The signal starts to increase in amplitude until it reaches the echo time (te).
Free induction decay is the scenario where the signal decreases in amplitude over time after a 90° RF pulse, while spin echo is the scenario where the signal increases in amplitude until it reaches the echo time (te) after a 180° RF pulse.
The periphery of k-space has a very rich and important amount of information encoded by high frequency information, which is essential for spatial resolution and edge definition in the image.
Understanding the distribution of data in k-space is important for accelerating the speed of imaging, manipulating the way information is placed in k-space, and exploiting conjugate symmetry in k-space to reduce imaging time.
Keyhole Imaging is a technique in MRI where the center piece of k-space is acquired separately, allowing for high quality studies with a reduced imaging time. This is achieved by understanding the nature of what is placed in the periphery and center of k-space and manipulating it accordingly.
Keyhole Imaging is used in clinical practice for imaging patients in real-time by acquiring a full resolution image of the entire brain before contrast is injected, and then only acquiring the central fifth of k-space during the dynamic part of the study. The peripheral part of k-space from the pre-contrast image is then used to fill in the missing information, allowing for high temporal resolution images of the contrast transit through the tissue.