Definition of "Ultrasound"

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Ultrasound is a Dx technique for visualizing soft tissue, including tendons, muscles, joints, vessels, and internal organs, for possible pathology/lesions.

Method
  • U/S is the use of sound waves with a frequency too high for human hearing
  • U/S involves sending a pulse of U/S into tissue using a U/S transducer (hand-held probe), that is directly placed on or moved over the Pt. This reflects from different parts of tissue, and these echoes are recorded and create an image
  • Typical Dx sonographic scanners operate between 1-18MHz. The choice of frequency is a trade-off between spatial resolution of the image and imaging depth:
    • Low frequencies (1-6MHz), produce less resolution but images deeper into the body. This provides lower axial and lateral resolution but greater penetration. This is used to image deeper structures including:
      • Liver
      • Kidney
    • High frequencies (7-18MHz), have a smaller wavelengths, and thus capable of reflecting/scattering from small structures. They also have a larger attenuation coefficient and thus are more readily absorbed in tissue, limiting the depth of penetration of the sound wave into the body. This provides better axial and lateral resolution. This is used to image superficial structures, including:
      • Muscles
      • Tendons
      • Testes
      • Breast
      • Thyroid and parathyroid glands
      • Neonatal brain
Classification
  • A-mode (amplitude mode), the simplest type of U/S. A single transducer scans a line through the body w/ the echoes plotteed on screen as a function of depth. Therapeutic U/S aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy
  • B-mode (aka 2D mode, brightness mode), the most well known, where a linear array of transdducers simultaneously scans a plane through the body that can be viewed as a 2D cross-section image of tissue on the screen
  • C-mode, formed in a plane normal (at 90 degrees) to a B-mode image. A gate that selects data from a specific depth from an A-mode line is used, then the transducer is moved in the 2D plane to sample the entire region at this fixed depth. When the transducer transverses the area in a spiral, an area of 100 cm^2 can be scanned in around 10 seconds
  • M-mode (motion mode), where pulses are emitted in quick succession, with each time, either an A-mode or B-mode image being taken. Over time, this produces a video recording showing motion of tissue over time, in U/S. As the organ boundaries that produce reflections move relative to the probe, this can be used to determine the velocity of specific organ structures
  • Doppler mode, which uses the Doppler effect to measure and visualize blood flow (Doppler U/S). It includes:
    • Color doppler, where velocity information is presented as a color-coded overlay on top of a B-mode image
    • Continuous Doppler, where Doppler information is sampled along a line through the body, and all velocities detected at each time point are presented (on a time line)
    • Pulse wave Doppler, where Doppler information is sampled from only a small sample volume (defined in 2D image), and presented on a timeline
    • Duplex, which is simultaneous presentatino of 2D and pulsed wave Doppler information. Triplex is the combination of color Doppler with pulsed wave Doppler, because modern U/S machines tend to naturally use color
  • Pulse inversion mode, where 2 successive pulses w/ opposite sign are emitted and then subtracted from each other. This implies that any linearly responding constituent will disappear while gases w/ non-linear compressibility stand out. Pulse inversion may also be used in a similar manner as in the Harmonic mode
  • Harmonic mode, where a deep penetrating fundamental frequency is emitted into the body and a harmonic overtone is detected. This way noise and artifacts due to reverberation and aberration are greatly reduced
  • Location of blood
  • Presence of specific molecules
  • Elastography (stiffness of tissue)
  • 3D ultrasound (anatomy of a 3D region)
Advantages
  • Images are produced in real-time
  • Portable
  • Low cost
  • Doesn't involve harmful ionizing radiation
  • Effective for imaging SOFT tissue of the body
  • Dx
  • Tx, using U/S to guide interventional procedures (e.g. biopsies or drainage of fluid collections)
Indications
  • Used to guide injecting needles, when placing local anesthetic solutions near nerves
  • Duplex U/S (B-mode vessels imaging combined w/ Doppler flow measurement), used in angiography to Dx arterial and venous disease
