Transducer arrays

Ultrasound transducers are assembled in an array (an array is the group of piezoelectric crystals within the transducer housing).

The array is created by slicing a large piezoelectric crystal into many smaller active elements. Each element is unique. They sit in different compartments that are shielded from each neighboring compartment. This prevents any acoustic or electric interference from distorting the signal sent and received by that particular element.

Types of arrays

  • Linear sequential array (switched array)
  • Linear phased array (vector, sector)
  • Curved sequential array (convex array, curvilinear sequential array).
  • Curved phased array (convex array, curvilinear phased array).
  • Annular array

The main differences to remember is that linear arrays are in a straight line (easy to remember) and curved are arranged on a curve.

The real difference though is between sequential (switched) and phased (sector) arrays. I associate certain words to help me remember them.

Sequential = Sequence Sequential arrays are lined up in a sequence. So the crystals are side by side and the face of the transducer is large. Each crystal creates a single scan line that runs straight from the transducer.

Phased = Phases In a phased array there are less actual crystals so they fire in phases to create an image that has more scan lines than actual crystals. Each crystal creates multiple scan lines. They are fired in phases.

I actually have a trick to be able to tell what type of transducer you are looking at. If the shape of the top of the image is the exact shape as the bottom then it is a sequential array. If the shape is different, i.e. The top is flat and the bottom is curved then you have a phased array.

Where this holds up still but is a little difficult to tell is in some transvaginal transducers as well as pediatric transducers. The top may be curved but they still cover more lines than there are crystals and you can see that the ratio will be different. Very curved on the top, with a normal curve on the bottom.

Linear sequential array

A linear transducer is something that most of us are familiar with for imaging small parts and shallow structures. The image is rectangular.

Each crystal is fired and creates  a single scan line. So the image corresponds to the width of the actual transducer.

A simple linear array will not have steering. As the beams are fired they run parallel to each other and straight from the face of the transducer. These beams stay parallel to each other no matter the depth. The space between the beam stays constant because they run parallel to each other.

Linear Transducer Sequential

Linear phased array

A phased array works by exciting all the crystals in the transducer at the same time. This creates many individual waves which constructively interfere with each other to create a single pulse. Since the crystals are able to be fired at different times with a small delay between each pulse the beam that is formed is able to be steered as well as focused.

Sector_transducer linear phased

 

Curved sequential array

A curved or convex sequential (switched) array is aligned the same as a linear array. Crystals side by side with each crystal creating a single line on the screen but the transducer shape is curved so you get a classic fan shaped image.

Beam steering is not necessary on this transducer because it naturally creates an image with an arc.

Convex_switched_transducer

 

You don’t need beam steering on this transducer because it naturally creates an arc shaped image.

A drawback to the curved array versus a linear array is that the beam at depth separates from the beam running parallel so you end up with gaps that the machine has to account for. So the deeper the image the greater the decrease in lateral resolution. A structure could sit in the gap between the beams and the machine would never know it exists.

 

Beam gaps in a sequential array

Annular Array

Annular arrays are unique in their composition. They are created by taking a single crystal and cutting it into rings. Think of the cross section of an onion. Smaller and smaller rings.

Because an annular transducer is steered mechanically it will have an image with a sector shape.

annular_transducer small

 

Annular beam profile

Annular beam profile

The benefit of an annular transducer is that the different crystal diameters have different acoustic properties.

The large diameter crystals on the outer edge have a natural focus that is deep and the small diameter crystals on the inside have a naturally shallow focus.

The transducer will use electronic focusing along the whole beam profile which creates a unique beam shape with a thin slice throughout.

 

 

 

All about Transducer Arrays

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Your answers are highlighted below.
Question 1
Array is a term meaning
A
the interference created by separate waves fired on a single pulse.
B
the group of active elements within the transducer housing.
C
transducer crystals that are cut in concentric rings.
D
beams from the transducer which run parallel to each other.
Question 1 Explanation: 
A large piezoelectric crystal inside the transducer housing is cut into many smaller pieces. These pieces are called "elements" An arrangement of these "elements" in the transducer housing is called an "array". TIP: Say the following aloud ARRAY=ARRANGEMENT (of active elements in the transducer)
Question 2
What causes phased array images to have more scan lines than actual crystals?
A
Phased arrays do not have more scan lines than actual crystals.
B
The crystal elements are actually larger.
C
The crystal elements are actually smaller.
D
The crystal fires in phases.
Question 3
If the shape of the top image is the exact same as the bottom image, then it is a
A
phased array
B
rectangular array
C
sequential array
D
mechanical transducer
Question 3 Explanation: 
If the shape of the top of the image is the exact shape as the bottom then it is a sequential array. If the shape is different, i.e. The top is flat and the bottom is curved then you have a phased array.
Question 4
Which of the following is true?
A
A linear array has less crystals than a phased array.
B
A phased array has less crystals than a sequential array.
C
A phased array has less crystals than a linear array.
D
A sequential array has less crystals than a phased array.
Question 4 Explanation: 
A phased array has less crystals than a sequential array.
Question 5
Linear Transducer SequentialThis is an image of a
A
a linear phased array
B
an annular array
C
a curved sequential array
D
linear sequential array (switched array)
Question 5 Explanation: 
The image of a linear sequential array is rectangular.Each crystal is fired and creates a single scan line. So the image corresponds to the width of the actual transducer.
Question 6
Using a curved array the beam at depth separates from the beam running parallel causing
A
a double structure to appear.
B
a false or "ghost" image.
C
gaps that the machine has to account for.
D
an aperture in the transducer.
Question 6 Explanation: 
A drawback to the curved array versus a linear array is that the beam at depth separates from the beam running parallel so you end up with gaps that the machine has to account for. So the deeper the image the greater the decrease in lateral resolution. A structure could sit in the gap between the beams and the machine would never know it exists. HINT: Learn to recognize the different by the drawings in this lesson.
Question 7
Which one of the following is not a type of transducer array?
A
Annular array
B
Curved phased array
C
Linear sequential array
D
Spatial phased array
Question 8
Two types of distinctions between array types are
A
elemental vs ambient and switched vs phased
B
curved vs sequential and phased vs linear
C
linear vs curved and sequential vs phased.
D
linear vs sequential and phased vs curved
Question 8 Explanation: 
Curved vs linear refers to the shape of the arrangement in the transducer. Sequential vs phased refers to the way in which the crystals fire.
Question 9
Which of the following is true?
A
A linear array has more crystals than a curved array.
B
A phased array has more crystals than a linear array.
C
A sequential array has more crystals than a phased array.
D
A linear array has more crystals than a phased array.
Question 9 Explanation: 
A sequential array has more crystals than a phased array.
Question 10
Convex_switched_transducer This image represents
A
a curved sequential array
B
a linear sequential array
C
an annular array
D
a lateral array
Question 10 Explanation: 
A curved or convex sequential (switched) array has crystals side by side with each crystal creating a single line on the screen like a linear array but the transducer shape is curved so you get a classic fan shaped image.
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I am the founder and lead Instructor here at ExamRefresh.com. I strive to help those that are entering the field of Ultrasound to be the best that they can be. Ultrasound is my passion.

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  1. Better explained than I previously learned .

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