Session F36: In-vivo Magnetic Measurements for Medical Diagnosis, Therapy and Discovery

Magnetic functional neuroimaging and transcranial magnetic stimulation

TBD   

MRI Magnetic Susceptibility Mapping In Vivo

The magnetic susceptibility of living tissues depends on their composition and microstructure. Therefore, the emerging, non-invasive, magnetic resonance imaging (MRI) technique of quantitative susceptibility mapping (QSM) is beginning to yield clinically useful information on pathophysiology-related changes in tissue composition and microstructure. I will outline the physical principles underpinning QSM and describe QSM applications we have developed in sickle cell anaemia, healthy brain ageing and head-and-neck cancer.

Weak tissue susceptibilities (Schenck, Med Phys 1996) cause small magnetic field perturbations seen in phase of the complex MRI signal. In QSM we calculate susceptibility maps from MRI phase images. MRI phase is a 0 to 2π angle in the complex plane and must be unwrapped. The unwrapped images are dominated by large-scale background phase variations caused by the relatively large air-tissue susceptibility difference. Once the background field variations are removed the result is the input for susceptibility calculation. This is an ill-posed inverse problem for which many regularisation methods have been proposed. There is a plethora of algorithms available for each stage in the QSM pipeline and the MRI QSM community is working to achieve consensus on the best methods.

QSM has important advantages over phase images and a widespread precursor known as susceptibility weighted imaging (SWI): it overcomes the non-local and orientation-dependent phase contrast (Shmueli, MRM 2009) to improve visualisation of tissue structure and composition.

Clinical applications are emerging based on QSM’s sensitivity to tissue calcifications, iron, myelin and deoxyhaemoglobin content. QSM highlights iron-rich brain structures in Parkinson’s disease, microbleeds and haemorrhages and distinguishes these from calcifications. QSM allows quantification of venous oxygenation with functional QSM now able to detect brain activity.   

Magnetoencephalography using optically-pumped magnetometers

Current magnetoencephalography (MEG) uses a helmet-shaped array of low-temperature superconducting quantum interference devices (SQUIDs) to image the magnetic field produced by the neural currents inside the head with millisecond temporal resolution. Moving the sensors closer to the scalp holds the promise of simultaneously increasing the spatial resolution of MEG. Our optically-pumped magnetometers (OPMs) are room-temperature sensors that work on the principle of laser spectroscopy of alkali atoms in a vapor cell. They can be microfabricated and placed in close proximity to the scalp for MEG. The MEG test system we present consists of 48 microfabricated OPMs, that are integrated into pairs on small flying lead sensor heads, such that they form 24 first-order gradiometers with a baseline of 2 cm. The gradiometer and magnetometer data are read out simultaneously. The sensors are assembled on a conformal 3D printed helmet with spokes that can be adjusted in the radial direction through a ratchet system. The magnetic field in the radial direction to the head is recorded with the magnetometers and radial gradiometers. We present first standard MEG recordings of resting-state measurements and evoked responses.
  

Smart magnetic probes for in-vivo metrology

Magnetic resonance imaging (MRI) represents one of the most outstanding examples of the successful application of something that began as fundamental physics research. Together with related NMR technologies, it now impacts many fields including food, pharmaceutical, chemical, energy, and of course medical industries, where it has rapidly become one of the most widely used medical imaging and diagnostic tools. Not requiring ionizing radiation, it can benignly probe deep within the body, offering excellent soft tissue contrast and, compared with other radiological methods, high resolution imaging.

But there remains much room to further develop the technologies. Specifically, this talk will discuss several recent examples [1,2] of how work in physics and engineering is enhancing the functionality of MRI through (i) new contrast agents 10-100x more powerful than existing alternatives that enable in vivo tracking of cells down to the single cell level, (ii) new micromagnetic structures that add “color” or multiplexing capabilities to traditionally black-and-white MRI, and (iii) smart polymer – magnetic composites that enable new radio-frequency (RF) addressable microsensors, or smart probes, for quantitative sensing. The talk will focus on the basic physics and engineering behind these imaging and sensing agents, including how they are made and some example first applications. Presentation will be at a general, introductory level; no prior knowledge of NMR/MRI required.

References:
[1] Zabow et al. Nature 453, 1058 (2008)
[2] Zabow et al. Nature 520, 73 (2015)   

Ultra-low field and unconventional MRI.

A promising approach to portable MRI is operation at ultra-low magnetic field where cost-effective electromagnets become practical. MRI in the ultra-low field (ULF) regime —when the magnetic field used for signal detection is below 10 mT—is inherently challenging mainly due to intrinsically low Boltzmann polarization. We will discuss hardware methods to improve attainable SNR in the Johnston-noise-dominated regime of ULF using improved coils such as quadrature volume coils at 276 kHz (6.5 mT). We will also discuss our work to reduce noise and increase attainable information per unit time using compute-based approaches that leverage low-cost GPU. These include magnetic resonance fingerprinting (MRF) to enable multiple quantitative contrasts at ULF, and the use of our neural network deep learning approach, AUTOMAP (AUtomated TransfOrm by Manifold APproximation), to reconstruct highly-undersampled low SNR imaging data..