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Ultrasound

Introduction

Ultrasound has become an almost ubiquitous imaging tool.  It is used for anatomical imaging, quantitation of flow (especially in cardiology), imaging in the emergency room with small handheld units, in surgery, and in many areas of the clinic and hospital, without and with contrast media.  It is capable of two-, three-, and even four-dimensional imaging using sophisticated transducer arrays.  It is also widely used in veterinary medicine.  However, it receives minimal attention in terms of medical physics support and quality control since it uses non-ionizing radiation.


Important Principles

The medical physicist must be familiar with ultrasound imaging and all of the factors affecting image quality, including the myriad adjustments made by the ultrasound technologist during the examination and how these impact the final image.  The bioeffects and safety aspects of ultrasound must also be understood.

Artefacts are much more prevalent in ultrasound imaging than in other areas of medical imaging.  The medical physicist must be able to investigate and identify causes of artefacts and to undertake routine quality control tests of ultrasound imaging systems, including systems used for recording of the images either with film or digitally.

The primary sophistication in ultrasound imaging is associated with the transducers and imaging techniques.  These range from single element transducers with continuous or pulsed operation, one-dimensional arrays, multi-frequency operation, apodization and dynamic aperture techniques, harmonic imaging, Doppler measurements, spectral analysis, and contrast agents.

It is important to be able to measure acoustic output, slice thickness, resolution, system sensitivity, image uniformity, depth penetration, distance accuracy, low contrast detectability, ring-down, and flow accuracy.


Introduction to References

There are several books available on the basics of ultrasound physics including Kremkau and Hedrick.  The text by Bushberg, et al., is a good resource for the physics and imaging.  The NCRP and AAPM reports in the Supplemental References provide excellent details on specific topics and are highly recommended.