Kidney Trauma Imaging

In the United States, trauma is the second leading cause of years of life lost and is also a cause of high morbidity in other countries. Renal injuries are estimated to occur in 1-5% of all traumas and in up to 10% of abdominal injuries. Renal trauma is more commonly seen in males, with a mean age of 30.8 years.[1] Blunt injuries account for 90-95% of renal traumas, mainly due to motor vehicle accidents. Other causes of blunt injuries include falls, assaults, and contact sports. Penetrating injuries are caused primarily by stab and gunshot wounds.[2]  Radiologists serve an integral role in the multidisciplinary approach to achieving diagnosis and management of traumatic renal injuries.[3, 4, 5, 6, 7, 8]

(Kidney trauma injuries are shown in the images below.)

Imaging modalities

A helical CT scan of the abdomen and pelvis with intravenous contrast and delayed images (10 minutes later) is the gold standard for assessment of renal trauma. However, immediate imaging allows for recognition of arterial extravasation, while delayed images identify renal collecting system injuries.[2]

Pediatric patients with any level of hematuria have historically been examined by CT. However, studies suggest that it may be acceptable to use similar guidelines as those used for adults.[9]  During the CT examination, if considerable perinephric fluid is noted (particularly on the medial side of the kidney) or if a deep laceration is noted, urinary extravasation should be investigated by using delayed CT images. While investigating the genitourinary tract, examiners should also thoroughly explore for active hemorrhaging, as urgent surgery or embolization is frequently needed in such situations to prevent exsanguination.

Before the widespread use of CT, the traditional tools used to search for genitourinary trauma were intravenous urography (IVU), standard cystography, and retrograde urethrography. Today, however, IVU plays a more limited role, as CT has replaced many of its applications. Occasions that may still warrant the use of intravenous urethrography include the imaging of hemodynamically unstable patients on their way to surgery or urologic imaging of a patient already in the operating room. This approach is used to confirm that 2 kidneys are present if nephrectomy might be needed.

If findings consistent with a bladder injury, such as gross hematuria or pelvic ring fracture, are present, conventional cystography or CT cystography should be performed after initial CT. Compared to a standard pelvic CT with intravenous contrast enhancement, cystography has a higher sensitivity for detecting bladder injuries. However, CT cystography is as good as or better than conventional cystography if adequate bladder distention can be achieved with contrast material. CT cystography also provides the ability to differentiate between intraperitoneal, extraperitoneal, and combined bladder rupture.[10]

If a trauma patient has blood at the urethral meatus, a high-riding prostate, or an inability to void, urethral trauma should be investigated by means of retrograde urethrography. In almost all cases, this should be done before the placement of a Foley catheter, unless the index of suspicion is low. In this case, a pericatheter urethrogram can be obtained later.

Ultrasonography also has limited clinical usefulness in the evaluation of renal trauma.[11] The main application of this technique in the trauma setting has been for focused abdominal sonography for trauma (FAST) scanning, with the goal of detecting any free fluid in an unstable patient. The primary advantage of this technique is that it can be performed in a matter of minutes in the trauma bay while a patient is being resuscitated. In many cases, the presence of fluid is an indication for exploratory laparotomy by surgeons.

Nuclear scintigraphy may be used to evaluate renal function after injury or to directly evaluate patients for urinary injury, especially patients with an extravasation of urine. Nuclear scans are useful for documenting and tracking functional recovery following renal reconstruction.[12] Scintigraphy is generally not useful in the acute trauma setting because of its low specificity and inability to evaluate for injuries outside the urinary tract. 

