A brief on Ultrasound Imaging for Biomedical Engineers

1. MEDICAL IMAGING
ULTRASOUND (US)
IMAGING TECHNIQUE
BY
DR RAMY ABDLATY

2. CONTENTS
• I. Introduction to Ultrasound
• A. Overview
• B. Basic Principles
• C. Theory of Ultrasound
• II. Types of Ultrasound Machines
• A. Portable or handheld ultrasound machines
• B. Cart-based ultrasound machines
• C. 3D/4D ultrasound machines
• III. Ultrasound Probes
• A. Transducer types (linear, convex, phased array, etc.)
• B. Frequency ranges and applications
• C. Transducer care and maintenance
• IV. Comparison Between Ultrasound and Other Radiology Techniques
• A. X-ray imaging
• B. Magnetic Resonance Imaging (MRI)
• C. Computed Tomography (CT)

3. I- INTRODUCTION TO
ULTRASOUND

4. I. A- OVERVIEW OF ULTRASOUND
(US)
• US is the most popularly used diagnostic
imaging technique.
• It accounts for ~25% of the imaging
examinations performed in the entire world.
• US waves are produced by a rapid push–pull
action of a probe (transducer) held against a
material (medium) such as tissue in human
being.
• Sound waves at frequencies less than about
20KHz are audible for human ears, above this
the threshold ultrasonic term is employed.
• In medical ultrasound, frequencies in the
range 20KHz to 50 MHz are used.

5. I. B- BASIC PRINCIPLES OF US
• US frequency is dictated by a trade-off between spatial
resolution and penetration depth, since higher frequency
waves can be focused more tightly but are attenuated
more rapidly by tissue.
• Audible acoustic waves are produced by a vibrating
source on air (vocal cords, loudspeaker, musical
instruments, machinery).
• In medical US the source is one or multiple piezoelectric
crystal mounted in a hand-held case and driven by
fluctuating voltage.
• Conversely, when US waves strike a piezoelectric crystal
causing it to vibrate, electrical voltages are generated
across the crystal, hence the US echo is said to be
detected.
• The hand-held transducers contain piezoelectric crystals
and some electronics. They convert electrical energy to
mechanical form and vice versa. They are fragile and
expensive.

6. I. B- BASIC PRINCIPLES OF US
• The medical US output are in the form of pulses or continuous waves . For a
continuous wave an alternating voltage is applied continuously whereas for a
pulsed wave it is applied for a short time.
• The basic data for most ultrasound techniques is obtained by detecting the echoes
which are generated by reflection or scattering of the transmitted ultrasound at
changes in tissue structure within the body

7. I. B- BASIC PRINCIPLES OF US
• The transducer causes regions of compression and rarefaction to
pass out from its face into the tissue. A waveform can be drawn to
represent these regions of increased and decreased pressure for an
ultrasound wave.
• The distance between equivalent points on the waveform is called
the wavelength and the maximum pressure fluctuation is the wave
amplitude. The number of oscillations per second is the frequency
of the wave.
• A transducer with a flat face will generate regions of equal
compression or rarefaction in planes. therefore, Plane waves or
wave-fronts are generated.
• The convex or concave transducer generates a convex or concave
wave-front. The latter can be used to provide a focused region at a
specified distance from the transducer face.
• The speed with which the wave passes through the tissue is very
high close to 1540 m/s for most soft tissue

8. I. C- THEORY OF US WAVES
• A pressure plane wave, p (x,t), propagating along one spatial dimension, x, through a homogeneous,
non-attenuating fluid medium can be formulated by using Euler’s equation and the Equation of
continuity.
• The strength of an US wave can be characterized by its intensity I, which is the average power per unit
cross sectional area evaluated over a surface perpendicular to the propagation direction. For acoustic
plane waves , the intensity is related to the pressure amplitude by:

9. I. C- THEORY OF US WAVES

10. I. C- THEORY OF US WAVES
The strength of an US wave can be characterized by its intensity I, which is the average power per unit cross
sectional area evaluated over a surface perpendicular to the propagation direction. For acoustic plane
waves , the intensity is related to the pressure amplitude by:

