Chapter 5-7, Physics Flashcards
Discovered in the 1880’s by Pierre and Jacques Curie
When a mechanical force is applied to certain materials they create a voltage or……
Some materials produce a voltage when “deformed” by an applied pressure
Conversely, these same materials produce a pressure wave when when an applied voltage deforms the materials
When voltage is applied to these materials, they change shape and vibrate
Piezoelectric Effect
Piezoelectric is also called ferroelectric
Refers to any device that converts one form of energy into another
Transducer
Active elements (cristals) in transducers are heated to a certain point to make them piezoelectric. What is this critical temperature called?
Curie Point
Transducer crystals lose their piezoelectric properties if they are heated above the Curie Point or Curie Temperature
The material becomes depolarized if heated to this point
Main component of a transducer is this with piezoelectric properties
Crystal
Natural materials: Quartz, Rochelle salts, tourmaline
Man-made: Lead zirconate titanate or PZT (most often used in diagnostic US transducers), barium titanate, lead metaniobate, lead titanate
Bonded to the back of the active element;
Shortens SPL and pulse duration
Improves image quality
Increases bandwidth (range of frequencies within the pulse)
Decreases the Quality factor (Q)
Decreases the transducers sensitivity to reflected echoes
Damping Element
Reduces reflections at transducer - tissue interface
Usually ¼ the wavelength of the ultrasound beam
Used to reduce the impedance difference between the transducer element and the skin itself
Helps send more US energy into the body rather than reflecting it because of the impedance difference
(Gel also helps to reduce the impedance difference)
Matching Layer or Impedance Matching Layer
Acoustic impedance of composite piezoelectric elements is closer to that of soft tissue, so matching is easier and more efficient with these transducers
Composites
Man Made
The operating frequency of a transducer
Also called natural frequency
Transducer frequency depends on:
- The thickness of the crystal (indirectly related), and
- Speed of sound in the crystal (directly related)
Resonant Frequency
Pulsed ultrasound transducers emit not a single ultrasonic frequency, but a spectrum of frequencies
IT describes the difference between the highest and the lowest frequency in a pulse
Bandwidth
The shorter the pulse, the wider the bandwidth
Resonant Frequency is at the center of the bandwidth
Advantages of Wide Bandwidth
Fewer cycles per pulse give longer listening time (allows acquisition of more echoes)
Wider bandwidth can receive a wider range of frequencies
Disadvantage of wider bandwidth
Decreased probe sensitivity
Element is not as responsive to the returning sound waves (pressure)
In pulsed ultrasound, a description of the width of the pulse as it travels away from the transducer
Width varies with distance away from the transducer
Beam
Sound beams are usually shaped like an hourglass; starts the size of the transducer, then gets smaller, then it diverges
Beam width at any location depends on:
Frequency
Aperture (size of the source)
Distance from the transducer
Region between the transducer and the focus
Also called the Fresnel Zone
Determined by the size and the operating frequency of the element
Much variation in beam intensity here because wavefronts are still coming together
Increases (longer) with increasing frequency, element size, or diameter squared
Near Zone (field)
Larger crystal diameter, longer focal length or near zone
Higher frequency, longer focal length
IT is the area of highest, most uniform beam intensity (anatomy of interest should lie here)
Focal Zone
Larger the crystal diameter, the farther or deeper the beam focus
Smaller the crystal diameter, the shallower or nearer the focus or focal depth
Near Zone Length or differently called…
Focal Length
The surface of the transducer face where ultrasound is transmitted and received.
Aperture
Also called the Fraunhofer zone
The region that lies beyond the distance of one near zone length
Place where the beam begins to diverge
Beam intensity tends to drop off here but is also more homogenous
Far Zone
Near Zone Length vs Frequency-Diameter
Directly proportional to near zone length
increasing frequency or diameter, increases near zone length
Divergence vs. Frequency and Diameter
Inversely proportional to divergence
smaller frequency or diameter, = more divergence
- Improves the accuracy of the ultrasound image
- Causes the focus of the beam (the waist) to become narrower
- Shortens the entire focal zone
- Creates a shallower focal depth
(near zone length is shorter) - Causes more divergence in the far field
Focusing
Describes the machine’s ability to image structures with accuracy
Resolution
As the numerical values of both types of resolution get smaller, the images are getting better (we’re distinguishing smaller structures)
Axial resolution is always better than lateral resolution because pulses are always wider than they are long (remember, smaller is better!)
