Thermal index
The thermal index (TI) is intended as a measure of an ultrasound beam's thermal bioeffects. It is often displayed on ultrasound screens (along with the mechanical index).
Absorption of sound waves may cause heating in tissue.
The thermal index depends on:
- a measure of time-averaged acoustic power
- assumptions of the properties of the tissue being heated
- assumptions about the path of the ultrasound beam
If beam focusing is unchanged, then the thermal index is directly related to the output power of the system:
thermal index = Wp / Wdeg
- Wp: relevant (attenuated) acoustic power at the depth of interest
- Wdeg: estimated power necessary to raise the tissue equilibrium temperature by one degree Celsius
So, if the acoustic power of the ultrasound beam is enough to raise the tissue temperature by 1.0°C, then the thermal index = 1.0. It should be noted that this is a relative risk of heating, not an absolute scale of tissue heating.
The thermal index at the skin surface, and in other special circumstances, is calculated differently 2.
The clinical relevance of the thermal index most often comes into play with imaging of the embryo. Color and spectral Doppler imaging calls for increased levels of ultrasound output power, and this in turn increases the thermal index. The use of color and spectral Doppler is discouraged when imaging an embryo to avoid crossing a theoretical thermal index threshold. Current guidelines recommend a thermal index <1.0 for first trimester screening (11 weeks to 13 weeks, 6 days) with exposure time to Doppler imaging for as little as possible, preferably less than 5-10 minutes.
The thermal index (and mechanical index) have been widely adopted in the radiology community as proxies for ultrasound bioeffects, but some recent discussion in ultrasound societies suggest that both indices may need revision to more accurately represent potential bioeffects 2.
The thermal index may be displayed in some ultrasound systems as:
- TIS: thermal index in soft tissue
- TIB: thermal index for bone at the focus
- TIC: thermal index for cranial bone
The conventional thermal index is not valid for 3D ultrasound or elastography.
Related Radiopaedia articles
Imaging technology
- imaging technology
- imaging physics
- imaging in practice
-
x-ray
- x-ray physics
- x-ray in practice
- x-ray production
- x-ray tubes
- tube rating
- filters
- automatic exposure control (AEC)
- beam collimators
- grids
- air gap technique
- cassette
- intensifying screen
- x-ray film
- image intensifier
- digital radiography
- digital image
- mammography
- x-ray artifacts
- radiation units
- radiation safety
- radiation detectors
- fluoroscopy
-
computed tomography (CT)
- CT physics
- CT in practice
- CT technology
- CT image reconstruction
- CT image quality
- CT dose
- CT contrast media
-
CT artifacts
- patient-based artifacts
- physics-based artifacts
- hardware-based artifacts
- ring artifact
- tube arcing
- out of field artifact
- air bubble artifact
- helical and multichannel artifacts
- CT safety
- history of CT
-
MRI
- MRI physics
- MRI in practice
- MRI hardware
- signal processing
-
MRI pulse sequences (basics | abbreviations | parameters)
- T1 weighted image
- T2 weighted image
- proton density weighted image
- chemical exchange saturation transfer
- CSF flow studies
- diffusion weighted imaging (DWI)
- echo-planar pulse sequences
- fat-suppressed imaging sequences
- gradient echo sequences
- inversion recovery sequences
- metal artifact reduction sequence (MARS)
-
perfusion-weighted imaging
- techniques
- derived values
- saturation recovery sequences
- spin echo sequences
- spiral pulse sequences
- susceptibility-weighted imaging (SWI)
- T1 rho
- MR angiography (and venography)
-
MR spectroscopy (MRS)
- 2-hydroxyglutarate peak: resonates at 2.25 ppm
- alanine peak: resonates at 1.48 ppm
- choline peak: resonates at 3.2 ppm
- citrate peak: resonates at 2.6 ppm
- creatine peak: resonates at 3.0 ppm
- functional MRI (fMRI)
- gamma-aminobutyric acid (GABA) peak: resonates at 2.2-2.4 ppm
- glutamine-glutamate peak: resonates at 2.2-2.4 ppm
- Hunter's angle
- lactate peak: resonates at 1.3 ppm
- lipids peak: resonates at 1.3 ppm
- myoinositol peak: resonates at 3.5 ppm
- MR fingerprinting
- N-acetylaspartate (NAA) peak: resonates at 2.0 ppm
- propylene glycol peak: resonates at 1.13 ppm
-
MRI artifacts
- MRI hardware and room shielding
- MRI software
- patient and physiologic motion
- tissue heterogeneity and foreign bodies
- Fourier transform and Nyquist sampling theorem
- MRI contrast agents
- MRI safety
-
ultrasound
- ultrasound physics
-
transducers
- linear array
- convex array
- phased array
- frame averaging (frame persistence)
- ultrasound image resolution
- imaging modes and display
- pulse-echo imaging
- real-time imaging
-
Doppler imaging
- Doppler effect
- color Doppler
- power Doppler
- B flow
- color box
- Doppler angle
- pulse repetition frequency and scale
- wall filter
- color write priority
- packet size (dwell time)
- peak systolic velocity
- end-diastolic velocity
- resistive index
- pulsatility index
- Reynolds number
- panoramic imaging
- compound imaging
- harmonic imaging
- elastography
- scanning modes
- 2D ultrasound
- 3D ultrasound
- 4D ultrasound
- M-mode
-
ultrasound artifacts
- acoustic shadowing
- acoustic enhancement
- beam width artifact
- reverberation artifact
- ring down artifact
- mirror image artifact
- side lobe artifact
- speckle artifact
- speed displacement artifact
- refraction artifact
- multipath artifact
- anisotropy
- electrical interference artifact
- hardware-related artifacts
- Doppler artifacts
- aliasing
- tissue vibration
- spectral broadening
- blooming
- motion (flash) artifact
- twinkling artifact
- acoustic streaming
- biological effects of ultrasound
- history of ultrasound
-
nuclear medicine
- nuclear medicine physics
- detectors
- tissue to background ratio
-
radiopharmaceuticals
- fundamentals of radiopharmaceuticals
- radiopharmaceutical labeling
- radiopharmaceutical production
- nuclear reactor produced radionuclides
- cyclotron produced radionuclides
- radiation detection
- dosimetry
- specific agents
- carbon-11
- chromium-51
- fluorine agents
- gallium agents
- Ga-67 citrate
- Ga-68
- iodine agents
-
I-123
- I-123 iodide
- I-123 ioflupane (DaTSCAN)
- I-123 ortho-iodohippurate
- I-131
-
MIBG scans
- I-123 MIBG
- I-131 MIBG
-
I-123
- indium agents
- In-111 Octreoscan
- In-111 OncoScint
- In-111 Prostascint
- In-111 oxine labeled WBC
- krypton-81m
- nitrogen-13
- oxygen-15
- phosphorus-32
- selenium-75
-
technetium agents
- Tc-99m DMSA
- Tc-99m DTPA
- Tc-99m DTPA aerosol
- Tc-99m HMPAO
- Tc-99m HMPAO labeled WBC
- Tc-99m MAA
- Tc-99m MAG3
- Tc-99m MDP
- Tc-99m mercaptoacetyltriglycine
- Tc-99m pertechnetate
- Tc-99m labeled RBC
- Tc-99m sestamibi
- Tc-99m sulfur colloid
- Tc-99m sulfur colloid (oral)
- thallium-201 chloride
- xenon agents
- in vivo therapeutic agents
- pharmaceuticals used in nuclear medicine