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Imaging Findings of Acute Appendicitis

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Overview and Indications for Imaging

• Atypical or equivocal clinical presentation
• High operative risk (elderly, pregnant, multiple comorbidities)
• Female patients of reproductive age (to exclude gynaecological pathology)
• Children (to reduce radiation exposure and negative appendectomy rate)

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1. Plain Abdominal Radiograph

• A right lower quadrant faecolith (appendicolith) is present in only 10.5% of patients with appendicitis and in 3.3% without appendicitis - it is neither pathognomonic nor sufficiently sensitive
• Present in 9% of non-perforated and 21% of perforated appendicitis specimens
• Plain films are indicated only in elderly patients where a perforated viscus must be excluded - an erect chest X-ray looks for free subdiaphragmatic air - Maingot's Abdominal Operations, p. 646

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2. Ultrasound (Graded Compression Ultrasound)

Positive Findings

Limitations

• Highly operator-dependent
• Cannot visualise the appendix in up to 30-40% of cases (gas, obesity, retrocaecal position)
• Limited in obese patients
• Poor negative predictive value if appendix not visualised
• If ultrasound is negative or inconclusive but clinical suspicion persists, proceed to CT - Rosen's Emergency Medicine

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3. Computed Tomography (CT)

Protocol

• IV contrast is essential - highlights mural hyperaemia and periappendiceal inflammation
• Oral/rectal contrast: not required; adds no significant diagnostic benefit and causes delay - Yamada's, Mulholland's
• Low-dose CT is non-inferior to standard dose in young adults (RCT, 891 patients) - Mulholland & Greenfield, p. 3610
• MDCT with multiplanar reconstruction (axial + coronal) is standard

Positive CT Findings - Uncomplicated Appendicitis

CT Findings - Complicated Appendicitis (Perforation/Abscess/Phlegmon)

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4. Magnetic Resonance Imaging (MRI)

Protocol

• T2-weighted sequences are most informative
• Gadolinium contrast is generally avoided in pregnancy (first trimester)
• If used in pregnancy, perform with minimal sequences and patient consent regarding minimal fetal risks

MRI Findings

• Thickened, fluid-filled appendix (T2 hyperintense lumen)
• Periappendiceal oedema (T2 signal in surrounding fat)
• Wall oedema and thickening
• Restricted diffusion on DWI correlates with gangrenous change

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5. Comparative Accuracy

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6. Recommended Imaging Strategy

Suspected appendicitis
        ↓
Is the diagnosis clinically clear in a young male?
  → YES: proceed to OR (imaging optional)
  → NO or equivocal: imaging indicated
        ↓
Pregnant patient? → Graded compression US → MRI if US non-diagnostic
Children? → Graded compression US first → CT if US negative/equivocal
Adult, non-pregnant → CT abdomen/pelvis with IV contrast (no oral/rectal contrast needed)
        ↓
CT negative, clinical suspicion persists → repeat CT at 24h or diagnostic laparoscopy

• Faecolith present
• Abscess or phlegmon
• Appendiceal diameter >11 mm
• Free fluid collection
• Suspected tumour - Rosen's Emergency Medicine

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7. Key Points for Postgraduate Surgery Examination

1. CT is the gold standard in adults - sensitivity/specificity ~96%/96%; no oral/rectal contrast required
2. Ultrasound first-line in children, pregnant women, and thin adults - operator-dependent; if non-diagnostic, proceed to CT
3. MRI is the preferred study in pregnancy when ultrasound is equivocal - no ionising radiation
4. Plain X-ray has no routine role; indicated only to exclude free perforation in elderly patients
5. Appendiceal diameter >6 mm on any modality is the primary threshold (some texts cite 10-13 mm for CT)
6. Periappendiceal fat stranding on CT is the most important associated finding
7. Appendicolith is neither pathognomonic nor reliably sensitive but increases risk of perforation
8. Failure to fill with contrast or air on CT makes appendicitis highly likely
9. Extraluminal gas or fluid, abscess, phlegmon = complicated appendicitis on CT
10. Low-dose CT is non-inferior to standard dose in young adults - relevant for radiation-conscious practice
11. CT performed very early (<12h) may be falsely negative - repeat at 24h is appropriate

