Ultrasound Physics Artifacts Hospital Physics Group George David

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

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 d. B/cm/MHz

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 d. B/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 compensation not enough to provide proper signal amplitude intensity under-compensated Attenuates more than. 5 d. B/cm/MHz Opposite of Enhancement Shadowed Reflector

Shadowing Attenuates more than. 5 d. B/cm/MHz Shadowed Reflector http: //raddi. uah. ualberta. ca/~hennig/teach/cases/artifact/noframe/imag 2 -f 2. htm

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

Enhancement Attenuates less. 5 d. B/cm/MHz Enhanced reflector http: //raddi. uah. ualberta. ca/~hennig/teach/cases/artifact/noframe/imag 6 -f 1. htm

Refraction Artifact refraction alters beam 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 1 st pulse reaches transducer after 2 nd pulse emitted scanner assumes this is reflection from 2 nd pulse places echo too close & in wrong direction 1 2

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 re-reflected toward “B” Result later false echoes heard scanner places dots behind reflector “B” 1 2 3 A 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 2 & reflectors Mirror Image common around diaphragm & pleura Real Mirror

Artifacts http: //raddi. uah. ualberta. ca/~hennig/teach/cases/artifact/noframe/imag 1 -f 1. htm Caused by Shotgun Pellets

Multiple Reflection Scenario Real Mirror http: //raddi. uah. ualberta. ca/~hennig/teach/cases/artifact/noframe/imag 5 -f 2. 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 3 D 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 180 o 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

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 #1 A #2 #2 A #3 #3 A

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

Coping with Aliasing decrease transducer frequency reduces Doppler shift proportional to operating frequency increase pulse repetition frequency decreases maximum imaging depth increases likelihood of range ambiguity for pulsed instruments 77 X f. D (k. Hz) 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 f. D (k. Hz) 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