Lecture 16 Flashcards
DOPPLER SHIFT=
REFLECTED FREQ - INCIDENT FREQ.
DOPPLER SHIFT EQUATION
DS = 2 x SPEED OF REFLECTOR x OPERATING FREQ x COSINE/ PROPAGATION SPEED
“2” IN DS EQUATION:
INITIAL ENCOUNTER W/SOUND WAVE AND REDIRECTION TO SOURCE.
OPERATING FREQUENCY IS ______ RELATED TO MAGNITUDE OF SHIFT.
DIRECTLY
VELOCITY OF REFLECTOR _______ PROPORTIONAL TO DOPPLER SHIFT.
DIRECTLY
THE GREATER THE ANGLE (TOWARD 90 DEGREES) THE ______ THE COSINE.
SMALLER
COSINE OF 90 DEGREES
0
DOPPLER SHIFT FREQUENCY IS ___________ TO VELOCITY OF FLOW.
PROPORTIONAL
- SENDS AND RECEIVES SOUND CONTINUOUSLY
- 2 ELEMENTS
- AUDIO AND WAVEFORM PRESENTED
- MEASURES HIGH VELOCITY FLOW ACCURATELY
- SMALL PROBE EASY TO MANIPULATE
- DISADVANTAGE - RANGE AMBIGUITY
CW DOPPLER
DETERMINES DIRECTION OF DS
QUADRATURE DETECTION
DETERMINES IF DS IS POS OR NEG.
BI-DIRECTIONAL DEVICE
CANNOT DETERMINE IF DS IS POS OR NEG.
UNI-DIRECTIONAL DEVICE
- 1 ELEMENT
- DS DISPLAYED AS WAVEFORM AND AUDIBLY
- DUPLEX SCANNING
- RANGE RESOLUTION (ABILITY TO DETERMINE DEPTH)
- DISADVANTAGE: ALIASING
PW DOPPLER
IMPROPER REPRESENTATION OF A POS SHIFT AS A NEG SHIFT
ALIASING
ALIASING OCCURS:
-WITH UNDERSAMPLING
-SAMPLING FREQ MUST = PRF
(PRF MUST BE 2x DS FREQ.)
-NYQUIST LIMIT EXCEEDED
NYQUIST LIMIT =
1/2 PRF
ELIMINATION OF ALIASING:
- CHOOSE SHALLOWER SAMPLING DEPTH
- INCREASE VELOCITY SCALE
- ADJUST BASELINE SHIFT
- CHOOSE LOWER FREQ TRANS.
- USE CW INSTEAD
SEPARATION OF MULTIPLE FREQUENCY COMPONENTS IN DOPPLER SIGNAL TO INDIVIDUAL ONES.
SPECTRAL ANALYSIS
PURPOSE OF SPECTRAL ANALYSIS:
-PROVIDE QUANTITATIVE DATA ON DIFFERENT REFLECTORS
SPECTRAL ANALYSIS IS ACCOMPLISHED BY:
FFT (FAST FOURIER TRANSFORM) PROCESSOR
COMPLEX MATHEMATICAL PROCESS ANALYZING SIGNAL AND PRESENTING IN REAL TIME.
FFT (FAST FOURIER TRANSFORM) PROCESSOR
INFO PW DOPPLER PRODUCES:
- VELOCITY FLOW
- DIRECTION OF FLOW
- RANGE OF FREQ.
- DURATION OF SIGNAL
- MAGNITUDE OF SIGNAL
QUANTITATIVE DOPPLER MEASUREMENTS RELATE TO______________________.
PULSATILITY OF FLOW.
PULSATILITY INDEX:
PI = MAX VELOCITY - MIN VELOCITY / MEAN VELOCITY
RESISTIVITY INDEX:
RI = MAX (SYSTOLIC) VELOCITY - MIN.(DIASTOLIC) VELOCITY/ MAX VELOCITY
