Study guide
9.3 Bioeffects & Output Display Measures
ARRT Sonography Primary
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1 Introduction & Exam Overview
1.1 Current Exam Facts & Logistics 1.2 Scoring, Section Minimums & the August 2026 Cut-Score Change 1.3 Official Blueprint & Study Strategy
2 Patient Care: Ethics, Legal Practice & Communication
2.1 Ethical & Legal Aspects of Patient Care 2.2 Interpersonal Communication & Patient Education 2.3 Physical Assistance, Monitoring & Sonographer Ergonomics
3 Patient Care: Safety, Emergencies & Infection Control
3.1 Medical Emergencies & Critical Findings 3.2 Infection Control & Hazardous Materials 3.3 Ultrasound Safety & Interventional Procedure Care
4 Image Production: Transducers & the Ultrasound Beam
4.1 Piezoelectric Effect & Transducer Construction 4.2 Beam Configuration: Near Field, Far Field & Focus 4.3 Transducer Types, Selection, Care & Malfunctions
5 Image Production: Pulse Characteristics & Resolution
5.1 Pulse Characteristics: PRF, PRP, SPL & Duty Factor 5.2 Resolution: Axial, Lateral, Temporal, Elevational & Contrast 5.3 Frequency, Bandwidth, Q Factor, Power, Pressure, Intensity & Amplitude
6 Image Production: Imaging Modes & Doppler Principles
6.1 B-Mode & M-Mode Imaging 6.2 Spectral Doppler: Pulsed Wave vs Continuous Wave 6.3 Color Doppler & Power Doppler
7 Image Production: Optimization Controls
7.1 Power, Gain, TGC, Depth & Focal Zone 7.2 Frame Rate, Line Density, Dynamic Range, Harmonics & Compounding 7.3 Doppler Optimization Controls
8 Image Production: Sound–Tissue Interactions
8.1 Speed of Sound & the Range Equation 8.2 Acoustic Impedance, Reflection, Refraction & Scattering 8.3 Attenuation, Absorption & Intensity
9 Image Production: Signal Processing, Display & Bioeffects
9.1 Receiver, Signal Processing & Analog-Digital Conversion 9.2 Postprocessing, Magnification, Panoramic & 3D/4D Imaging 9.3 Bioeffects & Output Display Measures
10 Image Production: Image Evaluation & Artifacts
10.1 Criteria for Diagnostic Quality & Annotation 10.2 Grayscale Artifacts 10.3 Doppler Artifacts
11 Image Production: Real-Time Interpretation & Informatics
11.1 Real-Time Imaging, Echogenicity & Echotexture 11.2 Color & Spectral Analysis: Measurements and Waveforms 11.3 Imaging Informatics: HIS, RIS, EMR, PACS & DICOM
12 Procedures: Abdominal & Transplant Vasculature
12.1 Aorta, Its Branches & the IVC 12.2 Portal Venous System & Hepatic Vasculature 12.3 Liver & Renal Transplant Sonography
13 Procedures: Liver
13.1 Liver Anatomy, Variants & Examination Technique 13.2 Diffuse Liver Disease 13.3 Focal Liver Lesions
14 Procedures: Biliary System, Pancreas & Spleen
14.1 Gallbladder & Bile Ducts 14.2 Pancreas 14.3 Spleen
15 Procedures: Urinary Tract, GI Tract & Other Abdominal Structures
15.1 Kidneys & Ureters 15.2 Bladder & Prostate 15.3 GI Tract, Adrenals, Lymph Nodes, Hernias & Peritoneal Cavity
16 Procedures: First-Trimester Obstetrics
16.1 Early Gestation: Normal Development & Structures 16.2 First-Trimester Measurements, Dating & Maternal Anatomy 16.3 First-Trimester Abnormalities
17 Procedures: Second/Third-Trimester Biometry & Fetal Survey
17.1 Standard Biometry: BPD, HC, AC, FL & Growth Assessment 17.2 Fetal Anatomic Survey: Head, CNS, Face & Spine 17.3 Fetal Anatomic Survey: Heart, Chest, GI, GU, Skeleton & Fetal Position
18 Procedures: Chromosomal, Genetic & Structural Abnormalities
18.1 Chromosomal & Genetic Abnormalities 18.2 Major Structural Anomalies by System 18.3 Fetal Infection, Neoplasms & Hydrops
19 Procedures: Placenta, Amniotic Fluid, Cord & Cervix
19.1 Placenta: Normal, Previa, Abruption & Accreta 19.2 Amniotic Fluid Assessment & Growth Disorders 19.3 Umbilical Cord & Cervical Assessment
20 Procedures: Fetal Assessment, Doppler & Multiple Gestation
20.1 Biophysical Profile & Fetal Surveillance 20.2 Fetal & Uteroplacental Doppler 20.3 Multiple Gestations, Assisted Reproduction & Maternal Disease
