Ultrasound Imaging

 

Introduction

  • Ultrasound is sound waves with frequencies which are higher than those audible to humans (>20,000 Hz).
  • Ultrasonic images, also known as sonograms, are made by sending pulses of ultrasound into tissue using a probe. 
  • The sound echoes off the tissue; with different tissues reflecting varying degrees of sound. These echoes are recorded and displayed as an image to the operator.
  • Frequencies in the range of 1 MHz to 20 MHz are used in diagnostic ultrasound.
  • Many different types of images can be formed using sonographic instruments. The most well-known type is a B-mode image, which displays the acoustic impedance of a two-dimensional cross-section of tissue. Other types of image can display blood flow, motion of tissue over time, the location of blood, the presence of specific molecules, the stiffness of tissue, or the anatomy of a three-dimensional region.

Ultrasound

Sound Wave Characteristics

  • Perio
  • Frequency
  • Wavelength
  • Velocity (propagation speed)
  • Amplitude/Intensity

Sound Wave Characteristics

  • Period 
    • Seconds per Cycle (units = μsec)
  • Frequency
    • Cycles per second (= Hertz, Hz)
      • Human Hearing 20 - 20,000 Hz
      • Ultrasound > 20,000 Hz
      • Diagnostic Ultrasound 1 to 20 MHz (this is what we use!)
    • Inverse of Period
    • Major Factor in Determining Depth of Beam Penetration
      • High frequency (5-10 MHz) 
        • Less penetration, greater resolution 
        • Shallow structures: vascular, abscess, t/v gyn, testicular
      • Low frequency (2-3.5 MHz) 
        • Greater penetration, less resolution 
        • Deep structures: Aorta, t/a gyn, card, gb, renal 

Period

  • Velocity (propagation speed)
    • Velocity of wave (= m/s)
    • Sound is energy transmitted through a medium
    • Each medium has a constant velocity of sound (c)
    • Product of frequency (f) and wavelength (λ) , c=f.λ
    • Frequency and Wavelength therefore are directly proportional- if the frequency increases the wavelength must decrease.
    • Propagation velocity
      • Increased by increasing stiffness
      • Reduced by increasing density
    • Function of medium
      • Air = 331 m/s
      • Water = 1430 m/s
      • Soft tissue = 1540 m/s
      • Bone = 4080 m/s

Propagation Velocity

Amplitude/Intensity Height of wave Function of source (transducer) Major determinant of power output Intensity is proportional to amplitude Intensity = power/area (units = mw/cm2) Theoretical concern of bioeffects of US mediated by amplitude/power

Amplitude

Physics of sound

Speed of Sound

  • Speed of sound in ‘norm‘ tissue:

c = 1540 m/s

  • Typical wavelength (f = 1..40 MHz):

λ = c / f                 λ = 1.54 .. 0.04 mm

  • Strong absorption (frequency dependent):

0.5..1.0 dB/cm/MHz

Target Characteristics

  • Echogenic (=bright)
    • Large reflection component
    • Waves returning to transducer
  • Anechoic or Hypoechoic (=dark)
    • Large attenuation component
    • Waves not returning to transducer
  • Mixed echogenicity

Physics of sound

Acoustic Window

 

  • Allows sound waves to penetrate into the body
  • Good acoustic windows
    • Liver, spleen
    • Urine-filled bladder
  • Poor acoustic windows
    • Gas (intestines, lung)
    • Strong reflectors (bone)

 

  • Acoustic impedance of a material is the product of its density and propagation velocity (For plane wave) 
  • Z  indicates resistance of the medium to being disturbed by the wave, and is location dependent
  • Differences in acoustic impedance create reflective interfaces that echo the u/s waves back at the probe

Acoustic Impedance

Interaction of Sound Waves With Tissue

  • Acoustic impedance (AI) is dependent on the density of the material in which sound is propagated
    • The greater the impedance the denser the material.

 

  • Reflections comes from the interface of different AI,s

 

  • Greater the AI, greater the returned signal
    • Largest difference is solid-gas interface
    • We do not like gas or air
    • We do not like bone for the same reason
  • Sound is attenuated as it goes deeper into the body

Interaction of Sound Waves With Tissue

Reflection

  • The ultrasound reflects off the tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance.
  • The ultrasound image is formed from reflected echoes.
  • The production of echoes at reflecting interfaces between tissues of differing physical properties. 
  • Reflection caused by a sound wave striking a large smooth interface at normal incidence (Large smooth interfaces (e.g. diaphragm, bladder wall) reflect sound like a mirror)

 

  • Specular Reflection – large smooth surfaces 
  • Diffuse Reflection – small interfaces or nooks and crannies
 

reflection

Reflected Echo’s

reflections

Transmission

  • Some of the ultrasound waves continue deeper into the body
  • These waves will reflect from deeper tissue structures

Reflection and Transmission Coefficients

  • A propagating wave will be partly reflected when encountered a medium with dissimilar acoustic properties (Z).