The role that angiography has played in the initial diagnosis of trauma to the renal vasculature has diminished with the advent of faster CT scanners. Despite this development, use of angiography in the management of vascular and exsanguinating solid-organ injuries has continued to increase, with the trend toward nonoperative management of trauma. Furthermore, angiography with transcatheter embolization is becoming the standard of care for treating stable patients with vascular injuries, such as traumatic pseudoaneurysms and active arterial bleeding.[13, 14]

Follow-up CT imaging is recommended for renal trauma with deep lacerations (AAST grade IV-V) or clinical signs of complications (eg, fever, worsening flank pain, ongoing blood loss, abdominal distention).[15, 12]

Guidelines

The Canadian Association of Radiologists published recommendations regarding contrast administration, screening, and risk stratification, including the following[16] :

• Recommend a current eGFR (estimated glomerular filtration rate) within 7 days for inpatients or upon presentation for emergent patients, but this should not delay emergency imaging examination.
• When the patient is presenting emergently (eg, suspected acute stroke, pulmonary embolism, acute aortic syndrome, bowel ischemia or perforation), contrast-enhanced imaging study should not be delayed.
• If eGFR is greater than 30 mL/min/1.73 m ² and there are no signs and symptoms of AKI (acute kidney injury), proceed with contrast imaging study.
• Discourage reduced dosing of IV contrast administration for CT examinations because that will lower parenchymal enhancement; recommend using the appropriate IV dose for high-quality CT imaging.
• Do not recommend restricting repeat contrast doses in lower-risk patients (eGFR >30, no AKI, IV route) or withholding repeat doses for emergency patients or inpatients who have life-threatening or acute presentation of illness.
• Recommend avoiding repeated contrast exposures within 48 hr for elective procedures if the patient is at higher risk of contrast-associated AKI (CA-AKI) (eGFR ≤30, AKI, intra-arterial ICM administration).
• Recommend that in patients with eGFR ≤30 mL/min/1.73 m ² or AKI, metformin should be held at the time of or before ICM administration. Metformin should not be restarted for at least 48 hr and only then if kidney function remains stable (< 25% increase compared to baseline creatinine) and the ongoing use of metformin has been re-assessed by the patient’s clinical team.
• Recommend a follow-up serum creatinine measurement 48-72 hr after intra-arterial ICM injection in patients with eGFR ≤30 mL/min/1.73 m ².

The American Urological Association (AUA) guidelines for the management for genitourinary trauma recommend that abdominal/pelvic computed tomography (CT) scanning with IV contrast (immediate and delayed images) be performed in all stable blunt trauma patients with hematuria (gross or microscopic) and a systolic blood pressure less than 90 mm Hg. Additional indications for immediate IV contrast-enhanced CT include penetrating abdominal injury and/or significant flank ecchymosis, rapid deceleration, and rib fracture.[15]  AUA guidelines recommend immediate intervention when radiologic findings identify a large perirenal hematoma (>4 cm) and/or vascular contrast extravasation in American Association for the Surgery of Trauma (AAST) grade 3-5 injuries in hemodynamically unstable patients.[17, 18]

The National Institute for Health and Care Excellence (NICE) has provided guidelines on preventing acute kidney injury in adults receiving intravenous iodine-based contrast media, including the following[19] :

• In the nonemergency setting, GFR measurements should be obtained within the preceding 3 months before IV iodine-based contrast media are offered.
• In the acute setting, adults should also have their risk of AKI assessed before offering IV iodine-based contrast media; however, this should not delay emergency imaging.
• Oral hydration should be encouraged in adults at increased risk of AKI, and volume expansion with IV fluids should only be considered for inpatients at particularly high risk.

Limitations of techniques

CT is the overwhelming leader for diagnosing and staging renal traumatic injuries. The main drawback to CT, however, is the time to complete a CT examination, especially if CT equipment is not available near the trauma bay. For the most critically injured patients, this time is extremely limited; therefore, ultrasonography has found some clinical appeal as a quick method of searching for critical injuries.

Ultrasonography, however, is limited in the types of injuries it can depict.[11, 20] First, although sonography can depict free fluid in the abdomen and pelvis, it lacks the ability to distinguish the type of fluid or source of fluid. Further, ultrasonography has not demonstrated significant sensitivities and specificities to adequately search for solid-organ injuries. In most cases, even if a solid-organ injury is found or if injury is clinically suggested but not found, CT examination is still indicated if the patient's condition is stable.

