3. Fundamentals of ultrasound physics. Ultrasound methods. Patient preparation before examination.

From greek.doctor
  • Fundamentals of ultrasound
    • High-frequency sound waves produced by a transducer are used to make images
      • 2 – 20 MHz
      • Higher frequency waves penetrate less deeply into the tissue but has increased resolution
        • High-frequency waves used for superficial structures
        • Low-frequency waves used for deep structures
    • Image is made by sending high-frequency sound into the patient and measuring the magnitude and time of returning echoes
      • The echoes depend on the density of tissue
    • Different tissues give different echoes
      • Cyst -> Hypoechoic/anechoic (because it’s mostly water)
      • Homogenous tissues (liver, spleen -> homogenous small echoes
      • Calcium, fat, air -> hyperechoic
    • Ultrasound interactions with tissues
      • Reflection
      • Refraction
      • Transmission
      • Attenuation
    • Acoustic shadowing
      • Some tissues reflect all sound waves back to the transducer
      • This means that tissues distal to these tissues are not exposed to sound waves and will therefore not be visible on ultrasound -> acoustic shadowing
      • Examples
        • Calcium (stones)
        • Air
    • Acoustic amplification
      • Some tissues refect very few sound waves back to the transducer
      • This means that the tissues distal to these tissue are exposed to more sound waves than normal and will be hyperechoic on ultrasound -> acoustic amplification
      • Examples
        • Cysts
        • Fluid
    • Advantages
      • Produce no ionizing radiation
        • Especially usefully in obstetrics
      • Quick
      • No long-term side effects
      • Portable
    • Real-time ultrasound
      • Allows images to be seen in sequential frames, just like a movie
      • Good for moving structures, like the heart
    • Patient preparation
      • Abstinence from food before abdominal studies
      • Only liquid is allowed
    • Types
      • A-mode (amplitude)
        • One-dimensional
        • Used in ophthalmology
      • B-mode (brightness)
        • Most often used
        • Produces a 2D image
      • M-mode (motion)
        • The motion of a structure is plotted along the x-axis
        • Used for movement of cardiac valves
      • Colour doppler
        • The machine analyses the frequency of the echoes -> blood can be coloured according to which direction it’s moving in
          • Blood flowing toward the transducer is red
          • Blood flowing away from the transducer is blue
        • Good to evaluate stenoses, direction of blood flow, valvular insufficiency
    • Indications
      • Examination of heart (echocardiography)
      • Examination of biliary system
      • Examination of urinary system
      • Asymptomatic, pulsatile abdominal masses
      • Female pelvic organs
      • Abdominal hernia
      • Appendicitis
      • Ascites