Which imaging technique uses a very rapid pulse sequence during patient rest or activity?

Functional MRI (fMRI) employs a very rapid pulse sequence to capture changes in brain activity by measuring blood flow and oxygenation levels. This technique is based on the principle that active brain regions consume more oxygen, leading to a change in the ratio of oxygenated to deoxygenated blood.

By using short and efficient pulse sequences, fMRI can capture these changes in real time, often during specific tasks or while the patient is at rest. The rapid acquisition of images allows for the analysis of dynamic processes in the brain, making it particularly useful for understanding brain function during various activities or states of rest.

In contrast, other techniques like perfusion primarily focus on blood flow without necessarily capturing functional changes in response to tasks. MR Spectroscopy analyzes the chemical composition of tissues rather than activity levels, and Diffusion Tensor Imaging examines the integrity of white matter pathways, which does not involve direct measurement of ongoing brain function. Thus, the specificity of fMRI in assessing brain activity makes it the most appropriate answer for this question.