Tuesday Case of the Day Physics History: Authors: Ryan Griesbach, MD and Lori Mankowski Gettle, MD, MBA Authors affiliation: UW Hospitals and Clinics, Madison WI History: Multiphase gadolinium enhanced MRI of the kidneys was performed. Delayed imaging shows a dark area within the right renal collecting system on T1 and T2 weighted images. No corresponding dark focus was seen on the pre-contrast images. Likewise no corresponding abnormality was present on recent prior CT. What is the source of the dark area? Punctate renal stone. Signal void due to T2 shortening at higher gadolinium concentrations. Aliasing of high signal from gadolinium T1 shortening. Inhomogeneity within the main magnetic field. T1 Pre T1 PV T1 5 min T2 Pre T2 5min CT

Diagnosis: B. Signal void due to T2 shortening at higher gadolinium concentrations.

Discussion: T2 shortening at higher gadolinium concentrations A. MR imaging of renal calculi generally shows nonspecific signal voids within the collecting system. This signal void should not increase over time or after contrast administration on either T1 or T2 weighted images. B. At lower gadolinium concentrations, such as those used in routine MR imaging, the T2 shortening effects of gadolinium are overwhelmed by the dominant T1 shortening effects. However as gadolinium concentrations increase, the T2 shortening properties of gadolinium begin to predominate. This results in signal drop out in areas of high gadolinium concentrations. This phenomenon can be seen within the gallbladder (from hepatic excreted gadolinium such as Gadoxetate disodium [Evoist®]) or with the urinary collection system (from renal excretion gadolinium such as Gadobenate dimeglumine [Multihance®]). C. Aliasing in MR imaging occurs when the field of view is smaller than the imaged body part. This results in wrap-around artifact where the portion of tissue beyond the edge of the field of view projects onto the other side of the image. This will occur on all pulse sequences but can be corrected by expanding the field of view, among other techniques. D. Inhomogeneity within the main magnetic field will produce a variety of artifacts including blurring, shading, curved slices, or Moiré fringes. These artifacts can be improved by magnetic shimming and by removing objects with intrinsic magnetic properties from the scanner area.

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