Ebook Point of care - Ultrasound: Part 1

Point-of-Care
Ultrasound
NILAM J. SONI, MD
Associate Professor of Medicine
Division of Hospital Medicine
University of Texas Health Science Center
San Antonio
San Antonio, Texas

ROBERT ARNTFIELD, MD
Assistant Professor
Division of Emergency Medicine and Division of Critical Care Medicine
Western University
London Health Sciences Centre
London, Ontario, Canada

PIERRE KORY, MPA, MD
Associate Professor of Medicine
Fellowship Program Director
Division of Pulmonary, Critical Care, and Sleep Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York


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POINT-OF-CARE ULTRASOUND   

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CONTRIBUTORS

Samuel Acquah, MD, FCCP
Assistant Professor
Director, Medical Intensive Care Unit
Division of Pulmonary, Critical Care, and
Sleep Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York
Sara Ahmadi, MD
Assistant Professor
Division of Endocrinology
University of Texas Health Science Center
San Antonio
San Antonio, Texas
Phillip Andrus, MD, FACEP, RDMS
Assistant Professor of Emergency Medicine
Division of Emergency Ultrasound
Icahn School of Medicine at Mount Sinai
New York, New York
Robert Arntfield, MD, FRCPC, FCCP, RDMS
Assistant Professor

Division of Emergency Medicine and
Division of Critical Care Medicine
Western University
London Health Sciences Centre
London, Ontario, Canada

Anita Cave, MD, FRCPC
Assistant Professor
Department of Anesthesia and Perioperative
Medicine
London Health Sciences Centre
London, Ontario, Canada
Gregg L. Chesney, MD
Fellow, Critical Care Medicine
Division of Pulmonary and Critical Care
Medicine
Stanford University School of Medicine
Stanford Hospital and Clinics
Stanford, California
Alan T. Chiem, MD, MPH
Assistant Clinical Professor
Director, Emergency Ultrasound
Department of Emergency Medicine
University of California, Los Angeles
Olive View–UCLA Medical Center
Los Angeles, California
Christopher Dayton, MD
Fellow, Critical Care Medicine
Division of Critical Care Medicine
Albert Einstein College of Medicine

Montefiore Medical Center
New York, New York

Michel Boivin, MD
Associate Professor
Division of Pulmonary, Critical Care, and
Sleep Medicine
University of New Mexico
Albuquerque, New Mexico

Orlando Debesa, DO
Assistant Clinical Professor of Medicine
Division of Pulmonary and Critical Care
Medicine
Virginia Commonwealth University
Richmond, Virginia

Pedro Campos, MD
Director, Emergency Ultrasound
Sutter Medical Center of Santa Rosa
Clinical Instructor
University of California, San Francisco
San Francisco, California

Eitan Dickman, MD, FACEP
Vice-Chair of Academics
Director, Division of Emergency
Ultrasound
Department of Emergency Medicine
Maimonides Medical Center

New York, New York

Carolina Candotti, MD
Assistant Professor
Division of Hospital Medicine
Penn State Hershey Medical Center
Hershey, Pennsylvania

Peter Doelken, MD
Associate Professor of Medicine
Albany Medical College
Albany, New York

v


vi

CONTRIBUTORS

Lewis A. Eisen, MD
Associate Professor of Clinical Medicine
Assistant Professor of Clinical Neurology
Division of Critical Care Medicine
Albert Einstein College of Medicine
Montefiore Medical Center
New York, New York

Patricia C. Henwood, MD
Instructor in Emergency Medicine

Harvard Medical School
Fellow in Emergency Ultrasound
Department of Emergency Medicine
Brigham and Women’s Hospital
Boston, Massachusetts

Daniel Fein, MD
Division of Pulmonary, Critical Care, and
Sleep Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York

Patrick T. Hook, MD, MS
Fellow, Rheumatology
Department of Rheumatology
Boston University School of Medicine
Boston, Massachusetts

Stephanie Fish, MD
Associate Professor
Department of Medicine, Endocrine
Division
Memorial Sloan Kettering Cancer Center
New York, New York
Ricardo Franco, MD
Associate Professor of Medicine
Division of Hospital Medicine
Rush University Medical College
John H. Stroger, Jr. Hospital of Cook County

Chicago, Illinois
Colin Gebhardt, BScPT, MD
Department of Adult Critical Care
University of Saskatchewan
Saskatchewan, Alberta, Canada
Ben Goodgame, MD, RDMS
Emergency Medicine Consultant
Rotorua Hospital, Lakes District
Rotorua, New Zealand
Behzad Hassani, MD, CCFP (EM)
Assistant Professor
Division of Emergency Medicine
London Health Sciences Centre
London, Ontario, Canada
Ahmed F. Hegazy, MD, FRCPC
Assistant Professor
Division of Critical Care Medicine
Department of Anesthesia and Perioperative
Medicine
Western University
London Health Sciences Centre
London, Ontario, Canada

J. Terrill Huggins, MD
Associate Professor of Medicine
Division of Pulmonary, Critical Care, Allergy,
and Sleep Medicine
Medical University of South Carolina
Charleston, South Carolina
Adolfo Kaplan, MD, FCCP

Pulmonologist
McAllen Health Sciences Center
McAllen, Texas
Chan Kim, MD
Clinical Instructor
Department of Rheumatology
Boston University School of Medicine
Boston, Massachusetts
Eugene Kissin, MD
Associate Professor of Medicine
Department of Rheumatology
Boston University School of Medicine
Boston, Massachusetts
Pierre Kory, MPA, MD, MHS
Associate Professor of Medicine
Fellowship Program Director
Division of Pulmonary, Critical Care, and
Sleep Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York
Daniel Lakoff, MD
Associate Director, Emergency Ultrasound
Department of Emergency Medicine
Icahn School of Medicine at Mount Sinai
New York, New York


vii

CONTRIBUTORS

Viera Lakticova, MD
Assistant Professor of Medicine
Director, Interventional Pulmonology
North Shore–Long Island Jewish Medical
Center
New Hyde Park, New York

Scott Millington, MD, FRCPC
Assistant Professor of Medicine
Department of Critical Care Medicine
University of Ottawa and The Ottawa
Hospital
Ottawa, Ontario, Canada

Catherine Y. Lau, MD
Assistant Clinical Professor of Medicine
Director, Neurological Surgery Patient
Safety and Quality
Department of Medicine
University of California, San Francisco
San Francisco, California

Paul K. Mohabir, MD
Clinical Associate Professor
Director, Adult Cystic Fibrosis Program
Division of Pulmonary and Critical Care
Medicine
Stanford University School of Medicine

Stanford, California

Alycia Lee, BS, RDCS, RVT
School of Medicine and Health Sciences
The George Washington University
Washington, DC

Arun Nagdev, MD
Director, Emergency Ultrasound
Alameda Health System
Highland General Hospital
Oakland, California

