[Acoustics / Ultrasound] Introduction to Ultrasound

Introduction

• Dr.Karl Theo Dussik, an Austrian neurologist, was the first to apply ultrasound as a medical diagnostic tool to image the brain

• Advantages

• Portable

• Free of radiation risk

• Relatively inexpensive compared with other imaging modalities such as MRI, CT, etc.

• Tomographic (cross-sectional view of anatomical structures)

• Images can be acquired in real-time

B-Mode Ultrasound

Pulse-echo approach with B-Mode(Brightness-Mode) display

• Transmitting small pulses of ultrasound echo from a transducer into the body

• As the ultrasound waves penetrate the body tissue of different acoustic impedances along the path of transmission, some reflected back to the transducer and some continue to penetrate deeper

• The echo signals returned from many sequential coplanar pulses are processed and combined to generate an image

Generation of Ultrasound Pulses

• Transducers contain multiple piezoeletric crystals

• Piezoeletric crystals are inter-connected electronically and vibrate in response to an applied electric current

• Pielzoelectric effect: mechanical vibration(stress) to electric charge

• Reverse piezoelectric effect: electrical signal to mechanical vibration

• Procedures

• Piezoelectric crystals in transducer of scan head produce pulses of ultrasound

• Transmission through tissue medium

• Reflection from tissue interfaces

• Signal(echoes) return to the system > electrical signal (reverse piezeoelectric effect)

• Signal processing : electrical signal to monitor / imaging of all reflections formed on the monitor

Ultrasound Wavelength & Frequency

• The upper limit of sound frequency for audible human hearing: 20 kHz

• Medical devices use sound waves: 1 - 20 MHz

• High-frequency ultrasound waves

• 10 - 15 MHz

• Short wavelength

• High axial resolution

• Accurately discriminate between two separate structures along the axial plane of wave propagation

• More attenuated than lower frequency waves

• Suitable for imaging mainly superficial structures

• Low-frequency ultrasound waves

• 2- 5 MHz

• Images of lower resolution

• Penetrate to deeper structures due to a lower degree of attenuation

[Fig 1.] Attenuation of ultrasound waves and it relationship to wave frequency.

[Fig 2.] A comparison of the resolution and penetration of different ultrasound transducer frequencies.

[Fig 3.] Schematic representation of ultrasound pulse generation.

* PRF: Pulse Repetition Frequency, the number of pulses emitted by the transducer per unit of time. PRF for medical imaging devices ranges from 1 to 10 kHz.

Ultrasound-Tissue Interaction

• Type of interactions
  
  

  

  
  

• Reflection

• Scatter

• Absoprtion: transformed into heat (increase in temperature)

• Refraction

• Acoustic impedance

• Intrinsic physical property of a medium

• Density of medium times the velocity of ultrasound wave propagation in the medium (ρ*c)
  
  

  

• Intensity of reflected echo  Difference in acoustic impedances between two mediums

• Reflection

• Specular reflection: Sound energy is reflected back to the transducer

• Echo intensity generated is proportional to the acoustic impedance gradient between the two mediums

• Refraction

• Change in the direction of sound transmission after hitting an interface of two tissues with different speeds of sound transmission

• Frequency is kept constant, wavelength must change

• One of the most important causes of incorrect localization of a structure

• Refraction artifacts are most prominent at fat/soft tissue interfaces

• Speed of sound is low in fat: approx. 1,450 m/s

• Speed of sound is high in soft tissues: approx. 1,540 m/s

• Scatter

• Ultrasound pulse encounters

• reflectors whose dimensions are smaller than the ultrasound wavelength

• Rough, irregular tissue interface

• Some echoes return to the transducer regardless of the angle of the incident pulse

• Appeared as tiny speckels

Recent Innovations in B-Mode Ultrasound

• Tissue harmonic imaging

• First observed in work geared toward imaging of ultrasound contrast materials

• Harmonic: Frequencies integral multiples of the frequency of the transmitted pulse (fundamental frequency or first harmonic)

• Second harmonic: Frequency of twice the fundamental

• An an ultrasound travels through tissues, the shape of the original wave is distorted from a perfect sinusoid to a "sharper," more peaked, sawtooth shape

• Reflected echoes of several different frequencies, of many higher order harmonics

• Used for the reduction of artifacts and clutter in near surface tissues

• Spatial compound imaging (Multi-beam imaging)

• Electronic steering of ultrasound beams from an array of transducer

• To image the same tissue multiple times by using parallel beams oriented along different directions

• Echoes from the different directions are averaged together into a single composite image

• Results in averaging out speckles, less "grainy", increasing the lateral resolution