Sonography physics and instrumentation

Sequential linear array

Elements are arranged in a linear pattern; timing of the electrical pulse to each element is used to create, focus and steer the US beam; CANNOT produce a trapezoidal image

Phased Linear Array

Groups of elements are fired at varied times to produce, focus and steer US beam; can produce trapezoid FOV

Linear array transducers offers improved skin _______ for entire transducer face

contact

Curvilinear phased array/ Convex array

Contains a large number of linearly arranged elements (120-250)

Sequentially pulsed elements

Curved/blunted sector

Electronic steering and focusing

Sector phased array

Contains large number of linearly arranged rectangular elements

Small footprint 100-300 crystals

Allows multiple focal point

Triangle sector

One pulse at a time

Acoustic lines are transmitted at an angle that is different from the previous acoustic line

Vector array

Combine sequenced linear array and phased array techniques to provide a trapezoidal image

Slightly larger footprint than a sector phased array

Sector with flat top

Electronic steering and focusing

Endocavity probes

Most commonly tightly curved phased arrays

Incorporates a curved array, phased or annular array

Advantage is increased FOV

Degraded lateral resolution at greater depths

Reflection

Return of the US wave to the transducer from a smooth surface; greatest amount occurs when beam is perpendicular to smooth surface

Scatter

Deflection of US waves from small structures; RBCs

Refraction

Deflection of US waves due to media with varied acoustic propagation speed and oblique incident angle of beam

Attenuation

Loss of US signal due to absorption in tissues as it travels; higher frequencies have higher attenuation rates

Deeper imaging __________ PRF because sound takes longer to travel in and out of pt

Reduces

__________ depth = increasing PRF = _______ frame rate

Decreasing; increasing

Axial resolution

Type of spatial resolution

Ability resolve two separate structures that lie parallel to the US beam

Improves with higher transducer frequency

Does not vary with depth

As frequency ___________, the number value for axial resolution _________-= improved axial resolution

Increases; decreases

Lateral resolution

Ability to resolve two separate structures than lie perpendicular to the US beam

Improves with higher transducer frequency, beam focusing, wider bandwidths and more superficial structures

Varies with width

Focusing improves the resolution

Lateral resolution is best at the narrowest portion of beam

Contrast resolution

Ability to differentiate two structures with varied echogenicity

Ability of the image display to distinguish tissues due to subtle variations in reflection intensity

Varies with impedance

Monitor contrast

Poor contrast resolution refers to a more black and white image

Increasing dynamic range/compression = more shades of gray = ________ contrast resolution

Improved

Elevational resolution

AKA slice thickness

Set by manufacturer

Improves with thinner image slices
More difficult to adjust

Slight improvement with increased transducer frequency and focusing

Mechanical focusing with lens can reduce slice thickness and improve resolution

Temporal resolution

Ability to accurately display moving structured over time

Determined by frame rate, lines per frame and image depth

Decreased frame rate may cause visible delay in display of info

Degraded temporal res will cause image to drag

Use of multiple focal zones will improve lateral but not temporal res

Rejection

Removal of low level echoes and non-diagnostic info

Uses wall filter

Spatial compounding

Can lines are electronically steered by the transducer in different directions to evaluate structures using multiple pulses from several different angles

Only used on phased arrays because requires ability to steer electronically

Spatial resolution is improved

Speckle and clutter are reduced

Shadowing/enhancement are reduced/eliminated

Contrast imaging

Microbubble contrast is used to enhance the Doppler signal from vessels

Requires harmonic imaging

Harmonic imaging

Reflections are received and processed at twice the fundamental frequency

Results from non-linear behavior

Improves image lateral and contrast res for lower frequency transducers

Variance

Degree of variability of mean velocity at different depths in the sample

Displayed as a green scale

Used to highlight flow turbulence

Harmonic imaging produces

Increased penetration with improved resolution

What structures in the abdomen are used to establish proper gain settings for an US exam

IVC and GB

What system should be turned off when evaluating a suspected renal calculi

Spatial compounding

Which of the following would be used to demonstrate both thyroid lobes on a single image

Panoramic

Dual

Trapezoidal

All of the above

All of the above

Which of the following is true regarding color Doppler system controls

A smaller color sample size is preferred

Lower persistence settings for venous evaluations

Lower wall filter settings to prevent artifactual color display

Lower priority settings are preferred for slower blood flow

A smaller color sample size is preferred

Which of the following is preferred for evaluating slow flow in a vessel?

A smaller color sample size is preferred

Higher persistence and priority settings

Higher wall filter settings prevent artifactual color display

All of the above

All of the above

Biggest disadvantage of PW compared to CW

PW is limited by the Nyquist limit

Which of the following is true regarding PW doppler system controls?

Sample volumes should be kept above 4mm for all exams

Lower wall filter settings are needed for high velocity flow

Faster sweep speeds are used for more accurate tracing of waveforms

Lower scale settings are preferred for higher velocity flow

Faster sweep speeds are used for more accurate tracing of waveforms

Most common reason for missing a stenosis

Doppler sample in wrong location