Peter M.J. Lee, MD, MHS
Division of Pulmonary, Critical Care,
and Sleep Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York
W. Robert Leeper, MD, FRCSC
Trauma and Acute Care Surgery Fellow
Department of Surgery, Division of Acute
Care Surgery
Johns Hopkins University School of Medicine
Baltimore, Maryland
Shankar LeVine, MD
Alameda Health System
Highland General Hospital
Oakland, California
Ken Lyn-Kew, MD

Assistant Professor of Medicine
Division of Pulmonary Science and Critical
Care Medicine
University of Colorado
National Jewish Health
Denver, Colorado
Daniel Mantuani, MD, MPH
Department of Emergency Medicine
Alameda Health System
Highland General Hospital
Oakland, California
Michael Mayette, MD, FRCPC
Assistant Professor
Critical Care Medicine
Université de Sherbrooke
Sherbrooke, Québec, Canada

Mangala Narasimhan, DO, FCCP
Section Head, Critical Care Medicine
Associate Professor of Medicine
North Shore–Long Island Jewish Medical
Center
New Hyde Park, New York
Paru Patrawalla, MD
Assistant Professor of Medicine
Division of Pulmonary, Critical Care, and
Sleep Medicine
New York University School of Medicine
New York, New York
Daniel R. Peterson, MD, PhD, FRCPC, RDMS

Clinical Assistant Professor
Academic Department of Emergency
Medicine
University of Calgary
Foothills Medical Centre
Calgary, Alberta, Canada
Nitin Puri, MD, FCCP
Assistant Professor of Medicine
Medical Director, Cardiac Surgery Intensive
Care Unit
Virginia Commonwealth University
Inova Fairfax Hospital
Falls Church, Virginia
Shideh Shafie, MD
Department of Emergency Medicine
Maimonides Medical Center
Brooklyn, New York


viii
Ariel L. Shiloh, MD
Director, Critical Care Consult Service
Assistant Professor of Clinical Medicine and
Neurology
Division of Critical Care Medicine,
Department of Medicine
Albert Einstein College of Medicine
Jay B. Langner Critical Care Service
Montefiore Medical Center
Bronx, New York

Michael Silverberg, MD
Division of Pulmonary, Critical Care, and
Sleep Medicine
Department of Emergency Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York

CONTRIBUTORS

Michael Stone, MD
Chief, Division of Emergency Ultrasound
Department of Emergency Medicine
Brigham and Women’s Hospital
Boston, Massachusetts
Christopher R. Tainter, MD, RDMS
Fellow, Critical Care Medicine
Massachusetts General Hospital
Harvard Medical School
Boston, Massachusetts
Stefan Tchernodrinski, MD, MS
Division of Hospital Medicine
John H. Stroger, Jr. Hospital of Cook County
Assistant Professor of Medicine
Rush University Medical College
Chicago, Illinois

Craig Sisson, MD, RDMS
Clinical Associate Professor
Department of Emergency Medicine

University of Texas Health Science Center
San Antonio
San Antonio, Texas

Ryu P.H. Tofts, MbCHb
Division of Pulmonary, Critical Care, and
Sleep Medicine
Mount Sinai Beth Israel Medical Center
Icahn School of Medicine at Mount Sinai
New York, New York

Diane Sliwka, MD
Associate Clinical Professor of Medicine
Director, Mt. Zion Medical Service
Division of Hospital Medicine
Department of Medicine
University of California, San Francisco
San Francisco, California

Christopher Walker, MD
Fellow, Critical Care Medicine
Division of Pulmonary and Critical Care
Medicine
Stanford University School of Medicine
Stanford, California

Nilam J. Soni, MD, FHM, FACP
Associate Professor of Medicine
Division of Hospital Medicine
University of Texas Health Science Center

San Antonio
San Antonio, Texas
Kirk T. Spencer, MD, FASE
Professor of Medicine
Section of Cardiology
Department of Medicine
University of Chicago
Chicago, Illinois

Ralph C. Wang, MD
Associate Professor of Emergency Medicine
Department of Emergency Medicine
University of California, San Francisco
San Francisco, California
Michael Y. Woo, MD, CCFP(EM), RDMS
Associate Professor
Director, Emergency Medicine Ultrasound
Program Director, Emergency Medicine
Ultrasound Fellowship
Department of Emergency Medicine
University of Ottawa and The Ottawa
Hospital
Ottawa, Ontario, Canada


This book is dedicated to all the compassionate and
dedicated clinicians who strive to provide the best bedside
care to their patients.

To those who inspire me — my parents, Jay and Niru, and

my children, Riya and Devak — and to the one whose
sacrificial support makes it all possible, Perla
NS

To my parents, Peg and David, for their lifelong support,
my children, Amelia and John, for reminding me what is
most important, and my wife, Shannon, for her boundless
love and encouragement
RA

To my angels, Amy, Ella, Eve, and Violet, along with
my dear parents, Leslie and Odile, for their unwavering
patience, support, and love
PK


PREFACE

Point-of-care ultrasound has been rapidly integrated into clinical practice in recent years, and its
role in medicine will continue to expand in coming decades. Point-of-care ultrasound has been
shown to make procedures safer, expedite diagnoses, and raise confidence in clinical decision
making. In addition to these clinical benefits, point-of-care ultrasound is one of few technologies
that brings providers closer to patients, putting them right at the bedside, raising the satisfaction
of providers and patients alike.
This book is intended for providers of any healthcare discipline interested in learning about
the principles and diverse applications of point-of-care ultrasound. Covered in detail are specific
point-of-care ultrasound applications that are most generalizable to healthcare providers from a
broad spectrum of specialties and practice settings. All chapters are based on the best available
evidence supplemented by expert opinion.
Nilam J. Soni

Robert Arntfield
Pierre Kory

xi


ACKNOWLEDGMENTS
For contributing ultrasound images:
Danny Duque, MD, RDMS, FACEP
Assistant Professor
Department of Emergency Medicine
Elmhurst Hospital Center
New York, New York
Laleh Gharahbaghian, MD
Clinical Assistant Professor
Department of Emergency Medicine
Stanford University Medical Center
Alycia Paige Lee, BS, RDCS, RVT
School of Medicine and Health Sciences
The George Washington University
Washington, DC
Jennifer Huang, DO, FACEP
Assistant Professor
Department of Emergency Medicine
Mt. Sinai Hospital
New York, New York
Bret P. Nelson, MD, RDMS, FACEP
Associate Professor
Department of Emergency Medicine
Mt. Sinai Hospital

