selection. A coarse matrix is required to obtain large voxels and therefore a high SNR. However, a fine matrix with small voxels and low SNR is not only necessary to maintain good spatial resolution, but also increases the scan time as more phase encodings are performed. The MRI practitioner must decide which factor (either SNR, phase resolution or scan time) is the most important, optimize this and sacrifice the others. When discussing these issues in Part 2, the importance of good SNR over the other factors is emphasized. There is little point in having an image with good spatial resolution if the SNR is so poor that the image has no diagnostic value.
The selection of an appropriately sized and tuned coil is also important, together with the proton density of the area under examination. For example, when examining the chest, which has a low SNR, protocol parameters must optimize the SNR as much as possible and spatial resolution and scan time are sacrificed. The importance of limiting the scan time for patient toleration is also discussed in Part 2. If the scan time is lengthy, all patients eventually become uncomfortable and move. The resultant motion artefact degrades any image regardless of its SNR or spatial resolution characteristics. Therefore, it is important to minimize scan times to acceptable levels. If patients are in pain or uncooperative, this strategy is even more important.
CONCLUSION
The variety of protocol parameters used in MRI is often bewildering, but their importance is undisputed, especially in determining image quality. A good working knowledge of these parameters and how they interrelate is necessary to ensure an optimum examination. Table 2.2 summarizes these trade‐offs. The choice of pulse sequence is also important in determining image contrast and these are discussed in the next section.
Table 2.2 Parameters and their trade‐offs.
Source: Catherine Westbrook and John Talbot, MRI in Practice, 5th ed., John Wiley & Sons, 2019.
| To optimize image | Adjusted parameter | Consequence |
|---|---|---|
| Maximize SNR | ↑NSA | ↑Scan time |
| ↓Image matrix (fixed FOV) | ↓Scan time (pMatrix) | |
| — | ↓Resolution | |
| ↑Slice thickness | ↓Resolution | |
| ↓Receive bandwidh | ↑Minimum TE | |
| — | ↑Chemical shift | |
| ↑FOV (fixed matrix) | ↓Resolution | |
| ↑TR | ↓T1 contrast (during incomplete recovery) | |
| — | ↑Number of slices | |
| ↓TE | ↓T2 contrast | |
| Maximize resolution (assuming a square FOV) | ↓Slice thickness | ↓SNR |
| ↑Image matrix (fixed FOV) | ↓SNR | |
| — | ↑Scan time (pMatrix) | |
| ↓FOV (fixed matrix) | ↓SNR | |
| Minimize scan time | ↓TR | ↑T1 contrast (during incomplete recovery) |
| — | ↓SNR (until complete recovery) | |
| — | ↓Number of slices | |
| ↓Phase matrix (fixed FOV) | ↓Resolution | |
| — | ↑SNR | |
| ↓NSA | ↓SNR | |
| ↑Movement artifact | ||
| ↓Slice number in volume imaging | ↓SNR |
Конец ознакомительного фрагмента.
Текст предоставлен ООО «ЛитРес».
Прочитайте эту книгу целиком, купив полную легальную версию на ЛитРес.
Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.
