ECHO BASICSPHYSICS AND INSTRUMENTATION - DR. NAIR ANISHKUMAR P.K.V
Mechanical vibration transmitted through an elastic medium. • Spectrum of sound Sound
Ultrasound can be directed as a beam and focused • As ultra sound passes through a medium it obeys laws of reflection and refraction • Targets of relatively small size reflect ultrasound thus can be detected and characterised. Advantages for Diagnostic utility
Ultrasound is poorly transmitted through a gaseous medium • Attenuation occurs rapidly, Especially at higher frequency. Disadvantages
Particles of the medium vibrate parallel to the line of propagation producing longitudinal waves. • Areas of compression alternates with areas of rarefaction. • Amount of reflection , refraction and attenuation depends on acoustic properties of medium • Denser medium reflect higher percentage of sound energy Mechanics :
The loss of ultrasound as it propagates through a medium is referred to as attenuation • It is the rate at which the intensity of the ultrasound beam diminishes as it penetrates the tissue. • Attenuation has three components: absorption, scattering, and reflection INTERACTION BETWEEN ULTRASOUND AND TISSUE
Always increases with depth • It is affected by the frequency of the transmitted beam and the type of tissue through which the ultrasound passes • The higher the frequency is, the more rapidly it will attenuate • Attenuation increases with increase in density of medium. Attenuation
Attenuation • Expressed as the half-power distance, which is a measure of the distance that ultrasound travels before its amplitude is attenuated to one half its original value. • As a rule of thumb, the attenuation of ultrasound in tissue is between 0.5 and 1.0 dB/cm/MHz.
Acoustic impedance • The velocity and direction of the ultrasound beam as it passes through a medium are a function of the acoustic impedance of that medium • Acoustic impedance (Z, measured in rayls) is the product of velocity (in meters per second) and physical density (in kilograms per cubic meter).
Acoustic impedance Importance : • The phenomena of reflection and refraction obey the laws of optics and depend on the angle of incidence between the transmitted beam and the acoustic interface as well as the acoustic mismatch, i.e., the magnitude of the difference in acoustic impedance’ • Use of a acoustic coupling gel during transthoracic imaging
The interaction between an ultrasound beam and a reflector depends on the relative size of the targets and the wavelength of the beam • As the size of the target decreases, the wavelength of the ultrasound must decrease proportionately to produce a reflection and permit the object to be recorded. Specular echoes and scattered echoes
Specular echoes are produced by reflectors that are large relative to ultrasound wavelength • The spatial orientation and the shape of the reflector determine the angles of specular echoes. • Examples of specular reflectors include endocardial and epicardial surfaces, valves, and pericardium Specular echoes
Targets that are small relative to the wavelength of the transmitted ultrasound produce scattering • Such objects are referred to as Rayleigh scatterers. • The resultant echoes are diffracted or bent and scattered in all directions. Scattered echoes
Scattered echoes contribute to the visualization of surfaces that are parallel to the ultrasonic beam and also provide the substrate for visualizing the texture of grey-scale images • The term speckle is used to describe the tissue-ultrasound interactions that result from a large number of small reflectors within a resolution cell. Scattered echoes
Without the ability to record scattered echoes, the left ventricular wall, for example, would appear as two bright linear structures, the endocardial and the epicardial surfaces, with nothing in between . • High-frequency ultrasound though has good resolution , is reflected by many small interfaces within tissue, resulting in scattering, much of the ultrasonic energy becomes attenuated and less energy is available to penetrate deeper into the body.. Importance :
Piezoelectricity • A period of quiescence during which the transducer listens for some of the transmitted ultrasound energy to be reflected back is known as dead time. • The amount of acoustic energy that returns to the transducer is a measure of the strength and depth of the reflector. • The time required for the ultrasound pulse to make the round-trip from transducer to target and back again allows calculation of the distance between the transducer and reflector
Piezoelectricity • Piezoelectric ceramics : ferroelectrics, barium titanate, and lead zirconatetitanate • Piezoelectric elements are interconnected electronically • The frequency of the transducer is determined by the thickness of these elements. • Each element is coupled to electrodes, which transmit current to the crystals, and then record the voltage generated by the returning signals.
