Ultrasound, just for its well-known advantages, which consist of low cost, lack of ionizing radiations, and its portability with the possibility to be rapidly performed at patient’s bedside, among others, is the most largely available imaging modality in emergency department.

Apart from its use in the emergency room with FAST (focused assessment with sonography for trauma) to highlight the presence of hemoperitoneum, ultrasound is often the first imaging of choice in evaluating a patient with localized low-energy trauma. In fact a standard ultrasound technique is able not only to detect free fluid but also to demonstrate a parenchymal lesion. In the specific case of kidney, the deep retroperitoneal position and the body type of patient can influence the detection of the lesion; moreover, the operator’s experience and patient’s collaboration are other factors that may affect the outcome of the exam. For these reasons US demonstrates high sensitivity for the detection of free intra-abdominal fluid; in the same study, it is reported more than CT exam for small amount, but fairly low sensitivity (even below 50%) for the detection of abdominal solid organ traumatic lesions [51].

Some studies report that the US, practiced in an emergency environment, has very low sensitivity in the detection of parenchymal renal injury (less than 22% in minor lesion) and perirenal collections [51–55]. The American College of Radiologists (ACR) Renal Trauma guidelines consider US usually not appropriate in renal trauma [46]. It also cannot be a reliable diagnostic tool for major vascular injuries and renal function.

Contrast-enhanced ultrasound (CEUS) in traumatic patients has been shown to be more sensitive than US for the detection of solid organ injuries, improving the identification and grading of traumatic abdominal lesions with levels of sensitivity and specificity similar to CT (up to 95%) [56]. With CEUS is also easier demonstrated even small amount of perirenal fluid and sometimes is possible to detect active bleeding. The principal limit of CEUS is the impossibility to evaluate the excretory phase, because microbubbles are not excreted into the collecting system; therefore, CEUS cannot demonstrate injuries to the urinary system: renal pelvis or ureter [57–60].

For these reasons both basic ultrasound and CEUS may not be the only investigations to evaluate a patient with trauma. Their role, especially that of CEUS, is to identify patients with a positive traumatic injury response and to send them a CT diagnostic completion test, differentiating them from negative traumatic injury patients who, if in agreement with the clinic, cannot continue the radiological diagnostic iter.

In this way it decreases the unnecessary radiation exposure, using the US-CEUS as a screening tool to select patients who require a CT or not.

This is very important especially in selected series of patients, such as pediatric patients, young women in reproductive age, and low-energy trauma patients, or in the follow-up of stable patients with kidney injury [39, 41, 61].

An US exam is performed using a convex-array multifrequency 3.5–8 MHz probe. During the US exam, a parenchymal renal injury can be seen as a slightly hyperechoic area with no defined margins that may be difficult to detect in renal parenchyma. Often even in case of minor trauma a surrounding hematoma is visible, as well as a recent hyperechogeneous hematoma, which sometimes can be confused with renal parenchyma (Fig. 19.1).

Over time, hematoma loses its echogenicity, becoming hypo-anecdotal and decreasing its volume (Fig. 19.2).

Usually CEUS exam, in case of low-energy localized renal trauma or during a follow-up of known injuries, is performed using a convex-array multifrequency 3.5–8 MHz probe after a previous basal ultrasound. CEUS uses second-generation ultrasound contrast agents and needs dedicated software operating at low mechanical index.

Like any contrastographic examination, the informed consent of the patient is required.

After contrast agent administration with quick bolus, the renal cortex enhances immediately in arterial phase (10–30 s), very brightly and evenly, and the pyramids enhance diffusely from the periphery to the center over about 30 s. The homogeneous phase of the kidneys generally lasts 2–2.5 min: this homogeneous phase (venous phase or nephrographic phase) is still the most effective for detection of traumatic injuries. At CEUS exam the injured area is detected as anechoic surrounded by normal strongly hyperechogenic renal parenchyma (Fig. 19.3). Perirenal or subcapsular hematoma is easily seen as perirenal hypo-anechoic zone, in case of subcapsular hematoma, with the typical imprint on the kidney profile (Fig. 19.4). In case of active bleeding, it is possible to detect in the injured area the hyperechoic spots. As we already said, it is impossible evaluate the excretory system [53, 60].

