A 52-year-old woman with bipolar disorder has been on lithium 900 mg daily for 3 years with good mood stabilization. She presents with coarse tremor, confusion, and ataxia. Serum lithium level is 2.8 mEq/L (normal therapeutic range 0.6–1.2 mEq/L). Which investigation is most appropriate to assess the extent of lithium-induced organ damage?