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Us physics 3

  1. 1. US PHYSICS (3) Dr. Kamal Sayed MSc US UAA OK indications/pt position/technique/planes/ acoustic shadowing/posterior transmission/
  2. 2. What is the principle of ultrasound? An electric current passes through a cable to the transducer and is applied to the crystals, causing them to deform and vibrate. This vibration produces the ultrasound beam. The frequency of the ultrasound waves produced is predetermined by the crystals in the transducer. • General • Ultrasound is a convenient and accessible tool for examination. It is relatively cheap and fast. Additionally, patients are not exposed to ionizing radiation. Table in slide (3) gives some idea of the many applications & indications of ultrasound technology. • The list includes only tests performed by the radiologist; prenatal ultrasound tests in pregnant women, for instance, are performed by specialized obstetricians.
  3. 3. • Figure 1. Broad outline of ultrasound indications. •
  4. 4. • Patient Position For carotid ultrasonography, there are two options for the relative position between the patient and examiner : • 1. One is the overhead position, in which the examiner sits beyond the patient’s head beside the end of the examination table and use two hands for ultrasonography. In this position, the examiner should use his right hand for the right carotid artery and use his left hand for the left carotid artery.
  5. 5. • The benefit of this position is that : @ the examiner can use both hands @ and there are plenty of positions possible for the probe. @ The sonic window can be made wider and offers a clear view of the carotid artery especially from the posterolateral projection. 2. Another position is the usual lateral sitting position, which is used for most other ultrasonography examinations. The examiner. uses his right hand for both carotid arteries. This position makes it easy to control the machines. However, the right posterior projection is a bit more difficult.
  6. 6. • 2. Another position is the usual lateral sitting position, which is used for most other ultrasonography examinations. The examiner. uses his right hand for both carotid arteries. This position makes it easy to control the machines. However, the right posterior projection is a bit more difficult.
  7. 7. • 3- # Between these two choices, the overhead position for Doppler ultrasonography of the carotid artery is recommended. # A pillow is not necessary. In fact, it produces a poorer window for the carotid artery. # The optimal patient head position is tilted about 45° away from the artery being examined. # The neck of the patients should be relaxed : Contractions of the sternocleidomastoid muscle cause poor sonic penetration and make positioning of the probes difficult.
  8. 8. • A significant benefit of ultrasound is that in some cases the clinical picture, e.g. local pressure pain or palpable swelling, can immediately be correlated with ultrasound findings. Additionally, it is a dynamic procedure with moving images. This may be useful, e.g. to demonstrate an inguinal hernia during Valsava or assess compressibility of the gallbladder or vessels
  9. 9. • Unfortunately, ultrasound also has its drawbacks : • @ Not all patients are suitable for ultrasound e.g • In adipous patients, it can be difficult to image everything clearly (slide 10). • @ Additionally, the quality of the examination largely depends on the experience of the person performing the ultrasound. • That is to say : it is operator dependent •
  10. 10. • Figure 3. Difference in image quality in an adipous versus slender patient. •
  11. 11. • Technique Ultrasound uses sound waves. • They are reflected, deflected or absorbed in the body. • The reflected sound waves produce the ultrasound image. • The more sound waves are reflected, the more hyperechogenic (= whiter) the tissue is imaged. • With reduced reflection, the image will be more hypoechogenic, and anechogenic if there is no reflection (= black). Both the speed of sound through the tissue and tissue density impact the quality of the ultrasound image. •
  12. 12. • High-density tissue generates multiple echo reflections (e.g. bone/calcareous structures), producing hyperechogenic images. • Fluid reflects no sound waves and therefore is anechogenic (= black). • Soft tissue (e.g. organs) is somewhere between hyperechogenic and anechogenic. • Isoechogenic is when the tissue has the same echogenicity as the surrounding tissue (slides 13/14).
  13. 13. • • Figure 4. Echogenicity with corresponding terms. •
  14. 14. • Figure 5. Sample abdominal ultrasound examination. Note the different echogenicities of the various structures.
  15. 15. • Various planes • A TXR is used to perform transversal and sagittal assessments. • By moving the TXR over the skin, a parallel series of ultrasound images is obtained, allowing systematic assessment of each part of the body. • Another technique is to Tip the TXR (The TXR is held in place but is rotated ninety degrees; only the sound beam changes direction). In this way structures can be evaluated in two directions :
  16. 16. • For instance : • @ in the craniocaudal direction (transverse plane) • @ and left-right direction (sagittal plane). • Important : location and direction of the TXR on the patient's skin determine anterior/posterior and left/right on the imaged obtained.