21 Procedures: Gynecology — Uterus
21.1 Uterine Anatomy, Myometrium & Fibroids 21.2 Endometrium: Cyclic Changes, Hyperplasia & Cancer 21.3 Cervix & Congenital Uterine Anomalies
22 Procedures: Gynecology — Adnexa & Pelvis
22.1 Ovaries: Normal, Functional Cysts & Benign Masses 22.2 Ovarian Malignancy, Torsion & Fallopian Tubes 22.3 Cul-de-Sac, Vagina & Pelvic Floor
23 Procedures: Neck & Scrotum
23.1 Thyroid & Parathyroid 23.2 Salivary Glands & Neck Lymph Nodes 23.3 Scrotum & Testes
24 Procedures: Breast & Peripheral Vascular
24.1 Breast & Axilla 24.2 Venous Extremity Doppler 24.3 Carotid Doppler & Post-Catheterization Complications
25 Procedures: Pediatric, Interventional & Miscellaneous
25.1 Neonatal Head, Spine & Hip Sonography 25.2 Pediatric GI & Noncardiac Chest 25.3 Ultrasound-Guided Interventions, MSK & Superficial Masses
9.3 Bioeffects & Output Display Measures
Key Takeaways Explain This Topic AI
Thermal bioeffects arise from absorption converting sound to heat and are worst in bone, whose absorption coefficient far exceeds that of soft tissue
The thermal index (TI) estimates maximum temperature rise and has three variants: TIS for soft tissue, TIB for bone at or near the focus, and TIC for bone at the surface such as cranial bone
Cavitation is the interaction of sound with gas bodies; stable cavitation involves sustained bubble oscillation while inertial (transient) cavitation involves violent bubble collapse
The mechanical index (MI) equals peak rarefactional pressure divided by the square root of frequency and indicates the likelihood of inertial cavitation
SPTA (spatial peak, temporal average) is the intensity most relevant to thermal bioeffects, and ALARA directs sonographers to minimize output power and dwell time, especially in obstetric and neonatal imaging
Last updated: July 2026
Thermal Bioeffects
Ultrasound loses energy to tissue primarily by absorption , and absorbed acoustic energy becomes heat . The temperature rise at any point depends on the local intensity , the tissue's absorption coefficient (which rises with frequency), the beam area and focusing , perfusion (blood flow carries heat away), and dwell time — how long the beam rests on one spot. Absorption in soft tissue is moderate, but bone absorbs far more strongly : its absorption coefficient is many times higher, and it converts a much larger fraction of incident sound into heat. Heating is therefore greatest in bone and in soft tissue immediately adjacent to bone — a particular concern once the fetal skeleton begins ossifying in the second trimester, and in neonatal cranial imaging. In gas-free soft tissue, diagnostic-level exposures produce rises generally well under 1°C; documented biologically significant heating begins with sustained elevations of several degrees.
The Thermal Index
The thermal index (TI) is the on-screen estimate of heating risk. Formally, it is the ratio of the acoustic power the system is actually emitting to the power that would be required to raise tissue temperature by 1°C under a conservative worst-case model. Reading a TI of 2.0 means the current settings could, under worst-case assumptions, produce up to about a 2°C rise — it is an estimate , not a thermometer. Three variants match the tissue model to the anatomy:
TIS — thermal index, soft tissue : assumes only soft tissue in the beam. Appropriate for first-trimester obstetric scanning before significant ossification, and for abdominal and gynecologic imaging.
TIB — thermal index, bone : assumes bone lies at or near the beam focus, where heating is worst. Appropriate for second- and third-trimester fetal imaging, where the ossified skeleton sits deep in the field.
TIC — thermal index, cranial : assumes bone lies at or near the tissue surface immediately under the transducer. Appropriate for adult transcranial studies and neonatal head imaging, where the skull is superficial.
