Grayscale images are obtained to evaluate for transplant size, echotexture, hydronephrosis, peri-transplant fluid collections, and masses and to measure renal cortical thickness. Color Doppler images evaluate the patency and direction of flow in transplant arteries and veins. Spectral analysis of vascular waveforms and velocities can provide information about a range of pathologies, including RAS and RVT. Advantages of US over CT include portability, no radiation exposure, and the lack of need for potentially nephrotoxic iodinated contrast agents. US is fast and real time, particularly when compared to MRI. Vascular evaluation with US instead of MRI can avoid the risk of nephrogenic systemic fibrosis (NSF) from gadolinium-based contrast agents. A relative limitation of US in comparison to CT or MRI is its operator dependence. Grayscale US abnormalities in dysfunctional renal transplants include a reduction in corticomedullary differentiation, reduction in renal sinus echogenicity, increased and reduced renal parenchymal echoes, and increased cortical echogenicity. Unfortunately, these findings are of limited value as the features are nonspecific and generally occur well after the onset of transplant dysfunction. Renal segmental or intralobar artery resistive indices (RI), measured by duplex Doppler US, are often used as a nonspecific parameter for allograft dysfunction. Although RI values differ between normal and abnormal allografts, studies have suggested that the RI is neither sensitive nor specific in identifying the cause of functional transplant dysfunction [11,12]. Using RI >0.90 in 145 examinations of 81 patients, Rifkin et al [13] found a sensitivity of 13%, a specificity of 100%, a positive predictive value (PPV) of 100%, and a negative predictive value (NPV) of 66% in making the diagnosis of acute rejection with duplex Doppler US.
Renal Transplant Dysfunction. Grayscale images are obtained to evaluate for transplant size, echotexture, hydronephrosis, peri-transplant fluid collections, and masses and to measure renal cortical thickness. Color Doppler images evaluate the patency and direction of flow in transplant arteries and veins. Spectral analysis of vascular waveforms and velocities can provide information about a range of pathologies, including RAS and RVT. Advantages of US over CT include portability, no radiation exposure, and the lack of need for potentially nephrotoxic iodinated contrast agents. US is fast and real time, particularly when compared to MRI. Vascular evaluation with US instead of MRI can avoid the risk of nephrogenic systemic fibrosis (NSF) from gadolinium-based contrast agents. A relative limitation of US in comparison to CT or MRI is its operator dependence. Grayscale US abnormalities in dysfunctional renal transplants include a reduction in corticomedullary differentiation, reduction in renal sinus echogenicity, increased and reduced renal parenchymal echoes, and increased cortical echogenicity. Unfortunately, these findings are of limited value as the features are nonspecific and generally occur well after the onset of transplant dysfunction. Renal segmental or intralobar artery resistive indices (RI), measured by duplex Doppler US, are often used as a nonspecific parameter for allograft dysfunction. Although RI values differ between normal and abnormal allografts, studies have suggested that the RI is neither sensitive nor specific in identifying the cause of functional transplant dysfunction [11,12]. Using RI >0.90 in 145 examinations of 81 patients, Rifkin et al [13] found a sensitivity of 13%, a specificity of 100%, a positive predictive value (PPV) of 100%, and a negative predictive value (NPV) of 66% in making the diagnosis of acute rejection with duplex Doppler US.