Transmission

Attenuation

  • The intensity of sound waves diminish as they travel through a medium
  • In real systems some waves are scattered and others are absorbed, or reflected
  • This decrease in intensity (loss of amplitude) is called attenuation.  
  • The deeper the wave travels in the body, the weaker it becomes 
  • 3 processes: reflection, absorption, refraction
  • Ultrasound wave propagating in tissue will be attenuated because of absorption and scattering.
  • Attenuation is linearly dependent on frequency (most materials)

ATTENUATION

Attenuation/Refraction

Attenuation . Refraction

What Are The Basic Components Of An Ultrasound Machine

Basic Components Of An Ultrasound Machine

Ultrasound Transducer

  • Converts one form of energy to another
  • This component (called Probe) of the ultrasound system that is placed in direct contact with the patients body 
  • Acts as both speaker and microphone
    • Emits very short sound pulse (transmitter)
    • Listens a very long time for returning 
  • Can only do one at a time

Ultrasound Transducer

  • Based upon the pulse-echo principle occurring with ultrasound piezoelectric crystals, ultrasound transducers convert:
    • Electricity into sound = pulse
    • Sound into electricity = echo

 

PULSE: Electricity into sound

  • Pulse of sound is sent to soft tissues
  • Sound interaction with soft tissue = bioeffects
  • Pulsing is determined by the transducer or probe crystal(s) and is not operator controlled

 

ECHO: Sound into electricity 

  • Echo produced by soft tissues
  • Tissue interaction with sound = acoustic propagation properties
  • Echoes are received by the transducer crystals
  • Echoes are interpreted and processed by the ultrasound machine

Transducer Principles: What is Piezoelectric/ Piezoelectricity?

The transducer contains a special type of crystal with in it named piezoelectric crystal.

  • In a electric field the alignment of dipole with in the crystal changes, which in turn causes the crystal to change the shape.
  • If the voltage is applied in a sudden burst the crystal vibrates and generates sound.

 

  • Piezo-electric crystal            
    • Converts electric signals to mechanical & vice versa
    • Transmits pulses of sound into tissue and listens for echoes
    • Most of the time is spent listening for echoes

 

  • Piezoelectricity means pressure electricity, which is used to describe the coupling between a materials mechanical and electrical behaviors. 
    • Piezoelectric Effect 
      • when a piezoelectric material is squeezed or stretched, electric charge is generated on its surface.
    • Inverse Piezoelectric Effect
      • Conversely, when subjected to a electric voltage input, a piezoelectric material mechanically deforms. 

Transducer Construction

Transducer Construction

Methodology 

Two different types of transducers are described

  • A sector transducer operates like a radar antenna that rotates to capture the echoes of aircraft. The probe can comprise three ultrasound crystals rotating at the transducer tip 
  • Can be made very small, about 1mm in diameter and can be used to examine blood vessels from the inside, such a coronary artery.

 

  • Linear transducers have a large number of small ultrasound crystals arranged in a line.
  • Use: such as a fetus inside the uterus
 

Methodology

Image Resolution

  • Image quality is dependent on
    • Axial Resolution
    • Lateral Resolution
    • Focal Zone
    • Probe Selection
    • Frequency Selection
    • Recognition of Artifacts
 

Image Resolution

Resolution

  • Spatial Resolution describes how physically close two objects can be and displayed separately.
  • The ability to differentiate between structures that are closely related, both in terms of space and echo amplitude.
  • All current equipment has an overall spatial resolution of 1.0 mm or less.
  • Wavelength (frequency) affects:
    • Gray Scale Resolution
    • Axial Resolution : (along the beam path)
    • Lateral Resolution : (perpendicular to beam path)

 

Axial Resolution

  • Axial Resolution = Ability of machine to distinguish between 2 distinct objects lying parallel to beam axis
  • Specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects

Axial Resolution

Lateral Resolution

  • The ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects
  • The ability to display two reflectors as two distinct reflectors when lying side-by-side, perpendicular to the sound beam’s main axis.
  • Beam width affects lateral resolution(width of the sound beam)
  • If two reflectors lying side-by-side are insonated at the same time due to the width of the sound beam, they will appear as one reflector.

