Final answer:
To appropriately modify a high mechanical index during an ultrasound, the most suitable action is to decrease the output power.
Step-by-step explanation:
If a sonographer notices that the mechanical index (MI) is too high during an ultrasound, the MOST appropriate modification that the sonographer can make is to decrease the output power. This action would reduce the pressure wave amplitude and subsequently the Mechanical Index, leading to a safer ultrasound examination. Adjusting the receiver gain would not directly affect the MI as it is related to the image brightness after the echo has been received. Increasing the period, which is the inverse of frequency, would not necessarily impact the MI, as the MI is a function of peak rarefactional pressure and frequency. Decreasing the frequency of the transducer would lower the resolution and is not directly related to the MI.