**2.1. Ultrasonography**

Ultrasonography (US) is the first choice for evaluating kidney allograft either in acute, immediate post-transplantation period or in the long-term follow-up [6, 7]. US is non-invasive, innocuous and due to its availability has a key hole when assessing complications of any nature in renal transplants. As the transplanted kidney usually lies in a superficial position in the iliac fossa, it is possible to use high-frequency transducers enabling images of high spatial resolu‐ tion. In addition, the ability of Color Doppler (CD) and Power Doppler (PD) to investigate blood flow helps to make the diagnosis of the most common functional complications as rejection acute tubular necrosis [8, 9].

#### **2.2. Magnetic resonance imaging**

When additional imaging is required, generally because the sonographic findings were indeterminate, Magnetic Resonance Imaging (MRI) emerges as the problem-solving method in kidney transplantation [10, 11]. MRI has several advantages when compared to Computed Tomography (CT); it has no ionizing radiation and the main contraindication to this method is the use of cardiac pacemakers. MRI has the highest contrast resolution among all imaging methods and is able to produce angiographic images (MR angiography) without the use of contrast media. And, when necessary the contrast media for MRI, Gadolinium-based salts, are safer than iodinated contrast media used in CT [12, 13]. In addition, the MRI technique to study the collecting system based on T2-weighted images, MR urography, has been used as an alternative to intravenous urography (IVU) and CT [14].

After initial concerning about the possible relation between gadolinium salts and Systemic Nephrogenic Fibrosis [15, 16], there is a consensus that some Gadolinium-based contrast media (GBCM), more stable, may be used in patients with depressed renal function, as long as recommendations regarding type and doses of contrast media were respected [17, 18]. The only absolute contraindication that still persists for GBCM is patients in a regular scheme of peritoneal dialysis [18].

#### **2.3. Computed Tomography**

Computed Tomography (CT) is scarcely used to evaluate kidney transplants, because MRI covers all the possible indications for CT, without ionizing radiation and the use of nephrotoxic contrast media [19]. Although CT angiography has great spatial resolution, this technique should be avoided whenever possible, due to the potential nephrotoxicity of iodinated contrast. CT will play a major role for evaluation potential donors for living transplantation as will be described later on in this chapter [20].