Most professional guidance suggests keeping TI as low as practicable, exercising extra caution when it exceeds about 1.0, and limiting exposure time at higher values.
Mechanical Bioeffects: Cavitation
The second mechanism is non-thermal. Cavitation is the interaction of the sound field with small gas bodies (microbubbles) in tissue or fluid, and it comes in two forms:
Stable cavitation : the bubble oscillates in size rhythmically with the pressure cycles but persists without collapsing. The oscillation drives local fluid motion called microstreaming , which can shear nearby cell membranes. Stable cavitation is the mechanism deliberately exploited by ultrasound contrast agents , whose injected microbubbles resonate strongly at imaging frequencies.
Inertial cavitation (historically called transient or collapse cavitation): the bubble grows during the low-pressure (rarefaction) half-cycles and then collapses violently during compression. Collapse generates localized temperatures of thousands of degrees, enormous pressures, shock waves, and chemically reactive free radicals — the potentially damaging event.
The mechanical index (MI) estimates the likelihood of inertial cavitation: MI equals the peak rarefactional pressure (in MPa, derated for tissue attenuation) divided by the square root of the center frequency (in MHz). A higher peak negative pressure and a lower frequency both raise MI — lower-frequency pulses give bubbles more time to grow during each rarefaction. MI is displayed on screen alongside TI; regulators cap MI at 1.9 for most applications, with much lower limits (0.23) for ophthalmic use.
Output Display Measures and SPTA
Federal regulation historically limited scanner output by intensity , and the variant that matters for bioeffects is the spatial peak, temporal average intensity (SPTA) . The name describes the measurement exactly: find the point in the beam where intensity is highest (spatial peak), then average the intensity there over the full time the beam dwells, including the dead time between pulses (temporal average). Because SPTA captures both where heating concentrates and how continuously energy is delivered, it is the intensity most relevant to thermal bioeffects — the classic exam answer. Current FDA (Track 3) limits cap derated SPTA at 720 mW/cm² for most applications and 50 mW/cm² for ophthalmic imaging, alongside the MI ceilings of 1.9 and 0.23 noted above. The older 17 mW/cm² ophthalmic figure belongs to the superseded Track 1 regime and is a common stale-source trap.
Since 1992, systems approved under the output display standard (ODS) instead show MI and TI on screen in real time , shifting responsibility to the operator: the machine reports estimated risk, and the sonographer must watch the indices and act accordingly. This is why the ARRT expects fluency with the meaning of each displayed number rather than memorized intensity limits alone.
ALARA
ALARA — as low as reasonably achievable — is the governing safety philosophy of diagnostic ultrasound. Because no exposure can be proven perfectly risk-free, prudent practice keeps exposure at the minimum that still yields a diagnostic image. In concrete terms:
Use the lowest output (transmit) power that answers the clinical question; brighten the image with receiver gain instead, which costs the patient nothing.
Minimize dwell time : keep the probe moving and do not linger on one structure, especially at high TI/MI settings.
Prefer the highest frequency that penetrates adequately when cavitation is a concern, and limit Doppler use — spectral and color Doppler use long dwell times and high pulse repetition, producing the highest TI values of any mode. This is why first-trimester Doppler of the fetus is discouraged unless clinically indicated.
Take special care with obstetric (especially first-trimester embryonic) and neonatal imaging, where tissues are most sensitive.
AIUM Position and Epidemiologic Evidence
The American Institute of Ultrasound in Medicine (AIUM) has reviewed decades of laboratory and epidemiologic data. Its official statements conclude that no independently confirmed harmful bioeffects have been demonstrated in mammalian tissue at diagnostically relevant exposures that do not produce significant temperature elevation, and epidemiologic studies of humans exposed in utero have revealed no confirmed causal relationship between diagnostic ultrasound and adverse outcomes. The AIUM simultaneously emphasizes that research continues and prudent use under ALARA — including avoiding non-medical 'keepsake' imaging — remains the professional standard.
- Index What it estimates When/where used
- TI (TIS/TIB/TIC) Maximum temperature rise (thermal risk) Soft tissue / bone at focus / bone at surface
- MI Likelihood of inertial cavitation (mechanical risk) Gas-body interaction; capped at 1.9
- SPTA Average intensity at the beam's hottest point Regulatory intensity limit; thermal relevance