Ultrasound & Artifacts

Transcription

1 ISSN Journal of Neurosonology 3(Suppl. 2):1-17, 2011 Ultrasound & Artifacts Siryung Han The Catholic University of Korea Artifacts False image- echoes without anatomic correlate US image dose not have one-to-one correspondence with regards to location, interface and intensity Artifacts could be due to Technology limitation Operator error Violation of assumptions Artifacts could lead to Misdiagnosis overcall Assumption of US Beam dimensions are infinitesimal Image from transmit line-of-sight only Speed of sound constant Amplitude of echo determined by reflective property and nothing else All detected echoes originate from main beam Journal of Neurosonology Volume 3 Suppl. 2,

2 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Beam dimensions Lateral and axial resolution finite Beam dimensions Out of plane dimension or slice thickness is most problematic Line of sight transmit US signal transmitted along line of sight in search reflectors Echoes returning to transducer along same transmission line of sight Speed of sound constant Soft tissue =1540 m/s Fat = ~ 1450 m/s Fluid = ~1500 m/s Differences in speed cause refraction of US beam >mis-registration, edge shadowing (defocusing) and ghosting Echo amplitude solely due to reflectivity Also dependent on Angle of insonation Frequency(acoustic scatterers) Interface(impedance mismatch) All detected echoes come from main beam Not true Side lobes and grating lobes interact with strong reflectors and generate lateral artifacts Slice thickness and refraction also cause image artifacts 2 신경초음파학회지제 3 권부록 2, 2011

3 Siryung Han Ultrasound & Artifacts Four types of artifacts Resolution Propagation Attenuation Doppler/color instrumentation Resolution artifacts Axial resolution Failure to resolve two separate reflectors located parallel to beam line V*PD/2 or SPL/2 Frequency and transducer 5MHz Wavelength=0.3mm=speed of sound/frequency=1,540 m/sec divided by 5*106 cycles/sec 2.5MHz Wavelength=0.6mm and pulse length =1.8mm; 3 cycles Lateral resolution Lateral Resolution Failure to resolve two separate reflectors located perpendicular to beam line A function of beam width Point objects merged into a line Filled in effect Journal of Neurosonology Volume 3 Suppl. 2,

4 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Diagram of an ultrasound beam. 4 신경초음파학회지제 3 권부록 2, 2011

5 Siryung Han Ultrasound & Artifacts Resolution artifacts Slice thickness: finite width of the beam producing extraneous echoes in normally anechoic or echo-free structures Partial volume effect Solutions for Resolution Artifacts Axial resolution-higher US frequency Lateral resolution-locate focal zone to or below level of interest, higher frequency Slice thickness-same fixes as for lateral resolution; also reposition patient Reverberation Propagation artifacts Repetitive reflections between two highly reflective layers Results in equally spaced bands of diminishing amplitudes A reflection phenomenon Can be useful artifact Journal of Neurosonology Volume 3 Suppl. 2,

6 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Reverberation artifact. Reverberation artifact. Reverberation artifact. Real First reflective echo interface D=v*t/2 d1 First reverberation D1=v*(2t)/2=2d d1 d2 D2=v*(3t)*/2=3d Reverberation artifact. Solutions for Reverberation Useful artifact, esp. during line placement, biopsy try changing angle of insonation, use different sonographic window, adjust gain or TGC 6 신경초음파학회지제 3 권부록 2, 2011

7 Siryung Han Ultrasound & Artifacts Four types of artifacts Resolution Propagation Attenuation Doppler/color instrumentation Resolution artifacts Axial resolution Failure to resolve two separate reflectors located parallel to beam line V*PD/2 or SPL/2 Frequency and transducer 5MHz Wavelength=0.3mm=speed of sound/frequency=1,540 m/sec divided by 5*106 cycles/sec 2.5MHz Wavelength=0.6mm and pulse length =1.8mm; 3 cycles Lateral resolution Lateral Resolution Failure to resolve two separate reflectors located perpendicular to beam line A function of beam width Point objects merged into a line Filled in effect Journal of Neurosonology Volume 3 Suppl. 2,

8 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Diagram of an ultrasound beam. 4 신경초음파학회지제 3 권부록 2, 2011

9 Siryung Han Ultrasound & Artifacts Resolution artifacts Slice thickness: finite width of the beam producing extraneous echoes in normally anechoic or echo-free structures Partial volume effect Solutions for Resolution Artifacts Axial resolution-higher US frequency Lateral resolution-locate focal zone to or below level of interest, higher frequency Slice thickness-same fixes as for lateral resolution; also reposition patient Reverberation Propagation artifacts Repetitive reflections between two highly reflective layers Results in equally spaced bands of diminishing amplitudes A reflection phenomenon Can be useful artifact Journal of Neurosonology Volume 3 Suppl. 2,

10 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Reverberation artifact. Reverberation artifact. Reverberation artifact. Real First reflective echo interface D=v*t/2 d1 First reverberation D1=v*(2t)/2=2d d1 d2 D2=v*(3t)*/2=3d Reverberation artifact. Solutions for Reverberation Useful artifact, esp. during line placement, biopsy try changing angle of insonation, use different sonographic window, adjust gain or TGC 6 신경초음파학회지제 3 권부록 2, 2011

11 Siryung Han Ultrasound & Artifacts Refraction artifacts Refraction artifact. Snell s law Bending of US beams at interface of two dissimilar material with different velocities Leads to mid-registration, edge shadowing (defocusing) and ghosting Refraction artifact. Misregistration Due to refraction, beam traveling from higher velocity medium to low velocity (or vice-versa) For example, sound waves transition from fat-rich to soft-tissue rich interface Edge Shadow (defocus) Beam may bend at curved surface and lose intensity, producing a shadow Beam traveling from higher velocity medium to low-velocity gives narrower shadow A wider shadow result from beam travel from lower to higher velocity region Journal of Neurosonology Volume 3 Suppl. 2,

