What are the different ultrasound transducer types, their operating frequency ranges, and their typical clinical applications?

Ultrasound Transducer Types, Frequencies, and Clinical Applications

Ultrasound transducers are selected based on the depth of penetration required and resolution needed: high-frequency transducers (7-50 MHz) provide superior resolution for superficial structures with limited penetration, while low-frequency transducers (2-10 MHz) enable deeper imaging at the cost of resolution. 1

High-Frequency Transducers (>15 MHz)

Intracoronary/Intravascular Applications

• Frequency range: 20-40 MHz for intracoronary ultrasound, enabling high-resolution vessel wall imaging with limited depth penetration 1
• Used for lesion assessment, plaque characterization, detection of vascular calcium, and guidance during balloon angioplasty and stent placement 1
• Utilizes miniaturized flexible ultrasound catheters with radially arranged piezoelectric elements or rotating element transducers generating two-dimensional radial images 1

Superficial Structure Imaging

• Frequency range: 15+ MHz for temporal arteries and 7-15 MHz for extracranial arteries, providing excellent resolution but limited penetration 2
• Frequency range: 35-100 MHz for ultrasound biomicroscopy of the anterior eye segment with exceptional detail 1
• Frequency range: 7-20 MHz for whole globe ophthalmic imaging 1
• Frequency range: up to 50 MHz for micro-ultrasound (µUS) in spinal surgery, improving spatial resolution from millimeter to micrometer scale but compromising depth penetration 3

Mid-Frequency Transducers (5-15 MHz)

General Vascular and Procedural Guidance

• Frequency range: 5-15 MHz linear array probes are recommended for ultrasound-guided central venous catheter insertion, with scanning surfaces of 20-50 mm 4
• Frequency range: 5-10 MHz for intracardiac ultrasound catheters, enabling greater depth of image penetration into blood or fluid-filled cavities and contiguous structures 1
• Intracardiac transducers use either radially arranged piezoelectric elements or linear/phased array transducers generating longitudinal two-dimensional images 1

Flexible and Wearable Applications

• Center frequency: approximately 5.3 MHz for flexible and stretchable ultrasound transducers designed for curved surfaces, with -6-dB bandwidth of 66.47% 5

Low-Frequency Transducers (2-10 MHz)

Deep Structure and Cardiac Imaging

• Frequency range: 3-8 MHz for convex probes, allowing deeper penetration while maintaining reasonable resolution for cardiac and abdominal applications 6
• Lower frequencies provide greater tissue penetration, making them suitable for deeper structures and obese patients 2
• Convex probes recommended for ascending aorta, aortic arch (with sector or linear probes), and abdominal aorta imaging 6

Specialized Therapeutic Applications

• Frequency range: 0.3-3 MHz optimal for microbubble-based drug delivery applications, corresponding to resonance frequencies of commercial microbubbles 2
• Frequency range: 0.02-3 MHz for trans-scleral drug delivery, with lower frequencies (40 kHz vs. 500 kHz) significantly improving penetration depth 1, 2
• Frequency range: 22-50 kHz for phacoemulsification in cataract surgery, using a "jackhammer" effect to physically degrade lens tissue 1

Critical Selection Principles

Frequency-Penetration Trade-off

• Higher frequencies (≥15 MHz) provide better resolution but limited penetration depth, making them unsuitable for obese patients or deep structures 2
• Lower frequencies increase tissue penetration but reduce resolution, with penetration depth inversely proportional to frequency 1
• For trans-scleral applications, 20 kDa dextran penetrated 100% of rabbit sclera at 40 kHz but only 46% at 500 kHz after 30 seconds 1

Transducer Configuration

• Focused transducers effectively draw the focal zone inward, increasing acoustic intensity by factors greater than 100 within the focal zone compared to outside, minimizing pressure on non-target tissues 1
• Planar transducers have longer Fresnel zones compared to focused transducers of the same diameter 1
• Linear arrays approach the quality of large annular arrays for superficial structures and allow visualization of flow within very small vessels 7

Common Pitfalls

• Avoid using high-frequency transducers (>5 MHz) in obese patients, as they produce inadequate images due to limited penetration depth 2
• Do not select transducers based solely on frequency; consider the mechanical index (MI), which combines frequency and peak negative pressure (PNP), as tissue damage correlates with increasing MI 1
• Ensure proper pulse repetition frequency (PRF) settings: 2-7 kHz for temporal arteries and 3-8 kHz for extracranial supra-aortic arteries to avoid underfilling or overfilling of vessel lumen 2
• For therapeutic ultrasound applications, power outputs above ISPTA.3 of 0.05 W/cm² require specific FDA approval, creating a potential barrier to clinical translation 1