Design and optimization for variable rate selective excitation using an analytic RF scaling function Neville D. Gai a, * , Yuval Zur b a GE Healthcare, Waukesha, WI, USA b GE Healthcare, Tirat-Carmel, Israel Received 1 February 2007; revised 16 August 2007 Available online 29 August 2007 Abstract At higher B 0 fields, specific absorption rate (SAR) deposition increases. Due to maximum SAR limitation, slice coverage decreases and/or scan time increases. Conventional selective RF pulses are played out in conjunction with a time independent field gradient. Var- iable rate selective excitation (VERSE) is a technique that modifies the original RF and gradient waveforms such that slice profile is unchanged. The drawback is that the slice profile for off-resonance spins is distorted. A new VERSE algorithm based on modeling the scaled waveforms as a Fermi function is introduced. It ensures that system related constraints of maximum gradient amplitude and slew rate are not exceeded. The algorithm can be used to preserve the original RF pulse duration while minimizing SAR and peak b1 or to minimize the RF pulse duration. The design is general and can be applied to any symmetrical or asymmetrical RF waveform. The algorithm is demonstrated by using it to (a) minimize the SAR of a linear phase RF pulse, (b) minimize SAR of a hyperbolic secant RF pulse, and (c) minimize the duration of a linear phase RF pulse. Images with a T1-FLAIR (T1 FLuid Attenuated Inversion Recovery) sequence using a conventional and VERSE adiabatic inversion RF pulse are presented. Comparison of images and scan parameters for different anatomies and coils shows increased scan coverage and decreased SAR with the VERSE inversion RF pulse, while image quality is preserved. Ó 2007 Elsevier Inc. All rights reserved. Keywords: SAR reduction; Analytic RF scaling function; Constant pulse width; Constant contrast 1. Introduction RF power deposition per unit weight (Specific Absorp- tion Rate or SAR) scales roughly as the square of the B 0 field. Therefore, high SAR can severely limit scan time and slice coverage in body imaging applications at higher field strengths. Various techniques have been proposed to reduce SAR. These include lowering the flip angle (from 180°) in a refo- cusing train (TSE) [1] and stretching the RF pulses and modulating the refocusing train [2–4]. Each of these tech- niques has drawbacks. Modulation of refocusing RF pulses in a train as described in [2] and [4] is useful only for TSE sequences with long echo trains and cannot be used for steady-state sequences (SSFP) and other SAR sensitive sequences. Lowering the flip angle results in relatively lower signal-to-noise ratio while stretching RF pulses can increase echo spacing resulting in blurring and unwanted increase in T 2 effects. In addition, decreased bandwidths result in increased sensitivity to off-resonance spins. A method to reduce SAR for any given RF pulse, termed ‘‘variable rate selective excitation’’ or VERSE, was originally proposed by Conolly et al. [5]. The VERSE technique is based on the idea of RF scaling [5], which will be described in more detail in the next section. When RF scaling is employed, the RF and gradient waveforms of a given RF pulse are modified, such that the excited slice pro- file is unchanged. The original RF pulse is referred to as the 1090-7807/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2007.08.017 * Corresponding author. Present address: DRD/CC/1N242, 10 Center Drive, National Institutes of Health, Bethesda, MD 20892, USA. Fax: +1 301 496 9933. E-mail addresses: gaind@mail.nih.gov, nevgai@yahoo.com (N.D. Gai). www.elsevier.com/locate/jmr Available online at www.sciencedirect.com Journal of Magnetic Resonance 189 (2007) 78–89