Medical Imaging Ultrasound

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Medical ImagingUltrasound

• Edwin L. Dove
• 1412 SC
• edwin-dove_at_uiowa.edu
• 335-5635

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3D Reconstruction
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Ultrasound Principle
When you shout into a well, the sound of your
shout travels down the well and is reflected
(echoes) off the surface of the water at the
bottom of the well. If you measure the time it
takes for the echo to return and if you know the
speed of sound, you can calculate the depth of
the well fairly accurately.
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Ultrasound Principle

• Ultrasound is sound having a frequency greater
  than 20,000 cycles per second, that is, sound
  above the audible range
• Medical ultrasound is sound having a frequency
  greater than 2-100 MHz
• Medical ultrasound imaging is sound that is
  converted to an image

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Medical Ultrasound

• Advantages of acoustic energy
• can be directed in a beam
• obeys the laws of reflection and refraction
• reflected off object borders
• no known unwanted health effects

• Disadvantages of acoustic energy
• propagates poorly through a gaseous medium
• reflected off of borders of small objects
• quickly dissipates (as heat)

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Why Ultrasound in Cardiology?

• Portable, relatively cheap
• Non-ionizing
• During the echocardiogram, it is possible for the
  cardiologist to
• Watch the hearts motion in 2D real-time
• Ascertain if the valves are opening and closing
  properly, and view any abnormalities
• Determine the size of the heart chambers and
  major vessels
• Measure the thickness of the heart walls
• Calculate standard metrics of health/disease
• e.g., Volume, EF, SV, CO
• Dynamic evaluation of abnormalities

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Ultrasound Theory

• Pressure (ultrasound) wave produced by vibrating
  source
• Listen for reflection
• Build image by sending wave in different
  directions

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Sinusoidal pressure source
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Quantitative Description

• p pressure
• applied in
• z-direction
• density
• ? viscosity

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Speed of Sound in Tissue

• The speed of sound in a human tissue depends on
  the average density ? (kgm3) and the
  compressibility K (m2N-1) of the tissue.

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Sound Velocity for Various Tissues
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Tissue Characteristics

• Engineers and scientists working in ultrasound
  have found that a convenient way of expressing
  relevant tissue properties is to use
  characteristic (or acoustic) impedance Z (kgm-2
  s-1)

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Pressure Generation

• Piezoelectric crystal
• piezo means pressure, so piezoelectric means
• pressure generated when electric field is applied
• electric energy generated when pressure is applied

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Charged Piezoelectric Molecules
Highly simplified effect of E field
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Piezoelectric Effect
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Piezoelectric Principle
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Vibrating element
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Transducer Design
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Transducer
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Reflectance and Refraction
Snells Law
(Assumes ?i ?r)
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Reflectivity
At normal incidence, ?i ?t 0 and
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Reflectivity for Various Tissues
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Echos
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Specular Reflection

• The first, specular echoes, originate from
  relatively large, strongly reflective, regularly
  shaped objects with smooth surfaces. These
  reflections are angle dependent, and are
  described by reflectivity equation . This type of
  reflection is called specular reflection.

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Scattered Reflection

• The second type of echoes are scattered that
  originate from small, weakly reflective,
  irregularly shaped objects, and are less
  angle-dependent and less intense. The
  mathematical treatment of non-specular reflection
  (sometimes called speckle) involves the
  Rayleigh probability density function. This type
  of reflection, however, sometimes dominates
  medical images, as you will see in the laboratory
  demonstrations.

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Circuit for Generating Sharp Pulses
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Pressure Radiated by Sharp Pulse
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Ultrasound Principle
When you shout into a well, the sound of your
shout travels down the well and is reflected
(echoes) off the surface of the water at the
bottom of the well. If you measure the time it
takes for the echo to return and if you know the
speed of sound, you can calculate the depth of
the well fairly accurately.
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Ultrasound Principle
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Echoes from Two Interfaces
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Echoes from Internal Organ
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Attenuation

• Most engineers and scientists working in the
  ultrasound characterize attenuation as the
  half-value layer, or the half-power distance.
  These terms refer to the distance that
  ultrasound will travel in a particular tissue
  before its amplitude or energy is attenuated to
  half its original value.

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Attenuation

• Divergence of the wavefront
• Elastic reflection of wave energy
• Elastic scattering of wave energy
• Absorption of wave energy

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Ultrasound Attenuation
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Attenuation in Tissue

• Ultrasound energy can travel in water 380 cm
  before its power decreases to half of its
  original value. Attenuation is greater in soft
  tissue, and even greater in muscle. Thus, a
  thick muscled chest wall will offer a significant
  obstacle to the transmission of ultrasound.
  Non-muscle tissue such as fat does not attenuate
  acoustic energy as much. The half-power distance
  for bone is still less than muscle, which
  explains why bone is such a barrier to
  ultrasound. Air and lung tissue have extremely
  short half-power distances and represent severe
  obstacles to the transmission of acoustic energy.

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Attenuation

• As a general rule, the attenuation coefficient is
  doubled when the frequency is doubled.

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Pressure Radiated by Sharp Pulse
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Beam Forming

• Ultrasound beam can be shaped with lenses
• Ultrasound transducers (and other antennae) emit
  energy in three fields
• Near field (Fresnel region)
• Focused field
• Far field (Fraunhofer region)

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Directing Ultrasound with Lens
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Beam Focusing

• A lens will focus the beam to a small spot
  according to the equation

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Linear Array
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Types of Probes
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Modern Electronic Beam Direction
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Beam Direction (Listening)
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Wavefronts Add to Form Acoustic Beam
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Phased Linear Array
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A-mode Ultrasound
Amplitude of reflected signal vs. time
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A-mode
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M-mode Ultrasound
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M-mode
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B-mode Ultrasound
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Fan forming
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B-mode Example
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Cardiac Ultrasound
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Standard Sites for Echocardiograms
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Conventional Cardiac 2D Ultrasound
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Short-axis Interrogation
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B-mode Image of Heart
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Traditional Ultrasound Images
End-diastole
End-systole
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B-mode
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Ventricles
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Mitral stenosis
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Results Possible from Echo
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Geometric problems
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New developments of Phase-arrays
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2D Probe Elements
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Recent 2D array

• 5Mz 2D array from Stephen Smiths laboratory,
  Duke University

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2D and 3D Ultrasound
a. Traditional 2D
b, c. New views possible with 3D
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3D Pyramid of data
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3D Ultrasound

• 2D ultrasound transmitter
• 2D phased array architecture
• Capture 3D volume of heart
• 30 volumes per second

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3D Ultrasound
Traditional 2D
New 3D
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Real-time 3D Ultrasound
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Real-time 3D Ultrasound
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Velocity of Contraction
Normal
Abnormal
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Normal artery
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Progression of Vascular Disease
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CAD
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Severe re-canalization
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Intravascular Ultrasound (IVUS)

• Small catheter introduced into artery
• Catheter transmits and receives acoustic energy
• Reflected acoustic energy used to build a picture
  of the inside of the vessel
• Clinical assessment based on vessel image

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IVUS Catheter

• 1 - Rotating shaft
• 2 - Acoustic window
• 3 - Ultrasound crystal
• 4 - Rotating beveled acoustic mirror

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Slightly Diseased Artery in Cross-section
Catheter
Plaque
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An array of Images
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3D IVUS
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Doppler Principle
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Doppler
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Doppler measurements
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Doppler angle
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Normal flow
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Diseased flow
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Blood Flow Measurements