Ultrasound

Ultrasound

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Ultrasound, Ultrasonography, Bedside Ultrasound, Point of Care Ultrasound, POCUS

  • Background
  • Physics
  1. Resolution increases with frequency
  2. Penetration decreases with increased frequency
    1. Low frequency (1 MHz to 6 MHz): Deep Structures
      1. Transabdominal: 3.5 Mhz to 5.0 Mhz
      2. Transvaginal: 5.0 Mhz to 7.5 Mhz
    2. High Frequency (7 MHz to 13 MHz): Superficial Structures
  3. Piezoelectric crystal
    1. Apply a mechanical force sends out electric signal
    2. Apply an electric current, sends out sound
  4. Probe Arrays
    1. Curvilinear Array
      1. Allows for smaller footprint to penetrate between ribs and other obstructions
      2. Curved surface sends signals in a fan shape covering a wider area than the footprint
      3. Ideal penetration for hepatobiliary and aorta evaluation (as well as Obstetric Ultrasound after 7 weeks)
    2. Linear Array
      1. Higher frequency, superficial probes
      2. Vascular access probes use a short, narrow linear array probe
      3. DVT evaluation probes use a longer (typically 4 cm), linear array probe
    3. Vector Array (or Phased Array)
      1. Low frequency, but high refresh rate
      2. Typically used in Cardiac Ultrasound (as well as E-FAST Exam) for excellent resolution and fast real-time updating
      3. Allows for deep penetration (up to 40 cm with some phased array probes)
      4. Among the most useful probes in the Emergency Department arsenal
  5. Modes
    1. A-Mode (Amplitude Mode)
      1. Original Ultrasound delivered only a wave form
    2. B-Mode (Brightness Mode)
      1. Typical 2-Dimensional Ultrasound image
    3. M-Mode (Time series of superimposed B-mode images)
      1. Typically used for heart imaging to demonstrate cardiac cycle activity and associated abnormalities
  6. Send/Receive Timing
    1. Continuous mode (e.g. Doptone)
      1. Probe has two crystals functioning simultaneously
      2. On crystal sends out signals, while the other crystal concurrently receives signals
    2. Pulsed echo mode (standard Ultrasound mode)
      1. One set of crystals that sends signals 1% of the time and receives signals 99% of the time
  7. Orientation: See precautions below (related to cardiac echo)
    1. Screen: Marker typically appears on the left side of screen
    2. Ultrasound probe marker
      1. Transverse view: Marker should point to 9:00
      2. Longitudinal view: Marker should point to 12:00
  8. Screen or Pixel brightness
    1. Fluid appears black
    2. Air appears white
      1. Due to reflection of sound waves (poor penetration)
    3. Bone appears bright white
      1. Due to near complete reflection of sound waves (very poor penetration)
  9. Resolution
    1. Axial Resolution
      1. Resolves objects which lie one in front of the other (one object closer to the Ultrasound probe than the other)
      2. With insufficient axial resolution
        1. Two objects (e.g. vessels, cysts), one in front of the other, will appear as a single object
    2. Linear Resolution
      1. Resolves objects which lie one beside the other (each at a similar distance to the Ultrasound probe)
  • Approach
  • Bedside Ultrasound
  1. Perform Ultrasound with a specific purpose
    1. Dyspnea evaluation for CHF (B-lines, IVC, contractility), Pleural Effusion, DVT
    2. Shock evaluation for Pneumothorax, Pericardial Effusion, hypovolemia (IVC), intraabdominal free fluid (FAST)
  2. Optimize Ultrasound settings
    1. Select appropriate probe (see probe arrays above)
    2. Set best gain, window/zoom (see Machine Settings below)
  3. Record abnormalities in 2 planes
  • Indications
  • Regions amenable to Ultrasound
  1. Cardiac Ultrasound
    1. Echocardiogram
      1. Parasternal Long-Axis Echocardiogram View ( PLAX View)
      2. Parasternal Short-Axis Echocardiogram View (PSAX View)
      3. Subcostal Echocardiogram View (Subxiphoid Echocardiogram View)
      4. Apical Four Chamber Echocardiogram View
      5. Suprasternal Echocardiogram View
    2. Echocardiogram in Congestive Heart Failure
    3. Echocardiogram in PE
    4. Transesophageal Echocardiogram
    5. Stress Echocardiogram
  2. Chest and Lung
    1. Breast Ultrasound
    2. Lung Ultrasound
    3. Lung Ultrasound for Pneumothorax (Sliding Lung Sign, Lung Point)
    4. Bedside Lung Ultrasound in Emergency (Blue Protocol)
    5. Volpicelli Dyspnea Evaluation with Ultrasound Protocol
  3. Vascular Ultrasound
    1. Inferior Vena Cava Ultrasound for Volume Status
    2. Abdominal Aorta Ultrasound
    3. DVT Ultrasound (including Focused Lower Extremity Venous Ultrasound)
    4. Ultrasound Guided Pericardiocentesis
    5. Ultrasound-Guided Antecubital Line
    6. Ultrasound-Guided Internal Jugular Vein Catheterization
  4. Abdomen: Gastrointestinal and Genitourinary
    1. Liver and gallbladder
      1. Gallbladder Ultrasound
    2. Pancreas
    3. Kidney and Bladder
      1. Bladder Ultrasound
      2. Nephrolithiasis Imaging with Ultrasound (including Limited Ultrasound for Acute Renal Colic)
    4. Pediatric Abdomen
