The US instrument generates, records and processes complex signals using assumptions that are simple and consistent, but flawed in all but ideal situations. Artifacts result when significant violations of these assumptions occur. These assumed principles include: Sound always travels in straight lines; only the properties of the imaged object directly determine the intensity of returning echoes; distance is directly proportional to the time it takes for an echo to make a round trip and return along the propagation path to the surface of the transducer.

Reverberation occurs as sound strikes a subjacent interface and is reflected many times between the interface and the transducer surface. When these multiple reflections are strong enough, they are detected by the receiver and given a spatial alignment that is a multiple of the depth of the original reflective interface. These artifactual echoes occur especially within soft tissue or fluid structures, and sianificant1v alter the echo texture of the object. Reverberations can simulate disease, such as a pseudomass or thrombus in a vessel, when the additional 'non-real' reflectors are placed on the image. The water-filled balloon around the EUS transducer can also be a cause of reverberation artifacts.

Short-path reverberations cause comet tail or ring down artifacts, seen behind a small but intense reflector such as air, metal, plastic or calcified objects. They appear as a series of closely spaced echoes that trail off in intensity as distance from the object increases. The short path is probably produced by microbubbles or crystalline structures that set up reverberation chambers.

Multipath artifacts occur when the reflected sound beam maintains its intensity and coherence at flat and smooth interfaces. Acoustic noise is produced by back scattering of many secondary sound waves from surrounding tissue which reflect again off the smooth interface and return signals to the transducer surface. The result is the visible acoustic noise or dirty shadowing behind a reflecting surface such as a large, smooth calcification or gas pocket. However, the coherence of the reflected beam depends on the diffractive nature of the presenting Surface. If the insonified surface is rough and/or has a small radius of curvature, the back-scattered beam will be diffuse, producing phase incoherence in the return beam. Since absorption, not reflection, of sound wave is the dominant process, phase cancellations and loss of signal occur, which cause clean shadowing.

Mirror image artifacts occur when sound takes longer, more indirect path from the primary interface to a secondary interface before finally returning to the transducer surface. The processor assumes a straight line path and places a phantom lesion at a location deep to the primary reflector due to misregistration of some secondary reflectors. Mirror image artifacts are commonly found around the diaphragm, pleura and bowel.. Similar phenomena are seen in colour Doppler.

Side lobe artifacts (from single element transducers) and grating lobe artifacts (from arrays) result from several low-intensity side lobe sound beams around the main ultrasound beam. These side beams can interact with reflectors and present sound back to the transducer face causing objects to be displayed incorrectly in a lateral position. The instrument, believing the integrity of a single main beam, assigns these side echoes a fictitious position within the path of the main beam. Side lobe signals are most significant at highly reflective interfaces and cause the true echo texture of the imaged object to be altered by low level echoes.

Focal zone banding artifacts occur because brighter shades on the grey scale are always assigned to higher amplitude echoes. However, a sound bearn varies in amplitude along its propagation path, resulting in focal zones of increased intensity. Electronic focusing can therefore create focal zone bands of alternating high and low intensity in an organ that is actual1v homogeneous. A pseudhypoechoic mass can be created in an organ through such banding.

Flat artifact is seen with colour Doppler and has not played a significant role in EUS. It occurs when colour is suppressed where grey-scale echoes are present, but assigned to anechoic or hypoechoic areas.

Spatial resolution limitations call result in artifacts. Objects that are separated by a small distance call merge on the screen if the pulse length is not short enough to distinguish two closely spaced points. Axial resolution is superior to lateral resolution because pulse length is normally much less than pulse width. Axial resolution generally improves ,with higher frequencies. Lateral resolution call range up to 3 cm, whereas axial resolution is usually no more than 2-3 mm. Therefore, small objects may appear larger or thicker when reflectors are parallel to the bearn compared to those that are encountered perpendicular to the beam. Measurements are best made in the axial direction as often as possible.

Image speckle, an interference pattern close to the transducer, causes a parenchymal echopattern, but also image degradation. This acoustic noise is produced by the constructive and destructive interference of rotating echoes from a scatter distribution. Speckle call be reduced by photographic averaging and deconvolution resulting in, improved images.

Slice thickness artifact, also termed section thickness, off-axis or beam-width artifact. results in the melding of the image of different tissues that may not belong together. A focused US pulse has a finite width in the direction perpendicular to the scan plane. The beam can therefore sonographically locate more than one tissue at the same location of a scan plane. The two simultaneously imaged tissues produce all image that has a combined echo texture.

Speed propagation artifact occurs because sound propagates through different body tissues or media at different velocities. Sound travels more slowly through soft tissue than fluid, and even more slowly through fat. As a result, sound that must traverse fatty tissue completes its round trip back to the transducer in a longer period of time. However, to produce an image, US equipment assumes an average tissue velocity of 1540 cm/s. Based on round-trip time and inaccurate depth assignment is made to reflectors posterior to tissue such as fat, fluid and cartilage which have acoustical velocities significantly different from the average.

Through-transmission artifacts occur when sound travels through a medium with low attenuation properties before reaching the imaged object. Scanning through fluid will cause objects to be more echogenic than usual. This is particularly relevant or EUS when cysts or peritoneal fluid are encountered around an area of disease.

