Final answer:
The key difference between gradient and spin echo sequences in MRI is the resultant image contrast, due to different handling of magnetic field inhomogeneities and relaxation mechanisms of nuclei within tissues.
Step-by-step explanation:
The difference between a gradient echo sequence and a spin echo sequence is Image contrast. These are two different MRI pulse sequences that create images with different contrasts and are useful for evaluating different tissue properties. A gradient echo sequence uses gradient fields to generate phase shifts in the spins, making the overall process faster, but more susceptible to magnetic field inhomogeneities. On the other hand, a spin echo sequence refocuses the spins using a 180-degree pulse, which corrects for inhomogeneities in the magnetic field and typically yields better T2 contrast images. The relaxation times involved, specifically T1 and T2, influence the contrast seen in the images produced by either sequence.