MRI contrast from relaxation along a fictitious field (RAFF)

Abstract

A new method to measure rotating frame relaxation and to create contrast for MRI is introduced. The technique exploits relaxation along a fictitious field (RAFF) generated by amplitude- and frequency-modulated irradiation in a sub-adiabatic condition. Here, RAFF is demonstrated using a radiofrequency pulse based on sine and cosine amplitude and frequency modulations of equal amplitudes, which gives rise to a stationary fictitious magnetic field in a doubly rotating frame. According to dipolar relaxation theory, the RAFF relaxation time constant (TRAFF) was found to differ from laboratory frame relaxation times (T1 and T2) and rotating frame relaxation times (T1ρ and T2ρ). This prediction was supported by experimental results obtained from human brain in vivo and three different solutions. Results from relaxation mapping in human brain demonstrated the ability to create MRI contrast based on RAFF. The value of TRAFF was found to be insensitive to the initial orientation of the magnetization vector. Finally, as compared with adiabatic pulse trains of equal durations, RAFF required less radiofrequency power and therefore can be more readily used for rotating frame relaxation studies in humans.

MRI rotating frame relaxation measurements for articular cartilage assessment

“…Imaging time was about 26 minutes for each measurement of T 1ρ-HS1 and T 1ρ-Hs4 . For RAFF [30], sine/cosine pulse trains of 0, 8, 16, 24, and 32 pulses were implemented with a pulse duration = 4.526 ms and ω 1 max /(2π) = 625 Hz (Figure 2b). For MT, hard pulses with amplitude ω 1 max /(2π) = 200 Hz and duration = 0, 0.3, 0.6, 0.9 and 1.2 s were irradiated at −10000 Hz offset.…”

“…To further alleviate SAR concerns for human applications, and to extend the sensitivity of relaxation to even slower molecular motions, our group has recently developed a novel rotating frame relaxation technique called relaxation along a fictitious field (RAFF) [29,30]. This method utilizes frequency-swept pulses operating non-adiabatically, and therefore generally requires less peak power than adiabatic pulses of similar pulse-length.…”

Functional MRI Measurements to Predict Early Adenoviral Gene Therapy Response in Ovarian Cancer Mouse Model

“…Relaxation along a fictitious field (RAFF) is a new relaxation time method that may offer a more sensitive marker to evaluate cell density and cell death in different tumor regions as a result of gene therapy as compared to T 1ρ or T 2 weighted imaging [37]. RAFF relaxation times measured in this ovarian cancer model were generally lower corresponding to previous results from the malignant glioma model [24], possibly caused by a more solid mature of the tumor.…”

“…Using these images, one slice showing the largest tumor in top part of the mice abdomen was selected for the relaxation time measurements and diffusion weighted imaging. The measured relaxation time parameters were the transversal relaxation time T 2 (adiabatic Hahn double echo preparation with TE = 8 -22 ms), the longitudinal relaxation time in the rotating frame T 1ρ (spin-lock time = 0 -45.4 ms, RF amplitude (γB 1 /(2π) )=1250 Hz) [36], and two novel rotating frame parameters; relaxation along a fictitious field in the 2 nd (T RAFF2 ) and 4 th frame (T RAFF4 ) (T RAFF2 or 4 -pulses [37,45], pulse train length of 0 -36 ms, γB 1 /(2π) = 1250 Hz for RAFF2 and 648 Hz for RAFF4). The radio frequency field Total tumor volumes from multiple tumor loci (mm 3 ) and area of the tumor (mm 2 ) were hand drawn on T 2 weighted anatomical images.…”

“…Additionally T 1ρ , T 2 , T RAFF2 and T RAFF4 relaxation times and water ADC were analyzed separately for vital and necrotic areas of tumors. All analyses were performed using Aedes software package on Matlab platform (MathWorks, Natick, MA) as in references [28,37]. Tumors were identified from surrounding healthy soft tissue with their deviant intensity and location.…”