Improving deuterium metabolic imaging (DMI) signal-to-noise ratio by spectroscopic multi-echo bSSFP: A pancreatic cancer investigation

Purpose Deuterium metabolic imaging (DMI) maps the uptake of deuterated precursors and their conversion into lactate and other markers of tumor metabolism. Even after leveraging H-2's short T(1)s, DMI's signal-to-noise ratio (SNR) is limited. We hypothesize that a multi-echo balanced steady-state free precession (ME-bSSFP) approach would increase SNR compared to chemical shift imaging (CSI), while achieving spectral isolation of the metabolic precursors and products. Methods Suitably tuned H-2 ME-bSSFP (five echo times [TEs], Delta TE = 2.2 ms, repetition time [TR]/flip-angle = 12 ms/60 degrees) was implemented at 15.2T and compared to CSI (TR/flip-angle = 95 ms/90 degrees) regarding SNR and spectral isolation, in simulations, in deuterated phantoms and for the in vivo diagnosis of a mouse tumor model of pancreatic adenocarcinoma (N = 10). Results Simulations predicted an SNR increase vs. CSI of 3-5, and that the peaks of H-2-water, H-2(6,6')-glucose, and H-2(3,3')-lactate can be well isolated by ME-bSSFP; phantoms confirmed this. In vivo, at equal spatial resolution (1.25 x 1.25 mm(2)) and scan time (10 min), H-2(6,6')-glucose's and H-2(3,3')-lactate's SNR were indeed higher for bSSFP than for CSI, three-fold for glucose (57 +/- 30 vs. 19 +/- 11, P < .001), doubled for water (13 +/- 5 vs. 7 +/- 3, P = .005). The time courses and overall localization of all metabolites agreed well, comparing CSI against ME-bSSFP. However, a clearer localization of glucose in kidneys and bladder, the detection of glucose-avid rims in certain tumors, and a heterogeneous pattern of intra-tumor lactate production could only be observed using ME-bSSFP's higher resolution. Conclusions ME-bSSFP provides greater SNR per unit time than CSI, providing for higher spatial resolution mapping of glucose uptake and lactate production in tumors.