Fast iZQC for In Vivo Spectroscopy
Early on in the (short) history of iMQCs, it was recognized that iZQCs (intermolecular zero quantum coherences) could produce high resolution spectra in inhomogeneous fields. Yet for several years, demonstrations of such resolution enhancement in vivo remained elusive. We were able to pinpoint one of the serious obstacles of this experiment by working with worms at low temperature: physiological fluctuations during the slow 2D acquisition were broadening the lines in F1. In designing a faster pulse sequence, we developed a new scheme for fast 2D spectroscopy which could also be generalized to speed up a wide range of conventional 2D sequences.
In vivo 1H magnetic resonance spectroscopy holds tremendous potential for the diagnosis and characterization of disease states. To fully exploit this potential, high resolution is essential for distinguishing metabolites crowded into a narrow region of the spectrum. However, the inhomogeneity of many organs makes such resolution inaccessible by standard one dimensional techniques.
Intermolecular zero quantum coherences (iZQC) give sharp lines along the indirect dimension, even in very inhomogenous fields. Since this signal evolves at the difference frequency between water and other molecules a few hundred microns apart, it is insensitive to larger scale inhomogeneity. Our work on cold-blooded animals at low temperatures (Figure 1) has increased resolution an order of magnitude over the direct acquisition. Though previous in vivo work has only shown a modest increase in resolution, we believe this is caused by t1 noise inherent to the long experimental times of two dimensional spectroscopy: each heartbeat and breath irregularly distorts the spectrum from scan to scan and ultimately broadens the indirect dimension. Ultrafast two-dimensional methods can overcome this problem in principle. However, the Frydman approach (time offset excitations of many different narrow slices) will not work for our application, since the molecules involved in an iZQC inevitably suffer some chemical shift misregistration, and their coupling would thus be eliminated. Instead, we developed a single scan approach that uses low flip angle mixing pulses paired with gradients to excite the entire indirect signal in a single scan. (Figure 2) The low flip angle pulses deplete just a small fraction of the magnetization, while the gradients serve the dual purpose of isolating signals from each t1 excitation and avoiding flip angle buildup. Preliminary work in vitro has demonstrated simultaneous acquisition of up to thirty-one indirect-dimension points per sequence. We have also demonstrated the extension of this approach to traditional two-dimensional sequences, such as intramolecular double quantum filtered spectroscopy.
Figure 1. In vivo spectra of worms show a significant improvement in resolution is possible when the t1 noise from physiological fluctuations is reduced.
Figure 2. The sequence for fast iZQC with four-fold speedup can be seen as a superposition of four t1 steps in a HOMOGENIZED experiment.
Figure 3. Spectrum of a 5% MEK solution in water, acquired with the fast iZQC pulse sequence with eight indirect acquisition points per scan. Water is suppressed with selective inversion pulses.