  • Echocardiography, used to Dx, e.g. dilatation of parts of the heart, and function of heart ventricles and valves
  • FAST exam, for assessing significant hemoperitoneum or pericardial tamponade after trauma. It is used in ED to exedite the care of Pt's w/ RUQ abdo pain who might have gallstones or cholecystitis
  • Abdominal U/S, to:
    • Image solid organs of the abdomen, although sound waves are blocked by gas in the bowel and attenuated in different degree by fat, so are limited Dx capabilities in this area. It includes:
      • Pancreas
      • Aorta
      • IVC
      • Liver
      • Gallbladder
      • Bile duct
      • Kidney
      • Spleen
      • Appendix can SOMETIMES be seen when inflammed, as in appendicitis
    • Endoanal U/S, is used particularly in the Ix of anorectal Sx, e.g. fecal incontinence or obstructed defecation. It images the immediate perianal anatomy and is able to detect occult defects e.g. tearing of the anal sphincter
  • Gyencological U/S (see page)
  • Obstetric U/S (see page), used during pregnancy to check the development of the fetus
  • Head and neck U/S, including:
    • Most structures of the neck, which are well visualized by high frequency U/S w/ exceptional anatomical detail. It includes the:
      • Thyroid gland, the preferred imaging modality for thyroid tumors and lesions, and is critical pre-op and post-op for Pt's w/ thyroid cancer
      • Parathyroid gland
      • Lymph nodes
      • Salivary glands
    • Many other benign and malignant conditions in the head and neck, for both Dx and U/S-guided Tx
  • In neonatology, transcranial doppler, for basic assessment of intracerebral structural abnormalities, bleeds, ventriculomegaly, or hydrocephalus, and anoxic insults (periventricular leukomalacia). The U/S can be performed through the fontanelle (soft spots in the skull of the newborn) until these completely close about 1yo, and form a virtually impenetrable acoustic barrier for the U/S. The most common site for cranial U/S is the anterior fontanelle. The smaller the fontanelle, the poorer the quality of the picture
  • In neurology, including:
    • Carotid ultrasonography, for assessing blood flow and stenosis in the carotid arteries
    • Transcranial doppler, to assess the big intracerebral arteries
  • Ocular U/S, imaging the eyes, including using A-scan U/S, and B-scan U/S
  • Endobronchial U/S, where probes are applied to standard flexible endoscopic probes, and used by pulmonologists to allow for direct visualization of endobrachial lesions and lymph nodes prior to transbronchial needle aspiration. It can aid lung cancer staging by allowing for lymph node sampling, w/o the need for major surgery
  • Pelvic U/S, which can be performed either transvaginally (in a woman) or transrectally (in a man). It can determine:
    • Amount of fluid retained in a Pt's bladder
    • Image the organs of the pelvic region, including the uterus, ovaries, or urinary bladder
    • In males, to check the health of their bladder, prostate, or testicles (e.g. to DDx epididymitis from testicular torsion)
    • In young males, to DDx more benign testicular masses (varicocele or hydrocele) from testicular cancer, which is highly curable but which must be Tx to preserve health and fertility
    • Imaging of the pelvic floor, to provide Dx information regarding the precise relationship of abnormal structures w/ other pelvic organs, and Ix pelvic prolapse, double incontinence, and obstructed defecation
    • Dx, and at higher frequencies, Tx (break up), kidney stones or nephrolithiasis (kidney crystals)
  • Musculoskeletal U/S:
    • Tendons, muscles, nerves, ligaments, soft tissue masses, and bone surfaces
    • Fracture sonography, as an alternative to x-ray to detect fractures of the wrist, elbow and shoulder for Pt's <12yo
  • Intravascular U/S:
    • Arterial sonography, to assess patency and posible obstruction of arteries
    • Thrombosonography, to Dx DVT
    • Venosonography, to determine extent and severity of venous insufficiency
Contraindications
  • Limit on its field of view (difficulty behind bone and air)
  • Limits on it's field of view, including Pt cooperation and body habitus
  • Difficulty imaging structures behind bone and air
  • Operator dependence on their skill
Epidemiology
  • U/S are increasingly used in both Dr's offices and hospitals
See also

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Definition of Ultrasound | Autoprac


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