Chapter 9

### INTRODUCTION

Magnetic resonance imaging (MRI) is an imaging modality for venous diseases that is an alternative to more conventional techniques such as ultrasonography, impedance plethysmography, computed tomography (CT), and invasive selective contrast venography. The advantages of MRI include its noninvasive nature, lack of ionizing radiation and exposure to iodine-based contrast, high spatial resolution, ability to acquire unlimited imaging planes, and its unique ability to perform multicontrast tissue characterization. Despite these advantages, however, routine magnetic resonance venography (MRV) is not widely performed because it generally requires a radiologic team that is familiar with this specialized technique. Also, MRV lacks portability and sometimes incur a higher cost compared with other conventional techniques.

### BASIC MAGNETIC RESONANCE IMAGING TECHNIQUES

MRI is based on the exploitation of the magnetization properties inherent in hydrogen protons of tissues to acquire images. In the presence of a strong external magnetic field, more commonly a 1.5-Tesla or increasingly 3.0-Tesla magnet, the hydrogen protons in the body precess along the direction of the magnetic field. Brief external radiofrequency (RF) pulses with a magnitude dependent on a proton's gyromagnetic ratio (an intrinsic property of the proton) and the field strength are applied, causing a disruption of the alignment, magnetization of the hydrogen proton, and a net absorption of energy. This pulse is rapidly turned off, and the protons return to their original alignment with the magnetic field. This shift from a higher energy to a lower energy state creates RF waves that are detected by receiver coils adjacent to the body and are converted into electrical signals that form the basis of an MRI image. Spatial localization of the signal is accomplished through gradient coils that produce subtle regional differences in resonant frequency in multiple directions.

MRV can be divided into two major groups: noncontrast MRV and contrast-enhanced MRV based on the use of gadolinium-based contrast agents. Noncontrast MRV, in turn, is performed most commonly using time-of-flight (TOF) or phase contrast (PC) techniques.1

#### Time-of-Flight Magnetic Resonance Venography

This is a type of MRI that relies on gradient echo or "bright blood" imaging. When subjected to a long series of RF pulses with a short echo time, flowing blood shows signal enhancement because motion allows these moving protons to experience only a restricted number of RF excitations before moving out of the imaged volume, and the continuous refreshment or inflow of spins into the imaging volume ensures a maximal alignment of protons with the external magnetic field before the RF pulses.2 In contrast, stationary tissues have partially relaxed signals that lead to lower signal intensity. Even without the use of contrast, the TOF images show hyperintense signals in the vessels, and the technique is robust as long as the protons in the blood experience only a few RF pulses before being refreshed. To differentiate venous from arterial signal, a presaturation band that nulls inflowing arterial blood ...

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