The following cases illustrate a range of acute retroperitoneal pathologies, emphasizing the role of imaging in diagnosis and management. Each case highlights key imaging findings in correlation with laboratory and clinical parameters.

1. Acute Pancreatitis 

• 42-year-old male with severe epigastric pain and nausea.
• Laboratory findings: Triglycerides 145.77 mmol/L (12,900 mg/dL), Lipase 9,600 U/L.
• Clinical diagnosis: Acute pancreatitis (Revised Atlanta Classification, 2/3 criteria met).
• Initial CT (IV contrast): 
• diffuse pancreatic parenchymal enlargement
• retroperitoneal fat stranding 
• fluid in the anterior pararenal space, around the pancreas.
• Progression (Follow-up CT, Day 3): fluid expansion and extension in the bilateral perirenal spaces, posterior pararenal spaces and in the peritoneal cavity.
  

  Fig 2: Acute pancreatitis - initial and follow-up CT - axial view - venous phase. Retroperitoneal and peritoneal fluid expansion and extension in 2 days time.
  

  Fig 3: Acute pancreatitis - initial and follow-up CT - axial view - venous phase. Retroperitoneal and peritoneal fluid expansion and extension in 2 days time.

2. Pancreatic Pseudocyst Complication

• 53-year-old male with melena and diffuse abdominal pain.
• History: Pancreatic cystic mass (non-enhancing on MRI) and biliary duct stenosis (treated with ERCP stent). Previous drainage via pigtail stent into the stomach.
• Laboratory findings: Leukocytosis with neutrophilia, anemia, elevated CRP and GGT, normal pancreatic enzymes.
• Eco-endoscopy: Stent migration into the stomach, fistula between duodenum (DI) and pancreatic cyst.
• CT (IV contrast):
• cystic cephalic pancreatic mass with a thick enhancing wall and heterogenous contents - fluid, central hyperdense material (average of 60 HU, possibly internal hemorrhage) and air.
• Stomach and duodenum (DI and DII) wall edema and mucosal enhancement.
• Pneumoretroperitoneum in the right anterior pararenal space and the great vessel compartment.
  

  Fig 4: Pancreatic Pseudocyst Complication - axial view - venous phase. Complicated pancreatic pseudocyst and pneumoretroperitoneum.
  

  Fig 5: Pancreatic Pseudocyst Complication - sagittal view - venous phase. Complicated pancreatic pseudocyst and pneumoretroperitoneum.
  

  Fig 6: Pancreatic Pseudocyst Complication - axial view - native and venous phase. Cystic cephalic pancreatic mass with a thick enhancing wall and heterogenous contents - fluid, central hyperdense material and air.

3. Complicated Pancreaticoduodenal Artery Aneurysm

• 59-year-old male with diffuse abdominal pain, nausea, vomiting – transferred to our hospital for further evaluation.
• History: Superior pancreaticoduodenal artery aneurysm and severe median arcuate ligament syndrome (MALS).
• Laboratory findings: Anemia.
• CT (IV contrast):
• Large retroperitoneal hematoma in the anterior pararenal space, between the pancreas and duodenum.
• Superior pancreaticoduodenal artery fusiform aneurysm with no visible rupture.
• Retroperitoneal fluid in the right anterior and posterior pararenal spaces.
  

  Fig 7: Complicated Pancreaticoduodenal Artery Aneurysm - axial view - native phase.. Large retroperitoneal hematoma.
  

  Fig 8: Complicated Pancreaticoduodenal Artery Aneurysm - axial view - arterial phase. Superior pancreaticoduodenal artery fusiform aneurysm with no visible rupture.
  

  Fig 9: Complicated Pancreaticoduodenal Artery Aneurysm - sagittal view - arterial phase.

4. Acute Pyelonephritis

• 68-year-old female with shivers, vomiting and dyspnea.
• History: congestive heart failure
• Clinical worsening: On day 2 - Oxygen saturation drop (70%) and peritoneal irritation.
• Lab findings: Leukocytosis with neutrophilia, rising blood urea and creatinine levels.
• CT (no contrast):
• Bilateral renal parenchymal enlargement.
• Thickened bilateral anterior and posterior renal fascias.
• Inflammatory perirenal fat stranding.
• Extraperitoneal fluid in the presacral space.
  