New York, New York
Roya Etemad Rezai, MD, FRCPC
Associate Professor
Department of Diagnostic Radiology and
Nuclear Medicine
Western University
London Health Science Centre
London, Ontario, Canada
Ee Tay, MD, FAAP
Assistant Professor
Department of Emergency Medicine (Pediatrics)
Mt. Sinai Hospital
New York, New York
Nate Teismann, MD
Assistant Clinical Professor
Department of Emergency Medicine
University of California San Francisco
San Francisco, California
Drew Thompson, MD, FRCPC
Associate Professor
Department of Emergency Medicine
Western University
London Health Sciences Centre
London, Ontario, Canada
xii

Brita E. Zaia, MD, FACEP
Director, Emergency Department Ultrasound
Department of Emergency Medicine
Kaiser Permanente San Francisco Medical

Center
San Francisco, California
For developing original illustrations and
photography:
Victoria Heim, CMI
Medical Illustrator
Loganville, Georgia
Jade Myers
Graphic Designer
Matrix Art Services
York, Pennsylvania
Lester Rosebrock
Photographer
University of Health Science Center
San Antonio
San Antonio, Texas
Sam Newman
Medical Animator
University of Texas Health Science Center
San Antonio
San Antonio, Texas
Jordan Hill, BA
Health & Fitness Consultant
San Antonio, Texas
For serving as a mentor and educator in the field:
Paul Mayo, MD
Professor of Clinical Medicine
Hofstra North Shore–LIJ School of Medicine
Long Island Jewish Medical Center
New Hyde Park, New York

Gary White, PhD
Editor, The Physics Teacher
American Association of Physics Teachers
Adjunct Professor of Physics
George Washington University
Washington, DC


C H A P T E R

1

Evolution of Point-of-Care
Ultrasound
Nilam J. Soni  n  Robert Arntfield  n  Pierre Kory

K E Y

POIN TS

• Point-of-care ultrasound is defined as a goal-directed, bedside ultrasound examination
performed by a healthcare provider to answer a specific diagnostic question or to guide
performance of an invasive procedure.
• Diagnostic ultrasound was first developed and used in medicine during the 1940s, but
point-of-care ultrasound has been integrated into diverse areas of clinical practice since
the early 1990s.
• Important considerations when using point-of-care ultrasound include provider training
and skill level, patient characteristics, and ultrasound equipment features.

Background

Point-of-care ultrasound has revolutionized
the practice of medicine, influencing how care
is provided in nearly every medical and surgical specialty. For more than a century, clinicians
had been limited to primitive bedside tools,
such as the reflex hammer (c. 1888) and stethoscope (c. 1816), but with bedside ultrasound,
providers are equipped with a tool that allows
them to actually see what they can only infer
through palpation or auscultation. The technologic miniaturization of ultrasound devices
has outpaced integration of these devices into
clinical practice. Many professional societies
and national organizations have recognized
the potent impact of point-of-care ultrasound
and have endorsed its routine use in clinical
practice. In 2001 the American Medical Association stated, “Ultrasound has diverse applications and is used by a wide range of physicians
and disciplines. Ultrasound imaging is within
the scope of practice of appropriately trained
physicians.”1 Thus, it has been well recognized for over a decade that providers from
diverse specialties can and should be trained to

use ultrasound within their scope of practice.
This chapter reviews the major milestones in
the history of medical ultrasound with a focus
on important considerations of point-of-care
ultrasound.

History
Acoustic properties of sound were well described
by ancient Greek and Roman civilizations. In
the twentieth century, the sinking of the Titanic
followed by the start of World War I served as

catalysts for the development of sonar, or sound
navigation and ranging, which was the first realworld application of the principles of sound.2,3
Although several physicians were simultaneously competing for recognition as the first
to use ultrasound in medicine, Karl Theodore
Dussik, an Austrian psychiatrist and neurologist, is credited as being the first physician to
use ultrasound in medical diagnostics when
he attempted to visualize cerebral ventricles
and brain tumors using a primitive ultrasound
device in 1942 (Figure 1.1).
During the 1940s and 1950s, many pioneers advanced the field of medical ultrasound.
3


4

Figure 1.1  Karl Theodore Dussik and the first
medical ultrasound device in 1946. (From FrentzelBeyme B. Vom Echolot zur Farbdopplersonographie. Der Radiologe. April 2005;45(4):363–370.)

Figure 1.2  Immersion-tank ultrasound machine
from the 1950s. (From Hagen-Ansert SL. Textbook of Diagnostic Sonography. 7th ed., St Louis,
Mosby, 2011.)

John Julian Wild described various clinical
applications of ultrasound, including the difference in appearance of normal and cancerous
tissues. Douglass Howry and Joseph Holmes
focused on ultrasound equipment technology. They built immersion-tank ultrasound
systems, including the “somascope” in 1954
(Figure 1.2), and they published the first twodimensional ultrasound images. Ian Donald
contributed significant amounts of research
in obstetric and gynecologic ultrasound.

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND

Inge Edler and Carl Hellmuth Hertz investigated cardiac ultrasound and established the
field of echocardiography in the early 1950s.
Shigeo Satomura, a Japanese physician isolated
from the pioneers in the United States and
Europe, is credited as being the first physician
to use Doppler ultrasound in his studies of cardiac valve motion.3
Advancements in ultrasound technology
accelerated the field in the 1960s and 1970s.
Early ultrasound machines used open-shutter
photography to capture screen images. Multiple
still images of moving structures were captured,
sequentially displayed, and interpreted by trying to imagine the structures in motion. In
1965, Siemens released the Vidoson, the first
real-time ultrasound scanner, which was able to
display 15 images per second. The Vidoson was
quickly incorporated into obstetric care over the
next decade and became a standard component
of assessing pregnant women. Sector scanning
became possible with development of phasedarray transducers in the early 1970s, giving rise
to echocardiography as an independent field.3
Ultrasound technology continued to
advance during the 1970s and 1980s with the
development of more sophisticated transducers along with refinements in image quality.
Following the “early adopters” of ultrasound,
namely, radiology, cardiology, and obstetrics/
gynecology, ultrasound began to be used in
emergency care, a role that marked the beginning of the era of point-of-care ultrasound use

by healthcare providers from diverse medical
specialties.3 Life-threatening conditions could
be assessed rapidly at the bedside with portable ultrasound. Frontline physicians, mostly
surgeons and emergency medicine physicians,
started assessing trauma patients with ultrasound in the 1970s, and the term FAST exam,
or Focused Assessment with Sonography in
Trauma, was coined in the early 1990s.4–6 The
FAST exam was incorporated into Advanced
Trauma Life Support (ATLS) guidelines in
the late 1990s.7,8 From its early description in
the 1970s in Europe to its incorporation into
ATLS guidelines in the 1990s in the United
States, the FAST exam established a precedent
for development of point-of-care ultrasound
applications and incorporation of these applications into routine clinical practice.
Since the 1990s, point-of-care ultrasound
has become a part of nearly every specialty’s
practice. In addition to development of specific
point-of-care ultrasound applications in the


