The straight and narrow path to good head and spine MRI.
The path to good head and spine images is narrow and treacherous. We have attempted to give the traveller a small but important set of basic rules, enabling him to cross with success. 1. Averaging can be used to achieve sufficient SNR for thin sections, but the cost in terms of scan time is high. Zooming the image (reducing the field of view) should generally be avoided, as the price in terms of SNR is very high. 2. Rectangular pixels and half-Fourier imaging are two methods for decreasing scan time. HFI, which produces high spatial resolution images, can be used when the SNR is not a limiting factor. Rectangular pixels improve the SNR, but decrease resolution. 3. To achieve good T1 contrast with spin echo imaging, set TE less than or equal to 20 msec. and TR less than or equal to 600 msec. For T2 weighted images, a TR between 2.0 and 3.0 sec. is preferred, typically with two echoes: for example, TEs of 25 and 90 msec. 4. Better slice profiles or gaps between slices can be used to combat slice-to-slice interference. This results in improved SNR on T1 weighted images and improved contrast on T2 weighted images. 5. Low bandwidth techniques may be used to improve the SNR on both T1 and T2 weighted images. Chemical shift artifact puts a finite limit on the extent to which this can be applied. 6. Motion compensating gradients are a tremendous boon to MRI and should be utilized in all possible head and spine applications. These reduce image degradation from CSF and vessel pulsation, as well as from involuntary motion. 7. Fast imaging techniques can be used in 2-D multislice mode to decrease scan time. Unfortunately the T2 contrast with this approach is far inferior to that of spin echo technique. 3-D FLASH, with 1 mm. sections, T1 contrast superior to spin echo technique, and the potential for high resolution reformatted images, may replace conventional 2-D, T1 weighted, spin echo imaging. Pulse techniques that combine all the advantages mentioned lie in the future. For example, one possible approach is a T2 weighted head screen that incorporates low bandwidth technique and HFI. This would produce high resolution images with reasonable SNR in approximately half the present scan time. Despite any further new developments, the trade-off between image quality and scan time will likely always remain.(ABSTRACT TRUNCATED AT 400 WORDS)
Runge
V M
VM
Department of Radiology, Tufs University, New England Medical Center Hospitals, Boston, Massachusetts 02111.
Wood
M L
ML
Kaufman
D M
DM
Traill
M R
MR
Nelson
K L
KL
D006257:Head / Q000033:anatomy & histology*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D013131:Spine / Q000033:anatomy & histology*