Modern MRI systems are extremely flexible so that optimal usage requires a solid understanding of the physics of the basic processes involved. In the end, determination of the “best” protocol to address a clinical problem requires careful consideration of trade‐offs between image contrast, signal‐to‐ noise ratio (SNR), spatial resolution, temporal resolution and patient safety. The major factors in considering MR image contrast are the relaxation times: T1 & T2, magnetization transfer mechanisms, physiological motion and the radio frequency (rf) pulsing scheme. The most important issues for SNR are field strength, rf coil properties, voxel size, and receiver bandwidth. Factors pertinent to spatial resolution include magnetic field homogeneity, gradient strength, matrix size, slice thickness and field of view. Imaging speed is determined by gradient slew rate, signal digitization rate, number of slices, parallel imaging mode & signal‐to‐ noise ratio. Patient safety issues are related to patient size, field strength, rf pulsing method and gradient slew rate. In this presentation the basic relationships amongst all of these parameters in MRI will be reviewed and their interactions discussed. For instance, the long TR values required for strongly T2‐weighted images is an example of how contrast requirements can limit imaging speed. High‐speed T2‐ weighted images can be obtained using Fast Spin Echo strategies, but only by compromising spatial resolution and the number of slices while increasing rf heating. Specialized clinical imaging protocols that push the envelope in several of these general areas will also be examined. The use of three‐dimensional MR angiography to maximize contrast while preserving good SNR at high imaging speeds will be explored. The conditions under which certain signals from particular types of tissues, such as adipose or cerebrospinal fluid, can be entirely eliminated will be surveyed. Roles for the use of exogenous agents to alter tissue contrast shall also be reviewed. Trade‐offs required to image the heart's motion in real time will be appraised. The implications of rf heating, as measured by the specific absorption rate, will also be considered. At the end of this presentation the attendee shall: 1. understand the primary types of image contrast used in clinical MRI and the relationships between pulse timing parameters and relaxation times of tissue on the acquired MRI signal. 2. know how to consider the interactions between SNR, spatial resolution and imaging speed and their relative weighting in defining image quality for a variety of clinical situations. 3. be conversant with circumstances under which patient physiology and magnet field strength may require modification of image acquisition parameters to achieve optimal image quality.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging