Lecture 1 Notes

Three important factors affect the choice of

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Unformatted text preview: (ZT) is ~15 times greater than Z of tissue (ZL) –  Placing crystal directly over skin would result a large amount of energy be reflected back from the boundary »  R= |(ZL-ZT)/(ZL+ZT)| ~1 Matching layer –  layer Zl = ZT Z L = λ/4 thickness –  ZI = √(ZTZL) –  Maximize energy transfer into the body –  Show as a homework(*) Problems: Finding material with exact Zl value Transducer   - Doppler Therapy and bandwidth. Three important factors affect the choice of frequency: 1) tissue attenuation, 2) required depth of penetration, and 3) system dynamic range. There is no strict rule for equating these variables and arriving at an optimum frequency. Generally, the highest frequency that provides adequate penetration is selected; this optimizes the lateral resolution [cf. (1)] . The pulse bandwidth needed to obtain good axial resolution poses an altogether different problem. Here, the tendency is to select the widest bandwidth (shortest pulse) because this provides the best axial resolution in water. Unfortunately, wide bandwidth sometimes leads to reduced sensitivity and loss of lateral resolution at depth in attenuating media [16]. The latter effect results from the selective attenuation of the higher Array types a)  b )  c)  d )  e)   Linear Sequential (switched) ~1 cm × 10-15 cm, up to 512 elements Curvilinear similar to (a), wider field of view Linear Phased up to 128 elements, small footprint → cardiac imaging 1.5D Array 3-9 elements in elevation allow for focusing 2D Phased Focusing, steering in both dimensions Homework   Reading:   Problems: –  Prince and Links, Medical Imaging Signals and Systems, Chap. 10 (Sec. 10.5 not required),11.2,11.3 –  –  –  –  –  –  –  P10.1 P10.3 P10.6 P10.8 P10.12 P10.13 Considering the (λ/4) matching layer in a transducer. Show that the transmitted energy into the tissue is maximized with an impedance of √(ZTZL) Z l = ZT Z L...
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