The kidneys can be imaged by plain radiography, ultrasound (gray scale and spectral or color Doppler), intravenous urography, nuclear medicine scans (using a variety of radionuclides tailored to assessing overall renal function or excretion and to searching for a source of sepsis), computed tomography (spiral CT is an informative and rapid scanning method), and magnetic resonance imaging (MRI). Ultrasound and, less frequently, CT are preferred in the critical care setting ( Levine 1994). The rest are inconvenient (intravenous urography, nuclear medicine scans, and particularly MRI), imprecise (the plain radiograph), or both (nuclear medicine scans and intravenous urography). The portability and ease of ultrasound makes it the method of choice in most situations.
Table.!, lists the most common indications for imaging the upper urinary tract and the relative value of each modality. It is impossible to quote meaningful figures for the reliability of any given imaging technique in the critically ill as patient status (the presence of multiple catheters or orthopedic metalwork and the often poor renal reserve precluding liberal, or even any, use of intravenous contrast agents) compromises image quality. For this reason, coupled with the dynamic nature of critical illness, repeated and frequent studies may be necessary to achieve a confident diagnosis.
Table 1 Common indications for renal imaging in the ICU
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