Ult phys ch 7 notes - ............ PDF

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Ultrasound Physics Chapter 7 Range equation To create an anatomic image, a sound pulse travels to a reflector and then returns to the tdx Time-of Flight or Go-Return Time Reflector depth determined by measuring time-of-flight The time needed for a pulse to travel to and from the tdx and the reflector is called the go-return time or time-of-flight Superficial reflectors = brief travel time Deeper reflectors = greater travel time The time-of-flight is directly related to the depth Greater distances prolong the time Smaller distances shorten the time Time of flight See fig. 7.1 Time-of Flight or Go-Return Time cont. US are programmed with the average speed of sound in soft tissue (1.54 mm/µs) Since soft tissue speed is known, the time-of-flight and distance that the pulse travels are directly related The depth of reflector can be accurately calculated from the go-return time Depth = 1.54mm/µs x go-return time (µs) 2 The 13-Microsecond Rule For every 13 s of go-return time, the object creating the reflection is 1cm deeper in soft tissue Since a pulse travels to the reflector and back to the tdx, the total distance that a pulse travels is twice the reflector depth Time of flight-cci Pulse Repetition Period (PRP) and Maximum Imaging Depth Remember, when imaging depth is adjusted it alters the PRP When the depth of view is shallow, PRP is short When the depth of view is deep, PRP is long PRP (µs) = imaging depth (cm) x 13µs/cm See fig. 7.2 Pulse Repetition Frequency (PRF) and Maximum Imaging Depth Sound can travel to and from a depth of 77cm a total of 1,000 times in 1 second When depth of view is shallow, PRF is high When depth of view is deep, PRF is low PRF (Hz) = 77,000 imaging depth (cm) Another equation to remember: Speed= distance/time...