1—EVOLUTION OF POINT-OF-CARE ULTRASOUND

1990s, such as the FAST exam, general medical ultrasound applications, broadly applicable
to many specialties, started to emerge. In the
mid-1980s, ultrasound artifacts of the lung
and pleura—an organ long felt to have little
utility in ultrasound diagnostics—began to be
described. The identification and codification
of these artifacts with discrete forms of lung

pathology was developed by Daniel Lichtenstein, a French critical care physician, which
gave rise to the field of lung ultrasonography.9
Even though lung ultrasound was first used by
intensivists to evaluate critically ill patients,
lung ultrasound is broadly applicable to evaluate any patient with pulmonary symptoms,
is more accurate than chest x-ray, and can be
used by any healthcare provider, regardless of
specialty.10 Another broadly applicable use of
ultrasound that evolved was in the guidance of
invasive bedside procedures. Multiple studies
since the 1990s have demonstrated reduced
mechanical complications when ultrasound is
used to guide common procedures, in particular central venous catheterization.11,12 Current guidelines recommend that all providers,
regardless of specialty, use ultrasound guidance when placing internal jugular central
venous catheters.
Ultrasound technology was well advanced
by the 2000s, which saw the development of
three-dimensional ultrasound for select diagnostic applications; however, two-dimensional
ultrasound has remained the standard for the
majority of indications. The more important change during the 2000s was continued
reduction in the size and price of ultrasound
machines. The increased portability and
affordability of ultrasound devices led to an
exponential increase in use of ultrasound by
all providers. Subsequently, many professional
societies published guidelines on use of pointof-care ultrasound, including the American
Institute of Ultrasound in Medicine (AIUM),
American College of Emergency Physicians
(ACEP), American College of Chest Physicians (ACCP), and American Society of Echocardiography (ASE). Furthermore, consensus
guidelines between imaging and specialty societies have been established, such as the guidelines on obstetrical ultrasound collaboratively

developed by the American College of Radiology (ACR), American College of Obstetricians and Gynecologists (ACOG), American
Institute of Ultrasound in Medicine (AIUM),
and Society of Radiologists in Ultrasound

5

(SRU) and the ACEP–ASE statement on
focused cardiac ultrasound in the emergent
setting.13,14 Specialty-specific guidelines also
emerged, such as the American Association
of Clinical Endocrinologists guidelines on
thyroid ultrasound that defined a pathway for
endocrinologists to earn a certificate of competency in thyroid and neck ultrasound.15
Medical educators recognized the importance of a basic understanding of ultrasound in
the early 2000s and started to explore how to
incorporate ultrasound training into curricula
for medical students, residents, and fellows.
The Accreditation Council for Graduate Medical Education (ACGME) has started to mandate certain residency and fellowship programs
in the United States include basic ultrasound
education; for example, critical care ultrasound
and ultrasound-guided thoracentesis and central venous catheterization are now required
components of pulmonary/critical care fellowship training. Several medical schools worldwide have started to expose their students
to the principles and practice of ultrasound,
most often in conjunction with anatomy and
physical examination courses.16–19 The coming generation of physicians will thus be more
adept at point-of-care ultrasound applications
and will consider use of bedside ultrasound to
be routine in most clinical encounters. While
past generations’ contributions established
the utility of ultrasound as a valuable bedside

tool in diagnostics and procedural guidance,
the next generation will advance the field by
studying how point-of-care ultrasound can be
best incorporated into patient care algorithms
and its impact on healthcare outcomes, cost-­
effectiveness, and patient satisfaction.

Key Considerations
Point-of-care ultrasound exams differ from comprehensive ultrasound exams in several aspects.
Point-of-care exams are generally employed to
detect acute, potentially life-threatening conditions where detection at the bedside expedites
patient care. An evaluation with point-of-care
ultrasound requires less time due to its focus on
a single or limited set of findings for a specific
clinical complaint or syndrome. In contrast,
comprehensive diagnostic exams thoroughly
evaluate all anatomic structures related to an
organ or organ system. The process of ordering,
performing, interpreting, and reporting such
comprehensive ultrasound exams usually takes


6

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND

hours, whereas acquisition and interpretation of
point-of-care ultrasound exams takes minutes,
providing more immediate information for
decision making.20 Although information can

be obtained rapidly, an important challenge
is overcoming the multiple time constraints
put on providers who are regularly performing
­point-of-care ultrasound exams.
Key considerations to improve the efficiency and quality of point-of-care ultrasound exams for different clinical applications
include optimization of provider training,
patient factors, and ultrasound equipment
features.





CLINICAL APPLICATIONS
A point-of-care ultrasound exam is aimed
at answering a specific question through a
focused, goal-directed exam and can be used
to assess most body systems (Figure 1.3).
Generally, the goal is to “rule in” or “rule out”
a specific condition or answer a “yes/no” question. Clinical applications can be categorized
as follows:

□ Procedural guidance: Ultrasound guidance
has been shown to reduce complications
and improve success rates of invasive bedside procedures. Procedures commonly
performed with ultrasound assistance
Lymph nodes

IVC

Bladder

Lungs

Gallbladder

Kidneys



include vascular access, thoracentesis,
paracentesis, lumbar puncture, arthrocentesis, and pericardiocentesis.
□ Diagnostics: Based on the patient’s presenting signs and symptoms, a focused
bedside ultrasound exam can narrow the
differential diagnosis and guide additional investigations, especially in urgent or
emergent situations. Focused ultrasound
exams are commonly performed to evaluate the lungs, heart, gallbladder, aorta,
kidneys, bladder, gravid uterus, joints, and
lower-extremity veins.
□ Monitoring: Serial ultrasound exams can
be performed to monitor a patient’s condition or to monitor the effects of an intervention without exposing patients to
ionizing radiation or intravenous contrast.
Common applications include monitoring inferior vena cava distention and
collapsibility as a surrogate for central venous pressure during fluid resuscitation,
monitoring left ventricular contraction
in response to inotrope initiation, and
monitoring for resolution or worsening
of a pneumothorax or pneumonia on lung
ultrasound.
□ Resuscitation: Use of ultrasound during

resuscitation for cardiac arrest is a unique
but underutilized application. Bedside
Eyes

Thyroid

Heart

Musculoskeletal

Male/female
reprod

Vascular

Figure 1.3  Common diagnostic applications of point-of-care ultrasound. IVC, Inferior vena cava.