Grading

Renal injuries are graded by the American Association for the Surgery of Trauma (AAST) on the basis of the depth of injury and the involvement of vessels or the collecting system as follows[21] :

• Grade 1: Subcapsular hematoma or contusion, without laceration

• Grade 2: Renal lacerations ≤1 cm depth that do not involve the collecting system; perirenal hematoma confined to Gerota fascia

• Grade 3: Renal lacerations >1 cm in depth that do not involve the collecting system; vascular injury or active bleeding confined to Gerota fascia

• Grade 4: Renal lacerations involving the collecting system with urinary extravasation; lacerations of the renal pelvis and/or complete ureteropelvic disruption; vascular injury to segmental renal artery or vein; segmental infarctions without active bleeding; active bleeding extending beyond the Gerota fascia into the retroperitoneum or peritoneum

• Grade 5: Shattered or devascularized kidney with active bleeding, ureteropelvic avulsions, complete laceration or thrombus of the main renal artery or vein.

These scores were initially developed to facilitate clinical research but have become a part of the language for imaging evaluation and triage used by the trauma surgeon. This system has also been adopted by the other members of the trauma team, for a universal team-based approach. The radiologist's role is crucial in diagnosing and confirming the staging of renal trauma at the initial imaging study and also in restaging the renal trauma on the follow-up CT.

As the initial imaging determines the optimal clinical approach, it is essential for the radiologist to understand this scoring system. Most renal injuries are of the minor types and include contusion, subcapsular and perinephric hematoma, and superficial laceration. A more significant injury, such as a deep laceration, infarction, or active hemorrhage, is more likely to require surgery.

The use of radiography for blunt abdominal trauma is nearly nonexistent, despite being an important tool in the primary evaluation of chest and skeletal trauma. In general, abdominal radiography has been replaced by CT because of its widespread accessibility and, to some degree, ultrasonography. However, radiography still plays a role in the assessment of penetrating trauma to the abdomen.[22]

Intravenous urography

The traditional tools for assessing genitourinary injury have been intravenous urography (IVU), standard cystography, and retrograde urethrography. The role of IVU, however, has become more limited as CT has become more available.

IVU may still be used if CT is not readily available, for unstable patients going to surgery, or for urologic imaging of a patient in the operating room. These studies are typically performed as a one-shot IVU, which consists of the acquisition of a scout radiograph, a radiograph taken immediately after the injection of contrast material, and a third radiograph obtained approximately 10 minutes after the injection (see the images below). Additional delayed radiographs may be necessary to assess delayed excretion of contrast material if present and to detect the presence of urinary contrast extravasation.

Findings that may be revealed by IVU include the loss of the renal outline or psoas shadow if there is perinephric hemorrhage, diminished or absent excretion (see the images below), or contrast extravasation. The ureters should also be visualized to evaluate for ureteral injury or displacement, and the presence of a contralateral functioning kidney should be confirmed in the event that significant unilateral renal injury warrants nephrectomy.[23, 24] However, the findings on IVU may not always accurately specify the cause or extent of renal involvement, while minor vascular injury or urinary extravasation may be missed. A nonvisualized kidney (nephrogram/pyelogram) does not necessarily represent significant renal trauma.

Retrograde pyelography

Retrograde pyelography is primarily useful when there is a suggestion of ureteral, ureteropelvic junction (UPJ), or renal pelvic injury and delayed images were not made or were not sufficient to exclude these injuries on CT or IVU. However, this is not routinely needed; drawbacks to retrograde pyelography include its impracticality in the emergent evaluation of a severely injured patient and the fact that it does not characterize renal parenchymal injuries.

Degree of confidence

Compared with CT, IVU has lower sensitivity for detecting renal injury and a lower sensitivity for detecting urinary contrast extravasation. In addition, it lacks the ability to detect nonurologic injuries; therefore, with the high availability of CT, IVU has taken a more limited role.