The machine’s ability to see and differentiate small structures and represent them (anatomically) correctly on the ultrasound image
Spatial or Detail Resolution
Along the axis of the ultrasound beam
The minimum distance 2 structures are separated from front to back or anterior to posterior, and still be distinguished as separate by the ultrasound machine
Axial Resolution
Determined by spatial pulse length
Shorter pulse lengths give better axial resolution
Increasing frequency shortens SPL, improves axial resolution
Also called transverse resolution
The minimum distance that two side by side structures can be separated and still show 2 separate echoes on the screen
Approximately equal to beam diameter
Lateral Resolution
Beam diameter varies with depth, so does lateral resolution
Lateral resolution is best at the focus of the beam because diameter is smallest there
Focusing improves lateral resolution
Scan plane width or beam diameter
Determines lateral resolution
A collection of active elements within a transducer housing
A single slab of piezoelectric material is cut into many separate elements
Each element has its own electrical connection to the US machine (a channel)
Elements can be activated or “excited” individually or in groups
Transducer Arrays
Linear, Curved (convex), Phased, Annular
Two Advantages:
Enable electronic beam steering (beam is swept across the imaged field with no mechanical motion of parts)
Enable electronic focusing and beam forming (this allows control of focal distance and beam width throughout the entire imaged field)
Commonly called Phased array or electronic sector
Elements are still arranged in a line but the array is very small
Beam steering and focusing is electronic
Multiple electronic signals are used to create a single pulse
Image is fan or sector shaped
Scans the beam in sector format with short time delays
Beam focusing – done by electronic curvature of the beam
Beam steering – done by electronic slope
Voltage is applied to groups of elements in such a way that allows steering or focusing of the beam
The number of crystals excited at once determines beam shape or lateral resolution
Linear Phased
Concentric rings cut from the same circular slab of piezoelectric material
Steering – done mechanically
Focusing – done electronically
Phased Array
Advantages
Superior image quality at all depths
Small footprint good for small windows
Disadvantages
Longer time to form image
Lower frame rates (reduced temporal resolution)
Mechanical steering (moving parts - can break)
Multiple elements arranged in a line Elements are fired in sequence Image consists of parallel scan lines Rectangular image shape Conventional focusing NO Beam Steering
Linear Sequential
Advantage: Beam is parallel at all depths
Disadvantage: Large footprint (bad for small windows)
Crystals arranged in an arc; sector shape format
NO Beam Steering
Elements are fired in a sequence
Upper region does not reach a point as in a sector or vector scan
Focusing is by conventional (lens or internal)
Convex Sequential (switched)
Advantages
Natural sector image with wider field of view superficially
Electronic – no moving parts
Disadvantages
Large footprint, hard to use on small windows such as rib spaces
Sound beams tend to separate from each other leaving gaps as you move farther out from the probe
Axial or Anterior to Posterior is one dimension
Lateral or side to side is a second dimension
Slice, Section thickness, or elevation is the third dimension
Third Dimension
Takes an odd number of rows of elements (at least 3) to be able to focus the third dimension electronically
Combination of phased array electronics and linear sequential array probes to provide electronic steering and multiple focal zones
Vector Array
Man Made;
Most often used in diagnostic US transducers
Lead zirconate titanate or PZT
Some of the energy from the transducer radiates at various angles to the transducer face known as…
Side Lobes
The non-linear excitation of crystal elements.
Apodization
Along the beam path
Axial
Perpendicular to the beam path
Lateral
Groups of piezoelectric material working singly or in groups
Electronic Arrays
crystals are placed parallel or in concentric rings
– transducer face is curved
– produces sector or pie-shaped image
Sector Array
crystals are placed parallel
– transducer face is flat
– produces rectangular image
Linear Array
The ability to select focal zones at different depths throughout the image. As the number of focal zones increases, the frame rate decreases.
Dynamic focusing
A small piece of pieozelectric material in a transducer assembly
Element
Additonal minor beams of sound traveling out in directions different from the primary beam. These result for the multielement structure of transducer arrays
Grating Lobes
“Slice Thickness”
- Thickness of the sound beam.
Determined by the construction of the transducer and can not be controlled by the sonographer
Elevational Resolution
Ultrasound beam from a flat aperture will get narrow and then spread out within and angle range. The depth where beam is most narrow is this….. of the aperture.
Natural Focus
Ultrasound transducers are referred to by:
Operating, Resonant, or Main Frequency
