An apparatus (100), control system (150) and methods are provided for directly measuring a pressure gradient, i.e. by real-time pressure measurements, with particular application for in situ measurement of transvalvular blood pressure gradients for the aortic valve and other heart valves, using minimally-invasive techniques. The apparatus takes the form of a multi-sensor assembly, e.g. enclosed within a micro-catheter or a steerable guidewire, and comprises a plurality of optical pressure sensors (10) is arranged along a length of the distal end portion (101), for measuring pressure simultaneously at each sensor location. For example, four MOMS optical pressure sensors (10), and optionally, a flow sensor (20), are incorporated into a distal end portion (101) having a diameter of 0.89 mm or less, and preferably 0.46 mm or less. Beneficially, all sensors are optically coupled, via respective optical fibers (11), to an optical coupler (112) at the proximal end of the multi-sensor apparatus, without requiring electrical connections.