Previously reported signs of hemoperitoneum on radiography are not sensitive or specific enough to be useful.

Minor extravasation from the UPJ or the ureter is difficult to diagnose on limited IVU and may result in a false-negative finding. Similarly, grade 1 or 2 renal injuries are not easy to detect on the IVU.

 Across all imaging modalities, CT is the most comprehensive diagnostic tool for assessing patients with blunt abdominal trauma. CT can be used to evaluate a large breadth of intra-abdominal injuries with accuracy, and hence, it has a primary role in evaluating the trauma patient. Further, the success of CT in staging abdominal injuries has contributed to the growing trend toward nonoperative management of traumatic abdominal injuries. As such, the CT scanner should be as close to the trauma bay as possible to minimize patient transport time.

Dugi et al identified CT findings associated with urgent intervention for hemostasis after traumatic renal injury according to the American Association for the Surgery of Trauma Organ Injury Scale for renal trauma. They retrospectively reviewed patients with renal injury grades 3 and 4. In the 102 patients studied, the authors found that increased perirenal hematoma size (perirenal hematoma rim distance greater than 3.5 cm), intravascular contrast extravasation, and a medial renal laceration site were radiographic risk factors significantly associated with intervention for bleeding after renal trauma. The findings showed that patients with 0 or 1 risk factor were at 7.1% risk for intervention and that those with 2 or 3 risk factors had a much higher risk of 66.7%. On the basis of the findings, the authors recommended that the American Association for the Surgery of Trauma grade 4 renal injuries be substratified into grades 4a (low risk) and 4b (high risk).[25]

CT technique

To best evaluate blunt abdominal trauma, the technique with which a CT is obtained must first be optimized. Conventional axial CT scanners can provide sufficient scans; however, helical CT scanners offer a considerable gain in speed and quality. Multidetector-row CT (MDCT) scanners are even more powerful than single-slice helical CT scanners because their thin-section, high-quality images can be obtained more quickly. As a result of shorter scanning times, less opportunity is available for motion or breathing artifact to appear.[26, 27]

Furthermore, MDCT is advantageous because of its improved ability to depict injuries such as active arterial extravasation. Apart from faster scanning time that MDCT provides, it also utilizes the tube-heat capacity in a more efficient manner. This allows multiple, successive CT examinations to be carried out without the need to wait for the CT tube to cool.

Intravenous and oral contrast material

Intravenous contrast enhancement is essential for abdominal CT.[28] Without intravenous contrast enhancement, solid-organ injuries such as renal lacerations can often be imperceptible. In addition, active arterial extravasation is only detectable with intravenous contrast material. Its usage with helical CT and MDCT scanners has further increased the frequency with which active arterial extravasation is seen.

For adults, the typical contrast dose is 100-150 mL, whereas for children, the dose is 1.5-2 mL/kg. The desirable injection rate is at least 2 mL/s; however, rates of 3-4 mL/s allow for optimal enhancement of the vasculature and parenchyma. A low-osmolarity, non-ionic contrast agent is standard at our institution.

Contrast material, when given orally, also has clinical utility in aiding in the detection of bowel injuries. Fortunately, oral contrast material is safe, even for children.[29, 30] As soon as an abdominal CT scan is requested, 400-600 mL of a dilute solution (for example, 4% diatrizoate meglumine in tap water) is given by mouth or by nasogastric tube. Images of trauma patients are then obtained without delay.

Thus, the stomach, duodenum, and proximal jejunum are typically the only structures opacified; fortunately, these are the most common sites of bowel injury. Withdrawing the nasogastric tube into the distal esophagus during the scan may reduce upper abdominal streak artifact.

For CT, an image thickness of 5 mm or less prevents major volume-averaging artifacts, and a scanner pitch of 1.5:1 for single-slice helical scanners optimizes speed while preventing excessive section-profile broadening. For an MDCT scanner, a pitch greater than 1 but less than 2 hastens image acquisition yet usually results in excellent image quality. Further, by scanning at speeds less than the maximum table speed or with a detector configuration narrower than the image thickness usually produced, thinner sections can be retrospectively reconstructed. This is sometimes needed to evaluate subtle injuries or associated spine or bony pelvic injuries.

As an example, an HS mode on a GE 4-slice scanner with 5-mm images and a table speed of 15 mm per rotation and 0.8-second scanning can be used. With this protocol, 2.5-mm sections can be obtained with retrospective reconstruction when needed. Depending on the scanning mode and the patient's size, 100-300 mA is typically used at a KVp of 140. The image acquisition start time begins 45 seconds after the injection of contrast material for the chest and 75 seconds for the abdomen. If a CT cystogram is being obtained, a pause of 180 seconds before pelvic scanning permits the bladder to opacify.

Some institutions regularly scan through the kidneys a second time during the urographic phase of enhancement to aid in detection of subtle injuries of the parenchyma and collecting system. Images of trauma patients can be regularly evaluated as they are obtained while the patient is still in the CT scanner. If noteworthy perinephric or periureteral fluid is found, urinary contrast extravasation is investigated by taking images delayed at 5-15 minutes.

If clinical findings such as gross hematuria or pelvic ring fracture are present, and if bladder injury is a concern, cystography or CT cystography should be performed. Standard CT with intravenous contrast enhancement has a lower sensitivity for these injuries. CT cystography offers a few advantages over conventional cystography. First, the patient can be evaluated by CT cystography after the initial scan without the need to move to another location. CT cystography can also distinguish intraperitoneal, extraperitoneal, and combined bladder rupture.[10]

When CT cystography is performed, the urinary bladder is first drained by a Foley catheter following the abdominal CT scan. The CT cystogram is done with either standard scans or scout imaging. The cystogram should be performed before intravenous injection of contrast media. In the adult, a minimum of 300 mL of dilute contrast media is necessary. If this is normal, drainage and wash out (bladder flushed with sterile fluid) may be performed but is not routinely needed.

CT interpretation

CT scans for blunt abdominal trauma must be meticulously reviewed for proper interpretation. On evaluation, urgent life-threatening injuries, such as a large hemoperitoneum, a large or tension pneumothorax, pneumoperitoneum, signs of hypovolemic shock, or active arterial extravasation, should be sought out first.

This should be followed by a thorough interrogation for injury of the abdomen and pelvis: liver and right paracolic gutter; spleen and left paracolic gutter; upper abdominal organs, including the stomach, duodenum, pancreas, gallbladder, and biliary tree; retroperitoneum, including the adrenals, kidneys, inferior vena cava, and aorta; small bowel, colon, and mesentery; pelvis, including the urinary bladder; muscles, including the abdominal wall, psoas, iliacus, and gluteals; bones, including the spine and pelvis; and thighs.

To perform a complete evaluation, the entire scan must be scrutinized with 3 different window/level settings: soft tissue, lung, and bone. The entire systemic review has been called the "every-organ-on-every-slice" approach.[31] With the Picture Archiving and Communication System (PACs) workstation, image review is accomplished by rapidly paging through the images multiple times, with special attention to one organ at a time—hence, "every slice of every organ." With this method, renal injuries can be readily identified and classified for proper treatment.

Grade 1 injuries

AAST grade 1 renal injuries include hematuria with normal imaging, contusions, and nonexpanding subcapsular hematomas; overall, this grade accounts for 80% of renal injuries. In CT images, contusions are perceived as ill-defined or sometimes sharply marginated areas of reduced enhancement and excretion. A segmental infarction, which is an AAST grade 4 renal injury, is differentiated from contusions by a lack of enhancement altogether, as depicted in the images below.