Refraction occurs when the sound beam at an oblique angle of incidence strikes a boundary between two media which conduct sound at different velocities. The transmitted part of the sound wave can be bent. The proportion of refraction and reflection that occurs depends on the angle of incidence. Complete bending of the beam can occur when the beam is parallel to the interface, obscuring the image of tissue posterior to the interface. Depending on the direction of the sound beam through the two adjacent media, sound waves will converge or diverge,

Posterior shadowing artifacts occur when sound wave penetration of a structure is very limited or absent. When impedance of two tissues differs to Such a degree that a near complete reflection of the Sound beam is produced, a dark shadow is noted where there are no echoes posterior to the interface. Shadowing also occurs at two apposed transitional zones in the same tissue. Refraction can also cause shadowing from the edge of a curved object, typically at the interface of calcium/soft tissue or gas/fluid.

Enhancement artifacts are due to contrasting acoustic impedance in adjacent structures. Posterior enhancement is a deceptive brightness behind a dark area. The higher amplitude of these echoes compared to those of equivalent adjacent tissue arises posterior to a contrastingly low attenuating area, such as a fluid structure. This sound transmission artifact can be used to distinguish cystic from solid structures. Edge enhancement is an infrequently seen artifact which can occur due to refraction at a curved edge. Focal enhancement can occur in the focal region of the transducer.

Phase cancellation occurs when reflected wave segments are out of phase and cancel each other in summation. When a coherent sound wave crosses boundaries between media that conduct sound at different velocities, the adjacent waveforms become distorted and out of phase. The destructive interference produces an area of cancellation resulting in a black streak or absence of signal on the screen.

Attenuation is a loss of sound wave intensity as a consequence of the behaviours of sound as it interacts with matter. The greatest loss is due to absorption. The higher the frequency, the greater the absorption of sound energy. The degree of attenuation is related to wave frequency and the tissue type and shape. Hypoechoic regions, which could be misinterpreted as masses, can occur behind certain tissues that significantly attenuate the sound beam. Because of the high frequency used with EUS, attenuation artifacts must always be considered. Accuracy in imaging the deep parts of soft tissue structures can be limited by shadows produced when a sound beam does not traverse the structure.

Anisotrophy artifacts result from changes in tissue plane orientation relative to the main second beam axis due to the modulating shape of an imaged organ. Tissue echogenicity changes as sound waves are transmitted at different angles through the same structures due to alterations of sound wave behaviour at the tissue reflectors.

Perivascular colour artifact occurs when intravascular blood flow becomes turbulent resulting in vibrations within disturbed tissue surrounding the blood vessel. When the tissue is examined with colour Doppler, the motion of this vibration will be colour-encoded producing a sonographic equivalent of a soft tissue thrill.

The composition of different tissues and organs determines their ultrasound image characteristics. Acoustic impedance of soft tissues can differ by as Much as 22%. The presence and uniformity of distribution of different tissue components in a soft tissue structure alters its US image. Changes in tissue density and uniformity due to disease can also alter sound wave/tissue interactions. Inherent complexity of tissue properties will be the major consideration for in vitro studies. For in vivo, and human studies, knowledge of anatomical relationships is also critical to proper EUS interpretation.

Important sources of artifacts and errors that arise from improper instrument placement are non-perpendicular scanning, object compression, insufficient contact at the appropriate anatomical acoustic window, and misinterpretation of anatomy. When the incident sound wave encounters a reflector at an angle, the reflected wave will emerge at an equal angle. Part of the incident wave may not reflect, but may be transmitted into the next tissue or medium. The angle of scanning will affect the amplitude of reflected sonographic echoes and the corresponding degree of refraction. Transducer position can affect these properties of sound waves and can produce artifacts.

The most important source of EUS transducer positioning artifact is oblique or tangential scanning, which can result in the appearance of pseudotumours or indistinct margins between parenchyma and vessels through acoustic artifacts such as reverberation, shadowing and enhancement. However, the most common distortion of non-perpendicular scanning is the appearance of a widening or thickening of the layers of the intestinal wall. The best measurements are obtained with the transducer at right angles to the target so that the sound beam is perpendicular to any boundaries between two tissues or media. In performing EUS, one is always trying to optimize image clarity by balancing the highest frequency to obtain the best spatial resolution with a frequency that penetrates deep enough to view the target area. The higher the frequency, the less divergence occurs, and the easier it is to focus the narrowed beam.

The amount of water placed in the balloon surrounding the transducer affects the way the image is generated and can alter the size of the field of optimal focus. Changing the amount of water in the balloon can also be used to move the focal point. However, changing the angle of the echoendoscope and the amount of water in the balloon can easily compress structures so that distortion occurs. Superficial layers of the wall are more sensitive to this distortion than other parts of the intestinal tract or surrounding structures.

Repeat scanning for the same as well as different angles during introduction and withdrawal of the instrument while varying the focal length with the balloon will limit misinterpretation of an EUS image.

The effects of artifacts can be minimized by scanning from different angles, using the narrowest sound beam possible, focusing properly on the target area, avoiding scanning at edges of objects, recognizing secondary images, and adjusting equipment settings (usually the gain). Misinterpretation of anatomy often results from improper transducer location and/or orientation. Misinterpretation also results if surrounding anatomy and structures of the area being scanned are not considered. This is particularly true at the distal oesophagus where the diaphragm complicates interpretation, especially if a hiatal hernia is present.