Introduction

We have seen that MRI is a digital, truly three-dimensional imaging modality of great flexibility with respect to image contrast and its spatial characteristics. However, one of the downsides of this flexibility is a greater complexity in terms of the choice of scanning parameters. This aside, does MR have any other weaknesses? Yes it does: in general scan times are not negligible and there is a certain tendency towards artefact (which we will investigate in the next chapter). However, most MR people would probably agree that the fundamental limitation in MRI is the signal-to-noise ratio (SNR). SNR is dependent upon the hardware, particularly the main field strength and radiofrequency (RF) coils, upon the relaxation properties of tissue and upon the choice of sequence parameters. SNR can be increased by injecting gadolinium or other paramagnetic contrast agents. But unlike X-ray imaging, there is no X-ray dose or milliampere-seconds (mAs) that can be increased to improve image quality. Good image quality depends upon making good scanning parameter choices. There are many parameters to tweak, buttons to press and dialogue boxes to click, and one way to learn about how each affects image quality would be to spend a lifetime tweaking! Alternatively you could read this chapter which will investigate the influence of various acquisition parameters and the practical trade-offs between SNR, contrast-to-noise, spatial resolution and scan time.