Subcapsular hematomas usually appear as a hyperattenuating fluid collection between the renal parenchyma and the renal capsule, at times deforming the underlying kidney. These hematomas are less common than perinephric hematomas in blunt abdominal trauma. Small, subcapsular hematomas often take on a crescent shape, whereas larger hematomas may appear elliptical and compress the renal parenchyma (see the images below). On rare occasions, the hematoma may progressively enlarge and compress the kidney enough to lower renal perfusion. This may result in reactive hypertension, or Page kidney.[32]

Grade 2 and 3 injuries

Renal injuries that are classified as grade 2 include nonexpanding perinephric hematomas contained by the retroperitoneum and superficial cortical lacerations less than 1 cm in depth without injury to the collecting system.

On CT, a perinephric hematoma often appears as an ill-defined, hyperattenuating fluid collection located between the Gerota fascia and the renal parenchyma (see the images below). More often than not, such a hematoma is associated with underlying injury, though they can occur in isolation. Thus, when a perinephric hematoma is discovered, a thorough investigation of the kidney should be undertaken to look for associated renal injury. Unlike a subcapsular hematoma, even a large perinephric hematoma does not traditionally deform the kidney.

Renal lacerations are seen on CT as jagged or linear parenchymal disruptions that can contain fresh or clotted blood (see the images below). The laceration may thus show attenuation higher than that of water, but this would occur without the contrast enhancement present in the renal parenchyma. By definition, grade 2 renal lacerations are less than 1 cm in depth, while grade 3 lacerations are greater than 1 cm in depth. Both grade 2 and grade 3 renal lacerations, however, do not involve the collecting system. As such, there would be no evidence of urinary contrast extravasation on delayed CT.

The treatment of most grade 1, 2, or 3 renal injuries is usually conservative, except when a vigorous active hemorrhage is present. In such cases, the active hemorrhage may be treated successfully with selective catheter embolization in an otherwise stable patient. Occasionally, continued bleeding or extravasation can lead to complications and higher morbidity if not identified and managed appropriately. Follow-up CT is useful for restaging the renal trauma and helps identify the patients with progressive worsening on conservative management. Appropriate intervention in these patients can help prevent complications.[33, 34, 35, 36]

Grade 4 injuries

Grade 4 renal injuries include renal lacerations that extend into the collecting system, injuries to the main renal artery or vein with contained hemorrhage, and segmental infarctions without associated lacerations. The first of these—renal lacerations with collecting system involvement— frequently produces extravasation of urine or contrast agent. Extravasation such as this should be thoroughly sought any time a laceration extends through the kidney or substantial perinephric fluid is seen on CT, especially if that fluid is around the renal hilum.[25] Delayed images allow contrast material to filter into the collecting system, providing adequate views of any urinary extravasation (see the images below).

Under many circumstances, the healing of even large urinary extravasations can occur with conservative treatment; however, stenting is sometimes necessary to facilitate the process. Surgical debridement or repair is usually necessary only when the laceration is accompanied by significant devitalized renal tissue, particularly when concomitant intraperitoneal injuries are also present. The main purpose of such a procedure is to prevent the development of urinoma, infection, or abscess. In the absence of such repair, a nephrectomy may be needed later to prevent sepsis.[37]

On CT, renal segmental infarctions appear as well-delineated, linear or wedge-shaped, often multifocal and nonenhancing areas that extend through the parenchyma in a radial or segmental orientation (see the images below). Thrombosis, dissection, and laceration of segmental renal arteries are primary causes of segmental infarctions, and such infarctions are frequently associated with other renal injuries. These injuries are treated conservatively, as they often resolve spontaneously or result in relatively minor renal scaring.[38] In 6-20% of patients, hypertension may develop as a delayed complication; however, this often resolves or can be medically managed.[13, 38]

Grade 5 injuries

Grade 5 renal injuries include a shattered or devascularized kidney, UPJ avulsions, and complete laceration or thrombosis of the main renal artery or vein. The first of these, a shattered kidney, essentially describes the extreme of multiple renal lacerations, often with devitalized areas due to infarction, and urinary extravasation resulting from injuries to the collecting system (see the images below).

