Guesswork... 40khz ultrasound has a wavelength of about 8mm. If your phase from end to end isn't at least 90 degrees, you don't have enough shift to form a beam with. I would imagine if you're clever, and your transducers don't drop off to fast on the sides, and you've got them fairly tightly constrained in X,Y,Z to a regular grid, you might be able to tweak out more resolution slightly off center, because the return phases will more quickly diverge as this angle increases.
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Guesswork... 40khz ultrasound has a wavelength of about 8mm. If your phase from end to end isn't at least 90 degrees, you don't have enough shift to form a beam with. I would imagine if you're clever, and your transducers don't drop off to fast on the sides, and you've got them fairly tightly constrained in X,Y,Z to a regular grid, you might be able to tweak out more resolution slightly off center, because the return phases will more quickly diverge as this angle increases.