mật độ cao / áp suất / nhiệt độ / chuyển động + Rarefactions: mật độ thấp / áp suất / nhiệt độ / chuyển động (Hình ) Transthoracic tiếng vang (TTE) ~ 2-5 MHz Transoesophageal echo (TOE) ~ 3,5-7 âm thanh MHz được mô tả bởi tốc độ lan truyền (m / s) Tần số (Hz) Bước sóng (m) | Physics of ultrasound 17 Types of focusing Fig. 1 external focusing 2 internal focusing 3 focusing mirror 4 electronic focusing phased array dynamic variable focusing adjustable by sonographer better resolution Arrays Array collection of active elements in one TX single slab of PZT-5 cut into small pieces Each active element is connected to its own electronic circuitry 18 Transoesophageal Echocardiography Fig. Linear elements in a line linear switched array linear phased array Annular elements with a common centre in a ring Convex curved collection in curved manner convex switched array convex linear array Linear switched array Fig. Large TX with elements arranged in a line Image no wider than TX with a rectangular image P E crystals fire in sequence to give 2-D image No steering fixed vertical focusing Defective crystal causes vertical dropout Phased arrays Fig. Collection of electric pulses delivered to the active elements in various patterns which focus and steer U S pulse Fan-shaped image Many signals excite multiple crystals one sound pulse If one element breaks erratic focusing steering Small time delays nearly simultaneous between electronic pulses delivered to array elements 20 Transoesophageal Echocardiography Steering slope Focusing .