roots organizations, and professional societies provide

leadership highlighting the important clinical utility and

safety of ultrasound contrast agents. In fact, based on

intense response of the community, a new international,

not-for-profit organization was created to provide

interdisciplinary and international information for those

108 Feinstein et al Journal of Nuclear Cardiology

Contrast enhanced ultrasound imaging January/February 2010

interested in CEUS. The International Ultrasound Con-

trast Society (www.icus-society.org) provides monthly

newsletters and timely updates to all members at no

charge. Currently, ICUS provides updates to several

thousand subscribers in [57 countries. (For a list of the

recently published reports see references.32-39)

VASCULAR IMAGING APPLICATIONS

Overview

Today, the clinical vascular applications for the use

of CEUS are legion. Similar to the chamber enhance-

ment applications for echocardiography, the vascular

applications include enhancement of aorta, carotid

arteries, and peripheral venous systems.

Mattrey and Kono initially identified the clinical

value of using CEUS as an alternative to more invasive

imaging technologies.35,36 Although, ultrasound contrast

agents are not currently FDA approved in the USA, this

is not the case in Europe, Asia, and South America. In

the autumn of 2009, it is expected that vascular FDA-

approved, clinical trials will be initiated in the USA.

Recently the use of CEUS has been identified as a

novel imaging system capable of producing high reso-

lution, real-time, images of microvascular perfusion,

including tumor angiogenesis. Specifically, imaging of

the neovasculature (vasa vasorum) within the carotid

artery atherosclerotic plaque has captured world-wide

attention.37,38,39 Thus, the unifying concept is that arte-

rial wall inflammation/hypoxia provides a source for the

generation of VEGF proteins and subsequent neovas-

cularization growth. Hypoxia and inflammation events

are routinely observed in diverse disease states including

diabetes, atherosclerosis, connective tissue diseases, and

cancer. And with great prescience, Judah Folkman

realized that neovascularization provides the requisite

tumor nutrient blood supply commonly observed in a

variety of disease states.40

VASCULAR APPLICATIONS

Currently, the applications for the use of ultrasound

contrast agents include the following: (1) Enhancement

of the carotid artery lumen (plaque/ulcer), (2)

Enhancement of the intima-media-thickness (IMT), and

(3) Identification of adventitial/intra-plaque angiogene-

sis (vasa vasorum).

Enhancement of the Carotid Artery Lumen

Ultrasound contrast agents serve as blood pool

agents and consequently provide enhancement of the

carotid arterial luminal surface which includes the

common, bifurcation, and internal carotid arteries. Often

the case, the ability to clearly define extent of the carotid

vascular tree eludes the examiner. The image quality is

often compromised by the physical habitus of the sub-

ject. Importantly, CEUS provides a non-ionizing

radiation alternative to more invasive and higher risk

procedures.36

Over the last few years, there are numerous reports

from international centers which described the use of

CEUS for vascular imaging, specifically for the ana-

tomic enhancement of the luminal structures leading to

the diagnosis of irregular surface lesions, ulcers, soft

plaque, etc.2,35,37,41-45

At Rush University Medical Center in Chicago,

Illinois, we have performed over 1000 CEUS clinical

carotid examinations since 2001. All the studies were

performed under physician supervision with appropriate

institutional approvals. No adverse events or untoward

occurrences were noted. Remarkably, over the last

9 years, frequently, luminal irregularities (i.e., ulcers

and ‘‘soft plaque’’) were not observed without the use of

ultrasound contrast agents (see Figures 1, 2, 3, and 4). In

order to confirm the value of CEUS for routine vascular

imaging, multi-center, prospective clinical trials will be

required to ascertain the value of using these agents for

enhanced lesion detection in ‘‘at risk’’ populations.37

Enhancement of the Carotid Intima-Media-Thickness (IMT)

The use of the ultrasound-derived c-IMT as a sur-

rogate marker of systemic atherosclerosis was first

described by Pignoli in 1986.46 Subsequently, over the

last 23 years, the use of c-IMT has become a widely

accepted clinical standard for the detection of premature

atherosclerosis. Numerous FDA approved clinical trials

utilized c-IMT as an efficient marker for therapeutic

efficacy.47

Traditionally, clinicians and researchers readily

acknowledge the technical limitations which surround

the precise measurement of c-IMT measurements, par-

ticularly associated with identification of the carotid

artery near wall IMT.48-50 The primary technical issues

revolve around acoustic physics and include the rever-

berant acoustic noise generated from the overlying soft

tissues; all of which limit a clear acoustic definition of

the near wall. In distinction to the near wall, the acoustic

definition of the carotid artery far wall is regularly and

routinely visualized. This is due, in part, to the overlying

presence of the uniform media (blood) and lack of tissue

reflectance and interference. Due to the fact that the

carotid artery far wall remains an acoustically strong

acoustic reflector, this target is generally used in most

clinical studies and related pharmaceutical trials.



