Echocardiography = a group of interrelated ultrasound applications used to examine the heart and great vessels.
- piezo-electric crystal which expands proportionally when a continuous alternating voltage is applied at 8MHz -> wave is generated by the compression of particles.
- these crystals are able to create ultrasonic waves and also sense how they interact with tissues -> construction of an image.
- Audible sound – frequency 20 and 20,000 Hz
- Ultrasound – >20 kHz
- Clinical U/S – 1 to 10 MHz – because of short wave length it is easily steered, focused and manipulated.
- Wave length – temporal duration between any two peaks or troughs in a cycle.
- Frequency – the number of cycles per second (Hz)
- Velocity = frequency x wavelength
C = f x wavelength
C = speed of sound (m/s)
f = frequency (Hz)
wavelength = (meters)
- Acoustic impedance = the ability to transmit sound (related to density of tissue) – as density increases -> sound travels faster.
- Amplitude (strength) of sound wave = peak pressure (measured in decibels).
- Blood flow measurements required the following information:
(1) cross sectional area of ascending aorta.
(2) transducer placed so beam falls in line parallel to aortic flow.
(3) U/S device integrates measured blood flow velocity over period of ejection to determine average value of RBCs at each heart beat (needs TOE).
(4) Q = average velocity value for each heart beat in aorta x aortic cross-sectional area x HR
V = (cfd/2ft) cos O
V = blood flow velocity
c = speed of sound in body tissue (1540m/s)
fd = doppler frequency
cos O = cosine of angle between sound beam & blood flow.
ft = frequency of transmitted u/s
Cardiac Output measurement:
Q = SV X HR
= (Aortic Area x V x Tej) x HR
Q = cardiac output
Aortic area = cross sectional area
V = velocity for each beat
Tej = time period during ejection
HR = heart rate
(1) 2D anatomical imaging
- used TTE or TOE
- cornerstone of imaging
- other modes use 2D as a reference
- image plane is determined by the axis of the heart not the spine.
- allows the display of structures along a beam as a function of time
- beam is fixed in position and there is a rapid sampling rate -> assessment of thin moving structures (valves)
- one dimensional ‘ice pick’ view
- time (x axis) and depth (y axis)
(3) Doppler techniques
- pulsed wave, continuous wave and colour-flow
Pulsed wave Doppler
- performed with a duplex transducer (2D and Doppler)
- single ultrasound transducer transmits and receives signals
- blood flow velocities of a small volume of blood are obtained at a specific depth
- useful for velocity measurements at specific sites such as the LVOT
Continuous wave Doppler
- transducer with two crystals (one continuously transmitting and one continuously receiving)
- can measure high velocity blood flow (AS)
- able to measure all the velocities along the beam
⇒ blood flow velocities from PW or CW Doppler can be converted into pressure gradients using the simplified Bernoulii equation (change in P = 4V2)
- change in frequency of sound waves with a moving object with colour superimposed
- blue away and red towards transducer(BART).
- information: direction of blood flow, timing of CFD signals, estimation of blood flow velocity and laminar vs turbulent flow differentiation.
- ‘real time’ 3D images
References and Links
- McAlister NH, McAlister NK, Buttoo K. Understanding cardiac “echo” reports. Practical guide for referring physicians. Can Fam Physician. 2006 Jul;52:869-74. PMC1781094.