Question: how many set of paired gradient are present in clinical mr scanner

title: Physics of magnetic resonance imaging
text: Using the k - space formalism , a number of seemingly complex ideas became simple . For example , it becomes very easy to understand the role of phase encoding ( the so - called spin - warp method ) . In a standard spin echo or gradient echo scan , where the readout ( or view ) gradient is constant ( e.g. , G ) , a single line of k - space is scanned per RF excitation . When the phase encoding gradient is zero , the line scanned is the k axis . When a non-zero phase - encoding pulse is added in between the RF excitation and the commencement of the readout gradient , this line moves up or down in k - space , i.e. , we scan the line k = constant .