The human ear can detect sound waves with a frequency between 20 Hz to 20, Hz. Strong reflectors near the image plane may appear "in" the image plane due to elevational beam width. Multiple scan lines are combined to one image. A 6-dB change results in a doubling or halving of the signal amplitude. However, the advantage of Color Doppler is that information from various depths are obtained immediately. B-mode B-mode imaging, or brightness modulation, displays the varying intensities of the returning echoes as varying degrees of brightness, instead of the vertical deflections used in A-mode. The latter signals are generally of low amplitude and do not interfere with the spectral display.
The ultrasound signal strength is progressively reduced due to absorption of the ultrasound energy by conversion to heat, a process called attenuation.
In Continuous wave Doppler ultrasound the transducer both emits ultrasound pulses and records the reflected ultrasound pulse simultaneously. The simplest type of ultrasound transducer has a single piezoelectric crystal and is often used for M-mode recordings. Beam width artifacts appear as a bright linear structure. Most clinical images have a "thickness" of 3 to 10 mm, depending on depth. When an ultrasonic beam travels through a homogeneous medium, its path is a straight line.
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