SPI cheat sheet

Propagation speed in average soft tissue is 1540 m/s (1.54 mm/microsecond); wavelength in millimeters = 1.54 divided by frequency in MHz.

Attenuation averages about 0.5 dB per centimeter per megahertz, so total attenuation (dB) = 0.5 x frequency (MHz) x path length (cm).

Acoustic impedance = tissue density x propagation speed; intensity reflection coefficient = [(Z2 - Z1) / (Z2 + Z1)] squared.

Axial resolution = spatial pulse length / 2 = (wavelength x number of cycles per pulse) / 2; higher frequency and heavier damping shorten the pulse.

Lateral resolution is beam width, narrowest at the focal zone; it improves with higher frequency, larger aperture, and a lower f-number.

Elevational (slice-thickness) resolution is usually the poorest dimension and is fixed by the lens or by a 1.5-D array's elevation aperture.

Backing (damping) material shortens ring-down, lowers Q, and broadens bandwidth; the quarter-wave matching layer improves transmission from crystal into tissue.

Frame rate = pulse repetition frequency / number of pulses per frame (lines per frame); greater depth, wider sector, and higher line density each lower frame rate.

Receiver order is amplification, time-gain compensation, compression, demodulation, and rejection; scan conversion and postprocessing occur in the display stage.

Doppler shift = (2 x transmitted frequency x velocity x cosine of angle) / propagation speed; the shift is maximal at 0 degrees and zero at 90 degrees.

Doppler angle should be kept at 60 degrees or less, because cosine changes steeply near 90 degrees and small angle errors create large velocity errors.

Nyquist limit = pulse repetition frequency / 2, the highest displayable shift; a shift above it aliases (wraps to the opposite channel or reverses in color).

Aliasing is reduced by raising PRF or scale, lowering transmitted frequency, shifting the baseline, or switching to continuous wave Doppler.

Continuous wave Doppler has no aliasing and no depth discrimination; pulsed Doppler provides a depth-selective sample gate but is subject to aliasing and range ambiguity.

Resistive index = (peak systolic - end-diastolic velocity) / peak systolic velocity; pulsatility index = (peak systolic - end-diastolic) / time-averaged mean velocity.

Pressure gradient across a narrowing is approximately 4 x velocity squared (Bernoulli); by continuity (A1V1 = A2V2), velocity rises where cross-sectional area falls.

Wall filters remove high-amplitude, low-frequency clutter from vessel wall and cardiac motion, but over-filtering erases genuine slow-flow signals.

Power Doppler encodes Doppler amplitude: it is angle independent, does not alias, and detects slow flow, but shows no direction or velocity; Reynolds number above about 2000 indicates turbulence.

Tissue-mimicking phantoms provide known attenuation, propagation speed, target depths, and cyst-like or solid lesions for reproducible measurements.

Axial resolution is the minimum separable reflector spacing along the beam; lateral resolution is measured across the beam at a stated depth.

Depth calibration should agree with true distance within about 2% or 2 mm, whichever is greater.

Dead zone is the distance from the transducer face to the first resolvable echo; a long dead zone obscures superficial structures.

Uniformity testing checks echo brightness across the image at fixed depth; vertical bands indicate failed elements and horizontal gradients indicate TGC errors.

Sensitivity (maximum penetration) is the deepest visible reflector at a given output, and anechoic cyst contrast tests contrast resolution.

Baseline QA is performed at installation, then on a scheduled interval such as semiannually, and always after transducer repair, drop, or replacement.

Transducer inspection looks for lens cracks, delamination, pitting, cable damage, and element failure, since these degrade images before electronics fail.

The output display standard requires real-time display of thermal and mechanical indices, supporting ALARA-based power reduction.

Thermal index estimates temperature rise as output power divided by the power needed to raise tissue 1 degree C; TIS is soft tissue, TIB bone at focus, TIC cranial.

Mechanical index = peak rarefactional pressure (MPa) / square root of frequency (MHz); values above roughly 0.5 raise cavitation concern when gas bodies exist.

FDA limits for general imaging: derated ISPTA at or below 720 mW/cm2 and MI at or below 1.9; ophthalmic limits are far lower (ISPTA 50 mW/cm2, MI 0.23, TI 1.0).

Stable cavitation is bubble oscillation; inertial cavitation is bubble collapse producing shock waves and local heating, with a lower threshold in the presence of contrast agent.

Reverberation produces equally spaced false echoes between two strong reflectors; comet tail and ring-down arise from fluid or gas traps.

Shadowing is reduced echogenicity deep to high-attenuation structures such as bone or calculi; enhancement is increased echogenicity deep to low-attenuation fluid.

Mirror image artifact duplicates a structure beyond a strong specular reflector (for example the diaphragm) because the extra path is not accounted for.

Speed-error artifact occurs when a tissue segment conducts faster or slower than the assumed 1540 m/s, displacing structures and refracting the beam.

Side lobes and grating lobes return echoes from off-axis reflectors that are displayed as if located along the main beam; color flash and twinkle artifacts arise from motion and rough calcific surfaces.