11. I. C- THEORY OF US WAVES
 The dB notation was employed in the past when it was
fairly difficult to measure absolute values of intensity
and power in units of mW/cm2 or mW respectively.
 The dB notation is essentially a historical hangover and
does not currently exist in machines designed for clinical
application.
 The recent machines have been related to possible
biological effects, in particular heating and cavitation.
 Cavitation is the violent response of bubbles when
subjected to the pressure fluctuations of an US wave.
Thermal (TI) and mechanical indices (MI) relate to these
phenomena and are displayed on screen.
MI & TI
Acoustic
Power
PRF
Frequenc
y
(1) Acoustic power is the primary determinant of TI
and MI, but the (2) US mode, (3) color Doppler blood
flow, (4) transmission frequency, and (5) pulse
repetition frequency (PRF), are also considered
control factors

12. I. C- THEORY OF US WAVES
REFLECTION AND TRANSMISSION

13. I. C- THEORY OF US WAVES
REFLECTION AND TRANSMISSION

14. I. C- THEORY OF US WAVES
ATTENUATION OF US WAVES

15. I. C- THEORY OF US WAVES
ATTENUATION OF US WAVES

16. I. C- THEORY OF US WAVES
ATTENUATION OF US WAVES

17. I. C- THEORY OF US WAVES
ATTENUATION OF US WAVES

18. I. C- THEORY OF US WAVES
ATTENUATION OF US WAVES

19. I. C- THEORY OF US WAVES
ATTENUATION OF US WAVES
330
1480 1455 1562.5 1555 1575
3720
0.4
1480 1360
1650 1630 1665
6900
12000
2.2
520
960
170
1200
11300
0
2000
4000
6000
8000
10000
12000
Air Water Fat Liver Blood Muscle Skull bone
Tissue Acoustic Properties
Average Sound speed (m/s) Acoustic Impedance (KRayl) Attenuation Coefficient (milli dB/cm @1MHz)

20. I. C- THEORY OF US WAVES
DISPLAY TECHNIQUES
 A-mode scanning: records the amplitude of
returning echoes from the tissue boundaries with
respect to time. In this mode of imaging the
ultrasound pulses are sent in the imaging medium
with a perpendicular incident angle.
 B-mode scanning: provides 2-dimensional images
representing changes in acoustic impedance of the
tissue.
 M-mode scanning: Provides information about the
variations in signal amplitude due to object motion

21. US IMAGES FOR HEART
Apical four-chamber (a) and five-chamber (b) views. The left (LV) and
right (RV) ventricles are easily identified, as are the left (LA) and right
(RA) atria. The tricuspid (TV) and mitral (MV) valves are closed in these
images during early systole. The five-chamber view allows viewing of
the aortic outflow tract and the aortic valve (AV).
M-mode echocardiogram via the
right (RV) and left (LV) ventricles.
The line of interrogation is shown
on the small 2D view at the top,
with the resulting M-mode at the
bottom. The right ventricular free
wall (RVW), interventricular septum
(IVS), and posterior left ventricular
wall (PW) are identified , and the
chamber dimensions can be
measured in either systole or
diastole . This view includes the
leaflets of the mitral valve (MVL).

22. I. C- THEORY OF US WAVES
RESOLUTION
 Axial resolution: the ability to
differentiate structures on axis with the
ultrasound beam.
 Lateral resolution: the ability to
differentiate structures side by side within
the ultrasound beam in the image plane.
 Transverse resolution: the ability to
differentiate structures side by side within
the ultrasound beam across the image
plane
 Contrast resolution: the ability to
differentiate closely grouped bright
reflectors

23. II- TYPES OF
ULTRASOUND MACHINES

24. II. A- PORTABLE OR HANDHELD US MACHINES
1. Hand-held portable ultrasound units cost approximately $2K to $10K, and
smaller handheld devices could further improve accessibility.
2. Hand-held ultrasound devices can potentially have a positive effect in
medical education and patient care, bringing ultrasound to classrooms,
clinics, sidelines of the playing field, the battle ground, rural locations, and
countries with limited resources.
3. Recently, ultrasound equipment has been developed that includes hand-
held devices, where a transducer is connected to a tablet or phone to view
images.
4. Such equipment has been used in several applications, such as trauma,
cardiorespiratory assessment, and invasive procedures.
5. One limitation of the portable hand-held ultrasound unit was the low
sensitivity of the color Doppler compared with cart-based ultrasound.
6. Another limitation of the portable ultrasound equipment was difficulty in
identifying small calcifications
Hand-Held Portable Versus Conventional Cart-Based Ultrasound in Musculoskeletal Imaging, Anna L. Falkowski, MD, Jon A. Jacobson, MD, Michael
T. Freehill, MD, and Vivek Kalia, MD, Investigation performed at the University of Michigan, Ann Arbor, Michigan, USA

