Ultrasound Physics
Welcome
Ultrasound Basics
Vibration and Wave
Ultrasound Parameters
Medium Acoustic Property
Ultrasound Reflection
Ultrasound Refraction
Ultrasound Scattering
Ultrasound Attenuation
Ultrasound Application
Ultrasound Transducer
Piezoelectric Effect
Transducer Cosntruction
Array Transducer
Beamforming
Ultrasound Beamformation
Beam Focus
Beam Steering
Imaging
Pulse-echo Method
Imaging Method
Imaging Resolution
Ultrasound Imaging Artifacts
Signal and Circuit
Unipolar Transmitter
Bipolar Transitter
Transceiverg
Time Gain Control
Conditioning
Preprocessing and Postprocessing
Flow Dection
Doppler Effect
Continue Wave Doppler (CW)
Pulse wave Doppler(PW)
Color Flow Imaging
Safety
Intensity
Mechanical Index
Thermal Index
Cavitation
Regulations
Ultrsound Imaging Artifacts
Definition:
In ultrasound imaging, an ultrasound beam scans the target, and generates echo lines based on the acoustic impedance mismatch of the target tissue. The ultrasound beam is assumed to be a short acoustic pulse, small in lateral and axial direction, traveling at constant speed within the tissue. In the ideal situation, a point target inside the tissue should generate a point image. When this assumption is not hold, for example, if the ultrasound pulse becomes too big, propagation speed changed due to tissue, or propagation directions changed due to reflection or diffraction, a point target will not generate a point image, but some kind of spread profile, called point spread function. The image from a target structure is the spatial convolution of the point spread function and the structure itself. As long as the point spread function is not a perfect point, then there are image artifacts. Image artifacts do not reveal the true target structure information.
Resolution related
Speckle: Speckle is generated by amplitude modulation, very typical in ultrasound imaging. Think about there are 10 point targets and each one will generate a short echo in time domain, and all these short echoes will be separated in time when the point targets are far away from each other. If the distances between these targets decrease, the echoes in time domain will also reduce accordingly. When the targets are close enough, all the echoes will be overlapped. Become the echoes are alternative signal, the overlapped final waveform will be very complicated and depends on the phase difference between each small echo. In ultrasound imaging, the target tissue is composed by infinite point targets, with all different size, and acoustic impedance, and randomly spatially distributed. The final echo waveform form soft tissue will be the overlap of all the small echoes, unpredictable in time domain and the amplitude doesn’t reflect the anatomy structure information, but only the statistic information of the soft tissue.
Propagation related
Mirror image: Ultrasound pulse is assumed propagate forward only and reflected acoustic pulse will be received by transducer, processed and showed on image. However if the reflected pulse is too strong, it will continue to travel in the reflected direction and generate echo along its path, and these echoes will be picked up by transducer, after process, shown as an mirrored image.
Most mirror images cannot be seen since they are weak and mixed with normal tissue images. They will become more noticeable in a big liquid pool behind a big planar surface, such as bladder or stomach.
Reverberation: Ultrasound imaging is based on pule-echo method, with assumption that the transmitted pulse will only propagate forward, all the returned the energy is treated as echo. The echo should disappear after it reaches the transducer surface. However, this assumption is not always true. When there is a flat surface structure in the field, such as diaphragm, or fully filled bladder wall, and its surface is parallel to the transducer surface, the echo will be so strong that it will be bounced back to the field after it reaches the transducer surface. The echo may be bounced back and forth several times between the surface structure and the transducer surface before it dies out. On image, the structure will appear several times at further depths, evenly distributed, getting darker each time.
Ring-down and Comet-tail: When ultrasound propagates and hits air targets, it appears as a comet tail. Most researchers believe it is caused by emission of continuous waves from this small structure.
Side lobe: side lobes of the ultrasound beam cause its cross-sectional area or diameter increasing. Imagining the main beam scans a point target and shows a
bright dot on the screen, and when the beam scan the next location, if the beam is very thin, it should generate nothing on screen. However, due to the side lobe, which has lower energy than the main lower, it will generate a less bright dot. Comparing to the main beam, the side lobe travels longer distance, and thus on the screen the dots generates from side lobes will be horizontally next to the bright dot by the main beam, but vertically further and further, and in this way, the total point spread function looks like a ‘flying bird’, with the main bright dot as the body, and the side lobe artifacts like wings.
Attenuation related
Shadowing: Ultrasound pulse will be attenuated dramatically when it encounters stone or bone structure, and the echo amplitude will decrease a lot. On the image it will appear as a dark shadow.
Enhancement: Ultrasound echo is amplified with time-gain-compensation (TGC) mechanism, i. e. the echo from deeper tissue is amplified more than that from the near field, assuming the ultrasound pulse is attenuated uniformly across the imaging depth. However, attenuation to ultrasound caused by different tissue is not the same. When ultrasound pulse passes through a liquid pool like a cyst, it will get less attenuation, and the tissue behind it will appear brighter due to TGC compensation.