  17. 17. • As a general rule, in the transversal plane (slide 17): • the top of the ultrasound image is the anterior side and the bottom is the posterior side. • left on the image is actually right and vice versa. • The body is seen from below as it were (as in a transversal section of a CT scan).
  18. 18. • Figure 8. Left kidney in the transversal plane. •
  19. 19. • As a general rule, in the sagittal plane (slide 20): • the top of the ultrasound image is the anterior side and the bottom is the posterior side. • right on the image is towards the feet (= caudal) and left is towards the head (= cranial). • The images can be read from the screen during the examination. Orientation tip when attending a live examination : the top of the image is the location where the sound waves enter the patient first. So irrespective of position and tipping, the top is the skin side.
  20. 20. • Figure 9. The left kidney in the sagittal plane. •
  21. 21. • Reflection/deflection/absorption/scatter • When sound waves move on the boundary surface between two media with different densities, part of the beam is reflected to the transducer. This phenomenon is called reflection. • The remainder of the beam continues on into the tissue, but under a different angle. This is called deflection. • As sound waves penetrate the tissue, part of the energy is converted into heat. This energy loss is called absorption. •
  22. 22. • Finally, part of the sound waves are lost in scatter. • This takes place when sound waves move through inhomogeneous tissue or in a 'hard’ boundary surface (= large density difference between two media). • Part of the sound waves are reflected in random directions, a small part of which towards the transducer.
  23. 23. • Artifacts • Ultrasound examinations are associated with a diversity of ultrasound artifacts and can be encountered during the examination. • Unfortunately, these artifacts cannot all be discussed in this course. Two important artifacts are explained here: • acoustic shadow and posterior sound transmission. Even though these are artifacts, they are valuable in practice. •
  24. 24. • Artifacts are any alterations in the image which do not represent an actual image of the examined area. • They may be produced by technical imaging errors or result from the complex interaction of the ultrasound with biological tissues. • Reverberation artifacts appear as a series of equally spaced lines.
  25. 25. • Acoustic shadow • Acoustic shadowing is caused by two different phenomena, total reflection or total absorption. • Total reflection occurs on the boundary surface between gas/tissue because of the large difference in density between gas and tissue. • Total absorption occurs when the sound waves are absorbed by calcareous structures (= including stones, bone).
  26. 26. • Acoustic shadowing contd • Sound waves are (virtually) all reflected/absorbed; • no sound waves reach the area behind these structures, making this part of the ultrasound image entirely anechoic (= black). • This is termed • acoustic shadow (slide 27).
  27. 27. • Acoustic shadowing caused by reflection by intestinal gas. •
  28. 28. • Acoustic shadowing is important in detecting disorders including tendon calcifications, stones or free air. • The artifact is also used to differentiate solid and calcified masses, e.g. gallbladder polyp • from bile stones. (slide 29) •
  29. 29. • Acoustic shadowing by a bile stone caused by absorption of sound waves by the calcareous stone. •
  30. 30. • Posterior sound transmission • In order to differentiate a cyst from a solid lesion, two artifacts are used : • 1- posterior wall amplification and • 2- increased sound transmission. • These phenomena occur when sound waves move through an anechogenic structure, usually a cyst.
  31. 31. • The sound wave loses little energy as it passes through the fluid in the cyst. • That is why there is more energy left in the sound wave in the posterior wall and behind the structure than at the same level in the surrounding area • (note: the surrounding tissue is more solid). More energy will therefore be left to reflect to the transducer. • This results in a echogenic posterior wall and echogenic area behind the cyst (slide 32/33).
  32. 32. • Figure 18. Renal cyst with posterior wall amplification and increased sound transmission. •
  33. 33. • Posterior sound transmission is a good tool to differentiate a cyst from a solid lesion . Figure 19. An hepatic cyst with posterior wall enhancement and sound transmission versus solid hepatic lesion. Note the cyst is anechogenic, as opposed to the echogenic solid liver lesion (the solid lesion was shown to be an hemangioma on a CT scan).
  34. 34. • Bioeffects • No known bioeffects at standard imaging intensities • Thermal Mechanism : • Temperature elevation via absorption resulting from • interaction of biologic tissue and US. • A second mode of • thermal injury may result from localized scattering of • acoustic energy, especially at inhomogeneities within the • medium (Rayleigh scattering.) •
  35. 35. • Cavitation Mechanism : • Microbubbles (gaseous nuclei) found in native tissues may be • excited by US, taking the form of shrinking and expanding • of the bubble. • Potential of near total energy absorption • where the nuclei exist may lead to thermal injury. • Stable cavitation :- microbubbles expand & contract • Transient cavitation:- microbubbles burst