title: Slice profile effects in 2D slice-selective MRI of hyperpolarized nuclei.
abstract: This work explores slice profile effects in 2D slice-selective gradient-echo MRI of hyperpolarized nuclei. Two different sequences were investigated: a Spoiled Gradient Echo sequence with variable flip angle (SPGR-VFA) and a balanced Steady-State Free Precession (SSFP) sequence. It is shown that in SPGR-VFA the distribution of flip angles across the slice present in any realistically shaped radiofrequency (RF) pulse leads to large excess signal from the slice edges in later RF views, which results in an undesired non-constant total transverse magnetization, potentially exceeding the initial value by almost 300% for the last RF pulse. A method to reduce this unwanted effect is demonstrated, based on dynamic scaling of the slice selection gradient. SSFP sequences with small to moderate flip angles (<40 degrees ) are also shown to preserve the slice profile better than the most commonly used SPGR sequence with constant flip angle (SPGR-CFA). For higher flip angles, the slice profile in SSFP evolves in a manner similar to SPGR-CFA, with depletion of polarization in the center of the slice.
authors.affiliation: Academic Radiology, University of Sheffield, C Floor Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF, United Kingdom.
authors.forename: Martin H
authors.initials: MH
authors.lastname: Deppe
authors.forename: Kevin
authors.initials: K
authors.lastname: Teh
authors.forename: Juan
authors.initials: J
authors.lastname: Parra-Robles
authors.forename: Kuan J
authors.initials: KJ
authors.lastname: Lee
authors.forename: Jim M
authors.initials: JM
authors.lastname: Wild
mesh_terms: D000465:Algorithms; D002138:Calibration; D003627:Data Interpretation, Statistical; D004574:Electromagnetic Fields; D006371:Helium; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods* / Q000706:statistics & numerical data; D015233:Models, Statistical; D019047:Phantoms, Imaging; D011846:Radio Waves; D012815:Signal Processing, Computer-Assisted
chemical_list: D006371:Helium