For most imaging systems, diffraction limits the resolution. That is, the resolution is limited to the wavelength divided by the angle subtended by the receiver aperture, which means that the ultimate resolution is approximately the wavelength itself. This is true for imaging systems based on optics, ultrasound, and x-rays (although there are other important factors, such as quantum noise, in x-ray). MRI is the only imaging system for which the resolution is independent of the wavelength. In MRI, the wavelength is often many meters, yet submillimeter resolution is routinely achieved. The basic reason is that no attempt is made to focus the radiation pattern to the individual pixel or voxel (volume element), as is done in all other imaging modalities. Instead, the gradients create spatially varying magnetic fields so that individual pixels emit unique waveform signatures. These signals are decoded and assigned to unique positions. An analogous problem is isolating the signals from two transmitting antenna towers separated by much less than a wavelength. Directive antenna arrays would fail because of diffraction spreading. However, we can distinguish the two signals if we use the a priori knowledge that the two antennas transmit at different frequencies. We can receive both signals with a wide-angle antenna and then distinguish the signals through frequency-selective filtering.