Over the course of the semester, our trainees are reviewing webinars in their given fields and preparing abstracts to help colleagues outside their discipline make an informed choice about watching them. As our program bridges diverse disciplines, these abstracts are beneficial for our own group in helping one another gain key knowledge in each other’s fields. We are happy to share these here for anyone else who may find them helpful.
Label free molecular imaging with spins, a path to high resolution through learned subspaces
Zhi-Pei Liang, Professor of Electrical Engineering, University of Illinois at Urbana-Champaign
Date: 4/5/2021
Analysis by Charles Marchini:
This webinar briefly describes an imaging method used to measure molecules in organisms using magnetic resonance spectroscopic imaging. It is important to know about because the method because it can be a very valuable tool in assessing the molecular composition of tissues in humans safely and has applications in neuroscience.
The field of biomedical imaging has advanced greatly in the past century with the invention of ultrasound, computed tomography (3-dimensional x-ray), and magnetic resonance imaging scanners. For imaging specific molecules, exogenous molecular probes are often used, which is invasive. Magnetic resonance spectroscopy (MRS) can measure molecules noninvasively from endogenous signals originating from the nuclear spin, a property found in the nucleus of many elements common in biology such as hydrogen, phosphorus, and carbon. The safety that comes from utilizing endogenous signals gives MRS a basic advantage when it comes to translating imaging methods from research studies to clinics. Magnetic resonance spectroscopic imaging (MRSI) not only the measures molecules but also identifies their location.
MRS works because it can differentiate between different frequencies of atomic nuclei, which is directly proportional to something known as the chemical shift. The frequencies of the nuclei depend on the number of protons and neutrons in the nuclei and the magnetic field they experience, which changes with the degree of electron shielding experienced by the nuclei. While applying a magnetic field as done with magnetic resonance imaging, the different frequencies that are measured can then be used to represent specific functional groups and specific molecules we are looking to image. The medical applications for MRSI technology include early detection of diseases, characterization of diseases and analyzing the metabolomic mechanism of diseases, such as imaging a decrease in choline in tumors after radiosurgery treatment.
Current challenges in the field of MRSI include the slow speed of imaging, the low signal to noise ratio, and the poor spatial resolution. One of the methods that can improve the problems is called spectroscopic imaging by exploiting spatiospectral correlation (SPICE). The method allows the imaging process to be significantly accelerated by using sub-Nyquist sampling (meaning the sampling rate is less than twice the frequency being resolved). It does this by assuming that the data can be broken down into a smaller set (smaller than a full set) of spatial basis functions and spectral basis functions. The spatial basis functions can be found for each of the molecules by using quantum simulations and training data.
Using the method, the imaging time to acquire a useful image was reduced from one hour to 6 minutes. Common molecules that can be imaged include N-acetyl aspartate, which is a marker of neuronal activity, and phosphocreatine, a marker of brain energy metabolism. Other molecules imaged include Myo Inositol, glutamate, glutamine, and amyloid beta.