1—EVOLUTION OF POINT-OF-CARE ULTRASOUND



ultrasound can direct emergent interventions by rapidly detecting tension
pneumothorax, cardiac tamponade, and
massive pulmonary embolism with acute
right ventricular failure. Additionally,
ultrasound can be used to assess cardiac
contractions to help determine when to
cease resuscitative efforts. Visualization
of cardiac standstill or clotting within the

heart chambers allows providers to stop
futile interventions, whereas visualization
of subtle or weak cardiac contractions
typically justifies continuation of resuscitative efforts.
□ Screening: Screening with ultrasound is
potentially advantageous because it is
noninvasive and avoids ionizing radiation. Although screening for abdominal
aortic aneurysm or asymptomatic left
ventricular function using point-of-care
ultrasound has been described, more
widespread screening applications have
been slow to develop due to the challenge
of weighing benefits of early detection
against the harms of false-positive findings that lead to unnecessary testing and/
or procedures.21–23

7

generations will focus on competency-based
education, with competency determined by
achievement of certain milestones rather
than completion of a predetermined number
of exams.
PATIENT FACTORS
Body habitus, positioning, and acute illness
are important considerations when imaging
patients. Similar to plain film radiography,
ultrasound waves are attenuated by adipose
tissue, and ultrasound has limited penetration
in morbidly obese patients. Lower frequencies

must be used for deeper penetration, resulting in lower-resolution images. Positioning
can limit ultrasound examination; for example, acquisition of apical cardiac ultrasound
images is often limited in patients who cannot
be placed in a left lateral decubitus position.
Similarly, providers often have to adjust their
own position to evaluate pleural effusions
and perform thoracentesis when patients are
unable to sit upright. On the contrary, ascites
and pleural effusions improve visualization
of deep organs due to deeper penetration of
sound waves.

PROVIDER TRAINING

ULTRASOUND EQUIPMENT

The amount of training required to achieve
competency in point-of-care ultrasound
applications varies by provider skill and exam
type. Prior experience with ultrasound greatly
facilitates learning new applications. The skills
required relate to the provider’s scope of
practice; for example, a rheumatologist may
be proficient with musculoskeletal ultrasound but less proficient with cardiac or
abdominal ultrasound, while the opposite
may be true for a critical care physician. Protocols from published studies on ultrasound
education have differed, but it is generally
accepted that training must include handson image acquisition and interpretation
practice supplemented by focused didactics.
Current studies have provided general guidance on the average number of exams needed

to acquire skills depending on the exam type;
for example, novice users have been able to
achieve an “acceptable” skill level in focused
cardiac ultrasound after performing 20–30
limited exams.24 Although a minimum
number of exams will likely continue to be
required to earn certain certificates, future

With the development of newer, more compact ultrasound devices, lack of familiarity with
the varying features and controls can present
a barrier to use. Fortunately, many machines
are designed specifically for point-of-care
ultrasound applications with “ease of use” as
a primary feature, an important consideration
when purchasing a machine. Providers must
be familiar with basic operations of available
ultrasound equipment, including entering
patient information, selecting the appropriate
imaging mode, and adjusting the image depth
and gain. The diminution in size of portable
ultrasound machines comes with certain limitations: smaller screen size, limited transducer
selection, fewer imaging modes, and fewer
adjustable parameters to optimize the image.
Transducer availability is an important consideration because certain exams can be performed
with multiple transducer types, whereas others
can be performed only with a single transducer
type; for example, a curvilinear or phased-array
transducer can be used to evaluate the abdomen but only a phased-array transducer can be
used to evaluate the heart.

8

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND

Vision
Point-of-care ultrasound use has increased
and spread rapidly over the past 20 years,
and we anticipate that all healthcare providers, including students, nurses, advanced care
providers, and physicians, will have integrated
point-of-care ultrasound into clinical practice
in the next 20 years (Figure 1.4). Healthcare

systems ­
throughout the world are striving to
provide high-quality, cost-effective healthcare,
and point-of-care ultrasound can contribute to
achieving these goals by reducing procedural
complications, expediting care, decreasing costly
ancillary testing, and reducing imaging that utilizes ionizing radiation. Realizing such objectives can further the ultimate goal of improving
patient satisfaction and healthcare outcomes.

All Providers
Nurses
Midlevel Providers
Med Students
Pediatrics
Internal Medicine + Subspecialties
Anesthesia
Critical Care

Emergency Med & Surgery
Obstetrics
Cardiology
1960
1970
1980

1990

2000

2010

2020

2030

Figure 1.4  Integration of point-of-care ultrasound in medicine specialties.

2040


References
1. Cardenas E. Emergency medicine ultrasound policies and reimbursement guidelines. Emerg Med Clin
North Am. August 2004;22(3):829–838, x–xi.
2. Woo J. A Short History of the Development of Ultrasound in Obstetrics and Gynecology. />3. Newman PG, Rozycki GS. The history of ultrasound. Surg Clin North Am. April 1998;78(2):179–195.
4. Eckel H. Sonography in emergency diagnosis of the abdomen, [author’s trans.] Rontgenblatter. May
1980;33(5):244–248.
5. Plummer D. Principles of emergency ultrasound and echocardiography. Ann Emerg Med. December
1989;18(12):1291–1297.

6. Jehle D, Guarino J, Karamanoukian H. Emergency department ultrasound in the evaluation of blunt
abdominal trauma. Am J Emerg Med. July 1993;11(4):342–346.
7. Han DC, Rozycki GS, Schmidt JA, Feliciano DV. Ultrasound training during ATLS: an early start for
surgical interns. J Trauma. August 1996;41(2):208–213.
8. Rozycki GS. Surgeon-performed ultrasound: its use in clinical practice. Ann Surg. July 1998;228(1):16–28.
9. Lichtenstein D. L’échographie générale en reanimation. Germany: Springer-Verlag; 1992.
10. Xirouchaki N, et al. Lung ultrasound in critically ill patients: comparison with bedside chest radiography. Intensive Care Med. 2011;37(9):1488–1493.
11. Weiner MM, Geldard P, Mittnacht AJ. Ultrasound-guided vascular access: a comprehensive review.
J Cardiothorac Vasc Anesth. April 2013;27(2):345–360.
12. Wu SY, Ling Q, Cao LH, Wang J, Xu MX, Zeng WA. Real-time two-dimensional ultrasound guidance for central venous cannulation: a meta-analysis. Anesthesiology. February 2013;118(2):361–375.
13. ACR-ACOG-AIUM-SRU practice guideline for the performance of obstetrical ultrasound. http://ww
w.acr.org/∼/media/ACR/Documents/PGTS/guidelines/US_Obstetrical.pdf; (revised 2013).
14. Labovitz AJ, Noble VE, Bierig M, et al. Focused cardiac ultrasound in the emergent setting: a
consensus statement of the American Society of Echocardiography and American College
of Emergency Physicians. J Am Soc Echocardiogr. December 2010;23(12):1225–1230.
15. Endocrine Certification in Neck Ultrasound Candidate Handbook and Application. American Association of
Clinical Endocrinologists; 2014. />16. Rao S, van Holsbeeck L, Musial JL, et al. A pilot study of comprehensive ultrasound education
at the Wayne State University School of Medicine: a pioneer year review. J Ultrasound Med. May
2008;27(5):745–749.
17. Hoppmann RA, Rao VV, Poston MB, et al. An integrated ultrasound curriculum (iUSC) for medical
students: 4-year experience. Crit Ultrasound J. April 2011;3(1):1–12.
18. Bahner DP, Royall NA. Advanced ultrasound training for fourth-year medical students: a novel
training program at The Ohio State University College of Medicine. Acad Med. February
2013;88(2):206–213.
19. Bahner DP, Adkins EJ, Hughes D, Barrie M, Boulger CT, Royall NA. Integrated medical school
ultrasound: development of an ultrasound vertical curriculum. Crit Ultrasound J. July 2, 2013;5(1):6.
20. Kory PD, Pellecchia CM, Shiloh AL, et al. Accuracy of ultrasonography performed by critical care
physicians for the diagnosis of DVT. Chest. 2011;139:538–554.
21. Frederiksen CA, Juhl-Olsen P, Andersen NH, Sloth E. Assessment of cardiac pathology by pointof-care ultrasonography performed by a novice examiner is comparable to the gold standard. Scand J
Trauma Resusc Emerg Med. December 13, 2013;21:87.

