1 / 33

Ultrasound Physics: Understanding and Preventing Artifacts

Learn about ultrasound artifacts caused by assumptions and how to prevent them. Explore shadowing, enhancement, refraction, lobe artifacts, and more. Understand resolution limitations and interference effects. This comprehensive guide by George David, M.S., provides valuable insights for radiology professionals.

mjennifer
Download Presentation

Ultrasound Physics: Understanding and Preventing Artifacts

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Ultrasound Physics Artifacts Hospital Physics Group George David, M.S. Associate Professor of Radiology

  2. Artifacts • Assumptions can cause artifacts when assumed conditions are not true • sound travels at 1540 m/s • sound travels in a straight line • All sound attenuation exactly0.5 dB/cm/MHz

  3. Distance from Transducer • Echo positioning on image • distance from transducer calculated from assumed speed of sound • can place reflector too close to or too far from transducer • can alter size or shape of reflector V = 1380 m/s X Actual Object Position Position of Object on Image X V = 1540 m/s

  4. Attenuation • For all scanning your scanner assumes • soft tissue attenuation • .5 dB/cm per MHz • Your scanner’s action • compensate for assumedattenuation • allow operator fine tuning • TGC

  5. Shadowing • Clinical Manifestation • reduction in imaged reflector amplitude • Cause • object between this reflector & transducer attenuates ultrasound more than assumed • assumed compensation not enough to provide proper signal amplitude • intensity under-compensated • Opposite of Enhancement Attenuates more than .5 dB/cm/MHz Shadowed Reflector

  6. Shadowing Attenuates more than .5 dB/cm/MHz Shadowed Reflector http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag2-f2.htm

  7. Enhancement • Clinical Manifestation • increase in imaged reflector amplitude • Cause • object between reflector & transducer attenuates ultrasound less than assumed • assumed compensation more than needed to provide proper signal amplitude • intensity over-compensated • Opposite of Shadowing Attenuates less .5 dB/cm/MHz Enhanced reflector

  8. Enhancement Attenuates less .5 dB/cm/MHz Enhanced reflector http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag6-f1.htm

  9. Actual Object Position Position of Object on Image X Refraction Artifact • refraction alters beam direction • direction of sound travel assumed to be direction sound transmitted X Refraction

  10. Refraction Artifact • refraction alters beam direction • scanner places dot in wrong location along line of assumed beam direction • can alter reflector shape

  11. Lobe Artifacts • Side Lobes • beams propagating from a single element transducer in directions different from primary beam • reflections from objects here will be placed on main sound transmission line • Grating Lobes • same as above except for transducer arrays X

  12. Range Ambiguity • Reflection from 1st pulse reaches transducer after 2nd pulse emitted • scanner assumes this is reflection from 2nd pulse • places echo too close & in wrong direction 1 2

  13. Scanner Assumptions Multipath Artifact Actual Object Position Position of Object on Image X X

  14. Multiple Reflection Scenario • reflection from reflector “B” splits at “A” • some intensity re-reflected toward “B” • Result • later false echoes heard • scanner places dots behind reflector “B” 2 3 1 A B real 1 2 false 3

  15. Real Mirror Artifacts • Reverberation (multiple echo) artifact • “comet tail” effect is 1 example • can have dozens of multiple reflections between • transducer & reflector • 2 reflectors • Mirror Image • common around diaphragm& pleura

  16. Artifacts http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag1-f1.htm Caused by Shotgun Pellets

  17. Real Mirror Multiple Reflection Scenario http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag5-f2.htm

  18. Resolution Artifacts • Axial and Lateral Resolution Limitations • results in failure to resolve 2 adjacent structures as separate • minimum image size equal to resolution in each direction

  19. Section Thickness Artifact • anatomy may not be uniform over its thickness • universal problem of imaging 3D anatomy • in CT & MRI this is known as partial volume effect Thickness

  20. Constructive Interference • 2 echoes received at same time • in phase • Result • higher intensity + =

  21. Destructive Interference • 2 echoes received at same time • Exactly 180o out of phase • Result • flat (zero) wave - =

  22. Acoustic Speckle • texture seen on image may not correspond to tissue texture • Results from interference effects between multiple reflectors received simultaneously which can • add together • constructive interference • subtract from one another • destructive interference

  23. Mirror Image & Doppler • Analogous to mirror image artifact discussed previously • mirrored structures can include mirrored vessel • duplicate image visible on opposite side of strong reflector • example: bone • Doppler data also duplicated • flow & spectrum copied from original vessel

  24. Spectral Duplication • mirror image of Doppler spectrum appears on opposite side of baseline • causes • electronic duplication caused by receiver gain set too high • overloads receiver • True sensing caused by too large Doppler angle • beam covers flow in both directions Blood flows toward transducer Blood flows away from transducer

  25. Aliasing • Results in detection of improper flow direction • occurs because sampling rate too slow • Similar to wagon wheels rotating backwards in movies

  26. Aliasing Sufficient Sampling Insufficient Sampling

  27. Aliasing • Which way is this shape turning? OR #1 #2 #3

  28. Aliasing Did the shape turn 1/4 turn right or 3/4 turn left? 1 1/4 turn right? #1 #2 #3

  29. Aliasing Does it help to sample more often? #2 #1 #1A #2A #3A #3

  30. Aliasing • Maximum detectable Doppler shift equals half the pulse repetition frequency • Sampling rate • Same as pulse repetition frequency • Must be at least twice highest frequency to be sensed • Aliasing occurs when Doppler shift exceeds 0.5 * PRF

  31. Coping with Aliasing • decrease transducer frequency • reduces Doppler shift • shift proportional to operating frequency • increase pulse repetition frequency • decreases maximum imaging depth • increases likelihood of range ambiguity for pulsed instruments 77 X fD (kHz) v (cm/s) = -------------------------- fo (MHz) X cosq

  32. q Coping with Aliasing • increase Doppler angle • Reduces relative flow rate between blood & transducer • Reduces Doppler shift sensed by scanner 77 X fD (kHz) v (cm/s) = -------------------------- fo (MHz) X cosq

  33. Coping with Aliasing:Baseline Shifting • operator instructs scanner to assume that aliasing is occurring • scanner does calculations based on operator’s assumption • scanner has no way of determining where in image aliasing occurs Yes No

More Related