Which method is based on BOLD contrast?
Functional MRI routinely uses blood-oxygen-level dependent contrast originating from intrinsic magnetic properties of hemoglobin. Oxyhemoglobin is diamagnetic while deoxyhemoglobin is paramagnetic and creates microscopic magnetic field inhomogeneity shortening T2* relaxation and reducing signal. Enhanced neuronal firing drives neurovascular coupling, increasing local blood flow and influx of oxygenated blood, washing out deoxyhemoglobin, prolonging T2* and raising fMRI intensity. This physiological cascade serves as surrogate for activation. PET relies on radiotracer positron decay, CT quantifies X-ray absorption, SPECT captures single gamma rays, only fMRI depends specifically on BOLD contrast.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.