Artifacts

Ultrasound Physics Artifacts Our Hospital Physics Group George David, M.S. Associate Professor of Radiology Nothing to do with Anything 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 exactly 0.5 dB/cm/MHz Distance from Transducer • Calculation of Distance scanner accurately measures time delay between sound generation & echo reception Distance = Assumed Speed X Measured Delay / 2 Actual Distance to interface 1380 m/s X 58usec / 2 = 4 cm Calculated Distance to interface 58 usec V = 1380 m/s 1540 m/s X 58usec / 2 = 4.47 cm (Average speed of sound in soft tissue) 4 cm 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 Actual Object Position X Position of Object on Image V = 1540 m/s X Attenuation • For all scanning your scanner assumes soft tissue attenuation » .5 dB/cm per MHz • Your scanner’s action compensate for assumed attenuation allow operator fine tuning » TGC Shadowing • Clinical Manifestation  reduction in imaged reflector amplitude • Cause  object between this reflector & transducer attenuates ultrasound more than assumed Attenuates  assumed compensation not enough to more than .5 provide proper signal amplitude dB/cm/MHz  intensity under-compensated • Opposite of Enhancement Shadowed Reflector Shadowing Attenuates more than .5 dB/cm/MHz Shadowed Reflector http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag2-f2.htm 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 Enhancement Attenuates less .5 dB/cm/MHz Enhanced reflector http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag6-f1.htm Shadowing / Enhancing • these artifacts not necessarily bad • can help in determining nature of masses “upstream of artifact which caused shadowing / enhancing Scanner Assumptions • Echo positioning on image direction of all sound travel assumed to be direction that sound was transmitted Actual Object Position X Position of Object on Image X Refraction Refraction Artifact • refraction alters beam direction • direction of sound travel assumed to be direction sound transmitted Actual Object Position X Position of Object on Image X Refraction Refraction Artifact • refraction alters beam direction • scanner places dot in wrong location along line of assumed beam direction • can alter reflector shape 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 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 Range Ambiguity • To improve any 1 of 3, at least 1 of other 2 must be reduced. • many scanners automatically reduce frame rate as depth increases Lines / Frame Depth Range Ambiguity Trade-off Frames / sec (dynamics) Scanner Assumptions Multipath Artifact Actual Object Position X Position of Object on Image X Multiple Reflection Scenario • reflection from reflector “B” splits at “A” • some intensity rereflected toward “B” • Result 1 2 3 A B later false echoes heard scanner places dots behind reflector “B” 1 2 3 real false 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 Real Mirror Artifacts http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag1-f1.htm Caused by Shotgun Pellets Multiple Reflection Scenario Real Mirror http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag5-f2.htm 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 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 Constructive Interference • 2 echoes received at same time • in phase • Result + higher intensity = Destructive Interference • 2 echoes received at same time • Exactly 180o out of phase • Result flat (zero) wave = 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 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 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 Doppler Artifacts • Doppler spectrum speckle • Cause same as acoustic speckle random constructive & destructive interference from sound scattered in blood Aliasing • Results in detection of improper flow direction • occurs because sampling rate too slow • Similar to wagon wheels rotating backwards in movies Aliasing Sufficient Sampling Insufficient Sampling Aliasing • Which way is this shape turning? #1 #2 #3 Aliasing Did the shape turn 1/4 turn right or 3/4 turn left? 1 1/4 turn right? #1 #2 #3 Aliasing Does it help to sample more often? #1 #1A #2 #2A #3 #3A 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 Aliasing • Maximum detectable Doppler Shift not limited for continuous wave Doppler • Maximum detectable Doppler Shift is limited for pulsed instruments Maximum Detectable Doppler Shift = half pulse repetition frequency 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 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 q 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 Any Questions?

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