ISMRM SYLLABUS: MR UROGRAPHY

Course: Pelvic MR Imaging (Weekday educational course): Monday, April 24 Speaker: Bobby Kalb, MD (bkalb@radiology.arizona.edu) Target audience: Radiologists, imaging scientists and MRI technologists Objective: To educate the attendee on the current state of MR Urography. Specifically, the attendees will learn technical aspects related to image acquisition and protocol optimization for two distinct, but important clinical scenarios: tumor detection and quantitative renal functional analysis by MRI. Clinical aspects of image interpretation will be covered to improve the ability of the attendee to differentiate between different renal and urothelial neoplasms. In addition, technical considerations regarding the ability to obtain glomerular filtration rates through latest generation image acquisition techniques focused on dynamic free-breathing analysis of renal perfusion will be discussed. Purpose: Chronic kidney disease (GFR < 60 mL/min) is significant public health issue, with an estimated prevalence of approximately 11% in developed countries. End-stage renal disease affects more than 650,000 patients per year in the United States, and consumes large amounts of per-capita healthcare spending (approximately 7% of the Medicare budget). In addition to medical renal disease, neoplastic diseases of the kidney and bladder (renal cell and urothelial carcinoma) is also a common health issue, with >60,000 new cases diagnosed each year, with >14,000 deaths. Appropriate diagnosis and management of patients with both end-stage renal disease and renal-urothelial neoplasms are important to guide appropriate therapeutic decision making. In patients with end-stage renal disease, especially after undergoing renal transplantation, accurate assessment of graft function in combination with anatomic assessment of the renal parenchyma, vascular and collecting systems are important to optimize therapies to target graft survival. In addition, patients with neoplastic disease of the renal and urothelial system benefit from optimized, non-invasive diagnosis of tumor type to guide appropriate management (surgical versus nonsurgical). In patients that require surgery, accurate pre-operative assessment of single kidney function helps guide surgical management decisions and approaches, especially related to partial nephrectomies. Methods: While MRI has shown the ability to calculate quantitative measures of renal function (such as GFR and renal blood flow), and has also been used to accurately diagnose neoplasms of the renal/urothelial system, hardware and sequence limitations have limited the ability of a single protocol to provide simultaneous assessment of both renal function and renal/urothelial neoplasm. Dynamic perfusion imaging of the kidney is used to assess quantitative renal function, and has typically required highly accelerated, low-resolution 3D gradient echo sequences that have not been optimized for renal or urothelial parenchymal analysis. Earlier studies have reported GFR values using mostly 2D or 3D Cartesian view sharing techniques, however these methods are susceptible to temporal blurring. Non-Cartesian techniques are more robust to motion artifact secondary to oversampling of the center of k-space, and hold promise for improvements in temporal and spatial resolution to allow for adequate parenchymal soft tissue analysis. Results: Recent advances in MR acquisition methods, including novel radial acquisition methods utilizing a golden angle stack of stars approach, have allowed for preservation of high-resolution, dynamic post-contrast imaging that can simultaneously acquire both the parenchymal tissue information important for tumor analysis in conjunction with perfusion data required for post-acquisition modelling to calculate quantitative renal function measures. In addition, compressed sensing methods of image analysis allow improvements in image quality that reduces artifacts related to sparse data collection during rapid image acquisition. Initial investigations utilizing this method suggests close approximation with standard methods of renal functional analysis that are reliant on laboratory analysis of estimated GFR. Discussion: Previously, analysis of both renal function and anatomic renal/urothelial imaging would require separate imaging studies, increasing the time, complexity and cost of the exam due to technical obstacles to simultaneous acquisition. Prior techniques, especially using Cartesian methods, have not allowed both rapid temporal resolution and adequate anatomic evaluation, precluding a comprehensive study. Oncologic surgeons that plan for resections of renal/urothelial neoplasms, or else transplant surgeons that depend on accurate analysis regarding the causes of degree of renal graft dysfunction, have relied on a variety of tests, in addition to clinical judgement, to make decisions regarding surgical intervention. However, the availability of a single test that can provide both anatomic and functional analysis to guide therapeutic decision making would be expected to impact the care of these patients. Conclusion: In summary, recent advances in sequence design and image processing have allowed for the simultaneous acquisition of dynamic perfusion imaging data (necessary for modelling of quantitative renal function) and also high-quality anatomic images of the renal and urothelial system (necessary for tumor diagnostics). This streamlined protocol would allow for a more comprehensive evaluation of patient with diseases of the renal and urothelial system, optimizing management decisions through non-invasive diagnostics.

Acknowledgements

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References

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