Introduction
This section refers mainly to the Technical issues subheading discussed under the Image 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 4 of MRI in Practice or an equivalent text.
The main considerations of image quality are:
- SNR
- contrast to noise ratio (CNR)
- spatial resolution
- scan time
Each factor is controlled by certain parameters, and each ‘trades off’ against the other (see later in Table 2.2). This section summarizes the parameters available and the trade-offs involved. Suggested parameters are outlined in Table 2.1, which can be found here and at the beginning of each anatomical region in Part 2. The parameters given should be universally acceptable on most systems. However, weighting parameters in particular are field strength dependent, and therefore, some modification may be required if you are operating at extremely low or high field strengths.
Table 2.1 Summary of parameters
| 1.5 T | 3 T | ||
| SE | SE | ||
| Short TE | Min–30 ms | Short TE | Min–15 ms |
| Long TE | 70 ms+ | Long TE | 70 ms+ |
| Short TR | 600–800 ms | Short TR | 600–900 ms |
| Long TR | 2000 ms+ | Long TR | 2000 ms+ |
| FSE | FSE | ||
| Short TE | Min–20 ms | Short TE | Min–15 ms |
| Long TE | 90 ms+ | Long TE | 90 ms+ |
| Short TR | 400–600 ms | Short TR | 600–900 ms |
| Long TR | 4000 ms+ | Long TR | 4000 ms+ |
| Short TEL | 2–6 | Short TEL | 2–6 |
| Long ETL | 16+ | Long ETL | 16+ |
| IR T1 | IR T1 | ||
| Short TE | Min–20 ms | Short TE | Min–20 ms |
| Long TR | 3000 ms+ | Long TR | 300 ms+ |
| TI | 200–600 ms | TI | Short or null time of tissue |
| Short ETL | 2–6 | Short ETL | 2–6 |
| STIR | STIR | ||
| Long TE | 60 ms+ | Long TE | 60 ms+ |
| Long TR | 3000 ms+ | Long TR | 3000 ms+ |
| Short TI | 100–175 ms | Short TI | 210 ms |
| Long ETL | 16+ | Long ETL | 16+ |
| FLAIR | FLAIR | ||
| Long TE | 80 ms+ | Long TE | 80 ms+ |
| Long TR | 9000 ms+ | Long TR | 9000 ms + (TR at least 4 × TI) |
| Long TI | 1700–2500 ms (depending on TR) | Long TI | 1700–2500 ms (depending on TR) |
| Long ETL | 16+ | Long ETL | 16+ |
| Coherent GRE | Coherent GRE | ||
| Long TE | 15 ms+ | Long TE | 15 ms+ |
| Short TR | <50 ms | Short TR | <50 ms |
| Flip angle | 20–50° | Flip angle | 20–50° |
| Incoherent GRE | Incoherent GRE | ||
| Short TE | Minimum | Short TE | Minimum |
| Short TR | <50 ms | Short TR | <50 ms |
| Flip angle | 20–50° | Flip angle | 20–50° |
| Balanced GRE | Balanced GRE | ||
| TE | Minimum | TE | Minimum |
| TR | Minimum | TR | Minimum |
| Flip angle | >40° | Flip angle | >40° |
| SSFP | SSFP | ||
| TE | 10–15 ms | TE | 10–15 ms |
| TR | <50 ms | TR | <50 ms |
| Flip angle | 20–40° | Flip angle | 20–40° |
| Slice thickness 2D | Slice thickness 3D | ||
| Thin | 2–4 mm | Thin | <1 mm |
| Medium | 5–6 mm | Thick | >3 mm |
| Thick | 8 mm | ||
| FOV | Matrix | ||
| Small | <18 cm | Coarse | 256 × 128 / 256 × 192 |
| Medium | 18–30 cm | Medium | 256 × 256 / 512 × 256 |
| Large | >30 cm | Fine | 512 × 512 |
| Very fine | >1024 × 1024 | ||
| NEX/NSA | Slice number 3D | ||
| Short | 1 | Small | <32 |
| Medium | 2–3 | Medium | 64 |
| Multiple | >4 | Large | >128 |
| PC-MRA 2D and 3D | TOF-MRA 2D | ||
| TE | Minimum | TE | Minimum |
| TR | 25–33 ms | TR | 28–45 ms |
| Flip angle | 30° | Flip angle | 40–60° |
| VENC venous | 20–40 cm/s | TOF-MRA 3D | |
| VENC arterial | 20–40 cm/s60 cm/s | TE | Minimum |
| TR | 25–50 ms | ||
| Flip angle | 20–30° | ||
The figures given are for 1.5 T and 3 T systems. Parameters are dependent on field strength and may need adjustment for very low or very high field systems.
Table 2.2 Parameters and their trade-offs
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| Parameter | Advantages | Disadvantages |
| TR increased (up to 2000 ms in SE) | Increased SNRIncreased number of slices per acquisition | Increased scan timeDecreased T1 weighting |
| TR decreased (below 2000 ms in SE) | Decreased scan timeIncreased T1 weighting | Decreased SNRDecreased number of slices per acquisition |
| TE increased | Increased T2 weighting | Decreased SNR |