ALARA
As Low As Reasonably Achievable
Basic general principle in Radiology: us the least amount of energy exposure possible to achieve adequate imaging.
There is no evidence of US causing harm to humans in any setting.
There is some evidence of teratogenicity in in pregnant mice which were exposed to US of unrealistically high intensity and/or duration (Child 1988, Hande 1995).
Therefore, diagnostic US safety concerns are purely theoretical.
There are only 2 areas of sonography where ANY safety concerns have ever been raised: OB and ophthalmology. Regardless of lack of evidence, OB and Opthalmic scan time and intensity should be kept to the minimum necessary – ALARA.
American Institute of Ultrasound in Medicine (AIUM) STRONGLY advises against “Entertainment US” – ultrasound done without any medical indication. More info here.
HM-POCUS does not include fetal US, and power output options are well hidden on most machines. Therefore there is not a lot one can mess up.
Here are a few practical points concerning ALARA in the context of HM-POCUS:
- It only applies to Ophthalmic US
- It only uses a linear (vascular) probe on the “ER -> Ophthalmology” settings
- Limit dwell time (move around)
- Limit scan time to minimal needed
- Be particularly mindful in cases of retinal detachment and foreign bodies. These can theoretically move around under the influence of US
- Avoid using Doppler, unless required (more energy)
- In ophthalmic preset TIS=1
- The concern is largely theoretical
- Even on theoretical level, any organ with robust blood flow, such the heart, is virtually immune to the thermogenic effect of US, because blood absorbs the heat and carries it away from the organ.
Theory
US waves transfer mechanical energy. This is familiar to most from therapeutic application of US in lithortipsy. Some of this mechanical energy is absorbed by the tissues, and is partly transformed into heat, and partly used up for a process called cavitation: formation of micro-bubbles in liquids. The heating effect (at much higher intensity of US than what is used for diagnostic purposes) is used in physical therapy. Cavitation can also be used for benefit (for example, for mixing liquids). See this video for an illustration. Again, the US energy delivered in these application much greater than that used in diagnostics. Herperthermia is known to be teratogenic, and it is the ability of US to heat up the tissues which is speculated to be of importance prenatally.
Accordingly, there are 2 common measures of the amount of US energy delivered to the tissue:
- mechanical index (MI)
- thermal index (TI)
TI is defined as delivered power (Watts) / power required to raise tissue Temp by 1C.
Specifically with OB applications in mind, 3 separate TIs are distinguished, based on different tissue properties:
- Soft Tissue (TIS)
- Bone (TIB)
- Cranial (bone) (TIC)
TI < 1 is considered generally safe, and TI < 0.7 is considered ABSOLUTELY safe.
In Ophthalmic preset TIS=1. Extrapolating from OB, this setting should be generally safe, but ALARA principle is still advised: limit scan time to as short as possible.
Mechanical index (MI), especially in the eye, is not well studied.