Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

i] ,A0.8 0.9 10 1.1flf O"[:f: L~0.50.0 I0.8 0.9 1.0 1.1 1.2f/f oFIGURE 16-11. Effect of damping block on the frequency spectrum. The dampingblock is adjacent to the back of the transducer and limits the vibration of the elementto a small number of cycles. Light damping allows many cycles to occur, whichresults in an extended spatial pulse length (number of cycles times the wavelength)and a narrow frequency bandwidth (range of frequencies contained in the pulse).Heavy damping reduces the spatial pulse length and broadens the frequency bandwidth.The Q factor is the center frequency divided by the bandwidth.faQ = Bandwidthwhere fo is the center frequency and the bandwidth is the width of the frequencydistribution.A "high Q" transducer has a narrow bandwidth (i.e., very little damping) and acorresponding long spatial pulse length. A "low Q" transducer has a wide bandwidthand short spatial pulse length. Imaging applications require a broad bandwidth transducerin order to achieve high spatial resolution along the direction of beam travel.Blood velocity measurements by Doppler instrumentation (explained in section 16.9)require a relatively narrow-band transducer response in order to preserve velocityinformation encoded by changes in the echo frequency relative to the incident frequency.Continuous-wave ultrasound transducers have a very high Q characteristic.An example of a "high Q" and "low Q" ultrasound pulse illustrates the relationshipto spatial pulse length (Fig. 16-11). While the Q factor is derived from the term quality]actor, a transducer with a low Q does not imply poor quality in the signal.The matching layer provides the interface between the transducer element and thetissue and minimizes the acoustic impedance differences between the transducerand the patient. It consists of layers of materials with acoustic impedances that are