NEX in MRI stands for Number of Excitations, and it's a crucial parameter affecting the signal-to-noise ratio (SNR) in magnetic resonance imaging. It's also sometimes referred to as NSA (Number of Signal Averages) or NAQ (Number of Acquisitions); these terms are often used interchangeably.
Understanding NEX
NEX essentially determines how many times each line of k-space data is acquired during an MRI scan. K-space is a temporary storage space for the raw data collected during the scan, which is then processed to create the final image.
Impact on Signal-to-Noise Ratio (SNR)
The primary reason to adjust NEX is to influence the SNR. Here's how it works:
- Increasing NEX: Averaging more signals (increasing NEX) effectively reduces random noise. Because the signal adds constructively with each excitation, while noise averages out, the SNR improves. The SNR improves proportionally to the square root of NEX. For instance, doubling NEX increases SNR by approximately 41% (√2 ≈ 1.41).
- Decreasing NEX: Reducing NEX speeds up the scan time. However, it also decreases the SNR, potentially making the image noisier and less clear.
Trade-Offs: Time vs. Image Quality
The use of NEX involves a fundamental trade-off:
- Higher NEX = Better Image Quality (Higher SNR) = Longer Scan Time
- Lower NEX = Lower Image Quality (Lower SNR) = Shorter Scan Time
Therefore, the appropriate NEX value is usually a balance between acquiring images with acceptable quality and keeping the scan time reasonably short.
Examples of NEX Usage
- High-Resolution Imaging: When detailed anatomical information is critical (e.g., imaging small structures or detecting subtle lesions), a higher NEX is often used to maximize SNR.
- Fast Imaging: In situations where speed is paramount (e.g., dynamic contrast-enhanced imaging or imaging of uncooperative patients), a lower NEX might be chosen, accepting a potential decrease in image quality.
Factors Influencing NEX Selection
The ideal NEX depends on several factors:
- Coil Type: Phased array coils with multiple elements usually have higher SNR compared to standard coils, potentially allowing for lower NEX values.
- Sequence Type: Different MRI sequences inherently have different SNR characteristics. For example, fast spin-echo sequences often have higher SNR than gradient-echo sequences.
- Anatomical Region: Regions with inherently low signal (e.g., areas with little fat or fluid) might require higher NEX.
- Clinical Indication: The specific clinical question being addressed influences the necessary image quality and, consequently, the NEX.
Conclusion
NEX (or NSA/NAQ) is a key MRI parameter controlling the number of times each line of k-space is acquired. Increasing NEX improves SNR but increases scan time, necessitating a careful balance to achieve optimal image quality within acceptable scan duration.