A different type of grade 5 renal injury, the UPJ injury, characteristically involves a medial or circumrenal urinoma on CT (see the images below). Such injuries are caused by a shearing force on the renal pelvis. Complete avulsion or partial tear of the UPJ occurs when rapid deceleration of the kidney pulls on the relatively fixed ureter and renal blood supply. Imaging can distinguish a partial tear from a complete avulsion by the presence of contrast agent in the distal ureter. In many cases, hematuria is absent or minimal.

Treatment for complete UPJ tears is surgical repair, but some partial tears can be managed with stenting and/or observation. When a UPJ injury is undiagnosed and when the proximal collecting system is not drained, a urinoma can develop. This may lead eventually to a nephrectomy.[39, 40, 41]

With CT imaging, a devascularized kidney appears nonenhancing (see the images below). Often, there is little hematoma or other sign of injury. In some cases, CT angiography shows a blind-ending renal artery. Retrograde opacification of the renal vein from IV contrast indicates an acute injury and the need for immediate emergency surgery to reestablish blood flow. In late evaluation, the renal vein is thrombosed, and this reverse flow is not seen. The cortical rim sign may be apparent, but not early. This usually indicates a dead kidney with rare recovery of renal function (if it ever occurs). In the absence of other associated injuries, hematuria is often nonexistent.[42, 43]

The most common cause of a devascularized kidney is an incomplete renal artery tear with thrombosis; a complete tear of the renal artery with an extensive hematoma or active bleeding is less common. When they occur, these injuries are often present with other renal injuries. This association contributes to the poor renal outcome after attempted repair; therefore, the care of stable patients is usually expectant.

For patients with active bleeding or major parenchymal disruption, treatment is usually nephrectomy except in the case where there is injury to or absence of the contralateral kidney. A potential complication of these injuries is hypertension; it can develop weeks to months after the initial injury in as many as 40-50% of patients. This often resolves or can be medically managed, but nephrectomy is sometimes necessary.[42, 44]

Another, less frequent form of vascular pedicle injury is damage to the main renal vein. One type of such an injury is laceration of the renal vein. On CT, this usually presents with medial or circumrenal subcapsular or perinephric hematoma. Thrombosis is a second type of renal vein injury that is depicted on CT as a filling defect or as nonenhancement of the vein. A delayed or persistent nephrogram may be present when thrombosis results in complete occlusion. This may be lethal in the adult.

Vascular extravasation of contrast material

Contained or active hemorrhage is indicated by bright enhancement with attenuation similar to that of nearby arteries within a laceration or around an injured kidney during the early phases of CT scanning. Active hemorrhage appears as an ill-defined, flame-shaped, or waterfall-shaped area on CT, with an associated fresh hematoma (see the images below).

The hematoma often demonstrates circumferential or dependent layering of older and fresher hemorrhage. On the contrary, contained hemorrhage or pseudoaneurysm is somewhat bound and contained within the renal parenchyma or laceration. If active extravasation of arterial contrast material from the main renal artery or lacerated kidney appears present, immediate transcatheter embolization or surgery may be needed to prevent exsanguination.[45, 46, 47, 48]

Degree of confidence

CT is highly accurate in identifying different types of renal injuries. CT is sensitive for most renal injuries and generally specific. It may be difficult to distinguish small contusions from lacerations or infarctions, but major injuries are easily distinguished from normal kidneys or minor injuries. Delayed scanning is needed to sufficiently evaluate urinary extravasation.

Standard CT with intravenous contrast enhancement can yield false-negative rates of 40% for bladder injury. A CT cystogram gives a higher accuracy rate, which equals or surpasses that of standard cystograms as long as appropriate distention of the bladder is obtained.

Delayed bleeding or rare cases of hypertension occasionally result from persistent pseudoaneurysms. Renal lacerations from blunt or penetrating trauma can also produce arteriovenous fistulas. Initially, these may be difficult to detect, but they can enlarge over time. The results of this may be delayed bleeding, hypertension, or high-output cardiac failure.

