Computed tomography (CT)
• Early and delayed phase imaging through the kidneys is required.
• Scanning should begin at the dome of the diaphragm.
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Table 7.1 American Association for Surgery of Trauma (AAST) organ injury severity scale for renal trauma
Grade Clinical/Imaging Findings
I Microscopic or gross hematuria without imaging abnormality.
Contusion
Non-expanding subcapsular hematoma without parenchymal laceration.
II Non-expanding perirenal hematoma confined to retroperitoneum.
Superficial cortical laceration<1 cm depth and not involving the collecting system.
III Renal laceration>1 cm depth and not involving the collecting system.
IV Deep parenchymal laceration involving the cortex, medulla, and collecting system Injury of the main renal artery and vein with contained hemorrhage.
Expanding subcapsular hematoma which compresses the kidney.
V Completely shattered kidney.
Renal hilar avulsion with devascularization of the kidney.
Adapted from: Moore EE, Shackford SR, Pachter HL (1989) Organ injury scaling: spleen, liver, and kidney.J.
Trauma29: 1664– 1666.
• Early phase
◦ approximately 70 s after intravenous contrast administration, of 150 cc of contrast (2.5–3 cc/sec)
◦ allows evaluation of the kidneys during the late cortical or homogeneous nephrographic phases of enhancement.
◦ allows optimal parenchymal enhancement so that major solid organ injuries such as hepatic or splenic laceration may be identified with greatest sensitivity.
• Excretory phase
◦ scanning is performed at least 3 min following the intravenous contrast infusion cov- ering the whole abdomen and pelvis to evaluate collecting system injury.
◦ Multiplanar reformatted images provide helpful adjunctive assessment.
Intravenous urography (IVU)
• In trauma assessment is generally relegated to gross evaluation of renal function in
◦ patients who are too hemodynamically unstable to undergo CT.
◦ patients brought to the operating room before CT can be performed.
◦ institutions where CT is not available 24 hours per day.
• An IVU is usually sufficient to screen for the presence of bilateral renal function and to identify major parenchymal and collecting system injury.
• Limited intravenous urography consists of a scout KUB followed by images obtained immediately following the intravenous infusion (2 cc/kg) or rapid bolus (100 cc) of contrast, as well as delayed images obtained 5–10 min later.
• Oblique projections and tomograms might are of value when CT is not readily available.
Ultrasound
• Focused abdominal sonograph is often performed in trauma centers to evaluate for hemoperitoneum.
• Ultrasound is less sensitive than CT, and serious injuries may be missed, especially renal arterial injury.
• Color and power Doppler techniques, technology which is not universally present in emergency rooms, can establish the presence of blood flow to the kidney.
• Ultrasound evaluation has certain limitations
◦ It is operator dependent.
◦ A dynamic ileus which is a frequent occurrence in trauma patients, may interfere with adequate imaging of the kidneys.
◦ Does not provide information about renal function.
Magnetic Resonance Imaging (MRI)
• MRI has not been extensively studied in patients with renal trauma and generally conceded to be an adjunctive assessment tool as opposed to a primary imaging modality in most cases.
• Major impediments to the routine use of MRI as a primary imaging modality in suspected cases of renal trauma include the more widespread availability of CT scanners in most emergency departments, as well as increased cost and imaging time.
• MRI has the following advantages:
◦ Can distinguish renal ischemia from infarction and accurately assess the viability of renal fragments.
◦ Able to suggest the age of a hematoma based on differences in signal intensity asso- ciated with the evolution of hemoglobin and its metabolites.
◦ Can accurately distinguish intrarenal from perirenal hematoma and depict laceration in cases complicated by additional abnormalities such as perirenal hematoma and renal infarction; a distinction not always reliably made by CT.
◦ Does not require ionizing radiation or the routine administration of intravenous contrast.
Imaging findings
The majority (75– 85%) of renal injuries are considered to be minor injuries (grade 1 and grade II injuries).
• Contusions may appear as a focal area of striation or persistent contrast staining on delayed imaging (Fig. 7.1a).
• Small intrarenal hematomasappear as round or ovoid areas of diminished attenuation on CT (Fig. 7.1b).
7.2 RENAL TRAUMA 125
(a)
(c)
(b)
Figure 7.1 (a) Renal contusion (grade I) of the right kidney. Excretory Phase of CT shows normal left kidney with contrast in collecting system. On the right, a focal area of striated nephrogram (arrows) due to contusion of the kidney. (b) Small intrarenal hematomas appear as a rounded area of diminished attenuation (arrows) in the left kidney. (c) A subcapsular hematoma appears as an elliptical fluid collection flattening the contour of the right kidney.
• A subcapsular hematoma appears as a round or elliptical fluid collection indenting or flattening the renal contour (Fig. 7.1c).
• A localized perinephric hematoma confined between the renal capsule and the bridging septum may simulate a subcapsular hematoma. A high-attenuation value of the fluid (40– 70 HU) is suggestive of acute clotted blood.
