Balanced Steady-State Free Precession (bSSFP) from an effective field perspective: Application to the detection of chemical exchange (bSSFPX)

Abstract

Chemical exchange saturation transfer (CEST) is a novel contrast mechanism and it is gaining increasing popularity as many promising applications have been proposed and investigated. Fast and quantitative CEST imaging techniques are further needed in order to increase the applicability of CEST for clinical use as well as to derive quantitative physiological and biological information. Steady-state methods for fast CEST imaging have been reported recently. Here, we observe that an extreme case of these methods is a balanced steady-state free precession (bSSFP) sequence. The bSSFP in itself is sensitive to the exchange processes; hence, no additional saturation or preparation is needed for CEST-like data acquisition. The bSSFP experiment can be regarded as observation during saturation, without separate saturation and acquisition modules as used in standard CEST and similar experiments. One of the differences from standard CEST methods is that the bSSFP spectrum is an XY-spectrum not a Z-spectrum. As the first proof-of-principle step, we have implemented the steady state bSSFP sequence for chemical exchange detection (bSSFPX) and verified its feasibility in phantom studies. These studies have shown that bSSFPX can achieve exchange-mediated contrast comparable to the standard CEST experiment. Therefore, the bSSFPX method has a potential for fast and quantitative CEST data acquisition.

Structure or Exchange? On the Feasibility of Chemical Exchange Detection with Balanced Steady‐State Free Precession in Tissue – An In Vitro Study

“…Standard CEST imaging at ultra‐high fields has not only demonstrated an easier separation of the peaks but also an increase in the effect strength for amide, amine, and NOE pools . BSSFPX, on the other hand, is expected to show a complex behavior with increasing field strength due to the periodicity of the bSSFP frequency response with 1/TR, which results in CEST‐related asymmetries at pool offsets larger than 1/TR folding back into the profile and consequently an overlap of CEST pools at different chemical shifts …”

“…5,6 On the other hand, bSSFP imaging was only recently reported to be intrinsically sensitive to chemical exchange processes leading to asymmetric profiles in isotropic probes of exchanging molecular species. 7 The authors demonstrated based on in vitro experiments that phase-cycled bSSFP, in this context termed bSSFPX, is able to detect chemical exchange processes in solute molecules with small chemical shift differences of the dominant pool close to 1 ppm, such as choline, glucose and glycogen water solutions. The provided contrast in the measured probes related to chemical exchange was observed to be comparable with standard chemical exchange saturation transfer (CEST) measurements.…”

“…The provided contrast in the measured probes related to chemical exchange was observed to be comparable with standard chemical exchange saturation transfer (CEST) measurements. 7 Based on these results, the authors hypothesized that the bSSFP sequence may perform well for studies investigating glucose infusion into brain tumors similar to glucoCEST. 8 The observed sensitivity of the bSSFP frequency response to chemical exchange raises a number of questions.…”

“…To date, the only reported experiments were performed in water solutions characterized by long T 1 and T 2 relaxation times unlike tissue. 7 It also appears interesting to investigate whether exchanging chemical moieties contribute to the pronounced bSSFP profile asymmetries observed in white matter or whether they are caused by purely structural effects related to the geometry of the fiber tracts. At low RF power as inherent to bSSFP imaging due to gradient hardware and SAR limitations, the chemical exchange processes in human brain tissues are governed by the nuclear Overhauser enhancement (NOE) effects occurring upfield from water at -3.5 ppm.…”

“…11 BSSFPX, on the other hand, is expected to show a complex behavior with increasing field strength due to the periodicity of the bSSFP frequency response with 1/TR, which results in CEST-related asymmetries at pool offsets larger than 1/TR folding back into the profile and consequently an overlap of CEST pools at different chemical shifts. 7 In this work, the above questions are explored by means of in vitro glucose and pig brain tissue experiments at field strengths of 3 T and 9.4 T. Similar experiments and simulations are performed based on probes containing homogenized fresh pig brain tissue from the brainstem to eliminate structural effects as possible source for the observed asymmetries. In addition, intact pig brain hemispheres are measured ex vivo at different angles with respect to B 0 providing reference asymmetry indices (AI) in white matter structures.…”