Echocardiography Overview

OVERVIEW

•  Echocardiography = a group of interrelated ultrasound applications used to examine the heart and great vessels.

ULTRASOUND

Piezo-electric crystal

• 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.

Frequency

• 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.

DEFINITIONS

• 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

MODES

(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.

(2) M-mode

• 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)

Colour-flow Doppler

• 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.

(4) 3D

• ‘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. Review. PubMed PMID: 16893150; PubMed Central PMCID: PMC1781094.