http://www.icus-society.org

However, based on recently published data, CEUS

provides a reliable and precise measurement of the near

c-IMT, particularly when compared to similar c-IMT

measurements performed without the use of ultrasound

contrast agents.51,52,45 In fact, and wholly consistent

with this data, previous pathology studies indicate that

the near wall is consistently thicker (20%) than the far

wall of the c-IMT; thus, implying that systemic athero-

sclerosis is preferentially distributed on the near wall

structures.53

Generally, the procedure for performing CEUS is

routine and straightforward. Specifically, after a patient

is referred for a clinically indicated carotid ultrasound

examination, it is reasonable to develop a consent form

for ‘‘off-label’’ use of ultrasound contrast agents. The

performance of a CEUS study requires a peripheral

intravenous injection of 0.5-1.0 mL of an FDA

approved contrast agent. Standard vascular imaging of

the carotid artery is routinely performed with a linear

array transducer. The mechanical index used for CEUS

is set at levels that are considerably lower levels than

those required for non-contrast studies (0.1-0.2 MI vs

0.3-0.5 MI, respectively). In order to measure the

c-IMT, it is recommended that one uses a readily

available semi-automated, computer-assisted c-IMT

software program.

The clinical utilization of the c-IMT as a surrogate

marker for the diagnosis of premature atherosclerosis is

increasing based on published clinical data bases.

Accordingly, the American Society and the Society for

Vascular Medicine have provided guidelines for clinical

use.54

Clearly, a current limitation for the use of per-

forming serial c-IMT measurements in individuals

remains the inherent error of measurement due to the

lack of a truly volumetric ultrasound scanner. While the

value if two-dimensional c-IMT analyses is unques-

tioned, the issues surrounding the routine clinical use

remain. Ultimately, with the introduction of real-time,

three-dimensional ultrasound scanners, the technical

issues associated with image alignment and registration

will be reduced.

Identification of Adventitial and Intra-Plaque Angiogenesis (Vasa Vasorum)

Historically, clinicians and researchers have repor-

ted upon the association of plaque vascularity and

‘‘vulnerability.’’39,55-74 Recently, CEUS imaging of the

carotid artery provided a novel, non-invasive method for

directly examining plaque vascularity; perhaps provid-

ing a ‘‘window’’ into plaque vulnerability.

Figure 1. Contrast-enhanced carotid ultrasound imaging—intra-luminal plaque. A, C Twodifferent carotid arteries with intra-luminal plaque on B-mode ultrasound imaging. B, DCorresponding arteries on contrast-enhanced ultrasound. The carotid intima-media complex(c-IMT) is depicted as a hypoechoic line and the adventitial layer appears echogenic.



Historical consideration of the vasa vaso-rum. Atherosclerotic plaques are believed to develop

from an initial endothelial cell insult often precipitated

by mechanical shearing, oxidative, and/or hypoxia

stresses from noxious substances. Following the initiat-

ing event, the subsequent deposition of intra-cellular

matter promotes focal migration of inflammatory cells

(monocyte derived macrophages), smooth muscle cells,

and fibroblasts, accumulating in the intracellular space

often resulting in foam cell development, raised lesions

and release of tissue hypoxic factors, and VEGF

proteins.

Sluimer et al, provided insight into the mechanism

surrounding the ongoing development of the tissue

hypoxia based on a description of the incomplete

endothelial junctions and inadequate structural integrity

of the immature and thin-walled microvessels.72 The

presence of the immature microvessels (‘‘leaky’’ ves-

sels) contribute inflammation materials by providing a

source of noxious plasma components (hemoglobin,

oxidized low-density lipoprotein cholesterol, lipopro-

tein[a], glucose, advanced glycation end products AGE)

and inflammatory cells.73,74

Ultimately, the atherosclerotic plaque, similar to the

other abnormal tumor growths, requires nutrient blood

flow supplied by arterial and venous vasa vasorum.

The anatomic structure of the vasa vasorum as

related to the growth of atherosclerotic plaques has been

well characterized by pathologists over 100 years ago.65

Based on a series of autopsy reports, intra-plaque

angiogenic vessels were identified within the vessel wall

Figure 2. Contrast-enhanced carotid ultrasound imaging—plaque ulceration. A, C Two differentcarotid arteries with plaque ulceration on B-mode ultrasound imaging. B, D Corresponding arterieson contrast-enhanced ultrasound.

Figure 3. Contrast-enhanced carotid ultrasound imaging—intraplaque neovascularization. Carotid artery with intra-lumi-nal plaques and intra-plaque neovascularization (arrow).



(media and intima) in subjects with known systemic

atherosclerosis.65,75 The seminal articles of Barger

et al55 and Beeuwkes et al56 and Winternitz in 1876,

provided important evidence directly linking adventitial

and intra-plaque vasa vasorum to the atherosclerotic

disease processes.