22. Martin LD, Mathews S, Ziegelstein RC, et al. Prevalence of asymptomatic left ventricular systolic
dysfunction in at-risk medical inpatients. Am J Med. January 2013;126(1):68–73.
23. Nguyen AT, Hill GB, Versteeg MP, Thomson IA, van Rij AM. Novices may be trained to screen for
abdominal aortic aneurysms using ultrasound. Cardiovasc Ultrasound. November 22, 2013;11(1):42.
24. Spencer KT, Kimura BJ, Korcarz CE, Pellikka PA, Rahko PS, Siegel RJ. Focused cardiac ultrasound:
recommendations from the American Society of Echocardiography. J Am Soc Echocardiogr. June
2013;26(6):567–581.

8.e1


C H A P T E R

2

Ultrasound Physics
Michael Mayette  n  Paul K. Mohabir

K E Y

POIN TS

• Understanding ultrasound physics is essential to acquire and interpret images accurately.
• Higher-frequency transducers produce higher-resolution images but penetrate shallower.
Lower-frequency transducers produce lower-resolution images but penetrate deeper.
• Basic modes of ultrasound include two-dimensional, M-mode, and Doppler.

Background
Ultrasound has been used for diagnostic purposes in medicine since the late 1940s, but
the history of ultrasound physics dates back

to ancient Greece. In the sixth century BC,
Pythagoras described harmonics of stringed
instruments, which established the unique
characteristics of sound waves. By the late
eighteenth century, Lazzaro Spallanzani had
developed a deeper understanding of sound
wave physics based on his studies of echolocation in bats. The field of ultrasonography would
not have evolved without an understanding
of piezoelectric properties of certain materials, as described by Pierre and Jacques Curie
in 1880.1 Multiple other milestones, such
as the invention of sonar by Fessenden and
Langevin following the sinking of the Titanic
and the development of radar by Watson-Watt,
improved our understanding of ultrasound
physics. Ultrasound use in medicine started in
the late 1940s with the works of Dr. George
Ludwig and Dr. John Wild2,3 in the United
States and Karl Theodore Dussik4 in Europe.
Modern ultrasound machines still rely on
the same original physical principles from
centuries ago, even though advances in technology have refined devices and improved
image quality. A thorough understanding of

ultrasound physics is essential to capture highquality images and interpret them correctly.
This chapter broadly reviews the physics of
ultrasound.

Principles
Sound waves are emitted by piezoelectric
material, most often synthetic ceramic material (lead zirconate titanate [PZT]), that is

contained in ultrasound transducers. When a
rapidly alternating electrical voltage is applied
to piezoelectric material, the material experiences corresponding oscillations in mechanical
strain. As this material expands and contracts
rapidly, vibrations in the adjacent material
are produced and sound waves are generated. Mechanical properties of piezoelectric
material determine the range of sound wave
frequencies that are produced. Sound waves
propagate through media by creating compressions and rarefactions of spacing between
molecules (Figure 2.1). This process of generating mechanical strain from the application
of an electrical signal to piezoelectric material
is known as the reverse piezoelectric effect. The
opposite process, or generation of an electrical
signal from mechanical strain of piezoelectric
material, is known as the direct piezoelectric
effect. Transducers produce ultrasound waves
9


10

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND

by the reverse piezoelectric effect, and reflected
ultrasound waves, or echoes, are received by the
same transducer and converted to an electrical signal by the direct piezoelectric effect. The
electrical signal is analyzed by a processor and,
based on the amplitude of the signal received,
a gray-scale image is displayed on the screen.
Key parameters of ultrasound waves include

frequency, wavelength, velocity, power, and
intensity.5
FREQUENCY AND WAVELENGTH
By definition, “ultrasound” refers to sound waves
at a frequency above the normal human audible
range (>20 kHz). Frequencies used in ultrasonography range from 2 to 18 MHz. Frequency
(f ) is inversely proportional to wavelength (λ)
and varies according to the specific velocity of

sound in a given tissue (c) according to the formula: λ = c/f. Two important considerations in
ultrasonography are the penetration depth and
resolution, or sharpness, of the image; the latter
is generally measured by the wavelength used.
For example, when wavelengths of 1 mm are
used, the image appears blurry when examined
at scales smaller than 1 mm. Ultrasound waves
with shorter wavelengths have higher frequency and produce higher-resolution images,
but penetrate to shallower depths. Conversely,
ultrasound waves with longer wavelengths have
lower frequency and produce lower-resolution
images, but penetrate deeper. The relationship
between frequency, resolution, and penetration
for a typical biologic material is demonstrated
in Figure 2.2. Maximizing axial resolution
while maintaining adequate penetration is a key
consideration when choosing an appropriate

Compression

Wavelength

Pressure

(ϩ)

Amplitude

Rarefaction

(Ϫ)

Figure 2.1  Sound waves propagate through media by creating compressions and rarefactions, corresponding with high- and low-density regions of molecules.

Wavelength (mm)
Figure 2.2  Relationship of ultrasound wave frequency, penetration,
and wavelength (image resolution).
High-frequency transducers produce higher-resolution images but
penetrate shallower. Low-frequency
transducers produce lower-resolution
images but penetrate deeper.