12 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Defocusing Occurs after striking large curved reflector Extends downward from curved reflector s edge A portion of incident sound beam lost refraction leaving less echo signal returning to transducer along beam path Solutions for Mis-registration & deficusing Artifacts Mis-registration and defocusing artifacts need to be considered, but difficult to eliminate completely Mirror image Mirror image artifact. Generated when objects present on one side of a strong reflector Shown on other side of reflector Examples; liver, tumor or hepatic vessels appearing on other side of diaphragm Mirror image artifact. Figure 6c. Mirror image artifact. 8 신경초음파학회지제 3 권부록 2, 2011

13 Siryung Han Ultrasound & Artifacts Solutions for Mirror image Change angle of insonation to vary reflectivity of interface Adjust focal zone or TGC at level of diaphragm to minimize its reflectivity Scan from multiple windows Propagation speed error Beam through structure consisting mainly of fat(velocity is only 1450 m/sec) structure will appear FURTHER away from transducer surface Speed displacement artifact. Structure consisting reflector that has higher velocity than 1540 m/sec CLOSER to transducer surface than it actually is Can also cause abnormal step-off of anatomic structures, such as diaphragmatic shift Speed displacement artifact. Speed displacement artifact. Journal of Neurosonology Volume 3 Suppl. 2,

14 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Speed displacement artifact. Solutions of Propagation speed error Be mindful of this possibility Newer US machines with multi-beam features and improved signal processing capabilities usually minimize this artifact Range ambiguity High PRF or shallow depth cause structures to appear closer to surface than they really are Avoid using excessively high PRF when measuring deep structures Solutions of Range ambiguity Avoid using excessively high PRF when measuring deep structures When measuring shallow structures, use high frequency linear probes or step-offs if necessary Side lobes If v>1540 m/sec(fast), object will appear closer TOWARDS transducer surface than it actually is If v<1540 m/sec(slow), object will appear further AWAY from transducer surface than it actually is Come up with acronym-e.g. SAFT Slow-Away-Fast-Towards Sound energy reflections generated at improper, off-axis locations alongside the main beam Added or assigned to main beam Occurs in both single and array transducers Appears as hyperechoic/hypoechoic density within hypoechoic/anechoic background 10 신경초음파학회지제 3 권부록 2, 2011

15 Siryung Han Ultrasound & Artifacts Side lobe artifact. Side lobe artifact. Solutions for Side lobes Apodization Differential amplification of excitation pulse of central vs. lateral elements Preferential receiver amplification Grating lobes Due to finite transducer array elements, worse with phase arrays Produce bursts of energy at oblique angle from main beam Cause reflections to appear at improper, off-axis locations in image Occurs frequently with strong reflectors near hypo/anechoic structure Solutions for Grating lobes Re-design transducer array such that individual elements < ½ wavelength apart Vary angle of insonation Use different window of approach Attenuation Artifacts Journal of Neurosonology Volume 3 Suppl. 2,

16 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Acoustic shadowing Shadowing. Reduction in echo strength of signals arising from behind a strong reflector or attenuating structure. Examples include gallstones, renal calculi and bone Shadowing. Comet tails Produced by small, strong reflector, similar to reverberation Due to acoustic mismatch minimal echo transmission Composed of thin, closely spaced discrete echoes clean shadows In lungs, rename B-lines if comet tails reach all way down Ring down Produced by small, weak reflectors In contrast to comet tails, has some echo transmission Causes by resonance phenomenon, and associated with gas bubble Also appears similar to reverberation, producing numerous parallel echoes Dirty shadows Ring-down artifact. 12 신경초음파학회지제 3 권부록 2, 2011

17 Siryung Han Ultrasound & Artifacts Ring-down artifact. Ring-down artifact. Ring-down artifact. Increased through transmission. Increased through transmission. Acoustic enhancement Increase in echo amplitude behind a weakly attenuating structure Example is reflection behind fluid-filled cyst Attenuation rate of structure LOWER than surrounding tissue A mostly useful artifact to help indentify fluid filled structures Journal of Neurosonology Volume 3 Suppl. 2,

18 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Focal banding Focal enhancement Occurs at or around focal zone with increase side-by-side intensity of structures Prominent with linear phase arrays Doppler Artifacts Excessive Doppler gain 90 o Doppler angle of insonation Reflection duplication of vessels aliasing Doppler angle of Insonation Location and angle of the sample volume in a diseased ICA with soft plaque. Location and angle of the sample volume in a diseased ICA with soft plaque. Location of the sample volume box in a tortuous artery. 14 신경초음파학회지제 3 권부록 2, 2011

19 Siryung Han Ultrasound & Artifacts Adjustment of the color Doppler sampling window. Adjustment of the color Doppler sampling window. Adjustment of the color Doppler sampling window in a tortuous ICA. Aliasing Spectral aliasing Color Doppler Aliasing Journal of Neurosonology Volume 3 Suppl. 2,

20 Journal of Neurosonology Vol. 3, Suppl. 2, 2011 Adjustment of the color scale in a carotid artery stenosis. Solutions for Alising Increase PRF such that Nyquist Criteria is satisfied PRF=2* actual Doppler frequency Adjustment of the color scale in a carotid artery stenosis. Adjustment of the color scale in a carotid artery stenosis. Adjustment of the color scale in a near occlusion. 16 Adjustment of the color scale in a near occlusion. 신경초음파학회지 제3권 부록 2, 2011

21 Siryung Han Ultrasound & Artifacts Excessive Doppler Gain Adjustment of the color gain. Adjustment of the color gain. Twinkle artifact Journal of Neurosonology Volume 3 Suppl. 2,