Scanhead contains one disc-shaped, element (like a coin)

Mechanical Scanning

The active element is moved by a motor, oscillating crystal or mirror through a pathway, automatically creating a scan plane.

Mechanical Steering

What type of focusing does mechanical scanning utilize?

Conventional or Fixed: curvature (internal) of the PZT or an acoustic lens (external) focuses the beam at a specific depth.

Which plane does focusing occur with mechanical scanning?

Focusing occurs in both the horizontal and vertical planes.

How do you change the focal depth with mechanical scanning?

Changing the focal depth requires changing the scanhead.

What image shape is mechanical scanning?

Fan or sector-shaped.

Scan lines spread apart creating gaps at greater depths.

What happens to image quality if there is a defective crystal in a mechanical scanhead?

The whole image is destroyed.

Define:

Transducer Array

A collection of active elements in a single transducer.

Describe the active element utilized in a transducer array.

A single slab of PZT cut into separate pieces called elements.

In  a transducer array, what is a channel?

Combination of electronic circuitry, the wire and the element.

Describe what a linear array is and the types.

A collection of elements in a line.

There are 2 types:

1. linear switched (or sequential) array

2. linear phased array

Describe what an annular array is.

A group of ringed elements (bull's eye) with a common center.

Describe what a convex, curved, or curvilinear array is and how many types?

Elements arranged in an arc.

There are 2 types:

1. convex switched (or sequential) array

2. convex phased array

What does this describe?Large transducers with approximately 200 rectangular-shaped elements arranged in a line side by side.

A few elements (3-10), but not all, are fired at exactly the same time to create a narrow directional beam. The improves lateral resolution.

Linear switched or sequential array

How are linear switched or sequential array transducers steered?

They are not steered. They utilize sloped time delays, pulses sent down in parallel lines. The scan lines are parallel and remain equally spaced at all depths.

How are linear switched or sequential array transducers focused?

The old method of focusing was fixed-focusing - lens (external) or curved crystal (internal); however, the modern transducer is focused electronically.

What is the image shape of a linear switched or sequential array transducer?

Rectangular, no wider than the transducer.

What happens to the image if there is a defective crystal in a linear switched or sequential array transducer?

Dropout extending from superficial to deep.

What type of array transducer does this describe?

Adjustable focus or multi-focus; achieved electronically.

Phased array

Describe the phased array active element

Approximately 200 rectangular-shaped elements arranged in a line.

Describe how the phased array transducer steers and focuses.

A collection of electric pulses is delivered to all of the transducer's elements in various patterns for each sound pulse. Thus, focusing and steering are electronic.

Electronic signals excite all of the transducer elements to create each sound pulse.

The phasing patterns focus and steer the sound beam during transmission.

Describe the image shape of a phased array transducer.

The image is fan or sector-shaped.

Where does phased array transducers get its name from?

There are miniscule time delays (10 ns), called phasing, between electronic pulses delivered to the individual array elements. The elements are fired nearly simultaneously.

What happens to the image if there is a defective crystal in a phased array transducer?

If one element malfunctions, the steering and focusing become erratic.

What type of array transducer does this describe?

Concentric rings cut from the same circular slab of PZT. The array looks like a bull's eye target.

Annular Phased Array

Describe the elements configuration of a annular phased array transducer and why they are arranged like this?

Small number of ring-shaped (like a donut) elements.

The strategy of the annular phased array is 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 and large diameter rings have a deep focal length.

Describe the image shape of an annular phased array transducer.

The image is fan or sector-shaped

Describe the focusing and steering of the annular phased array transducer.

Multi-focusing provides electronic focusing in all the planes at all depths; a core sample. This provides optimal lateral resolution at all depths throughout the image.

Steering is performed mechanically; which is unlike other phased array transducers.

What happens to the image of an annular phased array transducer if there is a defective crystal?

Defective crystal causes a horizontal (side-to-side) band of dropout.

 What type of array transducer is described?

Piezoelectric crystals arranged in a curve to provide a natural sector image.

May be sequential or phased array (just like a linear array).

Convex or curved array

Describe the active element design of a convex or curved array transducer.

Approximately 200, rectangular-shaped elements arraged in a curve to proved a natural sector image.

Describe the image shape of a convex or curved array transducer.

Blunted-sector, fan-shaped

Describe the difference between a large convex transducer from a small convex transducer.

A large convex sequential array transducer has a large acoustic footprint; whereas, the small transducer is typically a convex phased array.

Describe multidimentsional array transducers.

2-D arrays create 3-D or 4-D images. 3-D imaging accurately measures volumes of structures such as cysts. 2-D arrays have the same number of elements in the up and down and side-to-side directions. May contain thousands of elements. Remember, 4-D imaging is real time 3-D imaging.

1.5 dimensional arrays allow focusing in the plane of the image. This makes a thinner slice which improves elevational resolution. The transducer array contains more elements side-by-side than up and down.

Describe a vector array transducer.

Vector arrays combine linear sequential and linear phased array technologies. Phasing is applied to a linear sequential array.

The image shape is trapezoidal, basically a sector with a flat top that does not come to a point.

Because it is a phased array transducer, there are multiple focal points.