Introduction

Basic imaging concepts will be reviewed, including when to obtain specific imaging modalities, basic radiation safety, and anatomic specific imaging. The goal is to offer a quick reference for specific images. General trauma concepts include plain films as the typical initial study in evaluating fracture. Computed tomography (CT) scan is typically performed for periarticular fractures and fractures of the pelvic ring or acetabulum. Magnetic resonance imaging (MRI) is typically performed to further evaluate soft tissue injury or stress fracture if not seen on plain films when there is a high-clinical suspicion (▶Video 16.1).

I. Choice of Imaging

1. 
   

   Several factors affect the imaging that can be performed.

   
   
   
   
   1. 
      

      Patient condition—a patient may be unable to tolerate a CT or MRI based on current medical status.

      
      
   2. 
      

      Tissue or area of the body one wishes to image.

      
      
   3. 
      

      Radiation exposure—pregnancy may preclude a CT if other imaging is available.

      
      
   4. 
      

      Cost.

      
      
   5. 
      

      Availability of imaging modalities.

      
      
   6. 
      

      Patient implants—i.e., pacemaker, shrapnel, orthopaedic implants, ports, etc.

II. Imaging Modalities

• 
  

  A. Conventional radiography

  
  
  
  
  1. 
     

     Principles of image generation:

     
     
     
     
     1. 
        

        X-rays are short wavelength electromagnetic radiations that can pass through objects.

        
        
     2. 
        

        An X-ray source is aimed at a detector; the density and chemical composition of an object determines how much absorption occurs.

        
        
     3. 
        

        Bone is dense and composed of calcium, which readily absorbs X-rays. The denser the object, the more absorption occurs. Dense objects appear white on imaging.

        
        
     4. 
        

        Modern digital radiography facilitates instant electronic transfer of images through multiple media platforms and enhances portability of image capture.

     
     
  2. 
     

     Indications and characteristics:

     
     
     
     
     1. 
        

        Initial choice for imaging.

        
        
     2. 
        

        Relatively low cost.

        
        
     3. 
        

        Low-dose radiation.

        
        
     4. 
        

        High specificity and lower sensitivity in comparison to other modalities.

        
        
     5. 
        

        Produces a two-dimensional image.

     
     
  3. 
     

     Orthogonal (typically anteroposterior [AP] and lateral) views should always be obtained.

     
     
     
     
     1. 
        

        Additional specialized views can be obtained based on injury or clinical suspicion.

        
        
     2. 
        

        Care should be taken to ensure that the imaging is adequate and that true AP and lateral images are obtained. Inadequate imaging can lead to misdiagnosis.

        
        
     3. 
        

        Portable radiography is occasionally limited in acquiring appropriately oriented images.

        
        
     4. 
        

        Adequate imaging may be unattainable due to:

        
        
        
        
        • 
          

          i. Physiologic instability (unsafe for patient transport).

          
          
        • 
          

          ii. Difficulty with mobilization of an injured limb due to pain.

     
     
  4. 
     

     Image the entire bone:

     
     
     
     
     1. 
        

        Long bone fractures—the joint above and below the injury should be included to avoid missing other injuries.

        
        
        
        
        • 
          

          i. Approximately 5 to 10% of femoral shaft fractures have an associated femoral neck fracture. These injuries are missed 20 to 50% of the time.

          
          
        • 
          

          ii. Most missed injuries are due to a lack of appropriate imaging.

        
        
     2. 
        

        Periarticular injury radiographs should include the entirety of the bones involved.

        
        
        
        
        • 
          

          i. Ankle fracture—tibia/fibula and ankle X-rays.

          
          
        • 
          

          ii. Radial head fracture—elbow and forearm X-rays.

  
  
• 
  

  B. Computed Tomography (CT)

  
  
  
  
  1. 
     

     Principles of image generation:

     
     
     
     
     1. 
        

        Uses X-ray to build multiple cross-sectional images of an anatomic region.

        
        
     2. 
        

        Cross-sectional images, or “slices,” are reconstructed to create images that can be displayed in multiple planes and formats.

        
        
        
        
        • 
          

          i. Axial.

          
          
        • 
          

          ii. Sagittal.

          
          
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          iii. Coronal.

          
          
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          iv. Three-dimensional.

        
        
     3. 
        

        Much higher contrast resolution than conventional radiography which permits enhanced distinction of tissue types.

        
        
     4. 
        

        Displays images on a grayscale based on the physical density of the tissue type and measured by Hounsfield units (HU).

        

     
     
  2. 
     

     Indications and characteristics:

     
     
     
     
     1. 
        

        Improved characterization of select injuries and also useful for preoperative planning.

        
        
        
        
        • 
          

          i. Periarticular fractures.

          
          
        • 
          

          ii. Pelvic ring injuries.

          
          
        • 
          

          iii. Limb alignment studies.

          
          
        • 
          

          iv. Three-dimensional CT reconstructions can further improve physician understanding of an injury pattern.

        
        
     2. 
        

        Trauma scan (head, neck, chest, abdomen, and pelvis ± extremities) in multiple injured patients is primarily indicated for identification of life-threatening injuries.

        
        
        
        
        • 
          

          i. Allows diagnosis of injuries to the axial skeleton.

          
          
        • 
          

          ii. May aid in treatment decisions and preoperative planning for fracture surgery.

        
        
     3. 
        

        Higher radiation dose than X-ray.

        
        
     4. 
        

        Higher cost than X-ray but less than MRI.

        
        
     5. 
        

        Highly sensitive and specific for bony injuries.

        
        
     6. 
        