25. II. A- PORTABLE OR HANDHELD US MACHINES
ACEP_0719_pg14b.png (1200×859) (acepnow.com)

26. II. B- CART-BASED US MACHINES
• Conventional cart-based ultrasound equipment has been used, producing
detailed high-resolution images; however, the cost of such equipment (often
>$100,000 US) and lack of portability can be significant limitations.
• Cart-based US is used for the following purposes:
• Diagnose injuries with precision
• Guide treatment to the correct location with technology that aids in needle placement
• Monitor patient progress and response to therapy with clear and effective tools
• Cart-based US machine Assembly:
• Display: to show the physician the formed image by US imaging
• Transducer: It is the terminal that transfer the electrical energy into sonic one and vice
versa
• Pulse Controls: they tune the sound wave parameters (amplitude, frequency,…etc.)
• Keyboard: It is used to enter the patient’s data, add notes, and move the cursor to
various locations.
• CPU: It is responsible for processing the formed images
• Disk Storage: It is used to save the captured image by the physician during scanning
for advanced examination.

27. II. C- 3D/4D US MACHINES
• Further development of ultrasound technology led to the acquisition of
volume data, then integrated by high-speed computing software to
produce a three-dimensional (3D) image. The technology behind 3D
ultrasound thus has to deal with image volume data acquisition, volume
data analysis, and volume display.
• Volume data is acquired using three techniques:
1. Freehand movements of the probe, with/out position sensors to form the
images.
2. Mechanical sensors built into the probe head.
3. Matrix array sensors, which use one single sweep to acquire a
considerable amount of data, followed by data analysis that is used to
provide a 3D image. The operator can then extract any view or plane of
interest, which helps to visualize the structures in terms of their
morphology, size, and relationship with each other.
• There is also a tomographic mode which allows the viewing of
numerous parallel slices in the transverse plane from the 3D or four-
dimensional (4D) data set.

28. II. C- 3D/4D US MACHINES
• Advantages of 3D/4D ultrasound
1. Shorter time for fetal heart screening and diagnosis.
2. Volume data storage for screening, expert review, remote diagnosis in remote areas, and
teaching.
3. Enhanced parental bonding with the baby.
4. Healthier behavior during pregnancy as a result of seeing the baby in real-time and in 3D.
5. More support by the father after visualizing the baby’s form and movement.
6. Possibly accurate identification of fetal anomalies, those involving face, heart, limbs, and
skeleton.
7. In addition, these advanced ultrasound techniques share the benefits of 2D ultrasound.
• Disadvantages of 3D/4D ultrasound
1. Expensive machinery.
2. Longer training required to operate.
3. Volume data acquired may be lower-quality in the presence of fetal movements, which
will affect all later planes of viewing.
4. If the fetal spine is not at the bottom of the scanned field, sound shadows may hinder the
view.

29. II. C- 3D/4D US MACHINES
• 3D Ultrasound
• It is a more advanced technology in US imaging, which shows
3D images of the fetus or the heart. 3D US machines show a
more detailed picture than 2D US.
• 4D Ultrasound
• 4D is similar to 3D US machines, however, the main difference
is that the generated image is continuously updated. 4D is a
live stream of 3D image.
• When used in pregnancy scanning, it enables patients to watch
their baby in live motions. 4D US is usually performed to
discover structural congenital anomalies of the fetus.
• 5D Ultrasound
• 5D US machine technology focuses on workflow and
automation. 5D US images are of higher quality than traditional
machines, therefore, it gives the baby a life-like flesh tone color.
Most importantly, the medical facts of the fetus are clear and
abnormalities are more visible due to the high-quality images.