The dampening material shortens the ringing response of the piezoelectric substance after the brief excitation pulse. • An excessive ringing response (or ringdown) lengthens the ultrasonic pulse and decreases range resolution. • Thus, the dampening material both shortens the ringdown and provides absorption of backward and laterally transmitted acoustic energy Backing material
At the surface of the transducer, matching layers are applied to provide acoustic impedance matching between the piezoelectric elements and the body. • This increases the efficiency of transmitted energy by minimizing the reflection of the ultrasonic wave as it exits the transducer surface. Matching layers
An ultrasound beam as it leaves the transducer is parallel and cylindrically shaped beam. Eventually, however, the beam diverges and becomes cone shaped . • The proximal or cylindrical portion of the beam is referred to as the near field or Fresnel zone. • When it begins to diverge, it is called the far field or Fraunhofer zone. Wave motion
Imaging is optimal within the near field • The length of the near field (l) is described by the formula: • where r is the radius of the transducer and λ is the wavelength of the emitted ultrasound. Near field
Near field • From the above formula optimal ultrasound imaging : large-diameter & high-frequency transducer maximize the length of the near field.
Near field • Factors preventing this approach from being practical. 1) The transducer size is predominantly limited by the size of the intercostal spaces. 2) Although higher frequency does lengthen the near field, it also results in greater attenuation and lower penetration of the ultrasound energy
the ultrasound beam is both focused and steered electronically • it is primarily achieved through the use of phased-array transducers, which consist of a series of small piezoelectric elements interconnected electronically MANIPULATING THE ULTRASOUND BEAM
An undesirable effect of focusing is its effect on beam divergence in the far field. Because focusing results in a beam with a smaller radius, the angle of divergence in the far field is increased. • Divergence also contributes to the formation of important imaging artefacts such as side lobes
Resolution is the ability to distinguish between two objects in close proximity. • two components: • spatial • temporal. Resolution
It is defined as the smallest distance that two targets can be separated for the system to distinguish between them. • Two components: • Axial resolution • lateral resolution Spatial resolution
Ability to differentiate two structures lying along the axis of the ultrasound beam • The primary determinants are the frequency of the transmitted wave and its effect on pulse length. Axial resolution
the ability to distinguish two reflectors that lie side by side relative to the beam • affected by the width or thickness of the interrogating beam, at a given depth • lateral resolution diminishes as beam width (and depth) increases. Lateral resolution
Lateral resolution • The distribution of intensity across the beam profile will also affect lateral resolution • both strong and weak reflectors can be resolved within the central portion of the beam, where intensity is greatest.
Gain is the amplitude, or the degree of amplification, of the received signal. Gain
Contrast resolution • Contrast resolution refers to the ability to distinguish and to display different shades of grey For accurate identification of borders display texture or detail within the tissues. Useful to differentiate tissue signals from background noise. • Dependent on target size. • A higher degree of contrast is needed to detect small structures
Ability of the system to accurately track moving targets over time. • It is dependent on speed of ultrasound and the depth of the image as well as the number of lines of information within the image. • Greater the number of frames per unit of time, the smoother and more aesthetically pleasing the real-time image. Temporal resolution
The pulse, which is a collection of cycles traveling together, is emitted at fixed intervals TRANSMITTING ULTRASOUND ENERGY
Concept of dynamic range • Dynamic range is the extent of useful ultrasonic signals that can be processedto reduce the range of the voltage signals to a more manageable number • It is defined as the ratio of the largest to smallest signals measured at the point of input to the display • It is expressed in decibels
The range of voltages generated during data acquisition, by post-processing, is transformed to 30 shades of grey which the human eye is able to distinguish Grey scale :
The new frequencies generated due to nonlinear interactions with the tissue ,which are integer multiples of the original frequency, are referred to as harmonics. • The returning signal contains both fundamental and harmonic frequencies. By suppressing or eliminating the fundamental component, an image is created primarily from the harmonic energy Tissue harmonic imaging