Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 16-54. A comparison of color Doppler (left) and power Doppler (right) studies show theenhanced sensitivity of the power Doppler acquisition, particularly in areas perpendicular to thebeam direction, where the signal is lost in the color Doppler image. Flow directionality, however, islost in the power Doppler image.and quantitation. Images acquired with color flow and power Doppler are illustratedin Fig. 16-54.The system performance of a diagnostic ultrasound unit is described by severalparameters: sensitivity and dynamic range, spatial resolution, contrast sensitivity,range/distance accuracy, dead zone thickness, and TGC operation. For Dopplerstudies, pulse repetition frequency, transducer angle estimates, and range gate stabilityare key issues. To ensure the performance, accuracy, and safety of ultrasoundequipment, periodic quality control (QC) measurements are recommended. TheAmerican College of Radiology (ACR) has implemented an accreditation programthat specifies recommended periodic quality control procedures for ultrasoundequipment. The periodic QC testing frequency of ultrasound components shouldbe adjusted to the probability of finding instabilities or maladjustment. This can beassessed by initially performing tests frequently, reviewing logbooks over anextended period, and, with documented stability, reducing the testing rate.Ultrasound Quality AssuranceEquipment quality assurance is essentially performed every day during routine scanningby the sonographer, who should and can recognize major problems with theimages and the equipment. Ensuring ultrasound image quality, however, requiresimplementation of a quality control program with periodic measurement of systemperformance to identify problems before serious malfunctions occur. Tissue-mimickingphantoms are required, having acoustic targets of various sizes and echogenicfeatures embedded in a medium with uniform attenuation and the speed of soundcharacteristic of soft tissues. Various multipurpose phantoms are available to evaluatethe clinical capabilities of the ultrasound system.A generic phantom composed of three modules is illustrated in Fig. 16-55. Thephantom gel filler has tissue-like attenuation of 0.5 to 0.7 (dB/cm)/MHz (higherattenuation provides a more challenging test) and low-contrast targets within a