What happens to scan time as TR increases?

As the repetition time (TR) increases, the scan time also increases. This relationship stems from the fundamental principles of MRI imaging. TR is the time between successive excitations of the same slice. A longer TR allows for more time between successive pulse sequences, which means it takes a longer total time to complete the full sequence needed to generate the images for the scan.

When TR is extended, the system has to accommodate the delays in excitation and relaxation of the spins within the magnetic field, which extends the overall scan duration. Thus, as TR goes up, scan time increases because the time allocated to each phase of the imaging process is lengthened, directly affecting the total time needed to complete the imaging protocol.

It's important to note that this relationship is critical for understanding both the speed of image acquisition and the quality of the resulting images, as variations in TR can affect image contrast and the overall protocol efficiency.