Ultrasound in Practice - Amplified

Physics Basics

Ultrasound Principles

• Sound waves >20kHz (diagnostic: 2-15MHz)
• Higher frequency = better resolution, less penetration
• Lower frequency = deeper penetration, less resolution

Probe Selection

• Linear (high frequency 7-15MHz): superficial, vascular access
• Curvilinear (low frequency 2-5MHz): abdominal, FAST
• Phased array (low frequency 1-5MHz): cardiac

Image Orientation

• Marker on probe = left side of screen
• Depth adjusted to see target structure
• Gain adjusted for image brightness

US-Guided IV Access

Advantages

• Higher success in difficult access
• Fewer attempts
• Can access deeper veins

Technique

Short axis (cross-section):

• Vein appears as dark circle
• Easier to learn
• Harder to follow needle tip

Long axis (longitudinal):

• Vein appears as dark tube
• Can visualise entire needle
• Harder to stay in plane

Artery vs Vein

• Artery: pulsatile, thick wall, doesn't compress
• Vein: compressible, thin wall, non-pulsatile

FAST Examination

Views

RUQ (Morrison's Pouch)

• Probe: right flank, marker cephalad
• View: liver-kidney interface
• Positive: black stripe between organs

LUQ (Splenorenal)

• Probe: left flank, marker cephalad
• View: spleen-kidney interface
• Often harder to visualise

Subxiphoid (Pericardial)

• Probe: below xiphoid, marker right
• View: 4-chamber cardiac view
• Positive: black fluid around heart

Suprapubic (Pelvis)

• Probe: above pubic symphysis
• View: bladder and surrounding space
• Positive: free fluid around bladder

Thoracic (Pneumothorax)

• Probe: anterior chest, 2nd ICS
• Normal: lung sliding (glittery appearance)
• Positive for pneumo: absent sliding, barcode sign on M-mode

Limitations of FAST

• Operator dependent
• Negative doesn't rule out injury
• Can miss solid organ injury without free fluid
• Sensitivity improves with serial exams