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Imaging Findings of Acute Pancreatitis

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Diagnostic Criteria for Acute Pancreatitis

1. Typical epigastric pain radiating to the back
2. Serum lipase or amylase ≥3× upper limit of normal
3. Confirmatory imaging findings on contrast-enhanced CT, MRI, or ultrasound

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1. Plain Abdominal Radiograph

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2. Ultrasound

Findings

Limitations

• Sensitivity for pancreatitis itself is limited - bowel gas and ileus often obscure the pancreas
• Cannot visualise the distal body and tail reliably (retroperitoneal, gas overlay)
• Cannot reliably detect necrosis - this is a critical limitation
• Cannot visualise distal CBD stones reliably in acute setting

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3. Contrast-Enhanced Computed Tomography (CECT)

When to perform CT

• Mild pancreatitis with prompt clinical improvement: CT is not required
• CT performed in the first 24 hours may underestimate necrosis (which takes 2-3 days to develop fully)
• Optimal timing: 3-7 days after onset, when necrosis is fully demarcated
• Indications for urgent CT:
  • Diagnostic uncertainty
  • Failure to improve in 48-72h
  • New/persisting organ failure
  • Signs of sepsis
  • Clinical deterioration

CT Protocol

• IV contrast is mandatory - single portal venous phase at 70s (1.5-2 mL/kg at 4 mL/s)
• Arterial phase is not routinely needed unless vascular complication suspected
• Unenhanced images not required routinely
• Radiation dose minimisation is important (young patients may need multiple studies)

CT Findings - Interstitial Oedematous Pancreatitis (IEP)

CT Findings - Necrotising Pancreatitis (NP)

1. Pancreatic + peripancreatic necrosis - most common (75-80%)
2. Peripancreatic fat necrosis alone - better prognosis than parenchymal necrosis
3. Pancreatic parenchymal necrosis alone - rarest (5%)

1. Necrosis of more than one anatomical segment (head, body, or tail)
2. Distant fluid collections (posterior pararenal space, paracolic gutter)

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4. CT Severity Scoring Systems

A. Balthazar Grading (Original - 1985)

B. CT Severity Index (CTSI) - Balthazar + Necrosis Score

• Local complications developed in 54% of Grade D/E patients vs. 3.9% of Grades A-C

C. Modified CTSI (MCTSI) - Current Preferred

• Mild: 0-2 points
• Moderate: 4-6 points
• Severe: 8-10 points
• Organ failure incidence: 6% (mild), 9% (moderate), 50% (severe) - Grainger & Allison, p. 681

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5. Revised Atlanta Classification - CT Morphological Definitions

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6. MRI and MRCP

MRI

• Excellent alternative to CT when IV contrast is contraindicated (renal failure) or to reduce radiation in young patients
• Superior to CT for: detecting solid debris within collections (distinguishing pseudocyst from WON), detecting pancreatic ductal disruption, and identifying haemorrhage
• T2-weighted sequences: pancreatic oedema appears hyperintense; necrosis is heterogeneous with low T2 areas
• Gadolinium-enhanced sequences: necrosis appears as non-enhancing zones (same principle as CT)
• DWI (diffusion-weighted imaging): restricted diffusion in gangrenous/infected areas

MRCP (Magnetic Resonance Cholangiopancreatography)

• Gold standard non-invasive test for biliary pancreatitis - identifies CBD stones, ductal disruption
• Sensitivity and negative predictive value of 100% for ruling out CBD stones in resolving gallstone pancreatitis - patients with negative MRCP do not need preoperative ERCP - Grainger & Allison, p. 681
• Identifies disconnected pancreatic duct syndrome (duct disruption in necrotising pancreatitis)
• Avoids risks of ERCP (post-ERCP pancreatitis)

