Quality Assurance, Safety and Artifacts
Quality Assurance
Purpose and baseline
QA confirms that a system still performs as it did at acceptance and catches drift before it affects patients. Baseline values are recorded at installation and compared on a schedule (daily, monthly, annual, after transducer replacement or repair). Tests use tissue-equivalent phantoms with known 1540 m/s propagation speed and approximately 0.5 dB/cm/MHz attenuation.
Standard measurements
Axial resolution is the closest two reflectors along the beam that remain separate and is governed by spatial pulse length. Lateral resolution is the closest two side-by-side targets that remain separate and is governed by beam width. Dead zone is the nearest detectable echo, reflecting pulse length and ring-down. Maximum sensitivity (penetration) is the deepest visible echo at fixed settings. Uniformity compares brightness of identical targets at different depths and lateral positions; focal discrepancies suggest element failure or TGC malfunction. Distance accuracy compares a caliper reading with known pin spacing, typically with a tolerance of about 2% or 2 mm. Anechoic target detectability and gray-scale/dynamic-range targets assess contrast resolution.
Wider QA duties
Electrical safety testing, leakage current checks, transducer lens and cable inspection for cracks or delamination, monitor and printer calibration, and archiving of images. Accreditation programs such as ACR and AIUM require documented annual physics surveys.
Traps
Confusing distance accuracy with resolution (they are separate tests); changing machine settings between serial measurements, which invalidates comparison; scanning a phantom at an incorrect temperature, which shifts propagation speed; assuming a single passing test replaces ongoing monitoring.
Worked example
Two pins are specified 5.0 cm apart; the caliper reads 4.8 cm. Error = 0.2/5.0 = 4%, which exceeds a 2% tolerance, so the system warrants service before clinical use.
Safety and Artifacts
Output quantities
Acoustic output is described by pressure, intensity, power, and the mechanical index. SPTA intensity is the key thermal metric. Thermal index estimates temperature rise: TIS for soft tissue, TIB for bone at the focus, TIC for cranial bone. A TI of 1 approximates a 1°C elevation. Mechanical index = peak rarefactional pressure (MPa) ÷ √(f in MHz) and indexes cavitation risk. FDA limits for general use are MI ≤ 1.9, ISPTA ≤ 720 mW/cm², and TI ≤ 6, with tighter limits for ophthalmic and fetal scanning. ALARA governs every examination.
Bioeffects
Thermal effects arise from absorption and grow with exposure time, frequency, and output. Mechanical effects include stable cavitation, in which bubbles oscillate steadily, and inertial cavitation, in which bubbles collapse, producing high local pressures and free radicals. Gas bodies-lung, bowel, and especially microbubble contrast agents-lower the cavitation threshold. Spectral and color Doppler typically carry higher output than B-mode, so reduce dwell time and output when not needed.
Artifacts to recognize
Reverberation produces equally spaced echoes; comet tail and ring-down arise from two strong reflectors or resonance; mirror images appear behind a strong specular reflector such as the diaphragm; shadowing and acoustic enhancement reflect attenuation differences; refraction causes edge shadowing and duplicate images; side-lobe and grating-lobe clutter place echoes in wrong locations; slice thickness widens structures; speed error displaces structures along the beam; high-PRF range ambiguity places far echoes near the top of the image.
Worked example
Peak rarefactional pressure of 2.5 MPa at 4 MHz gives MI = 2.5/√4 = 1.25. That is below 1.9, but ALARA still requires the lowest output and shortest exposure that answer the clinical question.