Ultrasound 2D Images and Transducers

• High quality 2-D "slices" of anatomy are desired

• Sound travels only in a straight line so better lateral resolution sound beams must be narrow for better image quality

• 2-D images are constructed from multiple ultrasound pulses transmitted into the body in different directions

• Modern US systems send a narrow pulse, receive a reflection, store the data, then send another pulse in a slightly different location; this is repeated to create a 2-D image

• Explores different types of transducers and their functions

• Information to know about each type includes:

• Shape and number of active elements

• Automatic redirection of the sound beam to create an image (steering)

• Shape of a 2-D image

• Focusing technique

• Consequences of damaged crystal/active element

Mechanical Transducers

• Contains a single circular disc-shaped crystal
• The active element is moved by a motor, oscillating crystal, or mirror
• Image shape is fan or sector-shaped where scan lines diverge or separate with depth, creating gaps in the data
• Steering is mechanical
• Focusing is conventional fixed focus, either internal or external
• If crystal is damaged, the entire image is lost
• No longer in use as they are old, heavy, and have a fixed focus

Arrays

• Arrays are a more sophisticated design.

• An array is a single slab of PZT cut into tiny individual pieces called elements

• Array = collection of active elements in a single transducer

• Element = single slab of PZT cut into separate pieces

• Channel = electronic circuitry connected with each element

• Arrays are operated in two ways: sequencing and phasing; elements can be arranged in a straight or curved line

• Each active element is connected to its own electronic circuitry in the ultrasound system

• A channel is the combination of the active element, wire, and system electronics

• Each element has its own connection for excitation during transmission

• There are many types of array transducers

• The following arrays exists:

• Linear which have active elements arranged in a straight line and 2 types, Linear Switched/sequential, Linear Phased

• Annular which have elements arranged in circular rings with a common center

• Convex /Curvilinear which have active elements arranged in a bowed or arched line and is sometimes referred to as a curved or curvilinear array and has 2 types: Convex Switched/sequential, Convex Phased

Linear Phased Arrays

• Small square footprint with no moving parts

• Electronic focusing and steering is called phasing

• Contains approximately 200 rectangular elements

• Image produced is fan- or sector-shaped

• Damage to one element results in inconsistent or erratic beam steering and focusing with no dropout

• Phased array means adjustable or multi-focus where the focal zone can move up and down

• Sound beams are electronically transmitted in different firing sequences without any moving parts

• Steering is electronically controlled by sloping the firing sequence

• Focus is electronically controlled curvature of the firing sequence

• Steering and focusing can be done simultaneously, and fire can be curved shaped

• Electronic steering and focusing allows adjustment of the beam based on clinical circumstances

• Damage to one of the elements results in inconsistent or erratic beam steering and focusing

• Multiple Focus requires the ultrasound system to sends multiple sound beams down each scan line

• The firing patterns are used to determine if a beam is focused, steered, or both

• When voltage pulses are rapidly applied from left to right, an ultrasound pulse is produced and directed to the right

• Voltage pulses applied in rapid progression from right to left create a pulse directed to the left

• If a firing pattern has a slope, then the beam is steered

• Draw a line along the firing pattern of the beam, and then draw another horizontal line to display the direction of steering

• If the firing pattern is curved, the beam is focused

• If the firing pattern is curved and has a slope, the beam is steered and focused

Annular Phased Arrays

• Rings are concentrically cut from PZT.
• Elements are ring shaped
• Selected focal zones use inner crystals for shallow regions and outer crystals for deep regions
• Small diameter rings have a shallow focus but diverge rapidly
• Large diameter rings have a deep focal depth and less divergence
• Image shape is fan or sector
• Multi-focusing provides electronic focusing in all planes at all depths and optimal lateral resolution at all depths
• Steering is mechanical, where the element is rotated which is unlike other phased array transducers
• Defective crystal causes a horizontal (side to side) band of dropout
• If the inner crystal is defective, more superficial areas are lost
• If the outer crystals are defective, deeper areas are lost

Linear Sequential or Switched Arrays

• Large acoustic footprint, up to 10cm long
• Image produced is rectangular, never wider than the transducer; used for vascular and small parts imaging
• Approximately 200 rectangular shaped strips of PZT are arranged side by side in a straight line
• PZT are much larger than those in a linear phased array, about 1 wavelength in width
• Sound beams are parallel to each other and usually directed straight ahead
• Some, but not all, crystals are fired simultaneously or in groups
• A few elements (5-10), but not all, are fired at exactly the same time to create a narrow directional beam, improving lateral resolution
• Earlier models had a fixed focus, whereas modern arrays are electronic, for both transmit and receive focusing
• Damaged PZT affects only a portion of the image extending directly below the damaged element; they can be steered electronically
• Elements are in a straight line
• Voltage pulses are applied to groups of elements in succession
• Each element is about one wavelength wide

