State of the art in magnetic resonance imaging

As a clinical technology, MRI offers unsurpassed flexibility to look inside the human body.
In 1977, inspired by the observation that cancerous and healthy tissues produced different nuclear magnetic resonance signals, Raymond Damadian, Michael Goldsmith, and Lawrence Minkoff performed the first MRI scan of a live human body. In the early days of clinical MRI, scans took hours and provided low spatial resolution, but they have become essential for distinguishing between healthy and diseased tissues. By its 40th anniversary, MRI was a must-have tool in hospitals and clinics of all sizes. And it has found applications in image-guided interventions and surgeries, radiation therapy, and focused ultrasound. Advances in technology, meanwhile, have pushed the envelope of scanner performance with improvements to speed and spatial resolution.
At the frontiers of MRI development, work is focused on fast, quantitative imaging. Clinical needs increasingly demand functional information—on heart-muscle contractions, brain activity,3 chemical concentrations in tumors,4 and blood flow in and out of tissue5—in addition to anatomical structures. New approaches must also maintain a patient’s comfort and safety; MRI is well-known for sparing patients any exposure to ionizing radiation, yet it is not without hazards.

When biological tissue is placed in a magnetic field, nuclei with magnetic moments become magnetized. RF pulses are then applied that match the resonance, or Larmor, frequency of the nuclei, causing them to tip out of alignment with the external magnetic field and precess about it. The precessing nuclei, in turn, induce oscillating magnetic fields at the Larmor frequency; those oscillations are detected via Faraday induction of an electromotive force in a nearby coil of wire.
In practice, many nuclei must precess in phase with each other to produce a detectable signal. The loss of phase coherence among precessing nuclei over time is called T2 relaxation. And the orientations of the nuclei eventually return to their equilibrium orientation in the external magnetic field—a process called T1 relaxation. Early NMR experiments revealed that various tissues have distinct T1 and T2 relaxation times. For certain diseases, including cancer, changes in either time can distinguish between diseased and healthy tissue. That feature is useful in the case of lesions whose absorption of x rays is similar to that of surrounding healthy tissue, which makes them difficult to detect using radiography or x-ray computed tomography.
In NMR measurements, the timing of the applied RF pulse and of the RF readout signal from the tissue can be chosen so that the strongest signal is produced by tissue with the shortest T1 relaxation time. A measurement whose timing is chosen that way is called a T1-weighted measurement. Alternatively, the sequence timing can be chosen so that the strongest signal comes from tissue with the longest T2 relaxation time, a T2-weighted result.
In tissue, the hydrogen nucleus is the most abundant magnetizable nucleus. Its gyromagnetic ratio is 42.56 MHz/T, which results in operating at Larmor frequencies of roughly 64 MHz and 128 MHz for 1.5 T and 3 T MRI scanners, respectively. Magnets with a range between 0.2 T and 7 T are used for clinical scanning, and human scanning up to 10.5 T is currently available in research settings.
Using the NMR signals from tissue for clinical diagnosis requires that they be localized in three dimensions to form images. Three sets of electromagnetic gradient coils in the MRI scanner accomplish that task. Each produces a linearly varying magnetic field along one of three orthogonal axes. And each gradient can be switched on and off to produce different strengths depending on the current applied; the gradient fields are superimposed on the main magnetic field—usually 1.5 T or 3 T—to create a spatially dependent variation in Larmor frequency. If a gradient is applied for some time and then turned off, all signals have the same frequency, but their relative phase shifts, accumulated while the gradient was on, vary according to position along the gradient axis…..

Read more at https://physicstoday.scitation.org/doi/10.1063/PT.3.4408

A quick how-to user-guide to debunking pseudoscientific claims

Maxim Sukharev
Have you ever wondered why we have never heard of psychics and palm readers winning millions of dollars in state or local lotteries or becoming Wall Street wolfs? Neither have I. Yet we are constantly bombarded by tabloid news on how vaccines cause autism (hint: they do not), or some unknown firm building a mega-drive that defies the laws of physics (nope, that drive does not work either). And the list continues on and on and on. Sometimes it looks quite legit as, say, various natural vitamin supplements that supposedly increase something that cannot be increased, or enhance something else that is most likely impossible to enhance by simply swallowing a few pills. Or constantly evolving diets that sure work giving a false relieve to those who really need to stop eating too much and actually pay frequent visits to a local gym. It is however understandable that most of us fall for such products and news just because we cannot be experts in everything, and we tend to trust various mass-media sources without even a glimpse of skepticism. So how can we distinguish between baloney statements and real exciting scientific discoveries and breakthroughs? In what follows I will try to do my best to provide a simple how-to user guide to debunking pseudoscientific claims.
Read more at https://arxiv.org/ftp/arxiv/papers/1906/1906.06165.pdf

Einstein’s biggest mistake?


Gary J. Ferland
What, if any, was Einstein’s biggest mistake, the one most affecting our physics today? There is a perhaps apocryphal story, recounted by George Gamow, that he counted his cosmological constant as his biggest blunder. We now know his hypothesized cosmological constant to be correct. His lifelong rejection of quantum mechanics, an interesting side-story in the evolution of 20th-century physics, is a candidate. None of these introduced difficulties in how our physics is done today. It can be argued that his biggest actual mistake, one that affects many subfields of physics and chemistry and bewilders students today, occurred in his naming of his A and B coefficients…
Read more at https://arxiv.org/pdf/1905.09276.pdf

Two Notions of Naturalness

Porter Williams
My aim in this paper is twofold: (i) to distinguish two notions of naturalness employed in BSM physics and (ii) to argue that recognizing this distinction has methodological consequences. One notion of naturalness is an “autonomy of scales” requirement: it prohibits sensitive dependence of an effective field theory’s low-energy observables on precise specification of the theory’s description of cutoff-scale physics. I will argue that considerations from the general structure of effective field theory provide justification for the role this notion of naturalness has played in BSM model construction. A second, distinct notion construes naturalness as a statistical principle requiring that the values of the parameters in an effective field theory be “likely” given some appropriately chosen measure on some appropriately circumscribed space of models. I argue that these two notions are historically and conceptually related but are motivated by distinct theoretical considerations and admit of distinct kinds of solution.
Read more at https://arxiv.org/pdf/1812.08975.pdf

Relativistic spring-mass system

Rodrigo Andrade e Silva, Andre G. S. Landulfo, George E. A. Matsas, Daniel A. T. Vanzella

The harmonic oscillator plays a central role in physics describing the dynamics of a wide range of systems close to stable equilibrium points. The nonrelativistic one-dimensional spring-mass system is considered a prototype representative of it. It is usually assumed and galvanized in textbooks that the equation of motion of a relativistic harmonic oscillator is given by the same equation as the nonrelativistic one with the mass M at the tip multiplied by the relativistic factor 1/(1−v2/c2)1/2. Although the solution of such an equation may depict some physical systems, it does not describe, in general, one-dimensional relativistic spring-mass oscillators under the influence of elastic forces. In recognition to the importance of such a system to physics, we fill a gap in the literature and offer a full relativistic treatment for a system composed of a spring attached to an inertial wall, holding a mass M at the end.

Read more at https://arxiv.org/pdf/1810.13365.pdf