An indirect method for in vivo T₂ mapping of [1‐¹³C] pyruvate using hyperpolarized ¹³C CSI

Abstract

An indirect method for in vivo T mapping of C-labeled metabolites using T and T * information of water protons obtained a priori is proposed. The T values of C metabolites are inferred using the relationship to T ' of coexisting H and the T * of C metabolites, which is measured using routine hyperpolarized C CSI data. The concept is verified with phantom studies. Simulations were performed to evaluate the extent of T estimation accuracy due to errors in the other measurements. Also, bias in the C T * estimation from the C CSI data was studied. In vivo experiments were performed from the brains of normal rats and a rat with C6 glioma. Simulation results indicate that the proposed method provides accurate and unbiased C T values within typical experimental settings. The in vivo studies found that the estimated T of [1- C] pyruvate using the indirect method was longer in tumor than in normal tissues and gave values similar to previous reports. This method can estimate localized T relaxation times from multiple voxels using conventional hyperpolarized C CSI and can potentially be used with time resolved fast CSI.

Translation of Carbon‐13 EPI for hyperpolarized MR molecular imaging of prostate and brain cancer patients

“…Finally, in these initial studies, the total readout time for each slice was 9.9 ms for prostate studies and 16.5 ms for brain studies. Although this resulted in short echo‐spacing and made the acquisitions more robust to B 0 inhomogeneity, we anticipate that the SNR could readily be improved by increasing the total readout duration, given the relatively long T 2 * of 13 C substrates . However, this will place more of a burden on shimming and frequency calibration, as a further increase in echo‐spacing would make the acquisition more susceptible to geometric distortion and bulk shifts in the blip dimension.…”

“…Although this resulted in short echo-spacing and made the acquisitions more robust to B 0 inhomogeneity, we anticipate that the SNR could readily be improved by increasing the total readout duration, given the relatively long T 2 * of 13 C substrates. 30 However, this will place more of a burden on shimming and frequency calibration, as a further increase in echo-spacing would make the acquisition more susceptible to geometric distortion and bulk shifts in the blip dimension. In this case, alternating the blip polarity each time frame 31 or using a dual-echo EPI acquisition 32,33 could potentially be used to correct for distortion and signal loss arising from B 0 inhomogeneity.…”

3D hyperpolarized C-13 EPI with calibrationless parallel imaging

“…Blurring arising from T2*-related signal decay will ultimately limit the echo train length. Given that the in vivo T2* values are on the order of 20–100 ms at 3 T [27], readout durations on the order of 20–40 ms should be achievable without substantial resolution loss. While T2 has been reported to be substantially longer than T2* for 13 C substrates [28,29], spin echo approaches [30,31] are problematic for hyperpolarized studies because miscalibration of the B 1 power and B1+ inhomogeneity can lead to rapid RF decay from the refocusing pulses due to saturation at the edge of the coil [32].…”

Coil profile estimation strategies for parallel imaging with hyperpolarized 13C MRI

“…The digital phantom was designed as a replica of the physical phantom and simulated with an 8° tilt along the z‐axis (see Supporting Information Figure ). The following relaxation parameters were used in the simulation: T 1 = 1000 ms; T 2 = 200 ms; and = 50 ms (simulating properties of pyruvate) . The simulation was performed with a ±15 Hz macroscopic off‐resonance distribution and hard pulse singlet excitation.…”

“…The following relaxation parameters were used in the simulation: T 1 = 1000 ms; T 2 = 200 ms; and T * 2 = 50 ms (simulating T * 2 properties of pyruvate). 24 The simulation was performed with a ±15 Hz macroscopic off-resonance distribution and hard pulse singlet excitation. Off-resonance effects of the SPSP excitation were therefore not simulated.…”