For those of you who know absolutely nothing except there is such a thing as Doppler echocardiography, here are some basics. If you don't know any of the answers then skip to the answer section; otherwise test your skill :)

QUESTIONS

1. Describe the Doppler Effect (also known as the Doppler Principle).
    
2. In trying to measure RBC velocity by using the Doppler Principle, should the sound beam be parallel or perpendicular to the RBC path?
    
3. Why would any rational person want to measure RBC velocity anyhow?
    
4. There are 3 Types of Doppler Echocardiography:  Pulsed, Continuous Wave, and High Pulse Repetition (Paul, where do spectral & color flow fit in?)  Describe each briefly.
   
   
   
   
    

ANSWERS

1. I am going to use a pretty simple analogy for this since this was how the Doppler effect was described to me in high school.  Imagine you are standing on the sidewalk & a car approaches. As it approaches, the sound appears to get more & more high pitched & then as it passes you, the sound appears to get more & more low pitched.  We are all familiar with this phenomenon.
   
   Now switch perspectives. Relative to the car, you (yes, you standing on the sidewalk) are approaching & then passing by.  The car engine is actually emiting a sound wave with a constant frequency (pitch) but because the distance to your ear is changing, the pitch (frequency seems to be changing).  Frequency seems to be higher as the two objects are nearing & lower as they are passing.
   
   (One other way to picture this is the imagine a pool of water & you are dipping your finger in the water at a constant "frequency." The distance between the ripples is constant. Now move your finger forward, still dipping in at the same frequency.  The ripples are closer together at the forward edge of your trail & farther apart at the far end of your trail.) But you're still dipping at the same freqency.  The motion is what makes the frequency change.
   
   This is the Doppler effect.
   
   The difference between the frequency transmitted & the frequency perceived (or "received") is called the Doppler Shift.
   
   
   
   
    
2. Did I go too fast?  You can see from the above how there might be a mathematical relationship between the velocity of the perceiver ("receiver") & the change in frequency described.  There is such a relationship & it is described as:
   
   Velocity  =  (transmitted freq. - received freq) x beam speed
                        ---------------------------------------------------------------
                                    2 (transmitted freq) x cosine theta
   
   In the normal world, the beam speed is the normal speed of sound (in air) but in the body we aren't sending the beam through air anymore so the beam speed is a special number (fortunately, we don't  need to know the number; the gadget does these calculations & it knows the number).
   
   Theta is the angle that the sound beam is hitting the RBC path at.  We'd really like to know the velocity on the RBC on its path, which means the beam should come in as PARALLEL to the RBC path as possible.  And this makes Cosine theta = 1.
   
   So how the hell can we aim the beam at a zero degree angle :/
   
   Well, you don't exactly have to.  Anything less than 20 degrees turns out to be close enough.
   
   
   
   
    
3. (Okay, Paul, I'm a little confused on this part.  I definitely get it about measuring pressure across two points but I'm not so sure how it helps MR or VSD - the two conditions where I've read it is esp. helpful but I don't really see what it adds.)
   
   For one thing, according to the Bernoulli equation:
   
   4 x velocity = Pressure @ one point - Pressure @ a second point
   
   This means in English that an RBC velocity can be used to measure a pressure difference across 2 points, something that would be quite convenient if you wanted to rule in a stenotic lesion.
   
   (Er, Paul, my Physics book has Bernoulli's equation & it bears virtually no resemblance to the above. What is going on here?  And how the hell can you measure the pressure across 2 points with only one velocity.  Isn't velocity higher in a narrowed segment because of Bernoulli's principle?  Don't you need two velocities somehow? )
   
   
   
   
    
4. Pulsed Doppler:  in the gadget the transmitting crystal is the same as the receiving crystal.  This means that the crystal cannot transmit & receive at the same time.  Because it is sending out ultrasound in pulses, it can be very depth specific about the point where it is measuring velocity (this property is called "range resolution.") It is also unable to measure high velocities. When it tries to read a velocity that is too high, the readout does a funny thing.  The readout looks sort of like a bar graph w/an x & a y axis.  When a velocity is too tall to fit on the readout, it kind of wraps around the read out. This is called ALIASING.
   
   (Paul, what the hell is the Nyquist limit?)
   
   Continuous Wave Doppler uses separate crystals for transmitting & receiving so that receiving & transmitting can be continous.  This method cannot measure  velocity at a point; instead it measures along the entire depth (Paul, so does it average all these RBC velocities together or what?)  But there is no limit on the velocity that can be read.
   
   High Pulse Repetition is a compromise between the two.  An extremely rapid pulse is used so that higher velocities can be measured but there is still range resolution.  Not as acurate for range resolution as pulsed & cannot measure velocities as high as Continuous.
   
   (Is there anything else we gotta know about this?)