Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

One way to accomplish this is to acquire data along one beam line multiple times(depending on the number of transmit focal zones), and accept only the echoeswithin each focal zone, building up a single line of in-focus zones. Increasing thenumber of focal zones improves overall lateral resolution, but the amount of timerequired to produce an image increases and reduces the frame rate and/or numberof scan lines per image (see Image Quality and Artifacts, below).The elevational or slice-thickness dimension of the ultrasound beam is perpendicularto the image plane. Slice thickness plays a significant part in image resolution, particularlywith respect to volume averaging of acoustic details in the regions close to thetransducer and in the far field beyond the focal zone. Elevational resolution is dependenton the transducer element height in much the same way that the lateral resolutionis dependent on the transducer element width (Fig. 16-21). Slice thickness is typicallythe worst measure of resolution for array transducers. Use of a fixed focal lengthlens across the entire surface of the array provides improved elevational resolution at thefocal distance. Unfortunately, this compromises resolution due to partial volume averagingbefore and after the elevational focal zone (elevational resolution quality controlphantom image shows the effects of variable resolution with depth in Fig. 16-55B).Multiple linear array transducers with five to seven rows, known as 1.5-dimensional(1.5-0) transducer arrays, have the ability to steer and focus the beam in theelevational dimension. Elevational focusing is implemented with phased excitationof the outer to inner arrays to minimize the slice thickness dimension at a givendepth (Fig. 16-25). By using subsequent excitations with different focusing distances,multiple transmit focusing can produce smaller slice thickness over a rangeof tissue depths. A disadvantage of elevational focusing is a frame rate reductionpenalty required for multiple excitations to build one image. The increased widthof the transducer array also limits positioning flexibility. Extension to full 20 transducerarrays with enhancements in computational power will allow 3D imagingwith uniform resolution throughout the image volume.FIGURE 16-25. Elevational resolutionwith multiple transmit focusingzones is achieved with "1.5-D"transducer arrays to reduce the slicethickness profile over an extendeddepth. Five to seven discrete arraysreplace the single array. Phase delaytiming provides focusing in the elevationalplane (similar to lateraltransmit and receive focusing).