Overview
This lecture covers the main types of ultrasound transducers, their key characteristics, image formation processes, and essential concepts for understanding how transducers work in medical imaging.
Key Concepts & Definitions
- Transducers convert electrical energy to ultrasound waves and vice versa for imaging or Doppler applications.
- Field of view refers to the image shape produced by a transducer (sector, blunted sector, rectangular, trapezoid).
- The transducer footprint is the part of the probe touching the body, determining the width of the near field image.
- Crystals or PZT (lead zirconate titanate) elements are the core ultrasound-generating components; their arrangement defines transducer type.
Array Types & Arrangements
- 1D Array: Crystals in a single row; produces 2D images.
- 1.5D Array: Multiple rows, more control over beam thickness (elevational resolution).
- 2D Array: Crystals in a grid (checkerboard); enables 3D/4D imaging.
- Channel: Each element has a dedicated wire and electronics for activation and signal reception.
Focusing & Steering
- Multifocus (Fixed): Annular arrays use concentric rings for multiple focal points but have lower temporal resolution.
- Electronic Focusing: Voltage timing patterns on array elements create adjustable, multi-depth focus.
- Mechanical Steering: Motor physically moves crystals; used in older or specialized probes.
- Electronic Steering (Phasing): Sloped voltage patterns steer beams; curve introduces focus; both can combine for focused, steered beams.
Image Formation & Transducer Types
- Sequencing: Small groups of crystals activate in order—typically straight, unsteered beams.
- Phasing: All array elements activated together with time delays to steer/focus beam.
- Damaged crystals affect image quality; single-element loss may cause total dropout, arrays show partial dropout or image degradation.
Major Transducer Types
- PDof (blind Doppler): Continuous wave, no image, manual steering, two crystals, used in vascular/cardiac flow assessment.
- Mechanical Sector: Single circular crystal, sector-shaped image, mechanical steering, fixed focus, obsolete, good for cardiac applications due to small footprint.
- Annular Array: Multiple concentric rings, sector image, mechanical steering, multi-fixed focus, horizontal dropout if damaged, obsolete.
- Linear Switched Array: Rectangular image, crystals in a line, sequential activation, fixed focus, vertical dropout, obsolete.
- Phased Array: Sector image, all elements used, electronic steering/focusing, adjustable/multifocus, commonly for cardiac.
- Linear Sequential Array: Rectangular image, sequencing with small groups, electronic focus, used for vascular/small parts.
- Curved Linear Sequential (Convex): Blunted sector image, curved row of elements, wide field, used for abdominal, OB/GYN.
- Vector Array: Trapezoid/flat-top sector, combines phasing/sequencing, used for cardiac and abdominal imaging.
- 3D/4D Transducers: 2D array with motor, gathers data in all planes for 3D/4D images, used in OB/GYN and some cardiac work.
Key Terms & Definitions
- Sector Image — Pie-shaped FOV with narrow top.
- Blunted Sector — Curved top and bottom image.
- Rectangular Image — Flat-sided FOV, typically from linear arrays.
- Trapezoid Image — Flat top, curved bottom, from vector arrays.
- Footprint — Transducer area that contacts the patient.
- Channel — Combination of crystal, wire, and electronics for each array element.
- Phasing — Electronic method to steer/focus beams.
- Sequencing — Stepwise activation of small crystal groups.
- Voxel — Volume element in 3D imaging.
- Pixel — Picture element in 2D imaging.
Action Items / Next Steps
- Focus study on mechanical, annular, phased, linear, convex, and vector transducers—know image shape, steering, focusing, and application.
- Complete workbook activities and review nerd check questions for this unit.
- Review definitions and concepts from section 12a.1 as needed for clarity.