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Sodium MRI of the Human Kidney

Magnetic Resonance imaging (MRI) produces anatomical images of the human kidney that have exquisite resolution of the renal cortex, the medulla and the collecting systems (for reviews see: [1-5]). Over the past decade there have been several technical developments in MRI that have shown a great deal of promise in providing non-invasive methods for assessing various aspects of renal function in patients. These include the “Renal Bold“ method which produces images that can reflect renal hypoxia [6-10], arterial spin labeling (ASL) which provides quantitative maps of renal perfusion [11-13], and 23Na MR imaging [15], which can measure the cortical-medullary sodium gradient. Our institution has previously been involved in the development and validation of both the renal bold method and renal ASL.

Our current efforts in 23Na MRI of the kidney were motivated by the work of Maril et al [16-18] who showed that 23Na has the potential to provide important information about one aspect of renal function, namely the ability to concentrate sodium ions. Dr. Maril’s presence in our group from 2003-2005 coupled with our own interest in 23Na imaging [14, 19] led us to acquire sodium images of the kidney in normal volunteers using a previously published three dimensional gradient echo sequence (3D GRE) [14] which for the first time delineated the cortical-medullary sodium gradient in humans [15] See Figure 1.

LL_HK_fig1

We were also able to show that water deprivation led to the expected increase in the cortical-medullary sodium gradient [15]. These initial results establish a “benchmark” against which we can judge whether the different approaches produce improved 23Na images. We applied this method to produce the 23Na images of transplanted kidney, shown in Figure 2. Note that only some of the renal pyramids show a sodium gradient in the patient with CRF.

LL_HK_Fig2

The focus of this project is to complete the development and validation of 23Na imaging of the kidney for patients who have undergone kidney transplant and patients with chronic renal failure. This will set the stage for using 23Na MRI as part of a clinical integrated functional renal examination that can be used at 3T to evaluate patients with different kinds of renal disease that may have different underlying pathophysiological causes. The elements of the integrated renal examination are being developed and validated in close collaboration with our colleague (Dr. Orson Moe) in the Division of Nephrology, Department of Medicine, who is one of the world’s experts in renal physiology.

While our initial results show great promise, we believe that there are still significant improvements to be made in the acquisition methods, the reconstruction algorithms, and RF coil design that are employed. We believe that a thorough investigation of each of these three elements will improve the overall signal-to-noise of the 23Na study by at least a factor of two. This improvement can be used to either shorten the scan time, improve the spatial resolution or some combination of each. We will judge improvements in 23Na imaging by their influence on the precision with which we are able to measure the cortical-medullary sodium gradient in phantoms, normal volunteers and patients.

 

References

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