0.5

20

Wavelength (resolution)
Penetration

1.0

1.5
0

10

0 1 2.5 3

5

7.5

10

15

20

Penetration (cm)

30

0

0

Frequency (MHz)
Low-frequency
transducer

High-frequency

transducer


11

2—ULTRASOUND PHYSICS

Focal
zone

Focus

Far field

Axial resolution

Elevational
resolution

Near field

Lateral resolutio

n

Figure 2.3  Axial, lateral, and elevational
image resolution in relation to the ultrasound beam and display.

transducer frequency. Higher frequencies are
used in linear-array transducers to visualize

superficial structures, such as vasculature and
peripheral nerves. Lower frequencies are used
in curvilinear and phased-array transducers to
visualize deeper structures in the thorax, abdomen, and pelvis.
POWER AND INTENSITY
Average power is the total energy incident on a
tissue in a specified time (W). Intensity is the
concentration of power per unit area (W/cm2),
and intensity represents the “strength” of the
sound wave. The intensity of ultrasound waves
determines how much heat is generated in tissues. Heat generation is usually insignificant
in diagnostic ultrasound imaging but becomes
important in therapeutic ultrasound applications, such as lithotripsy (see “Safety”).

Resolution
Image resolution is divided into axial, lateral, elevational, and temporal components (Figure 2.3).
Axial resolution is the ability to differentiate two
objects along the axis of the ultrasound beam

and is the vertical resolution on the screen. Axial
resolution depends on transducer frequency.
Higher frequencies generate images with better axial resolution, but higher frequencies have
shallower penetration. Lateral resolution, or
horizontal resolution, is the ability to differentiate two objects perpendicular to the ultrasound
beam and is dependent on the width of the beam
at a given depth. Lateral resolution can be optimized by placing the target structure in the focal
zone of the ultrasound beam. The focal zone is
the narrowest portion of the ultrasound beam.
The ultrasound beam has a curved shape, and
the focal zone is the region of highest intensity

of the emitted beam. Lateral resolution decreases
as deeper structures are imaged due to divergence and increased scattering of the ultrasound
beam. Elevational resolution is a fixed property
of the transducer that refers to the ability to
resolve objects within the height, or thickness, of
the ultrasound beam. The number of individual
PZT crystals emitting and receiving ultrasound
waves, as well as their sensitivity, affects image
resolution, precision, and clarity. Temporal resolution refers to the clarity, or resolution, of moving structures. (See Chapter 3, Transducers, for
additional details about image resolution.)


12

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND

Refraction
Scattering
Transmission

Absorption

Reflection

Figure 2.4  Ultrasound waves are reflected, refracted, scattered, transmitted, and absorbed by tissues.

3
2

1

Electricity

4

5

Figure 2.5  Generation of ultrasound images. (1) Oscillating voltage is applied to piezoelectric crystals.
(2) Piezoelectric crystals vibrate rapidly, producing sound waves. (3) Ultrasound beam penetrates tissues.
(4) Echoes (reflected sound waves) return to transducer. (5) Echoes are converted to electrical signals that
are processed into gray-scale images.

Generation of Ultrasound
Images
Sound waves are reflected, refracted, scattered, transmitted, and absorbed by tissues
due to differences in physical properties of tissues (Figure 2.4). Ultrasound images are generated by sound waves reflected and scattered
back to the transducer. Transducers receive
and record the intensity of returning sound
waves. Specifically, mechanical deformation
of the transducer’s piezoelectric material generates an electrical impulse proportional to

the amplitude of these returning sound waves.
Electrical impulses cumulatively generate a
map of gray-scale points seen as an ultrasound
image. Depth of structures along the axis of
the ultrasound beam is determined by the
time delay for echoes to return to the transducer. The process of emitting and receiving
sound waves is repeated sequentially by the
transducer, resulting in a dynamic picture
(Figure 2.5). Reflection and propagation of
sound waves through tissues depend on two

important parameters: acoustic impedance
and attenuation.


13

2—ULTRASOUND PHYSICS

ACOUSTIC IMPEDANCE
Propagation speed is the velocity of sound in
tissues and varies depending on physical properties of tissues. Acoustic impedance is the resistance to propagation of sound waves through
tissues and is a fixed property of tissues determined by mass density and propagation speed
of sound in a specific tissue (Table 2.1). Differences in acoustic impedance determine
reflectivity of sound waves at tissue interfaces.
Greater differences in acoustic impedance
lead to greater reflection of sound waves. For
example, sound waves reflect in all directions,
or scatter, at air-tissue interfaces due to a large
difference in acoustic impedance between air
and bodily tissues. Scattering of sound waves at
air-tissue interfaces explains why sufficient gel
is needed between the transducer and skin to
facilitate propagation of ultrasound waves into
the body. Ultrasound machines are calibrated
to rely on small differences in impedance
because only 1% of sounds waves are reflected
back to the transducer. The majority of sound
waves (99%) do not return to the transducer.
ATTENUATION
As sound waves travel through tissues, energy

is lost, and this loss of energy is called attenuation. Attenuation is due to absorption, deflection, and divergence of sound waves and is
dependent on the attenuation coefficient of
tissues, frequency of sound waves, and distance
traveled by sound waves.9 Absorption, the most
important cause of attenuation, refers to energy
transferred from the ultrasound beam to tissues
as heat. Heat production is an important safety
limitation of ultrasonography,10 and each type
of tissue has an intrinsic attenuation coefficient
(Table 2.2). Absorption is the most important

determinant of depth of ultrasound wave penetration. High-frequency sound waves are
more readily absorbed and therefore penetrate
shallower compared to low-frequency sound
waves. Deflection, a second cause of attenuation, refers collectively to reflection, refraction,
and scattering of energy within tissues. Deflection results in a reduction in echo amplitude,
especially when the observed interfaces between
tissues is not perpendicular to the beam. Divergence refers to loss of ultrasound beam intensity as the beam widens, and a fixed amount
of acoustic energy is spread over a wider beam.
Attempts at overcoming attenuation can be
made by increasing the gain, or amplifying the
signal in postprocessing. However, increasing
gain affects both signal and noise. Adjusting
gain only manipulates the computer-generated
image and does not improve signal quality.

Modes
Multiple modes of ultrasound imaging have
been developed to enhance image acquisition. Here we discuss two-dimensional (2-D),
M-mode, color flow Doppler, and spectral

Doppler.

