So what exactly is a Reciprocal?

A reciprocal is basically any two variables that equal 1 when multiplied together.

For example, a / b x b / a = c

We see them in Ultrasound more frequently as a one variable equals a constant multiplied or divided by another variable.

For example, a = b / c, a = b x c

Ultrasound Physics Relationships

I use arrows next to any formula I write to be able to quickly glance and understand the change.

So, the main point that  I think that you should remember, especially pertaining to Ultrasound Physics, is that the numbers are not the important thing. Too many of us get stuck trying to memorize formulas and equations where if you understand the relationship between the variables you will find the answer. The formula is there to show you exactly why things happen the way that they do.

So in your studying when you come across a relationship make note of whether it is reciprocal (inverse) or direct relationship.

Examples of relationships

Reciprocal relationships – if one goes up the other goes down

  • frequency – wavelength
  • PRP – PRF
  • duty factor – PRP
  • frequency – penetration
  • frequency – period
  • beam area – intensity

Direct relationships – if one goes up the other goes up

  • wavelength – propagation speed (wavelength depends on the propagation speed, propagation speed is determined by the medium)
  • duty factor – pulse duration
  • duty factor – PRF
  • path length – attenuation
  • frequency – attenuation
  • beam power – intensity
  • density – propagation speed – impedance

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I am the founder and lead Instructor here at I strive to help those that are entering the field of Ultrasound to be the best that they can be. Ultrasound is my passion.

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  1. Esto ha sido lo mejor que a pasado,creo que el quiera estudiar para el examen de ardms esta es la mejor practica asi uno sabe en donde tiene que mejorar ya que te dice las correctas, esto esta fenomenal..
    Ayuda porque ayuda……

  2. Sharedamc says:

    I just came across this website as I’am preparing and studying for the SPI exam. I must say all the information so far has really been helpful. I’am looking forward to studying these practice questions on a daily basis along with my other SPI material. Thanks for making a great website with helpful material!

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