In 2004, Fleiner et al75 observed that the presence

and degree of neovascularization within vulnerable

plaque was associated with plaque rupture and clinical

occlusive cardiovascular events.

Similarly, Kumamoto et al in 1995 observed:

There was a significant positive correlation

between the density of new vessels in the intima

and the incidence of luminal stenosis, the extent of

chronic inflammatory infiltrate, the formation of

granulation tissue, or the atheromatous changes,

whereas the vascular density decreased in the

extensively hyalinized and calcified intima. The

newly formed intimal vessels originated mainly

from the adventitial vasa vasorum and also partly

from the proper coronary lumen. The intimal

vessels that originated from the adventitia occurred

approximately 28 times more frequently than those

that originated from the luminal side.65

In addition, Moreno and Fuster reported findings

which directly linked atherosclerosis and diabetes to the

formation of vulnerable plaques.76 Recently, Mauriello

et al examined 544 coronary segments in 16 patients

who experienced fatal coronary events.77 The results

revealed the presence of diffuse, active inflammation in

the entire coronary vascular system, in patients with

both stable and vulnerable plaques.

Using experimental animal models with dietary-

induced atherosclerosis, Williams et al showed regres-

sion of intima and media neovascularization after a

reduction of cholesterol feeding.78 In addition, Wilson

et al79 used micro-computed tomography techniques to

observe the induction of coronary adventitial vasa

vasorum in the pig and, subsequently, revealed regres-

sion after initiating statin therapy. Importantly, the

authors noted that the excessive growth of adventitial

vasa vasorum preceded the development of luminal

plaques. Similarly, Moulton et al studied anti-angio-

genesis therapies in an experimental animal model of

atherosclerosis.80

In 2007, Shah et al, at Rush University Medical

Center, in Chicago, published a clinical validation study

based on pathology specimens of subjects who under-

went CEUS prior to undergoing carotid endarterectomy

surgery.81 The results revealed a direct, positive corre-

lation between CEUS images and the surgically derived,

tissue specimens with regard to presence and degree of

angiogenesis within the human carotid plaques. For this

study, the histology consisted of hematoxylin and eosin

stains, with immunohistochemical markers: CD31,

CD34, von Willebrand factor, CD68, and were evalu-

ated for degree of vascularity.

Sets of stained slides were examined microscopi-

cally for evidence of neovascularization and inflam-

mation using a grading system as reported by Jeziorska

in 1999.61

Figure 4. Contrast-enhanced, vascular imaging. The image on the left revealed the un-enhanced,carotid ultrasound image. The images on the right revealed contrast-enhanced, vascular imaging.The circled region in red, highlights the intra-luminal plaque with associated intra-plaqueangiogenesis (microspheres appear as white objects within the plaque).



Similar to the limitation mentioned in performing

serial c-IMT measurements in the individual patient, the

ability to accurately quantify adventitial and intra-plaque

vasa vasorum, will necessitate the construction of an

ultrasound system with true, volumetric, image acquisi-

tion. Figure 5 is an image obtained from a real-time, 3D

mechanical ultrasound system during a clinical study

using CEUS. As noted, one can identify the luminal sur-

faces and the presence of the adventitial vasa vasorum.

FUTURE THERAPEUTIC APPLICATIONS

The current clinical applications of CEUS are

exclusively approved for diagnostic imaging. However,

future applications include a paradigm shift and will

include therapeutic options. Thus, the microscopic, gas-

filled, intravascular sphere that currently serves as a

‘‘stealth’’ agent has the potential to become an ideal

vehicle for delivering site-specific, drugs and/or genes to

the target organs. In fact, the ubiquitous presence of

angiogenesis in tumor growths provides an important

vascular conduit for the delivery of therapeutic

payloads.

The transformation of CEUS form a diagnostic

modality to a therapeutic option is created when in vivo

microspheres are acoustically disrupted via externally

applied acoustic energy resulting in a disruption of the

microspheres and subsequent, release of payload at a

target site. Based on published studies, which date to at

least 1995, Tochibana used ultrasound-directed therapy

for thrombolysis. More recently, the uses of CEUS

include site-specific delivery of drugs/genes.82-84

Thus, an entirely new field of non-viral, ultrasound-

mediated drug delivery appears to be unfolding. Leading

scientists throughout the world have successfully dem-

onstrated non-viral transduction through sonoporation in

a variety of pre-clinical scenarios. Clearly, the bur-

geoning scientific advancements in therapeutic options

are beyond the scope of this brief mention and command

future attention.

To imagine that the vascular conduits provide a

direct assess to tumor (or plaque) allows one’s imagi-

nation to speculate on the future of combined diagnostic

and therapeutic applications of CEUS. Stay tuned…

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Figure 5. Contrast-enhanced, 3D vascular imaging. Thelumen of the carotid artery is opacified following theintravenous injection of an ultrasound contrast agent (white).Note that the luminal-intima interface is highlighted. Theadventitial vasa vasorum are highlighted at the level of carotidbulb (white arrows).



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