{"id":654,"date":"2015-01-28T10:01:34","date_gmt":"2015-01-28T15:01:34","guid":{"rendered":"http:\/\/test.pegasuslectures.com\/blog\/?p=654"},"modified":"2015-01-28T10:57:48","modified_gmt":"2015-01-28T15:57:48","slug":"quick-concepts-grating-lobes","status":"publish","type":"post","link":"https:\/\/www.pegasuslectures.com\/blog\/index.php\/2015\/01\/28\/quick-concepts-grating-lobes\/","title":{"rendered":"Quick Concepts: Grating Lobes"},"content":{"rendered":"

Many of you may recall, when discussing artifacts I emphasize that not all artifacts you encounter are necessarily \u201cbad<\/em><\/strong>\u201d. In fact, artifacts can provide valuable, diagnostic information<\/span>.<\/p>\n

\"GALLBLADDER

GALLBLADDER WITH ACOUSTIC SHADOWING FROM A GALLSTONE<\/p><\/div>\n

Our recent blog on Acoustic Shadowing<\/a> pointed out\u00a0its underlying cause is the presence of a strong reflector or absorber. \u00a0Knowing this provides you with a valuable inference (i.e., the presence of a gallstone or kidney stone).<\/p>\n

Unfortunately, grating lobes is not a \u201cgood<\/em><\/strong>\u201d artifact. Side lobe <\/strong><\/em>(single element) or grating lobe<\/strong><\/em> (arrays) artifact results from two factors:<\/p>\n

\u2022 the true, more complex nature of sound beams versus our idealized visualization<\/strong><\/em><\/p>\n

\u2022 assumptions designed into ultrasound technology and imaging<\/strong><\/em><\/p>\n

\"\"<\/p>\n

Think about a situation where you shout across the office to a co-worker, rather than using the phone. Your idealized wish (and intention) is that your message will be carried in a straight line between you and the intended target. The reality is that everyone around you hears your message. And this is also (get ready for a bad pun) \u201cgrating<\/strong>\u201d \u2026on your co-workers\u2019 nerves.<\/p>\n

Despite the best efforts of transducer engineers and designers, the \u201cmain\u201d sound beam is accompanied by undesired, weaker sound beams pointing off-axis. These also propagate, reflect, and return to the transducer.<\/p>\n

Ultrasound presumes that the reflections returning to the transducer always come from the steered direction.<\/strong> However, when these (weaker) off-axis beams encounter a strong specular reflector, the returning reflected energy will be received and \u201cadded\u201d to the main beam \u2026often creating a spurious structure or clouding within an image.<\/p>\n

\"SPURIOUS

SPURIOUS SECOND AORTIC VALVE ROOT CAUSED BY GRATING LOBE ARTIFACT<\/p><\/div>\n

\"REFRACTION

REFRACTION ARTIFACT CREATING A “SECOND” ABDOMINAL AORTA<\/p><\/div>\n

From a conceptual standpoint, grating lobe artifact<\/span> and refraction artifact<\/span> have a great deal in common. In both these cases, the path of the ultrasound beam has not followed the intended, steered direction. They are both \u201clocational<\/strong>\u201d artifacts, which is to say their result within the ultrasound image is the structure\u2019s displacement from its true location<\/strong>. Generally, the spurious structure will be laterally displaced. As such, without a clear visualization of a refracting surface within the image, differentiation between grating lobe artifact and refraction artifact is relatively challenging.<\/p>\n

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This subject matter is discussed more thoroughly in Chapter 8: Artifacts, of Frank Miele\u2019s<\/a><\/em> Ultrasound Physics and Instrumentation<\/a> (5th<\/sup> edition)<\/strong>. \u00a0Grating Lobe artifact\u00a0is discussed specifically on page 282 and referenced in the context of discussion on the advantages of Harmonic Imaging in Chapter 10: Contrast and Harmonics on pages 329-330.<\/em><\/p>\n