Bảng tốc độ của âm thanh trong các phương tiện truyền thông khác nhau Mô phổi Air Fat não gan cơ xương tốc độ của âm thanh f = số chu kỳ trong một đơn vị thứ hai = U / S Hz 20 kHz Xác định nguồn âm thanh ảnh hưởng đến sự thâm nhập và độ phân giải trục | 34 Transoesophageal Echocardiography Velocity resolution depth line density frame rate Many pulses down each line averaged to give mean velocity n X PRP X N X F 1 n pulses line PRP 1 PRF N lines frame F frame rate Therefore increase in one parameter leads to decrease in others Tissue Doppler imaging TDI Three modalities Pulse wave-TDI PW-TDI 2-dimensional-TDI 2-D-TDI M mode-TDI MM-TDI Sample volume placed on myocardium or A-V valve annulus High frequency low amplitude signals from blood filtered out Measures peak velocities of a selected region Mean velocities calculated to give colour velocity maps PW-TDI Good temporal resolution Wave pattern S wave ventricular systole IVRT E wave rapid diastolic filling Diastasis A wave atrial contraction Tissue Doppler velocities 5-15 cm s Physics of ultrasound 35 2-D-TDI Poor temporal resolution good spatial resolution Uses colour flow imaging Low velocity myocardium coded with dark colours High velocity myocardium coded with lighter colours MM-TDI Excellent temporal resolution Uses colour flow imaging with M mode Artefacts Reverberations Secondary reflection along the path of the U S pulse due to the U S bouncing between the structure and another strong reflector or the transducer Creates parallel irregular lines at successively greater depths from the primary target Two types Fig. i linear reverberation ii ring down solid line directed away from TX due to merging of reverberations Ghosting Type of reverberation artefact when using colour flow Doppler Fig. Amplitude of ghost A of initial reflector if target is moving Mirror images Occurs with Doppler CW and PW dd A of fD spectrum signal in opposite direction normally below threshold therefore filtered out exceeds threshold Fig. 38 Transoesophageal Echocardiography False image .U S beam refracted Fig. Near field clutter In the near field strong signals are received from reflectors which dominate the image Amplitude of near field echoes reduced by near field
