SIGNAL‐TO‐NOISE RATIO (SNR)
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:
conventional spin echo (CSE) and fast or turbo spin echo (FSE/TSE) pulse sequences
a long repetition time (TR) and a short echo time (TE)
a flip angle of 90° in all spin echo type pulse sequences or the Ernst angle in gradient echo (GRE) pulse sequences
a well‐tuned and correctly sized receiver coil
a coarse matrix
a large FOV
thick slices
a narrow receiver bandwidth
high‐order signal averages (number of excitations (NEX)/number of signal averages (NSA)).
In Part 2, the following terms and approximate parameters are suggested when discussing the number of signal averages (NEX/NSA) (see also Table 2.1):
Short NEX/NSA is 1 or less (partial averaging).
Medium NEX/NSA is 2 or 3.
Long or multiple NEX/NSA is 4 or more.
CONTRAST‐TO‐NOISE RATIO (CNR)
The CNR is defined as the difference in SNR between two adjacent areas. It is controlled by the same factors that affect SNR. All examinations should include images that demonstrate a good CNR between pathology and surrounding normal anatomy so that pathology is well visualized. The CNR between pathology and other structures can be increased by the following:
administration of contrast agents
utilization of T2‐weighted pulse sequences
suppression of normal tissues or pulse sequences that null signal from certain tissues e.g., short TI inversion recovery (STIR), fluid alternated inversion recovery (FLAIR) and magnetization‐prepared pulse sequences
use of pulse sequences that enhance flow (see Pulse sequences).
A note on fat suppression techniques
The CNR can be improved by suppressing signal from tissues that are not important, thereby increasing the visualization of tissues that are. Fat is the most common tissue that is nulled or suppressed in MRI and this is assumed for the majority of protocols described in Part 2, where all of the techniques described below are referred to as fat suppression.
Fat suppression is most commonly used to distinguish between fat and enhancing pathology in T1‐weighted pulse sequences and in a FSE/TSE T2‐weighted pulse sequence where fat and pathology are often isointense. However, signal from any tissue can be suppressed using the inversion recovery (IR) pulse sequence and some saturation techniques are used to null signal from water or background tissue. Further details on suppression of tissues other than fat are provided where relevant in Part 2.
There are several ways in which fat and other tissues are suppressed, including:
Chemical pre‐saturation: a 90° RF pulse is delivered at the specific precessional frequency of the magnetic moments of spins in either fat or water. This pulse is delivered to spins inside the imaging volume before the RF excitation pulse is applied, saturating them. No signal is therefore received from either fat or water when the echo is read.
Spectral pre‐saturation: an RF pulse of a greater magnitude than 90° is applied and inverts the magnetization in a tissue as in inversion recovery (IR) pulse sequences (see Pulse sequences). At the time from inversion (TI) that corresponds to the null point of the tissue, a 90° RF excitation pulse is applied. No signal is therefore received from that tissue when the echo is read.
Dixon technique (either 2‐point or 3‐point): a reconstructed image is obtained from only the spin population in water. This ‘water‐only’ image has no contribution from spins in fat. Images look similar to the pre‐saturation techniques described above but rely on the chemical shift between fat and water (the difference in the precessional frequencies of the magnetic moments of the spin population in fat and water). Images are acquired
