Foundations: physics & knobology

The three workhorse probes

Frequency trades off resolution against penetration: higher frequency gives finer detail but shallow penetration; lower frequency reaches deep structures with less detail.

• Linear (~5–15 MHz, flat, rectangular image): superficial work — vascular/DVT, lung pleura, ocular, MSK, nerve blocks, soft tissue.
• Curvilinear / convex (~2–5 MHz, curved, fan image): deep abdomen — FAST, aorta, biliary, OB/pelvis.
• Phased array (~1–5 MHz, small footprint, sector image): fits between ribs — cardiac, subxiphoid, lung.

Specialty probes

Endocavitary (~5–9 MHz) for transvaginal/transrectal imaging; microconvex for neonatal and intercostal scanning.

B-mode and M-mode

B-mode (brightness) is the standard 2D grayscale image. M-mode plots one scan line over time, capturing motion precisely.

• B-mode: anatomy and most pathology.
• M-mode: lung sliding (seashore vs. barcode), cardiac wall motion, EPSS, fetal heart rate.

Doppler

Doppler detects motion/flow. Color Doppler overlays flow direction by convention BART — Blue Away, Red Toward the probe. Power Doppler is more sensitive to slow flow but shows no direction. Spectral (PW/CW) Doppler quantifies velocity.

The core controls

Optimize systematically rather than randomly twisting knobs.

• Depth: set so the target sits centered; trim excess depth to enlarge the region of interest.
• Gain: overall image brightness. Too high washes out; too low hides structures.
• Time-gain compensation (TGC): adjusts brightness at each depth to even out attenuation.
• Frequency: raise for resolution (shallow targets), lower for penetration (deep targets).
• Focus: place the focal zone at the depth of interest for the sharpest lateral resolution.
• Preset: select the correct exam preset (e.g., the low-output ocular preset for the eye).

Top of screen = nearest the probe

Every probe has an orientation marker that maps to an indicator on one side of the screen image.

• Radiology / abdominal convention: screen indicator on the LEFT — marker to the patient's right (transverse) or head (longitudinal).
• Cardiology convention: screen indicator on the RIGHT — the same anatomy appears mirrored.
• State which convention a page uses; this is the single most common source of confusion.

Know them — several are signs, not noise

Artifacts arise from assumptions in image formation. Recognizing them prevents errors and sometimes makes the diagnosis.

• Acoustic shadowing: dark band deep to stones/bone — helps confirm calculi.
• Posterior acoustic enhancement: brighter signal deep to fluid-filled structures (e.g., cysts, bladder).
• Reverberation: repeating parallel echoes (A-lines in lung).
• Mirror image: a duplicated structure across a strong reflector (e.g., diaphragm).
• Anisotropy: tendons/nerves falsely hypoechoic when the beam is off-perpendicular — fix with heel-toe angulation.
• Comet-tail / ring-down: short vertical echoes (B-lines are a ring-down variant).

How the image forms

A piezoelectric transducer emits sound pulses and listens for echoes returning from interfaces between tissues of different acoustic impedance. Echo timing gives depth; echo strength gives brightness.

• Resolution: axial (along the beam) improves with higher frequency; lateral improves with focusing.
• Attenuation increases with depth and frequency — the reason deep imaging uses low frequencies.

Safety (ALARA)

Use As Low As Reasonably Achievable output and time. Watch the thermal (TI) and mechanical (MI) indices, and use the dedicated low-output ocular preset when scanning the eye.