Which imaging approach typically uses less than 90-degree excitation pulses?

The imaging approach that typically uses less than 90-degree excitation pulses is Gradient Echo Imaging. This technique is designed to make efficient use of the available radiofrequency (RF) energy, and using smaller excitation pulse angles helps to minimize the amount of power needed while allowing for faster acquisition of images.

In Gradient Echo Imaging, the less-than-90-degree flip angles contribute to a more rapid repetition of the imaging sequence, which is advantageous for capturing dynamic processes or obtaining images with high temporal resolution. Additionally, this capability allows for variable contrast weighting and can provide unique visualization of different tissue types.

In contrast, other imaging techniques like Spin Echo Imaging mainly rely on 90-degree excitation pulses followed by 180-degree refocusing pulses to produce images with high-quality magnetic resonance signals and robust T1 and T2 contrast. This makes them less suited for the fast imaging applications where Gradient Echo Imaging excels. Phase Contrast Imaging uses a different principle altogether, focused on the flow of fluids rather than pulse angles, while Diffusion Weighted Imaging emphasizes water molecule diffusion rather than the specifics of flip angles. Therefore, Gradient Echo Imaging's method of using less than 90-degree excitation pulses is specific to its design and clinical applications, making it the correct answer.