      1. RLQ Abdominal Ultrasound (Ultrasound in Appendicitis)
      2. Intussusception Ultrasound (Running the transverse and ascending colon with Ultrasound)
      3. Pyloric Stenosis Ultrasound
    5. Male Genital
      1. Scrotal Ultrasound
    6. Female Pelvis
      1. Fetal Ultrasound
      2. Ultrasound Pregnancy Dating
      3. First Trimester Ultrasound
      4. Second Trimester Ultrasound
      5. Trisomy Findings on Fetal Ultrasound
      6. Pelvic Ultrasound Ovarian Mass Findings
      7. Preterm Labor Assessment with Ultrasound
  5. Musculoskeletal Ultrasound
    1. Shoulder Ultrasound
    2. Elbow Ultrasound
    3. Wrist Ultrasound
    4. Hip Ultrasound
    5. Knee Ultrasound
    6. Ankle Ultrasound
    7. Calcaneal Ultrasound
  6. Eye
    1. Orbital Ultrasound (Retinal Detachment, Increased Intracranial Pressure)
  7. Limited Trauma Ultrasound (FAST Exam or eFAST Exam)
    1. Subcostal Echocardiogram View (Subxiphoid Echocardiogram View)
    2. Right Intercostal Oblique Ultrasound View
    3. Right Coronal Ultrasound View
    4. Left Intercostal Oblique Ultrasound View
    5. Left Coronal Ultrasound View
    6. Suprapubic Ultrasound View (Long Axis or Longitudinal View)
    7. Lung Ultrasound for Pneumothorax (Sliding Lung Sign, Lung Point)
  • Advantages
  • Compared with other imaging modalities
  1. Functional
    1. Real-time imaging
    2. Directs image-guided procedures
    3. Confirms physical exam findings
    4. Good soft tissue imaging
  2. Accessible
    1. Portable
    2. Inexpensive when compared with other imaging modalities
    3. Widely available
  3. Safe
    1. Noninvasive
    2. No ionizing radiation
  • Disadvantages
  1. Steep learning curve (very operator dependent)
  2. Shadowing behind regions of poor penetration (bone or gas)
  3. Poor penetration of bone
  4. Poorly images gas filled regions
    1. Lungs
    2. Stomach and gastrointestinal tract
  • Documentation
  • Limited Regional Examination
  1. Headings broken out into discrete regional components
  2. Medical Necessity
  3. Interpretation
  4. Permanent Image Retention
  5. Final written report
  • Documentation
  • Ultrasound guided procedure
  1. Document laterality (e.g. right, left), probe direction and structure visualized or abbreviations (e.g. PLAX)
  2. Real-time visualization of needle entering vessel or cavity
    1. Stored image should show the vessel targeted (except Suprapubic Catheter)
      1. Documentation does not need to have an image in the record showing needle in vessel
    2. Important to document that real-time guidance was used
  • Precautions
  • Probe Direction Indicator
  1. Issue of confusion on learning Bedside Ultrasound (emergency department and critical care)
    1. All non-cardiac regional Ultrasound conventions and machine presets are with the indicator on screen left
      1. When in transverse orientation, probe points to 9:00 position on patient
      2. When operator stands at head of bed to place IJ, the probe indicator points to the patient's left arm
      3. When operator stands along patient's Abdomen, the probe indicator points to the patient's right arm
    2. Cardiac echo is, by convention, performed with direction indicator on the screen right
      1. Provider should direct the indicator when transverse to point to 3:00 or left arm (not 9:00, right arm) when performing Echocardiogram
      2. Ultrasound machines when on cardiac preset will automatically move the screen indicator to screen right
  2. Follow simple rule
    1. Probe indicator direction should match the screen indicator direction (with screen directly in front of operator)
  • Technique
  • Machine settings
  1. Sonosite Edge Image
    1. radUsSonoEdge.jpg
  2. Focal Zone
    1. Set a machines default (if possible) to be the center of the screen (and do not change)
      1. Adjust the image depth to bring objects of interest to match the default focal zone
    2. Avoid setting multiple focal zones (typically represented as arrows at screen right at various depths)
      1. Slows processing
  3. Gain
    1. Avoid increasing overall gain if possible
      1. Turning off the Ambient lights in the room is a preferred option
      2. Increasing overall gain reduces the available 256 gradations of grayscale, and the potential contrast
    2. Newer machines have fewer gain controls
      1. Older equilizer type gain controls have given way for machine/software optimized gain based on exam preset
      2. Typically auto-gain button on newer machines will refresh the image for ideal gain at various depths
      3. Fine-tune the gain with near gain and far gain settings to adjust for variations in echogenicity
        1. Example: Reduce far-gain behind the Bladder (due to attentuation of waves that pass through Bladder)
  4. Zoom
    1. Start with broad (non-zoomed) view
    2. Avoid zooming too early at the risk of missing deeper or peripheral findings
  • Resources
  • References
  1. Lin and Chou in Herbert (2017) EM:Rap 17(5):6-7
  2. Bhagra (2012) Mayo POIM Conferences, Rochester
  3. Reardon (2013) Emergency Ultrasound Course, 3rd Rock Ultrasound, Minneapolis, MN