  Fig 10: Acute Pyelonephritis - axial view - native phase. Thickened bilateral anterior and posterior renal fascias.
  

  Fig 11: Acute Pyelonephritis - axial view - native phase. Extraperitoneal fluid in the presacral space.

5. Pyonephrosis with Perinephric Abscess

• 70-year-old male with fever, acute lumbar pain and anuria.
• History: ESWL (Extracorporeal shock wave lithotripsy) in 2010.
• Lab findings:
• Leukocytosis with neutrophilia, anemia, elevated CRP
• Hypokalemia, metabolic acidosis
• Elevated blood urea and creatinine
• Urinalysis: Pyuria, proteinuria
• Urine culture: Escherichia coli
• Clinical diagnosis: Acute kidney injury (KDIGO Stage III), suspected pyonephrosis
• CT (no contrast):
• Grade III left hydronephrosis
• Left perinephric fat stranding and thickened anterior/posterior renal fascias
• Low-attenuating, well-defined mass on the medial aspect of Gerota’s fascia - suggestive for perinephric abscess.
  

  Fig 12: Pyonephrosis with Perinephric Abscess - axial view - native phase. Left perinephric fat stranding and thickened anterior/posterior renal fascias. Low-attenuating, well-defined mass on the medial aspect of Gerota’s fascia.
  

  Fig 13: Pyonephrosis with Perinephric Abscess - sagittal view - native phase. Grade III left hydronephrosis.

6. Spontaneous non-traumatic renal hemorrhage (Wunderlich syndrome)

• 73-year-old male with pallor, left flank pain with palpable mass and hematuria.
• History: Type II diabetes, dyslipidemia, hypertension, obliterative arteriopathy (Fontaine IV), left leg amputation, stroke with left hemiparesis
• Lab findings:
• Leukocytosis, severe anemia, coagulopathy
• Hypokalemia, hyperglycemia
• CT (IV contrast):
• Large subcapsular hematoma compressing the left renal cortex with active arterial-phase bleeding
• Retroperitoneal hemorrhage extending into the left perirenal and posterior pararenal spaces (along the psoas muscle)
• Additional findings on the left kidney: Non-obstructive calculi, simple cortical cysts, no contrast excretion (7-min delayed scan)
  

  Fig 14: Spontaneous non-traumatic renal hemorrhage - axial view - native and arterial phase. Large subcapsular hematoma compressing the left renal cortex.
  

  Fig 15: Spontaneous non-traumatic renal hemorrhage - axial view - native and arterial phase. Large subcapsular hematoma with active arterial-phase bleeding.
  

  Fig 16: Spontaneous non-traumatic renal hemorrhage. Retroperitoneal hemorrhage extending into the left perirenal and posterior pararenal spaces (along the psoas muscle).
  

  Fig 17: Spontaneous non-traumatic renal hemorrhage - coronal view - arterial phase.

7. Urinoma

• 82-year-old female with generalized abdominal pain and confusion.
• Lab findings: Iron-deficiency anemia, hypoalbuminemia, hyponatremia, normal renal function.
• Ultrasound: severe left hydronephrosis.
• CT (IV contrast + 30-min delayed images):
• Left kidney with grade II hydronephrosis and duplex collecting system with bifid ureter.
• Superior collecting system: Minimal renal excretion, obstructed distally by an ureteral calculus and a sigmoid colon enhancing asymmetrical thickening with linear soft tissue opacities radiating outwards (suspected malignancy).
• Inferior collecting system: Contrast excretion at 3 min with distal ureter leakage on delayed scans.
• Uriniferous fluid collections:
• Left perirenal space
• Expanding into great vessel compartment → Right perirenal space
  

  Fig 18: Urinoma - axial view - arterial phase. Left kidney with grade II hydronephrosis and duplex collecting system.
  

  Fig 19: Urinoma - axial view - 30-minutes delayed phase.
  

  Fig 20: Urinoma - axial view - arterial and 30-minutes delayed phase. Distal ureter leakage on delayed scans and uriniferous fluid collections.
  

  Fig 21: Urinoma - axial view - arterial phase. Sigmoid colon enhancing asymmetrical thickening with linear soft tissue opacities radiating outwards (suspected malignancy).