30. III-ULTRASOUND PROBES

31. III. A- TRANSDUCER TYPES
• Overview of the US transducer
1. The basic US transducer is composed of the head, the wire, and the connector.
2. In most machines, the transducer is interchangeable by detaching it completely from
the US machine base.
3. Many point-of-care (POC) US machines can be fitted with a transducer connector that
allows practitioners to select the appropriate probe for a study by simply pressing
button or touching the probe icon on a screen.
4. It is important to know the standard names given to the various parts of the machine
and probes. The tip of the probe head is referred to as the footprint. The footprint is
the part of the probe that is in direct contact with the patient through an acoustic
window (eg, US gel).
5. Larger footprints provide a more expansive scanning area. Smaller footprints are
preferred for examinations that require maneuvering of the probe in smaller anatomic
regions.
6. Piezoelectric crystals are located at the footprint of the probe and arranged according
to the shape of the probe tip. The footprint is a transmitter and receiver of the US beam
during scanning. Most modern probes use synthetic plumbium zirconium titanate (PZT),
compared with quartz crystals that were used in earlier units. PZT crystals can be
damaged or misaligned when probes are dropped, crushed, or thrown against other
objects.

32. III. A- TRANSDUCER TYPES
• Curvilinear Transducer
1. The curvilinear or convex array probe has a frequency range of 2
to 5 MHz.
2. It provides a wide, fan-shaped scanning area on the US screen.
This type of transducer is mostly used for evaluating deep
structures in the abdomen and pelvis.
3. Common clinical scenarios for this type of probe are patients with
abdominal pain to evaluate for gallbladder pathology, abdominal
pain in pregnancy, or the focused assessment with sonography in
trauma (FAST examination).
4. The intracavitary probe also has a curvilinear crystal array with a
wide view. However, the frequency is much higher (8–13 MHz)
than other curved probes. Because of the higher frequency, the
resolution of the images is better.

33. III. A- TRANSDUCER TYPES
• Linear Transducer
1. Linear transducer has a rectangular footprint shape
with a frequency range of 6 to 15 MHz.
2. This probe provides detailed anatomic resolution and is
ideal for evaluating superficial structures.
3. A wide variety of pathology can be seen at the bedside
with this type of probe, such as deep venous
thrombosis, musculoskeletal trauma, subcutaneous
foreign bodies and abscesses, testicular torsion,
pneumothorax, and ocular pathology.
4. The linear array probe can also be used to guide such
procedures as venous access (central and peripheral);
needle aspirations; and lumbar punctures.

34. III. A- TRANSDUCER TYPES
• Phased-array Transducer
1. The phased or sector array transducer has a frequency range of
1 to 5 MHz.
2. The crystal arrangement in the footprint is bundled in the
center and fans out creating a pielike image on the US machine
screen.
3. Because of the smaller footprint, this probe is commonly used
for echocardiography and is particularly useful in the
evaluation of pediatric patients.
4. The phased array probe can also be used for the FAST
examination in patients with tight intercostal spaces.

35. III. B- TRANSDUCER COMPARISON

36. IV-COMPARISON BETWEEN
ULTRASOUND &
RADIOLOGY TECHNIQUES

37. IV. A- ULTRASOUND VS. X-RAY
Aspect Ultrasound X-ray
Wave nature Mechanical wave Electromagnetic wave
Wave speed in space 330 m/s 300,000,000 m/s
Propagation Vs Oscillation In the same direction
(Longitudinal)
Perpendicular (Transverse)
Ionization Non-ionizing Ionizing
Main Tissue of interest Soft tissues Bones
Safety Safe Unsafe
Propagation media Requires a medium Propagate in space
Cost Less expensive Less expensive

38. IV. B- ULTRASOUND VS. MAGNETIC RESONANCE IMAGING (MRI)
Aspect Ultrasound MRI
Wave nature Mechanical wave Magnetic field + radio frequency
signal
Ionization Non-ionizing Non-ionizing
Main Tissue of interest Soft tissues Bones +Soft tissues
Safety Safe Safe
Cost Less expensive More expensive
Time of scanning Short Long
Limitation of Use Can’t get images through bones Can’t be used for:
1- claustrophobic people
2- patients with metallic parts
3- overweighed people
Image Resolution Low resolution High resolution
Portability Portable-movable Fixed in the room
Imaging plane Images are taken usual in one Images are taken in different planes

39. IV. B- ULTRASOUND VS. COMPUTED TOMOGRAPHY (CT)
Aspect Ultrasound CT
Wave nature Mechanical wave Electromagnetic wave
Ionization Non-ionizing Ionizing
Main Tissue of interest Soft tissues Bones +Soft tissues
Safety Safe Unsafe
Cost Less expensive More expensive
Time of scanning Short Short
Image Resolution Low resolution High resolution
Portability Portable-movable Fixed in the room
Imaging plane Images are taken usual in one
plane
Images are taken in different planes