Ultrasound I Flashcards
What does a small aperture cause?
Divergent beam (diffraction)
What does a large aperture cause?
Poor lateral resolution
Large wavelength (low frequency)
Poor axial resolution
Small wavelength (high frequency)
High attenuation
Low penetration depth
What is used for simple disc transducer?
The same transducer is used for both transmission and reception in
What does the points in transmission beam have?
Greatest intensity will also be the points in the receive beam where a point source would produce the greatest electrical signal at the transducer
What is beam transmitted by?
Simple disc transducer
What is the transducer?
A device that actually converts electrical transmission pulse into ultrasonic pulse and echo pulse into electrical echo signals
What is the piezoelectric material?
Used for transducers in medical imaging
Synthetic ceramic material: lead zirconate titanate PZT
What is Piezoelectric effect?
Electricity
Contraction/expansion of PZT plate
What is piezoelectric plate?
The actual sound-generating and detecting component
Piezoelectric material expand or contract when a postiitive or negative electrical voltage is applied actors them and generate a positive or negative voltages when compressed or stretched by an external force
What is the thin plate of PZT coated on both sides with?
Conductive paint
Forming electrodes to which electrical connections are bonded
Oscillating voltage is applied to the electrodes making the PZT element expand and contract to the required frequency
What does back and forth movement of front face send?
An ultrasonic wave into the patients tissue
In reception, the pressure variations of returning echoes cause the PZT plate to contract and expand
What is the voltage generated the electrodes directly proportional to?
Pressure variations, giving an electrical version of the ultrasonic echo
What is used for both transmission and reception?
Duality
What does oscillating voltage cause?
Changing thickness
What do all transducer element have?
Same basic component:
- A piezoelectric plate
- A matching layer
- Backing layer
- Lens
What is transmission beam?
Corridor along which pulse travels
Beam width = lateral pulse width
What is reception beam?
Region in Which the point source must lie to produce a detectable signal at the transducer
What are identical for disc transducer?
Transmission and reception beam
What are beak near field and far field called?
Near field - Fresnel zone
Far field - Fraunhofer zone
What are the two methods for ultrasound beam focusing?
- Concaved piezoelectric element
2. Acoustic lens in front of the element
What do modern transducers use?
Electronic focusing with an array of transducer elements
What are the two ways that focusing is achieved ?
- A curved source
2. An acoustic lens
How is curved source manufactured?
Radius of curvatures of F and hence produce curved wave fronts which converge at a focus F cm from source
What is acoustic lens attached to?
Face of a flat source and produces curved wave fronts by refraction at its outer surface
What are areas transducers?
Rows of small sources
How is image cross section obtained from the patient ?
The beam is moved electronically along the length of the transducer face sweeping out a flat scan plane
What are the transmitting and receiving apertures of array transducers?
Generally rectangular rather than circular
- beams will have a rectangular cross section close to the transducer - becoming roughly elliptical towards the focal region and beyond
What does array transducers allow for?
Beam to be moved instantly between positions and give the additional benefit of allowing the shape and size of beam to be changed to suit the needs of each examination
What happens in linear-array transducer ?
Focusing the scan plane is achieved entirely by electronic means - a cyclindrical plane perpendicular to scan plane
What happens in phased-array transducer?
The lens may have some curvature in the scan plane and elevation plane
What does linear arrays offer?
Rectangular FOV
Maintain its width close up to the transducer face - region of interest extend right up in to the surface
What are all the elements in the phased array used to form?
Transmit and receive be for every scan line
What is the principle behind beam steering?
It follows automatically from arranging for the transmission focus and the multiple-receive foci to lie on all oblique scan line
What is scattering largely responsible for?
Speckle
Grainy appearance of ultrasound images
Small scale variations in acoustic properties
Define backscatter
Energy scattered back in the direction of transducer
What are backscattered ultrasound signals ?
Very weak
What is also attenuated?
Echoes returning through tissue to the transducer
What is the aim for B-mode image?
Relate the display brightness to the strength of reflection at each interface regardless of its depth
What are weaker?
Echoes from more distant targets than those from closer ones
What is time-gain compensation?
Compensate for attenuation by amplifying echoes from deep tissue much more than those from superficial tissues
What is TGC?
Increasing the amplification echo signals with time
Makes use of an amplifier whose gains can be controlled electronically so that it can be changed with time
What is the process of TGC?
At the start of pulse-echo sequence, as echoes are being received from the most superficial interfaces, the gain is set to a low value. It is then increased with time to apply a higher gain to echoes arriving from greater depth
What happens after TGC?
Echoes from a similar interface should be equal in amplitude regardless of depth
What is the assumption of ultrasound image formation?
- Constant speed of sound (1540 m/a)
- Beam axis is straight
- Each pulse travels only along beam axis
- Attenuation coefficient is constant
- The beam has no thickness in elevational direction
When do artefact occur ?
When assumptions are broken down
Ultrasound image artefact are often subtle
What is acoustic shadowing?
Due to highly attenuating body
Occurs distal to a region of increased attenuation where all the echoes in the shadow are reduced compared to those arising lateral to the shadow
The undercompensated echoes are displayed at a reduced brightness
What is enhancement?
Occurs posterior to region of low attenuation
Compensation is applied to echoes behind the cyst
What is reflection artefact?
When an ultrasound wave propagated in the form of a beam - the direction of propagation is beam axis which may be deviated by a change in speed of sound
What is mirror image artefact?
Arises due to specular reflection of beam at a larger smooth surface
What are artefact related to variation in speed of sound?
- Target misregistration
- depth errors
- refraction - Distortion of interfaces
- Measurement errors
What is slice thickness artefact?
Observed in a longitudinal image of a blood vessel whose diameter is comparable to the slice thickness
Reduced by the use of a multi row transducer
