Ultrasound Transducers: 2-D Imaging

• High quality 2-D "slices" of anatomy are the goal for imaging

The Problem

• Sound travels only in a straight line
• Sound beams must be narrow for great lateral resolution and image quality

The Solution

• Construct 2-D images using multiple ultrasound pulses transmitted into the body at different angles
• Modern US systems send a narrow pulse, receive a reflection, store data, then send another pulse out in a slightly different location
• This is repeated numerous times

Transducers

• This chapter discusses the different types of transducers and their function
• It is important to understand the shape and number of active elements

Important Terms

• 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

• These contain a single, circular, disc-shaped crystal
• The active element is moved by a motor, oscillating crystal, or mirror
• These produce a fan or sector-shaped image; scan lines diverge/separate
• Mechanical steering is used
• Conventional fixed focus (internal or external) is utilized
• If the crystal is damaged, the entire image is lost
• They are not in use anymore; they are considered old, heavy, with fixed focus
• Image shape is a sector
• A defective crystal destroys the entire image

Arrays

• Single crystal transducers are replaced by more sophisticated designs called arrays
• An array is a single slab of PZT that is cut into tiny individual pieces called elements.

Definitions

• Array: a collection of active elements in a single transducer
• Element: a single slab of PZT cut into separate pieces called elements
• Channel: the electronic circuitry connected with each element

Arrays Operation

• Arrays are operated in two ways: sequencing and phasing
• Elements are 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 of electronics
• Each element has its own connection
• It can be excited in multiple ways to achieve varied goals

Types of Array Transducers

• Linear: active elements are arranged in a straight line
  • Two Types: Linear Switched/sequential and Linear Phased
• Annular: elements are arranged in circular rings with a common center
• Convex/Curvilinear: active elements are arranged in a bowed or arched line.
  • Two Types: Convex switched/sequential and Convex

Linear Phased Arrays

• Feature a small, square footprint with no moving parts
• electronic steering & focusing is called phasing
• There are approximately 200 rectangular elements
• They generate a fan or sector-shaped image
• Damage to one element results in inconsistent beam steering & focusing.
• Phased arrays mean adjustable or multi-focus to move the focal zone
• Sound beams are electronically transmitted in different firing sequences
• Utilize electronically-controlled sloping of the firing sequence to steer the beam
• Utilize electronically-controlled curvature of the firing sequence to focus the beam
• Steering & focusing can be done simultaneously
• When voltage pulses are applied in rapid progression from left to right, one ultrasound pulse is produced that is directed to the right
• Steering and focusing allows adjustment of the beam to clinical circumstances
• Multiple focus: require ultrasound systems to send multiple sound beams down each scan line

Focusing

• If the firing pattern has a slope then the beam is steered
• Draw a line along the firing pattern of the beam
• Draw another horizontal line out from that pattern and that will display the direction of steering.
• If the firing pattern is curved, then the beam is focused
• If the firing pattern is curved and has a slope then the beam is steered and focused

Annular phased array

• Consists of concentric rings cut from the same circular slab of PZT
• Elements are ring-shaped
• Select focal zones that 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
• Offer optimal lateral resolution at all depths
• Mechanical steering is used, unlike other phased array transducers
• A defective crystal causes a horizontal (side to side) band of dropout
  • If the inner crystal is defective, more superficial areas will be lost
  • If the outer crystals are defective, deeper areas will be lost

Linear Sequential or Switched Arrays

• Large acoustic footprint, up to 10cm long
• Generate a rectangular image (never wider than the transducer) for vascular, small parts
• Have approximately 200 rectangular shaped strips of PZT arranged side by side in a straight line
• PZT elements are large, about 1 wavelength in width
• Sound beams are parallel, and usually direct straight ahead
• Some but not all of the crystals are fired simultaneously, and may be fired in groups
• A few elements (5-10), but not all, are fired at the same time to create a narrow directional beam, which improves lateral resolution
• Earlier models had fixed focus; modern linear sequential arrays are electronic and focus
• Damaged PZT: only a portion of the image extending directly below the damaged element is affected or can be steered electronically
• Consist of a straight line of elements Operated by applying voltage pulses to groups of elements in succession
• Each element is about one wavelength wide
• Multiple elements are present in the transducer, and a small group of elements are fired at the same time
• A split second later, a second group of elements is fired, then 3rd, then 4th group etc
• Each group creates its own beam, for multiple beams firing one after the other

Differences Between Switched and Phased Arrays

• Phased array transducers: all of the elements are fired to create one sound beam
• The interference from every crystal creates one beam fired into the body
• The characteristics are determined by the minute differences in the way that elements are fired
• Phased arrays are adjustable with focus and steering, because the elements are fired in different ways
• All of the elements create sound waves that interfere to create only one sound beam.

