Fig. 4

Schematic representation of renal AML diagnosis on OP MRI. In larger lesions (a) the presence of india ink artefact around a nodule embedded within the renal parenchyma is diagnostic of gross fat. If the nodule is extremely small (b), india ink artefact may obscure the entire lesion and the AML will appear only as a spot of signal loss within the renal parenchyma. Similarly, if the base voxel resolution is too large (c), india ink artefact may obscure the centre of the lesion so that the entire lesion appears as a signal loss. A clinical example illustrates these principles in a 38-year-old woman with tuberous sclerosis. A small (5 mm) AML is present in the upper pole of the right kidney on axial T1-weighted IP, OP, and FS GRE images (left to right in d). The AML is depicted as a focus of increased T1 signal intensity on IP (black arrow) with an etching artefact around its circumference on OP (open black arrow) imaging and which loses signal intensity with chemical fat suppression (white arrow). The lesion was only prospectively identified on the opposed phase image and was indeterminate at CT (not shown). In the same patient at a lower level (e), two tiny AML (<5 mm in size) in the lower pole of the left kidney are only prospectively identified as areas of signal loss on the opposed phase images (white arrows) but in retrospect also demonstrate signal intensity loss with fat suppression (dotted arrows). A similar phenomenon will be seen if a larger voxel size is used (see Fig. 3)