        Soft tissue assessment, although better modalities are available.

        
        
        
        
        • 
          

          i. Computed tomography angiography (CTA).

          
          
        • 
          

          ii. The addition of contrast can aid in vascular/soft tissue assessment, but carries the risk of renal injury.

     
     
  3. 
     

     Limitations:

     
     
     
     
     1. 
        

        Morbidly obese patients may not fit on the scanner.

        
        
     2. 
        

        Metal inside or outside the patient can create “artifact” and obscure image detail.

        
        
     3. 
        

        Relatively contraindicated in pregnant patients due to radiation risk. Pregnant patients presenting as trauma activations with potentially life-threatening injuries still undergo whole body CT scanning.

  
  
• 
  

  D. Magnetic Resonance Imaging (MRI)

  
  
  
  
  1. 
     

     Principles:

     
     
     
     
     1. 
        

        Combines the use of strong magnetic fields and radiofrequency (RF) to create detailed images.

        
        
     2. 
        

        Hydrogen protons (present in water and thus nearly all human tissue) both align with and absorb the energy from the magnetic field (behave similarly to the way a magnet pulls the needle of a compass).

        
        
     3. 
        

        RF pulses disrupt proton alignment. When RF is turned off, protons realign at varying rates in different tissues and emit specific signals during this process.

        
        
     4. 
        

        Detectors measure the energy released during proton realignment and the machine subsequently creates the image.

        
        
     5. 
        

        RF pulse frequency can be manipulated:

        
        
        
        
        • 
          

          i. Repetition time (TR)—amount of time between RF pulses.

          
          
        • 
          

          ii. Time to Echo (TE)—time between RF pulse delivery and receipt of the signal.

        
        
     6. 
        

        Imaging sequences:

        
        
        
        
        • 
          

          i. T1 images—short TE and TR times (▶ Fig. 16.1 ).

          

          
          
          
          
          • 
            

            Better for evaluating many anatomic structures.

            
            
          • 
            

            Fat and bone marrow are bright; cartilage, tendon, and ligament are relatively darker.

            
            
          • 
            

            Does not highlight edema or water content.

          
          
        • 
          

          ii. T2 images—longer TE and TR times (▶ Fig. 16.2 ).

          

          
          
          
          
          • 
            

            Better for assessing fluid (bright) such as swelling and bone edema.

            
            
          • 
            

            Surrounding anatomic structures are darker.

            
            
          • 
            

            Remember: T ₂ and H ₂ O.

          
          
        • 
          

          iii. Fluid-attenuated inversion recovery (FLAIR): very long TE and TR times. Can distinguish between cerebrospinal fluid (CSF) (dark) and pathologic inflammation (bright).

          
          
        • 
          

          iv. STIR (Short T1 inversion recovery)–Suppresses fat.

     
     
  2. 
     

     Indications and characteristics:

     
     
     
     
     1. 
        

        Diagnosis and delineation of soft tissue injury, infection, and tumor. Trauma specific examples include:

        
        
        
        
        • 
          

          i. Pathologic fracture and extent of tumor involvement (covered in chapter 11, Pathologic Fractures).

          
          
        • 
          

          ii. Ligament and tendon injury.

          
          
        • 
          

          iii. Chondral injury.

          
          
        • 
          

          iv. Osteomyelitis.

        
        
     2. 
        

        Less useful for bony injury and is not the first-line imaging modality. Examples include:

        
        
        
        
        • 
          

          i. Occult fractures (hip).

          
          
        • 
          

          ii. Stress fractures (tibia, foot).

        
        
     3. 
        

        No radiation.

        
        
     4. 
        

        Highly sensitive and specific for soft tissue injuries–can be overly sensitive and should be interpreted with care.

     
     
  3. 
     

     Contrast-enhanced MRI—the addition of gadolinium can be helpful for studying tumors, inflammation, abscesses, and vascular pathology.–enhances and delineates structures and tissue associated with fluid.

     
     
  4. 
     

     Limitations:

     
     
     
     
     1. 
        

        Patients may be unable to obtain secondary to implanted defibrillators, pacemakers, artificial heart valves, aneurysm clips, cochlear implants, shrapnel, or other metal implants that are magnetic and can lead to injury. Questionnaires are administered prior to obtaining the scan to avoid complications.

        
        
     2. 
        

        Most implanted metal and ballistic may be “MRI safe” but contributes to signal artifact and obscures image detail making interpretation difficult.

  
  
• 
  

  E. Ultrasound (US)

  
  
  
  
  1. 
     

     Effective in the evaluation of many soft tissue conditions.

     
     
     
     
     1. 
        

        Tendon pathology such as rotator cuff tear and Achilles rupture.

        
        
     2. 
        

        Fluid collection—intra-articular and soft tissue.

        
        
     3. 
        

        US-guided injections—intra-articular and bursal.

        
        
     4. 
        

        Identify deep vein thrombosis (DVT) utilizing Duplex technology.

  
  
• 
  

  F. Dual-energy X-ray Absorptiometry (DXA) scans

  
  
  
  
  1. 
     

     Measures bone density.

     
     
  2. 
     

     Used to diagnose and monitor treatment of osteoporosis.

     
     
  3. 
     

     T-score (calculated against the score for an average 35-year-old woman) and Z-scores (age-matched) are given.

     
     
     
     
     1. 
        

        T-score:

        
        
        
        
        • 
          

          i. Less than −2.5 = osteoporosis.

          
          
        • 
          

          ii. Between −2.5 and 0 = osteopenia.

          
          
        • 
          

          iii. Greater than 0 = normal.