TABLE 2.2  n  Attenuation Coefficients
of Different Materials
Tissue or Material
Water
Blood
Soft tissues
Air
Bone

Attenuation
(dB/cm/MHz)
0.0022
0.15
0.75
7.50
15.00

TABLE 2.1  n  Acoustic Impedance of Different Tissues6–8

Tissue or Material

Density (g/cm3)

Speed of Sound (m/s)

Acoustic
Impedance
(kg/(s m2)) × 106

Air
Fat
Blood
Liver
Bone
Metal (e.g., titanium)

0.001225
0.95
1.055
1.06
1.9
4.5

340
1450
1575
1590
4080
5090

0.0004
1.38
1.66
1.69
7.75
22.9

14

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND

TWO-DIMENSIONAL MODE
Two-dimensional (2-D) mode is the default
mode of most ultrasound machines, and the
majority of bedside diagnostic ultrasound imaging is performed in 2-D mode. This mode is
also called B-mode, for “brightness,” because
echogenicity, or brightness, of observed structures depends on the intensity of reflected signals. Structures that transmit all sound waves
without reflection are called anechoic and appear
black on ultrasound. Most fluid-filled structures appear anechoic. Structures that reflect
some sound but less than surrounding structures appear hypoechoic, whereas structures
that reflect sound waves similar to surrounding
structures appear isoechoic. Both hypoechoic
and isoechoic structures appear as shades of gray
and are generally soft tissue structures. Hyperechoic structures reflect most sound waves and
appear bright white on ultrasound. Calcified
and dense structures, such as the diaphragm
or pericardium, are hyperechoic. Some hyperechoic structures, such as bones, create shadows
due to near-total reflection of sound waves and
often preclude visualization of distal structures.
Figure 2.6 illustrates different tissue echogenicities in the right upper quadrant of the abdomen.
M-MODE
M-mode, or “motion” mode, is an older mode
of imaging but is still frequently used today to
analyze movement of structures over time.11
After a 2-D image is acquired, M-mode imaging is applied along a single line within the 2-D
image. A single-axis beam is emitted along a

Anechoic
(blood vessel)

select line and gathers data on movement of all
tissues along that line. All points on the line
are plotted over time to evaluate the dimensions of cavities or movement of structures.
For example, M-mode is used to measure the
size of cardiac chambers or movement of cardiac valves throughout the cardiac cycle. Other
frequent point-of-care applications include
measurement of change in inferior vena cava
diameter with respiration, and evaluation of
the lung-pleura interface to rule out pneumothorax. Figure 2.7 is an example of M-mode
imaging.
DOPPLER IMAGING
The Doppler effect is a shift in frequency of
sound waves due to relative motion between
the source and observer.12 The primary source
of sound waves is the transducer, and the same
transducer is the observer for returning echoes.
Movement of tissues, such as blood flow, produces a shift in frequency of returning sound
waves. Blood flow moving toward the transducer shifts the echoes to a higher frequency
while blood flow moving away from the transducer shifts the echoes to a lower frequency
(Figure 2.8). The change in frequency between
the emitted and received sound waves is called
Doppler shift.13 Variables that determine the
amount of Doppler shift are:
1.
Frequency of ultrasound beam
2.
Velocity of blood flow

3.
Angle of insonation
The angle of insonation, or angle between
the ultrasound beam and direction of the measured flow, is critical (Figure 2.9). No Doppler
shift can be measured when the ultrasound
beam is perpendicular to the direction of blood

Hypoechoic

Isoechoic

Hyperechoic

Figure 2.6  Two-dimensional (B-mode) ultrasound
image with isoechoic, hypoechoic, and hyperechoic
tissues.

Figure 2.7  M-mode imaging of the left ventricular
chamber.


15

2—ULTRASOUND PHYSICS
Stationary
transducer

Stationary reflector

No change

in frequency
Reflector moving
toward transducer

Frequency increased
Reflector moving
away from transducer

Figure 2.8  Doppler shift in frequency of
sound waves. Movement of the source
or reflector of sound waves toward
or away from transducer causes an
increase or decrease in sound wave
frequency, respectively.

Frequency decreased

Angle of insonation
Beam

␪
Higher Doppler frequency obtained if:
1. Velocity is increased
2. Beam is aligned parallel to
flow direction
3. Higher frequency is used

flow. Ideally, the ultrasound beam should be
placed parallel to the direction of blood flow,
but a near-parallel intercept angle is more often

achievable. Angling the ultrasound beam toward
the direction of blood flow causes a positive
Doppler shift, whereas angling the ultrasound
beam away from the direction of blood flow
causes a ­negative Doppler shift (Figure 2.10). A
correctional factor for the angle of insonation is
used in the Doppler equation to better estimate
velocities.
Spectral Doppler
Doppler effect may be represented graphically using velocity (y-axis) plotted over time

Figure 2.9  Doppler interrogation and
angle of insonation.

(x-axis) in a display method called spectral
Doppler (Figure 2.11). By convention, the
frequency shifts displayed above the baseline
represent velocities toward the transducer, and
shifts below baseline represent velocities moving away from the transducer. Spectral Doppler
permits quantitative assessment of velocities
and is divided into two types: pulsed wave and
continuous wave.
Pulsed-wave Doppler refers to the emission of sound waves in pulses that allows
measurement of Doppler shift at certain
depths. After a pulsed signal is sent into tissues, the transducer must await the returning
echo before emitting another pulse. This cycle


16

1—FUNDAMENTAL PRINCIPLES OF ULTRASOUND
A

B

C

Skin
Beam
direction
Vessel
Flow

Figure 2.10  Relationship of angle of insonation and Doppler effect. Aligning the
ultrasound beam parallel with the direction of flow increases Doppler frequency.

Figure 2.11  Spectral Doppler imaging of femoral
artery velocities (using pulsed Doppler).

of emitting a wave into tissues and capturing the returning echo is repeated rapidly at
a rate called pulse repetition frequency (PRF).
Ideally, the maximum possible PRF is used;
however, the maximum PRF is determined by
wave travel time, and wave travel time is limited by tissue depth. Deeper depths require
longer wait times for returning echoes, reducing the maximum PRF before ambiguous
signaling, or aliasing, occurs. When aliasing
occurs, the true velocity and vector direction cannot be determined. The maximum
Doppler frequency or velocity that can be
measured before aliasing occurs is called the
Nyquist limit. The Nyquist limit is one half

of the PRF because ultrasound waveforms
must be sampled at least twice per wavelength to reliably assess velocity and direction

A

C

B
Doppler

(Figure 2.12).14 Significance of the Nyquist
limit is exemplified by severe aortic stenosis.
The aortic valve is a relatively deep structure,
which limits the the PRF and makes accurate
measurement of the high velocities of severe
aortic stenosis difficult. Techniques to avoid
aliasing include maximizing the PRF to raise
the Nyquist limit, reducing imaging depth,
selecting a lower-frequency transducer, or
switching to continuous-wave Doppler imaging. The advantage of pulsed-wave Doppler is
reduced interference from surrounding structures, but its main disadvantage is susceptibility to aliasing because of the Nyquist limit.
In contrast, continuous-wave Doppler
allows measurement of blood velocities along
the entire ultrasound beam. These transducers have two different sets of piezoelectric
crystals to continuously emit and receive
signals, and therefore PRF has no limit and
aliasing does not occur. Continuous-wave
Doppler is mostly used to measure high
velocities, such as in patients with aortic
stenosis, that pulsed-wave Doppler cannot

accurately measure. The main limitation of
continuous-wave Doppler is the inability to
measure velocities at specific depths because
Doppler signals are received from all tissues
in the path of the ultrasound beam. In both
pulsed- and continuous-wave Doppler imaging, the accuracy of measurements depends
on signal quality, which is determined by
the sharpness of peaks of the curves used to
determine velocities.