Biomedical Engineering Reference
In-Depth Information
￿ Explain the Larmor frequency and nuclear
magnetic resonance.
￿ Explain how flip angles affect recovery and
relaxation time constants.
￿ Distinguish between free induction decay
and spin echo signals.
￿ Explain how detected resonance signals are
spatially localized.
￿ List the steps involved in creating a magnetic
resonance image.
￿ Explain what is displayed in a magnetic
resonance image.
￿ Draw and explain the block diagram of an
MRI system.
￿ Explain the significance of k-space in MRI.
￿ Discuss applications of MRI including
fMRI.
￿ Discuss the principles of magneto-
encephalography.
￿ Compare contrast agents for different
imaging modalities.
￿ Explain image fusion.
￿ Compare major imaging modalities.
16.1 INTRODUCTION
Of the major diagnostic imaging modalities, ultrasound is the most frequently used, sec-
ond only to standard plane-view x-rays. Over the years, the cumulative number of ultra-
sound exams completed is estimated to be in the billions. Unlike x-rays and computed
tomography (CT) scanning, ultrasound imaging involves no ionizing radiation and there-
fore is considered to be noninvasive. Furthermore, it is portable, easy to apply, low in cost,
and provides real-time diagnostic information about the mechanical nature and motion of
soft tissue and blood flow. The basic principle of ultrasound imaging is the display of
pulse-echoes backscattered from tissues.
Magnetic resonance imaging (MRI) also obtains detailed anatomic information without
using ionizing radiation. MRI differentiates among types of organs by sensing the spin of
their atoms when a person is placed in a large, static, magnetic field. Static cross-sectional
images of the body include both bone and soft tissue, and the appearance of the images
can vary considerably by the selection of specific parameters. Of the major imaging modal-
ities, MRI is the most abstract and complicated technically. In addition to its precise ana-
tomical capability, it is often used for presurgery planning and for cancer detection.
Functional MRI (fMRI) provides images of brain activity in response to various stimuli.
In this chapter, the operation and principles of both ultrasound imaging and MRI will be
explained. At the end of the chapter, the main features of all major imaging modalities, including
CT and x-ray, will be compared. An understanding of the basic principles of the Fourier trans-
form, introduced inChapter 10, will enhance comprehension of the imaging concepts introduced
in this chapter. More information about the Fourier transform is provided in the next section.
16.1.1 Review of Fourier Transforms
Because Fourier transforms simplify the understanding of the imaging principles of
ultrasound imaging and MRI, their relevant properties are reviewed here. One important
Fourier transform concept from Chapter 6 is the equivalence of convolution in the time
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