When a renal lesion is detected at US or CEUS complete, the examination of the patient performing a CT with intravenous contrast medium is recommended.

Eco-color-Doppler imaging can be useful in monitoring vascular posttraumatic complications such as pseudoaneurysms, arteriovenous fistulas, and arterial or venous renal thrombosis.

With the technological development in the past two decades, contrast-enhanced MDCT has gained a central role in the evaluation of stable polytrauma patients, becoming the first choice examination. Integration of whole-body CT into the initial management of polytrauma patients significantly increases the probability of survival and a better prognosis [62].

CT can quickly and accurately identify and grade renal injury [63], can establish the condition of the contralateral kidney, and can demonstrate injuries to other organs.

In the setting of renal trauma, multiphase CT allows the most comprehensive assessment of the injured kidney. The standard protocol consists in an abdominal pre-contrast acquisition from the diaphragm to the pubic symphysis, followed by a post-intravenous contrast exam in arterial phase (delay around 40 s) and venous nephrographic phase (delay around 80 s). A pyelographic phase (at 5–10 min or more) is practiced only in the suspected urinary tract injury, e.g., in the presence of collections to differentiate an active bleeding from a urinoma (Fig. 19.5) [64, 65].

A volume of nonionic contrast medium of 100–150 mL is injected at a rate of 2–4 mL/s through an 18–20-gauge needle.

Concerns regarding contrast media worsening outcomes via renal parenchymal toxicity are likely unwarranted, with low rates of contrast-induced nephropathy seen in trauma patients [66].

However, in practice, trauma patients usually undergo standardized whole-body imaging protocols; it may happen that, caused by critical patient’s condition, it is not possible to perform an excretory phase; and in this case if there is suspicion that renal injuries have not been fully evaluated, repeating renal imaging when it is possible should be considered.

Magnetic resonance imaging (MRI) is not commonly used imaging modality in trauma patients, due to the time needed for examination, the difficulty to manage a traumatized patient in MR room, the limited access to the patient during the acquisition of imaging, the need for MRI-safe equipment, and the logistical challenges of moving a trauma patient to the MRI suite. Anyway the diagnostic accuracy of MRI in renal trauma is similar to that of CT with the benefit due to the lack of radiations exposure [67, 68]. Due to the lack of radiations, MRI can be useful, especially in the assessment of pediatric patients and young women, in cases in which the use of iodine contrast material is contraindicated or in follow-up of renal and urinary tract lesions, since with the administration of gadolinium the extravasations of urine can be visualized [69–71].

Intravenous urography is nowadays an obsolete imaging modality for the evaluation of renal trauma. It can be used only in rare situations, if the MDCT is not available or in the operating room, in hemodynamically unstable patients taken to the operating room without imaging, to confirm the contralateral renal function if nephrectomy is considered. The technique consists of an injection of intravenous contrast (2 mL/kg) followed by a single plain film taken after 10–15 min [46, 72]. In doubt or positive cases, MDCT is anyway necessary once the patient is stable.

Angiography has only a therapeutic role in renal vascular lesions in stable patients. This topic will be treated in Chap. 22.

Kidney scintigraphy with Tc-99m glucoheptonate, Tc-99m mercaptoacetyltriglycine, or Tc-99m-diethylenetriamine penta-acetic acid can be useful in the follow-up of renal injuries, to counsel the patient on the expected renal function [65].

It can be used also in studying the renal function in patients with contraindications to contrast media or in very selected cases.

This category comprises minor renal contusions and lacerations which don’t extend to the collecting system or medulla, subcapsular hematomas with less than 1 cm or more than 1 cm of thickness but without urinary excretal delay, and perinephric hematomas without active bleeding comprised in the perinephric adipose space and small subsegmental cortical infarcts.

Category I corresponds in the AAST renal injury scale to grades I and II. It includes most of kidney injuries (75–85%), which generally are treated conservatively.