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7. Imaging of Complications

Local Complications

Systemic Complications on Imaging

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8. Imaging Algorithm for Clinical Practice

Suspected Acute Pancreatitis
           ↓
Step 1: Establish diagnosis
  → Abdominal US (all patients, on admission)
     - Confirms gallstones/biliary aetiology
     - Detects early fluid collections
     - LIMITED for pancreatic assessment
           ↓
Step 2: Clinical assessment 48-72h
  → Mild, improving: NO CT required
  → Equivocal, non-improving, deteriorating:
     CECT at 72h-7 days (optimal timing)
           ↓
Step 3: Complications / Etiology workup
  → Biliary aetiology uncertain: MRCP (or EUS)
  → Suspected infection in necrosis: CECT ± CT-guided FNA
  → Suspected vascular complication: CT angiography
  → Ductal disruption / WON: MRI/MRCP
  → Renal failure (contrast-contraindicated): CEUS or MRI

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9. Key Points for Postgraduate Surgery Examination

1. Ultrasound first in all patients - primarily to detect biliary aetiology, not to diagnose pancreatitis
2. CT is NOT needed on admission in mild, resolving pancreatitis - clinical + biochemical diagnosis suffices
3. Optimal CT timing is 3-7 days - necrosis takes 48-72h to fully develop; early CT underestimates severity
4. IV contrast is mandatory for CT - without it, necrosis cannot be assessed
5. Necrosis = non-enhancing parenchyma on CECT - the critical finding that determines severity
6. Balthazar grading A-E + necrosis score = CTSI (0-10) - correlates with local complications better than mortality
7. Revised Atlanta 2012 defines 6 morphological entities: IEP, NP, APFC, pseudocyst, ANC, WON
8. Key distinction: APFC → pseudocyst (>4wks, IEP); ANC → WON (>4wks, NP)
9. WON vs pseudocyst: Both have a wall at >4 weeks, but WON is heterogeneous (solid debris) - MRI/EUS distinguishes them better than CT
10. Gas in a collection = infected WON/ANC until proved otherwise
11. MRCP NPV of 100% for CBD stones in resolving biliary pancreatitis - negative MRCP avoids ERCP
12. Pseudoaneurysm: Splenic artery > gastroduodenal artery; 2-5% incidence; managed by angioembolisation
13. Splenic vein thrombosis → sinistral portal hypertension → gastric varices
14. Infected necrosis accounts for 80% of deaths from acute pancreatitis

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CT Images of Necrotising Pancreatitis

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Image 1 - Acute Necrotising Pancreatitis (Classic Axial CECT)

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Image 2 - Infected Pancreatic Necrosis with Gas

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Image 3 - ANC Evolving into Walled-Off Necrosis (WON)

• (A - top): Early acute necrotic collection (ANC) in pancreatic head and body (arrow). The collection has no definable wall and contains heterogeneous necrotic material.
• (B - bottom): Follow-up CT at 4 weeks - the same collection has now developed a thick encapsulating wall = Walled-Off Necrosis (WON). The content remains heterogeneous with debris of fat attenuation.

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Image 4 - Extensive Necrotising Pancreatitis (Severe Form - Panel B)

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Image 5 - Different Morphological Types of Necrotising Pancreatitis

• (A): Combined intra- + extrapancreatic necrosis - most common type (75-80%); non-enhancing pancreatic parenchyma with adjacent fat necrosis
• (B & C): Extrapancreatic fat necrosis alone (arrows) - mass-like, dense fat collections distinguishable from simple oedematous stranding. Better prognosis than parenchymal necrosis
• (D): Intrapancreatic necrosis alone (arrow) - hypoenhancing focal zone without significant peripancreatic change
• (E): Tail necrosis alone (arrow) - localised non-enhancement confined to the pancreatic tail with minimal surrounding oedema

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Summary: Key CT Criteria for Necrotising Pancreatitis

Critical point: IV contrast is mandatory on CT to identify necrosis. Non-contrast CT cannot distinguish viable from non-viable pancreatic parenchyma.