Final answer:
The dosimetry index used to approximate the radiation dose for CT sections acquired during a helical scan is called the CT Dose Index (CTDI).
Step-by-step explanation:
The dosimetry index used to approximate the radiation dose for CT sections acquired during a helical scan is called the CT Dose Index (CTDI). CTDI quantifies the average radiation dose over a specific length of the patient's body. It is determined by measuring the radiation output from the CT scanner and the physical characteristics of the scanner's beam.