Switched vs Phased Arrays

• Switched or Sequential Arrays:
• Multiple elements are present and a small group of elements are fired at the same time; afterward, another group of elements is then fired.
• Each group creates its own individual beam resulting in multiple firing beams.
• Phased Array Transducers:
• All elements are fired to create each sound beam
• The interference from every crystal creates one beam fired into the body
• The beam's characteristics are determined by minute differences in the firing
• They feature adjustable focus and steering using different firing patterns
• All elements create interfering sound waves to create one sound beam

Convex/Curved/Curvilinear Array

• Crystals are arranged in a curve for a natural sector shaped image; it can use sequential or phased arrays
• Contains approximately 200 rectangular PZT strips arranged side by side in a bowed line
• Acoustic footprint is large, up to 10cm in length, describing the area of contact between transducer and skin
• Image shape is a blunted sector
• Beam steering is electronic sequential
• Focusing is electronic
• Sound beams radiate out and are not directly parallel
• Damaged crystal results in dropout from the defective crystal
• Larger footprint allows the creation of a wider field of view for abdominal imaging

Vector Arrays

• Combination of linear sequential and linear phased technologies
• Contains approximately 200 rectangular shaped crystals arranged side by side in a line
• May have a small footprint, often only a few centimeters
• Some, but not all, crystals are fired simultaneously like the linear arrays
• Electrical signals are delayed in a sloped pattern in the firing group of crystals like the phased arrays
• Transmit and receive focusing is done electronically
• Image shape is trapezoidal
• Crystal defect leads to poor steering and focusing
• Display is Parallelogram/Trapezoid-shaped
• Phasing can be applied to each element group in a linear sequenced array to steer pulses in various directions
• Pulses are initiated at various starting points across the array

Resolution and Artifacts

• Slice Thickness/Elevational Resolution:

• Ultrasound beams have a measurable thickness

• Elevational Resolution is considered the 3rd dimension

• Machines assume the imaging plane is razor thin, or has a measurable thickness

• Slice Thickness artifact occurs when a beam has a greater width than the reflector; narrow beams are desired

• Resolution in phased array, linear array, and convex array

• Rectangular shaped elements narrow the beam only side to side, not in the elevational plane through phasing

• An acoustic lens is placed on the probes to improve slice thickness, resulting in a fixed focal depth

• 1 ½ dimensional arrays are newer technology

• Improve slice thickness because it allows focusing in the image plane to produce a thinner slice

• Eliminates section thickness artifact

• Contains more elements side to side than up and down

• Elevational resolution depends on the transducer element height, similar to how lateral resolution depends on the transducer element width

• Multiple linear array transducers with five to seven rows, known as 1.5-dimensional (1.5-D) transducer arrays, can steer and focus the beam in the elevational dimension

Three Dimensional Imaging

• Volume Imaging uses a 2-D array with thousands of elements arranged in a checker board pattern/ the same number of elements up and down and side to side
• Accurately measures volume of structures such as cysts
• Electronically focused in both the lateral and thickness planes
• Beams are electronically steered and sweep through entire scan planes of anatomic structures
• 4-D Imaging – real time 3-D
• Rendering creates an image from the three dimensional data.
• Images are computer generated and performed after the ultrasound data has been collected
• Voxels are data points used in 3-D rendering
• Real time imaging is 4D

Other Key Concepts

• Side Lobes:

• Additional areas of sound energy at depths equal to and greater than the focal zone that extend outside the main beam

• Created by mechanical transducers

• Sufficient intensity can cause structures within them to generate reflections

• Degrades lateral resolution

• Grating Lobes:

• Similar to side lobes; created specifically by array transducers that degrades lateral resolution

• Additional weak beams result from multi-element array transducers.

• Fixes:

• Sub-dicing each element into a group of small crystals to weaken grating lobes

• Apodization: electronic adaptation reduces the strength of side lobes to allow various element to have excited voltages

• Elements are excited with different voltages and the ones in the center are excited with higher voltages while elements toward the edges have lower voltages, and this reduces the lobes

• Dynamic Aperture

• Performed by the machine

• Improves lateral resolution

• Called variable aperture

• Size of the transducer surface listening for echoes is varied as the returning sound beam strikes the transducer; varying the number of elements is how this is accomplished

• Echoes arising early (from superficial structures) are received using only a few crystals from the array

• As echoes return from deeper structures, the aperture is increased with more elements being used

• This optimizes image quality.

• Dynamic Receive Focusing

• Tiny time delays during reception are applied to electrical signals from the transducer

• Allows focusing at many depths

• Reception zone focusing relates to returning echoes and optimizes image quality

• Continual changes in the reception focus to receive echoes arriving from deeper locations -Similar mechanism to a video camera