• Lacerations
◦ Appear as linear parenchymal defects with associated perinephric hematoma (Fig. 7.2a).
◦ The depth of the laceration and associated involvement of the collecting system or vascular structures determines the severity of the laceration (Fig. 7.2b).
◦ Major renal lacerations may be associated with devascularization of the renal parenchyma or extension to the renal collecting system.
• Parenchymal infarctionmay be recognized as areas of diminished contrast enhancement without associated perinephric hematoma (Fig. 7.3). In distinction to contusions, which
(a) (b)
Figure 7.2 (a) A linear parenchymal laceration (arrow) associated with a small perinephric hematoma is shown in the left kidney. (b) A deep renal laceration extending from cortex into the pelvis of the left kidney with perinephric hematoma.
Figure 7.3 Parenchymal infarction of the right kidney appears as area of diminished contrast enhancement without associated perinephric hematoma.
may enhance to some degree on delayed imaging, areas of infarction remain hypoatten- uated.
◦ Subsegmental infarctsresult from stretching or thrombosis of the accessory renal, cap- sular, or intrarenal segmental arteries and have a wedge shape.
◦ Segmental infarctionsresult from occlusion of segmental renal arteries.
• Shattered kidney or renal rupture
◦ Multiple severe renal lacerations, usually associated with one or multiple devitalized fragments (Fig. 7.4a).
◦ lacerations extending to the renal pelvis and collecting system.
◦ Severe compromise in the excretion of contrast material.
◦ Extensive hemorrhage, and active arterial bleeding.
◦ A devitalized segment may be difficult to distinguish when it is surrounded by a hematoma of similar attenuation to the devitalized segment.
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(a) (b)
Figure 7.4 (a) Severe trauma causing shattering of the right kidney. (b) Active arterial extravasation of the right kidney on a CT scan immediately following contrast bolus administration shows contrast extravasation.
◦ Active arterial extravasation is often best distinguished on CT scans immediately fol- lowing contrast bolus administration as patchy areas of higher attenuation (85– 370 HU, mean of 132 HU) within a less dense hematoma (Fig. 7.4b). Not infrequently, the delayed images provide the only clue of injury as evidenced by pooling of contrast near the site of arterial injury.
◦ Urine extravasation may be detected on delayed scans, and urine leakage typically collects within the lateral perinephric space.
• Occlusion of the main renal artery
◦ Results as a consequence of intimal tearing and thrombus formation at the intimal flap following a rapid deceleration mechanism of injury.
◦ In most cases, contrast-enhanced CT is sufficient to confidently make the diagnosis of renal artery thrombosis.
An abrupt termination of the renal artery distal to its origin with global renal infarc- tion, with or without a cortical rim of enhancement (cortical rim sign).
A small amount of hematoma may be present around the proximal renal artery.
Arterial occlusion is characterized by the absence of a perinephric hematoma.
• Renal artery avulsion
◦ Caused by a laceration through the muscularis and adventitial layers
◦ CT demonstrates global infarction with extensive medial perirenal hematoma.
• Renal vein thrombosis
◦ Is a rare entity following blunt trauma.
◦ Pre-contrast CT
Thrombus may appear as a high attenuation intraluminal mass.
The renal vein may be distended.
◦ Enhanced CT
May show findings of acute venous hypertension
• nephromegaly
• delayed and diminished nephrogram
• decreased excretion of contrast into the collecting system.
◦ Venography may be needed in cases of suspected injury to the renal vein or inferior vena cava as venous laceration is not reliably detected by enhanced CT.
Commonly seen artifacts
• Patient motion during scanning can create an apparent zone of diminished attenuation along the surface of the kidney.
• Pseudocapsular hematomais readily distinguishable because
◦ similar findings may be seen on the same image in the anterior abdominal wall, liver, spleen, and contralateral kidney.
◦ This artifact will be absent on adjacent images or repeat scans.
• Arenal pseudofractureappears as a sharp cleft of the renal contour near the hilum, and unlike a true laceration, no perirenal hematoma is present.
Assessment of complications of renal injury
• CT and angiography are useful in detecting many of the possible urologic complications that may follow renal injury.
• Early complications generally occur within four weeks of injury and include urinary extravasation or urinoma formation, delayed bleeding, perinephric abscess or infected urinoma, sepsis, and arteriovenous fistula or pseudoaneurysm.
• Late complications include hypertension, hydronephrosis, calculus formation, and chronic pyelonephritis.
• The presence of devascularized segments are more often associated with delayed com- plications than are vascularized renal fragments.
• The combination of devascularized renal fragments and pancreatic or bowel injury places the patient at greater risk for infection and abscess formation.
• Selective renal angiography and renin sampling may effectively evaluate patients with suspected post-traumatic renovascular hypertension when vascular lesions are not depicted by CT angiography.
• Page kidney due to a post-traumatic subcapsular hematoma may appear as a subcapsular fluid collection or perirenal soft tissue thickening on CT with an asymmetrically delayed nephrogram in the affected kidney. The traumatic ‘page’ kidney is very rare and often resolves spontaneously.