| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Polyclinique Atlantique Saint Herblain 44800 Nantes, France
Kurihara et al. [1] explain
that the new coil sensitivity-encoding (SENSE) MR imaging technique permits
the acquisition of more slices and the use of longer TR without reduction in
scanning time compared with the usual sequences used for body imaging. These
results occur in part because the reduced scanning time of the SENSE technique
comes at the expense of a decrease of signal-to-noise ratio (SNR). The exact
reduction in signal-to-noise ratio with the SENSE sequence is
 |
(red) is the SNR with the SENSE
sequence, SNR(full) is the SNR without the
use of SENSE, g corresponds to a geometric factor
almost always equal to 1, and R refers to the chosen reduction factor
[2]. Therefore, to obtain a
reduction by 2 in the time sequence with an R factor of 2, the
resultant SNR is decreased by (1 - 1/
2) = 29%. This is exactly the same
reduction of SNR one should obtain by dividing the number of excitations by 2.
Thus, SENSE allows a reduced scanning time at the expense of a decrease in the
SNR that is exactly the same as the reduced number of excitations. Moreover,
SENSE requires an additional reference scan to be obtained before the SENSE
sequence imaging to determine the sensitivity of each individual synergy coil
that lasts between 20 and 50 sec, depending on the anatomy. The use of synergy coils has a real side advantage, such as the use of CLEAR (Constant LEvel AppeaRance sequences) (Philips Medical Systems, Shelton, CT) that allow a homogenization correction of the signal; in any case, the method provides a substantial gain in time compared with preexisting simple methods, such as using a decreasing number of excitations (Fig. 2).
References
St. Marianna University Kawasaki 216-8511, Japan
Philips Medical Systems Corporation Tokyo 108-8507, Japan
We thank Dr. Krief for the remarks about our article [1] and for pointing out the importance of the relation between coil sensitivity encoding (SENSE) reduction factor and signal-to-noise ratio (SNR).
It is true that, in general, SENSE reduces scanning time at the expense of a decrease in SNR that is equivalent to reducing the number of excitations. Thus, if the purpose is only to reduce scanning time, decreasing the number of excitations could be a good choice. However, simply decreasing the number of excitations and thus reducing the amount of data acquired in k-space can lead to either a decrease in real spatial resolution or to a higher sensitivity to artifacts, depending on the reduction strategy. Also, when the number of excitations is just one, we cannot further reduce the scanning time without using SENSE.
We use many scanning protocols with only one excitation, especially for body imaging, MR angiography, and diffusion studies. A reduction in scanning time can minimize image quality deterioration caused by respiratory motion or bowel peristalsis and can sometimes enable breath-hold technique; the resulting improvement in image quality overcomes the decrease in SNR. In dynamic imaging and MR angiography, dramatically improved temporal resolution with the SENSE technique provides more information about flow dynamics. SENSE may be used to reduce the length of the echo train by decreasing the number of phase-encoding steps and to correct the heterogeneous sensitivity of each coil, thereby improving the image quality, as we mentioned in our article. Therefore, the SENSE technique is useful not only for saving time but also for improving image quality by decreasing motion and susceptibility artifacts and for correcting the heterogeneous sensitivity of the coils—quite different from just decreasing the number of excitations.
In practice, obtaining a reference scan before performing the SENSE sequence does not add time to the examination because the scan can be obtained while choosing the subsequent protocols and setting up the scanning plan. Also, the reference scan needs to be obtained only once for each patient.
References
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
