ECHO BASICS PHYSICS 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 A s ultra sound passes through a medium it obeys laws of reflection and refraction
- DR. NAIR ANISHKUMAR P.K.V
Particles of the medium vibrate parallel to the line of propagation producing longitudinal waves.
The loss of ultrasound as it propagates through a medium is referred to as attenuation
The interaction between an ultrasound beam and a reflector depends on the relative size of the targets and the wavelength of the beam
Specular echoes are produced by reflectors that are large relative to ultrasound wavelength
Targets that are small relative to the wavelength of the transmitted ultrasound produce scattering
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
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 .
The dampening material shortens the ringing response of the piezoelectric substance after the brief excitation pulse.
At the surface of the transducer, matching layers are applied to provide acoustic impedance matching between the piezoelectric elements and the body.
An ultrasound beam as it leaves the transducer is parallel and cylindrically shaped beam. Eventually, however, the beam diverges and becomes cone shaped .
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
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.
Resolution is the ability to distinguish between two objects in close proximity.
It is defined as the smallest distance that two targets can be separated for the system to distinguish between them.
Ability to differentiate two structures lying along the axis of the ultrasound beam
the ability to distinguish two reflectors that lie side by side relative to the beam
Gain is the amplitude, or the degree of amplification, of the received signal.Gain
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.
targets over time.
The pulse, which is a collection of cycles traveling together, is emitted at fixed intervals
TRANSMITTING ULTRASOUND ENERGY
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 distinguishGrey 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.
After destructive interference the remaining harmonic energy can then be selectively amplified, producing a relatively pure harmonic frequency spectrum.
Side lobes occur because a portion of the energy concentrate off to the side of the central beam and propagate radially, a phenomenon known as edge effect
Doppler imaging is concerned with the direction, velocity, and then pattern of blood flow through the heart and great vessels.
The increase or decrease in frequency due to relative motion between the transducer and the target is referred to as the Doppler shift.
It is similar to echocardiography. Short, intermittent bursts of ultrasound are transmitted into the body and listens at a fixed and very brief time interval after transmission of the pulse.
This Imaging simultaneously transmits and receives ultrasound signals continuously.
A major advantage of continuous wave Doppler imaging is that aliasing does not occur and very high velocities can be accurately resolved.
A form of pulsed wave Doppler imaging that uses multiple sample volumes to record the Doppler shift
The primary determinant of jet size is jet momentum, which depends on both flow rate and velocity. Thus, factors that affect velocity, including blood pressure, will also affect jet size.
The eccentric jets that become entrained along a wall, making them appear smaller than they actually are (Chamber constraint ).
Doppler imaging records velocity, not flow. It cannot distinguish whether the moving left atrial blood originated in the ventricle (the filled triangles) or atrium (the filled circles), simply that it has sufficient velocity to be detected.
(billiard ball effect)
Related directly to the Doppler principle. For example, aliasing occurs when pulsed wave Doppler techniques are applied to flow velocities that exceed the Nyquistlimit
By adjusting gain and reject settings, the Doppler technique can be used to record the motion of the myocardium rather than the blood within it
One obvious limitation is that the incident angle between the beam and the direction of target motion varies from region to region.