Diagnostic Sonography

Transducers & Frequency Selection

📘 Table of Contents

Types of Transducers
Frequency Selection
Matching Transducer to Exam
Clinical Considerations

Types of Transducers

Ultrasound transducers come in various designs, each optimized for specific anatomical regions and clinical applications. The shape, frequency, and footprint of the probe determine its best use.

1. Linear Array Transducer

Frequency: High (7–18 MHz)
Shape: Flat, rectangular footprint
Use: Superficial structures — thyroid, breast, vessels, MSK, testes
Advantage: High resolution for shallow tissue imaging

2. Curvilinear (Convex) Transducer

Frequency: Medium (2–5 MHz)
Shape: Curved footprint for wide field of view
Use: Abdominal, obstetric, and pelvic scans
Advantage: Good penetration with broader view

3. Phased Array Transducer

Frequency: Low (1–5 MHz)
Shape: Small, square/triangular footprint
Use: Cardiac imaging and thoracic exams
Advantage: Small footprint fits between ribs, good for dynamic organs

4. Endocavitary Transducer

Frequency: High (5–9 MHz)
Shape: Long, narrow probe
Use: Transvaginal and transrectal scans
Advantage: Excellent detail of pelvic organs

5. 3D/4D Transducer

Frequency: Variable
Shape: Bulkier with matrix array
Use: Obstetrics, fetal anatomy, volume imaging
Advantage: Real-time 3D/4D visualization

6. Specialized & Intraoperative Probes

Use: Intraoperative, TEE (transesophageal), or interventional procedures
Advantage: Miniature design for tight spaces and specialized access

Tip: Always choose the highest frequency that gives adequate penetration for optimal resolution.

Diagram Suggestion: Display various probe shapes, labeled with clinical applications and frequency range.

Frequency Selection

🎚️ Frequency Selection

Selecting the appropriate ultrasound frequency is crucial for achieving the right balance between image resolution and penetration depth.

1. Frequency vs. Resolution & Depth

High Frequency (7–18 MHz): Better resolution, but shallow penetration
Medium Frequency (3–7 MHz): Balanced resolution and penetration
Low Frequency (1–3 MHz): Deeper penetration, but lower resolution

2. Choosing the Right Frequency

Superficial Structures: Use high frequency (e.g., thyroid, vessels, MSK)
Abdominal & OB/GYN: Use medium frequency (2–5 MHz)
Deep Organs / Obese Patients: Use low frequency for adequate depth

3. Frequency Trade-Off

Higher frequency: Better detail, limited depth.
Lower frequency: Greater depth, reduced detail.

4. Probe Selection Tip

Always select the highest frequency that provides sufficient penetration for your target area — this gives the best possible image clarity.

Diagram Suggestion: A spectrum showing frequency on one axis with increasing resolution and decreasing depth for illustration.

Matching Transducer to Exam

Choosing the appropriate transducer is essential for optimal image quality and diagnostic accuracy. Consider factors such as target depth, patient body habitus, and anatomical region.

1. Superficial Structures

Transducer: Linear array
Frequency: High (7–18 MHz)
Examples: Thyroid, breast, vessels, testicles, musculoskeletal (MSK)

2. Abdominal Imaging

Transducer: Curvilinear (convex)
Frequency: Medium (2–5 MHz)
Examples: Liver, kidney, gallbladder, spleen, pancreas

3. Obstetric & Pelvic Exams

Transducer: Curvilinear (for abdominal) or Endocavitary (for transvaginal)
Frequency: 5–9 MHz (TVS); 2–5 MHz (abdominal)
Examples: Fetal biometry, uterus, ovaries, adnexa

4. Cardiac & Thoracic Imaging

Transducer: Phased array
Frequency: Low (1–5 MHz)
Examples: Echocardiography, pericardial effusion, lung scans

5. Pediatric & Neonatal Scans

Transducer: High-frequency linear or microconvex
Frequency: 7–15 MHz
Examples: Brain (through fontanelle), hips, spine, bowel

Quick Tip: Match the footprint size to the anatomical window and the frequency to the depth of target.

Diagram Idea: A chart linking clinical exams to probe type and frequency range.

Clinical Considerations

When performing an ultrasound exam, it's important to adapt technique and settings based on the clinical scenario. Proper patient positioning, probe selection, and machine settings improve diagnostic value.

1. Patient Positioning

Use supine, left lateral decubitus, or upright positions based on the organ being assessed
Optimize access and reduce artifact by proper positioning
Use patient breathing or Valsalva to enhance vascular views

2. Probe Pressure and Angle

Apply appropriate gel and pressure to avoid discomfort
Adjust probe angle to avoid refraction and improve resolution
Use graded compression in abdominal or bowel exams

3. Gain and Depth Settings

Use Time Gain Compensation (TGC) to balance brightness

TGC Slider Panel with Grayscale Field

Gain 1

Gain 2

Gain 3

Gain 4

Adjust depth to keep target anatomy centered
Minimize depth and zoom in for superficial structures

4. Artifact Recognition

Identify common artifacts like shadowing, enhancement, reverberation
Understand when artifacts are diagnostic (e.g., gallstone shadow)

5. Safety & ALARA Principle

Use the lowest possible output power for diagnostic purposes
Limit scan time and avoid prolonged exposure in sensitive areas (e.g., fetal brain)
Monitor TI (Thermal Index) and MI (Mechanical Index)

Diagram Suggestion: Include illustrations of patient positions and probe placements for various exams.

Diagnostic sonography SCRS