What CT imaging findings may be encountered in renal trauma?
The kidney is injured in up to 10% of patients with blunt trauma and in up to 6% of patients with penetrating trauma. CT is the mainstay for imaging evaluation of patients with potential renal trauma. Currently, the most widely used CT grading system for renal injury is based on the American Association for the Surgery of Trauma (AAST) scale. The spectrum of renal injuries includes contusion, hematoma, laceration, avulsion, infarction, collecting system disruption, active hemorrhage, and vascular injuries including renal vascular pedicle injury, renal arterial pseudoaneurysm, and renal AVF. Patients with deep renal lacerations and vascular injuries are more likely to be unstable and in need of surgical or percutaneous intervention. However, the vast majority of patients with renal trauma respond well to conservative supportive treatment.
Renal contusions appear as focal areas of poorly defined hypoattenuation within the kidney, although they may sometimes be isoattenuating relative to surrounding renal parenchyma or hyperattenuating when there is associated hemorrhage. Perinephric fat stranding may also be seen.
Renal lacerations appear as linear, irregular, or branching nonenhancing low attenuation parenchymal defects in the kidney, often with adjacent areas of high attenuation hemorrhage. When severe, portions of renal parenchyma may even avulse from the remainder of the kidney.
Renal hematomas appear as foci of nonenhancing high attenuation (30 to 70 HU) hemorrhagic fluid on CT, either subcapsular or intraparenchymal in location. Subcapsular hematomas are well-circumscribed, are often crescentic in shape, and may distort the external renal contour. Because these hematomas are enclosed in a tight subcapsular space, they may cause ischemia of the underlying renal parenchyma due to mass effect, leading to hypertension; this is referred to as a “Page kidney.” Perinephric hematomas within the retroperitoneum are generally more ill-defined and may displace the kidney but less often distort the external renal contour. When active hemorrhage is present, one sees focal areas of increased attenuation on contrast-enhanced images similar to that of enhancing vessels which do not conform to known vascular structures and which persist or increase in size on more delayed phase acquisitions.
Renal arterial pseudoaneurysms also appear as well-circumscribed round or oval foci of contrast enhancement with attenuation similar to adjacent contrast-enhanced arteries but decrease in attenuation on more delayed phase acquisitions. Renal AVF manifests as early enhancement of the renal vein during the arterial phase of enhancement. These may occur from spontaneous trauma or from iatrogenic trauma such as from percutaneous biopsy, percutaneous ablation, or percutaneous nephrostomy procedures. These are generally treated with catheter-directed embolotherapy or with surgical repair.
Renal vascular pedicle injury involves avulsion or thrombosis of the renal artery or vein. With renal artery injury, abrupt termination or complete nonenhancement of the renal artery is seen, along with nonenhancement of part or all of the renal parenchyma. With renal vein laceration, a medially or circumferentially located subcapsular or perinephric hematoma is seen, although the lacerated vein itself is generally difficult to visualize. With renal vein thrombosis, an intraluminal hypoattenuating filling defect secondary to thrombus is typically seen in an enlarged renal vein, in association with renal parenchymal enlargement and delayed enhancement.
Collecting system disruption appears as extraluminal fluid, representing urine, adjacent to the kidney, collecting system, or proximal ureter. In the excretory phase, excreted contrast accumulation is seen within the fluid collections. With deep lacerations of the renal parenchyma, the collecting system may also be involved.