Lateral Resolution

Gray Scale Resolution

  • Dynamic Range determines how many shades of gray are demonstrated on an image. 
  • Adequate gray scale resolution allows for the differentiation of subtle changes in the tissues

Gray Scale Resolution

Frequency VS. Resolution

  • Transducer Frequency and Wavelength
  • Higher frequency transducers provide better image resolution
    • Better gray scale resolution
    • Improved ability to distinguish fine detail 
  • A 12 MHz scanhead has very good resolution, but cannot penetrate very deep into the body
  • A 3 MHz scanhead can penetrate deep into the body, but the resolution is not as good as the 12 MHz

Frequency VS. Resolution

Acquisition Modes

  • A-mode : Amplitude mode
  • B-mode: Brightness mode (64 to 256 channels)
    • Real time gray scale, 2D
    • Flip book- 15-60 images per second
  • B-Color
  • M-mode: Motion mode
    • Echo amplitude and position of moving targets
    • Valves, vessels, chambers
  • D-mode: Doppler mode (1 to 64 channels)
    • Spectral
    • Audio
    • Color
  • Color/Doppler/Power Angio (2D Doppler)-- slow flow (64 to 256 channels)
  • 3D and 4D Ultrasound (1024 to 4096 channels)

Acquisition Modes

Acquisition Modes (A-Mode)

  • A for Amplitude
  • Simplest mode, basically:
    • Clap hands and listen for echo :

A MODE

  • Time and amplitude are almost equivalent since sound velocity is about constant in tissue
  • Problem: don’t know where sound bounced off from:
    • Direction unclear
    • Shape of object unclear
    • Just get a single line
  •  A small size of ultrasound probe is required in this application.
  •  Due to small sizes in ophthalmology, the frequencies as much as 15 MHz may be used.

A mode 2

M-Mode

  • The M mode stands for Motion mode. 
  •  It is used for analyzing the motion of the body structures such as, heart valves both qualitatively and quantitatively.
  • It has some characteristics of A-Mode (being 1 dimensional) and some characteristics of B-Mode (displaying the brightness). 
  • Very high sampling frequency: up to 1000 pulses per second.
  • Useful in assessing rates and motion
  • Still used extensively in cardiac and fetal cardiac imaging 

M mode

B -Mode

  • B in B-mode stands for the Brightness, since the amplitude is converted into the brightness
  • Consider one line of the A-mode scan where the amplitude of the signal is converted to the amount of the brightness instead of causing vertical displacement of the sweeping beam in the CRT.

B mode 2

Doppler Ultrasound

  • Doppler ultrasound is based upon the Doppler Effect
  • When the object reflecting the ultrasound waves is moving, it changes the frequency of the echoes, creating a higher frequency if it is moving toward the probe and a lower frequency if it is moving away from the probe. 
  • How much the frequency is changed depends upon how fast the object is moving. 
  • Doppler ultrasound measures the change in frequency of the echoes to calculate how fast an object is moving. 
  • Doppler ultrasound has been used mostly to measure the rate of blood flow through the heart and major arteries. 

Doppler Ultrasound

Doppler Ultrasound

  • Continuous Wave (CW)
    • Continuous Sinusoidal wave, hence no depth information
  • Pulsed Wave (PW)
    • Pulsed waves along one scan line at constant pulse repetition frequency
    • Only information of one spatial position 
    • Sample each reflected pulse at a fixed time (range gate

 

Examples of Doppler Ultrasound 

  • Color Flow (CF)
    • Information of the whole image (Doppler equivalent of B-Mode)
    • Velocity is encoded as a color

Examples of Doppler Ultrasound

Benefits Over Other Imaging Modalities

  • Real-time imaging
  • Non-ionizing radiation
    • Used since the 1950s, US is considered one of the safest medical imaging modalities
  • Portability
  • Relative low cost
    • More affordable than CT and MRI
  • Excellent temporal resolution
  • Multi-channel in a single system
  • Color Doppler
    • Non-invasive flow measurement
  • Essentially non-toxic
  • It doesn’t need contrast material, X-ray or isotopes

 

Ultrasound Limitations 

  • Poor or no imaging through bone, gas
  • Noisy
  • Low spatial resolution
  • Operator dependant image quality 
  • Images more difficult to interpret than CT
  • It may be less effective due to
    • Obesity (subcutaneous fat)
    • Gas (gastric or intestinal)
    • In patients having ascitic fluid
    • Lack of co-operation (dyspnea)