Single shot 3D acquisitions are ideal for ASL because they provide insensitivity to motion and allow for effective background suppression. Current state of the art implementations use stack-of-spiral FSE based imaging for time efficiency and insensitivity to resonance offsets. We introduce here a further improvement in time efficiency by using spirals that are designed to approximate a spherically symmetrical variable density function. Spirals are constrained to elliptical shells, and are numerically optimized for time efficiency. An example trajectory is twice as fast as a cylindrical stack-of-spirals with similar density parameters (excluding refocusing time), and results in high quality ASL images.