Pulse
Repetition Frequency (PRF) - the number of pulses per second.
Duty Factor - the amount of time that the transducer is
transmitting sound energy. Most transducers are acting as the
receiver 99 % of the time. the duty factor is 1.
Medical Application PRFs -
High
PRF - the range for most commercial echocardiographs is
between 200 and 5000 pulses per second. The varies with the
type of mode in operation.
A
Mode - amplitude mode. Where the signals are displayed as
spikes that are dependent on the amplitude of the returning sound
energy.
B Mode - brightness mode. Where the signals are displayed
as various points whose brightness depends on the amplitude of
the returning sound energy.
Old format B-mode type scan
M Mode - motion mode. The application of B-mode and a strip
chart recorder allows visualization of the structures as a function
of depth and time.
2D Mode - 2 dimensional mode name is usually reserved
to indicate b-mode imaging of the heart. The spatially oriented
B-mode where structures are seen as a function of depth and width.
The beam is rapidly swept back and forth to create a cross section
of the imaged structures.
Scan
Types:
Mechanical
- transducers that use a combination of single element oscillation
back and forth, rotating multiple elements, or a single element
and set of acoustic mirrors to generate the sweeping beam for
2D imaging. This transducer type is sometimes called the "wobbler"
because of the vibration created as the mirrors rotate or oscillate
inside the housing.
Electronic or Phased
Sector
- creates a sector or pie shaped scan plane. This is most useful
for cardiac work where the beam is directed between the
ribs to image the heart. The beam is electronically
swept in a fan shape to transmit and receive ultrasound
signals.
Linear
- creates a linear or rectangular shaped scan plane. This is
most useful in abdominal, OB/GYN, and small parts work
where the organs are not blocked by bones or ribs. A
wide footprint allows the operator to image larger organs. This
method is simplified because there are no obstructions
to the signal by ribs and other bones.
Transesophageal
-
courtesy acuson corp
The piezoelectic device has been placed at the end of an
endoscope so that the transducer may be passed into the esophagus
to image the heart from inside the body.
Advantages
of Electronic over Mechanical - dynamic focusing, smaller
housing.
Disadvantages of Electronic over Mechanical - more expensive.
SIGNAL PROCESSING
Terms:
Decibel
(dB) - a unit describing the ratio of logarithmic power
amplification of the signal.
Dynamic
range - the range between the minimum low intensity and
maximum high intensity signals that a system is capable of displaying.
Reject - The control that allows
the operator to ignore the weak echoes that may clutter the
display and obscure the higher amplitude signals. It
filters out all signals below a fixed amplitude to improve
the recording
TGC - Time Gain Compensation. The control that allows
the operator to amplify the returning signal from deeper structures
in the body. The sector is divided into depths and altering
the TGC control can dampen or amplify the returning signals
Damping - a method of reducing the pulse duration, or
ringing of the transducer, and thereby increasing lateral resolution.
Compress - a control on some systems that affects the
gray scale and overall gain.
Axial Resolution - the resolution of objects above and below
each other.
Lateral Resolution - the resolution of objects that are
side by side.
Side-Lobe - produced from the side lobes of the ultrasound
beam. This artifact appears as false structures in the scan
plane.
Reverberation - produced from the multiple reflections
from an object as the sound energy bounces back and forth between
the object and the transducers face or dense structure.
Shadowing - the loss of data below a dense object because
the majority of the sound energy has been reflected. This
occurs typically in prosthetic valves.
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CALCULATOR
