We have used magnetic resonance imaging to study the structure of and convection within a solidifying mushy layer formed from an aqueous sucrose solution cooled from above. We focus on the situation in which dissolution channels, known as chimneys, are created by the action of buoyancy-driven convection. We have obtained high-resolution images of the microstructure formed by individual ice platelets and coarser-grained images that average over the platelets to show the geometry of the dissolution channels. We observe that the chimneys are branched and occur only in the lower part of the mushy layer. By acquiring low-resolution images rapidly, we have made detailed measurements of the thickness of the mushy layer, its porosity distribution and the number and total area of the chimneys. The mushy layer is seen to grow in a self-similar manner until internal convection begins, whereafter the solid fraction increases in the lower part of the layer.