Convex/Curved/Curvilinear Array

• Crystals are arranged in a curved line to create a sector shaped image, and may be sequential or phased
• Has around 200 rectangular shaped strips of PZT arranged side by side in a bowed line
• Has a large Acoustic footprint, up to 10cm in length -Acoustic footprint describes the area of contact between transducer and the skin
• Presents a blunted sector-shaped image
• It creates a wider field of view/larger footprint Electronic sequential steering allows for beam steering
• Sound beams radiate out, so beams are not parallel
• If a crystal is damaged, dropout will result from the defective crystal

Vector Arrays

• Combination of linear sequential & linear phased technologies
• Has around 200 rectangular shaped crystals arranged side by side in a line
• Has a small footprint, often only a few centimeters
• Some crystals are fired simultaneously (like the linear array)
• Electrical signals are delayed in a sloped pattern in the firing group of crystals (like the phased array)
• Transmit & receive focusing
• Produces a trapezoidal shape
• Has one crystal defect, which results in poor steering and steering
• Phasing can be applied to each element group in a linear sequenced array to steer pulses in various directions

Resolution

• Parallelogram/Trapezoid-shaped images
• Initiates pulses at various starting points
• Is a combination of linear and phased array technologies

Slice Thickness/Elevational Resolution

• Ultrasound beams have a measurable thickness
• Elevational resolution is the 3rd dimension of resolution
• Imaging plane is not always razor thin, the beam has a measurable thickness
• Slice Thickness artifact occurs when beam has a greater width than the reflector
• Consisting or rectangular shaped elements, phasing narrows the beam side by side, not in the elevational plane in phased array, linear array, & convex arrays:
• Acoustic lens is placed on the probes to improve slice thickness
• An acoustic lens is a fixed focal depth
• Improved slice thickness
• Eliminates section thickness artifact
• Contains more elements side to side than up and down

1 ½ dimensional arrays

• They improve slice thickness because it allows for focusing in the plane of the image and makes a thinner slice
• Elevational resolution is dependent on the transducer element height in which the lateral resolution is dependent on the width
• Multiple linear array transducers with five to seven rows/1.5-dimensional arrays, steer/focus the beam in the elevational dimension

Three-Dimensional Imaging

• Synonymous to Volume Imaging
• Employs uses a 2-D array with thousands of elements arranged in a checker board pattern.
• Same number of elements are used up and down and side to side making it possible to measures the volume of structures like cysts
• Electronically focused in the lateral and thickness planes
• Beams are electronically steered and sweep through scan planes of an atomic structures
• Achieves real-time 3-D or 4-D Imaging
• Rendering creates an image from the three dimensional data
• Images are COMPUTER GENERATED
• Often performed after the ultrasound data has been collected

Technical terms

• Voxels are the data points used in 3-D rendering

Side Lobes

• Additional areas of sound energy at depths equal to & greater than the focal zone extend outside of the main beam
• They are created by mechanical transducers
• If the lobes are of sufficient intensity, structures generate reflections
• Degrades lateral resolution of the sound beam

Grating Lobes

• Similar to side lobes, but created by array transducers.
• They degrade lateral resolution
• They are fixed using Subdicing and Apodization
• Utilizes Subdicing to weaken grating lobes
  • It is the process of dividing each element into a group of small crystals
• Apodization is used to reduce the strength of the side lobes
  • Electronic adaptation
  • Elements are excited with different voltages
  • Elements in the center are excited with higher voltages and towards the edges, lower voltages
  • These reduce the lobes

Dynamic Aperture

• Improves lateral resolution
• As the sound beam strikes the transducer, the size of the element listening to echoes is varied
• Accomplished by varying the number to elements used to signal
• Echoes are varying early
• Increasing the aperture improves imaging
• This optimizes image quality

Dynamic Receive Focusing

• Applied tiny delays
• Allows to focusing on different depth
• related to how it comes to the depth for the focus