A(n) scan uses radioactive markers in the blood to monitor blood flow and metabolic activity via x-ray

The answer is a pet scan

PET Scan is commonly used to explore the possibility of Cancer Metastasis.

One of the disadvantage of using a pet scan is that it has a high amount of Operational Cost.

Overall, PET Scan is very useful in detecting cancer possibility and heavily used in the field of oncology

It allows the reader to feel how lost Helen feels before she learns to communicate.

Explanation:

Helen Keller's The Story of My Life is an autobiographical account of her life as a blind and deaf person. Beginning from her childhood days, even before she learned how to communicate, she recounts how she felt as a handicapped child.

In the given passage, she recounts how frightened she was when she first started to be given special education under the supervision of Miss Annie Sullivan. She admits she felt like a ship, "tense and anxious, grop[ing] her way toward the shore with plummet and sounding-line, and waited with beating heart for something to happen". The passage shows how lost she felt and uncertain about what the future holds for her, especially the start if her education. This passage shows the reader to feel how lost Helen feels before she her education.