Catherine Westbrook - Handbook of MRI Technique

To use this book:

Find the required anatomical region and then locate the specific examination.

Study the categories under each section. It is possible that all the categories are relevant if the examination is being performed for the first time. However, there may be occasions when only one item is appropriate. For example, there could be a specific artefact that is regularly observed in chest examinations, or image quality is not up to standard in lumbar spine protocols. Under these circumstances, read the subsection entitled Protocol optimization.

If the terms used, or concepts discussed in Part 2are unfamiliar, then turn to Part 1and read the summaries described there.

Introduction

Signal-to-noise ratio (SNR)

Contrast-to-noise ratio (CNR)

Spatial resolution

Scan time

Decision strategies

Conclusion

This section refers mainly to the Technical issues subheading discussed under the Protocol optimization heading considered for each examination in Part 2. Only a brief overview is provided here. For a more detailed explanation, please refer to Chapter 7of the fifth edition of MRI in Practice or an equivalent text.

The main considerations of protocol optimization are:

signal‐to‐noise ratio (SNR)

contrast‐to‐noise ratio (CNR)

spatial resolution

scan time.

Each factor is controlled by certain protocol parameters, and each ‘trades off’ against the other. This section summarizes these parameters and their trade‐offs. Suggested protocol parameters are outlined in Table 2.1and should be universally acceptable on most systems. However, weighting parameters are field strength dependent, and therefore some modification may be required for extremely low‐ or high‐field systems.

Table 2.1 Summary of parameters.

Source: Catherine Westbrook and John Talbot, MRI in Practice, 5th ed., John Wiley & Sons, 2019.

SNR is defined as the ratio of the amplitude of signal received by the receiver coil to the average amplitude of the noise. The signal is the voltage induced in the receiver coil, and the noise is a constant value depending on the area under examination and the background electrical noise of the system. SNR may be increased by using: