MI and TI on the Ultrasound Screen: What They Mean and When to Act
The two small numbers in the corner of every image are the machine’s estimate of how much it could be affecting the tissue. What they measure, the limits safety bodies recommend, and the controls that bring them down.
What MI and TI measure
Both indices are calculated by the machine from the probe, mode and settings in use, and update as you change them. They are relative guides, not measurements inside this patient — the British Medical Ultrasound Society (BMUS) notes that TI can underestimate the real temperature rise by up to a factor of two.
| Mechanical index (MI) | Thermal index (TI) | |
|---|---|---|
| Estimates | The potential for non-thermal effects, mainly cavitation — gas bubbles oscillating or collapsing in the beam | The potential for tissue heating |
| Rises with | Higher output power and lower frequency (MI is peak negative pressure divided by the square root of frequency) | Higher output, spectral Doppler, narrow or deep write-zoom boxes, multiple focal zones and a probe held still |
| Matters most for | Ultrasound contrast agents, and gas-containing tissue such as neonatal lung and bowel | The embryo and fetus, the eye, the neonatal brain and spine, and febrile patients |
TIS, TIB and TIC: which one to watch
Many systems show MI and one TI at a time, and the preset usually decides which. Check that the TI shown suits the exam. Under IEC 60601-2-37, the safety standard for diagnostic ultrasound, a system able to exceed an index of 1.0 must display it.
TIS — soft tissue
Assumes only soft tissue in the beam. BMUS recommends it for pregnancy up to 10 weeks after the last menstrual period, before fetal bone forms, and for the eye.
TIB — bone at the focus
Assumes bone where the beam is most intense, which heats far more than soft tissue. Watch TIB in pregnancy after 10 weeks and, by BMUS advice, in most general abdominal and vascular scanning. TIB is always equal to or higher than TIS.
TIC — bone at the surface
Assumes bone right under the probe, as in transcranial scanning, or whenever bone lies within about 1 cm of the skin.
Recommended scan-time limits
BMUS publishes maximum scanning times by displayed TI. The obstetric column also covers gynaecology when pregnancy is possible, and the neonatal brain and spine; the general column covers abdominal, peripheral vascular and unlisted adult exams. The AIUM publishes its own table with different bands — follow the guideline your service adopts, and apply ALARA at every level.
| Displayed TI | Obstetric (TIS to 10 weeks, TIB after) | General abdominal and vascular (usually TIB) |
|---|---|---|
| 0 – 0.7 | No known reason to restrict time | No known reason to restrict time |
| 0.7 – 1.0 | 60 minutes | No known reason to restrict time |
| 1.0 – 1.5 | 30 minutes | 120 minutes |
| 1.5 – 2.0 | 15 minutes | 60 minutes |
| 2.0 – 2.5 | 4 minutes | 15 minutes |
| 2.5 – 3.0 | 1 minute | 4 minutes |
| 3.0 – 4.0 | Not recommended for an embryo or fetus, however briefly | 1 minute |
| Above 4.0 | Not recommended | 15 seconds to 5.0, 5 seconds to 6.0, not recommended above 6.0 |
Read next Obstetric ultrasound and ALARA
Situations that need extra care
Doppler in early pregnancy
The WFUMB/ISUOG statement on scans at 11–14 weeks or earlier says pulsed Doppler — spectral, power and colour — should not be used routinely. When it is clinically indicated, keep the displayed TI at 1.0 or below and the exposure as short as possible, usually no longer than 5–10 minutes and never more than 60.
The eye
The lens and vitreous have no blood supply to carry heat away. Use an ophthalmic preset, which caps output — in the USA at an MI of 0.23 and a TI of 1.0 — and BMUS does not recommend scanning the eye at a TIS above 1.0.
Neonates
BMUS notes a possibility of minor damage to neonatal lung or intestine above an MI of 0.3, so keep exposure short. Scans of the neonatal brain and spine follow the same time limits as the fetus.
Contrast agents
Above an MI of 0.7 there is a risk of cavitation when a microbubble contrast agent is in use, which is why contrast-specific imaging modes run at low MI.
Fever
A febrile patient’s tissue is already warmer, so reduce output and exposure time — especially in pregnancy.
Warm endocavity probes
International standards limit a probe’s surface temperature to 43 °C against the patient. BMUS advises against using an endocavity probe that heats noticeably while running in air.
How to bring MI and TI down
- Start each patient on a low output power setting and raise it only if the image needs it.
- Raise the receive gain before the output power: gain brightens the image without sending more energy into the patient.
- Choose the right preset — obstetric, ophthalmic, neonatal — because presets carry their own output limits.
- Freeze and review the cine loop instead of keeping the probe live on the patient.
- Find the vessel in B-mode or colour first, and switch on spectral Doppler only to measure.
- Keep the colour box small and shallow, and avoid long, narrow, deep write-zoom boxes and unnecessary focal zones.
- Watch the TI whenever you change mode — it can jump when pulsed Doppler is switched on.
- Have the displayed indices checked after installation and after hardware or software changes, as BMUS recommends.
Read next Knobology: gain, power, depth and focusDoppler modes and when you need them
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