Introduction 1 2 1 3 4 1 5 MRI technique 6 6 Coronal T2-weighted half Fourier single-shot turbo spin echo sequence (HASTE) (TR infinite, TE 120 ms, flip angle 90°, breath-hold), serving as a localizer, but also supplying valuable T2-weighted information. The limitation of this sequence is a relatively low signal-to-noise ratio. Axial T2-weighted turbo spin echo sequence with fat suppression (TR 2,000 ms, TE 100 ms, flip angle 90°, respiratory triggering). This sequence provides for more detailed T2-weighted information. The T2-weighted sequence is especially helpful in characterizing cysts and intraparenchymal abscesses and in evaluating hydronephrosis. Furthermore, the T2-weighted sequence is helpful in detecting solid lesions. 6 Axial T1-weighted gradient echo sequence for dynamic imaging (TR 130 ms, TE 1.0 ms, flip angle 90°), using 30 ml intravenous gadolinium contrast, immediately followed by three breath-hold periods with four scan series per breath-hold. In this way pre-contrast and post-contrast images in arterial and nephrographic phase are obtained. Gadolinium-enhanced images are used for lesion detection and characterization. Coronal 3D fast gradient echo with fat suppression, obtained immediately after the dynamic series for delayed contrast-enhanced images (TR 3 ms, TE 2 ms, flip angle 15°). This sequence can be used for renal venous anatomy, for the analysis of (tumor) thrombus and for evaluation of extent of the tumor in the perinephric fat. 7 8 Renal lesions 9 Malignant renal lesions Renal cell carcinomas 10 1 11 Fig. 1 a b c arrow a c  n 10 12 13 Assessment of enhancement 14 15 16 17 14 Subtypes of renal cell carcinoma 18 19 20 21 21 Fat-containing renal cell carcinomas 22 27 25 27 22 26 28 9 2 29 12 12 Fig. 2 a arrow b c  Staging 30 1 31 2 32 Table 1 30 Stage Tumor extent I Tumor confined to the kidney II Tumor extension through the capsule of the kidney in the perirenal fat including the adrenal gland, no involvement of Gerota’s fascia IIIa Tumor extension into the renal vein or inferior vena cava IIIb Involvement of regional lymph node(s) IIIc Involvement of regional lymph node(s) and extension into the renal vein or inferior vena cava IVa Tumor extension beyond Gerota’s fascia into adjacent organs IVb Distant metastasis Table 2 31 Stage Description Tx No information on primary tumor available T0 No evidence of primary tumor T1a Tumor size 4 cm or less, limited to the kidney T1b Tumor size more than 4 cm but no more than 7 cm, limited to the kidney T2 Tumor size more than 7 cm, limited to the kidney T3a Tumor extension into the perinephric fat and/or renal sinus fat or the adrenal gland, but not beyond Gerota’s fascia T3b Tumor grossly extends into the renal vein or its segmental (muscle-containing) branches, or inferior vena cava below the diaphragm T3c Tumor grossly extends into the inferior vena cava above the diaphragm or invasion of the IVC wall T4 Tumor extension beyond Gerota’s fascia Nx No information on regional lymph nodes available N0 No regional lymph node metastasis N1 Metastasis in a single regional lymph node N2 Metastasis in more than one regional lymph node Mx No information on distant metastases available M0 No distant metastases M1 Distant metastases 33 3 33 Fig. 3 T1-weighted gradient echo sequence after intravenous contrast of a large renal cell carcinoma in the upper pole of the left kidney with tumor thrombus extending into the IVC up to the level of the liver  34 35 36 33 33 37 MRI in nephron-sparing surgery 38 39 40 38 41 41 42 40 Transitional cell carcinoma 43 4 43 Fig. 4 a b arrow  Benign renal lesions Oncocytoma 5 44 38 45 38 Fig. 5 a b arrow  Angiomyolipoma 46 6 46 Fig. 6 a b c black arrow white arrow  7 47 Fig. 7 arrow a b c d  21 6 46 48 Attention should be paid to the possibility that carcinomas sometimes contain hemorrhage, causing high signal intensity on in-phase T1-weighted images. In these cases, opposed-phase images and spectral fat suppression will not show a drop in signal intensity. Xanthogranulomatous pyelonephritis Proteus E. coli 49 50 49 50 MR urography 51 51 52 53 53 51 51 54 55 54 55 51 MRI of potential donor kidneys 56 56 56 8 Fig. 8 a b  57 6 58 59 59 6 56 60 63 64 65 MRI for imaging of renal function 66 Perfusion 1 67 68 Glomerular filtration rate 69 \documentclass[12pt]{minimal}
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\begin{document}$${\text{GFR}} = {\text{EF}} * {\text{RBF}} * {\left( {{\text{1 - Hct}}} \right)},$$\end{document} 3 66 1 Diffusion-weighted MRI In diffusion-weighted MR imaging, the image contrast is influenced by the Brownian motion of water molecules. The signal intensity is high if water molecules are restricted in their motion, which can be caused by cell membranes or, in the case of free fluid, by high viscosity. The MR signal intensity is low if water molecules can diffuse freely. Diffusion-weighted imaging has found its place in neuroradiology, especially for the early detection of ischemic brain lesions. Diffusion MRI can also be used for fiber tract mapping in cerebral white matter, by measuring the directional components of the diffusion. Because diffusion-weighted images are inherently T2-weighted, the images are influenced by the T2-shine-through effect. This is the presence of high signal intensity in restricted water, caused by the T2 effect. To cope with this effect, the apparent diffusion coefficient (ADC) may be calculated from two images acquired with different gradient duration and amplitude (b-values) and used for ADC mapping. 70 9 71 72 73 Fig. 9 Diffusion-weighted tensor image of the right kidney on a 3T system. The renal pyramids show lower signal intensity than the surrounding parenchyma because of the radial orientation of the tubules in the pyramids, restricting the Brownian motion of the water molecules to one direction  Conclusion The role of MRI in renal imaging is still mainly in differentiating benign lesions versus malignant lesions in patients who cannot undergo CT scanning with intravenous iodinated contrast media, or in cases with nondiagnostic CT results. MRI and CT show comparable accuracy in detection and characterization of most renal lesions. MRI can have additional diagnostic value in the evaluation of lesions with minimal amounts of fat or with intracellular fat. Data suggest that MRI has a higher sensitivity in evaluating complicated cysts, however, the clinical implications still have to be studied. There is evidence to suggest that MRI has a higher accuracy than CT in the evaluation of early lymph node spread. MRI is a suitable tool in the preoperative work-up of potential kidney donors. Functional MRI of the kidney has not yet found broad clinical application, but it has great potential. Through the ongoing development of functional MRI techniques, we may expect an increasing role for functional MRI in the management of patients with renal disease.