9.1 – Introduction

Radiology encompasses a wide variety of modalities, anatomic regions, and clinical conditions.  The major imaging modalities include:

1. 1. X-rays (radiographs)
   2. Ultrasound (with Doppler)
   3. Mammography
   4. Fluoroscopy
   5. Angiography
   6. Computed Tomography (CT)
   7. Magnetic Resonance Imaging (MRI)
   8. Nuclear Medicine (PET/CT)

However, in this chapter we will only be discussing the first two modalities: X-rays and Ultrasound. X-rays utilize x-rays generated via an x-ray tube. The mechanical and technical features of the first five imaging modalities are very different but they all use ionizing radiation administered to the patient from a standardized distance and rely upon the differential absorption of the incident x-rays to create an image. In this chapter, the physical principles of X-rays are introduced. We start with a general definition of X-rays and introduce the c-arm x-ray machine.

Acoustic waves with frequencies (ξ) between 16 Hz and 20 kHz can be sensed by the human hearing and are thus called audible waves or audible sound. If ξ > 20 kHz, one speaks of ultrasound. Some animal species such as bats can perceive ultrasound and use it for echolocation: by measuring the time between sending and receiving (after partial reflection on a surface) ultrasonic waves, the distance of an object (e. g., a wall or prey) to the sender (bat) can be computed accurately, assuming that the sound velocity is known. Today, medical ultrasound often is the first-resort clinical imaging modality due to its cost-effectiveness and lack of ionizing radiation. Typical medical ultrasound frequencies are between 2 MHz < ξ < 40 Mhz. Traditionally, medical ultrasound is mainly put to use in diagnostic applications, however, more therapeutic applications are emerging.