ECHO 2013 / Artifacts of Imaging and Doppler Know Your Physics

Physical Properties of Sound

Sound wave = series of compressions and rarefactions

Cycle Length = length of one compression + one rarefaction

Wavelength ( ) =

length from the peak of one compression to the peak of next compression (range 0.1-0.8 mm)

Pulsed Ultrasound

Ultrasound images can only be created by pulsed ultrasound

Short bursts of ultrasound must be interspersed with periods in which the transducer receives reflected ultrasound

Shorter the pulse, higher the quality images created with greater detail

Shorter the receiving time (shorter depth), the shorter the pulse

Interaction of Ultrasound with Tissue

•Impedence: The acoustic resistance to sound traveling through a medium

•Reflection

– High impedance mismatch between two media

’d REFLECTION

–Allows creation of an ultrasound image

•Refraction

–occurs at the interface of media of different impedances when the incident beam is OBLIQUE

–Property which allows use of lenses for focusing the beam

•Attenuation

–The weakening of sound waves as it propagates through tissue

–Due to reflection, absorption and scatter

Physical Properties of Sound

•Image resolution is

•The shorter the wavelength (higher the frequency), the better the longitudinal resolution (ie: smaller the distance between two distinguishable objects)

•Depth of penetration is

•The shorter the wavelength (higher the frequency), the worse the penetration (i.e: the shorter the depth that the ultrasound travels)

Longitudinal (Axial) Resolution

Longitudinal resolution (mm) ~ spatial pulse length 2

–Ability to distinguish two objects, one in front of the other

–The minimal required separation distance between two

reflectors is one-half of the spatial pulse length (SPL)

–SPL/2 avoids the overlap of returning echoes since the distance traveled between two reflectors is twice the

separation distance.

– Improve longitudinal resolution with

•Shorter pulse length (shorter wavelength)

•Higher frequency transducer

•Wider bandwidth transducer

Effect of Focusing

Note:

1.Each focal zone requires separate pulse echo sequences to acquire data.

2.Increasing the number of focal zones improves overall lateral resolution, but the amount of time required to produce an image increases and reduces the frame rate and/or number of scan lines per image

Elevational Resolution

The elevational or slice-thickness dimension of the ultrasound beam is perpendicular to the image plane.

Elevational resolution is dependent on the transducer element height in much the same way that the lateral resolution is

dependent on the transducer element width.