1. Sound travels in a straight line
2. Narrow beam is needed for optimal resolution

Both overcome by using multiple pulses

1. Pulse transmitted into a body
2. Listening time
3. Reflections received
4. Data processed and stored
5. Another pulse transmitted at slightly different angle
6. Listening time
7. Reflections received
8. Data processed and stored
9. Rinse and repeat until entire sector image is completed

Takes 1/100 second for 90 pulses

• Active elements
  • Shape: disc or coin shaped
  • Number: 1
• Beam steering: physically moved by a motor, oscillating crystal or mirror through a pathway, automatically creating a scan plane
• Focusing: conventional/fixed -> curvature of the PZT or an acoustic lens focuses the beam at a specific depth
  • Internal focusing -> curved crystal
  • External focusing -> acoustic lens
  • Occurs in vertical and horizontal planes
• Image shape: true sector
• Damaged element consequence: destroys entire image

Linear -> Phased and Sequential

Annular -> Phased

Convex/Curvilinear -> Sequential

Compact transducer with a small, generally square-shaped footprint

All active elements are fired to create one big sound beam; multiple wavelets interfere constructively and destructively to form a beam with particular characteristics; the firing pattern determines the beam's direction and focus

2D images are created as subsequent beams are directed in a pattern similar to that of a windshield wiper

• Active elements
  • Shape: narrow rectangle
  • Number: 100-300 side-by-side along the face of the probe
• Beam steering: electronic phasing
• Focusing method: electronically
• Image shape: true sector
• Damaged element consequence: inconsistent or erratic beam steering and focusing; cannot be effectively determined by looking at an image

Phasing: Electronic steering process that transmits sound beams in different directions without the use of moving parts

Delays in electrical signals produce time differences between pulses emitted by adjacent elements, which create different slope patterns in the overall beam

Phased: Adjustable focus or multiple-focus; in reference to transducers

Determined by curvature of a beam

Steeper/sharper curve = shallower 

Dynamic Receive Focusing

aka Dynamic Aperture

Variable time delays to some of the electrical signals created by neighboring elements during reception depending upon the depth at which the reflection was created

Performed automatically by the ultrasound system

• Active elements
  • Shape: concentric ring-shaped with a common center to create a disc
  • Number: multiple
• Beam steering: mechanical
• Focusing method: multiple transmit focal zones according to diameter of elements; smaller diameter = shallower focus
• Image shape: true sector
• Damaged element consequence: horizontal band of dropout at single depth

Linear Sequential Array

aka Linear Switched Array

Sequential array with rectangular face

A small group of elements are fired simultaneously to create each beam

A 2D image is formed as groups of elements are fired sequentially

• Active elements
  • Shape: rectangle 1 wavelength in width
  • Number: 120-250 side-by-side along the face of the probe
• Beam steering: normally none, but each group can be steered electronically by the same degree to change image shape
  • Often used in vascular imaging to create a better reflector or to angle correct for spectral or color Doppler
• Focusing method: electronically per group of elements
• Image shape: same or narrower width as transducer footprint -> rectangle with no steering, parallelogram with steering
• Damaged element consequence: band of vertical dropout

Curvilinear Array Transducers

aka Convex aka Curved

Sequential array with bowed face

A small group of elements are fired simultaneously to create each beam

A 2D image is formed as groups of elements are fired sequentially

• Active elements
  • Shape: rectangle 1 wavelength in width
  • Number: 120-250 side-by-side along the face of the probe
• Beam steering: none since already arc-shaped
• Focusing method: electronically per group of elements
• Image shape: blunted sector with up to 10 cm long arc
• Damaged element consequence: band of vertical dropout

Combination of linear sequential and linear phased array technologies 

• Active elements
  • Shape: narrow rectangle to create small footprint
  • Number: 120-250 side-by-side along the face of the probe
• Beam steering: phased at different slope degrees in neighboring groups of elements
• Focusing method: electronically 
• Image shape: trapezoid
• Damaged element consequence: ?

Slice Thickness

aka Elevational Resolution

Above-to-below thickness of an image

Measured perpendicular to imaging plane

Structures above and beneath the plane create reflections that appear in the image

Thinner is better

Disc-shaped elements best; mechanical and annular arrays; height of circular beam (thickness) is identical to lateral resolution

Rectangular-shaped elements have best axial resolution, better lateral resolution, but do not improve thickness; linear phased, linear sequential, curvilinear arrays -> determined by a fixed focal length lens attached to the entire array

Create thinner beams with improved slice thickness resolution over a greater range of depths

Face of the transducer has numerous crystals arranged horizontally and vertically

Can comprise of 700+ elements capable of phasing

3D Imaging

aka Volume Imaging

2D array with thousands of elements arranged in a checkerboard pattern

Sound beams are electronically focused in both the lateral and elevational planes

Electronically steered to create entire scan planes that sweep through anatomic structures

Rendering is done post-processing to acquire dimensional image

Off-axis sound beams extending outside of the main beam in the far zone

Weaker than the main axis, but can generate reflections that create artifacts

Degrade lateral resolution

Reduces the strength of side and grating lobes

Stronger electrical signals are used to excite inner crystals and progressively weaker electrical spike excite outer crystals

Crystal is divided into a group of smaller sub-elements that are electrically joined to act as a single crystal

Reduces grating lobes

Dynamic Aperature

aka Variable Aperature

Narrows the beam over a greater depth range and optimizes lateral resolution

Accomplished by changing the number of crystals used to transmit pulses or receive reflections

Improves lateral resolution at a wide depth of ranges