note 5. Transducer Selection and Types in Ultrasound

Transducer Basics

• A transducer converts energy from one form to another.
• Modern transducers often combine elements into an array.
• Array scan heads have multiple small piezoelectric elements, each with its own electrical circuitry.

Components of a Single-Element Transducer

• Metal outer casing
• Backing block
• Power cable
• Acoustic insulator
• Electrodes that apply an alternating potential difference
• Piezoelectric crystal
• Plastic 'nose'

Array Transducers

• Elements on a transducer array scan head are very small, reducing beam divergence.
• Reduced beam divergence enables beam steering and focusing.
• Focusing occurs on both reception and transmission.
• Focusing is dynamic during reception.
• Some instruments use multiple transmit focal zones to improve resolution at specific depths.

Methods of Focusing

• Focusing narrows the ultrasound beam.
• Electronically scanned arrays can be curved or linear.
• Annular arrays have a large diameter with multiple rings of focus.
• Sector arrays have multiple small elements within the transducer face.

Transducer Selection Factors

• Type of examination
• Patient size
• Amount of fatty or muscular tissue

Transducer Types and Applications

• High-frequency linear array probes: Used for smaller structures like the thyroid and testes.
• Curved linear array and/or sector array: Used for abdomen scans; frequency depends on patient size.
• Phased array transducer: Used for echocardiography to scan between ribs due to its smaller size.
• Transesophageal probe: Used for transesophageal studies to image detailed cardiac anatomy.
• Linear or curved array transducer: Used for obstetric and gynecologic scans.
• Transvaginal and endorectal probes: Used to scan intercavity areas.

Multi-Element Transducers

• Contain groups of small crystal elements arranged sequentially.
• Transmitted sound pulses are created by summing multiple pulses from different elements.
• Timing and sequence of activation are altered to steer transmitted pulses and focus at multiple levels.

Phased-Array Transducers

• Each element participates in forming each transmitted pulse.
• Sound beams are steered at varying angles to produce a sector format.
• Smaller size is beneficial for scanning between ribs, making them useful in echocardiography.
• Provide a large, deep field of view.
• Limitations include reduced near-field focus and a small superficial field of view.

Linear-Array Transducers

• Activate a limited group of adjacent elements to generate each pulse.
• Pulses travel in the same direction (parallel) and perpendicular to the transducer surface.
• Produce a rectangular image.
• High resolution in the near field.
• Can be large and cumbersome.
• Frequently used in obstetric ultrasound.
• Pulses can be steered to produce a trapezoidal image.

Curved-Array Transducers

• Use a linear-array transducer with a curved convex shape.
• Produce a moderately sized, sector-shaped image with a convex apex.
• Wider far field of view but slightly reduced resolution.
• Can be formatted for various applications with varying frequencies, from abdominal to endoluminal scanning.

Intraluminal Transducers

• Small and can be placed into body lumens close to the organ of interest.
• Higher frequencies provide high resolution.
• Limited depth of field.
• Transesophageal (TEE) probes: Used by cardiologists for detailed views of cardiac anatomy.
• Intra-arterial probes: Used by interventional physicians to image the inside of coronary arteries.
• Transvaginal or endorectal probes: Used to image female organs and the rectum, respectively.

Transducer Comparison Chart

• Linear Array
  • Frequency range: Higher (8-15 \, \text{MHz})
  • Depth of imaging: Superficial (1-4 \, \text{cm})
  • Field of view: Linear, limited (depends on footprint)
• Sector (Phased) Array
  • Frequency range: Lower (2-6 \, \text{MHz})
  • Depth of imaging: Deeper (4-8 \, \text{cm})
  • Field of view: Trapezoidal, wider at depth, narrow at the surface
• Curved Linear Array
  • Frequency range: Higher (2-12 \, \text{MHz})
  • Depth of imaging: Intermediate (2-6 \, \text{cm})
  • Field of view: Trapezoidal, wide at surface and depth