For stable patients with a strong contraindication for iodinated contrast material, MRI with a gadolinium-based contrast agent can be helpful in assessing a renal injury; however, for the acutely injured patient, MRI is usually not practical because of motion artifacts and the examination time.

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

The use of abdominal ultrasonography for trauma patients remains controversial, particularly for detecting renal and urologic injuries. Despite this, in the United States, ultrasonography has achieved moderate acceptance for evaluating a patient with blunt abdominal trauma.[49]

Multiorgan point-of-care US aids in the objective assessment of volume status by allowing bedside evaluation of the hemodynamic circuit. Assessment of forward arterial inflow, venous outflow, and tissue congestion can be accomplished with integration of the data to help develop an individualized management plan.[50, 51, 52]

In the trauma setting, sonography is usually performed as a FAST scan for the primary purpose of identifying free fluid in the unstable patient. In most circumstances, FAST scans can be executed in the few minutes during patient resuscitation in a trauma bay. The examination includes probing 6 locations for the existence of free fluid: the right upper quadrant with the hepatorenal recess, the left upper quadrant with the splenorenal recess, both paracolic gutters, the pelvis and its various peritoneal cavity recesses, and the pericardial space. If the examination demonstrates the presence of fluid, surgeons will generally perform an exploratory laparotomy.

In situations in which sonography has been used to screen blunt abdominal trauma as part of a management algorithm, there is some variability in the training of the individual performing the examination. In some institutions, radiologists or sonologists perform the study, but in many centers, it is performed by the trauma surgeon or emergency physician. These specialists may have little training in clinical sonography and virtually no training in its technical aspects. Therefore, their ability to execute high-quality examinations has been seriously questioned. However, if the radiology staff is to perform trauma sonography, the service must be readily available at all times.

Although sonography can depict free fluid in the abdomen and pelvis, it cannot be used to make the clinically important distinction between extravasated urine, blood, or other types of fluid. Moreover, ultrasonography cannot depict the source of bleeding. A variety of groups also have proposed the use of ultrasound to search for solid-organ injury, but sufficient sensitivities and specificities have not been demonstrated.[11, 20, 53, 54]

Ultrasonography may demonstrate renal laceration, a change in echogenicity of an injured kidney, or a decrease in the usual perinephric echogenicity if perinephric fluid or hemorrhage is present. However, if sonograms are negative and if noteworthy hematuria is present, or if the sonogram is positive, CT is still indicated for evaluation of the injury if the patient is stable. For this reason, the use of sonography is probably best reserved for the rapid evaluation for intraperitoneal fluid in the unstable patient who may require urgent surgery.

In one study, ultrasonography depicted only 22% of solid-organ injuries, and in another, less than 50% were identified.[11, 55, 56] Although significant renal injuries are often associated with other abdominal injuries, peritoneal fluid is not present in as many as 65% of isolated renal injuries; this possibility increases the risk of missing a renal injury. In addition, ultrasonography is insensitive for retroperitoneal blood and injury to a hollow organ.[57]

In a study by Nepal et al, focused acute medicine ultrasonography (FAMUS) had a sensitivity of 90% and a specificity of 100% for hydronephrosis. Positive and negative predictive values were 100% and 99%, respectively.[51, 52]

Prior to the common availability of CT, angiography was often used to initially diagnose renal arterial or parenchymal aberrations found on IVU. Today, however, faster CT scanners and the increased detection of active arterial extravasation have limited the use of angiography for the initial diagnosis of traumatic injuries. Moreover, CT can also depict many injuries not seen on angiography.

On the other hand, interventional angiography can be considered in more stable patients as well as in postoperative patients with persistent or recurrent hematuria.[1] The main indications for angiography are embolization for active hemorrhage, pseudoaneurysm, and vascular fistula (see the images below). With advances in embolization techniques, angiography is also increasingly used in the management of grade 4 and grade 5 injuries.[58]