Pneumomediastinum

Fig. 1: A chest radiograph of a 29-year-old man with severe vomiting shows subtle streaks of lucencies in the mediastinum (along the left heart border - arrows), and into the cervical region (arrowheads).
Fig. 2: Coronal-reformatted chest CT image confirms the presence of pneumomediastinum (arrows) and cervical emphysema (arrowheads). A small left pneumothorax is noted on CT (not shown). No esophageal, tracheal or major broncheal perforation was noted on esophagoscopy and bronchoscopy.

Radiographic Findings of Pneumomediastinum (= normal mediastinal structures outlined by air)

• Thymic sail sign
• Pneumoprecardium (lateral chest radiograph)
• Ring around the artery sign
• Tubular artery sign
• Continuous diaphragm sign

Sources of Pneumomediastinum

• Intrathoracic: trachea, major bronchi, esophagus, lungs, pleural space
• Extrathoracic: head and neck (sinus), intraperitoneum and retroperitonum (hollow viscus)

In our case, pneumomediastinum was presumed to be due to alveolar rupture from severe vomiting.

Reference:
Zylak CM, Standen JR, Barnes GR, Zylak CJ. Pneumomediastinum revisited. Radiographics 2000;20:1043-1057.

Emphysematous Cholecystitis

Fig. 1: Transverse ultrasound image (with color flow) of an 82-year-old woman with RUQ pain shows thickening of the gallbladder wall (two-head arrow), distended gallbladder, and multiple hyperechoic dots with posterior reverberation artifacts (or 'ring-down' artifacts).
Fig. 2: Coronal-reformatted CT image performed subsequently reveals gas (arrow) in the gallbladder lumen, marked thickening of the gallbladder wall (double-head arrow), distention of gallbladder, enhancement of gallbladder wall (arrowheads). Conglomerate mass in the gallbladder wall represents sludge. Findings are consistent with emphysematous cholecystitis.

Facts

• Gas in gallbladder wall or lumen
• Clinical setting of acute cholecystitis
• No evidence of fistula between gallbladder and bowel
• Common organisms are Clostridium, E. coli, S. aureus and Streptococcus.

Imaging

• May be seen on radiography as curvilinear lucency or air-fluid level in RUQ
• Gas on ultrasound appears as hyperechoic dots/line with posterior reverberation artifact
• CT is the most specific and sensitive method to diagnose this condition.

Reference:
Smith EA, et al. Cross-sectional imaging of acute and chronic gallbladder inflammatory disease. AJR 2009; 192:188-196

Biological Effects of Radiation Used in Imaging (2): Stochastic Effect

Stochastic Effect of Radiation

• Effect of radiation that occurs by chance.
• Late effect such as cancer and genetic defects
• Probability (not severity) depends on dose. "As doses increase, the chance of cancer or genetic defect increases."
• No threshold dose
• Effects can occur even in human never been exposed to radiation above background level (remember normal background radiation from standing on Earth).
  

Effective Dose

• This term is used to determine probability of health effect from radiation (i.e. stochastic effects)
  
• Unit = Sv (sievert)
  
• Derived from calculation of i.e. entrance skin dose (based on dose indicator used in radiography), dose-area product (based on dose indicator in fluoroscopy), and CT dose index (based on dose indicator in CT)
• Good for dose optimization of procedures and balance radiation risks and information obtained from imaging
• Example: posteroanterior chest radiograph - <0.1>

Reference:
Verdun FR, et al. Radiation risk: what you should know to tell your patients. Radiographics 2008 (September)

Pulmonary Alveolar Proteinosis (PAP)

Fig.1: Chest radiograph of a 44-year-old immunocompetent woman with subacute dyspnea, short of breath and no fever shows diffuse, bilateral opacities predominantly in the perihilar and mid-to-lower lungs. Note normal-sized heart.
Fig.2: Axial CT of the chest shows extensive bilateral ground-glass opacities superimposed with interlobular septal thickening (arrows) in the upper, middle and lower lobes. There is no pleural effusion.

Clinical Features

• Rare disorder
  
• Lipoprotein material (probably surfactant) accumulates in alveoli
  
• Congenital, secondary (to impaired alveolar macrophage numbers or function i.e. immunosuppression, hematologic malignancy, inhaled silica, etc) and acquired forms
  
• Men > women, median age at diagnosis = 39 years
• Most common presenting symptom = progressive exertional dyspnea and cough
  
• Susceptibility to pulmonary infections (possible opportunistic organisms)
• Lavage Fluid: large and foamy alveolar macrophages + diffuse eosinophilic bodies on a PAS-stained background
  
• Gold standard: open-lung biopsy

Imaging Appearance

• Chest radiograph: bilateral airspace disease with perihilar predominance ("bat wing" appearance) but without signs of left-sided heart failure
• CT: ground-glass opacifications with superimposed interlobular septal thickening and intralobular lines "crazy paving".
  

Reference:

Trapnell BC, et al. Pulmonary Alveolar Proteinosis. N Engl J Med 2003 (December 25)

Double PCL Sign

Fig: Sagittal PD image of the knee shows 'double PCL sign'. Note the 'extra' dark band (arrows) beneath the true posterior cruciate ligament (arrowheads). This dark band represents 'bucket-handle tear of meniscus'. Joint effusion is marked by a red star.

Double PCL Sign

• Displaced fragment of meniscus in the intercondylar notch of the knee, resembling two posterior cruciate ligaments (PCL)
• Sign of bucket-handle meniscal tear
• Usually tear occurs in young active person
• Medial meniscus > lateral meniscus
• Confirmation on coronal image that shows displaced fragment in the notch
• Look for concomitant chronic ACL tear
  

Reference:
Stoller DW, et al. Diagnostic imaging: orthopedics. 2004