Question 2823
{"accessible":false,"alternatives":[{"id":13779,"text":"decreasing transducer frequency"},{"id":13780,"text":"decreasing wavelength"},{"id":13781,"text":"decreasing transducer power output"},{"id":13782,"text":"increasing transducer diameter"},{"id":13783,"text":"using a thinner piezoelectric crystal layer"}],"archived":false,"correctAlternativeId":13779,"explanation":"\u003cp\u003eNear field length is defined by the following equation\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eNear field length = d\u003csup\u003e2\u003c/sup\u003e (diameter of transducer\u003csup\u003e2\u003c/sup\u003e) / 4 x wavelength\u003c/strong\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eWavelength = speed of sound / frequency\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eTherefore (assuming speed of sound in soft tissue is 1540m/s):\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eNear field length (soft tissue) = d\u003csup\u003e2\u003c/sup\u003e x frequency / 4 x 1,54mm\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eReducing frequency reduces the near field length.\u003c/p\u003e\u003cp\u003eDecreasing wavelength (which can be achieved by using a thinner piezoelectric layer) and increasing transducer diameter will both increase near field length. \u003c/p\u003e\u003cp\u003eTransducer power output has no direct effect on beam geometry.\u003c/p\u003e","id":2823,"imageUrl":null,"imageAttribution":null,"imageAttributionCaseInfo":null,"firstQuestionPath":"/questions/2828","nextQuestionPath":"/articles/piezoelectric-effect/questions/2818","relatedArticles":[{"id":32088,"title":"Ultrasound (introduction)","link":"/articles/ultrasound-introduction?lang=us"},{"id":54038,"title":"Ultrasound transducer","link":"/articles/ultrasound-transducer?lang=us"},{"id":68369,"title":"Beam focusing","link":"/articles/beam-focusing?lang=us"}],"alsoUsedIn":[{"id":1615,"kind":"RestrictedPage","title":"Page 1615 (in no courses)","link":"https://radiopaedia.org/admin/restricted_pages/1615"},{"id":1979,"kind":"Course","title":"Imaging Physics: Ultrasound Peer Review - page 1979","link":"https://radiopaedia.org/courses/imaging-physics-ultrasound-peer-review/pages/1979"},{"id":1979,"kind":"Course","title":"Imaging Physics: Ultrasound - page 1979","link":"https://radiopaedia.org/courses/imaging-physics-ultrasound/pages/1979"}],"stem":"\u003cp\u003eWhich of the following will reduce the near field length of an ultrasound beam?\u003c/p\u003e","menuLinks":[{"text":"Report problem with question","url":"https://docs.google.com/forms/d/e/1FAIpQLSfO3soWYhOjJ7yErSysyCe5V4A1CqW7WK3rDA7MtAkecMGqNw/viewform?entry.1624461248\u0026entry.553583435=https://radiopaedia.org/questions/2823"}],"attemptsPercentages":[{"alternativeId":"13783","percentage":21},{"alternativeId":"13781","percentage":3},{"alternativeId":"13780","percentage":17},{"alternativeId":"13782","percentage":14},{"alternativeId":"13779","percentage":45}],"promptToLogin":false,"questionManager":false,"articleId":"piezoelectric-effect"}