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Diffusion MRI

13 December, 2010 | MRI

Diffusion MRIDiffusion MRI, the commonly used magnetic resonance imaging method,  produces in vivo images of biological tissues weighted with local microstructural characteristics of water diffusion. In diffusion-weighted imaging, each image voxel has an image intensity that reflects a single best measurement of the rate of water diffusion at that location. This measurement is more sensitive to early changes after a stroke than other MRI measurement methods, such as : T1 or T2 relaxation rates.

High signal to noise ratio is very important in diffusion MRI imaging (DWI). This is typically attained by implementing stronger gradient performance to reduce the time required for diffusion- sensitization and signal acquisition, thereby shortening the minimum attainable time-to-echo (TE) of the DWI sequence. Parallel imaging can also be used to further reduce the time required for signal acquisition.

Higher gradient performance and parallel imaging provide additional benefits to image quality, by reducing distortion and susceptibility artefacts.

DWI applications benefit from higher gradient performance, as DWI sequences tend to demand the full potential of gradient performance. However, gradient systems are only likely to improve marginally in the future compared to the high- specification MRI systems currently available, because of safety restrictions based upon physiological stimulation limits.

Overall image quality will additionally benefit from any measure to actively reduce the effect of eddy current-induced spatial distortions, such as: dedicated diffusion encoding schemes or optimized gradient coil design. In general, actions taken on the acquisition side are preferable, as compared to post-processing techniques.

A different approach to increasing the SNR of the DW image would be to perform DWI studies on an MRI system with a higher magnetic field. For example, the SNR is expected to increase twofold from 1.5 to a 3T system. DWI studies on 3T systems can therefore benefit from a more extensive image contrast range, than equivalent studies on 1.5T systems. However, with the increase of the magnetic field, image quality becomes more sensitive to susceptibility and distortion artefacts.

Therefore, higher gradient specifications and parallel imaging are more applicable and provide significant improvements to image quality in DWI studies on 3T  RI systems.

Diffusion MRI imaging is susceptible to signal misregistration from fat tissues. In order to minimize the fat signal, special fat-suppression techniques are used, whose effectiveness is likely to be dependent on main magnetic field homogeneity. DWI can be more sensitive to hardware defects than standard imaging applications. Degradation of image quality in DWI is typically associated with problems in the performance of gradient or shimming coils. DWI should, if possible, be included in a quality- control program, especially in cases of long- term, comparative or quantitative studies.

Diffusion MRI imaging sequences last only a few seconds and can therefore be used as an adjunct to conventional MRI studies without significant increase in scan duration. DWI sequences are also simple to implement as the b-value is the main determinant of image contrast. In addition, there is no need for contrast agent injection or physiological monitoring. However, advanced DWI analysis, for example: calculation of DTI, can benefit from electrocardiogram gating. Although the risks in this mode are minimized and scanning with DWI is extremely short, there is a small possibility of experiencing minor peripheral nerve stimulation. In addition, acoustic noise levels during DWI sequences tend to be higher than conventional MRI techniques and hearing protection should be provided to minimize patient discomfort and prevent temporary hearing loss.

Recommended  Main Specifications

For MRI 1.5 T
Gradient maximum amplitude                                                     ≥ 33mT/m
Gradient slew rate                                                                   ≥ 100mT/m/s
Number of channels                                                                  16 (18) – 32
Minimum TE                                                                                   ≤ 90ms
Minimum TE with parallel imaging                                                 ≤ 65ms
For MRI 3T
Gradient maximum amplitude                                                    ≥ 45 mT/m
Gradient slew rate                                                                   ≥ 150mT/m/s
Number of channels                                                                     16(18)- 32
Minimum TE                                                                                    ≤ 75ms
Minimum TE with parallel imaging                                                 ≤60ms