Dielectric pads and low- B1+ adiabatic pulses: complementary techniques to optimize structural T1 w whole-brain MP2RAGE scans at 7 tesla.
OBJECTIVE
To evaluate the combination of low-B1 (+) adiabatic pulses and high permittivity (εr ≈ 165) dielectric pads effectiveness to reproducibly improve the inversion efficiency for whole-brain MP2RAGE scans, at ultra-high field.


METHODS
Two low-B1 (+) adiabatic pulses, HS8 and TR-FOCI, were compared with the conventional HS1 adiabatic pulse in MP2RAGE acquisitions of four subjects at 7 Tesla. The uniform MP2RAGE images were qualitatively assessed for poor inversion artifacts by trained observers. Each subject was rescanned using dielectric pads. Eight further subjects underwent two MP2RAGE scan sessions using dielectric pads and the TR-FOCI adiabatic pulse.


RESULTS
The HS8 and TR-FOCI pulses improved inversion coverage in all subjects compared with the HS1 pulse. However, in subjects whose head lengths are large (≥136 mm) relative to the coil's z-coverage, the B1 (+) field over the cerebellum was insufficient to cause inversion. Dielectric pads increase the B1 (+) field, by ∼50%, over the cerebellum, which in conjunction with the TR-FOCI pulse, reproducibly improves the inversion efficiency over the whole brain for subjects with head lengths ≤155 mm. Minor residual inversion artifacts were present in three of eight subjects (head lengths ≥155 mm).


CONCLUSIONS
The complementary techniques of low-B1 (+) adiabatic RF pulses and high permittivity dielectric pads allow whole-brain structural T1 w images to be reliably acquired at ultra-high field. J. Magn. Reson. Imaging 2014;40:804-812. © 2013 Wiley Periodicals, Inc.
CIBM_IRM HUG, Department of Radiology, University of Geneva, Geneva, Switzerland; Advanced Clinical Imaging Technology, Siemens Healthcare IM S AW, Renens, Switzerland; CIBM AIT, École polytechnique fédérale de Lausanne, Lausanne, Switzerland.
Kieran R
KR
O'Brien
Arthur W
AW
Magill
Jean
J
Delacoste
Jose P
JP
Marques
Tobias
T
Kober
Hans-Peter
HP
Fautz
Francois
F
Lazeyras
Gunnar
G
Krueger
D000328:Adult; D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
brain; dielectric pads; morphometry; structural imaging; ultra-high field